410 ENGINEERING THERMODYNAMICS
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\M-therm\Th8-2.pm5(iii) Subsequently, the gas cools to the atmospheric temperature of 30°C, evaluate the final pressure of the
gas.
(iv) Evaluate the increase in specific internal energy, the increase in specific enthalpy, increase in specific
entropy and magnitude and sign of heat transfer.
[Ans. (i) 28 kg, 12.37 bar, 0.089 m^3 /kg ; (ii) cp = 1.038 kJ/kg K, cv = 0.745 kJ/kg K ;
(iii) 10.22 bar ; (iv) – 52.16 kJ/kg, – 72.67 kJ/kg, – 0.1536 kJ/kg K, 1465.1 kJ]- The pressure and volume of a gas, during a process, change from 1 bar absolute and 2 m^33 respectively to
6 bar absolute and 0.4 m^33 respectively. During the process the increase in the enthalpy of the gas is 200 kJ.
Taking cvvv = 10.4 kJ/kg K, determine cvpp, R and ∆U.[Ans. 13 kJ/kg K, 2.6 kJ/kg K, 160 kJ] - 1 kg of air at 27°C is heated reversibly at constant pressure until the volume is doubled and then heated
reversibly at constant volume until the pressure is doubled. For the total path find :
(i) The work ; (ii) Heat transfer ;
(iii) Change of entropy. [Ans. (i) 86.14 kJ, (ii) 728.36 kJ, (iii) 1.186 kJ/kg K] - A mass of air initially at 260°C and a pressure of 6.86 bar has a volume of 0.03 m^3. The air is expanded at
constant pressure to 0.09 m^3 , a polytropic process with n = 1.5 is then carried out, followed by a constant
temperature process which completes the cycle. All processes are reversible. Find (i) The heat received and
rejected in the cycle, (ii) The efficiency of the cycle.
Show the cycle on p-v and T-s planes. [Ans. (i) 143.58 kJ, – 20.3 kJ ; (ii) 38.4%]
REAL GASES- One kg-mol of oxygen undergoes a reversible non-flow isothermal compression and the volume decreases
from 0.15 m^3 /kg to 0.06 m^3 /kg and the initial temperature is 50°C. The gas obeys Van der Waals’ equation
during the compression. Find :
(i) The work done during the process ;
(ii) The final pressure. [Ans. (i) – 6706500 Nm/kg-mol, (ii) 13.825 bar] - Determine the compressibility factor for O 2 at (i) 100 bar – 70°C and (ii) at 5 bar and 30°C.
[Ans. (i) 0.71, (ii) 0.98] - Determine the pressure of air at 205°C having a specific volume of 0.00315 m^3 /kg by means of :
(i) Ideal gas equation.
(ii) Van der Waals’ equation ;
(iii) Beattie-Bridgeman equation. [Ans. (i) 435.7 bar, (ii) 557.3 bar, (iii) 525.8 bar]