Effects Of Axially Non-uniform Clearance On The Performance Of A Linear Turbine Cascade With Cavity Squealer Tip

Improving aerodynamic efficiency of aero-engine is of great economic significance.Turbine is one of the three core components of aero-engine and nearly one third of its single stage aerodynamic losses come from the leakage flows of the turbine rotor blade.Therefore,it is practical and economic to apply flow control methods to the tip clearance flow of turbine blades to reduce aerodynamic losses.Flow control methods include active flow control methods and passive flow control methods.The passive flow control methods are simple in structure and easy to implement in aero-engines because they usually need no additional complex structures.In the current research,two passive flow control methods:the squealer cavity,which has been widely studied,and the axially non-uniform clearance,which is relatively novel,were combined and studied by both experimental and numerical methods.Five-hole probe was used to obtain the flow information acquization in the flow field at the outlet plane.Meanwhile a modified end-wall static pressure measurement method was used to obtain the static pressure information acquization at the endwall.Oil flow was an additional method that was used to display the flow information acquization at the endwall.Numerical methods were used to take a further view in the turbine cascade,especially the flow field near the tip clearance.The influence of squealer cavity without considering the non-uniform axial clearance was analyzed.The results showed that the total pressure loss coefficient in the turbine cascade could be reduced by arranging squealer tip.The total pressure loss coefficient was decreased by 1.92% with a small clearance and 2.33% with a large clearance.The main mechanism was that the leakage flow in the squealer cavity moveed along the streamwise direction and formed vortices in the squealer cavity,which restricted the leakage flow to inject into the adjacent passage and delayed the formation of the passage vortices and leakage vortices.By investigating the variation of the total pressure loss coefficients under design and off-design conditions using the axially non-uniform clearance method combined with the squealer cavity method,it was found that the axially non-uniform tip clearance could further manage the leakage flows under the condition that the squealer cavity had already achieved flow control effects.Under different clearance size schemes and different attack angle conditions,the expanding clearances achieved better aerodynamic performance with the overall total pressure loss to be smaller,and the mass flow rate through the passage to be larger.This was due to that the expanding clearance could further delay the formation of passage vortices and leakage vortices and restrict the development of the passage vortices and leakage vortices.In the end wall flow field visualization results,it could be clearly recongnized that the expanding tip under small clearance could manage the leakage flow to move in the cavity along the streamwise direction,and then restrict the leakage flow over the tip of the blade to the adjacent passage and these effects still existed under large clearance schemes.The formation of passage vortices and leakage vortices was delayed under the expanding clearance,and the size of leakage vortices was smaller at the outlet.Meanwhile,the high loss area of leakage vortices was separated from the endwall.By investigating the schemes under off-design conditions,it was concluded that the expanding clearance was still applicable and the total pressure loss coefficient reduction effect was even more obvious in off-design conditions.Under both positive and negative attack angles,and both large and small clearances,the effects of expanding clearances under off-design conditions were strengthened.However,the mechanism under positive attack angle was different from that of negative attack angle.At negative attack angle,the effect was strengthened because the radial flow in the squealer cavity was more obvious,which lead to stronger resistance to the leakage flow.At positive attack angle,the leakage flows of uniform blade tip were stronger.As a result,the effect of non-uniform blade tip on delaying the generation of passage vortices and leakage vortices and restraining their development was stronger.The numerical methods were used to investigate the flow field near the tip clearance detailly.It was found that the leakage flows not only enter the tip clearance over the pressure side surface,but also enter the tip clearance over the suction side.This phonomenon was more obvious under the expanding clearance cases.The expanding clearance was observed to have effects of reducing the total pressure loss coefficient and increasing the mass flow rate in the passage with a larger leakage mass flow rate,which indicated that reducing the leakage mass flowrate didn’t lead to an improvement in the aerodynamic performance.The arrangement of expanding clearance could strengthen the radial flows in the squealer cavity and restrict the leakage flow over the tip clearance.