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° 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.

Effects of pulse flow and leading edge sweep on mixed flow turbines for engine exhaust heat recovery

“…The rotational speed of the turbine simulations was 50000 r/min, which was a main operation condition of the turbocharger while the design speed was 70000 r/min. Another reason for choosing this running speed is that it may have more obvious unsteady characteristics in the turbine at a lower running speed [15,16]. The static temperature imposed at the inlet boundary was 873 K. The residuals of steady simulations were less than 10 5 and errors between inlet and outlet mass flow rates were less than 0.1% after 4000 iteration steps, which meant that good convergence was obtained.…”

Trends in Pulsating Turbine Performance: Pulse-Detonation Driven Axial Flow Turbine

“…It is shown by numerous experimental and computational studies that for comparable time-averaged inlet and exit conditions, the two flows exhibit different time-averaged performance. 6,7,15,16,20,21,26,37,40,41 This deviation from steady behavior is well documented in parametric studies of pulse shape, amplitude, phase, and frequency, upstream geometry, as well as full and partial admission conditions.…”

“…Depending on the pulse shape and amplitude, substantial portions of the cycle may be spent at off-design incidence, and rotor designs with poor off-design performance will suffer accordingly. 20,21 During periods of little or no mass flow rate through the rotor, the turbine undergoes a state of "free-wheeling," and experiences what are known as windage losses. Friction between the approximately quiescent flow at the rotor boundaries effectively decelerates the rotor, generating additional losses.…”

Comparison of Flow Field Between Steady and Unsteady Flow of an Automotive Mixed Flow Turbocharger Turbine

“…They indicated that the method leads to more accurate prediction and proved it by extensive validation procedure with the work of Karamanis [3]. Palfreyman and Martniez-Botas indicated that the flow field in the turbine passage under pulsating flow condition is highly disturbed.…”