Experimental Analysis of Tip Clearance Effects for Performance Characteristics of Turbomachinery

Abstract

The current study presents extensive results obtained experimentally in order to investigate the tip clearance effects on performance characteristics of ducted fans. Three ducted fans having different tip clearance ratio and with same rotor size were examined under three different yawed conditions of slanted hotwire probe. Prior to measuring the flow fields, calibration of hotwire was done in the wind tunnel for several values of the velocity and yaw angle conditions. After performing the calibration experiments, correlation factors were gained by using a non-linear curve fitting method. Three dimensional flow fields were measured from hub to tip at 17 different positions at inlet and 21 locations at outlet of the ducted fans. The analysis of data acquired by the calibrated slanted hotwire anemometer were done by PLEAT (Phase locked Ensemble Averaging Techniques) and three non-linear differential equations were solved simultaneously by using Newton Rhapson numerical method. Flow field characteristics such as tip vortex and secondary flow were confirmed through axial, radial and tangential velocity contour plots. At the same time, the effects of tip clearance on other performance characteristics like axial thrust, propulsive efficiency and input power required to drive these ducted fans were also investigated over a wide range of advance ratio by varying fan rotational speed and free stream velocity. As expected, tip clearance effects have a big influence on the performance characteristics of the ducted fans. Thrust and propulsive efficiency values dropped rapidly as tip clearance level is increased. For achieving maximum value of performance characteristics; tip clearance level should be kept as small as possible.