The Influence Mechanism Of Unsteady Wakes On The Down Stream Turbine Cascades Flow

Unlike military aeroengines,economy is one of the key technical indicators in the design of civil turbofan engines with high bypass ratio.The low-pressure turbine is one of the heaviest components of a civil aviation engine,and reducing the weight of the low-pressure turbine can greatly improve the efficiency of the whole aircraft.To reduce the weight of a low-pressure turbine,a deeper understanding of its internal flow mechanism is required.In this paper,studies on the flow characteristics of low-pressure turbines are carried out at high exit Mach numbers,and in order to get closer to the operating conditions of high-altitude cruising.First,the effect of different Reynolds numbers on the flow characteristics of suction surfaces of low-pressure turbine blades is studied without wake interference.It is found that the lower the Reynolds number,the larger the thickness of the boundary layer,the less full the shape of the velocity planar surface,the closer the flow state is to laminar flow,the greater the velocity loss of the boundary layer is the easier the boundary layer separates under the action of the inverse pressure gradient,and the greater the loss.The effect of different attack angles on the boundary layer at suction surfaces of low pressure turbines is next investigated.Under low Reynolds number conditions,separation bubbles at the rear of the suction surface of the blade are not affected by changes of the attack angle.However,when the incoming flow is large positive attack angle,boundary layer separates in the front half of the suction surface.Compared to other attack angles,the separation of the front half of the suction surface at positive attack angles increases the loss of the attached layer,indicating that large positive attack angles should be avoided as far as possible during the operation of low-pressure turbines.Afterwards,the influence of a static round rod on the boundary layer of a low-pressure turbine cascade at low Reynolds numbers is observed by placing the rod in front of the leading edge of the blade.The analysis shows that when the round rod is placed directly in front of the leading edge of the blade,the rod deflects the airflow direction through the downstream blade to a positive attack angle,which results in flow separation in the front half of the blade suction surface and intensifies the loss of the attached layer.In addition,when the round rod is located directly in front of the leading edge of the blade,the wake of the rod will disturb the boundary layer of the blade suction surface.resulting in a high degree of main stream pulsation in the rear half of the blade and a near-turbulent flow state,which can significantly suppress the separation bubble in the rear of the blade then reduce the separation loss of the attached surface layer.The incremental loss due to separation at the front of the blade suction side is much smaller than the loss reduction due to suppression of the separation bubble in the rear,therefore,the perturbation of the flow field by the rod can effectively suppress separation bubbles and thus reduce the total pressure loss coefficient.Finally,the paper explores the impact of wake sweeping on the boundary layer flow characteristics at the suction surface of a downstream low-pressure turbine cascade at high exit Mach numbers.The results show that the velocity loss of the wake disrupts the expansion acceleration ability on the suction side of the blade,causing fluctuations in the backward pressure gradient,which is also the main reason for the change of separation bubbles on the suction surface under wake sweeping during the study.Although the effect is weak,reducing the diameter of the round rod and increasing the axial spacing can reduce the influence of wake loss on the boundary layer of the suction surface.