CFD-Based Multi-Point And Multi-Constraint Blade Optimization Approach And Experimental Investigation

The competitive pressure to improve performance, reduce research cost and design duration has always pushed turbomachinery designers to develop better and faster design methods. In recent years, CFD technology has been widely applied to numerical simulation of the three-dimensional viscous flow field inside turbomachinery, which has been verified by many researchers. Meanwhile, several effective optimization algorithms have been proposed. Fortunately, the rapid development of software and hardware of computers makes it possible for turbomachinery optimum design with the combination of CFD technology and those algorithms. To establish an optimum design system for turbomachinery impellers and blades, the author made an in-depth investigation of the related optimization methods, and developed several multi-point and multi-constraint optimization approaches for turbomachinery blades or impellers according to their individual characteristics. The developed approaches have been successfully applied to corresponding optimization cases.The balance of computation time frame and computation precision must be achieved so that CFD technology can be effectively applied to the aerodynamic optimization design of turbomachinery. The author has studied the discretization scheme and numerical solution of steady Reynolds-Averaged Navier-Stokes equations, illustrated the formulation of Baldwin-Lomax model, Spalart-Allmaras model and k-εmodel, and introduced the convergence criteria of the present study. The NASA Low speed compressor's impeller was selected as the reference case for numerical simulation and experimental validation. The influences of the turbulence models and grid distributions on the simulation results were discussed, which provided the ongoing optimum design with guidance.A CFD-based multi-point and multi-constraint blade optimization approach was developed, which can be convenient to develop different proper approach for specific optimum object with regard to different computation cost and the number of operating points. The approach was successfully applied into the single- and multi-point optimum design for cascade blade, the single-point and multi-constraint optimum design for a centrifugal compressor impeller with splitter, and the multi-point and multi-constraint optimum design for a very low-specific-speed industrial centrifugal blower blade.To realize these four optimization approaches, the corresponding optimization algorithms were developed. The parametrization methods for cascades and blades were developed to greatly reduce the number of the design variables. The proposed PANN has the improved mapping quality and training efficiency, which are superior to those of the traditional artificial neural networks. Inspired by the competitions occurring in populations and between populations in nature and society, HFCGA-DN was developed to overcome premature convergence. It is very efficient in global search. To improve the diversity of Pareto GA, the distribution function was introduced into NSGA-II algorithm, and INSGA-II was then developed. The data interface programs were also developed to combine the above algorithms with design of experiment (DOE) method and CFD technology.As for the optimization case for a two-dimensional cascade, the maximum lift-to-drag ratio was selected as the objective. That the leading edge metal angle and trailing edge metal angle are kept constant, the section area of the optimized is greater than or equal to that 20% of the original one, and the drag coefficient of the optimized is less than or equal to that of the original one, were selected as the constraints. From the results of the nondimensional aerodynamic forces, the drag coefficient of the optimized blade is 3.2% reduction over that of the original one, while the lift-drag ratio of the optimized blade is 8.3% higher than that of the original one.As for the same cascade, the minimum total pressure loss coefficients over three operating points, i=-10o,0o,10o, were selected as objective. And the constraints are the same as those of the above case. The multi-objective Pareto optimization approach based on approximate model was applied to the optimization process. The loss coefficients of the three points of the optimized cascade are decreased by 8.31%, 8.43% and 8.52%, respectively.As for a centrifugal compressor impeller with splitters, a single-objective optimization approach based on approximate model was developed. The maximum isentropic efficiency was selected as the objective. That the mass flow and total pressure ratio are greater than or equal to those of the original one was selected as the performance constraints, and the variation ranges of the design variables were selected as the geometric constraints. The optimized impeller was obtained by varying the profiles of the impeller hub, shroud and blade. The isentropic efficiency is improved by 1.06%, while the total pressure ratio is also increased by 0.52%.An optimization approach based on approximate model was proposed against many time-consuming CFD calculations during the numerical optimization process of turbomachinery. The core of this approach is a sample database, which is to build the approximate mode between the geometric information and its performance of samples. This proposed approach was applied to the optimum design of a set of three-dimensional compressor blades presented in this paper. The control points'ordinates of characteristic polygon of Bezier curve representing the profile of meridian plane and blade were selected as design variables, while maximum isentropic efficiency was selected as objective function. The maximum pressure rises at the three points were selected as the objective. That the mass flow and efficiency at each point are greater than or equal to those of the original one was selected as the performance constraints, and the variation ranges of the design variables were selected as the geometric constraints. The objective function of the optimized impeller is 1.06% higher than that of the original one on the condition that the pressure ratio does not fall, and flow quality on the meridian plane and in the flow channels is improved.A multipoint optimization approach to a very low-specific-speed industrial centrifugal blower blade was developed with the consideration of its structural characteristics and performance requirements. The total pressure rise at three specified operating points were selected as the objective function. That the flow rate and impeller total pressure efficiency are not less than those of the original one was selected as the performance constraints. The numerical optimization was carried out in two phases, the optimization phases of constant thickness blade and varying thickness blade. To validate their aerodynamic performances, the original and optimized blower blades were made into experimental prototypes. Their performances were tested on the same experimental facilities, and their internal flow patterns inside the impeller were measured by PIV system. The results of both the numerical optimization and performance experiment show that significant improvement has been obtained in the optimization objective, which validates the effectiveness of the developed optimization approach. In addition, the performance variation rule of this very low-specific-speed blower is different from that of the traditional ones.With the development of the multi-point and multi-constraint optimization approaches, successful application to different optimization cases, and numerical simulation and experimental investigation of a very low-specific-speed centrifugal blower, the author had a better knowledge of the influence of the blade profile on its performance and flow field, and made contribution to the optimization technology of turbomachinery.