Mingxu Qi

Shock, Leakage Flow and Wake Interactions in a Radial Turbine With Variable Guide Vanes

The clearances of the nozzle and volute have significant influences on the unsteady flow, blade load distribution and thermodynamic performance of downstream radial turbine. The unsteady flow and blade loads are intensified due to the interactions among shock, leakage flow and nozzle wake. Few studies have been conducted to date to investigate these effects in detail. This work focuses on the tip leakage flow effects on the nozzle wake and shock. Furthermore, the blade loads of the downstream rotor are considered by evaluating the effects of shock, leakage flow and nozzle wake, and the fluctuating pressures are presented in a novel space-time diagram. To reveal the flow mechanism in a variable radial turbine, two nozzle clearance sizes were chosen for investigation via numerical simulation of the unsteady flow and the interactions among the shock, leakage flow and wake, and the turbine performance was verified with test data. The results show that the interactions between leakage flow and nozzle wake are intensified with increasing nozzle clearances, and the nozzle wake deformations also become more severe. Additionally, the gas mixing speed between the nozzle wake and leakage flow is enhanced, thus inhibiting the gas from expanding in the nozzle channel and weakening the shock at the nozzle outlet. When the nozzle clearance leakage enters the downstream rotor channel, it interacts with the rotor tip clearance leakage in the blade tip region. These interactions have significant impacts on the formation of leakage vortexes and the distribution of the inlet flow angles. With the increase in tip clearances at both sides, the load fluctuation increases at the root and the tip of rotor leading edge, whereas the fluctuation decreases due to the weakening of the upstream shock in the middle span region of the blades.Copyright

Variable nozzle turbocharger turbine performance improvement and shock wave alternation by distributing nozzle endwall clearances

Effects of nozzle endwall clearances on variable nozzle turbocharger turbine performances have been widely studied, but improving the variable nozzle turbocharger efficiency utilizing an optimal distribution of nozzle endwall clearances between the hub and the shroud sides has not drawn wide attention. Meanwhile, with the various distributions, shock wave variations that are closely related to turbine reliability are rarely reported. To fill the gap, this research performed three-dimensional numerical simulations on a variable nozzle turbocharger turbine to analyze the effects of various nozzle endwall clearance distributions on both turbine performance and shock wave. The results showed that there is an optimal distribution of the nozzle endwall clearance that can improve turbine efficiency and shift nozzle trailing edge shock wave. Performed on a linear turbine nozzle and with detailed validations, both experimental measurements and numerical simulations provide evidence that supports the numerical analyses conducted on a variable nozzle turbocharger turbine.

Investigation on the flow characteristics of a VNT turbine under pulsating flow conditions

The turbines used in turbochargers naturally experience unsteadiness caused by inlet pulsating flow conditions and stator–rotor interaction. The unsteadiness has an influence on turbine performance. Meanwhile, under certain small-nozzle opening conditions, strong shock waves can be generated. The synergistic effect of turbine inlet pulsation and shock waves has a significant influence on the turbine performance, rotor blade loading as well as the excitation force exerted on the turbine rotor, which is responsible for turbine rotor high cycle fatigue. In order to understand the influence of pulsating flows on turbine performance and the shock wave characteristic at nozzle trailing edge as well as the incidence angle characteristic of the rotor blade, unsteady numerical simulations were performed to investigate the effect of pulsating flow conditions on the performance, flow characteristics in frequency domain and shock wave behavior in a variable nozzle turbine. The results indicate that the turbine inlet pressure pulsation has strong influence on the turbine performances. Meanwhile, the turbine inlet pulsation flow has a strong influence on the intensity of the shock wave and clearance leakage flow in the nozzle, which causes significant flow losses in the turbine. In addition, at the turbine rotor inlet, the unsteadiness caused by the turbine inlet pulsation varies significantly along the circumferential direction and spanwise. Up to two-thirds of the unsteadiness caused by the turbine inlet pulsation dissipates before entering the rotor due to the flow dissipation and mixing process along the nozzle streamwise. The excitation force exerted on the rotor blade leading edge caused by the turbine inlet pulsation is about the same level as that caused by the stator–rotor interaction.

Design and Engine Performance Simulation of a Turbo-Cooling System

With the purpose of further lowering the intake temperature of diesel engine, a turbo-cooling system was developed, which was matched with a diesel engine. The system consists of two turbochargers and an intercooler: one turbocharger is the traditional exhaust driven turbo, and the other is an air turbocharger, which consists of a low expansion ratio radial air turbine coupled with a low pressure ratio centrifugal compressor. The 1-D preliminary design and the 3-D simulation of the air turbine and the low pressure ratio compressor were carried out. The new designed air turbine and compressor were manufactured and tested to get the performance maps. Further, the computational model of the diesel engine matched with this turbo-cooling system was set up. The simulated result shows that the turbo-cooling system can lower the intake temperature effectively and potential of reducing NOx exhaust. It is also can be expected that exhaust gas recirculation could be realized more easily.Copyright