Q11. Nitrogen at a pressure of 80 bar and a temperature of 2000 K expands isentropically in a cylinder from a volume of 0.03 m3 to a volume of 0.36 m3. The pressure after expansion is 2.47 bar. The nitrogen is then cooled at constant volume until the temperature is 300 K.
(a) Sketch the processes on p-V and T-S diagrams.
(b) Calculate EACH of the following:
(i) The isentropic index
(ii) The work done during expansion
(iii) The heat transfer
(iv) The change in entropy.
Note: For nitrogen, R = 0.297 kJ/kgK.
Q14. In an air, standard Diesel cycle the volume compression ratio is 22/1. The maximum and minimum temperatures are respectively 2010 K and 305 K, and the minimum pressure is 1.05 bar.
(a) Sketch the cycle on p-V and T-s diagrams.
(i) The heat supplied per kg
(ii) The network output per kg
(iii) The thermal efficiency
(iv) The mean effective pressure.
Note: For air, γ = 1.4 and Cp = 1.005 kJ/kg K.
Q10. Pure benzene (C6H6) is burned in air, and the dry products are found to contain 4.2% oxygen and 1.2% carbon monoxide by volume. Determine the percentage excess air supplied.
Note: Atomic mass relationships: H = 1; C = 12; O = 16; N = 14
Air contains 21% oxygen by volume.
Q2. In a regenerative steam power plant, steam enters the turbine at a pressure of 60 bar and a temperature of 490°C. It expands to 0.10 bar with an isentropic efficiency of 84%. Some steam is bled from the turbine at a pressure of 10 bar and supplied to a direct mixing feed heater. There is no undercooling in the condenser, and the feed water leaves the feed heater at the saturation temperature of the bled steam.
(a) Sketch a line diagram of the plant.
(b) On Worksheet Q4, plot the expansion process. To estimate the bled steam condition, it may be assumed that the process line on the h-s chart is straight.
(c) Estimate the thermal efficiency of the cycle. The work required to drive the feed pump may be disregarded.
Q14. A vapour compression refrigeration cycle uses ammonia (R717) and operates between pressures of 2.680 bar and 14.70 bar. The refrigerant enters the compressor at a temperature of -8°C and is compressed with an isentropic efficiency of 85%. The refrigerant leaves the condenser as saturated liquid.
(a) Sketch the cycle on p-h and T-s diagrams.
(b) Determine EACH of the following:
(i) The temperature leaving the compressor
(ii) The coefficient of performance of the cycle.
Q12. The free air capacity of a reciprocating air compressor is 15 m3/min. Free air and suction pressure and temperature are 1.00 bar and 30°C respectively. The delivery pressure is 13.0 bar. Compression is carried out in two stages with perfect intercooling under minimum work conditions. The index of compression and expansion is 1.28.
(a) Sketch the p-V diagram for the compressor.
(i) The total indicated power
(ii) The rate of intercooling
(iii) The isothermal efficiency.
Note: For air, R = 0.287 kJ/kg K and Cp = 1.005 kJ/kg K.
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