# CHT problem between porous media and solid wall

Hello there,

I’m currently trying to simulate a case where a porous media is adjacent to a solid wall using Fluent. The goal is to predict the wall temperature profile. With non-isothermal flow passing through the porous media, there is heat transfer between the porous media and the wall.

Because of the presence of porous media, the heat transfer between the fluid and the wall will be augmented.

At the interface, is the thermal boundary condition write as CAF*Kf_eff*dTf/dn=Kw*dTw/dn ?

(CAF is the convective augmentation factor, Kf_eff is the effective conductivity of fluid phase in porous media, Tf is the fluid phase temperature, Kw is the wall thermal conductivity, Tw is the wall temperature)

Is it appropriate to use CAF in this situation ?

Is there any study use the convective augmentation factor to predict the temperature profile of the bounding wall?

(I didn’t find any study doing that, only several try to characterize the value of CAF.)

Thanks a lot for your help!

• UKForum Coordinator

Augmentation factor is generally used to mimic rough surfaces where the area is higher than the "flat" or "smooth" face modelled. I'm not aware of any studies, but also haven't looked into it: the term is covered in the Fluent documentation.

(It is strange that very few studies have mentioned this factor.)

Have a look at section 7.4.15.3.6 - Augmented Heat Transfer.

Thanks.

Karthik

Hi Karthik,

I've check the section 7.4.15.3.6. the CAF is written in function

qid = cid[0] + caf_fac*(cid[1]*C_T(c0,t0) - cid[2]*F_T(f,t)) - cid[3]*pow(F_T(f,t),4)

where cid[] is the fluid-side diffusive heat transfer coefficients.

If I use this CAF, how should I write my boundary condition at interface (fluid/solid interface)? Like this: CAF*Kf_eff*dTf/dn=Kw*dTw/dn ?

Yes, that is how I would write the condition. But, I have a question for you. If you are solving the flow and heat transfer in the porous media in Fluent, could I ask you what is your purpose for writing this condition between the solid / porous wall?

Thanks.

Karthik

Hello Karthik,

Actually, it is between porous media and solid wall (not porous wall).

As you know, to enhance the heat transfer between fluid and pipe wall, you can use "rough surfaces where the area is higher than the "flat" or "smooth" face" (like Rob mentioned). I guess this is reason of CAF exist in Fluent. My tutor would like to see whether porous media will have the same type of enhancement. Is it better to use a rough surface or porous media in order to augmented the heat transfer? That's why I'm asking about the interface condition.

(TBH, I perfer rough surface. Using rough surface seems much easier and straight forward. Otherwise you have to deal with the flow problem, heat transfer problem in porous media, interface condition, blablabla.....)

• UKForum Coordinator

They're different, porous media may mimic a mesh on the surface and augmentation a rough (corrugated) surface. Both will have an effect but "best" may not just be down to the heat transfer.