simulation results don't match with experimental results
 

Hello,
Everyone, I have a question in simulation on a blower which is used in vacuum cleaner home appliance, it comprises of back-ward impeller,diffuser and motor.

There is already testing result: vacuum(impeller inlet relative static pressure), volumeric flow rate at different rotating velocity.
At working condition the rotating velocity is around 36,000 RPM.

So I made a CFX simulation including impeller,diffuser and motor region to solve static pressure at impeller inlet under different working conditions

Following are the some parameters set up in CFX
Domains

1. Tunnel: a cylindrical tunnel with same diameter as impeller inlet and lengh about 3 times the impeller inlet diameter, stationary

1. impeller: 9 blades, all 9 fluid reigons are built in the model, rotationay

1. diffuser and motor domain, stationary

Environmental absolute Pressure: 1 Atm
Turbulence Model: K-epsilon
Inlet: mass flow rate, which is 1.2 multiplies tested volumeric flow rate
Outlet: opening with opeing pressure=0 (since CFX always suggest using opening as there is back flow in the motor exit )

interface between tunnel and impeller: general, stage with automatic pitch change;
inteface between impelle and diffuser: general ,stage with automatic ptich change;

so with those parametes, I look for the average relative static pressure at impeller inlet with an expression: massFlowAve(Pressure)@Impeller_inlet,

It turns out at working condition, which is 36,000 RPM and volumetric rate is 60 CFM, simulation agrees well with the testing result, which is around 20,000 Pa.

But at large flow rate condition(RPM is around 30,000 RPM,volumetric rate is more than 100 CFM), simulation result (8,000 Pa)is quite different from testing result(1,000 Pa).
At lower flow rate condition, simulation result is also quite diffrent with testing result.

Can anyone give me some clue on this mismatch?

Have you checked grid independence?

Thanks, Degroot, I will check this.