Thermo Q RE:isentropic efficiency through nozzles

[nickbone59]

Homework Statement

Exhaust gas expands through a nozzle whose isentropic efficiency is 88%. The inlet pressure and temperature are 1.6 bar and 400c respectively. At outlet the pressure has fallen to an ambient level of 1.013 bar. Given that the inlet velocity is negligible, and Cp and \gammaare 1.15kJ/kgK and 1.3 respectively, determine;

i)the outlet pressure and velocity
ii)the rate of generation of entropy, if the mass flowrate is 2.8 kg/s

Homework Equations

s2-s1=cp ln (T2/T1)-R ln(P2/P1)

The Attempt at a Solution

Outlet Temp

T2s/T1=(p2/p1)^(\gamma-1/\gamma)
Therefore,
T2s=673x(1.013/1.6)^(0.23076923)
=605.6266K

T2-673= - 67.373/0.88= -76.56

Therefore,
T2=673+-76.56
= 596.44K

This doesn't seem right? Is it just the first part of the formula i use, to get T2s?

After this i am stuck. Any help with this will be greatly appreciated!

 

[deepthishan]

to find outlet velocity (i don't understand how to 'find' outlet pressure since uve already stated it!):

T2s/T1=(p2/p1)^(-1/)
Therefore,
T2s=673x(1.013/1.6)^(0.23076923)
=605.6266K

for isentropic conditions (i.e. no external heat transfer),

Cp*T1 + .5*u1^2 = Cp*T2i + .5*u2i^2
since u1=0,
u2i=420.0m/s

.88=u2^2/u2i^2
therefore, u2=394.0m/s

________________________________________________________________________

 

[deepthishan]

p/rho=R*T

by bernoulli's principle,

u1^2/2 + R*T1 = u2^2/2 + R*T2
u2=394.0m/s (from previous solution)

T1= 1213.4K