[CellZone]

Hello ervybody and a happy new year 2021,

I would like to calculate the heat transfer coefficient in Fluent over a plate made of steel. So the steel has a starting temperature of assuming 20°C and is heated up by a 300°C warm air flow.

At the beginning my q_dot=htc*(T_Wall-T_Fluid) is high since the difference between T_Wall and T_Fluid is high. With increasing time q_dot is decreasing.

My question: does the heat transfer coefficient significantly change? The htc should be only depending on the properties of the air flow (which is more or less constant) and some flow phenomena like turbulent flow or not.

Thank you for your help

CellZone

[LuckyTran]

The heat transfer coefficient will definitely change in space (from plate leading edge to trailing edge). You are asking if the htc will vary in time?

If the properties are actually constant then it definitely should not change or you have a bug. How much and how significant is the change will depend on how your properties vary.


You can roughly estimate what the difference using the flat plat correlations for Nusselt number. If you recall, the properties for Nusselt number (but also Reynolds number and Prandtl number) are evaluated using the film temperature. The easiest way is to use your T_wall obtained from CFD, and at every time, calculate the local film temperature on the plate and plug that into the correlation to predict the Nusselt number everywhere, for all time. You don't have to use the solution from CFD, you could use a low-order model to predict the steel temperature (using a correlation for the htc of course) beforehand. But it's probably more straightforward to use what you have already computed.

For an even more crude but even more quick estimate, evaluate your properties at T_wall, T_Fluid to get 2 Nusselt number (or htc) profiles. That will tell you (roughly) how it will vary in time. T_wall and T_fluid are the most extreme temperatures, so that encompasses all the physical ranges of temperatures you can expect in your simulation.