Heat Flux Profile at Fluid-Porous Interface

Hitch8 · December 7, 2012, 10:13

Hi everybody,

I have some problems implementing a heat flux boundary profile at a fluid-porous interface. In realitiy this heat-flux is caused by thermal radiation which heats up the porous material. In my simulaiton the heat flux caused by the absorbed thermal radiation is just modeled by applying a heat flux boundary profile at the porous-fluid interface. I want that the heat flux is applied to the solid part of the porous domain. Therefore, I've implemented a boundary source at the interface of the fulid-porous domains.
Source -> Boundary Source -> Solid 1 -> Equation Sources -> Energy-> Flux => Boundary Profile....Wall Heat Flux (x,y,z).
What confuses me is the solid values tab; in there the boundary conditions for the solid also needs to be defined.
Does anybody now if I should define my boundary heat flux profile only in the source tab or only in the solid values tab or in both?

Thank you very much in advance!

audrey · December 14, 2012, 13:57

I'm not sure if anyone knows the answer to this question, but if you do, it would be greatly appreciated if you shared it. I'm also confused about a similar issue.

I am trying to model a kiln with a porous bed inside. The wall around the porous domain has a heat flux (providing heat to the porous solid and fluid inside the porous domain), but the gas is also heated in the fluid domain above the porous domain. I would like the porous solid temperature to be calculated by the solver, but CFX-Pre requires a heat transfer boundary condition at the interface between the porous and fluid domains. The options are: adiabatic, fixed temperature, heat flux in, heat transfer coefficient + outside temperature. None of these options seem to allow the solver to calculate the temperature of the solids at the interface. I am missing something?

cdegroot · December 14, 2012, 16:57

Okay, I will try to clarify the settings. In the details of the "porous" domain, under "porosity settings", the heat transfer coefficient is used for heat transfer between the fluid and solid phases within the porous domain. The options under the details of the domain interface, under "solid values" refer to heat transfer between the porous solid and the fluid in the adjacent domain.

So, audrey, you are right, there is a bit of a weakness in the CFX implementation of the non-equilibrium heat transfer model for fluid-porous domains. There is no way right now for it to automatically treat the heat transfer into the solid. You have to give it some more information. I think you should be able to get the heat transfer coefficient option to work for you though.

I've been thinking of a way to make the HT coefficient mimic a conduction process from the fluid using expressions but I haven't had a chance to try it yet.

audrey · December 15, 2012, 05:27

Thanks cdegroot for the response, it's helpful to know that I at least understand what I think I do. I'm interested in your suggestion of using an expression to simulate the heat transfer. I think I will stick to the heat transfer coefficient for the moment (assuming the temperature is what I expect it to be). But do let me know if you manage to make your expression work, I'd be very interested in finding out how.

Related comment: I've been finding the CFX documentation rather light on the topic of porous domains compared to other topics. Anyone else notice it? To be fair I wasn't originally planning on using porous domains, I am mainly using them to help progress a stubborn multiphase simulation.

Audrey

cdegroot · December 15, 2012, 10:57

The documentation on porous domains is a bit light in the theory and user guides. I find the best place to look is the CFX-Pre user guide for the specific menu option I am wondering about.

I think the heat transfer coefficient option can provide the most reasonable result for you. I'll let you know if I come up with a clever expression, but it doesn't seem to allow me access to the fluid temperature gradient so I'm not sure i can make something work like conduction from the fluid. Setting up an expression based on a Nusselt number correlation for a flat plate may not be a bad assumption either if it is mostly forced convection you are looking at.

December 7, 2012, 10:13	Heat Flux Profile at Fluid-Porous Interface	#1
Hitch8 New Member Philipp Join Date: Apr 2010 Posts: 27 Rep Power: 16	Hi everybody, I have some problems implementing a heat flux boundary profile at a fluid-porous interface. In realitiy this heat-flux is caused by thermal radiation which heats up the porous material. In my simulaiton the heat flux caused by the absorbed thermal radiation is just modeled by applying a heat flux boundary profile at the porous-fluid interface. I want that the heat flux is applied to the solid part of the porous domain. Therefore, I've implemented a boundary source at the interface of the fulid-porous domains. Source -> Boundary Source -> Solid 1 -> Equation Sources -> Energy-> Flux => Boundary Profile....Wall Heat Flux (x,y,z). What confuses me is the solid values tab; in there the boundary conditions for the solid also needs to be defined. Does anybody now if I should define my boundary heat flux profile only in the source tab or only in the solid values tab or in both? Thank you very much in advance!

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December 14, 2012, 13:57		#2
audrey New Member Join Date: Jan 2011 Posts: 21 Rep Power: 15	I'm not sure if anyone knows the answer to this question, but if you do, it would be greatly appreciated if you shared it. I'm also confused about a similar issue. I am trying to model a kiln with a porous bed inside. The wall around the porous domain has a heat flux (providing heat to the porous solid and fluid inside the porous domain), but the gas is also heated in the fluid domain above the porous domain. I would like the porous solid temperature to be calculated by the solver, but CFX-Pre requires a heat transfer boundary condition at the interface between the porous and fluid domains. The options are: adiabatic, fixed temperature, heat flux in, heat transfer coefficient + outside temperature. None of these options seem to allow the solver to calculate the temperature of the solids at the interface. I am missing something?

December 14, 2012, 16:57		#3
cdegroot Senior Member Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 414 Rep Power: 18	Okay, I will try to clarify the settings. In the details of the "porous" domain, under "porosity settings", the heat transfer coefficient is used for heat transfer between the fluid and solid phases within the porous domain. The options under the details of the domain interface, under "solid values" refer to heat transfer between the porous solid and the fluid in the adjacent domain. So, audrey, you are right, there is a bit of a weakness in the CFX implementation of the non-equilibrium heat transfer model for fluid-porous domains. There is no way right now for it to automatically treat the heat transfer into the solid. You have to give it some more information. I think you should be able to get the heat transfer coefficient option to work for you though. I've been thinking of a way to make the HT coefficient mimic a conduction process from the fluid using expressions but I haven't had a chance to try it yet.

December 15, 2012, 05:27		#4
audrey New Member Join Date: Jan 2011 Posts: 21 Rep Power: 15	Thanks cdegroot for the response, it's helpful to know that I at least understand what I think I do. I'm interested in your suggestion of using an expression to simulate the heat transfer. I think I will stick to the heat transfer coefficient for the moment (assuming the temperature is what I expect it to be). But do let me know if you manage to make your expression work, I'd be very interested in finding out how. Related comment: I've been finding the CFX documentation rather light on the topic of porous domains compared to other topics. Anyone else notice it? To be fair I wasn't originally planning on using porous domains, I am mainly using them to help progress a stubborn multiphase simulation. Audrey

December 15, 2012, 10:57		#5
cdegroot Senior Member Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 414 Rep Power: 18	The documentation on porous domains is a bit light in the theory and user guides. I find the best place to look is the CFX-Pre user guide for the specific menu option I am wondering about. I think the heat transfer coefficient option can provide the most reasonable result for you. I'll let you know if I come up with a clever expression, but it doesn't seem to allow me access to the fluid temperature gradient so I'm not sure i can make something work like conduction from the fluid. Setting up an expression based on a Nusselt number correlation for a flat plate may not be a bad assumption either if it is mostly forced convection you are looking at.