Steam Turbine Design Aid: Understanding Thermodynamics

[leon25034796]

Can somebody aid my understanding:
I am re-studying thermodynamics at the moment 6 years on from when I last studied it and could do with some clarification:
If at the inlet to a steam turbine the pressure is at say 150bar and 550 degrees then there will be a respective enthalpy which is a function of the following h=u+pv. Now if the steam progresses through the stationary nozzle of an impulse stage the pressure drops and the kinetic energy of the steam (velocity) increases. But pv remains constant i.e. the pressure drops and the specific volme increases. I believe the temperature would increase with an increase in kinetic energy resulting in an increase in internal energy and thus enthalpy, but I'm guessing that the temperature would drop by a greater amount associated with the pressure drop resulting in an overall decrease in enthalpy?
When progressing through the moving blades the pressure remains constant as does specific volume but the internal energy decreases as a result of the kinetic energy decreasing and temperature decreasing resulting in a decrease in enthalpy?

Something else I'm having difficulty with is throttling:
If the steam is throttled by a governor the pressure drops, specific volume increases but there is no change in internal energy as no heat energy is added and no work is done i.e. Q+W=0 so the enthalpy remains constant. What I don't understand is if the enthalpy is constant at the inlet then at the exhaust the enthalpy must be greater than if there was no throttling reducing the amount of work done by the turbine. Which I'm guessing means the temperature must be higher, but why?

thanks

 

[xxChrisxx]

EDIT: It seems I am talking crap, i'll have to read up on it some more.I don't like this thread, it beings back nightmarish memories of endless steam tables.

 

[Topher925]

Now if the steam progresses through the stationary nozzle of an impulse stage the pressure drops and the kinetic energy of the steam (velocity) increases. But pv remains constant i.e. the pressure drops and the specific volme increases.

No, the product of pressure and volume will not remain constant. Assuming inviscid flow, no system work, and a horizontal nozzle the product of pressure and specific volume must decrease, with an increase of kinetic energy.

I believe the temperature would increase with an increase in kinetic energy resulting in an increase in internal energy and thus enthalpy, but I'm guessing that the temperature would drop by a greater amount associated with the pressure drop resulting in an overall decrease in enthalpy?

If your still talking about an ideal nozzle here, no this would not happen as there would be no change in enthalpy across the nozzle.

When progressing through the moving blades the pressure remains constant as does specific volume but the internal energy decreases as a result of the kinetic energy decreasing and temperature decreasing resulting in a decrease in enthalpy?

Impulse turbines work by changing the momentum of a fluid. A change in kinetic energy does not constitute a change in internal energy, they are two separate mechanisms. The only way the internal energy of the fluid will change is if work is performed on the fluid or there is heat transfer.

If the steam is throttled by a governor the pressure drops, specific volume increases but there is no change in internal energy as no heat energy is added and no work is done i.e. Q+W=0 so the enthalpy remains constant. What I don't understand is if the enthalpy is constant at the inlet then at the exhaust the enthalpy must be greater than if there was no throttling reducing the amount of work done by the turbine. Which I'm guessing means the temperature must be higher, but why?

Are you referring to the inlet and outlet of the turbine? If you have an increase of enthalpy at the exit of a turbine, then you in fact do not have a turbine but a compressor. It is very possible to have a turbine that operates isothermally and isenthalpically. This is in fact how many impulse turbines operate. For example, the kind of turbine you would find at hoover dam.


On a side note, this is my 1,000th post. _w00t!