Endwall Motion Research Articles

Study of Relative Endwall Motion Effects in a Compressor Cascade Through Direct Numerical Simulations

Abstract Linear cascades are commonly used as surrogate geometries when performing fundamental studies of turbomachinery blading. Several effects are not accounted for in linear cascades, such as the relative motion between blade and endwall. In this study, three different relative endwall velocities are analyzed. The effect of the relative motion between endwall and blade in a linear compressor cascade is studied through direct numerical simulations. Results show a significant change in the secondary flow structure within the passage. Most notably, the tip leakage vortex is displaced away from the blade. Still, the blade spanwise range affected by the secondary flow field is similar to the case without relative endwall motion. At the outlet plane, a stratification of the total pressure losses and the exit flow angle is found, which overshadows any blade wake effects near the endwall.

Tip gap size effects on thermal performance of cavity-winglet tips in transonic turbine cascade with endwall motion

AbstractThe thermal performance of two cavity-winglet tips with endwall motion is investigated in a transonic high pressure turbine cascade, which operates at an engine representative exit Mach number of 1.2 and an exit Reynolds number of 1.7 × 106. The numerical method is first validated with experimental data and then used to investigate blade heat transfer at three different tip clearances of 1.1, 2.1 and 3.1% chord. The effects of relative endwall motion are considered. The present results show that as the size of the tip gap increases, the heat transfer coefficient and heat load on the tip increases. The winglet geometries on the blade tip mainly affect the tip flow structure close to them. At a larger tip clearance, the size of the separation bubble above the pressure side winglet increases. The heat transfer coefficient is high on the pressure side winglet due to the flow reattachment at all tip clearances. Within the tip gap, when the size of the tip clearance increases, the size of the cavity vortex increases and the cavity scraping vortex due to relative endwall motion becomes smaller. The impingement of the both two vortexes can lead to high heat transfer coefficient on the cavity floor surface. On the blade suction surface, when the size of the tip clearance increases, the heat transfer coefficient of the cavity tip increases, but those of the winglet tips decreases. The heat transfer coefficient is high on the side surface of the suction side winglet at all tip clearances because of the tip leakage flow impingement.

Effect of the Endwall Motion on a Hydrofoil with Various Widths of Clearance

This paper is devoted to evaluate the effects of the relative motion between the blade tip and casing wall with different widths of gap. The steady non-cavitating simulations around a sharp tip hydrofoil in tunnel with a moving endwall are implemented and another calculation with the stationary wall is carried out as a contrast. Overall, the endwall motion moves the trajectory of the tip leakage vortex (TLV) much closer to the foil but brings small effect on the streamwise vorticity and the minimum pressure in the TLV. The influence of the gap width is taken into account with the normalized gap width withinτ=0.02∼0.4. The flow features on a cross section inside the gap suggest that the endwall motion causes a little lower leakage flow rate and velocity in pitchwise direction. Inside a wider gap like τ=0.2, the pressure of the tip separation vortex (TSV) is a little higher and the direction of the velocity gradient for the streamwise flow is changed under the effect of the moving wall. The foil loading characteristics are revealed by the lift and drag coefficients which are a slightly lower under the effect of the endwall motion.

Heat Transfer of Winglet Tips in a Transonic Turbine Cascade

Understanding the heat transfer of winglet tips is crucial for their applications in high-pressure turbines. The current paper investigates the heat transfer performance of three different winglet-cavity tips in a transonic turbine cascade at a tip gap of 2.1% chord. A cavity tip is studied as the baseline case. The cascade operates at engine representative conditions of an exit Mach number of 1.2 and an exit Reynolds number of 1.7 × 106. Transient infrared thermography technique was used to obtain the tip distributions of heat transfer coefficient for different tips in the experiment. The CFD results were validated with the measured tip heat transfer coefficients, and then used to explain the flow physics related to heat transfer. It is found that on the pressure side winglet, the flow reattaches on the top winglet surface and results in high heat transfer coefficient. On the suction side winglet, the heat transfer coefficient is low near the blade leading edge but is higher from the midchord to the trailing edge. The suction side winglet pushes the tip leakage vortex further away from the blade suction surface and reduces the heat transfer coefficient from 85% to 96% span on the blade suction surface. However, the heat transfer coefficient is higher for the winglet tips from 96% span to the tip. This is because the tip leakage vortex attaches on the side surface of the suction side winglet and results in quite high heat transfer coefficient on the front protrusive part of the winglet. The effects of relative endwall motion between the blade tip and the casing were investigated by CFD method. The endwall motion has a significant effect on the flow physics within the tip gap and near-tip region in the blade passage, thus affects the heat transfer coefficient distributions. With relative endwall motion, a scraping vortex forms inside the tip gap and near the casing, and the cavity vortex gets closer to the pressure side squealer/winglet. The tip leakage vortex in the blade passage becomes closer to the blade suction surface, resulting in an increase of the heat transfer coefficient.

Effects of endwall motion on thermal performance of cavity tips with different squealer width and height

This paper investigates the thermal performance of cavity tips in a transonic high pressure turbine cascade. Numerical methods were first validated and then used to study more cases. The flow physics within the tip gap and the distribution of Nusselt number on the blade tip were presented. The effects of relative motion between the blade tip and the endwall were studied. For both cases with a stationary endwall or a moving endwall, a cavity vortex appears within the tip cavity. It is found that a cavity scraping vortex forms within the tip cavity as the endwall moves relatively to the blade tip, which was not observed before. The effects of the cavity scraping vortex on the distribution of Nusselt number are explained. Then, the effects of the squealer width and height on the thermal performance of the cavity tip were investigated. The flow physics within the tip cavity varies as the squealer geometry changes. As a result, the thermal performance of the cavity tips is different. The results show that increasing the height of the squealer reduces the average Nusselt number on the blade tip. The average Nusselt number and the heat flux on different parts of the blade tip, and the thermal performance of the blade surface near tip region and the loss of different blade tips are also discussed in this paper.

The Influence of Boundary Conditions on Tip Leakage Flow

Much of the current understanding of tip leakage flow has been derived from detailed cascade studies. Such experiments are inherently approximate since it is difficult to simulate the boundary conditions that are present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. The problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer of the leakage flow. More engine-representative tests can be performed on high-speed rotating turbines, but the experimental resolution achievable in such setups is limited. In order to examine the differences between cascade and engine boundary conditions, this paper presents a numerical investigation into the impact of inlet conditions and relative casing motion (RCM) on the leakage flow of a high-pressure turbine rotor. The baseline calculation uses a simplified inlet condition and no relative endwall motion, in typical cascade fashion. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or RCM. However, when both of these conditions are applied simultaneously, the pattern of leakage flow is significantly altered, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving the changes, the impact on performance and the implications for future experimental investigations.

Thermal Performance of Transonic Cooled Tips in a Turbine Cascade

Transonic high-pressure turbines are promising candidates for future aircraft engines. Recent studies in transonic turbine cascades showed that shock waves appeared over the uncooled flat tip, which affects its heat transfer. However, the effects of coolant injection on the thermal performance of transonic cooled tips are not clear. The current study aims to understand the thermal performance of a cooled flat tip and a cooled cavity tip in a transonic turbine cascade. The effects of tip film cooling are studied at five coolant pressure ratios (which are the stagnation pressure ratios of the coolant inlet and the cascade inlet) from 0.8 to 1.2. The flow physics over the tip, the heat transfer coefficient, the film-cooling effectiveness, and the heat flux of the tip are investigated. The effects of relative endwall motion are considered.

Aerothermal Performance of Different Tips in Transonic Turbine Cascade with End-Wall Motion

Modern high-pressure axial turbines of aircraft engines are often transonic. This paper investigates and compares the aerothermal performance of the tip leakage flow of two widely used tips, a flat tip and a cavity tip, in a transonic turbine cascade. Numerical methods were first validated and then used to study different cases. Shock waves appear within the tip gap of a flat tip and have a large effect on the aerothermal performance. Unlike the flat tip, the flow physics within the tip gap of the cavity tip is significantly affected by the cavity vortices, and the effects of the shock waves are small. The impact of tip leakage flow physics on the loss mechanism and heat transfer are investigated. The effects of the relative motion between the blade tip and the casing are also investigated. A tangential viscous force is imposed by end-wall motion and changes the flow physics within the tip gap. Furthermore, a scrapping vortex is formed in the main flow passage due to relative end-wall motion. The scrapping vortex has a large effect on the flow pattern downstream of the cascade.

The Aerothermal Performance of a Cooled Winglet Tip in a High Pressure Turbine Cascade

The aerothermal performance of a winglet tip with cooling holes on the tip and on the blade surface near the tip is reported in this paper. The investigation was based on a high pressure turbine cascade. Experimental and numerical methods were used. The effects of the coolant mass flow rate are also studied. Because the coolant injection partially blocks the tip leakage flow, more passage flow is turned by the blade. As a result, the coolant injection on the winglet tip reduces the deviation of the flow downstream of the cascade due to the tip leakage flow. However, the tip leakage loss increases slightly with the coolant mass flow ratio. Both the computational fluid dynamics tools and experiments using the Amonia–Diazo technique were used to determine the cooling effectiveness. On the blade pressure side surface, low cooling effectiveness appears around the holes due to the lack of the coolant from the cooling hole or the lift-off of the coolant from the blade surface when the coolant mass flow is high. The cooling effectiveness on the winglet tip is a combined effect of the coolant ejected from all the holes. On the top of the winglet tip, the average cooling effectiveness increases and the heat load decreases with increasing coolant mass flow. Due to its large area, the cooled winglet tip has a higher heat load than an uncooled flat tip at engine representative coolant mass flow ratio. Nevertheless, the heat flux rate per unit area of the winglet is much lower than that of an uncooled flat tip. The cycle analysis is carried out and the effects of relative tip-to-casing endwall motion are address.

Effects of Endwall Motion on the Aero-Thermal Performance of a Winglet Tip in a HP Turbine

In a gas turbine, the casing endwall moves relative to the blades. In this paper, numerical methods are first validated using experimental results for a stationary endwall. They are then used to study the effects of endwall motion on the aero-thermal performance of both winglet tips with and without tip film cooling at a tip gap of 1.9% C. The endwall motion imposes a tangential force on the flow. A scraping vortex is formed and the flow pattern within the tip gap changes significantly. The tip leakage mass flow rate that exits the tip gap from the suction side edge reduces by about 42% with endwall motion. Overall, the endwall motion reduces the tip leakage loss by 15%. The flow field downstream of the cascade also changes with endwall motion. With endwall motion, the changed flow pattern within the tip gap significantly changes the distribution of the Nusselt number on the winglet tip. For the winglet tip without tip film cooling, the Nusselt number and the heat load decrease with endwall motion. This is mainly due to the reduction in the tip leakage mass flow ratio, which reduces the leakage velocity over the tip. On the winglet tip with tip film cooling, the cooling effectiveness increases by 9% with endwall motion. Combined with the reduced Nusselt number, the heat flux on the winglet tip with tip film cooling reduces by 31% with endwall motion. The cooling effectiveness on the near tip region of the pressure side remains almost unchanged, however, the heat flux rate in this area reduces. This is because the reduced tip leakage mass flow ratio reduces the Nusselt number. With the moving endwall, the thermal performance of the suction side surface of the blade is affected by the scraping vortex. The effects of endwall motion should be considered during the design of the blade tip.

Thermal Performance of Different Cooled Tips in A High-Pressure Turbine Cascade

The thermal performance of the three turbine tips was investigated. The results are presented in terms of heattransfer coefficient (h), cooling effectiveness (η), net heat-flux reduction (NHFR), and heat load. The calculations were performed using a cooled flat tip, a cooled cavity tip, and a cooled suction-side squealer tip in a cascade at a tip gap of 1.6%C. The results were validated using experimental data where possible. For an uncooled flat tip, the fluid dynamics are dominated by flow separation at the pressure-side edge. A significant benefit of ejecting coolant inside this separation bubble is shown. At the coolant mass-flow ratio M c = 0:52%, both the cooled flat tip and the cooled cavity tip are well-protected by the coolant. For the cooled suction-side squealer tip, the coolant lifts off from the tip floor and leads to not only low cooling effectiveness, but also to high heat-transfer coefficients. The effects of the coolant mass-flow ratio on the thermal performance of the tip were presented. The effects of end-wall motion on the thermal performance of the blade tip are found to be small in the current study.

Wake of a Compressor Cascade with Tip Gap, Part 2: Effects of Endwall Motion

The effects of relative motion between the blade tips and endwall on the flow downstream of a linear compressor cascade have been studied. Endwall motion was simulated by using a 0.25-mm-thick Mylar belt propelled ove ra sliding surface beneath the tips of the cascade blades. Three-component mean-velocity and turbulence measurements made in cross sections downstream of the cascade reveal that the wall motion flattens and shears the turbulence and mean-velocity distributions of the vortex. Mean-helicity density plots also show that endwall motion smears the vortex center from a single point (when seen in cross section) into a ribbon that makes an angle of some 30 deg with the endwall. Despite these effects, many critical features of the vortex are almost unaffected by the endwall motion. The vortex produces almost the same magnitude of streamwise mean-velocity deficit, and this deficit still dominates both the mean velocity field and the production of turbulence. Thus, although endwall motion distorts and displaces the leakage vortex it does not fundamentally alter the mechanisms that govern the development of its mean flow and turbulence structure.

Effect of Endwall Motion on Blade Tip Heat Transfer

Local mass transfer measurements were conducted on the tip of a turbine blade in a five-blade linear cascade with a blade-centered configuration. The tip clearance levels ranged from 0.6 to 6.9% of blade chord. The effect of relative motion between the casing and the blade tip was simulated using a moving endwall made of neoprene mounted on the top of the wind tunnel. Data were obtained for a single Reynolds number of 2.7×105 based on cascade exit velocity and blade chord. Pressure measurements indicate that the effect of endwall motion on blade loading at a clearance of 0.6% of blade chord is to reduce the pressure gradients driving the tip leakage flow. With the introduction of endwall motion, there is a reduction of about 9% in mass transfer levels at a clearance of 0.6% of chord. This is presumably due to the tip leakage vortex coming closer to the suction side of the blade and ‘blocking the flow,’ leading to reduced tip gap velocities and hence lower mass transfer.

Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part I—Effect of Tip Clearance Height

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part I of this two-part paper focuses on the effect of reduced tip clearance height on the leakage flow and vortex. Reduced tip clearance results in less mass flow through the gap, a smaller leakage vortex, and less aerothermal losses in both the gap and the vortex. The shearing of the leakage jet and passage flow to which leakage vortex roll-up is usually attributed to is not observed in any of the simulations. Alternative explanations of the leakage vortex’s roll-up are presented. Additional secondary flows that are seen near the casing are also discussed.

Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part II—Effect of Outer Casing Relative Motion

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part II of this two-part paper series focuses on the effect of relative motion of the outer casing on the leakage flow and vortex development. Casing relative motion results in less mass flow through the gap and a smaller leakage vortex. The structure of the aerothermal losses in the passage change dramatically when the outer casing motion was incorporated, but the total losses in the passage remained very similar. Additional secondary flows that are seen near the casing are also discussed.

Investigation of Tip Clearance Phenomena in an Axial Compressor Cascade Using Euler and Navier–Stokes Procedures

Three-dimensional Euler and full Navier–Stokes computational procedures have been utilized to simulate the flow field in an axial compressor cascade with tip clearance. An embedded H-grid topology was utilized to resolve the flow physics in the tip gap region. The numerical procedure employed is a finite difference Runge-Kutta scheme. Available measurements of blade static pressure distributions along the blade span, dynamic pressure and flow angle in the cascade outlet region, and spanwise distributions of blade normal force coefficient and circumferentially averaged flow angle are used for comparison. Several parameters that were varied in the experimental investigations were also varied in the computational studies. Specifically, measurements were taken and computations were performed on the configuration with and without: tip clearance, the presence of an endwall, inlet endwall total pressure profiles and simulated relative casing rotation. Additionally, both Euler and Navier–Stokes computations were performed to investigate the relative performance of these approaches in reconciling the physical phenomena considered. Results indicate that the Navier–Stokes procedure, which utilizes a low Reynolds number k–ε model, captures a variety of important physical phenomena associated with tip clearance flows with good accuracy. These include tip vortex strength and trajectory, blade loading near the tip, the interaction of the tip clearance flow with passage secondary flow, and the effects of relative endwall motion. The Euler computation provides good but somewhat diminished accuracy in resolution of some of these clearance phenomena. It is concluded that the level of modeling embodied in the present approach is sufficient to extract much of the tip region flow field information useful to designers of turbomachinery.