CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on May 11, 2017 14:47:23 GMT -5
Hello All,
I just received a copy of Thomas Kamps book "Model Jet Engines" and have some questions on the thermodynamic equations.
Calculating Enthalpy rise across the compressor.
Delta H = Enthalpy increase in J/kg = T*Cp*((Pr^0.286)-1) Where T = inlet temperature 288 deg K Inlet pressure = 1.013 bar Outlet pressure = 2 bar Cp = Specific Thermal Capacity of air, 1000 (J/kg/K)
Pr = pressure ratio of the compressor is (final pressure/inlet pressure) = 1.974
Where (Pr^0.286)-1 = 0.2148
The exponent in the formula 0.286 is derived from the polytropic coefficient n. In the case of a adiabatic compressor n = 1.4 The exponent used is (n-1)/n = 0.28571 or .286 rounded.
I run the numbers and get 61.851 kj/kg. Kamps gives an example on the same page and gives a value of 83kj/kg.
Did I set up the equation incorrectly? My number comes in at 75% of Kamps. Perhaps Kamps example is factoring in 75% compressor efficiency that is not in the Delta H equation above?
Thanks, Tony
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Post by racket on May 11, 2017 17:03:49 GMT -5
Hi Tony
Yep , he's worked on 75% comp effic , which then raises the "energy" in the airflow due to the increased temperature rise across the stage .
Instead of the ~62 C degree rise at 100% effic , its ~83 C degrees at 75% , so T2 would be ~98 deg C if 15 C ambient.
Cheers John
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on May 11, 2017 18:56:34 GMT -5
Thanks John. I'll go through all the equations and look forward to learning and understanding them.
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