- #1
albertov123
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Homework Statement
A hot gas stream at 600K and 200 kPa is cooled at constant pressure to 300K in a pipe by direct thermal contact with the atmosphere. The mass flow rate of the stream is 0,1 kg/s and the atmospheric temperature and pressure are 300K and 100 kPa. Modeling the gas as an ideal gas with constant specific heat determine;
a)heat transfer rate to the atmosphere
b)entropy generation rate associated with the cooling process
c)if the hot gas stream is used to produce mechanical power by operating a heat engine between the stream and the atmosphere determine the maximum shaft power produced
d)demonstrate that the mechanical power produced in part c is proportional to the rate of entropy generation in part b
Homework Equations
The Attempt at a Solution
Since we don't know what the gas is, i write down c representing Cp (constant pressure problem)
dT = T2 - T1 (change in temp)
dS = S2- S1 (change in entropy)
a) Q=mcdt = 0,1c.(600K-300K) = 30c (heat transfer rate to the atmosphere)
b) dS = Sgen + (Q/T) so Sgen = dS - (Q/T)
dS = mcln(T2/T1) = 0,1cln(600/300) = 0.07c
Sgen = 0.07c - (-30c/300K) = 0.17c (entropy generation)
What did i miss? I am confused because i didn't make use of pressure throughout my calculations.
c) max efficiency n = 1 - (Tcold/Thot) = 1 - (300/600) = 0.5
Q.n=Work
30c.0,5=15c (maximum shaft work)
d) i couldn't come up with something on this.