Unsteady performance analysis of a twin-entry variable geometry turbocharger turbine

Abstract

The unsteady performance of a twin-entry variable geometry turbine for an automotive turbocharger is presented. A single-entry variable geometry turbine was initially designed based on a commercial nozzle-less turbine. The variable geometry single-entry volute was later modified to twin-entry through partitioning. A comprehensive range of steady testing and evaluation was also conducted as part of this project, which is discussed by Romagnoli et al. [1]. The unsteady curves of the twin-entry turbine exhibited the conventional looping characteristics representing filling and emptying effects, which was also the case for the nozzleless and single-entry nozzled turbine. The swallowing capacity of the twin-entry turbine, during full admission testing, was recorded to be inconsistent between the two entries, in particular they were at different pressure ratio levels -the shroud end entry was in most cases INTRODUCTIONA turbocharger turbine operates under unsteady conditions due to the pulsating nature of the exhaust gases. In consequence, twin-entry turbines are generally designed and used for better energy extraction from the pulsating exhaust gases. Two banks of exhaust manifolds feed each entry of the turbine so that the "rotor windmill" is minimized as the mass flow drops to zero. Thus, there is a need for experimental work to understand the unsteady-state performance of a twin-entry turbine in comparison to the single-entry and nozzleless unit.Wallace and Blair [2] and Benson and Scrimshaw [3] presented the earliest systematic study on the unsteady performance of a radial turbine. These were followed by Wallace et al. [4], Miyashita et al. [5], Benson [6] and Kosuge et al. [7]. All these investigations were limited by the inability of the instruments to instantaneously measure all the relevant performance parameters. Thus, only the static pressure was measured instantaneously, while the mass flow rate, temperature, speed and torque were measured as time-mean values. This remains similar for the investigation presented by Capobianco et al. [8,9] and Capobianco and Gambarotta [10]. Research work by Capobianco et al. [8,9] Fig. 1 illustrates the turbine configurations used for the current study, which was designed in progression from nozzleless single-entry to nozzled twin-entry. The details of the turbine design stages are described in Romagnoli et al. [1]. The experimental facility available in Imperial College EXPERIMENTAL FACILITYLondon is a simulated reciprocating engine test-bed for turbocharger testing. The facility has the capability of conducting steady and unsteady flow testing of single and twin-entry turbines. A schematic diagram of the turbine test rig is shown in Fig. 2. The test rig is equipped with an eddy current dynamometer, which enables turbine testing within a large velocity ratio range [7,8]. The test-rig is supplied by screw-type compressors, capable to delivering air up to 1.2 kg/s mass flow rate at a maximum pressure of 5 bars (absolute). The two separated streams of airflow in e...

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