
[Sponsors] 
January 19, 1999, 09:16 
Solve Enthalpy or Temperature?

#1 
Guest
Posts: n/a

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 

January 21, 1999, 08:47 
Re: Solve Enthalpy or Temperature?

#2 
Guest
Posts: n/a

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. 

January 21, 1999, 10:13 
Re: Solve Enthalpy or Temperature?

#3 
Guest
Posts: n/a

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. 

January 21, 1999, 13:14 
Re: Solve Enthalpy or Temperature?

#4 
Guest
Posts: n/a

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.


January 21, 1999, 18:22 
Re: Solve Enthalpy or Temperature?

#5 
Guest
Posts: n/a

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!!!


January 22, 1999, 10:40 
Re: Solve Enthalpy or Temperature?

#6 
Guest
Posts: n/a

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 )
******************* 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. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Enthalpy to temperature  adhiraj  OpenFOAM Programming & Development  16  May 31, 2017 02:26 
Changing patch temperature directly from the solver (buoyantPimpleFoam)  kriztof  OpenFOAM Running, Solving & CFD  11  January 23, 2012 18:55 
how to solve a problem involving both laminar and turbulent flow  seefd  FLUENT  1  June 3, 2011 03:20 
Bulk temperature Tf is obtained from total or static temperature?  NPU_conanxie  FLUENT  0  March 30, 2011 05:56 
use kiva to solve another temperature equation  zhzhguo  Main CFD Forum  0  June 6, 2006 20:59 