Impeller-Diffuser Interaction in Centrifugal Compressor

A low speed axial compressor with casing treatment of axial slots was numerically investigated. Time-accurate numerical calculations were performed to simulate unsteady flow in the rotor tip region and the effects of casing treatment on the flow. Since the compressor rotor had a large tip clearance, it was found that the tip leakage flow had an inherent unsteady feature that was not associated with rotor rotation. The unsteadiness of the tip leakage flow was induced by changes in the blade loading due to the pressure distribution formed by the tip leakage flow. This characteristic is called rotating instability or self-induced unsteadiness. The frequency of the flow oscillation was found to decrease as the flow rate was reduced. On the other hand, as expected, the operating range was improved by casing treatment, as shown by calculations in good agreement with the experimentally measured data. The unsteadiness of the tip leakage flow was alleviated by the casing treatment. The interaction between the flow in the tip region and the re-circulated flow through the axial slots was observed in detail. The removal and injection of flow through the axial slots were responsible not only for the extension of the operating range but also for the alleviation of the unsteadiness. Analyses of instantaneous flow fields explained the mechanism of the interaction between the casing treatment and the unsteady oscillation of the tip leakage flow. Furthermore, the effects of changes in the amount of re-circulation and the location of the removal and injection flow on the unsteadiness of the tip leakage flow were examined.

Self-recirculating injection, which bleeds air from the downstream duct of the last blade row and injects air as a wall jet upstream of the first rotor blade row, is experimentally investigated after the design of its structure in single- and three-stage axial flow compressors. External injection and outlet bleed air are selected for comparison. Results show that self-recirculating injection can improve the stall margin by 13.67% and 13% on the premise of no efficiency penalty in single- and three-stage axial flow compressors with only 0.7% and 4.2% of the total injected momentum ratio recirculated near stall, respectively. The self-recirculating injection is the best among all the three cases if the influence on pressure rise coefficient and efficiency is comprehensively considered. Moreover, findings indicate that the upstream injection plays an important role in terms of stability-enhancement. The details of the flow field are captured using a collection of pressure transducers on the casing with circumferential and chordwise spatial resolution. A detailed comparative analysis of the endwall flow indicates that the self-recirculating injection can postpone the occurrence of stalling in the proposed compressor by delaying the forward movement of the interface between the tip leakage flow (TLF) and main stream flow (MF), weakening the unsteadiness of TLF (UTLF), and sharply decreasing the circumferentially propagating speed dominated by the UTLF that triggers the spike-type stall inception. Finally, the stall control concept on the stage that first generates stall inception using self-recirculating injection is proposed. This study helps to guide the design of self-recirculating injection in actual application.

In this paper, an unsteady tip leakage flow phenomenon is identified and investigated in a centrifugal compressor with a vaneless diffuser at near-stall conditions. This phenomenon is associated with the inception of a rotating instability in the compressor. The study is based on numerical simulations which are supported by experimental measurements. The study confirms that the unstable flow is primarily driven by the Kelvin-Helmholtz instability inherent in the shear layer between the main-stream flow and the tip leakage flow. The shear layer instability induces large scale vortex shedding which propagates circumferentially, leading to pressure perturbations with short wavelength and high amplitude which rotates at about half of the blade speed. The shed vortex is also found to interact with the main blade leading edge, resulting in the reduction of the blade loading identified in the experiment. The paper also reveals that the downstream volute imposes a long wavelength circumferential non-uniform back pressure at the impeller exit, which is found to either excite or supress the vortex shedding within the blade passage depending on the relative circumferential position of the impeller and the volutes.

As the first step of the study of cavitation in the tip leakage flow of turbomachinery, experimental and numerical studies were carried out to investigate the characteristics of cavitation in the 2-D unsteady tip leakage flow. The numerical method was constructed by combining the vortex method with taking into account the effects of cavity growth. Three types of cavity models are examined. It is shown that one of them simulates the location and the size of the cavity fairly well. Comparisons with experiments are made for three values of initial pressure and for three values of tip clearance. The model can predict those effects and it is expected that the present model is useful for the estimation of cavitation characteristics of 3-D tip leakage flow.

Micro tip injection is a stall control technique in which engineers inject small-flowrate yet high-velocity jets into the tip region of axial compressor rotor blades to extend the stable operation range, while keeping the entire compressor characteristic line nearly unchanged. This paper summarizes the related research performed in Chinese Academy of Sciences in the past several years, ranging from understanding the fundamental mechanisms to demonstrating the technology in a laboratory compressor. A brief review in tip injection research indicates that for each compressor, there exists a critical injection momentum ratio, below which the injection flows may work differently from those over the critical value. Detailed casing unsteady static pressure measurements and the related sophisticated data processing proved the existence of self-induced unsteady tip leakage flow, which was also confirmed by unsteady CFD simulations. It is showed that this kind of unsteadiness in tip leakage flow might be responsible to make the auto-correlation coefficient drop significantly as the compressor operated at lower flowrate close to stall limit. This warning signal, which was first successfully used by other research groups with little care taken on its flow mechanism, appears much earlier than the classical modal-wave or spike stall precursors. Micro tip injection was effective if it interacts with the unsteady tip leakage flow long before the stall precursor emerges. As a technology demonstration, a DSP board with a build-in auto-correlation early-stall-warning algorithm and an on-off controller of injection valve worked well to drive a micro tip injection system and successfully delayed the stall in a low-speed compressor with much less jet flows than the continuous micro tip injection.

In compression systems the stable operating range is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to full scale instability: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown (so-called “spike” stall inception). The cause of the latter is not well understood; in axial machines it has been suggested that rotor blade-tip leakage flow plays an important role, but spikes have recently been observed in shrouded vaned diffusers of centrifugal compressors where these leakage flows are not present, suggesting an alternative mechanism may be at play. This paper investigates the onset of instability in a shrouded vaned diffuser from a highly loaded turbocharger centrifugal compressor and discusses the mechanisms thought to be responsible for the development of short-wavelength stall precursors. The approach combines unsteady 3D RANS simulations of an isolated vaned diffuser with previously obtained experimental results. The unsteady flow field simulation begins at the impeller exit radius, where flow is specified by a spanwise profile of flow angle and stagnation properties, derived from single-passage stage calculations but with flow pitchwise mixed. Through comparison with performance data from previous experiments and unsteady full-wheel simulations, it is shown that the diffuser is accurately matched to the impeller and the relevant flow features are well captured. Numerical forced response experiments are carried out to determine the diffuser dynamic behavior and point of instability onset. The unsteady simulations demonstrate the growth of short-wavelength precursors; the flow coefficient at which these occur, the rotation rate and circumferential extent agree with experimental measurements. Although the computational setup and domain limitations do not allow simulation of the fully developed spike nor full-scale instability, the model is sufficient to capture the onset of instability and allows the postulation of the following necessary conditions: (i) flow separation at the diffuser vane leading edge near the shroud endwall; (ii) radially reversed flow allowing vorticity shed from the leading edge to convect back into the vaneless space; and (iii) recirculation and accumulation of low stagnation pressure fluid in the vaneless space, increasing diffuser inlet blockage and leading to instability. Similarity exists with axial machines, where blade-tip leakage sets up endwall flow in the circumferential direction leading to flow breakdown and the inception of rotating stall. Rather than the tip leakage flows, the cause for circumferential endwall flow in the vaned diffuser is the combination of high swirl and the highly non-uniform spanwise flow profile at the impeller exit.

This study presents a numerical simulation of precursors and characteristics of rotating stall in a centrifugal compressor with vaned diffuser. The aim is to further understand the causes of stall inception, characteristics of stall process and to investigate reliable simulation of stall control methods and aid in their optimization. Initial numerical results show that the tip leakage flow and early separation in the impeller blades are causing weakness of radial velocity and deviation of fluid angle at impeller exit. Also, results show that the vaneless region between impeller and diffuser near shroud surface is the key factor of developing radial velocity deficiency into stall cells leading to pressure fluctuations on impeller blades. The present study confirms that injection of air at the shroud surface of the vaneless region has an effect on modifying fluid angle entering diffuser and on delaying of stall inception to occur at lower mass flow rates.

In this effort, 3D CFD simulations are carried out for real gas flow in a refrigeration centrifugal compressor. Both commercial and the in-house CFD codes are used for steady and unsteady simulations, respectively. The impact on the compressor performance with various volute designs and diffuser modifications are investigated with steady simulations and the analysis is focused on both the diffuser and the volute loss, in addition to the flow distortion at impeller exit. The influence of the tongue, scroll diffusion ratio, diffuser length, and cross sectional area distribution is examined to determine the impact on size and performance. The comparisons of total pressure loss, static pressure recovery, through flow velocity, and the secondary flow patterns for different volute designs show that the performance of the centrifugal compressor depends upon how well the scroll portion of the volute collects the flow from the impeller and achieves the required pressure rise with minimum flow losses in the overall diffusion process. Finally, the best design is selected based on compressor stage pressure rise and peak efficiency improvement. An unsteady simulation of the full wheel compressor stage was carried out to further examine the interaction of impeller, diffuser and the volute. The unsteady flow interactions are shown to have a major impact on the performance of the centrifugal stage.

It is well known that tip leakage flow has a strong effect on the compressor performance and stability. This paper reports on a numerical investigation of detailed flow structures in an isolated transonic compressor rotor-NASA Rotor 37 at near stall and stalled conditions aimed at improving understanding of changes in 3D tip leakage flow structures with rotating stall inception. Steady and unsteady 3D Navier-Stokes analyses were conducted to investigate flow structures in the same rotor. For steady analysis, the predicted results agree well with the experimental data for the estimation of compressor rotor global performance. For unsteady flow analysis, the unsteady flow nature caused by the breakdown of the tip leakage vortex in blade tip region in the transonic compressor rotor at near stall condition has been captured with a single blade passage. On the other hand, the time-accurate unsteady computations of multi-blade passage at near stall condition indicate that the unsteady breakdown of the tip leakage vortex triggered the short length-scale — spike type rotating stall inception at blade tip region. It was the forward spillage of the tip leakage flow at blade leading edge resulting in the spike stall inception. As the mass flow ratio is decreased, the rotating stall cell was further developed in the blade passage.

This paper discusses the various elements entering into the determination of the true maximum rise in pressure which may occur in closed conduits. It shows that, contrary to the usual belief, the maximum rise in pressure will not necessarily be produced when the full flow in a given conduit is cut off, but is a function of the initial flow from which the deceleration starts. The necessity of establishing the relation between the average conduit velocity and the rate of gate travel for various proportional strokes is cited. Comparison is made of the computations by several of the accepted methods based on the elastic wave theory, and a translation of important studies made by M. Maurice Gariel in France is included. The application to practical cases is outlined and specific examples are worked out in detail. A general summary of water-hammer computations is given with several simple formulas for the rapid calculation of the critical conditions.

For a centrifugal compressor with volute, the flow field is circumferentially nonuniform because of the volute asymmetrical structure and leads to a circumferential difference in the tip leakage flow. In this work, the compressor performance and the casing wall static pressure distribution are measured, and the results are compared with the time-averaged results of the unsteady calculation to verify the reliability of the simulation. The results show a relationship between the tip leakage vortex trajectory and the high static pressure region in the diffuser, based on which a prediction model is established for the reverse propagation of pressure waves caused by a volute tongue. Influenced by the volute asymmetric structure, the trajectory, shape, and strength of the tip leakage vortex at different circumferential positions differs significantly. The tip leakage vortex trajectory affected by the high static pressure is more inclined to a circumferential direction because the tip leakage flow velocity flowing out of the suction surface is reduced, and the tip leakage flow with low velocity is subjected to the high-pressure gradient in a passage. Moreover, the tip leakage vortex breakdown in different passages differs significantly. A tip leakage vortex core more inclined towards the streamwise direction is more likely to break down than a tip leakage vortex core inclined towards the circumferential direction because of the larger reverse pressure gradient.

As the most critical part of a hydropower station, reversible pump–turbines (RPTs) are facing the trend of high parameterization (e.g., high head and high rotation speed) with the rapid development of hydropower, which may cause more detrimental cavitation-induced hydrodynamic forces. Considering that cavitation occurs more easily under pump mode than turbine mode for the RPT, the characteristics and generation mechanism of the hydrodynamic radial force induced by cavitation of a RPT model at pump mode are investigated by computational fluid dynamics and experimental methods. Combined with the shear stress transport (SST) turbulence model, the three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved to calculate the cavitation flow of the RPT at various cavitation states. The blade loading and the pressure pulsations of the pressure/suction surfaces of the blade are monitored from the inlet to the outlet. Results show that with the development of cavitation, increasing cavitation bubbles covering the blade surfaces lead to the decrease of the blade loading, weaken the regularity of the pressure fluctuation, and increase the amplitude of the pressure pulsation, especially the suction surface. Furthermore, the characteristics of hydrodynamic radial forces under various cavitation states are discussed, and it is found that the values of the hydrodynamic radial forces become larger and the alternating hydrodynamic radial forces are more remarkable with the development of cavitation. Meanwhile, the symmetry of the radial force of the impeller presents a slight change due to the rotor–stator interaction between impeller and guide vanes and the symmetrical structure of the guide vanes. Then, to explore the generation mechanism of the hydrodynamic radial forces, the frequency spectrum of the pressure pulsation and hydrodynamic radial forces are analyzed comparatively. It is found that the hydrodynamic radial forces are affected by the rotor–stator interaction in the initial stage of cavitation, whereas the influence of the pressure distribution of suction surface of the blade increases with the development of cavitation.

State-of-the-art gas turbines operate at extreme turbine inlet temperatures to maximize thermal efficiency and specific work, which increases demands on sealing and cooling technologies. The stator-rotor cavity is the wheel-space between the stationary and the rotating disks of a turbine. A rim seal and a purge flow mitigate hot gas ingress from the mainstream annulus into the cavity. Appropriate sealing of the cavity is key to protecting the inner turbine disk from thermal damage. Additionally, the purge flow cools the blade roots and platform. There are two primary ingress types: externally induced and rotationally induced. Externally induced ingress is driven by circumferential variations in annulus pressure that create regions of ingress and egress at the seal lip. Rotationally induced ingress is driven by the rotating disk pumping effect, resulting in egress near the rotor disk and ingress near the stator disk. The priority of engine designers is to minimize the purge mass flow bled from the compressor to seal the cavity, which improves engine efficiency. Yet, accurate prediction of the minimum sealing flow still relies on costly experiments and high-fidelity simulations. The dissertation presents the development of experimental and numerical tools for the study of ingress in turbine rim seals. First, it presents a computational design strategy that combines two-dimensional Reynolds-averaged Navier-Stokes simulations of parametrized rim seal geometries with three-dimensional unsteady Reynolds-averaged Navier-Stokes simulations. The two-dimensional computations accelerate geometry screening, quantifying rim seal pressure drop and cavity heat transfer sensitivity to geometrical parameters. The three-dimensional computations serve to assess sealing effectiveness. The challenges of simulating turbine rim seal ingress and predicting minimum purge mass flow to seal the cavity are highlighted. Second, it presents the design, modeling, construction, and commissioning of PT3, a high-speed open-return continuous wind tunnel. The facility is modular with lightweight components for rapid multi-configuration testing campaigns. An auxiliary heated injection system with accurate temperature and mass flow control has also been developed for turbine rim seal studies. The performance and flow quality of the facility are characterized. A reduced-order model that predicts operating lines and component losses is implemented and validated. Third, it presents the design and aerothermal characterization of the Advanced Stationary Test Article for the Study of Externally Induced Ingress in Turbine Rim Seals (ASTER). It consists of a cylinder array in a linear test section that imposes a nonuniform pressure field upstream of an axial rim seal. Three-dimensional unsteady Reynolds-averaged Navier-Stokes simulations support the design of the test article. An extensive experimental campaign evaluates the effects of mainstream Mach number, purge mass flow, and rim seal clearance on temperature-based sealing effectiveness. The minimum purge mass flow required to seal the cavity for different configurations is computed, and the flow physics are studied, showing consistent behavior between experiments and computations. Thus, this dissertation provides a framework for the aerothermal characterization of ingress in turbine rim seals.

To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) kappa - omega simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90^{circ }-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.

This paper presents a numerical comparison of sealing performance between conventional radial rim seal and seven different kinds of groove radial rim seal with three coolant flow rates. Three-dimensional unsteady Reynolds-Averaged Navier-Stokes (URANS) equations, coupled with a fully developed shear stress transport (SST) turbulent model from ANSYS-CFX, are utilized to investigate the sealing effectiveness of rim seal and flow characteristics in the wheel-space cavity of gas turbines. The numerical method for the pressure distributions on the vane hub and sealing effectiveness of rim seal is validated on the basis of published experimental data. The seven kinds of groove rim seals designed in this work include four circumferential groove structures, one axial groove structure and two oblique groove structures. The sealing effectiveness of conventional and seven different groove rim seals are compared. Then the flow field in the disc cavity of conventional and groove rim seals is simulated and analyzed. Compared with conventional radial rim seal, the groove rim seals can increase the sealing effectiveness obviously. It is shown that the groove structures improve the damping dissipation of invading gas. The number of grooves has an obvious effect on sealing effectiveness of circumferential groove rim seals. The axial groove case results in less minimum sealing flow rate than oblique groove cases. In addition, the flow pattern of rim seal and disc cavity is used to describe rim seal gas ingestion flow characteristics.