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Homework Statement
The problem (tried my best to translate it):
A small high speed turbine is operating on compressed air. It deliveres dW/dT=100 W. At the inlet, the pressure is 400 kPa and the temperature 50*C.
At the exit, the pressure is 150 kPa and the temperature -30*C.
Neglect the velocity and assume an adiabatic process. Find the necessary mass flow of air through the turbine.
Homework Equations
First law for control volume
Definition ofg enthalpy
The Attempt at a Solution
I have derived the first law for a control volume:
dE/dt=(dQ/dt)-(dW/dt)+∑m_i (h_i+0.5v_i^2+gz_i)-∑m_e (h_e+0.5v_e^2+gz_e)
Where t is time, and m_i og m_e is rate of change of mass flow at the inlet and exit, respecitively.
Assumed steady state: dE/dt=0.
Adibatic dQ/dt=0.
Also m_e´=m_i´=m´
By neglecting kinetic and potential energy associated with gravity, i end up with:
dW/dt=m(h_i-h_e) <=> m=(dW/dt) / (h_i-h_e)
So far so good, but now I need to find the change of enthalphy. We were supposed to solve this task without the use of steam tables. I have tried to use the definition of constant volume heat capacity, but no luck so far. Any input?
Thanks in advance. :)