Thermodynamics Questions 2022

Thermodynamics
Thermodynamics
SEPTEMBER 30, 2019

Question: 1
P1 = 2 MPa = 2000 KPa
T1 = 350 °C = 350+273 = 623K
P2 = 500KPa
T2 = T1 = 623 K
P1 V1 = mRT1
Specific volume V1 = V 1
m
P1 V1 = RT1
V1 = RT 1
P1 = 287 x 623
2 X 106
V1 =0.0894 m^3/Kg
V2 = RT 2
P2 = (287 x 623)/(500x10^3) m^3/Kg
Initial specific volume = V1 = 0.0894 m^3 /Kg
Final specific volume = V2 = 0.3576 m^3 /Kg
T-V diagram
P-V diagram
V
T
1 2
P

Work done = P1V1ln ( P 1
P 2 )
= 2x10^6 x 0.0894ln ( 2
0.5 )
= 2.479 x 10^6 Nm/Kg
Heat transfer for isothermal process = work done
δQ = δW = 2.479 X 10^6 J/Kg
δQ = 2.479 MJ/Kg
Question: 2
M = 5 Kg
P1 = 800 KPa
T1 = 70 °C = 70+273 = 343 K
P2 = 400 KPa
Since container is rigid
V1 = V2
P1/ T1 = P2/ T2
800/ 343 = 400/ T2
T2 = 171.5 – 273 = -101.5° - 273 = -101.5°C
Since boiling temp of R134a is -26.1 °C and T2 < boiling temp therefore R-134a has
changed its phase from vapor state to liquid or solid state
CV = 0.23901 KJ/KgK
Q = ∫
T 1
T 2
CvdT (at constant volume)
Q = mCV (T2 – T1)
Q = 5 X 0.2390 X (171.5 -343) KJ
Q = 204.951 KJ
Negative sign shows heat transfer takes place outside the system