Evolution of vorticity from the endwall boundary layer P M V Subbarao Professor Mechanical Engineering Department Methods to Estimate Enhanced Losses along the Flow Path……

Classification of Vortices in Edge BL Flow HP: Pressure side leg of Horse-shoe vortex. HS: Suction side leg of Horse-shoe vortex. CV: Corner vortex. PV: Passage vortex. TEV: Trailing Edge vortex.

The Primary Cause : Think like H A Einstein The important cause of secondary flows is formation of horse- shoe vortex. The boundary layer fluid upstream of the leading edge is decelerated by the adverse pressure gradient and separates at a saddle point S 1. The boundary layer fluid elements form a reverse recirculating flow just before the leading edge.

Twin Legged HS Vortex This reverse flow separates at another saddle point S 2.. The upstream boundary layer rolled-up in the recirculating zone flows past the leading edge and is transported downstream in two legs – pressure-side and suction-side leg of the horse-shoe vortex.

The Independent Movements of HSVs The suction-side leg of the horse-shoe vortex moves near the suction surface of the blade. The pressure-side leg subject to the pressure gradient towards the suction surface moves across the blade-to-blade passage towards this surface.

Confluence of Vortices All main forms of secondary flows meet at the suction surface of the blade. The model that explains the transport of the horse-shoe vortex is more useful for estimation of losses. The pressure-side leg of the horse-shoe vortex together with the endwall cross flow form the main recirculating flow. The resulting passage vortex, stays apart from the counter-rotating vortex.

Change in Lift Due to Secondary Flow

Main Flow Symptoms due to Vortices The centres of secondary vortices are regions of a high turbulence level. The turbulence level at the exit section of the cascade in the passage vortex and trailing shed vortex estimated at 35% with respect to the inlet velocity. The turbulence level in the suction-side corner vortex is nearly as high as above. The increase of turbulent fluctuations in the region of secondary vortices can be attributed to the process of deformation of the endwall boundary layer. This depends on conditions of high streamline curvature and acceleration of the main flow in the cascade. Namely change in flow deflection angle & Degree of Reaction.

The EWBL is laminar in the major part (downstream of the horse shoe vortex ). Division of the endwall boundary layer : Region of laminar flow

Division of the endwall boundary layer : Region of intermittent flow The EWBL is also seen intermittent at few locations

Division of the endwall boundary layer : Region of laminar,intermittent and turbulent flow EWBL is turbulent only in the rear part of the blade-to- blade passage at the suction surface.

Occurrence of unreasonable Intermittent Flows The vortex flows wash away the endwall boundary layer towards the suction surface. This gives rise to relaminarisation of the downstream endwall boundary layer. The newly formed endwall boundary layer becomes thin. This will gradually increase in thickness but is constantly washed away. As a consquence, the endwall boundary layer has a highly three-dimensional character. Serious pressure pulsations are generated in the endwall boundary layer

Profile design considerations at the endwalls Design changes at the endwalls are made to counteract the growth of secondary loss and possibly to improve interaction of leakage flow with the downstream blade row. Secondary flows although undesirable, dominate the flow field in turbines consisting a major source of loss in axial turbines This aspect is considered to be an important additional degree of freedom that can be exploited to enhance overall design quality. Four aerodynamic aspects were found to be of great significance in controlling secondary loss generation: (b) higher degree of aft loading, (c) lower maximum circumferential pressure gradient, (d) lower diffusion on the suction side (thinner boundary layer).