Hub Leakage Flow Research Articles

Transonic Compressor Darmstadt Open Test Case: Experimental Investigation of Stator Secondary Flows and Hub Leakage

Abstract The future trend of clean sky aviation has led to a smaller core engine, resulting in highly loaded stages with increased relative tip gaps and leakage flows, which causes a reinforced influence of secondary flow phenomena. This article experimentally investigates the secondary flow effects in the 3D-optimized shrouded stator of the TUDa-GLR-OpenStage, representative of a transonic high-pressure compressor front stage, and its susceptibility to typical hub leakage flows. The impact of stator hub leakage on efficiency and total pressure ratio is investigated for several operating speeds, from subsonic to nominal transonic operating conditions. Steady five-hole probe and time-resolved virtual multi-hole probe measurements at different operating conditions at nominal speed are conducted to investigate the underlying loss mechanisms. The leakage mass flow is determined using the measured pressure difference within the fin seal. Steady results show that the optimized 3D stator effectively suppresses the hub corner separation compared to the predecessor 2D stator, but its performance is further limited by a near-tip separation. With the increased stator hub leakage (at a leakage-to-main flow ratio of about 0.4%), the compressor isentropic efficiency generally drops (by about −0.5%) at the full operating range due to an enhanced hub corner separation effect. However, the increased leakage also helps alleviate the tip-corner separation when operating near stall, leading to a smaller efficiency penalty under these conditions. Unsteady results reveal the sensitivity of transient compressor performance to the passing rotor wake, but limited interaction between this phenomenon and the increased leakage is observed. These findings provide insight into the stator hub leakage-related penalties on the compressor aerodynamics efficiency.

Investigation of transitional flow in a transonic compressor rotor with hub leakage using large eddy simulation

The hub leakage flow has been acknowledged as an important factor for performance deficiency of axial-flow compressors. Meanwhile, the laminar-turbulent transition in compressors is highly sensitive to the upstream flow state and significantly affects the flow loss. In the present work, quasi-wall-resolved large eddy simulation of a transonic axial-flow compressor rotor at the near-peak-efficiency point is carried out to investigate the effects of hub leakage as well as its absolute tangential velocity on the compressor performance and the laminar-turbulent transition in the blade passage. It is confirmed that the hub leakage with an absolute tangential velocity of 0.5 wheel speed can result in the near-hub total pressure deficit. With the hub leakage taken into account, the predicted total pressure ratio and adiabatic efficiency agree well with the experimental data. The simulation results indicate that increasing the absolute tangential velocity of hub leakage would intensify the near-hub vortices, elevate the endwall turbulence level, increase the near-hub flow loss, and cause a remarkable total pressure ratio drop. This work promotes the understanding of complex flow mechanisms in axial-flow compressors in the presence of hub leakage.

Effect of Moving Endwall on Hub Leakage Flow of Cantilevered Stator in a Linear Compressor Cascade

Effect of Moving Endwall on Hub Leakage Flow of Cantilevered Stator in a Linear Compressor Cascade

Effect of Rotor-Stator Spacing on Compressor Performance at Variable Operating Conditions

This paper investigates the effect of rotor-stator spacing and operating conditions on compressor performance and the rotor-stator interaction mechanism. We focus on the interaction between the rotor wake and the stator’s flow field through several unsteady numerical simulations aiming at a transonic compressor stage in the equal radius flow path with two-axial spacing at three operating conditions. The simulation results indicate that the time-averaged efficiency almost declined by 0.6 points due to mixing loss in the spacing ranging from close to far at nominal and low load operating conditions. The deep upstream wake leads to corner separation at high load conditions due to close spacing, imposing a fluctuating efficiency at the frequency where the stator wake near the shroud oscillates, as found by the dominant frequency screening method. Under all conditions, the wake transport can reduce the static pressure on the pressure surface and weaken the hub leakage flow afterward.

Approximation of hub leakage flow in a high speed axial flow compressor rotor with adjoint method

Abstract This paper focuses the effect of hub leakage flow on the performance of a high speed transonic rotor. The rotor considered here is NASA rotor 37. General computational fluid dynamics of this rotor have been unable to predict a total pressure deficit from the hub wall to the mid span height observed in the experiments. In this study, computations have been performed on the rotor with the boundary condition assumed the leakage flow from the tip gap at the upstream of the rotor. In the computations, the velocity distribution is determined on the computational domain corresponding to the location of the tip gap. The swirl velocity at the boundary is calculated with the rotational speed and the radial length of the hub wall. The radial velocity is estimated based on the leakage mass flow. However, the mass flow is an uncertain quantity, and it has effect to the prediction of the total pressure deficit. Therefore, an adjoint method has been applied for the approximation to the uncertain quantity. The simulation results using the leakage flow boundary and the result without the boundary condition have been compared with the experiment. The comparisons have shown that the boundary condition and the approximation with the adjoint method have enabled to predict the total pressure deficit.

Effect of hub clearance size and shape of a cantilevered stator on the performance of a small-scale transonic axial compressor

To deeply understand the hub leakage flow and its influence on the aerodynamic performance and flow behaviors of a small-scale transonic axial compressor, variations of the performance and the flow field of the compressor with different hub clearance sizes and clearance shapes were numerically analyzed. The results indicate that the hub clearance size has remarkable impacts on the overall performance of the compressor. With the increase of the hub clearance, the intensity of the hub leakage flow increases, resulting in more intense flow blockage near the stator hub, which reduces the compressor efficiency. However, the flow field near the blade mid-span is modified due to the more convergent flow as the reduced effective flow area caused by the passage blockage, and the flow separation range is narrowed, thus the flow stability of the compressor is enhanced. On this basis, two kinds of non-uniform clearance cases of expanding clearance and shrinking clearance with the same circumferential leakage area as the design clearance were investigated. The occurrence position of the double leakage flow which is closely connected with the flow loss and blockage is shifted backward by the expanding clearance, the flow capacity near the stator hub is enhanced, and the unsteady fluctuation intensity of the flow field is attenuated but fluctuation frequency remains. Similarly, the modification of the stator blade root flow field may result in the reduction of stall margin. The effect of the shrinking clearance on compressor performance is opposite to that of the expanding clearance, which reduces the peak efficiency and delays the stall inception.

Details of Shrouded Stator Hub Cavity Flow in a Multistage Axial Compressor Part 2: Leakage Flow Characteristics in Stator Wells

Abstract The flow in shrouded stator cavities can be quite complex with axial, radial, and circumferential variations. As the leakage flow recirculates and is re-injected into the main flow path upstream of the stator, it deteriorates the near-hub flow field and, thus, degrades the overall aerodynamic performance of the compressor. In addition, the windage heating in the cavity can raise thermal-mechanical concerns. Fully understanding the details of the shrouded-hub cavity flow in a multistage environment can enable better hub cavity designs. In the first part of the paper, the influence of the hub leakage flow on compressor performance and its interactions with the primary flow were investigated. While the impact of hub leakage flow on the primary passage is readily available in the open literature, details inside the cavity geometry are scarce due to the difficulties in instrumenting that region for an experiment or modeling the full cavity geometry. To shed light on this topic, the flow physics in the stator cavity inlet and outlet wells are investigated in this paper using a coupled computational fluid dynamics model with the inclusion of the stator cavity wells for the Purdue 3-stage (P3S) axial compressor, which is representative of the rear stages of a high-pressure-compressor in core engines. At the inlet cavity, the presence of at least one pair of vortices influences the trajectory of the cavity leakage flow. The amount of leakage flow also determines the size of the vortical structures, with larger clearances creating a smaller vortex and vice versa. After passing through the labyrinth seals, the leakage flow travels along the stator landing first and then transitions to the rotor drum. In general, a flow path closer to the rotor drum achieves higher circumferential velocity but also exhibits significant temperature rise. A rise in circumferential velocity directly corresponds to a rise in temperature. In addition, the windage heating increases with increasing seal clearance. Furthermore, the inlet well contributes the most to overall windage, nearly 50% of the total windage heating, while the labyrinth seals and outlet well account for very little.

Flow characteristics of a low-solidity cantilevered stator embedded in a 4-stage low-speed research compressor

This research presents the influence of the low-solidity design on the loss and stability of the cantilevered stator and the evolution of the hub leakage vortex. The highly loaded 4-stage low-speed research compressor (LSRC) with an embedded cantilevered stator (CS) of Stator 3 (S3) was experimentally measured at the blade row outlet and in the S3 passage, respectively, and the URANS simulation method was calibrated accordingly. Then, the prototype CS blade number was reduced, namely low-solidity cantilevered stator (LSCS), to increase the blade loading. The diffusion factor ( DF) of LSCS at the midspan at the design working condition is 5.2% higher than that of CS. The URANS results show that LSCS provides higher efficiency for the 1.5 stage compressor with a consistent stall margin. For S3, the total pressure loss of LSCS is lower than CS at the design point (DP) working condition, but it is the opposite at the near stall (NS) working condition. For LSCS at the NS point, the blockage region becomes large and occupies the lower half of the fore blade passage because the low-solidity stator could not provide the enough flow turning ability, and the core of the hub leakage vortex (HLV) moves forward and the intensity is strengthened. The high loss region of LSCS at the near stall condition is consistent with the size of two hub leakage vortices. The first HLV breakdown is caused by unsteady mixing flow, and then it twists downstream and contacts with the trailing edge of the adjacent blade pressure surface. The study of the hub leakage flow characteristics of the low-solidity cantilevered stator can help designers to control flow better.

Effect of hub clearance of cantilever stator on aerodynamic performance and flow field of a transonic axial-flow compressor

To investigate the effect of hub clearance of cantilever stator on the aerodynamic performance and the flow field of the transonic axial-flow compressor, the performance of single-stage compressors with the shrouded stator and cantilever stator was studied numerically. It is found that the hub corner separation on the stator blade suction surface (SS) was modified by introducing the hub leakage flow. The separation vortex on the SS of the stator blade root at about 10% axial chord length caused by the interaction of the shock wave and boundary layer was also controlled. Compared with the tip clearance size of the rotor blade, the stator hub clearance size (HCS) has a much less effect on the overall aerodynamic performance of the compressor, and there is no obvious effect on the flow field in the upstream blade row. With the increase of HCS, the leakage loss and the blockage degree in the flow field near the stator hub are increased and further make the adiabatic efficiency and the total pressure ratio of the compressor gradually decrease. Meanwhile, the stall margin of the compressor was changed slightly, but the response of the stall margin to the change of the HCS is nonlinear and insensitive. The stator hub leakage flow (HLF) can not only change the flow field near the hub but also redistribute the flow law within the range of the entire blade span. It will contribute to further understand the mechanism of the HLF and provide supports for the design of the cantilever stator of transonic compressors.

Flow characteristics on a 4-stage low-speed research compressor with a cantilevered stator

This paper presents the experimental and numerical investigation of flow characteristics on the highly loaded 4-stage low-speed research compressor (LSRC) with a Cantilevered Stator of Shanghai Jiao Tong University. A new three-dimensional design for the cantilevered stator in the LSRC was derived using an asymmetrical circumferential bow, and the cantilevered stator near the hub adopted the fore-loading design. The unsteady Reynolds averaged Navier-Stokes (URANS) numerical results were compared with experimental data and the differences were introduced. The evolution of the hub leakage flow at the near stall (NS) operating condition compared with the design point (DP) was analyzed, and design suggestions were provided for the two operating conditions. The present study details the effects of the hub leakage flow on the highly loaded LSRC, and provides some quantified data between the aerodynamic performance and hub leakage flow. Designers needing to better understand the hub leakage flow of a cantilevered stator in a multi-stage compressor will benefit from this paper.

Impact of hub gap leakage on stator endwall flow in an axial compressor stage with casing treatment

Abstract In an axial compressor stage equipped with slot-type casing treatment, simulation without hub gap between rotors and stators achieves much lower stall margin enhancement than experiment, which results from the over-predicted corner stall in stators. Aiming at clarifying the combined effect of hub leakage flow and casing treatment on operating range and the flow mechanism of leakage flow interacting with stator flow, a detailed numerical research is performed in this compressor stage. The effects of hub leakage mass flow are elaborated in terms of compressor performance and flow fields. Unsteady simulations with appropriate leakage mass flow condition are able to predict the stall margin improvement and total pressure characteristics preferably. Casing treatment slots tend to re-distribute the loading of downstream stator by decreasing loading above 70% span and increasing that below 70% span, which result in the exaggerated corner stall in stators. Hub leakage flow from cavity resists the tendency brought by casing treatment and succeeds in relieving the heavy loading near the hub. The flow mechanism for the effect of cavity leakage flow on stall margin is elucidated. Cavity hub leakage gives rise to a thickened boundary layer upstream stator inlet by producing a separation bubble and aggravating the boundary layer skewing. The thickened boundary layer produces a horseshoe vortex in the stator passage, which induces main flow to rolling up as the pressure-side branch and avoids the emergence of corner stall.

Hub clearance effects of a cantilevered tandem stator on the performance and flow behaviors in a small-scale axial flow compressor

The impacts of the increased hub clearance size (HCS) in the cantilevered tandem stator on the overall performance and flow behaviors have been investigated based on numerical simulations in a small-scale axial flow compressor stage. The results indicate that the HCS variation in the front blade of the tandem stator has a more significant effect on the compressor peak efficiency and stall margin. However, there is no obvious change of the compressor performance with the increase of the HCS in the rear blade. Detailed flow analysis shows that the increase of the HCS in the front blade has remarkable impacts on the flow fields in the front blade, while it has an only slight influence on the hub flow fields in the rear blade. It is found that the compressor efficiency penalty is due to the increase of the loss generation near the hub caused by the larger extent of hub leakage flow, while the stall margin improvement is due to the redistribution of the mass flow rate along the spanwise direction. As a result, the inlet flow condition within the upper blade span is improved and the flow blockage is reduced. The resulting influence of the increased HCS in the rear blade is mainly located near the blade leading edge of the rear blade, and there is no remarkable effect on the flow field in the front blade passage.

Experimental Investigation of Hub Leakage Flow Through Stator Knife Seals in an Axial Compressor

Experimental Investigation of Hub Leakage Flow Through Stator Knife Seals in an Axial Compressor

The effect of hub leakage on the aerodynamic performance of high-pressure steam turbine stages

In a steam turbine, leakage occurs at the gap between the rotor shafts and diaphragms and is injected into the main stream at the inlet of the rotor blade. Although this hub leakage flow is small, it affects the main flow, especially in the rotor passage. This study uses computational fluid dynamics to investigate the influence of hub leakage flow re-entry on the aerodynamic performance of turbine stages of a 10-stage high-pressure steam turbine operating at USC conditions. Each stage was individually investigated in regard to the influence of stage reaction on the efficiency drop. The flow field of the turbine without the leakage flow was analyzed first, and then the variations in flow phenomena and stage efficiency were investigated with increasing leakage flow ratio. Our analysis shows that the efficiency drop depends strongly on the stage reaction. The first stage designed with 21 % reaction showed an efficiency drop of approximately 1 % for 1 % increase in leakage flow, which is almost twice that of the last three stages with an average stage reaction of 48 %.

Cantilevered stator hub leakage flow control and loss reduction using non-uniform clearances

Hub leakage flow between the cantilevered stator and the hub wall is responsible for the deficit of stator aerodynamic performance, and the interaction between the leakage and main flows leads to leakage vortex and flow blockages in the hub region, thus deteriorating the stator or even the compressor performance. The paper conducts a numerical investigation on the influence of non-uniform clearances on the stator aerodynamic performance in a 1.5-stage axial-flow compressor. The main purpose is to find an aerodynamically preferable clearance shape to control the leakage flow and the associated aerodynamic losses. The clearance shapes considered include uniform, expanding and shrinking clearances from blade leading edge to trailing edge. The computed results show that, compared with the uniform clearance, the expanding clearance has powerful effect on weakening the leakage flow near the leading edge and also has better inhibition of the leakage vortex, thus achieving aerodynamic benefits gain in the hub region. But the shrinking clearance has the opposite effect on the leakage flow and leads to more losses than the uniform clearance. In addition, the overall aerodynamic loss of the stator with the expanding clearance is reduced with the increment of the clearance expansion ratio.

Aerodynamic impact of hub and shroud leakage flow on an axial turbine stage

This paper presents a study analyzing the aerodynamic effect and loss mechanism of hub and shroud leakage flow for an axial turbine stage. A series of computational fluid dynamics computations were performed to investigate the effect of various complexity of leakage configurations on the flow field. It is found that a non-linear relationship between different flow systems emerges in the vicinity of both the shroud and hub leakage flow exhibiting a deviation of flow direction and a radial shift of flow pattern, while the efficiency drop caused by the shroud and hub leakage configurations can be added linearly. By analyzing the expansion process in the cooled turbine and the spatial distribution of viscous dissipation term, the loss sources which can be directly traced back to the flow phenomena were indentified. Based on the aerodynamic feature of the turbine, an analytical approach to separate and quantify loss sources was proposed and applied to analyze the turbine. Four kinds of loss mechanisms, referred to as cavity losses, mixing losses, extra losses, and the leakage work reduction, were observed and their contributions were eventually represented by efficiency penalty.

Effect of the Stator Hub Configuration and Stage Design Parameters on Aerodynamic Loss in Axial Compressors

The choice of the stator hub configuration (i.e., cantilevered versus shrouded) is an important design decision in the preliminary design stage of an axial compressor. Therefore, it is important to understand the effect of the stator hub configuration on the aerodynamic performance. In particular, the stator hub configuration fundamentally affects the leakage flow across the stator. The effect of the stator hub configuration on the leakage flow and its consequent aerodynamic mixing loss with the main flow within the stator row is systematically investigated in this study. In the first part of the paper, a simple model is formulated to estimate the leakage loss across the stator hub as a function of fundamental stage design parameters, such as the flow coefficient, the degree of reaction, and the work coefficient, in combination with some relevant geometric parameters including the clearance/span, the pitch-to-chord ratio, and the number of seals for the shrouded geometry. The model is exercised in order to understand the effect of each of these design parameters on the leakage loss. It is found that, for a given flow coefficient and work coefficient, the leakage loss across the stator is substantially influenced by the degree of reaction. When a cantilevered stator is compared with a shrouded stator with a single seal at the same clearance, it is shown that a shrouded configuration is generally favored as a higher degree of reaction is selected, whereas a cantilevered configuration is desirable for a lower degree of reaction. Further to this, it is demonstrated that, for shrouded stators, an additional aerodynamic benefit can be achieved by using multiple seals. The second part of the paper investigates the effect of the rotating surfaces. Traditionally, only the pressure loss has been considered for stators. However, the current advanced computational fluid dynamics (CFD) generally includes the leakage path with associated rotating surfaces, which impart energy to the flow. It is shown that the conventional loss coefficient, based on considering only the pressure loss, is misleading when hub leakage flows are modeled in detail, because there is energy addition due to the rotation of the hub or the shroud seals for the cantilevered stator and the shrouded stator, respectively. The calculation of the entropy generation across the stator is a better measure of relative performance when comparing two different stator hub configurations with detailed CFD.

Leakage Uncertainties in Compressors: The Case of Rotor 37

This paper revisits an old problem of validating computational fluid dynamics simulations with experiments in turbomachinery. The case considered here is NASA rotor 37. Prior computational fluid dynamics studies of this blade have been unable to predict a total pressure deficit at the hub as observed in the experiments. A possible explanation for this discrepancy is a small hub leakage flow emanating fore of the leading edge, between the forward stationary center body and the rotating disk. In this work, a large-scale high-fidelity uncertainty quantification study is carried out to investigate whether this indeed was the case. Computations are carried out on a 4.5-million-cell rotor 37 mesh with a small cavity fore of the leading edge. This cavity has an inlet with three boundary conditions: all assumed to be uncertain. A nonintrusive, orthogonal polynomial-based technique using sparse grids is used to propagate these three uncertainties, namely, leakage mass flow, leakage whirl velocity, and radial flow angle. A total of 158 sparse-grid-based design-of-experiment computations are carried out at two flow conditions. The results of the uncertainty quantification study show that a small amount of leakage flow can account for the hub pressure deficit at both on- and off-design conditions. For the uncertainty supports selected, the total pressure that best matched experiment was found to lie between the second and third standard deviations over the assumed uncertainties.

The Effects of a Parametric Variation of the Rim Seal Geometry on the Interaction Between Hub Leakage and Mainstream Flows in High Pressure Turbines

The objective of this work was to assess and understand the effects of a parametric variation performed on a typical overlapping rim seal geometry. The datum geometry has been the focus of a detailed experimental investigation employing a large-scale linear cascade subjected to a range of the mass flow rates and swirl velocities of the leakage air. The parametric variations described in this paper were examined using validated computational fluid dynamics (CFD). As a part of the parametric studies, both the axial and the radial seal clearance between the rotor fin (angel wing) and stator platform were varied as well as the length of the overlap between stator and rotor platforms. In addition, the effects of forward and backward facing annulus steps were also investigated. It has been found that a backward-facing annulus step was detrimental for all conditions considered, while a forward-facing step offered improvements for smaller step heights and/or lower leakage fractions. Tightening of the seal clearances closer to the annulus line improved the sealing effectiveness but often at the expense of increased losses. On the other hand, increasing the overlap length led to improvements in the sealing effectiveness with very small effects on the overall losses. Moving the rim seal away from the blade-leading edges reduced the pressure asymmetry at the rim seal and increased the flow uniformity of the leakage air. However, this led to an increased cross-passage flow (more negative skew) and higher losses at all but lowest leakage fractions. The results presented in this paper highlight the fact that there may not be an optimum rim seal solution that would offer an improvement for the full range of leakage fractions and that, for different rim sealing flows, there may be a different optimum geometry. In addition, rotor disk movements in radial and axial directions at various off-design conditions should be considered as a part of the design process. Based on the presented results, it may be of a benefit to the turbine designer to consider rotor disk designs that would be more biased towards the upstream and outward disk movements, which would result in tightening of the seal clearances and avoidance of a backward-facing annulus step.

The Effects of Unsteadiness and Compressibility on the Interaction Between Hub Leakage and Mainstream Flows in High-Pressure Turbines

This paper investigates the effects of compressibility and unsteadiness due to the relative blade row motion and their importance in the interaction between hub leakage (purge) and mainstream flows. First, the challenges associated with the blade redesign for low-speed testing are described. The effects of Mach number are then addressed by analyzing the experiments in the low-speed linear cascade equipped with the secondary airflow system and computations performed on the low- and high-speed blade profiles. These results indicate that the compressibility does not significantly affect the interaction between the leakage and mainstream flows despite a number of compromises made during the design of the low-speed blade. This was due to the fact that the leakage–mainstream interaction takes place upstream of the blade throat where the local Mach numbers are still relatively low. The analysis is then extended to the equivalent full-stage unsteady computations. The periodic unsteadiness resulting from the relative motion of the upstream vanes appreciably affected the way in which the leakage flow is injected and the rotor flow field in general. However, the time-average flow field was still found to be dominated by the rotor blade's potential field. For the present test arrangement, the unsteady effects were not very detrimental and caused less than a 10% increase in the losses due to the leakage injection relative to the steady calculations.