Problem 1 Air at STP in a piston-cylinder device undergoes expansion by three di
ID: 2997493 • Letter: P
Question
Problem 1
Air at STP in a piston-cylinder device undergoes expansion by three different processes between same end states until the volume doubles. Find the work done, heat transfer and change in internal energy per unit mass.
Problem 2
A piston-cylinder device contains 0.49 lbm of water initially at 120 psi and 2 ft3. Now 200Btu of heat is transferred to it at constant pressure. Determine the final temperature of water.
Problem 3
Steam flows steadily through an adiabatic turbine. The inlet conditions of the steam are 12.5MPa, 450oC and 80 m/s and exit conditions are 15 kPa, 85 % quality and 50 m/s. The mass flow rate of steam is 10 kg/s. Determine the (a) change in KE, (b) Power Output and (c) Inlet area of turbine
Problem 4
Diffuser in a jet engine is designed to decrease the kinetic energy of air entering the engine compressor without any work done or heat transferred. Calculate the velocity at the exit of such a diffuser when air at 100 kPa and 20
Explanation / Answer
1.
STP conditions means P1 = 1 bar, T1 = 0 deg C = 273 K
Also, V2 / V1 = 2
a)
Pv^2 = c
P1 / P2 = (V2 / V1)^2
P1 / P2 = 2^2
P1 / P2 = 4
P2 = P1 /4 = 1/4 bar = 0.25 bar
Pv^2 = c and Pv = RT
P*(RT/P)^2 = c
T^2 / P = c
(T2 / T1)^2 = (P2 / P1)
(T2 / 273)^2 = 0.25
T2 = 136.5 K
w = (P1*v1 - P2*v2) / (n-1) = R(T1 - T2) / (n-1)
w = 287*(273 - 136.5) / (2 - 1)
w = 39175.5 J/kg
du = Cv*(T2 - T1)
= 717*(136.5 - 273)
= -97870.5 J/kg
q = w + du
= 39175.5 - 97870.5
q = -58695 J/kg
b)
Putting k = 1.4 in above equations we get
T2 / T1 = (P2 / P1)^((k-1)/k)
T2 / 273 = 0.25^(0.4/1.4)
T2 = 183.7 K
w = 287*(273 - 183.7) / (1.4 - 1)
w = 64061.7 J/kg
du = Cv*(T2 - T1)
= 717*(183.7 - 273)
= 64061.7 J/kg
q = w + du = 0
c)
Isothermal: du = 0
q = w
w = R*T1 ln (V1 / V2)
w = 287*273 ln(2)
= 54308.8 J/kg
q = 54308.8 J/kg
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