Question1
A piston cylinder device contains 25 g of saturated water vapor that is maintained at constant pressure of 300 kPa. A resistance heater within the cylinder is turned on and pass a current of 0.2 A for 5 minutes from a 120-V source. At same time, a heat loss of 3.7 kJ occurs. Determine the change of entropy of the steam.
1.6 kg of air is compressed according to the law pV1.3=C form a pressure of 1.2 bar and temperature of 20 oC to a pressure of 17.5 bar. Calculate the change in entropy.
Question 2
An engine works on ideal dual cycle draws air at 1 bar pressure and 30 oC. The compression ratio is 16 and the pressure ratio is 1.75. The total heat added is 2500 kJ/kg.
Sketch the cycle on a p-v diagram
Determine the following:
a. Pressures, temperatures and specific volumes at the salient points, heat added at constant pressure process and cut-off ratio.
b. Thermal efficiency of the cycle.
c. Mean effective pressure for the cycle.
Take Cp=1.005 kJ/kg K, Cv=0.718 kJ/kg K, g=1.4, R=287 J/kg K.
Question 3
A gas turbine unit has a pressure ratio of 9 and a maximum cycle temperature 900 K. the compression take place in two stages of equal pressure ratio with intercooling back to initial temperature of 15 oC. The isentropic efficiencies of turbine and compressor are 0.86 and 0.89 respectively . The unit is fitted with a heat exchanger of 70 % effectiveness.
1. Sketch the cycle on a T-s diagram
2. Determine the efficiency of the unit.