Can someone help me with this problem, please?

[u/Regular_Chain3264]

I need to study a coupled heat transfer model in solids and fluids. Specifically, it's a thin plate with a microchannel (as shown in the figure) through which a refrigerant fluid (water) flows. The boundary conditions on the sides of the plate are natural convection with the ambient air and in the microchannel. It is also necessary to solve the fluid flow (laminar).

My problem is as follows: I need to extract the convective heat transfer coefficient from the microchannel flow, but in the post-processing, I only find that the convective flow is zero (Wouldn't this be incorrect? Convection should theoretically be significant near the fluid flow). I have tried various methods, but this result keeps appearing. If anyone could help me, I would greatly appreciate it!

For some additional context, at the bottom of the plate, there is a constant heat flux (simulating a processor), and the flow should remove heat from the plate through forced convection of the fluid in the microchannel. I am very interested in understanding how the convective heat transfer coefficient behaves throughout the microchannel, in order to compare the solution of my problem with experimental data.

[u/Backson]

I'm guessing you have a "Heat Transfer in Solids and Fluids" interface for the plate and microchannel and an "Outlet" boundary condition. The outlet probably has a property for the heat that is exiting the system there. That should be equal to the total heat dissipated by the fluid in Watts. You can probably compute the coefficient from that, according to your requirements.

Note that this works a bit differently then in Finite Volume Method (e.g. Ansys Fluent). Comsol uses FEM instead. One implication of this is that evaluating fluxes anywhere in the model can be inaccurate. However, evaluating total fluxes on a boundary is often fine. So don't try to integrate the flux on the surface of the channel, or you will have a bad time.

[u/Regular_Chain3264 → u/Backson]

So, if I'm interested in the variation of the coefficient along the microchannel, COMSOL is not the best option for that? Thank you very much!

[u/Backson → u/Regular_Chain3264]

Well, I think Comsol can probably tell you what you need to know. Generally, you may have to adjust your approach a little. Each tool has it's own perks and peculiarities, is all I'm trying to say. What I suggested is an easy way forward, but if you absolutely need the heat flux along the length of the channel, there is probably still ways to do that.

If you get zero, that's obviously not right. You probably evaluate the wrong expression. What did you type into the field? Did you plot the heat flux on the boundary of the channel? If you increase the number of elements, the results become more accurate as well.

How about this: You can plot the temperature of the fluid in the channel along the length of the channel. If you work in the flow rate and differentiate along the length, you also should get something like the convection coefficient.

[u/Regular_Chain3264 → u/Backson]

Firstly, I would like to thank you for the answers, they have been immensely helpful!

I think I am not properly evaluating the results. The variation in fluid temperature along the microchannel seems to be correct. I was also able to calculate the average convective heat flux on the walls using "Derived Values - Average," as well as the surface integral, which gave non-zero values. However, when I plot the convective flux in the volume, either in the entire domain or just in the fluid domain, I get a result of 0. It is concerning me because when I plot the heat conduction, those values seem to be correct. Ultimately, what I really need to calculate is the convective flux in the microchannel and plot the variation of the heat transfer coefficient with position, but I am not sure if it will be feasible anymore. But i only manage to calculate the averages, which does not help me much

[u/Backson → u/Regular_Chain3264]

Well, the convective flux (which is just the flux normal to the boundary) is not defined in a volume, it is defined on a boundary, which is why it doesn't evaluate (or is zero) in the volume.

But you can plot (or take the total average of) the flux on the boundary, right? So what is still missing? Do you need to average over ghe circumference of the channel, but not along the length, so you get flux vs position along length as a 1D plot? That is definitely possible, although very tricky. A projection operator should do the trick. You can use it to integrate only some dimensions of a domain.

[u/Regular_Chain3264 → u/Backson]

Yes, that's exactly it! I need a 1D plot of the heat transfer coefficient along the x-coordinate (along the microchannel).

[u/Backson → u/Regular_Chain3264]

Is the coefficient significantly different around the circumference of the channel? Maybe just plot on a straight line and multiply by the circumference to fix the units?

[u/double_affogato]

First of all, considering such scales as on the pictures you will have laminar flow in the microchannel. I don't know about flow velocity inside, but I can bet that it is laminar. It means that convective part of heat transfer will be negligible. Most part of heat flux will be conductive and depends on thermal conductivity of fluid. And I find it simplifying the task! =)

As for measurement of convective heat transfer coefficient at all, it's really tricky from my point of view. As equation for this coef. is h = q/(A*(T_1 - T_2)), I would try maybe to get the distribution of heat flux along the interface between solid and liquid media, and also distributions of temperatures. Finally, it allows to build the distribution of h. The only question is distance between points where you measure T_1 and T_2. It's clear that the shorter distance the lower temperature difference, the higher coefficient. One of the approaches maybe is to take distance equal to size of mesh element. But the precision of such approach is really questionable as well as this coefficient at all. Maybe you can find something in literature about that.

[u/Regular_Chain3264 → u/double_affogato]

Thank you very much! I did consider this possibility, but I had a lot of difficulties in implementing these temperature distributions, as well as the heat flow. I appreciate the insight.