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-   -   Heat energy imbalance (http://www.cfd-online.com/Forums/cfx/20774-heat-energy-imbalance.html)

Coriolius November 4, 2004 01:46

Heat energy imbalance
 
I have built a rectangular domain with 1 inlet, 1 outlet (elongated to prevent backflow at the outflow opening), some constant heat flux heat sources, and adiabatic room surface. The working fluid is room air.

When I solve it in CFX-Solver, the residual of momentum and energy always converge much slower than the mass and even oscilate. The result for imbalance of quantites show that all variables (except the heat energy) have a value of less than 0.1%. But the energy term has an imbalance of more than 50%. Even the adiabatic wall have some energy value.

Is it always the case that the energy equation is much more difficult and slower to converge?

NymphadoraTonks November 4, 2004 07:57

Re: Heat energy imbalance
 
Sounds as if you have specified unphysical BCs and/or the solution is inherantly transient.

Coriolius November 4, 2004 21:53

Re: Heat energy imbalance
 
The result is like the following:

OUTER LOOP ITERATION = 500 CPU SECONDS = 2.13E+04 ---------------------------------------------------------------------- |Equation | Rate | RMS Res | Max Res | Linear Solution | +----------------------+------+---------+---------+------------------+ | U-Mom | 1.01 | 1.8E-04 | 2.7E-03 | 7.7E-04 OK| | V-Mom | 1.00 | 1.1E-04 | 2.7E-03 | 1.3E-03 OK| | W-Mom | 1.01 | 1.7E-04 | 2.8E-03 | 8.3E-04 OK| | P-Mass | 1.01 | 1.4E-06 | 7.0E-05 | 9.2 3.5E-02 OK| +----------------------+------+---------+---------+------------------+ | H-Energy | 0.99 | 1.3E-03 | 2.6E-02 | 5.5 5.4E-04 OK| +----------------------+------+---------+---------+------------------+ | K-TurbKE | 1.00 | 1.2E-04 | 4.3E-03 | 5.5 2.7E-03 OK| | E-Diss.K | 1.01 | 1.1E-05 | 7.0E-04 | 12.9 4.2E-06 OK| +----------------------+------+---------+---------+------------------+

+--------------------------------------------------------------------+ | H-Energy | +--------------------------------------------------------------------+ Boundary : Inlet -2.3883E+03 Boundary : Motor 1.4602E+04 Boundary : Outlet -5.4289E+03 Boundary : Panel 1.9813E+03 Domain : PlantRoom 2.7376E-09

----------- Domain Imbalance : 8.7659E+03

Domain Imbalance, in %: 60.0332 %

=================================

The "Inlet" is an opening supplying pre-conditioned at 18 deg-C and constant flow rate into the domain, the "Outlet" is a free outlet, "Motor" and "Panel" are surface with constant heat flux, the "PlantRoom" are given adiabatic condition. Simulation condition is steady state.

Since all these boundaries are fix zsurfaces of the flow domain, can we say that the imbalance is actually the heat gain of the supply air in the flow domain before leaving the outlet?

tuki72@hotmail.com November 5, 2004 14:00

Re: Heat energy imbalance
 
1) Check the mesh independence 2) Make sure the Y+ value (depending on the turbulence model you are using)..

What is the mass convergence criteria?

Tuki

Coriolius November 5, 2004 23:29

Re: Heat energy imbalance
 
The convergence criteria is 1 e-4

I have checked with the calculated surface temperature of the boundary with heat flux (represent a lift traction motor) and found that it wss about 300 - 500 deg-C which is very much unrealistic (the maximum value is more than 1000 deg-C!) . In practice, the temperature of the motor stator is about 100 deg-C. The surface temperature should at least lower than that value.

In the model, I simply model the motor by making a rectangular block on the floor and subtract it from the domain by "Cut material". Then a constant heat flux of about 600 W/m was applied to the motor surface boundary.

Generally, is there any tricks or points to note when modelling surface boundary with heat flux?



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