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 sheng January 19, 1999 09:16

Solve Enthalpy or Temperature?

Hi all,

In order to model pure natural convection flow (heat loss to air) of two completely stratified liquid in a tank, which scalar is better to be solved (enthalpy or temp.)?

These two fluids are of different density, heat capacity and viscosity....

If solving the enthalpy equation, how can I set up the initial condition? In the reality, the two fluid keep the same temperature at the beginning, so this means to set the diffent initial enthalpy values. However, based on this setting, the convective enthalpy is very big during computation, then it will lead to derive a big temperature gradient at the interface of the two layer. Is this correct? Could you give some help on this?

Thanks a lot

Sheng

 I. Dotsikas January 21, 1999 08:47

Re: Solve Enthalpy or Temperature?

You can't solve any "temperature equation", because there is no temperature conservation law. You can only nummerically approximate the energy ( ~enthalpy ) conservation law.

Beginning from this equation you may omit many terms such as enthalpy change with pressure etc. If you do so, you may introduce a negligible error if you observe a system under "normal" conditions. (These cases affect your solution if you calculate compressible flows with very high velocities where these effects are important)

From your energy equation you obtain a "temperatur" equation that usually describes your system quite well. Don't ever forget that you use the energy conservation law in a simplified version.

 sheng January 21, 1999 10:13

Re: Solve Enthalpy or Temperature?

Thanks a lot, Dotsikas!

I am really confused that there are always very strange numercially transfer of the enthalpy due to its gradient jump at layer interface. This will lead to derive wrong temperature fields.

 John C. Chien January 21, 1999 13:14

Re: Solve Enthalpy or Temperature?

Looks like that you are talking about the interface problem instead of enthalpy vs temperature. If the interface is a discontinuity, then the properties can be discontinuous. The temperature is likely to be continuous, the velocity can be nearly continuous depending on whether the slip at the interface exists or not. If you are not treating the interface directly, then it is a different story.

 mahesh prakash January 21, 1999 18:22

Re: Solve Enthalpy or Temperature?

Hi, One solve only the thermal energy balance equation and this contains all information about thermal energy no matter in what form this thermal energy is in. So there is no question of having two different equations to express thermal energy!!!

 I. Dotsikas January 22, 1999 10:40

Re: Solve Enthalpy or Temperature?

As I said: you may treat "thermal energy" or enthalpy. In a simplified form you are still treating thermal energy or enthalpy but formly it seems so as you treat a temperature equation. So far to enthalpiy vs. temperature. (thanks to John Chien )

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As far as I understand your problem: you have two layers. Every layer has an other viscosity , density, temperature. I suppose that you have marangoni convection, byuoancy forces so that you must first define your problem.

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