Aerodynamic optimization of high pressure turbine and interstage duct in a two-stage air system for a heavy-duty diesel engine

Abstract

The automotive industry is turning to turbochargers for engine downsizing in order to adapt to more stringent CO2 emissions regulations every year. For smaller-size engines, turbochargers are able to provide similar boost power as their larger counterparts by allowing more compressed air into the engine. However, turbocharger turbines will experience a different level of unsteadiness depending on different engine speed and load combinations. Part of the unsteady effect is due to pulsating flow from valve opening and closing, pulsating flow due to cylinder interactions and a high frequency-turbomachinery unsteadiness due to blade passing at the volute tongue. Therefore, it is important to increase turbocharger turbine efficiency at design conditions as well as in the wider range of engine operating conditions.

This thesis presents development of an aerodynamic optimization method for engine air system geometry using a 3D CFD method. It focuses on a high pressure turbine (HPT) wheel and an interstage duct for a two-stage air system. The optimization method covers geometry parameterization of the components, automatic meshing and post-processing, and employs a genetic algorithm based optimizer. Prototypes of optimized designs were manufactured and tested. CFD simulations and experimental test data for the optimized wheel design show an increase of around 2 percentage points of efficiency at peak condition as well as increased overall performance at off-design between 1.5 to 10 percentage points efficiency. The optimized interstage duct shows a small benefit of 0.18 percentage points efficiency increase for the HPT but importantly reduces static pressure at the volute inlet by up to 1.3 kPa. Although small, this pressure reduction represents lower exhaust back pressure and thus reduced pumping work at the engine level