## Fluids

#### Dimension of heat of reaction

• Shiyao
Subscriber

Hi,

I am running a simulation with volumetric reactions, and exporting the data "heat of reaction". The dimension of it is presented as Watts.

I am wondering if it means that it is the total heat of reaction in that cell, or if it is actually W/m^3.

• Rob
Ansys Employee

What exactly are you exporting? The units in Fluent tend to be what you're getting, but the reports may not be doing what you think.

• Shiyao
Subscriber

It is called "heat of reaction" in "file - export - solution data". You can also find it by plotting a contour of "reaction - heat of reaction", where the dimension is shown to be W.

It seems to be the heat release/absorption of all the reactions at the node, but that does not make sense with W instead of W/m^3 or W/m^2.

So you are probably right that it might be something else, and I am curious about what it is, and how I can get the heat of reactions at each node.

• Rob
Ansys Employee

The units are correct, you may need to create a Custom Field Function (under User Defined tab). The definition of heat of reaction is here,  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_fvdefs.html   It makes sense from a solver/computational point of view.

• Shiyao
Subscriber

That makes things clear, as it is defined as the volumetric heat of reactions.

Thank you for the help!

• Rob
Ansys Employee

You're welcome. There are a few "odd" volume related definitions. In reality a W/m3 would be averaged over a whole reactor or the like. In Fluent values are often cell based as the solver doesn't really know diameters or zone volumes: the post processing can calculate on zone volumes but the solver works at the cell level.

• Shiyao
Subscriber

That makes sense.

Talking about the cell volume, I am not clear about it. I have a 2D axisymmetric mesh, and I am trying to figure out what Fluent considers as "cell volume".

Based on the data, I found it to be the "cell-volume-2d" times "y-coordinate" (radial coordinate). That means it considers 1 unit rad in axisymmetric angular dimension.

May I ask if I am correct on this?

• Rob
Ansys Employee

It's covered in the manual, but yes, multiply the face area by 2PI https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_sec_report_conventions.html

• Shiyao
Subscriber

Thanks!

It is told to be computed for an angle of 2PI rad, but what I found from the output data is that the "cell-volume" is the integral value of "cell-volume-2d" for an angle of 1 rad.  For example, when "cell-volume-2d" is 2.5e-9 m^2 and "y-coordinate" is 1e-3 m, the "cell-volume" is 2.5e-12 m^3, which has not timed a 2PI.

For other quantities, I can't verify them as I only find integral data. I am wondering if they are integral values for 2PI rad or 1 rad.

• Rob
Ansys Employee

I've checked area (surface) and volume (volume report) and both suggest it's for the full 2PI (360 degree) domain.

Check https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_fvdefs.html  for "cell volume".  Looks like some of the post processing variables have a different basis.

• Shiyao
Subscriber

I see. So some specific quantities have a different reference cell depth.

Can I assume that the quantities without specified axisymmetric cell depth in the instruction use full 2PI?

• Rob
Ansys Employee

Generally, yes. It's always worth checking the documentation though.

• Shiyao
Subscriber

Got it. Many thanks for your help!