BEng (Hons) Electrical & Electronic / Mechanical Engineering (3+0) In Collaboration with University of Sunderland Level 2 FINAL EXAMINATION MAY 2015 SUBJECT: THERMOFLUID & ENGINE SUBJECT CODE: EAT 223 TIME ALLOWED: 3 HOURS DATE: INSTRUCTIONS TO CANDIDATE 1. Answer ANY FIVE (5) questions out of EIGHT (8). 2. This is a closed book examination – this means you are not permitted to use any text books or study aids in the examinations. 3. Thermodynamics Properties table and graph paper are provided in the examination. 4. All symbols have their usual meanings unless otherwise defined in the text. 5. You are forbidden to use programmable calculators. 6. You must answer the required number of questions only. Any additional answers will not be marked. You should put a cross through any work you do not wish to be marked. (3 marks) (e) The cycle COP. (3 marks) (d) Specific work done by compressor. (12 marks) Part B Briefly discuss the following.8 bars and the specific fuel consumption is 0. (3 marks) (b) Specific refrigeration effect at the evaporator. explain two reasons for this. it has a minimum temperature of -20 oC and a maximum pressure of 7 bars. The fuel used has a calorific value of 44. it operates on an ideal standard refrigeration cycle.01 bar. (4 marks) Question 2 An old freezer unit runs on R12 as the working fluid.67. (a) Enthalpy value of the refrigerant upon leaving the compressor. When tested at full load at 720 rpm the bmep is 4. (3 marks) (c) Specific heat reject by the condenser. (a) Most of the large ship diesel engines are 2 strokes instead of four strokes. Determine the following. Calculate the brake thermal efficiency and volumetric efficiency of the diesel engine.Question 1 Part A A experimental four cylinder four stroke diesel engine has a bore of 180mm and a stroke of 250 mm. (3 marks) (f) The refrigeration capacity in ton RT ? ( 3.200 kJ/kg. The air-fuel ratio is 25:1.18 kg/kWh. The test was carried out in an environment with air condition at 15 oC and 1.000 btu/hr or 1 ton RT cooling) (5 marks) .48 kW of cooling = 12. The compressor circulates 6 kg/min of refrigerant and has an isentropic efficiency of 0. (4 marks) (b) What are two main advantages if a turbocharged diesel engine instead of a petrol engine is used in a sedan car. A new same geometrical turbine design is made for another hydro dam at lower altitude. (4 marks) .0 MW under 7m head. under normal circumstances most of the gas turbine plants are shut down during off peak load.333 and c p = 1. Determine the following.01 bar. (3 marks) (d) Calculate the air exit temperature. Explain briefly why gas turbine plants should be served as standby plants instead of coal fire plants. (3 marks) (f) In most countries there are mixtures of coal fire & gas turbine power generation plants.50 MW at 250 rpm under a head of 10 m from a hydro dam. It supposed to generate 2. The isentropic efficiencies for compressor & turbines are both 0. (4 marks) Question 4 Part A A water turbine is to generate 3.. The maximum cycle temperature is 1127 oC . (3 marks) (e) Calculate the net work done & cycle thermal efficiency.4 and cp = 1. operates at the pressure ratio of 12:1.15 kJ/kgK (a) Draw the T-S diagram for the above process (2 marks) (b) Find the air exit temperature from the compressor. (4 marks) (c) Calculate the compressor specific work done.005 kJ/kgK. for the combustion and expansion process take γ = 1.Question 3 A power generating gas turbine takes in air at 17 deg C and 1. For compression process take air properties of γ = 1.82. (a) the new turbine operation speed (4 marks) (b) the diameter ratio of the new turbine to the old turbine (4 marks) (c) the specific speed for both turbine. Part B A mini hydro dam uses a pelton wheel to generate electricity. (4 marks) (c) At this operating point calculate the power delivered by pump hence the pump efficiency.075 Total Head (m) 70 68 Input power kW 97 127 0. Test results: Flow (m3/s) 0. The friction head loss in pipe can be calculated by Hf = fL Q2 3 3d all in SI unit (a) Plot the graph of head & power versus discharge for the pump & piping system.250 0. (4 marks) .300 64 58 49 147 163 170 The above piping system is a 300mm diameter pipe with total length of 120m. The pipe friction of coefficient f = 0. The bucket mean speed is 10 m/s and water is deflected through 160 deg.06. The pump is driven by a AC motor at 1500 rpm in a piping system that delivers river water to a reservoir 60m above.150 0. Determine the power & efficiency of the wheel. Neglect all friction losses in pipe flow. the water is supplied at 0.68 m3/s from a reservoir 20m above it.200 0. (8 marks) QUESTION 5 A centrifugal pump was tested and produces the following performance results. (12 marks) (b) From the graph determine the pump discharge and power absorbed at 1500 rpm in the system. 005 kJ/kg.4 and Cp=1.01325 bar as thrusters. c v = 0. (6 marks) (e) Calculate the ratio of inlet to exit area (3 marks) Question 8 Part A . a) Sketch the P-V diagram with necessary labelling for the above diesel cycle.9 bar. Do the following. (4 marks) f) Calculate the maximum pressure and the mean effective pressure. γ=1. (a) Calculate the critical temperature at the throat.5. (4 marks) (b) Calculate the critical velocity at the throat. ( 3 marks) c) Calculate the engine maximum temperature. The air properties are R=287 J/kg.K. (3 marks) b) Calculate the air temperature after compression.005 kJ/kgK. Air at 3 bar and 167 oC flows into the inlet at a velocity of 145 m/s. Assume all processes are isentropic. (4 marks) (d) Calculate the exit and inlet Mach numbers.4. It operates at a hill top resort where the air is at 0. (3 marks) (c) Calculate the air exit tempearture. (3 marks) e) Calculate the net work done and the overall thermal efficiency. (4 marks) Question 7 An experimental light air craft uses a convergent-divergent nozzle to produce supersonic air discharge at atmospheric pressure of 1.K. It has a cut of ratio of 2. 300K.Question 6 A diesel engine for a generator set has a compression ratio 18:1.718 kJ/kgK and cp = 1. (3 marks) d) Calculate the exhaust air temperature. Air properties are γ = 1. The transition Reynolds number may be taken as 5x105. (4 marks) (b) Calculate the drag coefficient. Discuss briefly one way to counter this unwanted effect. Laminar.5 =4. (8 marks) . ρ=1. µ=1. (4 marks) (c) Calculate the drag force of the flag and power to overcome the drag. The rectangular flag is fully extended and is 0. x −0. (4 marks) The following equations may be used as required.A car moving at 110 km/hr has a small flag attached on the roof.161 kg/m3.5 Cd = 1.377 ℜx . x Turbulent.64 ℜ x .2 =0.2 Cd = 0. (a) Calculate the Reynolds number. δ −0. The ambient air temperature is 30oC and has the following properties.296 ℜ x −0.3m tall and 1m long (in the direction of motion).875 x 10-5 N. δ −0. The flag can be assumed acting as a flat plate moving through the air with two surfaces generating the drag.074 ℜx Part B Discuss briefly why when a car is moving at high speed it gets lighter and why this is dangerous for driving.s/m2. in pipe. Euler Turbomachinery Equation. P P = (Mass flowrate)x(V1 cos θ1v1 + V2 cos θ2v2 ) where v2 & v1 are blade velocity and V1 & V2 are fluid absolute velocity 4. Head gH 2 2 N D . General power equation for turbine blade Power. Euler head H= 1 g( v 2 V t 2 – v 1 V t 1 ) where v2 & v1 are blade velocity and Vt1 & Vt2 are fluid absolute tangential velocity 5. Pump/Turbine dimensionless coefficient Flow Q 3 ND .EAT 223 THERMOFLUID & ENGINE FORMULAS SHEET 1. Pelton wheel power P = (Mass flowrate)x (V -v)x(1 + cosθ ) v. F Fx = (Mass flowrate)x(V1 –V2cos θ) + ( p1A1 . Force exerted due to direction flow change θ deg to x axis. Power P 3 5 ρN D .p2A2 cos θ ) Fy = (Mass flowrate)x(0+V2sinθ) + ( 0 + p2A2 sin θ ) 2. 2 power in supply water = V × Mass flowrate 2 3. (velocity profile) 7.1 N Q2 Specific speed for pump Ns is H 3 4 1 N P2 Specific speed for turbine Ns is 5 H4 6. n = number of cylinder N is engine speed rotation per second. Pm = S × Ai Li based on experimental chart plot obtained. A is cylinder area. Boundary Layers δ δ* = Displacement thickness 0 δ Ө = Momentum thickness ( ¿∫ 1− ¿ u dy U ) u u 1− dy ∫ U 0 U ( ) ¿ Shape factor number H = δ θ . Indicated power = (n cylinders) x Pm x A x L x 0.5× rpm 60 or ¿ bp ηm . Engine Performance parameters Mean Effective Pressure. L is stroke length. (for 4 strokes engine) Mean piston speed = 2× stroke length time for one revolution for 4 stroke engine Mechanical efficiency = shaft power ∨brake power indicated power Brake power = bp . Mass fuel rate = Mfuel Brake mean effective pressure. bmep = b p ×2 A × L× N × n for 4 stroke engine Specific fuel consumption = mass fuel rate consumption brake power air air mass flowrate ratio= fuel fuel mass flowrate Volume efficiency= actual volume∨mass air into engine Volume∨mass of air filled by swept vol ume Thermal efficiency = bp M fuel ×Qcaloric . 287 kJ/K R= Ro/M where Ro is molar gas constant at 8.3143 kJ/kg K M is relative molecular mass of gas R = C p – Cv 9. q =Cp (T1-T2) P ×V T is constant for all processes . Ideal gas PV = nRoT where N is number of moles gas present Ro is molar gas constant at 8.Indicated Thermal Efficieny= indicated power Fuel flow rate× fuel calorific value 8.3143 kJ/kg K PV =m RT where R is the specific gas constant For normal air R is 0. q = Cv (T1-T2) Constant pressure process. For gasses Constant volume process. q = Cp (T1-T2) Isentropic/Adiabatic process. r = V1/V2 Work done = Qin – Qout/reject 11. P1 V 1k =P 2 V 2k T 1 V 1k−1=T 2 V 2k−1 P P T 2 (¿¿ 2) K K−1 K T 1 (¿ ¿1) K −1 =¿ ¿ Where k or γ = Cp/Cv specific heat ratio 10. c=√ kRT . Standard Diesel cycle Clearance volume = V2 Swept volume = V1 – V2 Compression ratio. Compressible flow Speed of sound .Adiabatic process. Where k or γ = Cp/Cv specific heat ratio Stagnation Temp . T o=T 1+ T 0 V 0k−1=T 1 V 1k−1 P P T 1 (¿¿ 1) K K −1 K T 0 (¿¿ 0) K−1 =¿ ¿ V2 2 Cp .