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22 3. TURBOCHARGERS, WITH FOCUS ON THE TURBINE for different rotational speeds of the wheel and with pulse frequencies of 40 Hz and 60 Hz. The results showed that the cycle averaged isentropic efficiencies were higher for a mixed flow turbine compared to a radial turbine. By using a cycle averaged efficiency, no phase shifting of the shaft torque is needed, which is beneficial, since the time-resolved isentropic efficiency is strongly affected by the size of the phase shift. Lam et al. (2002) performed a time resolved 3-dimensional numerical study of the pulsatile flow in a radial nozzled turbine. They used the frozen rotor technique to model the rotation of the wheel. The results showed that in- stantaneous performance of the rotor under pulsatile conditions did not vary significantly from the non-pulsatile conditions, and Lam et al. concluded that the wheel can be treated as a quasi-steady device, while the volute must be treated as a non quasi-stationary flow device. Palfreyman & Martinez-Botas (2005) investigated the pulsatile flow in a mixed flow turbocharger with numer- ical methods. They used a medium sized mixed flow nozzle-less turbine. They concluded that the used method with explicit rotation of the wheel, better cap- tures the non quasi-stationary behaviour of the turbine than the method used by Lam et al. (2002). But, this might also be an affect of that Palfreyman and Martinez-Botas used a nozzle-less turbine, without the damping the nozzles can introduce, leading to a more uniform flow into the rotor. At the inlet to the rotor, the incidence angle varied from −92◦ to +60◦, which will give raise to losses, due to strong tip vortices at the leading edge of the blades. The blade torque and the work output fluctuated substantially and with the fre- quency of the pulse. A perturbation with same frequency as the blade passage frequency was also superimposed on the shaft torque trace, which is an effect of the blades passing the tongue. The velocity field within the turbine wheel also varied substantially during the pulse due to poor flow guidance at the en- trance to the turbine wheel. The trajectory of the mass flow versus efficiency showed a hysteresis loop, which encapsulates the quasi-steady values. This is, according to Palfreyman and Martinez-Botas, due to the imbalance between inlet and outlet mass flow during the pulse and the “filling and emptying” of the volute as the turbine acts as a restriction. They also reported a phase lag between the computed isentropic work and the actual work. One way to increase the work output of a turbine working in pulsatile flow is to use some type of active control of the turbine. This can be done by controlling the throat inlet area to the wheel in accordance to the exhaust pulse. This improves the pressure ratio, especially at low mass flow. Pesiridis & Martinez-Botas (2006) tested a mixed flow turbine with a nozzle that was able to alter to the throat inlet area to the turbine wheel. The variable nozzle was of a sliding-wall type restrictor and it was possible to control the throat area in phase and out of phase with the incoming pulses. The actuator wasPDF Image | Numerical computations of the unsteady flow in a radial turbine
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