Hi, I'm working on a simulation of hydrocyclone, it's a tube with one inlet, two outlets. After a best simulation (i.e. get a good flowfield profile, compareing with test result),but i find a conflict of Mass Flow Rate: the inlet mass flow rate obtained from "Report/Fulx/Mass Flow Rate" is 0.8kg/s (it is true), another from "Report/Surface Integrals/Mass Flow Rate" is -4.1kg/s(it is not true). So, if anyone can give me a explanation about this conflict. The boundary i used is: the inletis "Velocity Inlet", two outlets are "Outflow".

1. Which surface do you use for incorrect mass flow rate calculation ?

2. For two outlets, 'outflow' boundary condition is, in general, not good. It is valid only if mass flow rate to the each outlet is known. How about to use 'pressure outlet' ?

Sincerely, Jinwook

Hi, Mr. Jin-Wook Lee,

Thanks for your response.

I'd like to present more details of my question .

1) I used a 2D Axis Symmetrical Model in my hydrocyclone simulation. The inlet should be a circle cylindrical surface in 3D model, and two outlets shoud be disk surfaces located at the oppsite two ends of hydrocyclone tube. In "Report" pulldown menu, the result of the "Mass Flow Rate" is not correct, i.e. the inlet mass flow rate does not meet the sum of two outlets.

2) Indeed, I don't know exactly the mass flow rate of each outlet before simulating. But I can give a mass flow ratio to each outlet,such as one is 0.1, another is 0.9, and so on. I must do it, because this one of my goals to inspect different flowfields at different operating conditions.

3) Using 'pressure outlet' is my another case. If I have any trouble, I'll ask for advice. Thank you again.

Yours Sincerely, Paul

1. In order to obtain mass flow rate to each outlet, pressure outlet is the best approach.

2. For your reference, for axisymmetric condition, please remember that the axes have the rule. X-axis is for axial direction and Y-axis is for radial direction.

Sincerely, Jinwook

Hi, Dear LEE,

Just as you said, I used X-axis as axial direction and Y-axis as radial direction.

I have found my problem is that although i set the inlet radial velocity component is negative as the boundary condition, but it becames positive value at many points on the linlet boundary after simulation. Why ?

Sincerely yours, Paul

Paul,

You have to be careful when you define an outflow boundary condition. The weighting factors for each outlet must be calculated with care. If you use this type of boundary condition, you don't need to prescribe any conditions at the outlet (velocities or pressures), but you need to establish the percentage of mass flow which go to each boundary. Why not run the case compressible and trying with mass-flow rate inlet boundary condition and pressure boundary conditions at the outlet?

Cheers

Peter,

Thanks for your reply. 1) Could you tell me the advance of using mass-flow rate inlet boundary condition and pressure boundary conditions at the outlet, comparing with "Velocity Inlet boundary , Outflow boundary at outlets"? 2) I recognise the inmportance of seting the weighting factors for each outlet carefully. But I can only give them assumptions, and those values are all my expected parameters in actual operation conditions, which i want to simulate. I mean i hope to divide the same inlet flow rate into two outflows at many different given ratio. So, I think how I can set the split ratio of two outflows exactly, if i use the pressure outlet boundary conditions.

Sincerely, Paul

Paul,

outflow boundary condition can only be used for incompressible cases. That is, if you case involve compressibility effects, you may need to run it with compressible flow. Then you will need to specify a pressure outlet boundary condition. The difference is that with an outflow boundary you don't know anything about the flow condition at this location, just the proportion of the flow going to each outlet. If you have no pressure data at the outlet, probably you need to use and outflow boundary condition.

Cheers

Hi Paul !

I was reading your first problem : The problem with the calculation of mass rate at the oulet. Had you two different values with different methods ? (Was it?)

I had the same problen in a 2D model. In :

Report/Surface Integrals/Mass Flow Rate or Area the calculatino is not well donde.

Do you know why is this ?

Thank You in advance.

Sergio Ferraris

Hi, Sergio Ferraris,

Unfortunately to tell you, I have not gotten any answer for my early question. Well, if you have some good idea, let me share it. Thanks!

Sincerely, Paul