Flow Mechanism With Non-Axisymmetric Endwall Profiling In High Pressure Turbines

Turbomachinery is a kind of widely used equipment in industrial engineering and is a core component in modern gas turbine engines.Axial flow turbines are probably the most complicated component in turbomachines due to their complex configuration associated with internal flow field,and the complexity becomes further aggravated as a result of fewer stages,higher blade loads,greater pressure and temperature gradients,the metal material and the cooling system required by the development of high-performance gas turbines.The majority of high-pressure turbines use cylindrically shaped endwalls between the blades on the hub or shroud,three-dimensional non-axisymmetric endwall profiling is a reasonably recent technique that relaxes this constraint,and allows the geometry of the endwalls to depart from that of a plain cylinder to mitigate secondary flows.At present,the non-axisymmetric endwall method as an effective technology to control secondary flow has become one of the promising technical reserves for the development of high-performance gas turbines.Although a number of studies have shown non-axisymmetric endwall profiling to be an effective mechanism for the reduction of secondary flows,within the published literature there still remains a general lack of relevance of endwall profiling in two-dimensional cascades to the flow mechanism in real turbines.Among some of the other most important issues which are less addressed such as the majority of previous studies involved the endwall optimizations of a single row ‘of which mostly the profiling was achieved for the stator row’,and the detailed information relating to the optimal design solutions.Thus it is important to carry out the research on the non-axisymmetric endwall method and to explore in great depth the influence of non-axisymmetric endwall on the internal flow field in the real turbine stage and the flow mechanism of improving the flow field quality by non-axisymmetric endwall profiling.Based on some of the reasons for the lack of considerations as described above,this thesis documents and presents an extraordinary contribution towards the non-axisymmetric endwall optimizations of two different high-pressure turbines: a high subsonic high-pressure turbine stage and a transonic high-pressure turbine stage.The different non-axisymmetric endwall configurations were intended to produce flow conditions optimized using a selection of metrics not necessarily commonly found in the literature.The first part of this thesis focuses and presents a unique and novel study in which all the endwalls of a high-pressure turbine stage were subjected to three-dimensional contouring.The proposed method aims to optimize the end wall secondary flow by selecting different criteria.Both the endwalls of the stator row were optimized together in one optimization cycle.The individual optimization of the rotor endwall was also presented.The optimization of the rotor had also been achieved in succession,in the environment of the optimized stator.Every optimization run included full stage simulations.It was adopted through a fully automated design and optimization environment.The optimization was quantified by using optimization algorithms based on the pseudo-objective function.The challenge was to find the optimal endwall shape that best satisfies the objectives of reducing secondary losses in terms of increasing total-to-total efficiency while constraining the mass flow rate on the target value of the design condition.In order to ensure that global optimum had been achieved,the function of parameters was first approximated through the artificial neural network,and then optimum was achieved by implementing the genetic algorithm.The steady simulations were carried out during the optimization.Other than obvious global performance parameters defined in the objective function,the effectiveness of the optimization was quantified in terms of loading on the blade surfaces,the entropy generation,and the secondary kinetic energy.The result of the investigation showed that the optimized shape of the endwalls could significantly help to increase the efficiency up to 0.18% with the help of a reduction of the transverse pressure gradient and the aforementioned metrics.The size of the pressure side leg of the vortex was reduced,less contributing to the formation of passage vortex.The phenomenon of non-axisymmetric endwall had more influence on the suction surface rather than the pressure surface of the blade.In order to investigate the periodic effects,the design of the optimized turbine under steady simulations was confirmed through time-accurate unsteady simulations,and an answer to the question whether the unsteady phenomena that possibly might not be captured within the optimization had any negative influence on the flow characteristics of the optimized design,was presented to get a further confidence for the new designs.The second part of this thesis presents a detailed numerical analysis of secondary flows in a transonic turbine.For this purpose,the stator hub and the rotor hub of a transonic turbine were optimized individually by mitigating secondary flows through the method of non-axisymmetric endwall design.The contoured endwalls of the stator and the rotor hub were designed based on equidistant Bézier curves along the camber line in the blade channel.The initial design samples as ten times as the design variables were generated through the Latin Hypercube Sampling method for the database generation.The optimization of the endwalls was achieved by using a state of the art latest multi-objective optimization algorithm,NSGA-Ⅱ,connected with the ANN to increase the performance.For this purpose,two optimization formulations were constructed.Firstly,the stator endwall was optimized to enhance the isentropic efficiency and decrease the secondary kinetic energy,while the mass flow was constrained to remain on the datum value as in the original geometry.Secondly,the rotor endwall was optimized to increase the isentropic efficiency while constraining the mass flow and the degree of reaction(a novel metric for the objective function)on the original value.The individual optimization of hub endwalls of the stator and the rotor produced an increase in the efficiency of 0.27% and 0.25%,respectively.After the individual optimization of both rows,they were simulated together resulting in a cumulative improvement of 0.46% in the efficiency,and finally,the time-accurate simulations were carried out to authenticate our design.The formation of pseudo-objective and multi-objective optimization techniques revealed that both methods were compatible and equally acceptable.The penalty setting of the pseudoobjective optimization using weighing factors had an influence on optimization results in which weight factors were required to be adjusted many times to get the optimal result,while the multi-objective optimization gave freedom to choose from the pool of Pareto-front samples.The periodic fluctuation of performance metrics was somehow increased in the subsonic turbine after the optimization,but the time-averaged result was satisfactory.In addition to the mitigation of passage vortices and eradication of the corner vortices in both the turbines,the flow mechanism unveils that the shockwave in the stator row of the transonic turbine was weakened due to the adaptation of non-axisymmetric endwalls.