Original article Proc IMechE Part D: J Automobile Engineering 1–11 Ó IMechE 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954407018757620 journals.sagepub.com/home/pid Variable nozzle turbocharger turbine performance improvement and shock wave alternation by distributing nozzle endwall clearances Ben Zhao 1,2 , Mingxu Qi 1 , Harold Sun 1 and Xin Shi 1 Abstract 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 noz- zle 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 turbochar- ger turbine. Keywords Variable nozzle turbocharger, nozzle endwall clearance, performance, shock wave, unsteady Date received: 28 July 2017; accepted: 8 January 2018 Introduction In a variable nozzle turbine (VNT) used in an internal combustion engine, the nozzle vanes are located upstream of the turbine wheel, adjusting nozzle pas- sages through which the fluid accelerates and turns the flow direction. A survey of current VNT products reveals that for many VNTs, variation of nozzle vane flow passage is achieved by pivoting the nozzle vane around its spindle. Nozzle endwall clearances between the nozzle vane and the endwalls have to be designed to achieve continuous and reliable variation of the noz- zle vane flow passage. Pivoting the nozzle vanes to improve the turbine performance has been investigated by many researchers. 1 But the distribution of the nozzle vane endwall clearances at both the shroud and the hub sides has not attracted many attentions. In current VNT products, this endwall clearance distribution is not controlled. As a consequence, the distribution may change when a VNT operates under uneven aerody- namic loading condition and with strong vibrations. This changed distribution is possible to affect turbine performances and alter the nozzle vane trailing edge shock wave. Nozzle endwall clearances provide space for fluid to cross the nozzles and form strong leakage flow. Hu et al. 2 performed numerical simulations on a VNT to investigate the nozzle endwall clearance effect on tur- bine performances. Their results revealed that the pres- ence of nozzle endwall clearances deteriorates turbine performances, especially at small nozzle vane open con- ditions, and that the leakage flow not only leads to total pressure loss in nozzle vane passages but also creates more mixing loss in a turbine wheel. Walkingshaw et al. 3 carried out numerical simulations of flow fields in a highly off-design VNT and indicated that the reason why a nozzle endwall clearance leakage flow affects tur- bine performances is that leakage vortices change the circumferential distribution of the approaching flow to 1 School of Mechanical Engineering, Beijing Institute of Technology, Beijing, China 2 Michigan State University, East Lansing, MI, USA Corresponding author: Mingxu Qi, School of Mechanical Engineering, Beijing Institute of Technology, No 5, South Zhong Guan Cun Street, Beijing 100081, China. Email: qimx@bit.edu.cn