Unsteady Responses of the Impeller of a Centrifugal Compressor Exposed to Pulsating Backpressure

Abstract

The flow in intake manifolds of internal combustion engine becomes more unsteady because of engine downsizing which is achieved by reducing cylinder number and increasing the boosting. Turbocharger compressor is thus exposed to the enhanced pulsating backpressure. This paper studies responses of a centrifugal compressor to the pulsating backpressure via experimentally validated numerical method. Firstly, CFD model with the volute and all impeller passages is established and validated by experimental measurements. Then the unsteady three-dimensional simulation is conducted on a single passage imposed by the pulsating backpressure conditions which are obtained by 1-D unsteady simulation. Results show that the performance of the passage evidently deviates from steady performance. Hysteresis loops of the performance appear at pulsating backpressure conditions, which encapsulate the performance at steady conditions. Moreover, the unsteadiness of the impeller performance is enhanced as the mass flow rate reduces. The performance and flow structures of the impeller near stall for the pulsating case are more favorable than that at corresponding constant backpressure. Furthermore, flow structures at points with the same instant mass flow rate are also notably different when they are located at different strokes of the pulse. The flow in the impeller is determined by not only the instant boundary condition but also the evolution history of flow field. The dynamic stall which is analogue to the phenomenon on pitching airfoil happens in the compressor and delays the instability of the device when it is exposed to pulsating backpressure. This study provides useful insights in the influence of pulsating backpressure on compressor performance in actual engine situations, from which better turbo-engine matching might be benefited.

Investigation on the unsteadiness of centrifugal compressor exposed to pulsating backpressure

“…According to previous research, the secondary flow can evidently influence the inertia of flow in the impeller. 20,23 Therefore, a coefficient β is introduced here to represent the influence of secondary flow structures on the characteristic time. A value equals 1 is applied when compressor is in stable operating region, then it increases when compressor is unstable at near surge condition, with the highest value 2 according to CFD results.…”

“…Furthermore, in most of the 1D models, the performance of turbomachinery is considered as quasisteady. 18,19 However, it has been confirm that the times scale of the evolution of flow structures in rotor cannot be neglected, 20,21 which implies that the performance of compressor is no longer quasi-steady. On the other hand, the existence of secondary flow structures leads to a ''lagging effect'' of the respond of compressor to the pulsation, resulting in a discrepancy of compressor performance from the quasi-steady state.…”

“…As pointed out by some researches, the flow field in the turbomachinery under pulsating condition deviates from the quasi-steady scenario due to the comparable time scales between the evolution of flow structures and the pulsation period. 20,23 Hence, the steady performance cannot be used directly, and the time lag between compressor responding and the pulsating boundary condition should be considered. Therefore, an one-order lag equation is implemented in the code to calculate the source terms at transient condition, as shown in equation ( 7):…”