Convective heat transfer coefficient issue

[sbaw88]

I am working on the simulation of a heater in ANSYS. I'll explain the geometry:
- Top plate - Ceramic Porcelain material - 2.65in x 4.8in x 0.06in - carries a small heater (2in x 0.125in x 0.01in) on top of it which generates 18W of heat.
- Bottom heater - 5.8in x 3.65in x 0.03in - Stainless steel plate
- Air gap between the two heaters - 0.01in

The top small heater power runs constantly for 4.5 minutes and the plate has a convection coefficient of 30 W/m²C. The bottom heater start running 30 seconds after the top heater starts and goes from ambient temperature (24.85 C) to 400C.

I am including radiation effect between the bottom layer of top plate and upper layer of bottom plate. I am mainly concerned about the convection coefficient to be used for air. I am unable to get a proper value of the coefficient to be used. Can someone help me with the calculations? Am I missing something very basic?

Thanks!

 

[Achy47]

Thank you for sharing your simulation project on the heater in ANSYS. It sounds like a complex and interesting problem. I am a scientist with experience in thermal analysis and I would be happy to help with your question about the convection coefficient for the air gap.

Firstly, it is important to note that the convection coefficient is influenced by various factors such as fluid properties, flow conditions, surface roughness, and geometry. In your case, the air gap between the two heaters is only 0.01in, which means that the flow conditions are likely to be laminar. This is important to consider when determining the convection coefficient.

One approach to calculating the convection coefficient for laminar flow is to use the Nusselt number correlation for laminar flow over a flat plate. This correlation takes into account the Prandtl number, which is a dimensionless number that describes the ratio of momentum diffusivity to thermal diffusivity. For air, the Prandtl number is approximately 0.7 at 25°C.

Using the Nusselt number correlation, the convection coefficient can be calculated as follows:

Nu = 0.664*(Re^0.5)*(Pr^0.33)

Where Nu is the Nusselt number, Re is the Reynolds number (calculated using the air gap thickness and the velocity of the air), and Pr is the Prandtl number.

Once the Nusselt number is known, the convection coefficient can be calculated using the following equation:

h = (Nu*k)/L

Where h is the convection coefficient, k is the thermal conductivity of air, and L is the characteristic length (in this case, the air gap thickness).

It is also worth noting that the convection coefficient is dependent on the temperature difference between the surface and the surrounding fluid. In your case, the temperature difference will vary as the bottom heater heats up. Therefore, it may be necessary to use an average convection coefficient over the entire simulation time.

I hope this information helps with your calculations. Please let me know if you have any further questions or if you need any clarification on the above. Good luck with your simulation project!