[arindamsantra7]

Originally Posted by ghost82

If you treat steam as ideal gas, ideal gas equation for density and Sutherland equation for viscosity (for example).

I am not getting water vapour density at 373 k by solving the sutherland's law. please specify me how to calculate manually the water vapour density at 373 k.

[ghost82]

Please re-read carefully what I wrote.
Sutherland equation has nothing to do with density.

[arindamsantra7]

Originally Posted by ghost82

Please re-read carefully what I wrote.
Sutherland equation has nothing to do with density.

sorry sir, this is my mistake. I have written density instead of dynamic viscosity.
I am not getting the dynamic viscosity of water vapour at 373 k by solving the sutherland's law. please specify me how to calculate manually the dynamic viscosity of water vapour at 373 k.

[ghost82]

What's your problem?
Sutherland's equation fits well experimental data...

[thermal energy]

Originally Posted by ghost82

Hi,
I don't understand your question...you are stating that:
- we can assume the temp is constant throughout the system, that is 400 k
- So I want to know, how I can incorporate the temp of the fluid in this cavitation mode

If you want to simulate at constant temperature all you need to do is to specify the fluid properties at that temperature, you don't need to set temperature in fluent; this means you have to set constant density, viscosity, vaporization pressure and surface tension (for Singhal et al model), which are unique values at that operating temperature.

dear ghost82,

ı try to model melting/solidification for phase change material in enclosure. so thermophysical properties are depend on temperature. ı mean ı have values both solid and liquid phase (piecewise or polynomal functions for cp, k). also ı have udf for density that depend on temperature.
my question what should be operating temperature for this case. my initial temperature for phase change material is 300K. operating temperature should be 300K or default value is ok. ı am confisued. thanks for your help.

[ghost82]

Originally Posted by thermal energy

dear ghost82,

ı try to model melting/solidification for phase change material in enclosure. so thermophysical properties are depend on temperature. ı mean ı have values both solid and liquid phase (for cp, k). also ı have udf for density that depend on temperature.
my question what should be operating temperature for this case. my initial temperature for phase change material is 300K. operating temperature shold be 300 or not. ı am confisued. hanks for your help.

Hi,
as I know the reference temperature (T0) plays only a role if you are using the Boussinesq approximation:
(rho-rho0)*g=-rho0*beta*(T-T0)*g

[thermal energy]

Originally Posted by ghost82

Hi,
as I know the reference temperature (T0) plays only a role if you are using the Boussinesq approximation:
(rho-rho0)*g=-rho0*beta*(T-T0)*g

hi. thank you dear friend. well, ı also have an open ended natural convection in vertical channel as an referance geomerty. ı use boussinesq appr. initial temperature for air is 300 K.

should we activate operating density and how to find it?

also, ı wonder that how should ı set operating temperature. ı mean ı already specify the fluid properties at that temperature (300K). so do ı need to specify again operating temperature? default value is ok or not?

operating temperature means initial temperature value of air and operating density is density at that temperature, does not it?

thanks ghost82 for guidance.

[ghost82]

As you know, Boussinesq model is appropriate for not too large variation in temperature: if you plan to have air that enters at 300 K and exits for example at 310 K, I would choose 305 K as operating temperature, set the density to Boussinesq in material properties and enter a constant value for density and thermal expansion coefficent(corresponding to the operating temperature).
It is not a must to activate operating density, but if you have multiple phases you can activate it and set to that of the lighter phase, as recommended in the fluent guide.

[thermal energy]

Originally Posted by ghost82

As you know, Boussinesq model is appropriate for not too large variation in temperature: if you plan to have air that enters at 300 K and exits for example at 310 K, I would choose 305 K as operating temperature, set the density to Boussinesq in material properties and enter a constant value for density and thermal expansion coefficent(corresponding to the operating temperature).
It is not a must to activate operating density, but if you have multiple phases you can activate it and set to that of the lighter phase, as recommended in the fluent guide.

thank you for guidance dear friend. ı have a constant heat flux BC on one of the vertical surface other one is adiabatic. ı have some solution for steady and air exits max 330K near the heated wall. ı set operating temp 300 K. also, ı set all material properties on panel for 300 K. ı do not have multiple phase. ı have only one fluid (air).
ı am confused to set operating temperature. Operating temperature is used only for correct initialization of solution, is not it? my initial condition is certain. Tair is 300 K. so why do not I set operating temperature 300 K. ı am really confused.

thanks a lot again for answers and your time .

[ghost82]

No, operating temperature is not used to initialize the solution, unless you input the same value in the initialization panel. The operating temperature is defined as the characteristic temperature of your domain. If your domain is all at 300 K than your operating temperature would be 300 K. In your case air enters at 300 K and it should exit at 330 K so the operating temperature is between 300 and 330, for example 315 K.
Again operating temperature is used only in the Boussinesq Equation and it is a sort of reference point to calculate densities at other temperatures.
So better to choose a reference temperature in the middle of your temperature range.

[thermal energy]

Originally Posted by ghost82

No, operating temperature is not used to initialize the solution, unless you input the same value in the initialization panel. The operating temperature is defined as the characteristic temperature of your domain. If your domain is all at 300 K than your operating temperature would be 300 K. In your case air enters at 300 K and it should exit at 330 K so the operating temperature is between 300 and 330, for example 315 K.
Again operating temperature is used only in the Boussinesq Equation and it is a sort of reference point to calculate densities at other temperatures.
So better to choose a reference temperature in the middle of your temperature range.

ok, ı see. thanks again. maybe air exit 330K but this temperature occur only near the heated surface (total length of surface=0.1m, 0.005<=heatedsurface<=0.085). ı have a thin thermal boundry layer. you can see this in attach. so it does not represent all, does not it? if ı check the average temp in domain from "reports-area weighted average-facet average" ı see nearly 297 K. so, can ı set the operating temperature to 290 K. it would be wrong? or other words how does it have influnce on solution based on your experience? it should be tried

finally, if ı know inlet and exit temperature or guess domain temperature, as you said average temp best choice. but if ı do not know, how to guess operating temp? Because ı will have transient simulation, so ı will stopped calculation if heated surface temp reaches the Tcr.

also, what is the operating density? is it required to set. as far as ı know it is also used only in boussinesq.

thank a lot for your time and inform.

[ghost82]

Hi,
yes, you can set it to 290 K.
As I said it is a reference temperature, so I think that if it is chosen in the middle of the range the calculation of the density is better approximated but as the change in temperature is small I think it does not matter too much if this T0 is in the middle of the range or at its limits.
For operating density: set it to density corresponding to the operating temperature.