[Sarutochi]

Hello,

I am currently attempting to simulate fluid flow and heat transfer in a non-uniformly heated pipe. I have modelled a pipe with a fluid inside meshed it using generalised cylinder model. For the fluid my physics models are: Implicit unsteady, Reynolds stress turbulent, Elliptic Blending, Segregated flow, Segregated fluid temperature, RANS, 3-D, Constant density and passive scalar. My pipe physics models Constant density, implicit unsteady, segregated solid energy and 3-D. The reason for using passive scalar was to assume the fluid as in-compressible which I was hoping would make it converge as it wasn't before. Unfortunately that seems to have made it worse and the values being asked are not helping. So here are my questions:

Firstly, a more general question, I'm trying to find the Schmidt number but cannot find the mass diffusivity of my fluid, which is Syltherm 800, would anyone be able to tell me where I can find such information?

Secondly, it is now asking me for the heat transfer coefficient, between the pipe and the fluid, since there is a heat flux, the temperature difference changes with time, is it asking me for initial heat transfer coefficient? Or something else?

Lastly, what are the Passive scalar values in the boundary physics values branch?

Any help would be greatly appreciated, if any clarification is needed feel free to ask.

[ashokac7]

Originally Posted by Sarutochi

Hello,

I am currently attempting to simulate fluid flow and heat transfer in a non-uniformly heated pipe. I have modelled a pipe with a fluid inside meshed it using generalised cylinder model. For the fluid my physics models are: Implicit unsteady, Reynolds stress turbulent, Elliptic Blending, Segregated flow, Segregated fluid temperature, RANS, 3-D, Constant density and passive scalar. My pipe physics models Constant density, implicit unsteady, segregated solid energy and 3-D. The reason for using passive scalar was to assume the fluid as in-compressible which I was hoping would make it converge as it wasn't before. Unfortunately that seems to have made it worse and the values being asked are not helping. So here are my questions:

Firstly, a more general question, I'm trying to find the Schmidt number but cannot find the mass diffusivity of my fluid, which is Syltherm 800, would anyone be able to tell me where I can find such information?

Secondly, it is now asking me for the heat transfer coefficient, between the pipe and the fluid, since there is a heat flux, the temperature difference changes with time, is it asking me for initial heat transfer coefficient? Or something else?

Lastly, what are the Passive scalar values in the boundary physics values branch?

Any help would be greatly appreciated, if any clarification is needed feel free to ask.

I don't understand use of passive scalar here. If you are using constant density then it is already in-compressible flow, isn't it?
We mostly use passive scalor to see the mixing of two fluids etc. I am unable to understand few sentences of your question.

What kind of convergence error are you getting?


Ok. For assigning passive scalar, if we have two inlets with different fluid, we assign passive scalar as 1 at one fluid inlet and then 0 at other inlet.

[Sarutochi]

Originally Posted by ashokac7

I don't understand use of passive scalar here. If you are using constant density then it is already in-compressible flow, isn't it?
We mostly use passive scalor to see the mixing of two fluids etc. I am unable to understand few sentences of your question.

What kind of convergence error are you getting?


Ok. For assigning passive scalar, if we have two inlets with different fluid, we assign passive scalar as 1 at one fluid inlet and then 0 at other inlet.

Thanks for the reply and sorry if I badly explained my problem, I'll try to explain differently


So the objective of my passive scalar is to stop the change in temperature of the fluid from impacting the density of the fluid inside the pipe. If I were to allow temperature to impact density then my fluid wouldn't be incompressible. I'm doing this because my energy equation wasn't converging to an answer when I was considering it to be compressible, I then looked into considering passive scalars.



My energy residual is fluctuating between 0.93 and 0.002 (roughly).


Again, thank you for the reply.

[LuckyTran]

Some pics would be nice. I can't follow which boundaries you are referring to.


I have no idea what you are trying to accomplish with the passive scalar. If density is constant then it is incompressible. Check. The passive scalar doesn't do anything to the density, it's passive! You need to give boundary conditions for the passive scalar. Walls usually have the zero gradient condition. At inlets, people normally take 0's and 1's.


The convection boundary condition is normally only used for solid surfaces for where you are not solving the fluid flow. You should be using a fixed temperature or fixed flux boundary condition. Again I can't tell what your domain is to say which one is appropriate.