ON THE PERFORMANCE OF MINIATURE CENTRIFUGAL FANS WITH VARYING BLADE CORD LENGTH

This paper focuses on optimizing the blade design of miniature centrifugal flow fans for application in processor cooling solutions of portable power electronics. The design parameter varied is the blade chord length and the resulting fan performance is gauged by examining flow rate, pressure rise and power consumption characteristics. The former two of these are measured using a BS848 fan characterization rig and the latter by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15mm, 24mm and 30mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number; chord length to diameter of fan ratio; flow coefficient; pressure coefficient and power coefficient. When these characteristic parameters are plotted against Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data appears to be identical to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans and provides guidelines for designers to aid in selecting the optimum fan design for a specific application.