REf- Air Craft System

April 3, 2018 | Author: rag2604756437 | Category: Gas Compressor, Refrigeration, Atmosphere Of Earth, Heat, Gas Technologies



Advanced RefrigerationMaster of Engineering (Thermal Engineering) SEM – 1st air cycle refrigeration system Faculty Name: Dr. R.G.Kapadia S.V.M.I.T. College , Bharuch Presented By: Patel Devang S. Patel Mayank D. Patel Harsh U. 130450721002 130450721007 130450721006 0utline           Introduction to air cycle Refrigeration Air standard cycle analysis- Assumptions Reversed Carnot cycle Ideal reversed Bryton cycle Actual reversed Bryton cycle Advantages of air cycle cooling system Aircraft cooling system Simple aircraft refrigeration cycle Bootstrap cycle Dry air rated temperature At the end of the lesson the student should be able to:  Describe various air cycle refrigeration systems  State the assumptions made in the analyses of air cycle systems  Show the cycles on T-s diagrams  Perform various cycle calculations  State the significance of Dry Air Rated Temperature  The gas does not undergo any phase change during the cycle.  Gas cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases. in which a gas is used as the working fluid. consequently. . all the internal heat transfer processes are sensible heat transfer processes.Introduction  Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems. assumptions  The working fluid is a fixed mass of air that behaves as an ideal gas  The cycle is assumed to be a closed loop cycle  All the processes within the cycle are reversible. the cycle is internally reversible  The specific heat of air remains constant throughout the cycle . i.Air standard cycle analysis ..e. isothermal heat absorption in a turbine . adiabatic compression in a compressor Process 2-3: Reversible. (a) Schematic of a reverse Carnot refrigeration system Process 1-2: Reversible.Reversed Carnot cycle employing a gas Fig. isothermal heat rejection in a compressor Process 3-4: Reversible. adiabatic expansion in a turbine Process 4-1: Reversible. (b) Reverse Carnot refrigeration system in P-v and T-s coordinates .Reversed Carnot cycle employing a gas Fig. Reversed Carnot cycle employing a gas .  All actual processes are irreversible. the volumetric refrigeration capacity of the Carnot system is very small leading to large compressor displacement. . hence completely reversible cycles are idealizations only.  In addition. which gives rise to large frictional effects.Reversed Carnot cycleLimitations  One of the main difficulties with Carnot cycle employing a gas is the difficulty of achieving isothermal heat transfer during processes 2-3 and 4-1. adiabatic expansion in a turbine Process 4-1: Reversible. (a) Schematic of a closed reverse Brayton cycle Process 1-2: Reversible. adiabatic compression in a compressor Process 2-3: Reversible. isobaric heat absorption in a heat exchanger . isobaric heat rejection in a heat exchanger Process 3-4: Reversible.Ideal reverse Brayton cycle Fig. This cycle is also called as Joule or Bell-Coleman cycle.Ideal reverse Brayton cycle Fig. as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes. (a) Schematic of a closed reverse Brayton cycle This may be thought of as a modification of reversed Carnot cycle. . Ideal reverse Brayton cycle . Ideal reverse Brayton cycle . Ideal reverse Brayton cycle . Ideal reverse Brayton cycle . Ideal reverse Brayton cycle . Actual reverse Brayton cycle . the compressor work input increases and turbine work output reduces.Actual reverse Brayton cycle  Figure shows the ideal and actual cycles on T-s diagram. Due to these irreversibilities. The actual work transfer rates of compressor and turbine are then given by: . the COP of actual reverse Brayton cycles will be considerably lower than the ideal cycles.  The refrigeration effect also reduces due to the irreversibilities.  As a result. .  Design of efficient compressors and turbines plays a major role in improving the COP of the system.Actual reverse Brayton cycle  thus the net work input increases due to increase in compressor work input and reduction in turbine work output. Advantages of Air cycle refrigeration  Air is cheap. safe. Leakage of air is not a problem  Cold air can directly be used for cooling thus eliminating the low temperature heat exchanger (open systems) leading to lower weight  The aircraft engine already consists of a high speed turbo-compressor. non-toxic and non-flammable. This reduces the weight per kW cooling considerably. less than 50% of an equivalent vapour compression system  Design of the complete system is much simpler due to low pressures. Typically. Maintenance required is also less. . hence separate compressor for cooling system is not required. .2 bar and temperature of 223 K (at 10000 m altitude) is compressed to 1 bar. This effect adds heat to the cabin. If the cabin is maintained at 0. For example. equipment etc.Why Aircraft cooling system is Required?  Large internal heat generation due to occupants. its temperature increases to about 353 K. the outside pressure will be sub-atmospheric.  Heat generation due to skin friction caused by the fast moving aircraft  Solar radiation  At high altitudes.8 bar. when outside air at a pressure of 0. the temperature will be about 332 K. which needs to be taken out by the cooling system. This effect is called as ram effect. the temperature increases significantly. When air at this low pressure is compressed and supplied to the cabin at pressures close to atmospheric. Simple Aircraft Refrigeration cycle Fig. Schematic of a simple aircraft refrigeration cycle . and is cooled to state 4 in the air cooler. The cold air at state 5 is supplied to the cabin. the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2). as a result its temperature drops from 4 to 5.  As shown in the T-s diagram.  This air is compressed in the main compressor to state 3. This is an open system. During this process its temperature increases from 1 to 2. .  Its pressure is reduced to cabin pressure in the turbine (state 5).Simple Aircraft Refrigeration cycle  Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram. . The power output of the turbine is used to drive the fan. which maintains the required air flow over the air cooler. This simple system is good for ground cooling (when the aircraft is not moving) as fan can continue to maintain airflow over the air cooler.Simple Aircraft Refrigeration cycle  Cold air picks up heat as it flows through the cabin providing useful cooling effect. which is the ratio of velocity of the aircraft (C) to the sonic velocity a= 2 .Simple Aircraft Refrigeration cycle  By applying steady flow energy equation to the ramming process. i.. . the temperature rise at the end of the ram effect can be shown to be: where M is the Mach number.e. Simple Aircraft Refrigeration cycle  Due to irreversibilities. .. ηRam.e. the actual pressure at the end of ramming will be less than the pressure resulting from isentropic compression. The ratio of actual pressure rise to the isentropic pressure rise is called as ram efficiency. i. For air Cp=1.o.004 Kj/kg k and Cp/Cv = 1. This air is then further compressed in a compressor to 4.75 bar. The air leaves the cabin at a temperature of 27°c. The atmospheric temperature Is 17°c.75 bar T4 = 67° c = 340 k P5 = P5’ = 1 bar T6 = 27°c = 300 k .95 bar and temperature of 30°c due to ram action. The atmospheric air is compressed to a pressure of 0. The isentropic effectiveness of both compressor and turbine are 0.9.p. expanded in a turbine to 1 bar pressure and supplied to the cabin.4 Q = 30 TR T1’ = 17° c = 290 K P2 = 0.Simple Aircraft Refrigeration cycle  An air craft refrigeration plant has to handle a cabin load of 30 Tonnes. Calculate the mass of air circulated per minute and the c. cooled in a heat exchanger to 67°c.95 Bar T2 = 30°c = 303 K P3 = p3’ = 4. Simple Aircraft Refrigeration cycle . Bootstrap Cycle . this system consists of two heat exchangers (air cooler and aftercooler). in stead of one air cooler of the simple system.Bootstrap Cycle  Figure shows the schematic of a bootstrap system. as a result a separate fan is not required.  As shown in the figure.  It also incorporates a secondary compressor. where in the velocity of the aircraft provides the necessary airflow for the heat exchangers. which is a modification of the simple system. . which is driven by the turbine of the cooling system.  This system is suitable for high speed aircraft. The heat rejected in the air cooler is absorbed by the ram air at state 2. ambient air state 1 is pressurized to state 2 due to the ram effect.  The air is then cooled to state 4 in the air cooler. This air is further compressed to state 3 in the main compressor.  It is then cooled to state 6 in the after cooler. .  The air from the air cooler is further compressed from state 4 to state 5 in the secondary compressor.Bootstrap Cycle  As shown in the cycle diagram. expanded to cabin pressure in the cooling turbine and is supplied to the cabin at a low temperature T7. The isentropic efficiency of each of the compressor is 80%. Assuming ramming action to be isentropic.O. the C. The discharge pressure of air from auxiliary compressor is 4 bar. .9 bar and temperature of the air leaving the cabin not more than 20°c . The pressure of air discharged from the main compressor is 3 bar.85 bar respectively. The ambient air temperature and pressure are 20°c and 0. while that of turbine is 85%.Bootstrap Cycle  A Boot strap cooling system of 10 TR capacity is used in an aeroplane.Find 1. the required cabin pressure of 0. The pressure of air increase from 0.85 bar to 1 bar due to ramming action of air. of the system.P.the power required to operate the system 2. 50 % of the enthalpy of air discharged from the main compressor is removed in the first heat exchanger and 30% of the enthalpy of air discharged fro the auxiliary compressor is removed in a second heat exchanger using rammed air. Bootstrap Cycle . Recuced Ambient Air Coolling Cycle . Recuced Ambient Air Coolling Cycle . Recuced Ambient Air Coolling Cycle . Recuced Ambient Air Coolling Cycle . Regenerative Air Coolling Cycle . Regenerative Air Coolling Cycle . Regenerative Air Coolling Cycle . Regenerative Air Coolling Cycle . Regenerative Air Coolling Cycle . the dew point temperature and hence moisture content of the air should be very low. the air should be very dry. i.e.. The cooling capacity is then given by: .Dry air rated temperature (DART)  The concept of Dry Air Rated Temperature is used to compare different aircraft refrigeration cycles.  For condensation not to occur during expansion in turbine.  The aircraft refrigeration systems are rated based on the mass flow rate of air at the design DART.  Dry Air Rated Temperature is defined as the temperature of the air at the exit of the cooling turbine in the absence of moisture condensation. supersonic aircrafts .Dry air rated temperature (DART) A comparison between different aircraft refrigeration systems based on DART at different Mach numbers shows that:  DART increases monotonically with Mach number for all the systems except the reduced ambient system  The simple system is adequate at low Mach numbers  At high Mach numbers either bootstrap system or regenerative system should be used  Reduced ambient temperature system is best suited for very high Mach number. Comparision of various Air cooling systems used for Aircraft .
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