Heat transfer BC at wall- why need wall thickness?
 

Helo all

I am rather disturbed about something

I checked frm the Fluent Manuals that for thermal boundary conditions inputs for Wall,

for temperature BC: Fluent calculates the heat flux into the fluid using

q=h(Tw-Tf)+qrad

where Tw is the temperature at the surface of the wall applying heat transfer knowledge, Tw is the temperature at the surface of wall that is IN contact with the fluid. and Tf is the temp of the fluid. so it will be a purely convection equation, when qrad=0

for heat flux BC: Fluent calculates temp at wall Tw using

Tw= (q-qrad)/h +Tf

so its the same.And all we haf to do is to input the value of heat flux, that is how much heat flux is added into the fluid.

My question is... since Tw is the temperature of internal wall surface in contact with the fluid. Why on earth does Fluent needs the information of Wall thickness??

It is not as if Fluent asks us to input the outside surrounding temperature i.e. the temp of external surface of wall exposed to the outside environment, thus it needs the wall thickness to calculate the heat flux thru conduction thru walls, and then the heat flux thru convection thru the fluid.

I hope someone understands what im tryin to say here. Have I misunderstood wat Fluent is tryin to do or misunderstood the concept of heat transfer? Since Tw is temperature of INTERNAL surface in contact wif the fluid, why need us to input the thickness of wall?

Re: Heat transfer BC at wall- why need wall thickn
 

i guess for some cases like mine, when u have an annulus in the wall..it becomes wall thickness.. mayb you can leave the wall thickness as zero for your case..

i didnt know about those equations, mayb its time i read up too..:P

Re: Heat transfer BC at wall- why need wall thickn
 

Hi,

Hmm, after I posted this question I also thought about the possibilities in some cases where the equations could be valid. I'm not totally sure of what you mean, but it could be more or less similar to what I have thought...

If I'm not wrong,

wat u are sayin is perhaps in some cases, there is a surface in the fluid flow where, fluid passes thru and thus in contact with BOTH sides of the face, hence there may be a need of wall thickness?

Re: Heat transfer BC at wall- why need wall thickn
 

This is useful like modeling effect of insulation where you want to use the thermal resistance by this wall thickness.

Ashu

Re: Heat transfer BC at wall- why need wall thickn
 

But I thought we already have a wall thickness when we draw it out in Gambit...

So I guess some did not draw any thickness in gambit then..so need to specify it in Fluent..

Re: Heat transfer BC at wall- why need wall thickn
 

This is in case you don't model that explicitly in Gambit and only assign property in Fluent. Material and thickness needs to be ssigned. It would compute 1D heat transfer to get the temperature. In case you use planar conduction you can get the same effect in planar dimension. But planar conduction is not available in 2D model ( available only for 3D models)

Ashu

Re: Heat transfer BC at wall- why need wall thickn
 

Hello,

I think the wall thickness will be more useful when you are doing a Conjugate Heat transfer analysis i.e, including the effect of heat conduction thru the solid. In such a case to calculate the conduction fluxes, thermal resistance etc you need to have the wall thickness. Moreover, I am of the opinion that, even if you have an annulus sort of a situation, there is no question of a wall thickness coming into picure because you are only solving for the fluid flow (once again- unless you are interested in the conjugate heat transfer)- in which case the solid poirtion doesnot at all form a portion of your domain.

I hope I am clear.

Regards, Sridhar.

Wall thickness CFD
 

Based on my information, I think the wall thickness is too important when you supply a heat flux on the outside of the wall (in contact with surrounding). this amount of heat flux is not the same at the inside surface of the wall (in contact with fluid) due to the thermal conductivity of the wall material. in contrast, if you supply temperature on the outside wall surface, the temperature on the inside wall surface will not be the same due to the same reason. eventually, if you have to examine the effect of the wall thickness you have to consider it.
TQ