Numerical Study of the Unsteady Flow Inside a Centrifugal Fan and its Downstream Pipe Using Detached Eddy Simulation

The 3-D unsteady turbulent flow inside a centrifugal fan and its downstream pipe is investigated at the best efficiency point flow rate using the computational fluid dynamics (CFD) package ANSYS Fluent. The impeller with an outlet diameter of 400mm has 12 forward curved blades. The computational domain comprises four parts: the inlet part, the impeller, the volute, and the downstream pipe. The flow domain was meshed in ANSYS ICEM-CFD with structured hexahedron cells, and totally 8.72 million cells were used. The Detached Eddy Simulation (DES) turbulence modelling approach was employed with this fine enough mesh scheme. The impeller was set as the rotating domain at a speed of 2900rpm. A sliding mesh technique was applied to the interfaces in order to allow unsteady interactions between the rotating impeller and the stationary parts; the unsteady interactions generate pressure fluctuations inside the centrifugal fan. One impeller revolution is divided into 2048 time steps, in order to capture the transient flow phenomena with high resolution. Monitoring points were set inside the volute, at volute tongue, and along the downstream pipe. When the numerical simulation became stable after ten impeller revolutions, data sampling was started with a total of 20480 time steps (10 impeller revolutions) sampled. The time history data of the pressure and velocity magnitude at the monitoring points were saved and with Fourier transform applied to obtain the frequency spectra.

The time-averaged flow fields show clearly the static pressure rises gradually through the impeller, and further recovers from the velocity in the volute, and becomes to be constant along the downstream pipe. Large velocity region exists around the impellor exit, and the maximum velocity near the trailing edge can reach 1.5v₂, where v₂ is the circumferential velocity at the impeller outlet. Vorticity with large magnitude is found around both the leading and trailing edges of the blades, indicating intensive turbulence wherein.

The root mean square value distribution of pressure fluctuations show that the tongue region and the suction side of blades undergo large pressure fluctuation with the magnitude about 15% of the reference dynamic pressure 0.5ρv₂²; the maximum value locating at the tongue tip can reach 30% of 0.5ρv₂². The pressure fluctuation magnitude decreases quickly along the outlet pipe: after 5D (D is the outlet pipe diameter) the magnitude is 1% of 0.5ρv₂², and 0.1% after 20D. The pressure and velocity fluctuation spectra at the monitoring points in the volute show striking discrete components at the blade-passing frequency (BPF) and its 2^nd, 3^rd harmonics. The BPF component has the maximum value of 15% of 0.5ρv₂² at the tongue region, and it decreases dramatically along the downstream pipe with the amplitude less than 0.1% of 0.5ρv₂² after 10D distance.