The centrifugal compressor of a turbocharger is affected by the opening and closing of the engine intake manifold and consequently experiences a pulsating backpressure. The imposed pulsating conditions generate internal losses and reduce the overall compressor performance and efficiency. However, previous studies have shown that these dynamic conditions can also lead to a surge margin improvement, benefiting the lower limit of stability, in certain conditions. The stability of the compressor arises from the flow behaviour in the inlet region and the interaction between the rotating and stationary components at the inlet. In this paper, the computational flow field of the inlet region of a centrifugal compressor under pulsating flow has been studied. A full stage, 3D URANS model of a compressor exposed to pulsating backpressure has been solved using ANSYS-CFX for the near surge operating point. The transient behaviour of this operating point has been studied at six instances in time within one cycle of the pulse. The flow field is assessed using Mach number, static entropy, static pressure and local flow direction in the inlet region. Flow separation regions have been identified. The authors propose that the variation in flow field at the inlet due to the dynamic response to the imposed boundary condition could explain the improvement in surge margin under pulsating flow conditions for this compressor system.

A high-speed torque meter has been designed and tested for turbocharger applications in order to assess the effect of unsteady boundary conditions. The torque meter is designed using the shaft-twist method by measuring the change in relative angle between the two ends of the drive shaft. Two encoder wheels are mounted on either end of the shaft and a high-speed FPGA measures the variation of time between edges from optical switches. The angular twist in the shaft is calibrated against a reference torque transducer over a range of torque values. The torque meter is installed on an existing compressor rig testing facility at Imperial College London equipped with a pulse generator capable of producing different pulse frequencies and amplitudes. To test the response to unsteady conditions, a centrifugal compressor is driven under a pulsating backpressure while torque is measured at the drive shaft. The torque measurement is compared against static pressure measurements corresponding to the pulsating conditions. Twelve combinations of frequency and pulse amplitudes have been tested. Due to its simple construction and development, the authors believe that where high-speed measurement equipment is already in place, effective torque measurements of this kind can be added with a low cost to help explore unsteady performance in turbomachinery.