Mixed Flow Turbines: Inlet and Exit Flow Under Steady and Pulsating Conditions

Abstract

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.

Unsteady Flow Loss Mechanism and Aerodynamic Improvement of Two-Stage Turbine under Pulsating Conditions

“…Based on the understanding of the unsteady turbine characteristics, a number of models were developed to capture the unsteady effects. These models are helpful in turbine preliminary design and performance evaluation [14][15][16][17][18][19][20][21]. Also, the performance of engines matched with different turbines can be predicted before the experiment.…”

“…The abovementioned research disclosed the unsteady characteristic of turbine pulsating flows and proposed modelling methods, while the flow details inside the turbine were not revealed. To explore the flow field inside the turbine, Karamanis firstly measured the flows at the inlet and exit of a mixed-flow turbine under pulsating flow conditions by using a laser Doppler velocity system [18]. The experiment showed that the changing range of the inlet incidence angle during a pulse period is −83 to 52 degrees.…”

Effect of Pulsating Flow Characteristics on Performance and Surge Limit of Automotive Turbocharger Compressors

“…Only steady flow maps, defined in a restricted operating zone, are generally provided by the turbocharger manufacturer hence causing errors in the curve extrapolation. Besides, only few research facilities are able to experimentally reproduce the typical unsteady flow generated by the opening and closing engine valves [6,7,8,9,10], due to the unavailability of adequate experimental equipment and to the difficulty in performing measurements of instantaneous parameters.…”

Experimental Investigation on the Unsteady Performance of Automotive Turbocharger Turbine

“…The difficulty now is to define an exact position, where the flow can be assumed to reach the rotor inlet. There are different locations suggested by some researchers: 180° from the volute tongue by Winterbone and 130° from the volute tongue by Karamanis. But there are no theoretical or experimental evidence to support these definitions of the rotor inlet.…”

“…The first is based on bulk flow velocity and the other is on sonic velocity. Karamanis assessed the two approaches through velocity measurements and the results show that the pulse travels at local sonic velocity . The difficulty now is to define an exact position, where the flow can be assumed to reach the rotor inlet.…”