The development of a novel unsteady flow control method: Controlling the rotating nozzle ring

The turbocharger is continuously fed with highly unsteady exhaust flow from a reciprocating engine. Despite that, the pulsating exhaust flow can provide more kinetic energy to the turbine compared with the moderated flow in a constant pressure turbocharging, it still significantly deteriorates the efficiency of the turbocharger, as the turbocharger turbine works at off-design point at most instances in an exhaust cycle. In order to address the issue, a novel mechanism named ‘rotating nozzle ring’ has been developed. It was shown that by rotating a nozzle ring around the turbine, the deviation of the flow angle from the design point can be reduced and, therefore, the performance of the turbocharger can be improved. This novel idea is further presented in this paper, by introducing a passive control method to control the speed of the rotating nozzle ring. It will be demonstrated that the rotating nozzle ring can be controlled by the exhaust flow by means of pre-setting a particular nozzle angle, and the rotation will stabilise at an approximately constant speed. An optimised nozzle profile will also be presented, with the intention to reduce the incidence loss on the rotating nozzle ring. A detailed full-stage computational fluid dynamics model will be built to investigate this passive control method. Results of both quasi-steady and transient calculation will demonstrate that, the passively controlled rotating nozzle ring can effectively suppress the unsteadiness level of turbine’s unsteady operation. As a result, the performance of the turbocharger turbine is improved, with the variation of the velocity ratio through a pulse cycle reduced by 8.5% and the isentropic energy weighted cycle averaged efficiency increased by 4.7%, compared to a traditional stationary nozzle ring.