Unsteady Effect in a Nozzled Turbocharger Turbine

The unsteady behavior of a nozzled turbocharger turbine under pulsating flow conditions has been studied experimentally in a cold flow test facility that replicates engine pulses. The results presented are obtained at a turbine speed of 48,000 rpm for pulsating frequencies of 40 Hz and 60 Hz (which correspond to 1600 rpm and 2400 rpm in a twin turbocharger six cylinder internal combustion engine). The turbine unsteady behavior is compared for nozzle vane angles ranging between 40 deg and 70 deg. A nozzled turbocharger turbine is found to behave differently from a nozzleless turbine under pulsating flow. The existence of a nozzle ring “damps” the unsteady flow leading to a reduced level of flow dynamics affecting the turbine wheel for all vane angles. The bigger volume in the nozzled turbine is also another contributing factor to the observed characteristics. The results are more pronounced in the higher frequency and maximum vane opening condition. Given this “damping” behavior, the concept of unsteady efficiency is questioned. The level of unsteadiness in the flow is characterized by the relevant nondimensional parameters, and the onset of the unsteadiness in the flow and its effect on a nozzled turbocharger turbine is discussed. The onset of the unsteady effect is suggested to be at 40 Hz flow condition. However, the nozzled turbine exhibits more of filling and emptying characteristics for both the frequency conditions, especially at close nozzle position cases. The effect of unsteadiness on the instantaneous efficiency calculation is more pronounced in the nozzled turbine compared with a nozzleless turbine.