Anna University, Chennai



Reg. No. :

Question Paper Code : T3050


Third Semester

Mechanical Engineering


Time : Three hours Maximum : 100 Marks

(Use of Standard Thermodynamic tables, Mollier diagram, Psychrometric chart and

Refrigerant tables are permitted) Answer ALL Questions

PART A — (10 × 2 = 20 Marks)

1. What is the difference between the classical and the statistical approaches to thermodynamics?

2. State the zeroth law of thermodynamics.

3. What is the Kelvin-Planck expression of the second law of thermodynamics?

4. Why is the second law, called a directional law of nature?

5. Why is excessive moisture in steam undesirable in steam turbines?

6. Why is the Carnot cycle not a realistic model for steam power plants?

7. What does the Joule-Thomson coefficient represent?

8. In a gas mixture, which component will have the higher partial pressure-the one with the higher mole number or the one with the larger molar mass?

9. What is the difference between dry air and atmospheric air?

10. When are the dry-bulb and dew-point temperatures identical?

PART B — (5 × 16 = 80 Marks)



A reciprocating air compressor takes in 2 m3/min air at 0.11 MPa, 293 K

which it delivers at 1.5 MPa, 384 K to an after cooler where the air is

cooled at constant pressure to 298 K. The power absorbed by the

compressor is 4.15 kW. Determine the heat transfer in (i) the compressor

(ii) the cooler. State your assumptions. (16)


(b) In a turbo machine handling an incompressible fluid with a density of

1000 kg/m3 the conditions of the fluid at the rotor entry and exit are as given below :

Inlet Exit Pressure 1 .15 MPa 0.05 MPa Velocity 30 m/sec 15.5 m/sec Height above datum 10 m 2 m

If the volume flow rate of the fluid is 40 m3/s, estimate the net energy

transfer from the fluid as work. (16)



The interior lighting of refrigerators is provided by incandescent lamps

whose switches are actuated by the opening of the refrigerator door.

Consider a refrigerator whose 40W light bulb remains on continuously as

a result of a malfunction of the switch. If the refrigerator has

a coefficient of performance of 1.3 and the cost of electricity is

Rs. 8 per kWh, determine the increase in the energy consumption of the

refrigerator and its cost per year if the switch is not fixed. (16)




A Carnot heat engine receives 650 kJ of heat from a source of

unknown temperature and rejects 250 kJ of it to a sink at 297 K.

Determine the temperature of the source and the thermal efficiency

of the heat engine. (6)

(ii) A Carnot heat engine receives heat from a reservoir at 1173 K at a rate of 800 kJ/min and rejects the waste heat to the ambient air at

300 K. The entire work output of the heat engine is used to drive a refrigerator that removes heat from the refrigerated space at 268 K and transfers it to the same ambient air at 300 K. Determine the maximum rate of heat removal from the refrigerated space and the total rate of heat rejection to the ambient air. (10)



Consider a steam power plant operating on the ideal Rankine cycle.

Steam enters the turbine at 3 MPa and 623 K and is condensed in the

condenser at a pressure of 10 kPa. Determine (i) the thermal efficiency of

this power plant, (ii) the thermal efficiency if steam is superheated to

873 K instead of 623 K, and (iii) the thermal efficiency if the boiler

pressure is raised to 15 MPa while the turbine inlet temperature is

maintained at 873 K. (16)


(b) Consider a steam power plant operating on the ideal reheat Rankine cycle. Steam enters the high-pressure turbine at 15 MPa and 873 K and is condensed in the condenser at a pressure of 10 kPa. If the moisture content of the steam at the exit of the low-pressure turbine is not to exceed 10.4 percent, determine (i) the pressure at which the steam should be reheated and (ii) the thermal efficiency of the cycle. Assume the steam is reheated to the inlet temperature of the high-pressure turbine. (16)




Using the Clapeyron equation, estimate the value of the enthalpy of


vaporization of refrigerant R –134a at 293 K, and compare it with



the tabulated value. (10)

Show that c p cv = R for an ideal gas. (6)





Show that the Joule-Thomson coefficient of an ideal gas is zero. (6)

Using the cyclic relation and the first Maxwell relation, derive the

other three Maxwell relations.





What is the lowest temperature evaporative cooler if it enters at 1 relative humidity?

that atm,



can attain in an K, and 40 percent (4)