Evaluation of steady and pulsating flow performance of a double-entry turbocharger turbine

Abstract

The turbocharger remains one of the best means available to the engine developer to satisfy the power density demands on a modern internal combustion engine. This simple device uses the otherwise waste exhaust gas energy to provide significant improvements in the volumetric efficiency or ‘breathing capacity’ of an engine. In order to maximize the energy of the exhaust driving the turbine, most applications utilize pulse turbocharging where a compact exhaust manifold feeds the highly pulsating exhaust flow directly into the turbine wheel. This thesis considers the influence that this pulse-charging has on a double-entry turbocharger turbine. The design of this turbine plays an important role in much of the research presented in this thesis. The turbine is equipped with a mixed-flow rotor with 12 blades that are fed by a 24 blade nozzle ring. The circumferentially divided volute is designed with two gas inlet passages that each feed a separate 180° section of the nozzle ring. Thus, there is no communication between the entries from the volute inlet to the exit of the nozzles. At the exit to the nozzle, the fluid from both inlets expands into an interspace that spans the circumference of the rotor inlet. This small volume that is formed between the nozzle and the mixed flow rotor is the first area where interaction between the flows can occur. The core of this report contains three main divisions: Steady flow experimental results, CFD modelling, and unsteady flow experimental results. These sections are preceded by an introduction explaining the background of the research study, and an essential outline of the equipment and the method of experimentation. The aim of this work is to use a combination of experiments and computational modelling to build up a picture of the performance of the turbine under a wide variety of flow conditions that will eventually lead to further insight into its unsteady performance. First, a comprehensive steady-state experimental data set was obtained to establish the base-line turbine performance. Steady, equal admission tests yielded excellent performance, peaking at 80% efficiency. Owing to the double-entry arrangement, steady flow could also be introduced in the two inlets unequally. During unequal, steady-state operation a notable decrease in performance was observed. The correlation between the ratios of entry pressures and the efficiency of operation was apparent but essentially independent of which flow was varied. In the extreme, when the turbine was only partially supplied with air, the consequence was a 28 point decrease in performance at the optimal velocity ratio. Despite the division between the two entries, the experiments showed that the flows through each inlet were interdependent. Compared to full flow,of the performance of the turbine under a wide variety of flow conditions that will eventually lead to further insight into its unsteady performance. First, a comprehensive steady-state experimental data set was obtained to establish the base-line turbine performance. Steady, equal admission tests yielded excellent performance, peaking at 80% efficiency. Owing to the double-entry arrangement, steady flow could also be introduced in the two inlets unequally. During unequal, steady-state operation a notable decrease in performance was observed. The correlation between the ratios of entry pressures and the efficiency of operation was apparent but essentially independent of which flow was varied. In the extreme, when the turbine was only partially supplied with air, the consequence was a 28 point decrease in performance at the optimal velocity ratio. Despite the division between the two entries, the experiments showed that the flows through each inlet were interdependent. Compared to full flow, 4 when the pressure in one entry was low, the second entry could swallow more mass, and when it was high, the second entry swallowed less. A three-dimensional CFD model was constructed in order to permit a detailed study of the flow in the double-entry design and answer specific questions regarding the observed steady-state performance. For both equal and unequal admission simulations, the model showed close agreement with the experimental mass flow behaviour and reproduced the measured efficiency trends quite well. The interdependence of the swallowing capacity of the two inlets was also predicted by the model, thereby allowing the analysis of the physical flow effects that drive this trend. It was found that the interspace region near the tongues was the site of much of the interaction between inlets. A major emphasis of this modelling work was also to discover areas of loss generation that could lead to the decrease in performance. By focussing on partial admission, this study found that the windage loss in the interspace region of the non-flowing entry proved to be one of the more significant areas of loss generation. Pulsating air flow was then introduced using the range of frequencies typically produced by an internal combustion engine. The operating point of the turbine, traced an orbit within a 3-D space defined by three non-dimensional parameters: velocity ratio, pressure ratio across inlet one, and pressure ratio across inlet two. Direct comparison between steady and unsteady values at the same pressure ratios and velocity ratio was possible due to the large amount of steady data measured. Thus, a quasi-steady versus unsteady comparison was made on the basis of efficiency, mass flow and output power. In general, under pulsating flow conditions, the turbine behaved quite differently than that predicted by the quasi-steady assumption. Lower frequency, higher amplitude pulsations produced the lowest unsteady cycle-averaged efficiency and also produced the most significant departure from quasi-steady behaviour.