[SSG_NJ]

Hi everyone!

How are you all doing?

I am working on a thermal model in SW Flow Simulation. I noticed a discrepancy in the value of the convective heat transfer coefficient ('h') obtained directly from the simulation (i.e. by using a surface goal) vs. calculating it via an equation goal using Newton's law of cooling, and I am unable to get to the bottom of this.

To verify which of these values are correct, I calculated the heat generated in the model (with an equation goal) using the 'h' obtained directly from the simulation (i.e. by using a surface goal) and compared it to the known value of heat generation which was inputted in to the model, and the two values differ greatly.

The flow is turbulent, forced, and the heat convects from a heat generation source directly onto the fluid.

Kindly help.

Thanks !

[native]

Hi SSG_NJ

I have two questions regarding your problem:

1. Did you take radiation into account in your model?
2. The heat transfer coefficient isn't equal on the whole geometry. Did you compare Min and Max value to you calculated value somehow? Is it at least in the range between Min and Max?

Best regards
Florian

[SSG_NJ]

Hi Florian,

How are you? Apologies for the late response.

To answer your questions,
1. There is no radiative heat transfer in the model
2. The calculated heat transfer coefficient (using an equation goal) is 3 orders of magnitude higher than the heat transfer coefficient obtained directly from the simulation. In the equation goal, I calculated 'h' as : h = SG Average heat flux /(SG Average temperature of solid - SG Average temperature of fluid), {where SG denotes Surface Goal}

Please advise!

[native]

Alright, so I think the deviation results from your definition of the fluid temperature as a surface goal. For these kind of proportionality constants or dimensionless numbers, usually the free stream temperature is used. The fluid temperature close to the wall is (almost) the same as the temperature of the wall itself. Thus, this results in a way higher heat transfer coefficient compared to the value calculated by using the free stream temperature.

I modeled the problem myself, using FloEFD for Creo. The copper sphere has a diameter of 0.1 m and is heated by 5 W. The free stream air velocity is 1 m/s and freestream temperature is 20 °C. As shown in the attached figure, the heat transfer coefficients from a surface goal and from an equation goal are the same. By the way, they also compare well to a literature value of 15.9 W/(m^2 K) for this case. The second equation goal is calculated with the fluid temperature on the surface of the sphere. This one doesn't fit at all.

Hope this helps.
Best
Florian