Enthalpy related dillema in a steam piston engine

[AndreiM]

I am modeling a steam generator and it features a boiler, a monotube boiler, a steam uniflow motor, which has an admission stage, which is a constant pressure and temperature, and then it has an expansion stage, which expands the steam isentropically. And then it's a condenser and a pump, which pumps the condensed water back into the steam monotube boiler. So my problem right now is that I've calculated the enthalpy lost to the condenser per stroke of the motor and the enthalpy extracted from the motor per stroke. And those together sum up to a bigger number than I expected. It's more than the enthalpy that was used in the boiler and pump per stroke. And the difference between the enthalpy used in the boiler and the water pump per stroke (the difference between that and the enthalpy which was lost in the condenser and extracted from the motor) is precisely the energy that was extracted from the motor in the admission stage, so basically in the constant temperature and pressure stage. Why is this? What am I understanding wrongly?

 

[BrianPetersen]

Did you account for the quality (mass fraction of vapour in the vapour-liquid mixture) through to the end of the expansion phase?

(If your expansion starts with saturated steam, a portion of it is going to condense to water, in the form of mist dispersed in the remaining vapour, during the expansion stroke)

 

[Snickster]

I believe there is something wrong with how you are modelling the admission stage of the motor. The admission stage should not be at constant temperature and pressure since if this was so it would also be at constant enthalpy, and no energy would be extracted during this stage. Because of this you are starting the expansion stage at the same fluid conditions and energy content as the admission stage, but the expansion stage steam should be at a lower enthalpy than the admission stage. I believe in reality what happens during the admission stage with inlet steam valve open, the entire vapor space of the boiler is open to the cylinder so the entire volume of the steam space in the boiler and connecting piping to the cylinder gives up energy through an isentropic expansion of that entire volume. I am not sure how to model this but it must be accounted for some way in your model.

This is why I believe you are getting precisely the energy/work obtained in the admission stage as the "extra" energy of the system. The actual energy in and out should balance so pump work plus heat input to the fluid in boiler (not total heat of combustion) should equal engine work plus heat out in condenser. You are not deducting the work/energy done in the admission stage from the steam before it is entering the expansion stage if you don't change its state conditions - pressure, temperature, enthalpy, etc.

 

[crshears]

I'm no thermodynamic engineer, but from what I've read, saturated steam expansion in a cylinder, even a uniflow one, is [ once the admission valve is fully closed ] neither isentropic, nor adiabatic, and definitely not isothermal, but polytropic.

And what cylinder wall temperature gradient are you using? And of what material are the cylinder and piston made? These will both affect initial condensation and re-evaporation.

 

[Snickster]

My understanding is that he was modeling the system ideally for simplification.

 

[AndreiM]

My understanding is that he was modeling the system ideally for simplification.

precisely, this model Is meant to give me accurate results without taking into consideration some processes that are hard to account for and really are not worth modelling. Therefore I won't be taking into consideration certain friction losses, external losses and such.