Turbochargers are finding increasing application to automotive diesel engines as cost effective means for improving their power output and efficiency, and reducing exhaust emissions; these requirements have led to the need for highly loaded turbocharger turbines. A mixed-flow turbine is capable of achieving its peak isen-tropic efficiency at reduced velocity ratios compared to a typical radial inflow turbine; it is therefore possible to improve the turbocharger/engine matching. These turbines differ from the commonly used radial turbines in that the flow approaches the rotor in the non-radial direction; in the extreme a mixed-flow turbine would become an axial machine. The steady and unsteady performances of a mixed-flow turbocharger turbine with a constant blade inlet angle have been investigated. The steady flow results indicated that the mixed-flow turbine obtains a peak efficiency (total-to-static) of 75 per cent at a velocity ratio of 0.61, compared with that of a typical radial-inflow turbine which peaks at a velocity ratio of 0.7. The performance and flow characteristics were found to deviate significantly from the equivalent steady state values commonly used in turbocharger turbine design.
The performance and detailed flow characteristics of a high pressure ratio mixed flow turbine has been investigated under steady and pulsating flow conditions. The rotor has been designed to have a nominal constant incidence (based on free vortex flow in the volute) and it is for use in an automotive high speed diesel turbocharger. The results indicated a departure from the quasi-steady analysis commonly used in turbocharger turbine design. The pulsations from the engine have been followed through the inlet pipe and around the volute; the pulse has been shown to propagate close to the speed of sound and not according to the bulk flow velocity as stated by some researchers. The flow entering and exiting the blades has been quantified by a laser Doppler velocimetry system. The measurements were performed at a plane 3.0 mm ahead of the rotor leading edge and 9.5 mm behind the rotor trailing edge. The turbine test conditions corresponded to the peak efficiency point at 29,400 and 41,300 rpm. The results were resolved in a blade-to-blade sense to examine in greater detail the nature of the flow at turbocharger representative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been shown. The unsteady flow characteristics have been investigated at two flow pulse frequencies, corresponding to internal combustion engine speeds of 1600 and 2400 rpm. Four measurement planes have been investigated: one in the pipe feeding the volute, two in the volute (40 deg and 130 deg downstream of the tongue) and one at the exit of the turbine. The pulse propagation at these planes has been investigated; the effect of the different planes on the evaluation of the unsteady isentropic efficiency is shown to be significant. Overall, the unsteady performance efficiency results indicated a significant departure from the corresponding steady performance, in accordance with the inlet and exit flow measurements.
The performance and detailed flow characteristics of a high pressure ratio mixed flow turbine has been investigated under steady and pulsating flow conditions. The rotor has been designed to have a nominal constant incidence (based on free vortex flow in the volute) and it is for use in an automotive high speed diesel turbocharger. The results indicated a departure from the quasi-steady analysis commonly used in turbocharger turbine design. The pulsations from the engine have been followed through the inlet pipe and around the volute; the pulse has been shown to propagate close to the speed of sound and not according to the bulk flow velocity as stated by some researchers. The flow entering and exiting the blades has been quantified by a laser Doppler velocimetry system. The measurements were performed at a plane 3.0 mm ahead of the rotor leading edge and 9.5 mm behind the rotor trailing edge. The turbine test conditions corresponded to the peak efficiency point at 29,400 and 41,300 rpm. The results were resolved in a blade-to-blade sense to examine in greater detail the nature of the flow at turbocharger representative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been shown. The unsteady flow characteristics have been investigated at two flow pulse frequencies, corresponding to internal combustion engine speeds of 1600 and 2400 rpm. Four measurement planes have been investigated: one in the pipe feeding the volute, two in the volute (40° and 130° downstream of the tongue) and one at the exit of the turbine. The pulse propagation at these planes has been investigated; the effect of the different planes on the evaluation of the unsteady isentropic efficiency is shown to be significant. Overall, the unsteady performance efficiency results indicated a significant departure from the corresponding steady performance, in accordance with the inlet and exit flow measurements.
A detailed flow investigation downstream of two mixed-flow turbocharger turbines has been carried out at 50% and 70% design speeds, equivalent to 29,400 and 41,300 rpm respectively. The measurement technique used was laser Doppler velocimetry (LDV). The measurements were performed at a plane 9.5 mm behind the rotor trailing edge, they were resolved in a blade-to-blade sense to fully examine the nature of the flow. The results confirmed the performance tests and indicated the improved performance of the rotor with a constant inlet blade angle relative to the rotor with a nominally constant incidence angle.
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