ABSTRACTA LOCAL FLOW ANGLE APPROACH TO CENTRIFUGAL COMPRESSOR VANELESS DIFFUSER STABILITYByChristopher ClarkeThe vaneless diffuser is found in many applications of centrifugal compressors. Therefore, it has been the subject of numerous scientific investigations over the last several decades. While this has produced many results the issue of vaneless diffuser rotating stall still exists. This is because rotating stall is a dynamic instability tied directly to the compressor stage geometry. Most previous investigation have focused on determining the physical triggers that lead to rotating stall onset. This investigation is not meant to do that.Previous investigations of centrifugal compressor stability have been focused on the time-dependent (transient) nature of the phenomenon. This investigation focuses on predicting the onset of rotating stall. In the preceding decades vaneless diffuser stability has been based upon the determining of a critical flow angle at the diffuser inlet based on the predictive Senoo – Kobayashi equation. However, it has been found that this one-dimensional method of predicting the critical flow angle is insufficient to properly determine the critical conditions for all diffuser models.Using a steady state simulation the flow characteristics of fourteen unique geometries have been simulated at shaft speeds of 13100 RPM, 19240 RPM, and 21870 RPM. The local flow angle profile at the diffuser inlet as a function of span was determined and compared against the critical flow angle predicted by the Senoo – Kobayashi equation and the experimentally determined flow angle profiles provided by Solar Turbines Inc. This gave several interesting results.It was found that the width ratio of the vaneless diffuser is the dominant parameter in predicting vaneless diffuser stability. For width ratios of 0.067 and above the local flow angle profile breached the line determined by the Senoo – Kobayashi equation (henceforth Senoo line) at the point of rotating stall onset. For cases where the width ratio was 0.045 and smaller the local flow angle did not breach the Senoo line. For stages with width ratios between 0.045 and 0.067 the results showed that secondary influences help to determine whether or not the local profile is capable of breaching the Senoo line. It was discovered that it is possible to capture localized velocity reversal at the diffuser inlet for cases where the diffuser width ratio is 0.078 and greater.Secondarily, it was found that the local flow angle approach was capable of capturing localized flow reversal inside of the diffuser. Through the use of a geometric parameter, b4/dpitch, it was determined that for geometries with values of 0.177 and above that localized flow reversal could be captured inside of the vaneless diffuser. However, for parameter values of 0.152 and below it was not possible to capture localized flow reversal in the diffuser. Nothing could be said about the region with parameter values between 0.152 and 0.177. This result leads to two very interesting conclusions. First, the results showed that there are two regions of flow breakdown. In the case where the parameter is above 0.177 the flow will breakdown in the span-wise direction allowing the steady state simulation to capture the localized flow reversal. In the region where the parameter is less than 0.152 the flow breaks down in the circumferential direction. This type of breakdown is washed out by the mass flow averaging process of the steady state simulation and does not allow for the detection of localized flow reversal inside of the diffuser.Second, it has been taught that localized flow reversal is the trigger for rotating stall onset. However, it was determined that this is not the case. By use of the results showing localized flow reversal it was found that localized flow reversal preceded the onset of rotating stall and was not the trigger. Thus, it was determined that localized flow reversal is necessary for rotating stall onset, but not sufficient to be the primary trigger.
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