Shaft Power Matching and Performance Prediction for Contra-Rotating Axial Flow Fans

Abstract

The purpose of shaft power matching of contra-rotating axial flow fans (CRAFF) is to ensure the best utilization of the motors at their rated power while not to overload them at the operation.Usually it requires a lot of time and effort due to lack of theoretical guidance and, thus, becomes a major problem for the application of CRAFF. Through analyzing velocity triangle, the authors have found a nondimensional parameter k, defined as the tangential ratio of two relative outflow angles for two impellers, to be critical for successful shaft power matching and aerodynamic design. The authors then performed a numerical simulation of the flow field for a design of CRAFF, of which the performance prediction was validated very well by test data. Based on the analysis of the flow filed data, the result obtained from the velocity triangle analysis is further improved and some values for k and deviation angle δ are recommended. In order to determine the approximate optimum of blade pitch angles of two impellers, a guideline for shaft power matching is then presented. Finally, a comparison of the blade pitch angles obtained from the guideline with that obtained from the optimum using numerical simulation are carried out for six designs of CRAFF. The errors are only within 2 degrees.