Numerical Modelling of a Radial Inflow Turbine with and without Nozzle Ring at Design and Off-Design Conditions

Abstract: The design of a radial turbine working at peak efficiency over a wide range of operating conditions is nowadays an active topic of research, as this constitutes a target feature for applications on turbochargers. To this purpose many solutions have been suggested, including the use of devices for better flow guidance, namely the nozzle ring, which are reported to boost the performance of a radial turbine at both design and off-design points. However the majority of performance evaluations available in literature are based on one-dimensional meanline analysis, hence loss terms related to the three-dimensional nature of real flows inside a radial turbine are either approximated through empirical relations or simply neglected.In this thesis a three-dimensional approach to the design of a radial turbine is implemented, and two configurations, with and without fixed nozzle ring, are generated. The turbine is designed for a turbocharging system of a typical six-cylinder diesel truck engine, of which exhaust gas thermodynamic properties are known. The models are studied by means of a CFD commercial software, and their performance at steady design and off-design conditions are compared.Results show that, at design point, the addition of a static nozzle ring leads to non negligible increments, with respect to the vaneless case, of both efficiency and power output: such increments are estimated in +1.5% and +3.5% respectively, despite these data should be compared with the uncertainty of the numerical model. On the other hand both turbine configurations are found to be very sensitive to variations of pressure and temperature of the incoming fluid, hence off-design performances are dependent on the particular off-design point considered and a “best” configuration within all the combustion cycle does not exist.