Circumferential Groove Casing Research Articles

Effects of the circumferential casing groove configuration on aerothermal performance of a transonic turbine stage

In order to control tip leakage flow and improve the tip aerothermal performance, a circumferential casing groove (CG) configuration is proposed. Taking the typical transonic turbine stage TTM (Thermal Turbomachinery and Machine Dynamics) as the research object, the effects of CG with different heights (b), initial locations (IL) and widths (w) on the aerodynamic loss of the turbine stage and heat transfer characteristics of the tip region are investigated numerically. Results indicate that due to the introduction of CG, the vortex system topology in the tip region changes remarkably. Tip leakage vortex (TLV) is separated into two parts, with TLV-1 being entrained and dissipated by upper passage vortex (UPV) and TLV-2 rebuilding behind the groove and extending to the trailing edge. The interaction between TLV and UPV changes with the development of both. Also, complicated flow-thermal interplays lead to the change of thermal load in the tip region, which attributes to the difference of local turbulence mixing intensity. More quantitatively, a CG with the groove of b = 3.0 g, IL = 0.20 Cax and w = 0.05 Cax can increase ηTS by 0.38%, and reduce the area-averaged heat transfer coefficient (HTC) by 5.28% and 1.49% for the tip and casing surface, respectively, with a slight increase of area-averaged HTC on the blade by 0.29%. Overall, CG with appropriate geometrical parameters can improve aerothermal performance of the turbine tip significantly, without any serious increase of thermal load on the blade.

Numerical Investigation of the Interaction of a Circumferential Groove Casing Treatment and Near-Tip Modifications for a Highly Loaded Low-Speed Rotor Under the Influence of Double Leakage

Abstract Eckel et al. (2023, “Numerical Investigation of Near-Tip Modifications for a Highly Loaded Low-Speed Rotor Under the Influence of Double Leakage,” ASME J. Turbomach., 145(4), p. 041003) proposed using a convex-profiled pressure side region close to the tip, known as belly, as an effective method of extending the operating range of low-speed axial compressor rotors. In the literature, circumferential grooves are another well-described technique for improving the stable working range of a compressor rotor. No research has been conducted to date to determine which modification is more effective and how they interact when used together. This paper numerically investigates the influence of circumferential casing grooves and near-tip modifications on the flow field in the tip region of a highly loaded, low-speed axial compressor rotor. The simulated rotor consists of a hybrid blade configuration with a tandem profile in the mid-span region and single blade profiles near the endwalls. The single blade profile close to the tip features three different convex-profiled elements, which differ in their respective thicknesses. The aim of the numerical analysis is to explain the interaction of the secondary flow phenomena when applying the circumferential grooves and the belly geometries. For this purpose, eight different axial positions of the circumferential groove are investigated for each of the three belly configurations. These are arranged in 10% increments from −7% to 63% along the axial rotor tip chord. The potential of the concept is evaluated by a numerical investigation in the 1.5-stage setup with an inlet guide vane and tandem stator. It is shown that a circumferential groove can further increase the operating range for all belly configurations when positioned axially correctly. In this respect, equalization of the near-casing deceleration in the circumferential direction leads to an extension of the stall margin with both modifications. Concentrated regions of low-momentum fluid with a large extent in the radial direction should be avoided consequently. A tip vortex stability factor is introduced to quantitatively evaluate this effect. The operating range can thus already be estimated in a first approximation at the design point. In general, the groove and belly should be positioned where the tip leakage vortex meets the pressure side of the adjacent blade. If the groove is used together with the belly, the leading edge of the former should be situated at the location of maximum thickness of the near-tip modification. The effects of the circumferential groove and the belly are then superimposed. If using only one modification, the belly appears better suited for ensuring an extension of the operating range while maintaining high efficiencies.

Influence of Casing Groove on Rotating Instabilities in a Low-Speed Axial Compressor

AbstractRotating instability (RI) can be regarded as a pre-stall signal because such phenomena are usually detected when a compressor operates at a near-stall point. Since RI is closely relevant to aerodynamic, acoustic, and structural issues, its mechanism has attracted industrial concerns and in-depth analysis. In this paper, a low-speed axial single compressor rotor row is selected as the baseline research object. A circumferential casing groove is mounted over the shroud. The solid casing and the casing treated configurations were simulated using the zonal large eddy simulation model, and the simulations present remarkable predictions in capturing the averaged and dynamic characteristics of the investigated compressor. By implementing a data-driven method called dynamic mode decomposition, the mode with the highest amplitude is proved to be correlated to the dominant RI frequency. The spatial and temporal property of the RI is visualized. The results show that the casing groove has limited influence on the averaged flow field but manipulates the dynamic features. The groove allows the RIs to shift from around 0.5 blade passing frequency to higher frequencies in the upstream region and effectively suppresses the unsteadiness in the zones under and downstream of the groove. The RIs are triggered by the flow interaction of the tip leakage flow at the front part of the blade. It can be depicted as pressure waves staggering with the rotor blade and propagating over the blade tip region at around half of the blade speed. With the existence of the casing groove, the dominant wavelength changes, and the fluctuation is damped under the groove, which finally modifies the properties of RIs.

Stall Margin Improvement in a Transonic Compressor With a Casing Treatment: Flow Mechanism

Abstract A compressor casing treatment with circumferential casing grooves (CCGs) is studied in detail. The primary objective of the current paper is to unearth the main driving fluid mechanism that changes the stall margin in a transonic compressor with CCGs. Large eddy simulation (LES) is applied to calculate the transonic compressor flow fields with and without CCGs. The present investigation shows that CCGs reduce the mass flow through the tip gap by about 16% near the stall condition. Calculated flow fields show that most of this reduction of tip leakage flow occurs near the CCGs. Reinjected flow from the CCGs pushes the tip leakage flow radially inward below the casing and changes how the tip leakage flow collides with the incoming main passage flow. However, a detailed examination of the calculated flow in the tip region shows that the reinjected flow does not contribute to the reduction of the overall blockage generation. The primary driver for reducing blockage generation with CCGs is the reduction of overall mass flowrate through the tip gap. In the present investigation, measurements show a very small decrease in efficiency with CCGs at the design flow condition, although the difference in efficiency is within the measurement uncertainty. Results from the LES simulation at the design condition with CCGs show that the tip leakage vortex (TLV) is pulled toward the blade suction side and double leakage flow is eliminated. The result is that the simulated efficiencies with and without CCGs are almost the same.

Stall margin improvement in a low-speed axial compressor rotor using a blockage-optimised single circumferential casing groove

The stall margin of tip-critical axial compressors can be improved by using circumferential casing grooves. From previous studies, in the literature, the stall margin improvement due to the casing grooves can be attributed to the reduction of the near casing blockage. The pressure rise across the compressor as the compressor is throttled intensifies the tip leakage flow. This results in a stronger tip leakage vortex that is thought to be the main source of the blockage. In this paper, the near casing blockage due to the tip region aerodynamics in a low-speed axial compressor rotor is numerically studied and quantified using a mass flow-based blockage parameter. The peak blockage location at the last stable operating point for a rotor with smooth casing is found to be at about 10% of the tip chord aft of the tip leading edge. Based on this information, an optimised single casing groove design that minimises the peak blockage is found using a surrogate-based optimisation approach. The implementation of the optimised groove is shown to produce a stall margin improvement of about 5%.

CFD Investigation of an Axial Compressor with Casing Treatment for the Enhancement of the Stall Margin

Casing treatment is an efficient technique that is used to increase the compressor stall margin with a minor reduction in efficiency. The interaction between the blade passage and the groove is the main cause of the stall margin improvement by various researches. A numerical study is conducted on a single circumferential casing groove using NASA rotor 37 in the current study. The performance of the two circumferential groove models is studied by discretizing RANS 3D equations using a finite volume technique. The inception of the stall is predicted according to the criteria of convergence. Two models of the circumferential groove have been suggested and numerically tested. A single passage simulation is selected for the two models. The performance of the smooth casing and the two models are analyzed. Moreover, the stall margin, total pressure ratio, and peak adiabatic efficiency of the normal casing, and the two models are analyzed to determine the influence of the groove on the axial compressor performance and stability. Models 1 and 2 stall margins are enhanced by 6.62 % and 4.45% respectively. The adiabatic efficiency of model 1 and model 2 are decreased by 0.79 % and 1.08 % respectively.

Recirculation-groove coupled casing treatment for a transonic axial compressor

This paper investigates a novel casing treatment for an axial compressor that combines a circumferential casing groove with flow recirculation channels. This casing treatment is located in the rotor's tip region and comprises a single shallow circumferential groove with 36 recirculation channels that are distributed equally around the annulus. The effects of the casing treatment on the stability and aerodynamic performance were evaluated numerically on a NASA Stage 37 single-stage transonic axial compressor. The results show that the addition of the recirculation channels improved the gain in stall margin while mitigating the penalty in the adiabatic efficiency and pressure rise compared to a casing groove without recirculation channels. Six geometric parameters were examined in a parametric study. The maximum increase in stall margin was 42.5% with small reductions in the efficiency and pressure ratio. To maximize the stall margin and minimize efficiency loss, two design variables were selected to perform a multi-objective optimization using a multi-objective genetic algorithm coupled with surrogate models using a radial basis neural network. The optimization successfully improved both the objective functions for all representative designs compared to the reference design.

Experimental Investigations of Cavitation Performance Breakdown in an Axial Waterjet Pump

Abstract This experimental study examines the mechanisms causing cavitation breakdown in an axial waterjet pump. The database includes performance curves, images of cavitation, measured changes to endwall pressure as the blade passes, as well as velocity and pressure distributions inside the blade passage, the latter estimated using Bernoulli's Equation in the rotor reference frame. They show that cavitation breakdown is associated with a rapid expansion of the attached cavitation on the blade suction side (SS) into the blade overlap region, blocking part of the entrance to this passage, increasing the velocity and reducing the pressure along the pressure side (PS) of the blade. Initially, expansion of the SS cavitation compensates for the reduced PS pressure, resulting in a slight increase in performance. Further reduction of the inlet pressure causes a rapid decrease in performance as the SS pressure remains at the vapor pressure, while the PS pressure keeps on decreasing. In addition, during the breakdown, entrainment of the cloud cavitation by the tip leakage vortex generates the previously observed perpendicular cavitating vortices (PCVs) that extend across the passage and reduce the through-flow area in the tip region. Tests have been repeated after installing circumferential casing grooves aimed at manipulating the tip leakage flow and reduce the formation of PCVs. These grooves indeed reduce the tip region blockage during early phases. However, they have a small effect on the performance degradation by cavitation breakdown, presumably owing to their limited effect on the attached SS cavitation and tip region cloud cavitation.

Performance Enhancement of a Transonic Axial Flow Compressor with Circumferential Casing Grooves to Improve the Stall Margin

The stability range of the gas turbine engine compressors is being challenged in the modern days due to the intention of increasing per stage maximum loading. Casing treatment has been widely adopted as a realistic passive flow control means to improve the stall margin with a slight decrease of efficiency at the same time by various grooves of which shape (location, angles and so on) has a significant influence on controlling effect. However, the influence of some details in grooves is ignored in most of the case that may impact the specific flow field in the groove. A research on the impact of chamfer and fillet corners on the performance of casing treatment was proceeded by numerical simulation in this paper. The performance of different models of grooves on NASA Rotor 37 was investigated by discretizing 3D RANS based on finite volume method. Firstly, steady simulations were performed on NASA rotor 37 for validation. The CFD results and experimental data for adiabatic efficiency and pressure ratio were in good agreement. According to convergence criteria, the initiation of the stall was predicted. Few numbers of circumferential grooves casing treatment (CGCT) models have been proposed and tested numerically. Rectangular CGCT shape and smooth wall casing performances were analyzed. Moreover, the highest adiabatic efficiencies and stall margins of smooth wall casing, rectangular grooves and rectangular grooves with chamfer and fillet corners shapes models were compared to evaluate the influence of the shape of grooves on the stability and performance on axial flow compressor. The rectangular circumferential casing grooves and rectangular grooves with chamfer and fillet corners enhanced significantly stall margin and an operational range of the transonic axial flow compressor but adiabatic efficiency was slightly decreased.

Effect of part sweep on axial flow compressor performance in the presence of circumferential casing grooves

Modern axial compressors demand high performance and increased operating range. High performance is generally obtained by employing 3D design features, such as sweep and lean. To improve operating range, use of circumferential casing grooves is quite common. An extensive numerical study is carried out to understand performance change due to swept rotor blade on axial compressor performance and stall margin, in the presence of circumferential casing grooves. Numerical methodology used in the current work is validated with experimental data of NASA Rotor37. Grid sensitivity as well as turbulence model validation is carried out to validate numerical methodology used in this work. A baseline rotor is created without any sweep. Sweep considered in this study is employed only at part span of the blade. Impact of part sweep with circumferential casing grooves is not reported by many in open literature, which is the focus of this work. Different magnitudes of sweep are considered in this study. The current study indicates existence of an optimum combination of magnitude of sweep and span location at which sweep starts from. Sweep in the presence of circumferential grooves results in considerable increase of operating range with nominal decrease in efficiency. A detailed flow field investigation is presented to understand the underlying flow physics.

The impact of circumferential casing grooves on rotating instability in a transonic axial compressor

The impact of circumferential casing grooves on rotating instability is first assessed for both design and part speed operations in a transonic axial compressor, with the purpose of developing the next generation casing treatments for vibration control. Multi-passage time-resolved computations are performed to capture the origination and propagation behavior of the instability for cases with and without casing grooves. Probed pressure signals in different passages show a nonsynchronous fluctuation of tip flow. It proves tip leakage vortex and its self-excited oscillation is responsible for this type of inconsistence, regardless of the compressor operation speed. Although flow separation on blade suction surface and the consequent shedding vortex contributes to another origin of instability, the resulted flow appears to be consistent. Casing grooves are able to enhance the synchronization by greatly suppressing both tip leakage vortex oscillations and the intermittently shedding separation vortex, especially in the front part of blade passage. Both types of instability are constrained in several separated axial scope by casing grooves, which essentially increase the damping of flow oscillations. Thus, further improvement of casing treatment design can be expected if the axial transport of the instability in the tip region is restrained more efficiently, for both extending stall margin and enhancing aerodynamic stability.

A numerical analysis of casing groove parameters on the performance of wave energy conversion device

Casing grooves in Wells turbine can enhance its performance. The present computational work has been carried out to understand the effect of casing-groove's width-to-chord (w/C) ratio on the performance of a wave energy conversion device. In this study, four different values of w/C ratio such as 0.24, 0.48, 0.64 and 0.96 were investigated. The circumferential casing groove depth (3% of the chord length) and tip clearance (1% of the chord length) were kept constant for all the cases. The flow field was analysed through discretisation of three-dimensional steady incompressible Reynolds-averaged Navier-stokes (RANS) equations with two-equation eddy viscosity turbulence closer model. The simulation results show that the w/C ratio has a significant impact on the turbine performance and the w/C = 0.64 was found to be the optimal one among the several values investigated in the present study.

Experimental and Numerical Investigation of a Circumferential Groove Casing Treatment in a Low-Speed Axial Research Compressor at Different Tip Clearances

The demand of increasing pressure ratios for modern high pressure compressors leads to decreasing blade heights in the last stages. As tip clearances (TC) cannot be reduced to any amount and minimum values might be necessary for safety reasons, the TC ratios of the last stages can reach values notably higher than current norms. This can be intensified by a compressor running in transient operations where thermal differences can lead to further growing clearances. For decades, the detrimental effects of large clearances on an axial compressor's operating range and efficiency are known and investigated. The ability of circumferential casing grooves in the rotor casing to improve the compressor's operating range has also been in the focus of research for many years. Their simplicity and ease of installation are one reason for their continuing popularity nowadays, where advanced methods to increase the operating range of an axial compressor are known. In the authors' previous paper, three different circumferential groove casing treatments were investigated in a single-stage environment in the low-speed axial research compressor at TU Dresden. One of these grooves was able to notably improve the operating range and the efficiency of the single stage compressor at very large rotor TC (5% of chord length). In this paper, the results of tests with this particular groove type in a three stage environment in the low-speed axial research compressor are presented. Two different rotor TC sizes of 1.2% and 5% of tip chord length were investigated. At the small TC, the grooves are almost neutral. Only small reductions in total pressure ratio and efficiency compared to the solid wall can be observed. If the compressor runs with large TC, it notably benefits from the casing grooves. Both, total pressure and efficiency can be improved by the grooves in a similar extent as in single stage tests. Five-hole probe measurements and unsteady wall pressure measurements show the influence of the groove on the flow field. With the help of numerical investigations, the different behavior of the grooves at the two TC sizes will be discussed.

Numerical analysis of flow in a centrifugal compressor with circumferential grooves: influence of groove location and number on flow instability

As an effective and economic method for flow range enhancement, circumferential groove casing treatment (CGCT) is widely used to increase the stall margin of compressors. Different from traditional grooved casing treatments, in which the grooves are always located over the rotor in both axial and radial compressors, one or several circumferential grooves are located along the shroud side of the diffuser passage in this paper. Numerical investigations were conducted to predict the performance of a low flow rate centrifugal compressor with CGCT in diffuser. Computational fluid dynamics (CFD) analysis is performed under stage environment in order to find the optimum location of the circumferential casing groove in consideration of stall margin enhancement and efficiency gain at design point, and the impact of groove number to the effect of this grooved casing treatment configuration in enhancing the stall margin of the compressor stage is studied. The results indicate that the centrifugal compressor with circumferential groove in vaned diffuser can obtain obvious improvement in the stall margin with sacrificing design efficiency a little. Efforts were made to study blade level flow mechanisms to determine how the CGCT impacts the compressor’s stall margin (SM) and performance. The flow structures in the passage, the tip gap, and the grooves as well as their mutual interactions were plotted and analysed.

Multi-objective optimization of groove casing treatment in a transonic compressor

The paper presents a multi-objective optimization of circumferential casing grooves geometries for the NASA Rotor 37 transonic compressor. The depth normalized by the tip clearance and the width normalized by the tip chord are selected as the design variables. The stall margin and peak efficiency are used as the objective functions. The Latin Hypercube Sampling technique was used to select the sample points in the design space. Based on the numerical results of the sample points, the radial basis function network model of the artificial neural network was constructed. The NSGA-II multi-objective evolutionary algorithm is then employed to search for Pareto-optimal solutions. The leave-one-out cross validation method was also used to evaluate the precision of the radial basis function network model. The results of the optimization show the present method can be effectively used for the design of circumferential casing grooves to take account of the stall margin and efficiency. From the Pareto-optimal solutions, two groove configurations are selected and the internal flow fields are compared with the smooth casing. The effect mechanism of the circumferential casing grooves on the performance of the transonic compressor is discussed by the analysis of the flow in the blade tip region.

The Self-induced unsteadiness of tip leakage flow in an axial low-speed compressor with single circumferential casing groove

Numerical investigation on the self-induced unsteadiness of tip leakage flow (TLF) for an axial low-speed compressor with smooth wall and six single grooved casings are presented. A ten-passage numerical scheme is used to solve the unsteady Reynolds averaged Navier-Stokes (URANS) equations. It is found that the single grooves at various axial locations could have a large impact on the self-induced unsteadiness and the stall margin improvement (SMI) of compressor. The trend of SMI with groove center location demonstrates that the groove located near the mid of blade tip chord generates the best SMI. The worst groove is located about 20% Cax after the blade leading edge. The root-mean-squre of static pressure (RMSP) contours at 99.5% span and fast Fourier transform for the static pressure traces recorded in the tip clearance region for each casing are analyzed. The results demonstrate that the single groove location not only affects the oscillating strength but also the frequency of the unsteady tip leakage flow. At the near-stall point of smooth casing, the self-induced unsteadiness of TLF is enhanced most by the best grooved casing for SMI. While, the self-induced unsteadiness disappears when the worst groove for SMI is added. The characteristic frequency of TLF is about 0.55 blade passing frequency (BPF) with smooth casing. The frequency components become complicated as the single groove moves from the leading edge to the trailing edge of the blade.

Exploration of a new-type grooved casing treatment configuration for a high-speed small-size centrifugal compressor

The main design target for a casing treatment device is to increase the stall margin of a given compressor without sacrificing efficiency. In this article, numerical and experimental investigations about a new-type grooved casing treatment configuration are presented for a high-speed small-size centrifugal compressor. Different from traditional grooved casing treatments, in which the grooves are always placed over the rotor (for both axial and radial compressors), one or several circumferential casing grooves are placed along the shroud side of the diffuser passage in this configuration, to enhance the stall margin of the compressor. Computational fluid dynamics analysis is performed under stage environment in order to find the optimum location of the circumferential casing groove for the subsequent experiments in consideration of stall margin enhancement and peak efficiency gain, and the impact of groove number to the effect of this new-type grooved casing treatment configuration in enhancing the stall margin of the compressor stage is studied. It is found that stall margin gain due to the existence of circumferential casing grooves arises from the suction–reinjection effect of the low momentum fluid and its transportation along the circumferential and streamwise directions in the grooves and the grooves located at the middle and rear part of the diffuser casing wall have more pronounced effect in expanding the stall margin of the centrifugal compressor. In order to verify the effectiveness of this new-type grooved casing treatment configuration, the solid casing compressor and the compressors with circumferential casing grooves are tested in our test rig. The results indicate that this new-type grooved casing treatment configuration can obtain obvious improvement in the stall margin without sacrificing peak efficiency. Also, the numerical investigation is conducted for the entire compressor with circumferential casing grooves and the result is compared with the experimental data.

Aerodynamic analysis and optimization of a transonic axial compressor with casing grooves to improve operating stability

A transonic axial compressor with circumferential casing grooves is optimized to improve operating stability. Numerical analysis is conducted by solving three-dimensional Reynolds-averaged Navier–Stokes equations using the shear stress transport turbulence model. An optimization process based on a weighted-average surrogate model and steady flow analysis are performed with three design variables defining the tip clearance, blade tip angle, and depth of the grooves. The steady stall inception point is identified from the last converged point, and values of the stall margin as the objective function are predicted using steady flow analysis at the design points sampled by Latin hypercube sampling in the design space. The surrogate model is constructed based on these objective function values. Optimization of this model found the optimum design, which yields a considerable increase in the stall margin compared to the smooth casing. To investigate unsteady behavior of the flow in the optimized compressor with casing grooves, an unsteady flow analysis is performed, and the stall inception point is re-predicted using this analysis.

Effects of Number of Circumferential Casing Grooves on Stall Flow Characteristics of a Transonic Axial Compressor

This work investigates the effects of circumferential casing grooves on stall flow characteristics of a transonic axial compressor. Numerical analysis is conducted by solving three-dimensional steady Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model. The results of flow analysis for an axial compressor with smooth casing are validated in comparison with experimental data for the pressure ratio and adiabatic efficiency. The numerical stall inception point is identified from the last converged point by convergence criteria, and the stall margin is predicted numerically. The peak adiabatic efficiency point is also obtained by reducing the normalized mass flow in the high mass flow region. In order to explore the influence of number of the circumferential casing grooves on the performance of the compressor, the stall margins and peak adiabatic efficiencies are evaluated compared to the case smooth casing. The stability of the axial compressor with circumferential casing grooves is found to be sensitively influenced by the number of grooves.

Performance evaluation of a transonic axial compressor with circumferential casing grooves

This article presents an investigation on the performance characteristics of a transonic axial compressor with the application of the circumferential casing grooves. Three-dimensional Reynolds-averaged Navier–Stokes equations with the shear stress turbulence model are discretized by finite volume approximations and solved on hexahedral grids for the flow analysis. The numerical results for the pressure ratio and the adiabatic efficiency show good agreement with the experimental data. The numerical stall inception point is identified from the last converged point by the convergence criterion, and the stall margin (SM) is numerically predicted. The performances of an axial compressor with rectangular shape of the circumferential casing grooves are analysed in comparison with the case of smooth casing. Additionally, the SMs and the peak adiabatic efficiencies with various shapes of the circumferential casing grooves are compared and evaluated in order to explore the influence of groove shape. The application of the circumferential casing grooves leads to significant improvement in the operating stability of compressors with slight reduction in the peak efficiency.