Q9. Air at a pressure and temperature of 1 bar and 32ºC respectively is compressed in an isentropic process from BDC to TDC in an engine. Heat is then added reversibly at constant volume until the pressure reaches 35.5 bar.
The clearance volume is 1.82 litres and the bore and stroke are 250 mm and 350 mm respectively.
(a) Sketch the p-V and T-s diagrams.
(b) Calculate EACH of the following:
(i) The compression ratio
(ii) The work transfer
(iii) The total change in entropy.
Note: For air Cv = 0.718 kJ/kgK and R = 0.287 kJ/kgK
Q10. Air enters the compressor of a simple gas turbine plant at a pressure and temperature of 1.03 bar and 305 K respectively. It is compressed to 6.18 bar with an isentropic efficiency of 83%.
The hot gases leave the combustion chamber and enter the turbine at a pressure of 6.18 bar and a temperature of 1210 K.
The turbine exhausts at a pressure of 1.03 bar and a temperature of 805 K.
The fuel air ratio is 0.0125.
(a) Sketch the cycle on a Temperature-specific entropy diagram.
(i) The isentropic efficiency of the turbine
(ii) The network output per kg of exhaust gas
(iii) The thermal efficiency of the cycle.
Note: For air: γ = 1.4 and Cp = 1.005 kJ/kgK
For the hot gas: γ = 1.33 and Cp = 1.15 kJ/kgK
Q9. Octane (C8H18) is burned in 15% excess air.
The dry flue gas analysis shows they contain 2.5% carbon monoxide by volume.
Calculate, using molar volumes for 1 kmol of fuel, EACH of the following:
(a) The actual amount of oxygen supplied
(b) The full combustion equation
(c) The volumetric analysis of the total flue gas.
Note: Air contains 21% oxygen by volume.
Q11. A vapour compression refrigeration cycle operating between pressures of 2.006 bar and 10.163 bar is to be used as a heat pump supplying a heating load of 10 kW.
The working fluid is R134a which enters the compressor as a dry saturated vapour and leaves at a temperature of 50ºC. The liquid enters the expansion valve with 5 K of sub-cooling.
(a) Sketch the cycle on p-h and T-s diagrams.
(b) Determine EACH of the following:
(i) The mass flow of refrigerant required
(ii) The cooling load based on the mass flow obtained Q6 b (i)
(iii) The heat pump co-efficient of performance
(iv) The isentropic efficiency of the compressor.
Q10. The free air capacity of a two-stage reciprocating air compressor is 18 m3/min.
The free air suction pressure and temperature are 1.05 bar and 25ºC respectively.
The delivery pressure is 9.45 bar.
The stage pressure ratios are equal, inter-cooling is perfect and the polytropic index for EACH expansion and compression is 1.26.
(a) Sketch the cycle on a p-V diagram.
(i) The total indicated power
(ii) The rate of inter-cooling
(iii) The power saved by inter-cooling.
Note: For air R = 0.287 kJ/kgK; Cp = 1.005 kJ/kgK
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