When a turbocharged engine which was designed to work at sea level works at high altitude area, its power will decrease because of the decrease of the air density. And the expansion ratio of the turbine will increase a lot because of the decrease of the out pressure, leading to a bad efficiency of the turbine. In order to recover the power of the turbocharged engine when it worked at the high altitude area, the efficiency of the turbine should be increased when it worked at high expansion ratio condition.

This paper shows a numerical investigation of the flow fields in a radial inflow turbine at design and off design condition. The comparison of the flow characteristics between the design and off design conditions is researched. When the turbine work at a high expansion ratio, there will occur a supersonic zone at the leading edge on the suction side and a shock wave will occur at the trailing edge, causing a flow separation and making a lot of losses, leading the decrease of the turbine efficiency.

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