EFFECT OF ROUGHNESS ON SECONDARY FLOW IN A RECTILINEAR TURBINE CASCADE

Abstract

ABSTRACT The aerodynamics of the flow in a turbine stage (stator/rotor) is a complex issue and always been a subject of research. The flow through a cascade is inherently three dimensional and usually viscous. This characterization leads to the phenomenon of boundary layer. Due to the blade profile the flow becomes unstable and also subjected to separation. Both of these phenomenon leads to development of vortices and these vortices are the source of cross or circulatory flow which is termed as secondary flow. This report gives a detailed understanding of secondary flow and effect of roughness on secondary flow. Three dimensional geometry of rectilinear cascade of four blades (reaction) is created in the Gambit® 2.2.3 software and flow behavior has been studied using FLUENT 6.2. Air with an inlet velocity of 102m/s is passed through the cascade. The cascade is open to atmosphere at the exit. Initially, both surfaces of the blade of the cascade are kept as smooth and secondary loss is analyzed in the span wise direction. This secondary flow loss is then compared with the blades on which a roughness of 500 μm is applied on suction surface and pressure surface individually as well as on both the surfaces together. It is observed that in a smooth blade average total loss is 14.7% whereas in case of blades having both the surfaces rough this loss gets almost doubled and becomes 27.7%. When roughness is applied to all the suction surfaces only then average total loss is 24.7% and if roughness is present only on the pressure surfaces then average total loss is 18.2%. But the corresponding average secondary loss decreases from 1.7% in case of smooth blades to 1.5% for rough blades. This average secondary loss is 1.9% for the blades on which roughness is present on all the pressure and 1.3% in case when roughness is applied to only suction surfaces of the blades