pressure difference - which surface integral?
 

Hi,

I want to calculate the pressure drop of an apparatus. Now, how to calculate the pressure difference?

I take ->report->surface integrals and select the inlet and the outlet of my apparatus.

But what report type is the right one? "Integral" or "Area-weighted-Average" or any other?? There are different values resulting...

Thanks

Ralf

Re: pressure difference - which surface integral?
 

I think "Area-weighted-Average" is right. In addition, pay attention to the "static pressure difference" and "total pressure difference".

Re: pressure difference - which surface integral?
 

Hi, You have to work with Facet Average (choose total pressure). It will compute the average total pressure on the desired surface. If you take inlet and outlet at the same time, fluent will compute the average on surfaces inlet + outlet. So you have to compute the total pressure (Facet Average) on inlet, and then on the outlet. You get 2 values, one for inlet and one for outlet. dp= paverage inlet - paverage outlet mAx

Re: pressure difference - which surface integral?
 

Hi,

I'm a little bit confused... When I take total pressure for pressure drop calculation, it is the static + the dynamic pressure... but the velocities in the inlet and in the outlet are different (different cross-sections)!

So the dynamic pressure difference will NOT give me right results... isn't it?

Second, when Fluent reports the pressures at different times, the values are the same than reporting at the same time. The only difference is that when reporting at the same time, a third value appears: "net".

Does anybody know what kind of value that is? It is NOT the difference between inlet and outlet! It is about factor 1,626 smaller than the inlet pressure value (Area-Weighted Average Static Pressure) calculated for 9 different geometries…

Any Help is appreciated!!

Ralf

Re: pressure difference - which surface integral?
 

Hi, I suppose the "Net" value is the summ of both values. But as you computed your pressure with Area Weighted Average, the "Net" value you get should be like: paverage = (p1*A1 + p2*A2) /(A1 + A2) where p1,A1 is the pressure and Area on inlet and p2,A2 on outlet. Once again compute your pressure on inlet, and then re-compute the pressure on outlet. Then substract both values. This is my way for computing drop pressures in hydraulic valves.

Re: pressure difference - which surface integral?
 

I have doubt about using facet averages for pressure on a surface. I think area weighted is more appropriate since it will work with non-uniform meshes.

Re: pressure difference - which surface integral?
 

I agree with Muhammad and lcw - area weighted average is the option to choose, facet average will give you results based on the size of the mesh facets. This was a mistake that I initially made when I started with Fluent.

Re: pressure difference - which surface integral?
 

ok, but in my cases, it doesn't change my results ( I checked and compared with facet average / area weighted average / Mass weigthed average) I get always the same results.

Re: pressure difference - which surface integral?
 

Thank you a lot. It works!

Total or static
 

The pressure you measured at the inlet is total pressure or static pressure? Can someone clarify this ? I'm guessing it's total pressure as the surface is perpendicular to the flow direction. But on the other hand, the surface is not like the Pitot tube but only doing the average. I got confused. Please advice.