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-   -   Don't know where energy lost from my domain (https://www.cfd-online.com/Forums/cfx/21120-dont-know-where-energy-lost-my-domain.html)

 Atit Koonsrisuk March 27, 2005 13:11

Don't know where energy lost from my domain

I model the flow in solar chimney. The flow gains thermal energy from energy source, afterward the energy is extracted by the model of turbine. I use the actuator disc as the turbine approximation. The actuator disc is very thin such that flow field properties change discontinuously across it. I create the fluid sub-domain, this sub-domain is the actuator disc representation. The ratio of thinkness to diameter of sub-domain is 0.0125, and I use mesh control with 1.2 expansion factor to make the finer grid in sub-domain. I set this sub-domain as the energy source and momentum source simultaneously.

The energy source is (-dW/volume)

and the momentum source is (-dW/(volume x velocity)),

where dW is the power that I want the actuator disc to extract from the flow, and 'volume' is the volume of sub-domain which I input as the number. So only 'velocity that I input as 'v' variable. After CFX converge, I found that the difference of total enthalpy before and after actuator disc doesn't equal 'dW'. For example,

- dW=165; mdot x ( static enthalpy + 0.5x velocity^2)=158

- dW=320; mdot x ( static enthalpy + 0.5x velocity^2)=304

- dW=590; mdot x ( static enthalpy + 0.5x velocity^2)=560 where I get 'mdot x static enthalpy' from the integration of CFX.

I do not understand why they don't equal. I set the wall as the adiabatic free-slip wall. So no other way for energy to be lost, isn't it? Then I specify that when the flow past this sub-domain, the power must be extracted 'dW', the total enthalpy must be lost 'dW'. Do anyone have idea why it doesn't? Thank you very much for your kind consideration.

 Glenn Horrocks March 28, 2005 18:35

Re: Don't know where energy lost from my domain

Hi,

Have you checked the imbalances? Your simulation might not have converged. Converging on imbalances and residuals is often a more reliable convergence measure than residuals alone when you have user sources.

Glenn

 Atit Koonsrisuk March 29, 2005 06:30

Re: Don't know where energy lost from my domain

Dear Glenn, Actually I never check the imbalance, thank you that you introduce me to know about 'imbalance'. After reading your suggestion, I found that the maximum global imbalance occur for H-Energy. It is 0.0036%. So we can be sure that it already converged. What do you think?

Atit

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