Turbochargers are widely used in Diesel engines as a means of increasing the output power. Most of them are fitted with radial or mixed flow turbines. In applications where high boost pressure is required, radial turbines are replaced by mixed flow turbines which can achieve a maximum efficiency at a lower value of blade speed to isentropic expansion velocity ratio than the usual 0.7 (for radial turbines). This study, performed with the ANSYS-CFX software, presents a numerical performance prediction of a mixed flow turbine under inlet pulsating flow conditions. In addition, the influence of the pulse frequency is studied and the numerical results are compared with those of a one-dimensional model and experimental data.

Radial and mixed flow turbines which are an important component of a turbocharger consist essentially of a volute, a rotor and a diffuser. Vaneless volute turbines, which have reasonable performance and low cost, are the most widely used in turbochargers for automotive engines. Care has to be done in the design of the volute, whose function is to convert a part of the engine exhaust gas energy into kinetic energy and direct the flow towards the rotor inlet at an appropriate flow angle with reduced losses. Turbulent compressible flow analysis and performance prediction using the finite volume method with two turbulence models (RNG k-ε and SST) implemented in the ANSYS-CFX software, are carried out on two different volute types. Four volutes, with different cross section areas used for radial turbines, are studied and the computed results are compared with the available experimental data. The second volute studied is the one used for a mixed flow turbine in the turbocharger test rig at Imperial College. In this part of the study, the interest is focused on the influence of the volute inlet flow conditions on its performance (efficiency, exit flow angle, etc).

Turbochargers are widely used in Diesel engines as a means of increasing the output power. Most of them are fitted with radial or mixed flow turbines. In applications where high boost pressure is required, radial turbines are replaced with mixed flow turbines with positive rotor inlet blade angle so that they can achieve a maximum efficiency at a lower value of blade speed to isentropic expansion velocity ratio than the usual 0.7 (for radial turbines). This study, performed with the ICEM and CFX softwares of ANSYS, presents a numerical performance prediction of a mixed flow turbine for a wide range of rotational speeds and pressure ratios. The influence of the clearance between the rotor tip blades and the casing on the turbine performances is also investigated. A simulation of the turbine under pulsed inlet flow conditions is also presented.