Discussion Forum

Hi

in the heat transfer T is the dependent variable so you solve for T that means that T=T(x,y,z,t) (T only if you are in a transient case, so if you select a boundary and use the variable T it will scan over the edge coordinates, whatever they are. The formula applies to an element of size dx,dy,dz (in 3D). This is part of the implit notation of COMSOL, it is important to understand it, see the doc.

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Good luck
Ivar

Hi Ivar,

Thank you very much for your reply and help.

So that means I don't need to specifically find an expression for T on the boundary and {q_suf = h(T_ext-T) + other terms} is just perfectly fine. And when it comes to that boundary, COMSOL will automatically evaluate T at those coordinates. Am I correct?


Thanks a lot again.

Hi

You need to add your "other terms" but if you use T in your equations it will evaluate T along the boundary for each mesh element when using the formula you enter

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Good luck
Ivar

Hi Ivar,

Thank you very much!! This is very helpful.

Hi Dr.Ivar

I am using comsol 3.5a for simulating Li ion battery.I am a new user ,so I am trying to redo the same model which is given in comsol already.But I get the following error,

Error:
Time 3.746334577378822:
Error in residual computation.
Power_of_negative
Function: ^
Failed_to_evaluate_variable
Variable: i0, Defined_as: (((k_pos*((c1max_pos-c1s_pos)^aA_pos))*(c1s_pos^aC_pos))*(c2^aA_pos))
Failed_to_evaluate_variable
Variable: i_loc, Defined_as: ((exp((((Far*aA_pos)*eta)/(Rg*T)))-exp(((((-aC_pos)*Far)*eta)/(Rg*T))))*i0)
Failed_to_evaluate_expression
Expression: d(((-K1eff*phi1x*test(phi1x)/L_pos-i_loc*Sa_pos*L_pos*test(phi1))-(0))*(dvol),{test@1})
Last time step is not converged.

Can you please help me out in this?

Thanks,

Rajeswaran.

Hi Ivar,

I actually have another question related to this.

Suppose that I have a coupled problem, and for that same boundary for the heat transfer problem, the expression for boundary condition includes a flux n from the module Transport in Diluted Species, where the dependent variable is the concentration c.

I believe I could obtain this flux as a result from that module. However, I need to specify it only on the boundary. Is this case the same?

So the expression for the boundary condition reads:

q_surf = h(T_ext - T) -lambda*n_surf - c_p*T*n_surf

This n_surf is the flux from the species transport. How could I obtain this for the boundary?


Thanks a lot again.

I have the same error as you. It is really annoying, Mine has sign '^''. But yours does not have this sign.

I don't know if the T can be used to boundary directly. I would like to define a boundary variable in Definitions.