July 22, 2019 at 4:43 pmMRMRSubscriber
In my case I'm simulating an air flow through a nozzle - and the problem is that in energy balance which I try to define I have some missing values and I cannot understand where is the problem - nozzle drawing included (c - velocity, i - enthalpy, Q - heat added, t - temperature).
So I have the boundary conditions and no matter what kind of mesh I use (RMS heat transfer residual always lower than 10^(-4) and most often lower than 10^(-5) or reaching 10^(-6); velocity and other residual nearly always reaching 10^(-6) I'm missing something in energy balance.
I use CFX, SST with incl. viscous heat term, pressure inlet and outlet as boundary conditions, air as ideal gas, outside wall of nozzle is heated - defined by a temperature of wall. No wall thickness - simple model.
So I start with CFD-post calculator to get massflowAve enthalpy and velocity for inlet and outlet. Then I check what is the mass flow and by multiplating it with enthalpy values I get the values to the energy balance. Then I calculate kinetic energy (mv^2)/2 and to get the wall heat flux first I calculate it for the wallave and then multiple it with wall area.
And in this way I get every needed values on the left and right side of the equation, but there is always a little bit missing (maybe 1 to 10J heat more is added - in the whole energy balance is nearly nothing). I thought about friction but it should only create heat which is included in energy balance (pressure losses also counted). I also thought about some approximation mistakes but the bigger model is - the bigger are these "losses" so it's not a coinsidence.
If you dont know the answer, could you help me where should I look to get to know where to search to fill my lack of knowledge?
July 23, 2019 at 8:22 amDrAmineAnsys Employee
Do rely on the solver out file for checking imbalances.
In Post use the Heat Flux variable in post processing which is available on walls and non wall boundaries.
July 23, 2019 at 5:41 pmMRMRSubscriber
Thank you Amine for your answer.
In CFD-post I checked the areaAve heat flux for inlet, outlet and wall and multiplied them with the area of each surface - the differences in the whole balance are very little so it works, thank you.
Do you/anyone else have an idea why the values of massflow doesnt work? Approximation mistakes?
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