Performance Investigation Of A Variable Radial Turbine With Non-uniform Arranged Guide Vanes

Variable geometry turbocharger(VGT) can effectively improve the economical, dynamical characteristics and the emission of vehicle engine, and it can adapt the operating requirements of vehicle engine in a wide operating range. With a wide application prospect, VGT has become one of research focuses of vehicle turbocharging technology. Variable radial turbocharger is most widely used in VGT. The adjusting mechanisms of the turbochargers are all installed in the intermediate parts. Previous studies on the performances of the variable radial turbine mainly focused on the effects of the shape, the opening change and the tip clearance of variable guide vane.Non-uniform flow distributions along circumferential direction exist not only at the volute outlet but also in the nozzle ring because of the structural constraint, the structure of volute tongue and the complexity flow in the volute, which induces that the flow angle of volute outlet does not match well with the installation angle of guide vane in circumferential direction. As a result, the flow losses are increased and the efficiency of turbine is reduced. Considering the non-uniform flow in the volute, the present study on a variable radial turbine proposes a solution that includes the optimization of the connecting arm of the nozzle seat and the non-uniform arrangement of guide vanes in circumferential locations to enhance the turbine efficiency. On the base of that, a novel design of the turbocharger regulating mechanism was developed by integrating it with the turbine housing. This paper investigated the performance of the radial inflow turbine with non-uniform arranged variable guide vanes by numerical calculation and experimental methods. The main contents and results are listed as follows:(1)The numerical method of variable radial turbine. The numerical method and the turbulent model in the numerical computation are confirmed. The geometric model of variable radial turbine is built. The computational domains of the volute, the nozzle ring and the impeller are meshed, and the links and matching of the computation grids of the whole turbine stage are implemented. The relevant calculation parameters are selected and set. The boundary conditions of numerical calculation are determined according to turbine performance test results. Three-dimensional numerical computations of turbine are performed. The comparison between the experimental and the computational results is performed to validate the numerical calculation.(2)The effects of the circumferential non-uniform flow of volute on the performances of variable radial turbine. Three-dimensional numerical calculation of the turbine is performed on the basis of the boundary conditions determined by the test data. The influence rule of the volute circumferential flow non-uniformity on the performances of the variable radial turbine is discussed. The results show that, corresponding to the engine rated power and maximum torque operating conditions, the circumferential distribution ranges of both the volute outlet parameters and the internal flow field parameters in the volute and nozzle ring are large, and the effect ranges of the nozzle connection arm and the volute tongue structure are obvious. The volute circumferential flow non-uniformity leads to the circumferential flow non-uniformities in both the nozzle ring and the impeller channel. The variation range of circumferential distribution of the guide vane inlet flow angle is large. Both the blade loads of the impeller and the mass flow in every channel present circumferential non-uniform distribution. The above phenomena in response to the circumferential non-uniform flow of the volute increase the flow losses of the turbine, and lead to the decrease of the turbine efficiency.(3)The improved design for a variable radial turbine based on the circumferential non-uniformity. According to the circumferential non-uniform flow distribution in volute, a design for improving the nozzle seat connecting arm’s structure and non-uniform arrangement of variable vanes is proposed to decrease the turbine stage’s flow losses and improve the efficiency of turbine. The results show that the efficiency of the turbine improved 5.18% at the reduced speed condition corresponding to the engine rated power working condition, and the efficiency increased 3.57% at the reduced speed condition corresponding to the maximum torque working conditions. The flow angle at volute outlet of the optimized turbine gets to be more uniform. The flow angle is close to the opening angle of guide vane, which decreases the flow loss in the nozzle ring and explains the reason for the improved turbine efficiency. After optimization, the distribution of the blade load is more uniform, and the flow non-uniformity among impeller channels is significantly reduced, and the relative flow angle distribution at leading edge of blade is more stable. The results prove that the retrofit scheme is helpful to reduce the attack angle loss of impeller blade, to increase the efficiency of the turbine, to reduce blade vibration and to extend the working life of the turbine.(4)Experimental performance comparison between the prototype turbine and the redesigned one. To further validate the effectiveness of the optimal scheme, the performance tests of both the prototype variable radial turbine and the redesigned one are conducted. Firstly, the turbine performance testing principle is analyzed, the power measuring method of the turbine efficiency is determined, and the experimental data processing method is provided. The modified nozzle seat and drive ring are manufactured. According to the actual structure of turbocharger, the install position of the turbocharger on the test rig is adjusted. The opening adjustment and control device of the guide vane are designed. The experiments of two kinds of turbines are conducted to validate the effectiveness of the optimization scheme by means of comparing turbine efficiency.(5)Novel design for the regulating mechanism of variable radial turbocharger. Based on the above analyses, a design scheme of turbocharger regulating mechanism integrated on the turbine housing is proposed to solve the problems of the current turbine regulating mechanism. This provides a novel thinking for the design of regulating mechanism of the variable radial turbochargers. In the design scheme, the spacer sleeve or the nozzle seat structure in the traditional turbocharger is replaced by three blunt aerodynamic fixed guide vanes to control the width of the nozzle ring, and the installation angles of fixed guide vanes match with the turbocharger design point. The design scheme is expected to realize the decrease of the local disturbance in the volute or in the nozzle ring field caused by the special structure, the decrease of the flow loss and the improvement of the turbine stage efficiency. The designed turbocharger regulating mechanism is installed on the exhaust end of the volute housing, and the structure of the intermediate part keeps no change, which brings convenience to the engineering application. The numerical calculations of the designed model, the nozzle seat structure model and the spacer sleeve structure model are conducted under the same operating condition. Comparisons among the efficiency of these three types of turbines validate the feasibility of the novel design scheme in theory.