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3.4. TURBINE PERFORMANCE UNDER PULSATILE FLOW 21 the turbine volute can be treated as being quasi-stationary, since the rate of change of pressure with respect to distance was much greater than it was with respect to time. Since the shaft torque measurements were time-resolved, they could compare them to the time resolved pressure traces at the inlet, and the phase lag between the pressure trace and the torque trace was about 40◦. The time this phase lag corresponds to, is larger than the pressure wave travel time from the inlet to the rotor, and hence the phase lag do not only depend on the wave propagation time. Winterbone et al. also reported that the mass flow pulses were approximately in phase with the pressure pulses, but the mass flow trace was quite noisy, so it was hard to determine any phase shift. The rotational speed of the wheel varied with approximately 2% during the pulse. Wallace et al. (1969) investigated the effects of different frequencies, mass flows and turbine speeds. They used a dynamometer to measure the shaft torque of the turbine, and this dynamometer could be operated up to 100 000 rpm. The frequency of the pulsatile flow was varied from 16.7 Hz to 50 Hz, and they concluded that the turbine shaft torque first increased and then decreased with increasing frequency. The shaft power diminished slightly with increasing mass flow for constant rotational speed of the turbine wheel and mean inlet pressure to the turbine. Benson & Scrimshaw (1965) conducted experiments where the pulse frequency was varied from 30 Hz to 70 Hz, and the rotational speed of the turbine wheel was varied from 30 000 rpm to 60 000 rpm. The used turbine was a nozzled radial turbine with twin entries. The results showed also that the efficiency increased up to a pulse frequency of 60 Hz, and then decreased with further increase in pulse frequency. Dale & Watson (1986) measured the shaft power of a twin entry turbine working in pulsatile flow. They concluded that the efficiency varied with vary- ing admissions in the two entries, where the lowest efficiency occurred when the flow entered only one of the two entries. For an operation point with a pulse frequency of 40 Hz, the maximum deviation of the instantaneous efficiency from the quasi-stationary assumption was up to 10%, with both higher and lower values than the efficiency from the quasi-stationary assumption. Kara- manis et al. (2001) concluded from an experimental study, that the deviation of isentropic efficiency from the quasi-stationary assumption was reduced as the pulse frequency was increased for a mixed flow turbine. The frequency of the pulsatile flow was 40 Hz and 60 Hz, respectively. When they computed the isentropic efficiency, they applied a correction for the phase lag, which corre- sponded to the sonic travel time from the inlet to the wheel. The cycle-averaged efficiency was lower than the corresponding steady-state efficiency, due to the large variation of flow conditions. The incidence angle varied from -80◦ to 40◦ during the pulse. In the experiment conducted by Arcoumanis et al. (1999) the steady and unsteady performance of a mixed flow turbine was investigatedPDF Image | Numerical computations of the unsteady flow in a radial turbine
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