Q4. An ideal gas is heated in a cylinder at constant volume from a temperature of 300 K to a temperature of 1208 K. The initial pressure and volume are 1.02 bar, 0.084 m3 respectively.
It is further heated at constant pressure to 1474 K. The gas then expands isentropically to ten times the initial volume.
The change in specific entropy for the first process is 1.0 kJ/kgK and in the second process it is 0.2 kJ/kgK.
(a) Sketch the p-V and T-s diagrams.
(b) Calculate EACH of the following:
(i) The values of Cv, Cp, and γ for the gas
(ii) The final temperature of the gas
(iii) The specific work transfer.
Q4. In an air standard diesel cycle the volume compression ratio is 18:1 and the volume cut off ratio is 1.8:1. The minimum pressure and temperature are 0.95 bar and 303 K respectively.
(a) Sketch the cycle on a T-s diagram.
(i) The temperatures and pressures at the cardinal points in the cycle
(ii) The thermal efficiency
(iii) The mean effective pressure.
Note: For air γ = 1.4 and Cp = 1.005 kJ/kgK
Q5. A fuel with a mass analysis of 85% carbon 15% hydrogen is completely burned in air. The dry flue gas shows they contain 3.5% oxygen by volume.
(a) Use molar volumes to formulate the complete combustion equation in kmol per kg of fuel.
(i) The percentage excess air by volume
(ii) The fuel air ratio by mass.
Note: Relative atomic masses H = 1, C = 12, N = 14, O = 16. Air contains 21% oxygen by volume
Q19. A single acting vertical bilge pump operates at 40 rev/min against a constant discharge pressure of 2.8 bar. The pump delivers 10 m3/hour of fresh water for the single 120 mm diameter piston. The combined mass of the piston and entrapped water is 18 kg and the piston can be assumed to move with simple harmonic motion. The piston is moving upwards and is 80 mm away from the top of the stroke. Calculate EACH of the following:
(a) The force required to move the piston;
(b) The input power to the pump motor at the given instant if the motor efficiency is 80%.
Q5. A reversed Carnot cycle uses Ammonia as the working fluid. The Ammonia is dry saturated at the end of compression and leaves the condenser as a saturated liquid. The cycle operates between temperatures of -12ºC and 34ºC respectively.
(a) Sketch the cycle on P-h and T-s diagrams.
(b) Determine EACH of the following:
(i) The dryness fraction at the compressor inlet
(ii) The coefficient of performance
(iii) The refrigerating effect per kg of ammonia.
(c) Give TWO reasons why such a cycle would not be used in practice.
Q3. In a two-stage, single acting reciprocating air compressor, the LP suction pressure is 1.05 bar, the inter-stage pressure is 3.15 bar and the HP delivery pressure is 10.2 bar. The LP suction temperature is 298 K and the HP suction temperature is 311 K. The index of compression and expansion for both stages is 1.29.
(a) Sketch the process on a p-V diagram indicating the areas which represent work saved by intercooling and the isothermal curve.
(b) Calculate EACH of the following per kg of induced air:
(i) The indicated work
(ii) The heat removed in the intercooler
(iii) The work saved by intercooling.
(c) Calculate the isothermal efficiencies with and without intercooling.
Note: For air R = 0.287 kJ/kgK and Cp =1.005 kJ/kgK
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