Hotwire-based Experimental Study of Secondary Flows in Turbine Cascades

The periodical three-dimensional flow field at the outlet of a turbine plane cascade was measured by using rotary single-wire slanting hotwires and a mathematical solution to the three-dimensional speed average value was accomplished by using a least square fit of the hot-wire measured data with the help of the LSQNONLIN most optimized function of Matlab.A sub-sonic cascade test wind tunnel was set up to study the transient speed fields measured by hotwires at three different blade heights and two different inlet flow rates and analyze the secondary vortex flow conditions at the cascade outlet.By comparing different operating conditions of the cascade,it has been found that the cascade with a high inlet flow speed has a more conspicuous wake zone and more intensive secondary flows than those with a low inlet flow speed.The cascade with a relatively small blade height,however,has more drastic secondary flows,resulting in a quick increase of both radial flow speed (u) in the outlet plane of the cascade and axial flow speed (w) perpendicular to the outlet plane and a dramatic decrease of circumferential flow speed (v) in the outlet plane.Both the decrease of blade height and the increase of aerodynamic load (speed) will greatly increase the secondary flow losses of the cascade,which can be substantially ascribed to an enhancement of transversal pressure gradient of the cascade.