Numerical analysis on the effect of varying number of diffuser vanes on impeller - Diffuser flow interaction in a centrifugal fan

It is quite well known that the diffusers are required for efficient conversion of the dynamic head generated by the impeller in a centrifugal fan. Hence the flow into the diffuser passage plays a crucial role in determining the efficiency of conversion. The flow in the region bounded by the impeller exit and the diffuser entry i.e. the radial clearance space is generally considered to be highly complex. With the development of PIV as well as versatile numerical CFD tools such as moving mesh techniques, it has become possible to arrive at a prudent solution compatible with the physical nature of the flow. Hence, in this work a numerical solution with moving mesh technique is made use of in predicting the real flow behavior, as exhibited when a target blade of the impeller is made to move past a target blade on the diffuser. Many research works both experimental and numerical on the impeller diffuser interactive phenomenon have been undertaken so far. But it is found from the literature that the study on the impeller diffuser interaction as well on the performance of the fan by varying the number of diffuser vanes has not been the focus of attention in these works. Hence a numerical analysis has been carried out in this work to extensively explore impeller-diffuser fluid interaction as well as to predict the flow characteristics of the fan by changing the number of diffuser vanes while keeping the number of impeller blades same. It is found from the analysis that there is an optimum number of diffuser vanes which would yield maximum static pressure recovery and when the diffuser vanes are increased beyond certain number, rotating stall occurs in diffuser flow passages corresponding to the blade passing frequency. Further it is observed from the analysis that smaller the number of diffuser vanes, larger is the pressure fluctuations at the exit flange of the fan which eventually would even out as the number of diffuser vanes are increased.