In an internal combustion engine, the centrifugal compressor is placed upstream of the inlet manifold and therefore, it is exposed an unsteady flow regime caused by the inlet valves of the cylinder arrangement. This valve motion sets a pulsating state at the compressor exit, having greater influence when the operation is near the surge margin of the compressor. This paper presents the experimental results of the evaluation of the surge dynamics on a compressor with induced downstream pulsating flow. Different pulsation levels are achieved by the variation of three different parameters on the induced pulse: pulse frequency, amplitude, and system storage volume (plenum). Each pulse parameter was evaluated independently in order to assess its effect on the compressor stability limit. The main effect on the surge margin of the compressor was found to be due to the presence of a storage volume in the system for all cases (steady/pulsating condition) and at all frequencies. It was found that the magnitude of the pulse frequency determines the hysteresis behavior of the system that leads to a phase difference between the convected terms and the acoustic dominated terms, and therefore this affects the onset of flow instability, surge, in the compression system under study.

It is well known that compressor surge imposes a significant limit on the flow range of a turbocharged internal combustion engine. The centrifugal compressor is commonly placed upstream of the inlet manifold, and hence, it is exposed to the intermittent flow regime of the inlet valves. Surge phenomena have been well studied over the past decades, and there still remains limited information with regard to the unsteady impact caused by the inlet valves. This study presents an experimental evaluation of such a situation. Engine representative pulses are created by a downstream system comprising a large volume, two rotating valves, a throttle valve, and the corresponding pipe network. Different pulsation levels are characterized by means of their frequency and the corresponding amplitude at the compressor inlet. The stability limit of the system under study is evaluated with reference to the parameter B proposed by Greitzer (1976, “Surge and Rotating Stall in Axial Flow Compressors—Part II: Experimental Results and Comparison With Theory,” ASME J. Eng. Power, 98(2), pp. 199–211; 1976, “Surge and Rotating Stall in Axial Flow Compressors—Part I: Theoretical Compression System Model,” ASME J. Eng. Power, 98(2), pp. 190–198). B describes the dynamics of the compression system in terms of volume, area, equivalent length, and compressor tip speed as well as the Helmholtz frequency of the system. For a given compressor, as B goes beyond a critical value, the system will exhibit surge as the result of the flow instability progression. The reduced frequency analysis shows that the scroll diffuser operates in an unsteady regime, while the impeller is nearly quasi-steady. In the vicinity of the surge point, under a pulsating flow, the instantaneous operation of the compressor showed significant excursions into the unstable side of the surge line. Furthermore, it has been found that the presence of a volume in the system has the greatest effect on the surge margin of the compressor under the unsteady conditions.

Turbocharging has become a fundamental technology to realize engine downsizing, which is an attractive strategy for low carbon vehicles in the near term. The stable operation of turbocharger compressors at low and high flow rates is crucial to provide peak torque demand and rated power for turbocharged automotive engines. The scroll or volute is a key component in centrifugal compressors as its design not only impacts the compressor efficiency but also affects the operating range. This component causes a distorted pressure field upstream which can contribute to stall on the impeller, inducing surge. As the flow inside the volute is fully three dimensional and turbulent, a better understanding of flow mechanisms is key to enable a volute design methodology. In this study, a centrifugal compressor stage has been modelled numerically and validated by experimental results, to identify the geometric parameters of the volute which contribute to the main flow losses. By solving Reynolds average Navier-Stokes (RANS) equations using a commercial code, the threedimensional flow field of the compressor was modelled. Based on detailed analysis of this flow field, and the impact of various geometric parameters, an optimized volute was developed. The results showed that the total-to-total isentropic efficiency and surge margin could be improved by 1.5% and 4.5%, respectively at design speed.

It is well known that compressor surge imposes a significant limit on the flow range of a turbocharged internal combustion engine. The centrifugal compressor is commonly placed upstream of the inlet manifold and hence, it is exposed to the intermittent flow regime of the inlet valves. Surge phenomena has been well studied over the past decades, there still remains limited information with regards to the unsteady impact caused by the inlet valves. This study presents an experimental evaluation of such a situation. Engine representative pulses are created by a downstream system comprising a large volume, two rotating valves, a throttle valve and the corresponding pipe network. Different pulsation levels are characterized by means of their frequency and the corresponding amplitude at the compressor inlet. The stability limit of the system under study is evaluated with reference to the parameter B proposed by Greitzer [7–9]. B describes the dynamics of the compression system in terms of volume, area, equivalent length and compressor tip speed as well as the Helmholtz frequency of the system. For a given compressor, as B goes beyond a critical value, the system will exhibit surge as the result of the flow instability progression. The reduced frequency analysis shows that the scroll-diffuser operates in an unsteady regime, while the impeller is nearly quasi-steady. In the vicinity of the surge point, under a pulsating flow, the instantaneous operation of the compressor showed significant excursions into the unstable side of the surge line. Furthermore, it has been found that the presence of a volume in the system has the greatest effect on the surge margin of the compressor under the unsteady conditions.

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.