ME 6301 ENGINEERING THERMODYNAMIICS, Second year, Department of Mechanical Sandwich Engineering, Model Exam Questions

PART – (10 x 2 = 20 marks)

1.      What is heat?

2.      Prove that c_p-c_v=R.

3.      State zeroth law of thermodynamics. What is its application?

4.      Deduce the relation between the COP of heat pump and refrigerator.

5.      What is meant by thermodynamic temperature scale? How do you device such scale?

6.      What do you understand by pure substance? Give some typical examples.

7.      What is critical point? What are the properties of water at critical point?

8.      What are the unique features of van der Waals equation of state?

9.      What is compressibility factor? What does it signify? What is its value for van der Waals gas at critical point?

10.  What is dew point temperature? How is it related to dry bulb and wet bulb temperature at the saturation condition?

                                    PART B – (5 x 16 = 80 marks)

11.  (i) Prove that internal energy is a property?                                                                (4)

(ii) 1 kg of gas at 1.1 bar, 27°C is compressed to 6.6 bar as per the law  pu ^1.3= const. Calculate work and heat transfer, if

(1)   When the gas is ethane (C₂H₆)  with molar mass of 30 kg/k mol and c_p of 2.1 kJ/kg K.                                                                                                      

(2)   When the gas is argon (Ar) with molar mass of 40 kg/k mol and c_p of 0.52 kJ/kg K.                                                                                                (12)

12. (a) (i)   “Two reversible adiabatic lines cannot intersect”. Is this statement true or false? Justify  the answer.                                                                                                        (4)            (ii)A reversible engine operates between a source at 972°C and two sinks, one at 127°C and another at 27°C. The energy rejected is same at both the sinks. What is the ratio of heat supplied to the heat rejected? Also calculate the efficiency.         (12)

                                                                  (Or)

      (b) (i)   what are the conditions for reversibility?                                                       (2)

            (ii) Differentiate between heat pump and refrigerator.                                        (2)

            (iii)            50kg of water is at 313 K and enough ice at -5°C is mixed with water in an adiabatic vessel such that at the end of the process all the ice melts and water at 0°C is obtained. Find the mass of ice required and the entropy change of water and ice. Given cp of water = 4.2 kJ/kg-K, c_p of ice = 2.1 kJ/kg-K and latent heat of ice = 335 kJ/kg.

                                                                                                                                          (12)

13.(a)(i)     Draw the p-T diagram of a pure substance and label all the phases and phase changes.

                                                                                                                                          (4)

        (ii)     What do you understand by dryness fraction? What is its importance?        (2)

        (iii)    A rigid tank of 0.03 m³ capacity contains wet vapour at 80 kPa. If the wet vapour mass is 12 kg, calculate the heat added and the quality of the mixture when the pressure inside the tank reaches 7 MPa.                                                                (10)

                                                                  (Or)

(b)(i)          What are the major problems of Carnot vapour cycle?                                   (2)

     (ii)        What are the methods for improving the performance of Rankine cycle        (2)

     (iii)       Steam enters the turbine at 3 MPa and 400°C and is condensed at 10 kPa. Some quantity of steam leaves the turbine at 0.6 MPa and enters open feed water heater. Compute the fraction of the steam extracted per kg of steam and cycle thermal efficiency.                                                                                                        (12)

14.(a)(i)           Write down the Dalton’s law of partial pressure and explain its importance.(6)

(ii)            0.45 kg of CO and 1 kg air is contained in a vessel of volume 0.4 m³  at 15°C. Air has 23.3% of O₂  and  76.7% of N₂ by mass. Calculate the  partial pressure of each constituent and total pressure in the vessel. Molar masses of CO, O₂ and N₂ are 28,32 and 28 kg/k mol.                                                                          (10)

                                                                (Or)

(b)(i)          What is the use of Clapeyron equation? And write it down for liquid-vapour region.                                                                                                                      (6)

(ii)           Explain the flow process of a real gas through a throttle value. Drive the expression for Joule Thomson coefficient and deduce its value an ideal gas.(10)

15.(a)(i)           Differentiate between             Dry bulb temperature and wet bulb temperature.       (4)

                        Wet bulb depression and dew point depression                                             (4)

(ii)            Air at 16°C and 25% relative humidity passes through a heater and then through a humidifier to reach final dry bulb temperature of 30°C and 50% relative humidity. Calculate the heat and moisture added to the air. What is the sensible heat factor?                                                                                                                            (8)

(b)(i)            In an adiabatic mixing of two streams. Derive the relationship among the ratio of mass of streams, ratio of enthalpy change and ratio of specific humidity change.                                                        (8)

     (ii)   Saturated air at 20°C at a rate of 1.167 m³/Sec is mixed adiabatically with the outside air at 35°C and 50% relative humidity at a rate of 0.5 m³/sec. Assuming adiabatic mixing condition at 1 atm, determine specific humidity, relative humidity,dry bulb temperature and volume flow rate of the mixture.              (8)