Difference between Volume average and Mass flow average?
 

I need help with mass flow average and volume average in Star CCM+. Could someone please help me by explaining what is the difference between the two? I have been doing simulations to see how well they compare to my experimental trials and when I choose mass flow average the numbers are off but with volume average they are almost exact.

The system I am trying to analyze is a plate heat ex changer.

There could be more to it than this, but...

Mass Flow Averaged-
This takes the mass averaged value of a field function on a face. This face can be either a part, region boundary or derived part. What makes it mass averaged is that the value is weighted by the mass flow rate through each cell. It gives a better understanding of the bulk behavior of the fluid passing through the surface.

sum(rho*V*A*<Field Function>)/sum(rho*V*A)

Where rho and V are the velocity and density in each cell of are A comprising your 'surface'.

Volume Averaged-
Takes the average of a field function in a given volume. Here you cannot specify a surface, it has to be an entire volume. This can either be a region or derived part but must be 3D in nature. Here each value of the field funciton is weighted by cell volume rather than by the amount of fluid passing through the cell as before.

sum(vol*<Field Function>)/sum(vol)

Where vol is the volume of each individual cell.