A piston-cylinder assembly contains 2 lb of air at a temperature of 540 °R and a pressure of 1 atm. The air compressed to a stat
e where the temperature is 840 °R and the pressure is 6 atm. During the compression, there is a heat transfer from the air to the surroundings equal to 20 Btu.
1. Using the ideal gas model for air, determine the work during the process in Btu.
1. Using the ideal gas model for air, determine the work during the process in Btu.
1 answer:
Answer:
123.9 Btu
Explanation:
The energy balance on the air is:
∆E = E2 − E1 = ∆KE + ∆PE + ∆U = Q + W
ignore ∆KE and ∆PE,
W = ∆U − Q = m(u2 − u1) − Q; (u2 − u1 = 51.94 Btu/lb)
ideal gas properties is attached
W = (2 lb)(143.98 − 92.04) Btu/lb − (− 20 Btu) = 123.9 Btu
u2 − u1 ≈ cv(T2 − T1) = (0.173 Btu/lb°R)(840 − 540) °R = 51.9 Btu/lb
You might be interested in
Answer:
A) condenser pressure = 9.73 kPa,
B) 10242 kw
C) 36.9%
Explanation:
given data
entrance pressure of steam = 12.5 MPa
temperature of steam = 550⁰c
flow rate of steam = 7.7 kg/s
outer pressure = 2 MPa
reheated steam temperature = 450⁰c
isentropic efficiency of turbine( nt ) = 85% = 0.85
isentropic efficiency of pump = 90% = 0.90
From steam tables
at 12.5 MPa and 550⁰c ; h3 = 3476.5 kJ/kg, S3 = 6.6317 kJ/kgK
also for an Isentropic expansion
S4s = S3 .
therefore when S4s = 6.6317 kJ/kg and P4 = 2 MPa
h4s = 2948.1 kJ/kg
nt = 0.85
nt (0.85) = =
making h4 subject of the equation
h4 = 3476.5 - 0.85 (3476.5 - 2948.1)
h4 = 3027.3 kj/kg
at P5 = 2 MPa and T5 = 450⁰c
h5 = 3358.2 kj/kg, s5 = 7.2815 kj/kgk
at P6 , x6 = 0.95 and s5 = s6
using nt = 0.85 we can calculate for h6 and h6s
from the chart attached below we can see that
p6 = 9.73 kPa, h6 = 2463.3 kj/kg
B) the net power output
solution is attached below
c) thermal efficiency
thermal efficiency = 1 - = 1 - ( 2273.7/ 3603.8) = 36.9% ≈ 37%
Answer:
Explanation:
the solution is well stated
Answer:
TBC thickness of 4 mm is insufficient to prevent fire hazard
Explanation:
Given:
- Temperature of hot-fluid inner surface T_i = 333°C
- The convection coefficient hot-fluid h_i = 7 W/m^2K
- The thermal conductivity of engine cover k_1 = 14 W/mK
- The thickness of engine cover L_1 = 0.01 m
- The thermal conductivity of TBC layer k_2 = 1.1 W/mK ... (Typing error)
- The thickness of TBC layer L_2 = 0.004 m
- Temperature of ambient air outer surface T_o = 69°C
- The convection coefficient ambient air h_o = 7 W/m^2K
Find:
Would a TBC layer of 4 mm thickness be sufficient to keep the engine cover surface below autoignition temperature of 200°C to prevent fire hazard?
Solution:
- We will use thermal circuit analogy for the 1-D problem and steady state conduction with no heat generation in the cover or TBC layer.
The temperature at each medium interface and the Thermal resistance for each medium is given in the attachment schematic and circuit analogy.
- We will calculate the total heat flux for the entire system q:
q = ( T_i - T_o ) / R_total
- R_total is the equivalent thermal resistance of the entire circuit. Since all resistances are in series we have:
R_total = 1 / h_i + L_1 / k_1 + L_2 / k_2 + 1 / h_o
- Plug in the values and compute:
R_total = 1 / 7 + 0.01 / 14 + 0.004 / 1.1 + 1 / 7
R_total = 0.2900649351 T-m^2 / W
- Calculate the Total heat flux q:
q = ( 333 - 69 ) / 0.2900649351
q = 910.141 W / m^2
- Just like the total current in a circuit remains same, the total heat flux remains same. We will use the total heat flux q to calculate the temperature of outer engine surface T_2 as follows:
q = ( T_i - T_2 ) / R_i2
Where,
R_i2 = 1 / h_i + L_1 / k_1
R_i2 = 1 / 7 + 0.01 / 14 = 0.14357 T-m^2 / W
Hence,
( T_i - T_2 ) = q*R_i2
T_2 = T_i - q*R_i2
Plug the values in:
T_2 = 333 - 910.141*0.14357
T_2 = 202.33°C
- The outer surface of the engine cover has a temperature above T_ignition = 200°C. Hence, the TBC thickness of 4 mm is insufficient to prevent fire hazard
