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Worked Example 2.10:
The gas in an internal combustion engine, initially at a temperature of 1270°C; expands polytropically to five times its initial volume and one-eight its initial pressure. Calculate:
a) The index of expansion, n, and
b) The final temperature
Answer:
1.292;
698K
Worked Example 3.1:
A closed rigid container has a volume of 1 m3 and holds air at 345 kPa and 20°C. heat is added until the temperature is 327°C. determine the change in Internal energy:
a) Using an average value of the specific heat
b) Taking into account the variation of specific
heat with temperature
Answer:
932 kJ;
1018.7 kJ
Worked Example 3.19:
A reciprocating internal combustion engine has a clearance volume of 0.0001 m^3 and a compression ratio (volume ratio) of 10. The pressure and temperature of the combustion gases when the
piston is at top dead centre are 4000 kN/m^2 and 1800°C respectively. Assuming that the expansion process follows PV^1.3 = constant, calculate:
a) The work transfer in this process; and
b) The temperature of the gases at the end of the
process.
Answer:
66.7 J;
1039 K
Worked Example 3.36:
A burner heats air from 20 to 40°C at constant pressure. Determine the change in entropy for a unit mass of air going through the heater, assuming that for air Cp = 1 kJ/kg-K.
Answer: 0.03356 kJ/kg-K
A boiler receives feed water at 20 kPa as saturated liquid and delivers steam at 2 MPa and 500oC. If the furnace of this boiler is oil fired, the calorific value of oil being 42000 kJ/kg determine the efficiency of the combustion when 4.2 tonnes of oil was required to process 42000 kg of steam.
Ans: 96%
10 kg/s steam at 6 MPa and 500oC, expands isentropically in a turbine to a pressure of 100 kPa. If the heat transfer from the casing to surroundings represents 1 per cent of the overall change of enthalpy of the steam, calculate the power output of the turbine. Assume exit is 5 m above entry and that initial velocity of steam is 100 m/s whereas exit velocity is 10 m/s.
Ans: 96%
Worked Example 2.8: (With Table)
300 kg/minute of steam at 3 MPa and 400°C is supplied to a steam turbine. Determine the potential heat released from steam if it is condensed at constant pressure. Can you deduce the specific heat of the steam under this condition?
Answer:
Q = 2133.5 kW;
Cp = 4.009 kJ/kg-K
Worked Example 3.2: (With Table)
An adiabatic steam turbine expands steam from a pressure of 6 MPa and a temperature of 500°C to a pressure of 10 kPa. The isentropic efficiency of the turbine is 0.82 and changes in kinetic and potential energy may be neglected. Determine the state of the steam at exit from the turbine and the specific work transfer.
Answer: Mixed phase with dryness factor x =
0.917; 849 kJ/kg
Worked Example 3.15: (With Table)
Steam at a pressure of 6 MPa and a temperature of 500°C enters an adiabatic turbine with a velocity of 20 m/s and expands to a pressure of 50 kPa, and a dryness fraction of 0.98. The steam leaves with a velocity of 200 m/s. The turbine is required to develop 1 MW. Determine
a) The mass flow rate of steam required, when KE
is neglected, and
b) What is the effect of KE on the answer?
Answer:
1.216 kg/s;
1.187 kg/s or an error of 2.4%
Worked Example 3.25: (With Table)
Steam at a pressure of 2 MPa and a temperature of 240°C enters a nozzle with a velocity of 15 m/s. The steam expands reversibly and adiabatically in the nozzle to a pressure of 100 kPa and a dryness fraction of 0.9. Calculate the velocity of the steam at exit from the nozzle.
Answer: 715 m/s
