340 ENGINEERING THERMODYNAMICSDHARM
M-therm\Th6-2.PM5
- 1 kg of air is compressed polytropically from 1 bar and 300 K to 7 bar and 380 K. Determine the irreversibil-
ity and effectiveness, assuming temperature and pressure as 300 K and 1 bar.
Take for air : cp = 1.005 kJ/kg K, cp = 0.718 kJ/kg K, R = 0.287 kJ/kg K. [Ans. 1.25 kJ ; 0.924] - The moment of inertia of a flywheel is 0.54 kg-m^2 and it rotates at a speed of 3000 r.p.m. in a large heat
insulated system, the temperature of which is 15°C. If the kinetic energy of the flywheel is dissipated as
frictional heat at the shaft bearings which have a water equivalent of 2 kg, find the rise in the temperature
of the bearings when the flywheel has come to rest.
Calculate the greatest possible amount of the above heat which may be returned to the flywheel as high-
grade energy, showing how much of the original kinetic energy is now unavailable. What would be the final
r.p.m. of the flywheel, if it is set in motion with this available energy?
[Ans. 3.19°C ; 0.1459 kJ ; 222 r.p.m.] - In a steady flow air enters the system at a pressure of 10 bar and 200°C with a velocity of 100 m/s and leaves
at 1.5 bar and 25°C with a velocity of 50 m/s. The temperature of the surroundings is 25°C and pressure is
1 bar. Determine reversible work and actual work assuming the process to be adiabatic.
Determine also the irreversibility and effectiveness of the system on the basis of one kg of air flow.
Take for air : cp = 1 kJ/kg K ; R = 287 J/kg K. [Ans. 200.65 kJ ; 178.75 kJ/kg, 21.90 kJ/kg ; 0.894]