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Q16. Air at a pressure of 1 bar and a temperature of 300 K is compressed polytropically in an engine cylinder from a volume of 0.4 m³ to a volume of 0.03 m³. The pressure after compression is 48.7 bar. Heat is then supplied at constant pressure until the temperature is 1800 K.

(a) Sketch the processes on p-V and T-S diagrams.

(b) Calculate EACH of the following:

(i) The index of compression

(ii) The temperature after compression

(iii) The total heat transfer

(iv) The total change in entropy.

Note: For air, γ = 1.4 and R = 0.287kJ/kg K.

Q23. Air enters the compressor of a simple gas turbine plant at a pressure of 1 bar and a temperature of 300 K. It is compressed to 6 Bar with an isentropic efficiency of 85%. The hot gases leave the combustion chamber at a pressure of 6 bar and a temperature of 1200 K, the turbine exhaust at a pressure of 1 bar and a temperature of 810 K.

(a) Sketch the cycle on T-s diagram

(b) Determine EACH of the following

(i) The isentropic efficiency of the turbine

(ii) The network output per kg of gas

(iii) The thermal efficiency of the cycle.

Note: For air, g = 1.4 and Air. C_p= 1.005 kJ/kg K for hot gas in combustion chamber and turbine, g = 1.33 and Exh. C_p= 1.150 kJ/kg K

Q15. Benzene (C₆H₆) is burned in 20% excess air

Determine the volumetric analysis of the dry flue gases given that they contain 2% carbon monoxide by volume

Note: Relative atomic masses H = 1, C = 12, N = 14, O = 16 air contains 21% oxygen by volume

Q8. In a regenerative steam power plant, steam enters the turbine at a pressure of 120 bar and a temperature of 520°C. It expands to 0.05 bar with an isentropic efficiency of 80%. Some steam is bled from the turbine at a pressure of 2 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. (3)

(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. (5)

(c) Estimate the thermal efficiency of the cycle. The work required to drive the feed pump may be disregarded. (8)

Q9. In a two row velocity compounded impulse turbine stage, steam leaves the nozzles with a velocity of 1000 m/s at an angle of 180 to the plane of rotation. The mean blade velocity is 200 m/s. All the blade rows are symmetrical, and the blade velocity coefficient is 0.9 for both moving rows and for the fixed row.
Determine EACH of the following;
(a) the total blade work per kg of steam; (13)
(b) the diagram efficiency. (3)

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.

Q20. A wire of diameter 2 mm carries an electric current, and each metre length generates 2 watts of heat. The surrounding air is at 25°C and the surface heat transfer coefficient is 10 W/m²K.

(a) Determine the temperature of the wire. (5)

(b) Determine the new temperature of the wire if it is to be covered with insulation 1 mm thick and of thermal conductivity 0.1 W/mK. The heat transfer coefficient at the outer surface may be assumed to remain the same. (6)

(c) Command on the values of temperature obtained in Q7(a) and Q7(b). (5)

Q20. The free air capacity of a reciprocating air compressor is 15 m³/min. Free air and suction pressure and temperature are 1 bar and 27°C respectively. The delivery pressure is 9 bar. Compression is carried out in two stages with perfect intercooling. The stage pressure ratios are equal. The index of compression and expansion is 1.28.

(a) Sketch the p-V diagram for the compressor. (3)

(b) Determine EACH of the following:

(i) The total indicated power; (6)

(ii) The rate of intercooling; (3)

(iii) The isothermal efficiency. (4)

Note:  For air, R = 0.287 kJ/kg K and C_p = 1.005 kJ/kg K.

Q2. (a) Explain the term choked flow, with reference to a convergent nozzle. (4)

(b) Air leaks into an evacuated vessel from the surroundings which are at a pressure of 1.00 bar. The passage through which the air leaks may be considered as a convergent nozzle with exit area 0.5 mm², and the flow within the passage may be assumed isentropic. The temperature in the vessel is constant at 25°C.

Determine the mass flow rate when the pressure in the vessel is:

(i) 2.00 bar; (6)

(ii) 1.5 bar. (6)

Note:  For air, g = 1.4 and R = 0.287 kJ/kg K