Aerodynamic and thermal characterization of turbocharger turbines: experimental and computational evaluation

Abstract

Turbochargers are widely used in both passenger and commercial vehicle applications to increase power density, improved fuel economy leading to significant emissions reductions. In recent years, car manufacturers have introduced turbochargers widely in the diesel market in response to the stricter regulations in exhaust emissions. Although investment in turbocharger technology has made it possible to overcome issues related to reliability and cost, research is much needed in the area of design, testing methodologies and model development. This is particularly the case when considering unsteady flow effects. Computational codes are used by engine manufacturers to predict its performance and size components; prediction accuracy is crucial in this process. This thesis contributes to this process in several ways: steady modelling and heat transfer predictions. Furthermore, most aero-thermal design and analysis codes need data for validation; often the data available falls outside the range of conditions the engine experiences in reality leading to the need to interpolate and extrapolate excessively. The current work also contributes to this area by providing extensive experimental data in a large range of conditions. A further contribution of this work is the understanding of the turbine performance under pulsating flow; it shows that this performance deviates from the commonly used quasi-steady assumption in turbocharger/engine matching. A turbocharger is subjected to high temperature conditions; heat transfer within the turbine and the compressor severely affects the compressor performance at low rotational speeds and mass flow rates. Compressor maps provided by turbocharger manufacturers do not usually take into account the effects of heat transfer; this causes a mismatch when fitting the maps into engine codes which is detrimental to the overall engine performance prediction. The experimental investigation was conducted on three different turbine designs for an automotive turbocharger. The design progression was based on a commercial nozzleless unit modified into a variable geometry single as well as a twin-entry turbine configuration. The main geometrical parameters of these turbines were kept constant to allow equivalent performance assessment. The mixed-flow rotor used in this study consists of 12 blades with a constant inlet blade angle of +20°, a cone angle of 50° and a tip diameter of 95.2mm. The variable geometry stator consists of 15 vanes fitted into a ring mechanism, capable of pivoting in the range of 40° and 80° (with reference to the radial direction). The design progression into twin-entry turbine was completed by fitting a divider (accounting for only ≈6% of the overall internal volume) within volute. The turbine response for different vane angles (40° to 70°) and mass flow ratios between the two entries of the turbine was assessed. The turbine was tested under steady and pulsating flow conditions for two rotational speeds, 27.9 rev/s·√K and 43.0 rev/s·√K, a velocity ratio (U2/Cis) of 0.3 - 1.1 and a pulse frequency of 40 - 80Hz under both in-phase and out-of-phase conditions. A meanline aerodynamic model capable of predicting the performance parameters was developed for the nozzleless and the variable geometry single-entry turbine. The former was validated against experimental results spanning an equivalent speed range of 27.9 rev/s·√K and 53.8 rev/s·√K while the latter validated against one single speed (43.0 rev/s·√K) and three different vane angle settings (40°, 60° and 70°). The wide range of tests data from the Imperial College High Speed Dynamometer enabled the evaluation of the model in areas of the maps where currently no data exists. Based on the model prediction, a breakdown aerodynamic loss analysis was performed. As for the twin-entry turbine, the interaction between the two entries was investigated. Based on experimental evidence, a map-based method was developed to uniquely correlate the flow capacity within the entries for both partial and unequal admission. An investigation into the effects of heat transfer on a turbocharger was performed using a commercial turbocharger mounted on a 2.0 litre diesel engine. The global objective of these tests was to improve the understanding of heat transfer in turbochargers under realistic engine conditions. Measurements were obtained for engine speeds between 1000 and 3000 rpm at a step of 500 rpm – for each engine speed the load applied was varied from 16 to 250 Nm. In addition to the standard set of measurements needed to define the turbo operating point, the turbocharger was equipped with 17 thermocouples positioned in different locations in order to quantify the temperatures of the components constituting the turbocharger. A simplified 1-D heat transfer model was also developed and compared with experimental measurements. The algorithms calculate the heat transferred through the turbocharger, from the hot to the cold end by means of lump capacitances. The compressor performance deterioration from the adiabatic map was then predicted and based on the data generated by the model a multiple regression analysis was developed in order to assess the main parameters affecting the compressor non-adiabatic performance.