Homepage: http://www.geocities.com/abhijitsathe Experimental and Theoretical Studies on a Bubble Pump for a Diffusion-Absorption Refrigeration System Project work completed for the award of the degree of Master of Technology in Mechanical Engineering by ABHIJIT SATHE at Institut für Thermodynamik und Wärmetechnik Universität Stuttgart Germany Refrigeration and Air-Conditioning Laboratory Department of Mechanical Engineering Indian Institute of Technology Madras India Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) Homepage: http://www.geocities.com/abhijitsathe List of Contents Abstract Acknowledgement INTRODUCTION The Diffusion-Absorption Refrigeration Cycle The Bubble Pump Two Phase Flow Literature Review MATHEMATICAL ANALYSIS The Maximum Pump Tube Diameter Mathematical Model SELECTION OF PROPERTIES Selection of Working Fluid Selection of Dimensions Selection of Other Parameters EXPERIMENTAL SET-UP Overall Set-up Description Detailed Component Description Test Procedure RESULTS AND DISCUSSIONS Observations with a Transperant Bubble Pump Tube Evaluation of Bubble Pump Operating Parameters Comparison with the Mathematical Model Conclusion List of Symbols References Appendix Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) Homepage: http://www.geocities.com/abhijitsathe ABSTRACT A diffusion-absorption refrigeration cycle or a pumpless vapour absorption refrigeration cycle holds a great significance in noiseless refrigeration applications. The diffusion-absorption cycle is unique in that it runs without any mechanical work input. The cycle utilizes ammonia-water-hydrogen as working fluids. The diffusion-absorption cycle relies on a bubble pump to pump the solution from the absorber to the boiler. A bubble pump is a fluid pump that operates on thermal energy to pump liquid from lower level to the higher level. It does not contain any moving parts. The bubble pump operates on the same principle that lifts coffee to the top of a coffee percolator. The liquid in the liquid reservoir initially fills the tube to the same level (h). Heat is applied at the bottom of the tube at a rate sufficient to boil some of the liquid in the tube. The resulting vapour bubbles rise in the tube. Due to the small diameter of the pump tube, the vapour bubbles occupy complete cross-section of the tube and are separated by small liquid slugs. Each bubble acts as a gas piston and lifts the corresponding liquid slug to the top of the pump tube. The bubble pump operates most efficiently in the slug flow regime in which the vapour bubbles are approximately the diameter of the tube. The important parameters of the bubble pump are pump tube diameter (dp), driving head (h), pump lift (L) and pump heat input (Qp). The bubble pump was built and tested in a test-rig. The test-rig did not comprise of a refrigeration system. The working fluid used was methyl alcohol (methanol). Methyl alcohol has a boiling point of 64 ° C which was suitable for the given set-up. It is non-reactive with copper at all temperatures. Liquid methanol was stored in a liquid reservoir. It was first pre-heated to the saturation temperature in a liquid pre-heater. Heat was supplied at the bottom of the bubble pump tube by means of an electrical heater. A small portion of liquid boiled off and the remaining liquid was lifted to the top by the rising vapour bubbles. The liquid that was pumped by the bubble pump was separated from the accompanying vapour bubbles in a liquid-vapour separator. The vapour was condensed and the flow rate of the condensate was measured. Flow rate of the pumped liquid was also measured separately. The bubble pump was tested extensively for varying heat inputs and different pump tube diameters and driving heads at constant ambient pressure. The influence of these parameters on the flow rate of the pumped liquid is discussed in detail. Pumping ratio is another important parameter to judge the performance of the bubble pump. The variation of the pumping ratio with the pump heat input for different driving heads and different pump tube diameters is also discussed. The frequency of pumping action is observed to increase with increase in pump heat input. The mass flow rate of the vapour increases linearly with the heat input while the mass flow rate of the pumped liquid first increases, attains a maximum value and then decreases with increase in the heat input. There exists an optimum value of the heat input for each bubble pump where the pump renders the maximum amount of pumped liquid. This value of heat input increases with increase in the pump tube diameter. The pumping ratio decreases almost linearly with the heat input. Submergence ratio, defined as a ratio of driving head to pump lift, also influences the pump behavior. Higher the submergence ratio, more is the amount of the pumped liquid for the same heat input. A mathematical model of the bubble pump is established by using simple analytical equations such as the continuity equation and the momentum equation. The model assumes slug flow in the bubble pump. The model is compared with the experimental results. A correction factor is necessary to account for the discrepancies observed between the actual experimental observations and the assumptions made in the theoretical studies. The correction factor established is a function of the vapour flow rate. It is also a function of the pump tube diameter and the correlation between the two can be established by conducting more tests. Keywords:- Diffusion-absorption refrigeration, bubble pump, two-phase flow, driving head, pump lift, pumping action. Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) Homepage: http://www.geocities.com/abhijitsathe ACKNOWLEDGEMENT I wish to express my sincere gratitude to Dr. -Ing. K. Spindler for his invaluable suggestions and guidance throughout my work at the University of Stuttgart. The eager involvement on his part in my dissertation work at every step has really been encouraging. I heartily thank Dipl. -Ing. Thomas Brendel for his wonderful guidance throughout the project work. He always guided me through the difficulties and made me understand the concepts needed for the project. His experimental and theoretical knowhow was indeed very helpful. I express my deep gratitude to Dr. M. P. Maiya for advising and guiding me through e-mails. Without his timely help and advice it would not have been possible for me to complete this project. I am very much thankful to Prof. S. Srinivasa Murthy for his timely help during the early days and also throughout the project work. I am also grateful to Prof. Dr. -Ing. E. Hahne for his timely encouragement. I also heartily thank all my co-workers in ITW for their co-operation and help. A special thank is owed to Deutscher Akademischer Austauschdienst (German Academic Exchange Service) or DAAD for providing financial support as well as arranging for our stay and other related things in Germany. I would also like to thank the staff of Internationale Angelegenheiten (Office of International Affairs) for their kind and timely help during our stay in Germany. Last but not the least, I thank my family members and friends for giving me moral support and advice whenever needed. Abhijit Sathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) In reality. This is achieved by pumping the fluids using a bubble pump driven by heat. Another unique feature of this cycle is that it is essentially noise free. The cycle is unique in that it runs without any mechanical work input.geocities. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The diffusion-absorption cycle utilizes ammonia-water-hydrogen as working fluid. In the diffusion-absorption cycle.e. The auxiliary gas provides pressure equalization for the working fluid between the condenser and evaporator. The schematic arrangement for a typical diffusion-absorption refrigerator is shown in Fig. Water absorbs the refrigerant at low temperature and low partial pressure and releases it at high temperature against a high partial pressure. the partial pressure of the ammonia gas varies from point to point instead of the overall system pressure.com) . Weak solution flows from the boiler to the absorber because of the difference in height between the top of the boiler and the top of the absorber. The roles of ammonia and water are familiar from absorption cycle experience. The circulations in the system are produced solely by gravity and density differences as follows: l l l Hydrogen circulates between the absorber and the evaporator because of the greater density of the ammonia-rich gas column. The refrigerant serves as a transporting medium to carry energy from a low temperature source to a high temperature sink.Homepage: http://www.com/abhijitsathe INTRODUCTION The Diffusion-Absorption Refrigeration Cycle The diffusion-absorption refrigeration cycle was pioneered around 1920 by two Swedes named von Platen and Munters. The cycle utilizes a regenerative gas heat exchanger between the evaporator and the absorber which is driven by gravity-induced pressure differences. Strong liquid coming out of the absorber is carried to the top of boiler by the action of the bubble pump.1 The cycle uses a three-component working fluid consisting of the refrigerant (ammonia). The widest commercial use is ammonia-water-hydrogen. the one descending from the evaporator. Hydrogen is used as a capping gas to equalize the pressure throughout the cycle to allow the low-head bubble pump to operate as a liquid circulator. there are small variations in the system pressure that are quite important for operation. Heat applied to the pump causes formation of bubbles and the density of strong solution in the vertical pump tube is reduced so that the solution is forced to the top by the static head of solution in the absorber vessel. the absorbent (water) and the auxiliary gas (hydrogen). 1. i. Helium can also be used as the auxiliary gas with a performance penalty. The liquid in the liquid reservoir initially fills the tube to the same level (h).com/abhijitsathe Fig.geocities.1. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. 1. system pressure and properties of the pumped solution. The Bubble Pump As discussed in section 1. Heat is applied at the bottom of the tube at a rate sufficient to evaporate some of the liquid in the tube. The bulk density of the liquid and vapour mixture in the pump tube is reduced relative to the liquid in the liquid reservoir.1 Schematic arrangement of a diffusion-absorption refrigerator The diffusion-absorption cycle has inherent irrreversibilities that are larger than those found in typical vapour absorption cycles. the vapour bubbles occupy complete cross-section of the tube and are separated by small liquid slugs. In the latter case. The bubble pump. The energy sources generally used are (i) electric heat and (ii) flame heat. In particular. the diffusion-absorption cycle relies on a bubble pump to pump the solution from the absorber to the boiler. Due to the small diameter of the pump tube. the entire length of the bubble pump or boiler is heated to increase the heat transfer area. Each bubble acts as a gas piston and lifts the corresponding liquid slug to the top of the pump tube. as shown in Fig. is nothing but a vertical tube of small circular cross-section. Depending on the bubble pump tube. 1. A bubble pump is a fluid pump that operates on thermal energy to pump liquid from lower level to the higher level. The resulting vapour bubbles rise in the tube. there is an increased mass transfer resistance on the vapour side due to the presence of hydrogen. The bubble pump operates on the same principle that lifts coffee to the top of a coffee percolator.Homepage: http://www.2. These factors explain why the cycle performance is fairly low. thereby creating an overall buoyancy lift. It does not contain any moving parts.com) . There is also an additional heat exchanger called the auxiliary gas heat exchanger. g. The important parameters of the bubble pump are pump tube diameter (dp). g. · Void fraction:. The rising vapour bubble acts like a piston and lifts a corresponding liquid slug to the top of the bubble pump tube. vapour-liquid mixture in a bubble pump) while gas-liquid mixtures where the vapour and liquid are different fluids are referred to as two-phase two component systems (e.com) . the flow behavior changes from the slug flow regime to that of the mixed flow. where the vapour and liquid are phases of the same fluid are referred to as two-phase single component mixtures (e. 1. pump lift (L) and pump heat input (Qp). Vapourliquid mixtures.The void fraction is the ratio of the gas flow cross-sectional area to the total flow cross-sectional area.2 The bubble pump Two Phase Flow Basic Definitions A two-phase flow is defined as a flow of two separate parts of a heterogeneous body or system. · Dryness fraction:. air-liquid mixture in an air-lift pump). Following are some commonly used terms in two-phase flow. After a certain pump tube diameter is exceeded. Fig.geocities. The main characteristic values to judge the performance of the bubble pump are solution flow rate and the pumping ratio.It is defined as a ratio of mass flow of gas to the total mass flow. driving head (h). namely slug flow and mixed vapour bubble-liquid (bubbly) flow. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. At the bottom of the pump tube small bubbles form and join together forming bigger vapour bubbles.com/abhijitsathe two different kinds of flow are possible.Homepage: http://www. Two sets of basic equations can now be written.In this approach the two phases of the flow are considered to be artificially segregated..The calculation of two-phase pressure drop involves some complex calculations. At one extreme. The Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www. This information is inserted in the basic equations. the gas or vapour phase is distributed as discrete bubbles in a continuous liquid phase. 1. the two-phase flow is assumed to be a single-phase flow having pseudo-properties arrived at by suitably weighting the properties of the individual phases. 1. Alternatively.com) . It is the ratio of mass flow rate to the total flow cross-section area of the mixture. The flow pattern model:. the equations can be combined.3. Three main types of assumptions have been made.In two-phase flow literature. 2. the simplest approach to the problem. Various correlations and charts are used to calculate the pressure gradients developed due to friction in the flow and change in momentum. momentum. In slug flow the gas or vapour bubbles are approximately the diameter of the pipe.1. In either case information must be forthcoming about the area of the channel occupied by each phase and about the frictional interaction with the channel wall. In bubbly flow. Slug Flow. In order to apply these models. Following flow patterns are encountered when a mixture of vapour and liquid flows through a vertical pipe. it is necessary to know when each should be used and to be able to predict the transition from one pattern to another. The separated flow model:. Bubbly flow. and energy.geocities. Methods of Analysis The methods used for analyzing a two-phase flow are extensions of those already well tried for singlephase flows. l Pressure drop:.In this more sophisticated approach the two phases are considered to be arranged in one of three or four definite prescribed geometries. either from separate empirical relationships in which the void fraction and the wall shear stress are related to the primary variables. The basic equations are solved within the framework of each of these idealized representations. Flow Patterns The flow patterns encountered in vertical upwards co-current flow are shown in Fig. the bubbles may be small and spherical and at the other extreme the bubbles may be large with a spherical cap and a flat tail. The procedure invariably is to write down the basic equations governing the conservation of mass. or on the basis of simplified models of the flow. 3. The homogenous flow model:.In this. there may be some confusion with slug flow. mass velocity is extensively used. often in a one-dimensional form and to seek to solve these equations by the use of various simplifying assumptions. 2. viz. In this latter state although the size of bubbles does not approach the diameter of pipe.com/abhijitsathe l Mass velocity:. one for each phase. These geometries are based on the various configurations or flow patterns found when a gas and a liquid flow together in a channel. In this case. as distinct from the wispy-annular pattern. and extensive references.3 Flow patterns in vertical co-current flow 3. Bubble pumps are also known as vapour lift pumps. Churn flow. Convective Boiling and Condensation by Collier and Thome [3] gives some useful information Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. hence the descriptive name ?churn? flow. 4. Fig. However. The length of the main gas bubble can vary considerably. The flow in this region takes the form of a relatively thick liquid film on the walls of the pipe together with a considerable amount of liquid entrained in a central gas or vapour core. An extensive search revealed that the book which provides the best starting point for a bubble pump analysis was Two Phase Flow in Pipelines and Heat Exchangers (Chisholm. 1983). since a bubble pump is really just a pipe containing two phase fluid flow. 1. Churn flow is formed by the breakdown of the large vapour bubble in the slug flow. Chisholm [2] provides the basic definitions and terminology. Large amplitude coherent waves are usually present on the surface of the film and the continuous break up of these waves forms a source for droplet entrainment which occurs in varying amounts in the central gas core. These slugs may or may not contain smaller entrained gas bubbles carried in the wake of the large bubble..geocities. This region occurs at high mass velocities and because of the aerated nature of liquid film could be confused with high velocity bubbly flow. 5. the droplets are separate rather than agglomerated. Literature Review The most common applications of bubble pumps are electric drip and percolating coffee makers. The gas or vapour flows in a more or less chaotic manner through the liquid which is mainly displaced to the channel wall. The flow has an oscillatory or time varying character. In annular flow a liquid film forms at the pipe wall with a continuous central gas or vapour core. The liquid in the film is aerated by small gas bubbles and the entrained liquid phase appears as large droplets which have agglomerated into long irregular filaments or wisps. The liquid flow is contained in liquid slugs which separate successive gas bubbles.com) .Homepage: http://www. literature on bubble pumps is nearly non-existent. This region is also sometimes referred to as semi-annular or slugannular flow.com/abhijitsathe nose of the bubble has a characteristic spherical cap and the gas in the bubble is separated from the pipe wall by a slowly descending film of liquid. books and papers on two phase flow provide more than sufficient information for the analysis of a bubble pump. Wispy-annular flow has been identified as a distinct flow pattern. the flow patterns encountered in vertical pipes. While commonly used. Wispy annular flow. Annular flow. com) . The bubble pump is tested experimentally in a test-rig and the bubble pump behavior is analyzed in detail. Saravanan. Investigations on Triple Fluid Vapour Absorption Refrigerator. 1996) [6] provides a few references to papers which mention bubble pumps. G. Design Analysis of the Einstein Refrigeration Cycle.geocities. Cattaneo [1] with a title Über die Förderung von Flüssigkeiten mittels der eigenen Dämpfe (About the pumping of liquids by means of their own steams) provided a useful information about the constructional details of the liquid-vapour separator. pump lift etc. Absorption Chillers and Heat Pumps (Herold. [9] Triple Fluid Vapour Absorption Refrigerator: Investigations on Solution Circuit. He has analyzed the performance of the bubble pump for parameters such as system pressure. An improved solution circuit is also suggested and the performance of the bubble pump for simple and improved solution circuits is compared. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The model is also compared to the bubble pump built and tested in the conceptual demonstration prototype. thesis.D. throws some light on selection of pump tube for a given pump discharge and heat input. and it is shown to provide a reasonable estimate of the heat input necessary to provide a given liquid flow rate.com/abhijitsathe about two-phase flow models and two-phase pressure drop correlations. A paper on Studies on bubble pump for a water-lithium bromide vapour absorption refrigerator by Pfaff. An old German paper by A.Homepage: http://www.gatech.edu/energy/andy_phd) has done a mathematical modeling of the bubble pump in his Ph. Delano [4] (http://www. Maiya [8] has analyzed the solution circuit of the triple-fluid vapour absorption refrigeration system and has developed a mathematical model for the solution circuit in his Ph.me. Another paper by Maiya. thesis. Maiya and Srinivasa Murthy [10] provides a modeling of the bubble pump using the manometer principle.D. The liquid is at a saturation temperature at the entry of the bubble pump. for a given fluid in a tube of diameter greater than that predicted by the above equation. 4. 3. The maximum diameter tube in which slug flow occurs is given by the following equation (Chisholm.geocities. The liquid level in the liquid reservoir does not oscillate during the operation. Note. 2. Further increase of vapour flow causes a highly oscillatory flow with a tendency for each phase alternatively to fill the tube. A bubble pump operates most efficiently in the slug flow regime.Homepage: http://www. slug flow will never occur.edu/energy/andy_phd) has modeled the bubble pump using simple analytical equations such as Bernoulli’s equation. Increasing the vapour flow causes the vapour bubbles to coalesce into bullet shaped slugs of vapour which rise in the liquid phase. 2.1 Bubble pump schematic Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) . 5. All the properties are measured at steady state. reached by even further increase of vapour flow. finely dispersed vapour bubbles will rise in a continuous liquid phase. The last flow regime. This is a bubble flow regime. This is a slug flow regime. The variation in the ambient conditions is negligible. Following assumptions were made in the modeling.me. For low vapour flow rates.gatech. Modelling of the bubble pump Andy Delano (http://www. small. 1. This is a churn flow regime. Fig. is annular flow regime in which the liquid forms a film around the pipe wall and the vapour rises up the core. 1983): where vf and vg are the specific volumes of the liquid and vapour respectively. the Continuity equation and the momentum conservation equation. there are four flow regimes for two phase up flow in a fixed diameter vertical pipe. and s is the surface tension. The liquid is uniformly heated at the bottom of the bubble pump.com/abhijitsathe MATHEMATICAL ANALYSIS The maximum tube diameter As already discussed. CV.1. conservation of momentum is applied to CV in Fig.geocities. Combining equations 2-5. Assuming that the mixture of vapour bubbles and liquid exit this control volume at a mixture velocity. 2-2. V2. Substituting equation 2-9 into equation 2-10.com/abhijitsathe In Fig. point 1 represents the inlet of the bubble pump. The specific volume at point 2 can be expressed as and. continuity equation yields: or.com) . continuity equation is applied to the control volume to which heat is applied. 2-6 and 2-7. Equation 2-8 now becomes: Next. Neglecting the friction pressure drop over this short distance. the mass flow rate of the vapour is assumed negligible relative to the mass flow rate of liquid and the specific volume of the liquid is assumed negligible relative to the specific volume of the vapour. The specific volume at point 2 is assumed to be the specific volume of a vapour-liquid mixture with a quality x. rearranging the terms. 2. Substituting equation 2-11 into equation 2-2.Homepage: http://www. Applying Bernoulli’s equation between the surface of the reservoir and point 1 yields: Next. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. Now. Now substituting equation 2-9 into equation 2-20. Weight W can be expressed as the combined weight of liquid and vapour in the tube. where Af is the superficial area through which the liquid flows and Ag is the superficial area through which the vapour flows. Finally equation 2-21 is equated with equation 2-12. Substituting equation 2-19 into equation 2-13. Substituting these equations into equation 2-15.geocities. Master's thesis work of Abhijit Sathe (e-mail:
[email protected]/abhijitsathe Now. applying the conservation of momentum to the bubble pump tube connecting the lower and upper reservoir. We can also write down the following equations. where B is the perimeter of the bubble pump tube and W is the weight of fluid in the bubble pump tube.Homepage: http://www.com) . Equation 2-14 is simplified by assuming that the density of the vapour phase is negligible as compared to that of the liquid. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. But the comparison with the experimental results is necessary for estimation of K. where f is a laminar friction factor. In the conventional diffusion-absorption refrigeration system. Furthermore.com) . The amount of the liquid pumped by the bubble pump can be expressed as This mass flow rate of liquid can be expressed as a function of the heat input using the above equations. Pipe elbows and entrance effects may be accounted for by increasing the value of K.geocities. Assuming the fluid in the lower reservoir and the tube to be saturated. and no heat transfer over the length of the pump tube. The equations established in this chapter are used in chapter 5 for the comparison with the experimental results. f is calculated assuming only liquid flow throughout the pipe. The friction factor for laminar flow is and.com/abhijitsathe where. since the flow assumed through the bubble pump is laminar. the velocity of the liquid. heating power required to produce the desired vapour flow rate is. Neglecting the mass of the condensate. V1 at the entrance of the bubble pump (point 1) can be calculated as The pumping ratio is calculated as Thus all the bubble pump parameters can be calculated mathematically. K can be an adjustable parameter to account for losses other than friction in the tube. vapour bubbles are produced by the addition of heat to the lower portion of the bubble pump tube. K may also be adjusted to match experimental data since losses are sometimes difficult to quantify analytically.Homepage: http://www. 5 °C at atmospheric pressure. The working fluid must have the following properties. the use of acetone was ruled out. Hence methanol was used as a working fluid for testing the bubble pump without any danger. So with water used as a heating medium. 1.5 °C. Methyl Alcohol :. For a smooth operation of the system. The aim of the experiments were to measure the amount of liquid pumped by the bubble pump for a given heat input. 3. The working fluid used for testing the bubble pump was methyl alcohol (methanol). they were not big enough to cause any danger to the human health. It is readily available at low prices. It is non-toxic in nature. The saturation temperature of the working fluid had to be above ambient temperature at all times if the heat supplied to the system were to be measured. the saturation temperature of the working fluid had to be less than 70 °C. Also the exhaust system of the room was good which meant all the methanol vapour that leaked out. Ethyl alcohol has a boiling temperature of 78.Methyl alcohol (CH3OH) or methanol has a boiling temperature of around 65 °C at atmospheric pressure. A counter flow heat exchanger was used for heating up the working fluid to its saturation temperature.com/abhijitsathe SELECTION OF PROPERTIES Selection of Working Fluid The most important job was the selection of the working fluid. 3. It does not react with copper. The sealing of the system was not tight enough to tolerate higher pressures. It is highly flammable.geocities. which meant water cannot be used as a heating medium. 1. 2. Though there were small losses in the system. Following fluids were considered for the selection. 1. 3. 4. The boiling point or the saturation temperature must be in the range of 40 to 70 °C at atmospheric pressure. was removed from the room. 4. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The working fluid must be non-reactive and chemically stable at all temperatures. 2. Also it is extremely toxic and presence of 5 % of acetone of vapour in air may cause fire hazards. Higher temperatures were possible only when some other heating medium such as heating oil were used. It should be easily available. Considering the considerable leakage in the set-up and the flammable nature of acetone. But methyl alcohol is toxic in nature and can be fatal if imbibed or inhaled in excess quantities. Following restrictions were encountered in the test set-up. Hence use of the refrigerants in the given system was ruled out because the saturation temperature of the working fluid had to be higher than 40 °C. But with the available system.Acetone (C3H6O) has a boiling temperature of 56 °C at atmospheric pressure. The heating source for the liquid pre-heater was selected as water.Homepage: http://www. The system was not capable of operating at pressures higher than atmospheric pressure. water heater was used to make the system simple and easy to operate. So the wall temperature of the heat exchanger had to be necessarily higher than the saturation temperature by 5-10 °C owing to heat losses. Acetone:. Ethyl Alcohol :. it was not possible to use a working fluid whose boiling temperature was higher than 100 °C.Ethyl Alcohol (C2H5OH) or ethanol has a boiling temperature of 78. Hence methyl alcohol was the only working fluid that can be used in the system considering all the constraints. 2.com) . It must be non-toxic and non-flammable. Table I in Appendix gives the properties of methyl alcohol. It is toxic in nature. the thermal load was too small. the thermal expansion of the liquid if any.Homepage: http://www. Too large separator meant the heat losses from the separator were very high. The density of methanol was accurately known at 20 °C. The maximum allowed height of the cooling unit for a domestic application was 2 m since it was the height of the room. 3. the diameter of the separator was chosen as 64 mm and the height was selected as 80 mm Selection of Other Parameters The temperature of the sub-cooled methanol liquid was maintained at 20 °C for following reasons. 2. It was essential to cool the liquid in order to put some load on the system. room temperature. 1. At 20 °C. Too small separator meant the pumped liquid got flooded inside the separator body thereby hampering smooth operation of the bubble pump. Hence the liquid was sub-cooled after condensation and then again mixed with the liquid in the liquid reservoir.e. i. Since there was no evaporator in the system.geocities. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. 1. was negligible so the volume of the liquid measured in the condensate flow meter was accurate. 2.6 m.com) . Selection of Liquid-Vapour Separator Dimensions The liquid-vapour separator formed a critical component of the bubble pump test set-up. Considering these two factors combined by the availability of the material. Following factors were considered for the selection of its dimensions. Hence the height of the bubble pump tube was selected as 1.com/abhijitsathe Selection of Dimensions The dimensions of the different components of the test set-up were determined keeping in view several factors. Selection of Pump Lift The experimental investigation of the bubble pump was a part of a project "Design of a Solar Driven Cooling Unit based on the Diffusion-Absorption Principle" for domestic air-conditioning purpose by European Union. Hence the system became thermally inefficient. Also a smaller prototype meant more inaccuracies in the results. Water was heated using the solar energy and this hot water was used to drive the refrigerator. All the components where heat leakage was anticipated were insulated by using a black foam (Armaflex) of thickness 10 mm. The vapour was condensed in a water-cooled condenser and flow rate of the condensate was measured.e.. Fig.Homepage: http://www. i.com) . 4. It was first pre-heated in a liquid pre-heater and then was boiled at the bottom of the bubble pump tube using an electrical heater. 4. (iii) temperature of the pumped liquid. (ii) temperature of the methanol liquid coming out of the liquid pre-heater. liquid and vapour were separated.1. (i) temperature of the methanol liquid entering the liquid pre-heater. The working fluid used was methyl alcohol. the condensate and the pumped liquid were passed back to the liquid reservoir. 4.The temperature was measured at six different locations in the test setup.2.com/abhijitsathe EXPERIMENTAL SET-UP Overall Description The bubble pump which forms a critical component of the diffusion-absorption refrigeration cycle. The liquid that was pumped by the bubble pump was separated from the accompanying vapour bubbles in a liquid-vapour separator.geocities. The flow rate of the pumped liquid was also measured separately.2 The actual experimental set-up Temperature Measurement:. The methanol liquid was stored in a liquid reservoir. The test-rig did not comprise of a refrigerating system. Fig. After flow measurements.1 The Schematic arrangement of the test set-up The actual experimental set-up with all components is shown in Fig. 4. The full test-rig setup is described in Fig. Both the phases. (iv) Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. was built and tested in a test-rig. Water in the reservoir was heated by an electrical heater and circulated by a small water pump. The temperature in the hot water reservoir was measured to an accuracy of 0. The efficiency of the heat exchanger was assumed to be around 70 %.geocities. The accuracy in temperature measurement was ± 0. The liquid pre-heater was a counter flow tube-in-tube heat exchanger (Fig. A NiCr-Ni tube-in-tube thermocouple was used for the temperature measurement.4). The output was measured in microvolts and was converted in °C. (v) temperature inside the liquid-vapour separator and (vi) temperature of the condensed liquid. Methanol flows through the inner tube while hot water flows through the outer tube. Both the tubes were made up of copper.3 Arrangement of thermocouple Detailed Component Description The setup has following components. Liquid Pre-heater:.1 °C by a mercury thermometer. 2. The thermocouple wires were inserted in small diameter tubes which were placed in flow whose temperature was to be measured (Fig.com) . 4. 1.Methyl alcohol was filled in a liquid reservoir made of circular glass tube of outer diameter 48 mm to a predetermined level h. Liquid Reservoir:. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. Fig.The liquid methanol then passes through a liquid pre-heater where it was heated to the saturation temperature by a hot water stream.1 °C. A vertical scale of accuracy 1 mm was fixed to the reservoir to check the level of liquid during initial conditions as well as during the tests. The hot water used for heating the methanol liquid was supplied from a constant level hot water reservoir.3). The inner tube had an inner diameter of 10 mm and a thickness of 1 mm while the outer tube had an inner diameter of 20 mm and a thickness of 1 mm. The maximum and the minimum level experienced by the liquid during the operation was noted down for each test. 4. 4.com/abhijitsathe temperature of the methanol vapour.Homepage: http://www. The pump consumed approximately 300 W. The heat supplied by the hot water to the methanol was approximately 50 W. The liquid level in the reservoir was accurately maintained. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The detailed separator design is shown in Fig.The saturated methanol liquid then entered the bubble pump tube which was also made up of copper.6 m. The Bubble Pump:.com) . Fig. The total height of the bubble pump as measured from the bottom of the electrical heater was 1. The heater was connected to a single phase variable power supply and a power meter.Homepage: http://www.5 The bubble pump tube with heater 4.geocities. Liquid-Vapour Separator:. A vertical copper plate was fixed to the top surface. Heat losses were minimized by insulating the whole vertical pump tube. 4. 4. 4. The separator was a hollow cylinder made of copper having a diameter 64 mm and a height of 80 mm.com/abhijitsathe Fig. The liquid fell down and was removed from the separator using an outlet situated at the bottom side of the separator.5).The two-phase fluid pumped by the bubble pump entered the separator. The liquid pumped by the bubble pump was made to fall down as it forced out of the pump tube into the separator. Only the vapour was allowed to travel upwards. Plastic flexible tubes were used to connect the bubble pump with the heating chamber and also with the liquid-vapour separator.4 The Pre-heater 3. The error in applying and measuring the electrical power was approximately 1 %. 4. An electrical heater was placed at the bottom of the pump tube (Fig. The heating element was a cylindrical stick of diameter 8 mm and length 80 mm and was made of stainless steel. This arrangement ensured that no liquid droplets were present in the vapour section and all the liquid pumped by the bubble pump was measured by the mass flow measuring instrument. The maximum heating power withstood by the heating element was 500 W.6. 7) and an electrical power of 10 W was supplied to the heater.8 Separator plate with thermcouple groove Fig. a small electrical heater was used to heat the separator body.15 % for measuring the mass flow of the liquids. A heating wire was coiled around the whole periphery of the separator (Fig. 4. This power was determined from a number of extensive tests which determined the exact point at which the heat lost by the separator equaled the heat supplied to it and the temperature of the separator was constant and was equal to the saturation temperature of methanol.9 Cross-sectional view of separator 5.Homepage: http://www. 4. The instrument had a wide range of operation and the mass flow rate was displayed digitally.10). Fig.6 Constructional details of the separator To estimate and to compensate for the heat losses of the separator. 4.geocities. This heating ensured that no methanol vapour condensed in the separator as the separator was fully adiabatic.com) .com/abhijitsathe Fig.7 Heating arrangement for the separator Fig. 4. Master's thesis work of Abhijit Sathe (e-mail:
[email protected] measuring the mass flow rate and the density of the pumped methanol liquid. 4. 4. The flow-meter had an accuracy of ± 0. a Coriolis flow-meter was used (Fig. Pumped Liquid Flow Measurement:. com) . are made to oscillate in antiphase so that they act like a tuning fork.Homepage: http://www. As the mass flow rate increases. The measuring system accurately determines and evaluates the resulting effects on the measuring tubes.com/abhijitsathe Fig. with fluid flowing through them. with the fluid standing still.e.11 Principle of Coriolis flow-meter The Coriolis forces produced at the measuring tubes cause a phase shift in the tube oscillation (Fig. the tube oscillation is decelerated at the inlet (2) and accelerated at the outlet (3). 4. The amplitude of the Coriolis force depends on the moving mass Dm. its velocity v in the system and therefore its mass flow. · When there is mass flow.11): · When there is zero flow. the phase difference also increases. i. The instrument uses an oscillation instead of a constant angular velocity and two parallel measuring tubes. 4.geocities. 4. The oscillations of the measuring tubes are determined using electrodynamic sensors at the inlet and outlet. These forces are always present when both translational (straight line) and rotational (revolving) movement occur simultaneously. Fig. both tubes oscillate in phase (1). The Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The measuring principle is based on the controlled generation of Coriolis forces.10 The mass flow measuring instrument Measuring Principle A mass flow dependent Coriolis force occurs when a moving mass is subjected to an oscillation perpendicular to the flow direction. The condenser operated at atmospheric pressure.The volume flow rate of the condensed methanol liquid was measured in a calibrated plastic cylinder. When the system achieved a steady state. Testing Procedure The bubble pump was tested for three different diameters and three different reservoir levels.com/abhijitsathe measurement principle operates independently of temperature. Condensate Flow Measurement:. conductivity or flow profile. Test Procedure:1. All the three procedures were conducted simultaneously. 2. The condensate was allowed to collect in the measuring cylinder. 7. 6. 2. The bubble pump electrical heater was switched on and a steady electrical power was supplied to the heater by using a variable power supply. The initial liquid level in the liquid reservoir was adjusted to the pre-determined value. pressure. It was also a tube-in-tube counter-flow heat exchanger. Both the tubes were made of copper. with cooling water flowing from the inner tube and methanol vapour flowing from the annular space. the mass flow rate of the methanol vapour pumped by the bubble pump was easily estimated. Condenser:.Homepage: http://www. Time required for filling 40 ml condensate in the measuring jar was noted and from this.geocities. The liquid-vapour separator was heated electrically by supplying a power of 10 W. 6. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The valve for the condensate flow meter was closed. The condensed liquid was then passed to the flow-meter for flow measurement.com) . The variation in the applied power was minimized by continuously monitoring and adjusting the input power. 5. The hot water bath temperature for the liquid pre-heater was set to 64 °C. The condensed liquid was sub-cooled to a temperature of 20 °C by adjusting the flow rate of the cooling water. 7.meter was noted. the stop-watch was started and the initial reading of the liquid mass flow. The condenser cooling water was started and the temperature of the sub-cooled liquid was maintained at 20 °C. the test was started. New liquid was filled to compensate for the losses. 4. viscosity.The construction of the condenser was similar to that of the liquid pre-heater. 3. The general test procedure was as follows: Initial Procedure:1. The temperatures at all the six locations were measured. For each diameter and each reservoir level.Homepage: http://www. two sets of readings were taken. The results that were obtaining from the readings. Following formulae were used to calculate the different parameters. 6. one with increasing power and the other with decreasing power. the stop-watch was stopped and the final reading of the liquid mass flow-meter was noted.com/abhijitsathe 3. The electrical power was then varied to the next value. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The fluctuations in the liquid level in the liquid reservoir were noted down by noting the maximum and the minimum level achieved by the liquid. When 40 ml (DV) condensate was collected.com) .geocities. 4. are presented and discussed in the next chapter. The above procedure was repeated to take 15 readings for a given electrical power. 5. The power was changed in the step of 50 W from 500 W to the minimum value when the pump stopped. The pumping action was not continuous. the liquid oscillated in the tube violently. In the period between two pumping actions. the process of forming a vapour slug at the surface of the heating element takes a finite time. At very low heating powers (below 20 W). higher amount of liquid was evaporated and a huge amount of condensate was observed to flow through the condensate flow measurement device. smaller is the residence time for formation and transportation of the vapour slug. At very low heat inputs. The results are presented and analyzed in the subsequent discussion. The time interval between two consecutive pumping actions was not constant and varied with the heat input.1. This is because. Many factors influence the performance. The flow of the condensate. The liquid in the liquid reservoir also oscillated. Pumping frequency is more for a smaller diameter pump tube for the same heat input. With increase in the heating power. The diameter of the pump tube also affects the pumping frequency. a much higher heat input was needed to start the pumping action which resembles to a higher starting torque required to start a mechanical pump. The general trend of variation of the mass flow rate of the pumped liquid with the pump heat input is shown in Fig.2. the time interval between the two consecutive pumping actions is not constant for a given heat input. At higher heat inputs. At higher heating powers. the duration for each pumping action reduces and the frequency increases. As the heating power was increased. Since the vapour slugs form quickly. The mass flow rate Performance of the bubble pump is evaluated on the basis of the amount of liquid pumped by the bubble pump for a given heat input. 5. this time interval was small which meant the liquid was pumped more frequently. Evaluation of Bubble Pump Parameters Following bubble pump parameters were analyzed from the experimental results: The frequency of pumping action The pumping action is intermittent and the frequency of pumping varies with the heat input. The amplitude of oscillation was high at high heating powers. The flow behavior of the fluid inside the pump tube was observed by using a transparent plastic tube of 6 mm diameter. more and more liquid was evaporated and the size of the bubbles formed increased. The smaller the diameter of the tube. Though the bubble pump operated at a heating power as low as 50 W. it was very much useful to visualize the flow of the two-phase fluid inside the pump tube. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. The frequency of pumping action also depends on the geometry of the heating element and other heating onditions. At low heating power.geocities. a large amount of liquid is pumped. however was not intermittent. The bubble pump was tested for three different pump tube diameters and three different levels of liquid in the liquid reservoir.com) . the liquid is lifted more frequently thereby increasing the pumping frequency. this height was reduced though the amplitude of oscillation was increased. The pumping frequency increases linearly with the heat input. The maximum height to which the liquid oscillated inside the tube increased with increase in heat input. The amount of liquid pumped in a single pumping action is also not constant. 5.com/abhijitsathe RESULTS AND DISCUSSIONS Observations with a transparent bubble pump tube The bubble pump test procedure is described in the last chapter. At higher heating powers. But it is reasonably constant at higher heat inputs. A typical variation of the pumping frequency with the bubble pump heat input is shown in Fig. Extensive tests were conducted to obtain the characteristic curves for the bubble pump. As the boiling rate increases with the heat input. the height to which the liquid raised inside the reservoir was more. Though no readings were taken with this set-up.Homepage: http://www. Also at low heat inputs. The flow of condensate started a considerable time after the first pumping action was recorded. This was due to the inability of the vapour bubbles to lift the solution. The level of violence was higher at higher heating powers. the frequency of pumping is extremely small. the solution simply oscillated inside the pump tube without being lifted to the top. but was intermittent. At low heat input. Fig. for both of these driving heads. As discussed above. as the heat input to the liquid in the bubble pump increases. however. Since the system was open system.45 m driving heads is approximately the same.5. Hence the liquid flow rate increases as the heat input increases though the rate of increase decreases. the mass flow rate of the pumped liquid first increases with the heat input. This maximum flow occurs when the increase in the frictional pressure drop caused by increased vapour flow rate exceeds the increased buoyancy effect of the vapour to pump the liquid. i. the force exerted by the liquid column is higher. where the liquid is trapped between the slugs of vapour. all the three curves run almost parallel to each other.4 and Fig.8 describe the pump behavior for three different tube diameters at the Master's thesis work of Abhijit Sathe (e-mail:
[email protected] show variation of pumped liquid flow rate with the heat input for different driving heads for the same pump tube. The height of the pump tube (pump lift L) was not varied. the maximum mass flow rate occurs at a heat input of 250 W. The submergence ratio of the bubble pump is a measure of how far the pump is submerged relative to its length. Thus at higher heat inputs. 5. the two curves separate. 5. the mass flow of the pumped liquid should theoretically be the same. reaches a maximum value and then starts decreasing. the bubble pump gives nearly the same mass flow rate for a given heating power. Further increase in the heat input. i. With increasing submergence ratio. Increase in the pump tube height. The results indicate that a higher driving head leads to a higher volume flow rate for the same pump lift. 5. however.4 and Fig. the behavior of the pump for 0. a submergence ratio. It is much lower as compared to the mass flow of pumped liquid. the pump lift also results in decrease in the mass flow because the liquid has to be lifted to a higher level.com) .3. Fig. If both. results in decrease in the mass flow of the pumped liquid. This is due to the fact that. the pump tube diameter (dp) also forms a design parameter for the bubble pump. Fig. the pump lift and the driving head are increased without altering the submergence ratio. i. not the driving head or the pump lift alone. so the liquid flow rate increases. but a ratio of the driving head to the pump lift (h/L). 5. For the pump tube of 10 mm diameter.7 and Fig. At heat inputs higher than 300 W. i. it operated at an atmospheric pressure. From Fig. The lower driving heads render lower mass flow rates. the variation in the condensing temperature was not possible. the pump behavior is similar for all the three driving heads.e. this value is 250 W. 5. The variation of the mass flow rate of the pumped liquid with that of the vapour should essentially be same as the variation of pumped liquid flow rate with the heat input. at a higher driving head. Fig. At heat inputs higher than 225 W. it can be seen that at low heat inputs. 5. 5. with more or less the same gradients (increase of volume flow rate per increase of heat input) at any heat input. Along with the driving head (h) and the pump lift (L).Homepage: http://www. The mass flow rate of vapour.e. For the pump tube of diameter 6 mm. Thus the region to the left side of the maximum mass flow rate can be called as a buoyancy force dominated region while the one to the right side of the maximum mass flow rate can be termed as pressure drop dominated region.5 m and 0. because the increase in frictional pressure drop on the account of increase in the pump tube length is offset by the increased velocities due to increase in the driving head. From all the above discussed graphs. more and more number of vapour bubbles form which lift more and more amount of liquid. This is because.geocities. the maximum mass flow rate of the pumped liquid is observed at a heat input of approximately 275 W. This is a slug flow regime.3. The occurrence of the maximum mass flow rate of pumped liquid for a given pump tube diameter is fairly at a constant heating power. 5. the relative height to which the pump must lift the liquid decreases. while for the pump tube of 8 mm diameter.e. A reduced driving head means in the same way a reduced driving force developed by the bubble pump. Effect of pump tube diameter on the mass flow rate of pumped liquid The pump tube diameter played a very important role in the pump behavior.com/abhijitsathe the mass of liquid pumped by the bubble pump per unit time increases almost linearly with the heat input. 5. increases proportionately with the heat input. Fig. The mass flow reaches a maximum value. If the bubble pump tube is inadequately insulated. the vapour flow rate is very high which means the frictional losses are heavy resulting in lower amount of pumped liquid. The important parameter is.e. it is clear that the general trend of variation is the same. at very high heat inputs. This is because. some vapour may condense as it travels upwards thus further lowering the vapour flow rate. Effect of driving head on the mass flow rate of pumped liquid The bubble pump was tested for three different tube diameters and three levels of liquid in the liquid reservoir (driving head h). This results in increased fluid velocities in the bubble pump tube thereby rendering a higher amount of pumped liquid. This is due to the increased flow rate and consequently the increased pressure head loss at higher heat inputs. Thus vapour is vaporized and less solution is pumped. Also at high heat inputs. the rate of decrease in pumping ratio is decreased.10.Homepage: http://www.e. At a heat input of 500 W. i.the maximum liquid mass flow rate occurs when the increased buoyancy effect of the vapour to pump the liquid is balanced by the increased frictional pressure drop caused by increased vapour flow rate. the heat input required to produce the maximum liquid mass flow is less.com) . each cycle (pumping action) takes more time and correspondingly more vapour escapes from the pump. It is prominent at higher heat inputs. Also observed from the graphs is the fact that at lower heat inputs. The vapour mass flow rate Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. 5. the frictional pressure drop decreases thereby increasing the efficiency of the bubble pump which results in increased liquid mass flow rate.. This behavior may be explained as . The behavior of the bubble pump remains the same. The bubble pump should always be operated at this maximum liquid flow rate in order to maximize its performance. as the diameter increases. this value is 300 W. This is because of a very high friction loss experienced by the 6 mm diameter pump tube which results in a much reduced liquid flow rate and a very low pumping ratio. A higher driving head results in a slightly higher vapour flow rate. the general behavior of the pumping ratio with respect to the heat input seems similar. As the pump diameter increases. the curves seem to flatten a bit which indicates that that the rate of decrease in pumping ratio is decreased.geocities. This is because. a similar behavior is observed. a smaller driving head. the heat input at which this maximum mass flow rate occurs is not the same for all the pump tubes. The vapour flow rates at higher heat inputs are nearly constant for all the driving heads. the frictional drop is higher. Irrespective of the diameter of pump tube and the driving head. Also at low heat inputs. it is clear that for the same heat input. For a pump tube of lower diameter. gives a lower pumping ratio. the maximum liquid mass flow occurs at a heat input of 225 W. the difference in the liquid mass flow rates for different diameters of pump tube is smaller. shows a departure from the other two curves. i. A bigger diameter pump tube renders higher pumping ratio for the same heat input. however. The pumping ratio decreases almost linearly with the increase in heat input. The variation of the pumping ratio with the heat input for a constant pump tube diameter and different driving heads is given in Fig. at lower heat input. From Fig. The curve for 10 mm diameter tube.12 shows the variation of vapour flow rate with heat input for different pump tube diameters. the value of heating power required to produce the maximum liquid flow is 250 W while for 10 mm diameter pump tube.. When the driving head is reduced.10. 5. the frictional pressure drop decreases and the occurrence of the maximum liquid flow rate is shifted to the right side. the difference in the pumping ratio for 6 mm diameter tube and 8 mm diameter tube is as much as twice the difference in the pumping ratio for 10 mm and 8 mm diameter tubes.. Fig.11 that the driving head does not influence the vapour flow rate much. However. 5. The effect of pump tube diameter on the pumping ratio is explained in Fig. Increase in the diameter of the pump tube results in increased liquid mass flow rate. It is clear from Fig. the difference between the liquid mass flow rates for 6 mm pump tube and 8 mm pump tube is much higher (200 %) than the difference between liquid mass flow rates for 10 mm pump tube and 8 mm pump tube. Fig. i. reaches the maximum value and then starts decreasing with further increase in heat input.9 while that for a constant driving head and different pump tube diameters is shown in Fig. the mass flow rate of pumped liquid increases with heat input. This is because of the increased force offered by the liquid column. smaller is the pressure drop. The pumping ratio The pumping ratio is the ratio of volume flow rate of the pumped liquid (Vf) to volume flow rate of the vapour (Vg).e.. The temperature inside the pump will also increase to a higher level before the cycle completes.7. So the maximum liquid flow rate occurs at a relatively lower vapour flow rate. Thus in case of 6 mm pump tube. 5. The curves for 8 mm and 6 mm diameter pump tubes run parallel to each other for all the heat inputs. for the pump tube of 6 mm diameter. For lower diameters. Bigger the diameter. 5.9. to the higher heat input side. For pump tube of 8 mm diameter.e. resulting in lower pumping ratio. i. 5. In Fig. 5.9 reveals that all the three curves run almost parallel to each other for all the heat inputs. The vapour mass flow rate is directly proportional to the heat input.com/abhijitsathe same driving head and pump lift. The difference increases with increase in heat input. 5. At higher heat inputs.e. higher is the mass flow of the pumped liquid and so higher is the pumping ratio. the laminar forces are predominant and the friction factor values are too high which results in a much reduced liquid mass flow rate. Homepage: http://www.15. The liquid in the reservoir oscillated violently which was assumed to be at a constant level. The values of the constants for the different pump tube diameter are given in Table 5-1. some vapour escaped from the pump tube without lifting any liquid. This aspect was not included in the analytical model.037 x 1012 -7.419 x 1015 3. 2-26. Master's thesis work of Abhijit Sathe (e-mail:
[email protected] -4. A typical correlation for a given pump tube diameter is of following structure where A. Also for modeling.749 x 105 10.49 -1.14 gives the procedure for finding out the liquid mass flow rate of a given bubble pump. It is therefore essential to establish a correction factor that compensates for all the above stated discrepancies. 2-23 as is found to have a relation with the mass flow rate of the vapour.284 x 1014 B 9. 2-27 and 5-6. It is also possible to calculate the geometrical parameters of the bubble pump for a given liquid mass flow rate. Thus it is possible to predict the performance of the bubble pump for a given driving head. It is important to compare this model with the results obtained from the experiments in order to validate it.874 x 108 -1.141 x 1015 8. the flow of the two-phase fluid in the bubble pump tube was intermittent. C. During these oscillations. B. Fig.361 x 108 -2. The correlation established for one pump tube diameter is not valid for another pump tube diameter.13 gives the variation of the factor K with the mass flow rate of liquid. Thus all the vapour did not contribute effectively in lifting the solution.718 x 104 1. D and E are constants and are functions of the pump tube diameter. it was clear that the flow pattern was not stable and varied with the heat input. However.com) . Fig 5. As a consequence of this. it is possible to simulate the bubble pump performance. the liquid oscillated in the pump tube.091 x 109 2. A dimensionless factor K which was defined in Eqn. during the tests.688 x 104 3.geocities. Table 5-1 Pump Tube Diameter 10 mm 8 mm 6 mm A -1. Also as discussed earlier. The assumptions made in the modeling are stated in Chapter 2.056 From Eqn. But in the actual testing. In between two consecutive actions.594 x 1011 -3. some discrepancies were noticed with the assumptions. The correlation is found out by applying the experimental results to the analytical model for a given pump tube diameter and a given driving head. the model fails to give satisfactory results for all the heat inputs. the kind of flow in the pump tube was assumed to be always slug flow. pump lift and pump tube diameter.974 x 1011 C D E 6. 2-22. The procedure for the same is described in Fig.com/abhijitsathe Comparison with the Mathematical Model A mathematical model for a bubble pump has already been established in Chapter 2. 5. 5. The mass flow rate of vapour increases linearly with the heat input whereas the mass flow rate of the pumped liquid first increases. Selection of bubble pump tube The bubble pump must give the desired pump discharge (mass flow rate of pumped liquid) at the rated heat input. then the diameter and vapour flow rate of the pump will be chosen such that this liquid flow rate is the maximum. the lesser is the amount of heat to be supplied to the bubble pump to get the required liquid mass flow rate. for the same value of liquid mass flow rate there exist different heat inputs depending on the pump tube diameters. The important bubble pump parameters are the driving head (h). Thus it may seem that a large diameter pump tube would always be advantageous.Homepage: http://www. the driving head and the pump lift can be combined to form a single parameter known as the submergence ratio (h/L). The results reveal that the frequency of pumping action increases with increase in pump heat input. As seen from the graphs for the bubble pump behavior.com/abhijitsathe Conclusion The bubble pump has been tested both analytically and experimentally and the results are presented in the previous sections. the bubble pump operates most efficiently in the slug flow regime and should operate at its maximum liquid flow rate.geocities. the pump lift (L) and the pump tube diameter (dp). Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. As discussed earlier. pump lift (L) and the pump tube diameter (dp). The important geometrical parameters which govern the bubble pump behavior are the driving head (h). The higher the tube diameter. The bubble is best operated at the maximum liquid mass flow rate when the efficiency is the highest. increasing the diameter with a fixed liquid flow will eventually cause transition from the assumed slug flow to bubbly flow. reaches a maximum value and then decreases with the increase in the heat input. But as discussed earlier. However.com) . If the liquid flow rate needs to increase or decrease. The pumping ratio decreases almost linearly with the heat input. com/abhijitsathe PERFORMANCE CURVES FOR BUBBLE PUMP Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www.com) . com) .geocities.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www. com) .Homepage: http://www.geocities.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. geocities.com) .com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www. com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) .geocities.Homepage: http://www. com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www.com) .geocities. Homepage: http://www.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) .geocities. com) .geocities.Homepage: http://www.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]: http://www.geocities.com) . Homepage: http://www.geocities.com) .com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. com) .geocities.Homepage: http://www.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. geocities.Homepage: http://www.com) .com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. Homepage: http://www.geocities.com/abhijitsathe Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) . 14 Calculation of liquid mass flow rate Master's thesis work of Abhijit Sathe (e-mail:
[email protected]. 5.com/abhijitsathe Fig.com) .Homepage: http://www. 5.com) .15 Calculation of pump tube diameter Master's thesis work of Abhijit Sathe (e-mail:
[email protected]/abhijitsathe Fig.Homepage: http://www. Homepage: http://www. Master's thesis work of Abhijit Sathe (e-mail:
[email protected]) .com/abhijitsathe List of Symbols Symbol Description SI Unit A B dp f g G h Cross-sectional area Perimeter of the pump tube Diameter of the pump tube Friction factor Acceleration due to gravity Mass velocity Driving head Height of the bubble pump tube (Pump lift) Mass flow rate m2 m m m/s2 kg/m2s m m kg/s N/m2 W L P Pressure Heating power Re s Reynolds number Velocity constant Velocity Volume flow rate m/s m3/s N V W x Greek Letters n s r m a Weight Dryness fraction Specific volume Surface tension Density Dynamic viscosity Void fraction m3/kg N/m kg/m3 N.s/m2 Subscript f denotes liquid while subscript g denotes vapour.geocities. Homepage: http://www. 1980. London and New York. Radermacher and S. 1998. Klein. Indian Institute of Technology Bombay. 20th International Congress of Refrigeration. and Thome John R. “Two-phase Flow in Pipelines and Heat Exchangers”. 1987. B2. Oxford Science Publications. 1996. 10. Pfaff M. IIF/IIR-Commissions B1.. Belgium. Herold K. IIR/IIF.. 1993. vol. R. “Some Characteristics of Thermal Siphons for Ammonia-water Solutions”. Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo... “Studies on Bubble Pump for a Water-Lithium bromide Vapour Absorption Cooler”. International Journal of Refrigeration.. E2-Mons. CRC Press New York. Collier John G.. Sydney.geocities. 9.. Hahne E. George Godwin. 6.com/abhijitsathe REFERENCES 1. Srinivasamurthy S.com) . G. 8. Grigull.edu/energy/andy_phd). “Triple Fluid Vapour Absorption Refrigerator: Investigations on Solution Circuit”.. Maiya M. 452-462. Delano Andrew. Saravanan R. 1999. Thesis. “Convective Boiling and Condensation”. 1998. 4. E1. P.. Cattaneo A. 1983. E. 7.. 21. Oxford Science Publications.me. 1935. “Design Analysis of the Einstein Refrigeration Cycle”. Chisholm D. Ph.. Department of Mechanical Engg. 1998. Maiya M. 5: p.gatech. (http://www. “Absorption Chillers and Heat Pumps”.D. Maiya M. Maczek K. “Über die Förderung von Flüssigkeiten mittels der eigenen Dämpfe”. “Investigations on Triple Fluid Vapour Absorption Refrigerator”. P. Zeitschrift für die gesamte Kälte-Industrie. 5. Georgia Institute of Technology. Thesis. no. “Heat Transfer in Boiling”.D.. Ph. P. 2. 3. and Zoltaniecki A. 04 kg/kmol 0.65 °C 103 kJ/kg 64.11 (Air=1) -97.7 °C 1100 kJ/kg 19.geocities.6 °C) 81 bar 0.Homepage: http://www. General Properties Chemical name Chemical formula Molecular weight Specific gravity Vapour density Melting temperature Enthalpy of fusion Boiling temperature Enthalpy of vapourisation Enthalpy of combustion Vapour pressure (at 20 °C) Surface tension at boiling temperature Flash point Solubility Flammability limits Lower Upper Nature Methanol CH3OH 32.0172 N/m 12 °C Miscible with water 6 % in air by volume 36% in air by volume Toxic 2.6 K (239.930 kJ/kg 12.118 m3/kmol Master's thesis work of Abhijit Sathe (e-mail:
[email protected] 1.3 kPa (97 mm of Hg) 0.com/abhijitsathe APPENDIX Properties of Methyl Alcohol 1.com) . Critical Properties Critical temperature Critical pressure Critical volume 512. 193 3.87x10-5 25 °C 1.75x10-4 50 °C 765 2.39x10-4 Units kg/m3 kJ/kg K W/m K Ns/m2 4.16 1.798 0.374 0.28x10-4 150 °C 643 -0.201 5.85x10-5 100 °C 1.178 2.com/abhijitsathe 3.com) .534 0.049 1.68 0.036 0.0157 0.geocities.22x10-5 200 °C 1. Liquid Properties Properties Density Specific heat Thermal conductivity Dynamic viscosity 0 °C 813 2.77x10-4 20 °C 792 2.88x10-5 Units kJ/kg K W/m K Ns/m2 Master's thesis work of Abhijit Sathe (e-mail: abhijitsathe@yahoo. Vapour Properties Properties Specific heat Thermal conductivity Dynamic viscosity 0 °C 1.495 0.207 7.0137 0.034 1.33 0.85x10-4 100 °C 714 -0.56x10-5 300 °C 2.Homepage: http://www.386 0.227 1.