Xinqian Zheng

Influence of the volute on the flow in a centrifugal compressor of a high-pressure ratio turbocharger

Abstract The asymmetric influence of the volute on the flow in a transonic, high-pressure ratio centrifugal compressor at off-design conditions was investigated. Fully three-dimensional viscous steady-state computational fluid dynamics (CFD) was applied to simulate the flow in a 4.2:1 design pressure ratio compressor for automotive application. Computed performance characteristics are presented for low- and high-pressure ratio operating conditions, with and without an overhung volute. The volute was found to severely harm aerodynamic stability of the investigated compressor when operating at lower than design mass flow. The relative narrowing effect of the volute on compressor map width increases with pressure ratio up to a 42 per cent drop in stable flow range at design speed. The inter-passage variations in performance quantities and the influence of the volute tongue region are discussed in detail. The circumferential variations of incidence angle correlate with rotational speed, which, in combination with the higher sensitivity to incidence angle at transonic inflow conditions, seems to deteriorates stability when transonic inflow conditions are reached.

Development of an advanced turbocharger simulation method for cycle simulation of turbocharged internal combustion engines

Abstract An advanced turbocharger simulation method for engine cycle simulation was developed on the basis of the compressor two-zone flow model and the turbine mean-line flow model. The method can be used for turbocharger and engine integrated design without turbocharger test maps. The sensitivities of the simulation model parameters on turbocharger simulation were analysed to determine the key modelling parameters. The simulation method was validated against turbocharger test data. Results show that the methods can predict the turbocharger performance with a good accuracy, less than 5 per cent error in general for both the compressor and the turbine. In comparison with the map-based extrapolation methods commonly used in engine cycle simulation tools such as GT-POWER®, the turbocharger simulation method showed significant improvement in predictive accuracy to simulate the turbocharger performance, especially in low-flow and low-operating-speed conditions.

Stability Improvement of High-Pressure-Ratio Turbocharger Centrifugal Compressor by Asymmetric Flow Control-Part I: Non-Axisymmetrical Flow in Centrifugal Compressor.

This is Part I of a two-part paper documenting the development of a novel asymmetric flow control method to improve the stability of a high-pressure-ratio turbocharger centrifugal compressor. Part I focuses on the nonaxisymmetrical flow in a centrifugal compressor induced by the nonaxisymmetrical geometry of the volute while Part II describes the development of an asymmetric flow control method to avoid the stall on the basis of the characteristic of nonaxisymmetrical flow. To understand the asymmetries, experimental measurements and corresponding numerical simulation were carried out. The static pressure was measured by probes at different circumferential and stream-wise positions to gain insights about the asymmetries. The experimental results show that there is an evident nonaxisymmetrical flow pattern throughout the compressor due to the asymmetric geometry of the overhung volute. The static pressure field in the diffuser is distorted at approximately 90 deg in the rotational direction of the volute tongue throughout the diffuser. The magnitude of this distortion slightly varies with the rotational speed. The magnitude of the static pressure distortion in the impeller is a function of the rotational speed. There is a significant phase shift between the static pressure distributions at the leading edge of the splitter blades and the impeller outlet. The numerical steady state simulation neglects the aforementioned unsteady effects found in the experiments and cannot predict the phase shift, however, a detailed asymmetric flow field structure is obviously obtained.

Stability Improvement of High-Pressure-Ratio Turbocharger Centrifugal Compressor by Asymmetrical Flow Control—Part II: Nonaxisymmetrical Self-Recirculation Casing Treatment

This is part II of a two-part paper involving the development of an asymmetrical flow control method to widen the operating range of a turbocharger centrifugal compressor with high-pressure ratio. A nonaxisymmetrical self-recirculation casing treatment (SRCT) as an instance of asymmetrical flow control method is presented. Experimental and numerical methods were used to investigate the impact of nonaxisymmetrical SRCT on the surge point of the centrifugal compressor. First, the influence of the geometry of a symmetric SRCT on the compressor performance was studied by means of numerical simulation. The key parameter of the SRCT was found to be the distance from the main blade leading edge to the rear groove (Sr). Next, several arrangements of a nonaxisymmetrical SRCT were designed, based on flow analysis presented in part I. Then, a series of experiments were carried out to analyze the influence of nonaxisymmetrical SRCT on the compressor performance. Results show that the nonaxisymmetrical SRCT has a certain influence on the performance and has a larger potential for stability improvement than the traditional symmetric SRCT. For the investigated SRCT, the surge flow rate of the compressor with the nonaxisymmetrical SRCTs is about 10% lower than that of the compressor with symmetric SRCT. The largest surge margin (smallest surge flow rate) can be obtained when the phase of the largest Sr is coincident with the phase of the minimum static pressure in the vicinity of the leading edge of the splitter blades.

Investigation on a Type of Flow Control to Weaken Unsteady Separated Flows by Unsteady Excitation in Axial Flow Compressors

By solving unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme, the results showed that the disordered unsteady separated flow could be effectively controlled by periodic suction and blowing in a wide range of incidences, resulting in enhancement of time-averaged aerodynamic performances of an axial compressor cascade. The effects of unsteady excitation frequency, amplitude, and excitation location were investigated in detail. The effective excitation frequency spans a wide spectrum, and there is an optimal excitation frequency that is nearly equal to the characteristic frequency of vortex shedding. Excitation amplitude exhibits a threshold value (nearly 10% in terms of the ratio of maximum velocity of periodic suction and blowing to the velocity of free flow) and an optimal value (nearly 35%). The optimal excitation location is just upstream of the separation point. We also explored feasible unsteady actuators by utilizing the upstream wake for constraining unsteady separation in axial flow compressors.

Flow Control of Annular Compressor Cascade by Synthetic Jets

An experimental investigation conducted in a stationary annular cascade wind tunneldemonstrated that unsteady flow control using synthetic jets (zero mass flux) could effec-tively reduce flow separation in the axial compressor cascade. The synthetic jets drivenby speaker were introduced through the casing radially into the flow-field just adjacent tothe leading edge of the compressor cascade. The experimental results revealed that theaerodynamic performance of the compressor cascade could be improved amazingly bysynthetic jets and the maximum relative reduction of loss coefficient was up to 27.5%. Theoptimal analysis of the excitation frequency, excitation location was investigated at dif-ferent incidences. In order to obtain detailed information on flow-field structure, thedigital particle image velocimetry (DPIV) technique was adopted. The experimental re-sults indicated that the intensity of wake vortices became much weaker and streamlinesbecame smoother and more uniform with synthetic jets.

Improvement in the performance of a high-pressure-ratio turbocharger centrifugal compressor by blade bowing and self-recirculation casing treatment

Blade bowing together with a self-recirculation casing treatment was introduced into a high-pressure-ratio turbocharger centrifugal compressor in order to improve the performance. Experiments were conducted to investigate the effects of blade bowing and the self-recirculation casing treatment on the compressor performance. The results showed that, in comparison with the baseline (with radial blades), negative blade blowing increased the choke mass flow rate and the peak efficiency by 3.41% (relative value) and by 4.31% (absolute value) respectively at the designed speed. Furthermore, negative blade bowing together with the self-recirculation casing treatment improved the stable flow range of the compressor by 5.85% at the designed speed. Numerical simulation was conducted to analyse the flow mechanism. The result showed that, in the choke condition, bowing affects the actual throat area and thus changes the choke mass flow rate. Negative bowing enlarges the throat area. In the design condition, bowing affects the migrating process of the secondary flow at the suction surface and redistributes the low-energy fluids along the span. Negative bowing tends to advance the separation of the boundary layer flow at the suction surface and to alleviate accumulation of the low-momentum fluid near the blade tip; these lead to improvement in the compressor efficiency. The self-recirculation casing treatment decreases the effective flow area of the impeller passage and introduces a jet into the flow field near the blade tip, thereby increasing the axial velocity of the fluid near the shroud. This reduces accumulation of the low-energy fluid in the blade tip, thus delaying impeller stall.

Influence of volute-induced distortion on the performance of a high-pressure-ratio centrifugal compressor with a vaneless diffuser for turbocharger applications:

As the geometry of the volute of a turbocharger centrifugal compressor is non-axisymmetric, it causes a distortion at the outlet of the diffuser and influences the upstream components to make the distribution of the flow parameters non-axisymmetric. To quantify the circumferential distortion induced by the volute, a distortion model was proposed to replace the volute by imposing a circumferentially asymmetric pressure distribution. The simulation results prove that this model could reproduce the distortion induced by the volute and, therefore, was sufficient to allow research on the volute’s effect. Based on the distortion model, variable amplitudes of distortion were realized, and the influence of the amplitude of the distortion on the performance of a centrifugal compressor was studied in detail. The results show that the distortion severely harms the aerodynamic stability of the compressor. Larger amplitude distortion cause worse compressor performance. The distortion induced by an asymmetric volute propagates to the upstream components and causes local flow separations in the diffuser and impeller, contributing to compressor surge. When the amplitude of the distortion increases up to 15%, the stable flow range of the compressor decreases 37.7%. The results provide useful guidelines to improve the performance of turbocharger compressors by decreasing the distortion induced by the asymmetric volute.

Separation Control of Axial Compressor Cascade by Fluidic-Based Excitations

Flow separation control was explored on a compressor cascade usi ng three types of fluidic-based excitations: steady suction, steady blowing and synthetic jet. By solving unsteady Reynolds-averaged N-S equations, the effect of excitation parameters (amplitude, angle and location) on the performance were presented. The results showed that the separated flow could be controlled by the fluidic-based actuators effectively and the time-mean performance of flow filed could be remarkably improved. Generally, the positive effect is more visible when the excitation amplitude is increased. The optimal direction varies with each type of excitations and is related to physical mechanisms underlying the separation control. For two types of steady excitations, the most effective jet location is at a distance upstream of the time-mean separation point and the synthetic jet is just at the separation point. NOMENCLATURE

Effect of temperature on the strength of a centrifugal compressor impeller for a turbocharger

High pressure ratio turbocharger technology is used to decrease fuel consumption, reduce emissions and improve power density of an internal combustion engine. The centrifugal compressor is the turbocharger’s core component. The reliability of its impeller becomes critical as the pressure ratio gets higher and the temperature starts playing an important role. In order to study the effect of the flow temperature on the reliability of a centrifugal compressor impeller, solid–fluid coupling is used to calculate the temperature distribution on the impeller surface. This temperature distribution is then applied as boundary condition in three-dimensional finite element analysis to analyze impeller stress. The results show that the percentage of impeller stress caused by thermal load remains approximately constant (about 2%) at different pressure ratios, which does not increase with increasing pressure ratio. Centrifugal load plays an absolutely critical role in the impeller stress at different pressure ratios. High pressure ratio also leads to an increase of air temperature, which causes higher material temperature and consequently the lower ultimate tensile strength of the impeller material. The maximum compressor pressure ratio which the impeller can bear decreases from 4.6 to 4.2 for the researched compressor if the effect of temperature on the ultimate tensile strength was considered. That means the effect of the temperature on compressor impeller strength and reliability at high pressure ratio should be considered while it can be ignored at low pressure ratio.