Search Results (14)

Turbines experience pressure losses from various sources, one of which is the tip leakage flow in the rotor blades. This is one of the main factors responsible for the decrease in turbine efficiency. This leakage is caused by pressure differences between the blade pressure and suction sides. High-pressure turbines with low aspect ratios and high-pressure loading face critical tip clearance losses, impacting turbine performance. One way to reduce tip leakage flow is to apply the desensitization technique to modify the rotor blade tip geometry. This study aims to apply the desensitization technique to the Energy-Efficient Engine developed by NASA. Different Winglet geometries with varying extensions along the blade tip chord (A—$100\%$, B—$80\%$, and C—$60\%$), three types of Squealers with different rim dimensions and cavity heights (Squealer A and B), and the same rim thickness and cavity height of Squealer A with a decreased trailing edge region down to $1\%$ (Squealer C) were numerically tested. Additionally, the study simulates blending Winglet A with Squealer A (Squealer–Winglet A), Squealer A with Winglet B (Squealer–Winglet B), and Winglet A with Squealer B (Squealer–Winglet C). Numerical simulations are conducted and compared with experimental data. Comparing the various geometries at the design-point pressure ratio, the Winglet A configuration demonstrates an increase of $0.30\%$ in efficiency, Squealer C an increase of $0.20\%$, and for cases involving all Squealer–Winglet models, no improvement was obtained. For $80\%$ N at the design-point pressure ratio, Winglet B demonstrates an increase of $1.47\%$ in efficiency, Squealer C an increase of $1.43\%$, and Squealer–Winglet A an increase of $1.43\%$. These are interesting results in the case of the engine operating at cruise condition, in which the rotational speed is around $80\%$ N. Full article

This paper presents an experimental and computational investigation of novel, ejector-based, Coandă-surface-assisted tip leakage mitigation schemes. The predicted changes in the key performance metrics are presented after explaining the aerodynamic concept development for the novel tip geometries. The performance metrics are the stage total-to-total isentropic efficiency, tip-gap mass flow rate, and a figure of merit based on rotor exit total pressure. The schemes are based on direct geometric modifications to the turbine blade tip, effectively promoting an effective redirection of tip leakage fluid via specific channels. The proposed ejector channels operate in conjunction with strategically located Coandă surfaces to alter the path of the leakage fluid, promoting an effective leakage fluid delivery into the blade’s wake. Multiple schemes are assessed, including single-ejector, single-ejector with “hybrid” squealer, double-channeled, and triple-channeled designs. The designs are evaluated computationally for the HP stage of the Axial Flow Turbine Research Facility AFTRF at Penn State University. Extensive experimental validation of the baseline flow computations for the HP stage is also presented. Upper-bound efficiency gains of 0.49% and mass flow reductions of 14.80% compared to an untreated flat tip for the large-scale turbine test rig AFTRF are reported. Evaluation of the current tip designs in a high-speed turbine cascade environment with a transonic exit flow has also been completed. The detailed results from the high-speed investigation and heat transfer impact are in the process of being published. Implementation in the high-speed environment of the same design concepts also returned non-negligible performance gains. Full article

A clearance necessarily exists between the blade tip and the casing in turbines. A leakage flow, formed by the accelerated gas through the tip clearance, is a major cause of turbine stage efficiency loss. Severe heat loads on the blade tip surface also result from a leakage flow, a primary cause of blade damage. Although the understanding of leakage flow mechanisms is mature after years of research, the continuous rise in turbine inlet temperature, pursuing higher engine thrust, requires more effective cooling methods for the blade tip region. This paper presents a review of research on three fundamental tip structures (flat tip, squealer tip, and winglet tip) to explain their design concepts, analyze their respective flow mechanisms as well as heat transfer characteristics, and introduce various modified designs. Various film cooling arrangements applied to these tip structures are examined to identify effective strategies that strengthen the advantages of structural optimization. In view of engineering applications, this paper reviews research on unsteady wake interactions as the aforementioned framework, hoping to provide readers a more comprehensive understanding. Full article

This study deals with the effectiveness of casing-injection for a few squealer tip designs in a turbine stage to mitigate tip leakage penalties. Seventy-two Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations were conducted. Five factors were examined: number of injection holes, axial position, jet inclination, blowing ratio, and hole diameter. The ideal configuration demonstrated the highest aerodynamic loss reduction compared to the baseline flat tip by 2.66%. The optimal injection scheme was integrated with three tip-rim topologies: complete channel squealer, suction-side partial squealer, and pressure-side partial squealer. The channel squealer enhances the advantageous effects of injection; the injected jets produce a counter-rotating vortex pair that disturbs the tip leakage vortex core, while the cavity formed by the squealer rim captures low-momentum fluid, thus thermally protecting the tip surface. The injection combined with channel squealer had the highest stage isentropic efficiency and the lowest total-pressure loss, thereby validating the synergy between active jet momentum augmentation and passive geometric sealing. The best configuration shows a 2.87% total pressure loss decrement and a 4.49% total-to-total efficiency increment compared to the baseline design. The best configuration not only improved stage efficiency but also achieved a 43.9% decrease in the tip heat transfer coefficient. Full article

Turbines are rotating machines that generate power by the expansion of a fluid; due to their characteristics, these turbomachines are widely applied in aerospace propulsion systems. Due to the clearance between the rotor blade tip and casing, there is a leakage flow from the blade pressure to the suction sides, which generates energy loss. There are different strategies that can be applied to avoid part of this loss; one of them is the application of so-called desensitization techniques. The application of these techniques on gas turbines has been widely evaluated; however, there is a lack of analyses of hydraulic turbines. This study is a continuation of earlier analyses conducted during the first stage of the hydraulic axial turbine used in the low-pressure oxidizer turbopump (LPOTP) of the space shuttle main engine (SSME). The previous work analyzed the application of squealer geometries at the rotor tip. In the present paper, winglet geometry techniques are investigated based on three-dimensional flowfield calculations. The commercial CFX v.19.2 and ICEM v.19.2 software were used, respectively, on the numerical simulations and computational mesh generation. Experimental results published by the National Aeronautics and Space Administration (NASA) and data from previous works were used on the computational model validation. The parametric analysis was conducted by varying the thickness and width of the winglet. The results obtained show that by increasing the winglet thickness, the stage efficiency is also increased. However, the geometric dimension of its width has minimal impact on this result. An average efficiency increase of 2.0% was observed across the entire turbine operational range. In the case of the squealer, for the design point, the maximum efficiency improvement was 1.62%, compared to the current improvement of 2.23% using the winglet desensitization technique. It was found that the proposed geometries application also changes the cavitation occurrence along the stage, which is a relevant result, since it can impact the turbine life cycle. Full article

In high-pressure turbines, there is a large difference in temperature between the mainstream and the turbine blade surface. Most of the turbine blade tip heat transfer studies were conducted under the assumption that the Over-Tip-Leakage (OTL) flow field is independent of the wall thermal condition. Recent numerical and experimental studies have revealed that the two-way coupling effect between aerodynamics and heat transfer should not be neglected. The heat transfer coefficient obtained by the conventional method is not able to match the realistic engine condition accurately. This study investigates the impact of the wall thermal boundary condition on the tip cooling performance of squealer turbine blades. The RANS CFD result was validated against experimental tip heat transfer data obtained from a high-speed test rig with the effect of high-speed relative casing motion. The aerothermal performance for both uncooled and cooled squealer tips was studied at two different gas-to-wall temperature ratios, 1.7 and 1.1; the reference temperature is 204 K. It was found that the location and strength of cavity vortices varied with different wall thermal boundary conditions, leading to different signatures in tip heat transfer and cooling performance. It is recommended that the experimental heat transfer data and film cooling effectiveness obtained at the near-adiabatic wall boundary condition should be corrected before their application to the tip cooling design process. It would be more reliable to match the wall-to-gas temperature ratio during the tip experimental study. Full article

Cavity width and height are two key geometric parameters of squealer tips, which could affect the control effect of squealer tips on tip leakage flow (TLF) of gas turbines. To explore the optimal values and the control mechanisms of cavity width and height, various cases with different cavity widths and heights are investigated by solving the steady Reynolds Averaged Navier–Stokes (RANS) equations. In this study, the range of cavity width is 9.2–15.1 τ, and that of cavity height is 1.0–3.5 τ. The results show that the optimal value of cavity height is 2.5–3.0 τ, and that of cavity width is about 10.0–10.5 τ. The small cavity width could restrain the breakdown of tip leakage vortex (TLV) and reduce the extra mixing loss. Both small cavity width and large cavity height could enhance the blocking effect on the TLF, reducing the corresponding mixing loss. However, both of them will inhibit the length of the scraping vortex (SV), which is bad for the control of loss. In addition, large cavity height could reduce the loss inside the clearance, while small cavity width could not. This work could provide a reference for the design of squealer tip. Full article

This study experimentally investigated the effects of cascade inlet velocity on the distribution and the level of the heat transfer coefficient on a gas turbine blade tip. The tests were conducted in a transient turbine test facility at Korea Aerospace University, and three cascade inlet velocities—30, 60, and 90 m/s—were considered. The heat transfer coefficient was measured using the transient IR camera technique with a linear regression method, and both the squealer and plane tips were investigated. The results showed that the overall averaged heat transfer coefficient was generally proportional to the inlet velocity. As the inlet velocity is increased from 30 m/s to 60 m/s and 90 m/s, the heat transfer coefficient increased by 11.4% and 25.0% for plane tip, and 26.6% and 64.1% for squealer tip, respectively. However, the heat transfer coefficient near the leading edge of the squealer tip and the reattachment region of the plane tip was greatly affected by the cascade inlet velocity. Therefore, heat transfer experiments for a gas turbine blade tip should be performed under engine simulating conditions. Full article

Rotor tip film cooling is investigated at the Large Scale Turbine Rig, which is a 1.5-stage axial turbine rig operating at low speeds. Using pressure sensitive paint, the film cooling effectiveness $\eta$ at a squealer-type blade tip with cylindrical pressure-side film cooling holes is obtained. The effect of turbine inlet swirl on $\eta$ is examined in comparison to an axial inflow baseline case. Coolant-to-mainstream injection ratios are varied between 0.45% and 1.74% for an engine-realistic coolant-to-mainstream density ratio of 1.5. It is shown that inlet swirl causes a reduction in $\eta$ for low injection ratios by up to 26%, with the trailing edge being especially susceptible to swirl. For injection ratios greater than 0.93%, however, $\eta$ is increased by up to 11% for swirling inflow, while for axial inflow a further increase in coolant injection does not transfer into a gain in $\eta$ . Full article

The present research focused on the analysis of the leakage flows developing from advanced blade tip geometries. The aerodynamic field of a contoured blade tip and of a high-performance rimmed blade were investigated against a baseline squealer rotor. Time-resolved numerical predictions were combined with high-frequency pressure measurements to characterize the tip leakage flow of each tip design. High spatial and temporal resolution measurements provided a detailed representation of the unsteady flow in the near-tip region and at the stage outlet. Numerical computations, based on the nonlinear harmonic method, were employed to assess the unsteady blade row interactions and identify the loss generation mechanisms depending on the tip design. The space- and time-resolved flow field was analysed by modal decomposition to identify the main periodicities of the near-tip and outlet flow and classify the most relevant sources of aerodynamic unsteadiness and entropy generation across the stage. Full article

The turbine tip geometry can significantly alter the performance of the turbine stage; its design represents a challenge for a variety of reasons. Multiple disciplines are involved in its design and their requirements limit the creativity of the designer. Multi-Disciplinary Design Optimisation (MDO) offers the capability to improve the performance whilst satisfying all the design constraints. This paper presents a novel design of a turbine tip achieved via MDO techniques. A fully parametrised Computer-Aided Design (CAD) model of the turbine rotor is used to create the squealer geometry and to control the location of the cooling and dust holes. A Conjugate Heat Transfer Computational Fluid Dynamics (CFD) analysis is performed for evaluating the aerothermal performance of the component and the temperature the turbine operates at. A Finite Element (FE) analysis is then performed to find the stress level that the turbine has to withstand. A bi-objective optimisation reduces simultaneously the aerodynamic loss and the stress level. The Multipoint Approximation Method (MAM) recently enhanced for multi-objective problems is chosen to solve this optimisation problem. The paper presents its logic in detail. The novel geometry offers a significant improvement in the aerodynamic performance whilst reducing the maximum stress. The flow associated with the new geometry is analysed in detail to understand the source of the improvement. Full article

This article provides an overview of gas turbine blade tip external cooling technologies. It is not the intention to comprehensively review all the publications from past to present. Instead, selected reports, which represent the most recent progress in tip cooling technology in open publications, are reviewed. The cooling performance on flat tip and squealer tip blades from reports are compared and discussed. As a generation conclusion, tip clearance dimension and coolant flow rate are found as the most important factors that significant influence the blade tip thermal performance was well as the over tip leakage (OTL) flow aerodynamics. However, some controversial trends are reported by different researchers, which could be attributed to various reasons. One of the causes of this disagreement between different reports is the lacking of unified parametric definition. Therefore, a more appropriate formula of blowing ratio definition has been proposed for comparison across different studies. The last part of the article is an outlook of the new techniques that are promising for future tip cooling research. As a new trend, the implementation of artificial intelligence techniques, such as genetic algorithm and neural network, have become more popular in tip cooling optimization, and they will bring significantly changes to the future turbine tip cooling development. Full article

The decrease in the performance of centrifugal compressors operating at low Reynolds numbers (e.g., unmanned aerial vehicles at high altitudes or small turbomachines) can reach 10% due to increased friction. The purposes of this review are to represent the state-of-the-art of the active and passive flow control methods used to improve performance and/or widen the operating range in numerous engineering applications, and to investigate their applicability in low-Reynolds-number centrifugal compressors. The applicable method should increase performance by reducing drag, increasing blade loading, or reducing tip leakage. Based on the aerodynamic and structural demands, passive methods like riblets, squealers, winglets and grooves could be beneficial; however, the drawbacks of these approaches are that their performance depends on the operating conditions and the effect might be negative at higher Reynolds numbers. The flow control method, which would reduce the boundary layer thickness and reduce wake, could have a beneficial impact on the performance of a low-Reynolds-number compressor in the entire operating range, but none of the methods represented in this review fully fulfil this objective. Full article

An experimental and numerical investigation into the tip leakage flow of a turbine rotor is carried out using a particle image velocimetry (PIV) system and the commercial software ANSYS CFX 14.0. The specimen used in this work is a typical GE-E³ model with a new squealer tip design. The experimental data are used to create a turbulence model and numerical strategy. Through the validated turbulence model and numerical strategy, simulations are carried out to compare the characteristics of the tip leakage flow in three cases: (1) the blade is rotating, but the shroud is stationary, which is the real status of turbine rotor operation; (2) the blade is stationary, but the shroud moves, to simulate their relative movement; (3) the blade is stationary, and the shroud is also stationary, this is a simplified case, but has been widely used in the experiments on rotor tip leakage flow. Detailed analysis of the flow phenomena shows that the second case is a reasonable alternative approach to simulate the real state. However, the flow patterns in the third case exhibit some evident differences from the real status. These differences are caused by the inaccurate viscous force arising from the stationary blade and shroud. In this work, a modification method for the experiments conducted in the third case is firstly proposed, which is realized through adding an imaginary roughness at the shroud wall to be close to the real viscous effect, and to thereby reduce the deviation of the experiment from the real case. According to the results calculated by ANSYS CFX, the flow structure in the modification case is very close to the real status. Besides, this modification case is an easy and cheap way to simulate the real tip leakage flow. Full article