Expert: Paul Soderman - 10/28/2011

Question
Sir,  
I am really stuck with these doubts. Kindly sort them out:

1.What is the dependency ( or relation) between static enthalpy and pressure? How are static enthalpy and Kinetic energy related? How does doing work actually increase pressure?


2.How is the process of increase in pressure in rotor and stator blades different? ( I know that in stator, KE is converted to Pressure energy, what about rotor?)

3.Enthalpy is a function of temperature. How exactly is enthalpy added? By adding temperature? By doing work?( But stator does no work even though it has a definite increase in enthalpy!)

Answer
Sangeeth - You are taking me back to graduate school.  Enthalpy is a state property of a fluid:  h = u + pv.  That is, enthalpy is the sum of internal energy per unit weight and the product of its pressure and specific volume.  It is a convenient function for thermodynamic problems involving heat quantities (heat capacities, latent heat, heats of reaction) when pressure is the variable being controlled.  For example, in a constant volume process in a hydrostatic system, heat is the flow of internal energy; whereas for a constant pressure process, heat is the flow of enthalpy.

In an open thermodynamic system, that is with flow, we have to introduce kinetic energy (KE) into the energy equation.  But enthalpy is only affected if the terms in the above equation change. In other words, enthalpy and KE are only related indirectly through changes in the state properties of the fluid.

Work is any mechanical process that changes the state property of a fluid exclusive of heat.  From the ideal gas law we know that pressure, temperature and volume are related. To change pressure, the volume has to change (work) or the temperature has to change or both.

Rotors and stators change pressures in a fluid in the same way.  They change the momentum of the fluid by changing its direction such that (from Newton's law) forces are created on the airfoils.  Forces on a surface create pressures that convect throughout the field.  With a coordinate system attached to the airfoil, it does not matter that the rotor is moving and the stator is fixed. All that matters is that they may see different flow fields coming at them.

Yes, enthalpy changes if temperature or work are changed.  The stator does do work.  It converts kinetic energy of the flow into heat by creating drag via friction.  Imagine a stator vane set in a wind tunnel.  If you turned the fans off, the stators would cause the air to heat up, slow down and eventually stop even neglecting losses in other parts of the circuit.
Paul