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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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compressible flow with heat transfer

Posted Sep 11, 2012, 1:29 a.m. EDT Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Heat Transfer Version 4.3 3 Replies

Dan Gould
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I am attempting to model hydrogen flowing through a heat exchanger at high speeds and temps (250m/s and 2000K). In order to do this, the change in density of the hydrogen must be taken into account as it travels down the exchanger and heats up. I have been using the conjugate heat transfer model to do this, but it does not seem to be taking into account the density change. Rather, it seems to be modeling it as an incompressible flow. I know that choosing incompressable or compressible flow is an option for the fluid transfer modules, but I have not been able to find it for the conjugate heat transfer module. Does anyone know how enable this function?
3 Replies Last Post Sep 11, 2012, 8:19 p.m. EDT
Jesus Lucio
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Posted: 1 decade ago Sep 11, 2012, 1:36 p.m. EDT
Hi,

Apparently, COMSOL usually supposes that the fluids are compressible, of that type are the Navier-Stokes equations that COMSOL uses by default, even in the "conjugate heat transfer" physics. The complication seems to be the very high velocities, exactly high (> 0.3) Mach numbers, see "The Mach Number Limit" in the COMSOL's Multiph. User's Guide. For that case, COMSOL has the "High Mach number flow" physics, at least in CFD Module. I think you would have to manually couple that physics with heat transfer.

Bye,

Jesus.
Hi, Apparently, COMSOL usually supposes that the fluids are compressible, of that type are the Navier-Stokes equations that COMSOL uses by default, even in the "conjugate heat transfer" physics. The complication seems to be the very high velocities, exactly high (> 0.3) Mach numbers, see "The Mach Number Limit" in the COMSOL's Multiph. User's Guide. For that case, COMSOL has the "High Mach number flow" physics, at least in CFD Module. I think you would have to manually couple that physics with heat transfer. Bye, Jesus.
Dan Gould
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Posted: 1 decade ago Sep 11, 2012, 3:20 p.m. EDT
I had thought of modeling the flow seperatly and then feeding it into heat transfer model, (although I had never thought of the high mach number model, thank you) but my problem is that the change in temperature is the major driving force behind the flow speed. When hydrogen goes from 300k to 2000k, its density decreases by a factor of 6ish. This then should increase the velocity by a factor of 6 assuming constant mass flow. How could I model this speed up without heat transfer?
I had thought of modeling the flow seperatly and then feeding it into heat transfer model, (although I had never thought of the high mach number model, thank you) but my problem is that the change in temperature is the major driving force behind the flow speed. When hydrogen goes from 300k to 2000k, its density decreases by a factor of 6ish. This then should increase the velocity by a factor of 6 assuming constant mass flow. How could I model this speed up without heat transfer?
Dan Gould
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Posted: 1 decade ago Sep 11, 2012, 8:19 p.m. EDT
So per your suggestion, I tried to model the heat exchanger in the high mach number module... but it didn't like it very much. I was returned more simultaneous errors than I have ever seen before. Now this could certainly could be my not setting it up right (the boundary conditions for the inlet had me especially confused), but I am thinking that it just doesn't like it because it goes from a velocity of Ma = .0587 at the inlet to something that should be around Ma = .7 at the end. In other words, it is subsonic the entire time. Does anyone see anything I did wrong in setting this up or can I assume the this scenario just doesn't work in the High Mach model?
So per your suggestion, I tried to model the heat exchanger in the high mach number module... but it didn't like it very much. I was returned more simultaneous errors than I have ever seen before. Now this could certainly could be my not setting it up right (the boundary conditions for the inlet had me especially confused), but I am thinking that it just doesn't like it because it goes from a velocity of Ma = .0587 at the inlet to something that should be around Ma = .7 at the end. In other words, it is subsonic the entire time. Does anyone see anything I did wrong in setting this up or can I assume the this scenario just doesn't work in the High Mach model?

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