[Raza Javed]

Hello Everyone,

I have a question related to boundary conditions. As we know that we can put different boundary conditions on the inlet and outlet of our geometry.

Is it possible to put power as a boundary condition on the inlet OR on the walls of the geometry? OR If we can derive the power from the existing boundary conditions?

Actually, for my problem, I must give some defined power at the inlet of my geometry, So I want to know the possible way to do that?

Any help will be highly appreciated.

Thank you.

[LuckyTran]

Your boundary conditions will always be in terms of the primitive variables and their derivatives. For example, in momentum equation it will be velocity and velocity gradients.

Originally Posted by Raza Javed

Is it possible to put power as a boundary condition on the inlet OR on the walls of the geometry? OR If we can derive the power from the existing boundary conditions?

You need to back-calculate what is the equivalent variables that result in this power since there isn't a power transport equation in traditional CFD. There could be, if you used a power transport equation. Each transport equation is written in terms of some primitive variables, and these are the variables you fix in the boundary conditions.

Power is energy flux. What "energy" are you talking about? Thermal power, mechanical power, etc? And what are the physical mechanisms that transports this power? That determines the the mathematical expressions (in terms of values and their derivatives) that you need to apply.

[Raza Javed]

Thank you so much for your reply.



It will be thermal power, that I have to give as an initial boundary condition at the inlet.


Actually, the brief description of my problem is here:


I have a pipe from which some fluid is flowing. At the outside wall of a pipe, I have some electrical devices that are touching the pipe from the outside. The power dissipation of those devices are given and I want to know that how much heat will be transferred to the fluid from these devices and what is the temperature of those devices?

[FMDenaro]

This kind of information could be related as a non-homogeneous Neumann condition for the flow computation. Specifically, the normal derivative of the temperature gradient at the walls k*dT/dn must be prescribed equal to the heat flux that you need to know. But if you have only the power dissipation of the device that is not exactly the heat flux at the wall. It seems you need to model a conjugate heat tranfer process.

[LuckyTran]

Ok so you have a total power in terms of Watts. You need to convert this into a heat flux. To get the heat flux you take the total power and it by the surface area.

If your energy equation is cast in internal energy or enthalpy, then your boundary condition is a enthalpy gradient or internal energy gradient. If your energy equation is written in temperature, then you have more work and need to convert the heat flux into a temperature gradient. Most CFD software will take heat flux as a boundary condition because this is a very common condition. Under-the-hood, the software will convert this value of heat flux into a temperature gradient constraint.

[Raza Javed]

Originally Posted by LuckyTran

Ok so you have a total power in terms of Watts. You need to convert this into a heat flux. To get the heat flux you take the total power and it by the surface area.

Do you mean that I need to multiply the power (watts) with the surface area of the electronic device to convert it to heat flux?

Originally Posted by LuckyTran

If your energy equation is cast in internal energy or enthalpy, then your boundary condition is a enthalpy gradient or internal energy gradient. If your energy equation is written in temperature, then you have more work and need to convert the heat flux into a temperature gradient. Most CFD software will take heat flux as a boundary condition because this is a very common condition. Under-the-hood, the software will convert this value of heat flux into a temperature gradient constraint.

What are the parameters that decide which type of energy equation we should have? Either it will be written in terms of internal energy and enthalpy OR it should be in terms of temperature?

[LuckyTran]

Divide the power by the pipe surface area to get heat flux


The energy transport equation is usually written in terms of either enthalpy, internal energy, or temperature but only one of these these. You have to figure out which one you have.


In general, the energy transport equation can be written in terms of internal energy, enthalpy, and temperature altogether in one equation with constitutive models provided related three to each another. But this is cumbersome because you have to implicitly solve constrained equations and I don't know of any software that actually solves it this way.

[Raza Javed]

Thank you so much for your reply.



I have found one article about energy transport equation in terms of internal energy, enthalpy and temperature, but I am not so sure that is exactly what you are talking about.



I am attaching the link below for your reference.


https://www.comsol.com/multiphysics/...tion-of-energy


It would be helpful If you would have a look of this.


Thank you