[mehelyavuzselim]

Hi everyone,

My question is;
I have a centrifugal fan and I want to calculate its massflow rate and maximum total pressure for a given rotational speed using CFX. What should I enter as boundary conditions? I can't give massflow for inlet or outlet as boundary condition because I don't know it.

Could you help me please?

[Jared1986]

Hello, If I understand clear, you want to know fan curve. Pressure on one axis and volume flow on the other axis. You should try to set few points for your calculation. First you should choose right direction of rotation, if you don't have straight blades (90degree angle)... Next step is setup of low mass flow rate on the outlet and 0 Pa on inlet side. You obtain value of pressure close to highest value of the total pressure and after that you can try to set next value of mass flow rate and as soon as you obtain different polarity of the pressure, you will have value close to maximum of mass flow rate. I hope that this answer will help you. Good luck

[mehelyavuzselim]

Hi,
First of all thanks for your reply.

Actually I want to only calculate maximum massflowrate and maximum total pressure for a given rpm. In your method, I have to run 4-5 different boundary conditions to get max flowrate. Is it possible to do it with 1 run?

[highorder_cfd]

Originally Posted by mehelyavuzselim

Hi,
First of all thanks for your reply.

Actually I want to only calculate maximum massflowrate and maximum total pressure for a given rpm. In your method, I have to run 4-5 different boundary conditions to get max flowrate. Is it possible to do it with 1 run?

Is your simulation steady state or transient?
Create a monitor in CFX pre (Output control) printing MFR and TotPressure at the boundary of your interest. The expression for the MFR is MassFlow()@Boundary and total pressure is MassFlowAve(Total Pressure in Stn Frame)@Boundary. You can get the maximum for a transient run in the solver monitor when running the solution (or the steady state converged value)

[mehelyavuzselim]

Originally Posted by highorder_cfd

Is your simulation steady state or transient?
Create a monitor in CFX pre (Output control) printing MFR and TotPressure at the boundary of your interest. The expression for the MFR is MassFlow()@Boundary and total pressure is MassFlowAve(Total Pressure in Stn Frame)@Boundary. You can get the maximum for a transient run in the solver monitor when running the solution (or the steady state converged value)

My simulation is Steady State. So what should I enter as boundary conditions for Inlet and Outlet before run? Normally in this kind of studies Total Temp and Total Pressure for inlet, Static pressure for outlet are given. but I already dont know total pressure and total temperature. Because total temp= Static temp+ v^2/(2*cp) and I dont know the velocity. I just want to give an rpm to frozen rotor and want cfx to calculate the massflow and pressure.

[shivasluzz]

The maximum flow depends on the maximum pressure ratio at which the fan will not stall.
So run by setting different value of pressure ratio as boundary condition (assume the values).
As you keep on increasing the pressure ratio, the change in mass flow rate will keep on reducing and it will become asymptotic. That's the point of maximum mass flow rate which is near the stall point. Further increasing the pressure ratio will induce stall and you cannot solve that in steady state condition. If you want then you can run the transient simulation and confirm.

Then you can, your centrifugal fan gives a maximum of flow of X at the applied pressure ratio or difference of Y and at the applied speed of Z.

Hope you understand

[mehelyavuzselim]

Originally Posted by shivasluzz

The maximum flow depends on the maximum pressure ratio at which the fan will not stall.
So run by setting different value of pressure ratio as boundary condition (assume the values).
As you keep on increasing the pressure ratio, the change in mass flow rate will keep on reducing and it will become asymptotic. That's the point of maximum mass flow rate which is near the stall point. Further increasing the pressure ratio will induce stall and you cannot solve that in steady state condition. If you want then you can run the transient simulation and confirm.

Then you can, your centrifugal fan gives a maximum of flow of X at the applied pressure ratio or difference of Y and at the applied speed of Z.

Hope you understand

If I explain what I understood by an example;
Frozen rotor domain RPM will be constant. Fluid domain reference pressure will be 1atm. As boundary condition first I will try to give Total Pressure (TP)=300 Pa, for Total Temperature (TT) 20°C. These were for the inlet BC. For outlet BC I will give static pressure 0 and run it. After run finishes, I will calculate the flowrate. Then in the second step I will give inlet BC TP=350 Pa and TT=20°C, for outlet BC Static pressure=0 Pa and I will run it. After run finishes I will calculate the massflow rate. I will repeat these steps till flowrate almost does not change. Right?