Experimental And Numerical Study Of Nonaxisymmetric Endwalls Considering The Midgap

With the continuous development of aviation propulsion technology,the turbine blade load gradually increases.Reducing the loss of the endwall secondary flow effectively is the key to improve the isentropic efficiency of the turbine.Nonaxisymmetric endwall design can restrain the secondary flow intensity and reduce the related loss and has been applied in some real aircraft engines.However,the loss reduction mechanism of nonaxisymmetric endwall is not well understood and the relationship between flow structure and loss generation need to be investigated to improve the endwall design efficiency.At the same time,to further improve the efficiency,more details should be considered in the blade passage aerodynamic design,such as considering the midgap flow effect in nonaxisymmetric endwall designs.With test and numerical simulation methods,a large-scale low-speed turbine cascade is taken as an object to study the loss reduction mechanism of the nonaxisymmetric endwall design.Meanwhile,the midgap is added to investigate the midgap leakage flow influence on the aerodynamic performance of blade passage with nonaxisymmetric endwall.Finally,the midgap structure is added to the optimization process to redesign the nonaxisymmetric endwall to reduce the loss including the midgap leakage flow loss.The loss reduction mechanism of the redesigned nonaxisymmetric endwall is also analyzed.Following are the main work and findings of this research thesis:Nonaxisymmetric Endwall Optimization Design:A sinusoidal function in the circumferential direction and a B-spline in the axis direction is used to generate the profile.The CGNS mesh editor is developed to improve the mesh generation efficiency.The endwall has been optimized using an efficient global optimization based on an improved kriging surrogate model to minimize the total pressure loss coefficient at the passage exit.Loss Reduction Mechanism Analysis:The aerodynamic performance of the designed endwall and the flat endwall cascade exit are measured and compared to validate the loss reduction effect of the nonaxisymmetric endwall.The flow structure and loss distribution in the two passages are also measured to reveal the vortex structure development and the relationship between the vortex structure and the loss source.By comparing the flow structure and loss distribution in two blade passages,the loss reduction mechanism of the endwall design is explained.Midgap Leakage Flow Impact Analysis:The midgap is added to both the flat endwall and the designed endwall passage.The secondary flow structure and the loss characteristics of the two blade passage under different net leakage flow conditions were analyzed.The flow structure of the midgap leakage flow itself and its influence on the other vortex structures and loss distribution are revealed.Redesigned Nonaxisymmetric Endwall.The midgap structure is added to the nonaxisymmetric endwall design process to redesign the nonaxisymmetric endwall.Comparing the loss distribution in the flat endwall and the two design endwall passages.It is found that the new design achieves a further improvement in loss reduction.The mechanism is also revealed through the CFD method.