Learning Forum

[sjyang]

Third party link removed  Public Ansys Help Seeing as most of the correct, current, documentation is available now there's no reason to use an unofficial source! Rob

In the explanation of "Diffusion at Inlets" in the theory guide at the bottom of the linked page, it states, "The convection component is fixed by the inlet temperature specified by you." However, how are the heat transfer coefficient and free stream temperature determined? Isn't it necessary for both the heat transfer coefficient and the free stream temperature to be defined in order to calculate the convection heat transfer rate?

[SRP]

Hi,

The convection component of energy transport at inlets in Ansys Fluent is determined by the inlet temperature that you specify. This is part of the net transport of energy, which also includes a diffusion component.

[sjyang]

Hi, Thank you for reply

Could you provide the equations used by ANSYS Fluent to calculate convection and diffusion heat transfer at the inlet? It's difficult to understand from the text alone.

 

[Rob]

We can only refer you back to the documentation due to the constaints of working on the Forum. Check the User's Guide and Theory Manual. 

[sjyang]

Hi,

Is there a User's Guide or Theory Manual that explains the content of my question? If so, could you provide the link? When I searched, I couldn't find this information in the User's Guide or Theory Manual.

[sjyang]

 

The manual does not provide equations for convection and diffusion heat transfer at the inlet, but based on my interpretation, could it be represented by the following equations?

In ANSYS Fluent, does convection refer to the energy carried by the fluid as it enters through the inlet (i.e., advection), while diffusion refers to the propagation of energy (heat) through molecular movement according to the temperature gradient (i.e., conduction)?

[Rob]

Convection is moved with the fluid, yes. Diffusion is thermal diffusivity, and can be turned off at boundaries. 

[sjyang]

Thank you.

As far as I know, thermal diffusivity represents how quickly heat spreads through a material, and it is represented as follows. Are you referring to the same thing?

[Rob]

It's the molecular component, ie other than conduction. 

[sjyang]

What is the difference between diffusion and conduction in terms of equations?

[Rob]

Probably not a lot. 

[sjyang]

Thank you for the response. It has resolved a long-standing question for me.

Additionally, I would like to ask if, at the outlet, only convection heat flux is considered, and diffusion heat flux is not calculated?

[Mark Owens]

Hi, There will be diffusion anywhere there is a temperature gradient.  The same applies to species transport with the diffusion determined by the species gradient.

We allow you to  turn it off for inlets because for low velocity inlets the diffusion can be greater than the advection and so you get more heat in than you really want.  Turning it off gives you more control over what is coming in. It would be unphysical to turn it off for an outlet. Nevertheless, for very low velocity inlets where the heat in (or species in) is critical, it is recommended not to use an inlet but a volumetric source.

[sjyang]

Hi, Thank you sincerely for the detailed explanation:)

(https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/flu_ug/flu_ug_sec_hxfer_report.html%23x1-110100014.4.1)

 

However, as per the above manual, I understand that by using the 'Surface Integrals', choosing 'Flow Rate' for the 'Report Type', and selecting 'Enthalpy' (under the Temperature category), the convective heat flux can be calculated.

In the attached image below, I have listed the values for the Total Heat Transfer Rate calculated in the Fluxes section and the Enthalpy Flow Rate calculated in the Surface Integrals section at the inlet and outlet. If you look at the values in detail, the outlet shows that the heat transfer rate and the enthalpy flow rate (convective heat flux) are the same. However, at the inlet, these two values differ.

I understood that the heat flux at the outlet only accounts for convective heat flux, which is why the two values are the same, whereas at the inlet, both convection and diffusion are accounted for, resulting in different values for the Total Heat Transfer Rate and Enthalpy Flow Rate (convective heat flux). Could you point out where I might have misunderstood?

 

[Mark Owens]

Hi, surface integrals are not guranteed to reproduce the values in a flux report. See the section in the user manual on reporting fluxes which says "Note that the fluxes are reported exactly as computed by the solver. Therefore, they are inherently more accurate than those computed with the Flow Rate option in the Surface Integrals Dialog Box (described in Surface Integration)." If you plot the solution for temperature and/or species mass fraction you can see if there are any significant gradients near the inlet or outlet. If there is no significant gradient then the flux report should be similar to the convective flux.