This paper provides an overview of published literature on the effect of inlet Flow Distortion on the performance of centrifugal fans, with an emphasis on a recently completed testing program [8] on the effect of inlet distortion on the performance of airfoil bladed Centrifugal fans, at various inlet vane positions. The results of this testing program [8] show that a model fan, when equipped with variable inlet vanes, is generally subject to lower levels of performance degradation, for a given level of inlet distortion, than the same model when the variable inlet vanes are not present. This indicates that the variable inlet vanes are acting to condition the distorted flow prior to entry into the fan wheel in a beneficial manner. The reduction in performance degradation seems to occur when the variable inlet vanes are wide open (parallel to the fan shaft axis), and does not seem to improve with closure of the variable inlet vanes. The maximum reduction in Total Pressure development and Total Efficiency measured for fans without variable inlet vanes in this test was −8.4% and −6.1% respectively. The maximum reduction in Total Pressure development and Total Efficiency measured for fans with variable inlet vanes in the wide open position was reduced to −2.9% and −3.5% respectively. In fact, the beneficial effects of the variable inlet vanes appear to be greater with the vanes in the wide open position, and are possibly less beneficial when the inlet vanes are partially closed. (The maximum reduction in Total Pressure development and Total Efficiency measured for fans with variable inlet vanes in a partially open position was −2.5% and −4.1% respectively.) Refer to Table 2 for a summary of the Maximum Distortion Observed for the testing program. An unanticipated result of the tests conducted [8] is the discovery that the effects of distortion seem to vary with the position of the rating point along the fan curve, with distortion effects being greater at higher flow conditions and lower at lower flow conditions. An actual field case study on the negative effects of inlet distortion on a centrifugal Induced Draft (ID) fan is also presented. Recommended steps to take in designing a fan system to minimize the potential for any negative effects on fan performance from inlet distortion are also presented. The conclusion compares the results of previously conducted tests [1, 2] and recently conducted tests [8], and concludes that the recent tests do not support many of the earlier findings. This leads to authors to believe that the actual impact of inlet distortion is highly dependent on the specific geometry of the airfoil-bladed centrifugal fan involved, and the relationship to various fan design parameters is not currently well understood.

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