convective + radiative heat transfer coefficient boundary condition

sortik · August 12, 2019, 07:01

Hi,

Is there any not so complicated way how could I write an expression that would estimate overall heta transfer coefficient (both for radiation and convection) according to boundary surface temp? If it is too complicated I could be satisfied only with radiative htc. I can estimate that convective htc for my simulation is somewhere between 5-10 W/m2K but I would like to sum it with htc for radiation and put it inside boundary condition as heat transfer coefficient. How could I do that?

I am using cfx for the simulations

Thanks

sortik · August 12, 2019, 08:06

Ok so for the radiative htc I did something like this 5.67e-08[W m^-2 K^-4]*Temperature^3*Emissivity+8[W m^-2 K^-1]. I think thats it. The problem is if I would like to be more precise with the htc for natural convection I would have to calculate Grashof number and for that I need characteristic length for either vertical or horizontal plane. I think I am not able to get this value in cfx expression because something like MaxVal(y)@meshregion - MinVal(y)@meshregion doesn't work.

I would be also interested in knowing how other calculate models that are larger and heat from surfaces is lost to the ambient. Do you just use air enclosure with buyoancy turned on and let ansys do the natural convection calculation? This is not option for me because I use large and rather complicated models.

August 12, 2019, 07:01	convective + radiative heat transfer coefficient boundary condition	#1
sortik New Member Join Date: Nov 2017 Posts: 25 Rep Power: 8	Hi, Is there any not so complicated way how could I write an expression that would estimate overall heta transfer coefficient (both for radiation and convection) according to boundary surface temp? If it is too complicated I could be satisfied only with radiative htc. I can estimate that convective htc for my simulation is somewhere between 5-10 W/m2K but I would like to sum it with htc for radiation and put it inside boundary condition as heat transfer coefficient. How could I do that? I am using cfx for the simulations Thanks

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August 12, 2019, 08:06		#2
sortik New Member Join Date: Nov 2017 Posts: 25 Rep Power: 8	Ok so for the radiative htc I did something like this 5.67e-08[W m^-2 K^-4]Temperature^3Emissivity+8[W m^-2 K^-1]. I think thats it. The problem is if I would like to be more precise with the htc for natural convection I would have to calculate Grashof number and for that I need characteristic length for either vertical or horizontal plane. I think I am not able to get this value in cfx expression because something like MaxVal(y)@meshregion - MinVal(y)@meshregion doesn't work. I would be also interested in knowing how other calculate models that are larger and heat from surfaces is lost to the ambient. Do you just use air enclosure with buyoancy turned on and let ansys do the natural convection calculation? This is not option for me because I use large and rather complicated models.