Flow Mechanism And Flow Control Of A Variable Geometry Turbocharger Turbine With Variable Operating Conditions

Turbocharging is one of the key technologies for increasing power, lowering fuelconsumption and meeting the future emission legislations for internal combustionengines. One of the importance trends in the development of turbocharger technology isimproving the turbine variable conditions adaption. The turbine variable conditionadaption is gradually improved from the fixed geometry turbine, to the wastegateturbine, to the variable geometry turbine(VGT). The wastegate turbine in conjunctionwith others measures have been taken to meet Euro2emissions standard for the egine.In order to meet Euro3and Euro4emissions standard, the variable geometryturbocharger technology must be used to. The VGT modifies its effective flow areacontinuously to adapt to different engine exhaust gas flow conditions and provides agood match between the turbocharger and the engine. The engine transient responeability can be improved and the transient emissions can be lowered.Because of the wide operating and MFR(Mass Flow Rage) range of an automotiveengine, the VGT is working at a wide U/C(Velocity Ratio) range. The performane of theturbine changes a lot due to the different inlet conditions of nozzle and rotor passage.Especially the engine under low speed and high load conditions, where the VGT isoperating at low U/C values, the rotor will experience high values of positive incidence,which causes large flow losses and results in reduced efficiency. The investigation ofthe flow mechanism and flow control of the VGT with variable operating conditions isan important way to improve the turbine variable condition adaption.CFD model of the VGT nozzle and rotor passage was conducted and validated bythe variable condition performance experiment of the VGT. Detailed flow structures aswell as the loss distributions in the rotor passage at both high U/C values(nozzle openposition) and low U/C values(nozzle closed position) in the VGT were analyzed by thenumerical simulation. At the nozzle open position and high U/C value, the rotor blade isunder a considerable negative incidence. The tip leakage vortex is formed in the bladetip on suction surface and extends to the middle of the passage when propagating todownstream of the passage. Main losses are causd by the tip leakage vortex and wallboundary layer. At closed nozzle position and low U/C value, the flow into the rotor haspositive incidence, and a recirculation zone is generated on the suction surface of the blade leading edge, which is the main cause for loss increase and efficiency penalty forthe VGT.The flow control methods of changing rotor blade leading edge geometry and usingsplitter-blade were investigated respectively by numerical simulation. The results showthat backward curved blade could yield an increase in turbine efficiency at low U/Cvalue while the forward curved blading has the opposite effect. At nozzle open position,the negative blade lean shifts the peak efficiency to higher U/C value. At nozzle closedposition, the influence of blade lean on the turbine performance varies according todifferent swept blading. Using splitter-blade in the rotor inlet can improve the efficiencyof the VGT effectively at variable operating conditions. The splitter-blade has littleeffect on the flow field and increases flow losses due to the extra wall friction and wakeflows at nozzle open position. At nozzle closed position splitter-blade reduces thenegative effect of the passage vortex on the suction side of the main blade, lowers theflow losses in the passage and improves the efficiency of the VGT effectively. Theforward curved blading rotor with splitter-blade could improve the performance of theVGT on all operation conditions.