GATE Mechanical Engineering Thermodynamics Set1

1. 2 moles of oxygen is mixed with isentropically with another 2 moles of oxygen, so that final pressure and
temperature becomes same as that of their individual constituents at initial stage. The universal gas constant is
given as R. the change of entropy per mole of oxygen is given by
(A) –Rln2
(B) 0
(C) Rln2
(D) Rln4
Ans B: for isentropic process the heat transfer is zero hence change in entropy is zero.
Isentropic means No Change in Entropy, hence in above question the answer is Zero.

2. If a closed system undergoes irreversible process, the Entropy of the system
(A) must increase
(B) always remains constant
(C) must decrease
(D) can increase, decrease or remain constant
Ans: A: irreversible process means increase in entropy hence answer is "A".

3. In an ideal vapour compression refrigeration cycle, the specific enthalpy of the refrigerent (kJ/kg) at different states are given as
Inlet of Condenser = 283
Exit of Condenser = 116
Exit of evaporator = 232
the COP of this cycle is
(A) 2.27
(B) 2.75
(C) 3.27
(D) 3.75
Ans: A:
Enthalpy @ exit of condenser = Enthalpy @ inlet of evaporator
Desired Output = Heat absorbed during Evaporation = Exit of evaporator - Exit of Condenser
Desired Output = 232-116 = 116 kJ/kg
Input to ref system = Workdone by compressor = Inlet of Condenser - Exit of evaporator
Input to ref system = 283-232 = 51 kJ/kg
COP = Desired Output/Input to ref system = 116/51 = 2.27

4. Which of the following statement is true for water. 
(A) Latent heat of vaporization decreases with increase in pressure and increase in temperature 
(B) Latent heat of vaporization decreases with increase in pressure and decrease in temperature
(C) Latent heat of vaporization decreases with decrease in pressure and increase in temperature
(D) Latent heat of vaporization decreases with decrease in pressure and decrease in temperature

Ans: A: Latent heat of vaporization decreases with increase in pressure and increase in temperature .