Squealer Cavity Research Articles

Experimental study on aerodynamic performance of turbine blade squealer tip with different trailing edge designs under high-speed relative casing motion

Over-tip leakage (OTL) flow leads to great aerodynamic performance reduction in high-pressure turbine, reasonable blade tip design can effectively control the loss caused by OTL flow. The relative casing motion is one of the key boundary conditions that can significantly influence OTL flow. In this study, aerodynamical tests were conducted for a high-pressure squealer tip with different trailing edge designs of full cavity squealer tip, pressure-side cutback and suction-side cutback at both stationary and rotating conditions. Loss distribution and blade near tip loading of different trailing edge structures at high-speed rotating condition are firstly reported and evaluated. The result indicates that pressure-side cutback design significantly increases the aerodynamic loss compared with full cavity tip, while suction-side cutback design has close overall loss to full cavity tip. This conclusion was consistent with numerical simulation based on Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) analysis, which reveals that pressure-side cutback design causes the cavity vortex leaks earlier compared with full cavity tip, a small vortex formed at cut region forms and results in higher total pressure loss. The effect of relative motion between blade tip and shroud reinforces this trend. The total pressure loss difference can reach 42 % compared with 8 % in stationary condition, which indicates that relative casing motion needs to be take into consideration when ranking different tip sealing designs.

Investigation of aerodynamic effects and dominant design variables of cavity squealer-tip in three-dimensional performance of a centrifugal compressor

Enhancing the performance of turbomachines has been a longstanding area of interest, with numerous geometry modification methods proposed in the literature. One such method, historically prevalent in turbines, is the cavity squealer tip. However, its application in compressors, particularly of the centrifugal type, remains relatively underdeveloped. This study employs the Taguchi method's L-9 orthogonal array as a sensitivity analysis (utilizing Design of Experiments) to investigate the impact of cavity squealer design variables on the performance of the NASA-CC3 compressor at three key operational points: choke, design, and near stall regions. Computational fluid dynamics is utilized to derive performance parameters, and various sets of design variables result in improvements across pressure ratio, isentropic efficiency, mass flow rate, and surge margin. Notably, a significant 20 percent enhancement in surge margin is achieved in one instance. The performance at the remaining operating points varies among the nine cases, with improvements observed in some areas and declines in others. Processing the data yields three optimal geometries, with the best configuration enhancing all performance parameters (0.59% for design mass flow rate and 2.20% for efficiency, maintaining a constant pressure ratio compared to the base). As anticipated, the optimal impellers exhibit reduced flow leakage compared to the baseline.

Design Optimization of Blade Tip in Subsonic and Transonic Turbine Stages—Part II: Flow Physics and Augmented Aerothermal Integral Objective Function

Abstract In Part I, a companion paper of the two-part article, a subsonic turbine stage and a transonic one conditioned at the same Reynolds number, flow coefficient, loading coefficient and reaction, but two different exit Mach numbers are designed to provide a direct contrast between a high-subsonic and a transonic flow conditioning for rotor blade squealer tips. In this paper as Part II, further analyses are carried out to address the main issues of interest arising from Part I: first, to identify the driving flow physical mechanisms for the contrasting aerodynamic efficiency sensitivities of the two stages; second to seek a more suitable heat transfer objective function for the tip aerothermal design optimization, given the seemingly strong conflicts among those conventionally adopted heat transfer objective functions. Two counter-rotating tip vortical structures, the pressure side vortex (PSV) and the casing-driven cavity vortex (CCV), are shown to impact the aero-performance differently between the two stages. For the subsonic stage, the leakage flow is strongly affected by a stronger residual PSV at the squealer cavity exit. For the transonic stage however, the tip choking in limiting the over tip leakage (OTL) mass flow and favorable pressure gradient in a transonic flow over a separation bubble led to a much stronger and more persistent CCV and thus lower aero-effectiveness of squealer tip for the transonic stage. The two vortices also show major heat transfer signatures on the cavity surfaces by impingement. For the PSV, the impingement impact is mainly on the cavity floor. For the CCV, on the other hand, its impact is mainly on the inner side-wall of the suction side rim. The latter is found to be mainly responsible for the overall linear variations of the heat load with the squealer height. Again, the relative strength between PSV and CCV serves as an effective differentiator in the heat transfer performance. The stronger and more persistent CCV in the transonic stage results in a strong signature on a large portion of the suction side cavity inner sidewall, thus a much higher increment in heat transfer with the squealer height than that for the subsonic stage. In seeking to establish a more consistent heat transfer objective function for combined aerothermal design and optimization, a coolability weighted nonuniformity parameter is proposed to integrate the local heat transfer and the coolability. The proposed objective function is shown to lead to consistent Pareto fronts for the combined aerothermal performance sensitivities, particularly for the present cases with strong heat transfer nonuniformity which are particularly challenging to those conventional treatments as shown in Part I. The coolability-augmented objective function should thus serve as an enabler to help practical applications of blade tip aerothermal design optimizations.

Investigation of the Interaction Between Tip Leakage and Main Annulus Flow in the Large Scale Turbine Rig: Comparison of Different Rotor Tip Geometries

Abstract Shroudless rotor blades are state-of-the-art in modern high pressure turbines. Tip leakage flow has a crucial impact on turbine efficiency. Specific blade tip designs are a key factor to handle tip leakage losses by controlling tip leakage flow and its re-entry into the rotor passage. Comparative measurements of a cavity squealer type tip and a notch type tip have been conducted at the Large Scale Turbine Rig at Technical University of Darmstadt. The test rig has been operated at the blade tips design point. Experimental data have been acquired at rotor inlet and outlet as well as within the rotor passage. For cavity squealer tips, a tip leakage vortex develops at the suction side as the tip leakage flow is rolled-up and further mixed with main annulus flow. The tip leakage vortex determines the blockage of main annulus flow at the blade tip. The design of the suction side of the notch tip benefits a jet-like re-entry of tip leakage flow into the passage. Results are a tip leakage vortex system with smaller sized vortices and a more homogeneous mass flow redistribution in the outer annulus of the rotor. The zone of affected main annulus flow at the blade tip increases through the dominant tip leakage jet.

Experimental Investigation of a High-Speed Turbine With Rainbow Rotor and Rim Seal Purge Flow

AbstractThe present paper addresses the experimental and numerical study of the unsteady flow established in a high-pressure turbine stage with rim seal purge. The HPT test section, operated at engine-relevant flow conditions in the high-speed turbine rig of the von Karman Institute, is heavily instrumented for high-resolution, high-bandwidth aerothermal measurements. A rainbow rotor setup allows the simultaneous testing of six different sectors, each hosting a specific tip and platform geometry optimized for enhanced aerodynamic performance. This paper focuses on the flow over the baseline sector, equipped with an axisymmetric hub platform and a squealer tip, with a purge flowrate matching 1.74% and 1% of the stage mass flow. The numerical study relies on Reynolds-averaged Navier–Stokes (RANS) computations with test-calibrated boundary conditions. Unsteady pressure and heat transfer measurements are performed at the rotor shroud. The maximum heat transfer is achieved along the front pressure side rim, whereas the squealer cavity generates a region of uniformly low static pressure and heat flux. The experimental adiabatic wall temperature is derived to quantify the thermal contribution to the global heat flux, demonstrating an increase in over-tip gas temperature up to 1.2 T01 above the pressure side rim. A Euler-based model is proposed to evaluate the temperature-driving work-exchange mechanism in the tip gap. The peak in casing heat transfer coefficient (750 W/m2K) is found at the tip leading edge. Time-resolved measurements of outlet total pressure, Mach number, and flow angle confirm the predicted phase and intensity of the tip leakage and upper passage vortex in the near-casing region. At 20% hr, the total pressure minimum is highly underpredicted by the RANS, indicating an inaccurate modeling of the interaction between rim seal purge and main flow. The measured impact of the purge flow variation is more significant than predicted by the RANS computations, with a negative offset of about 0.5% P01 in the lower 50% hr at lower purge flowrate.

Aerothermal Performance of Axially Varying Winglet-Squealer Blade Tips

Abstract High-pressure turbine blades are usually susceptible to secondary flow losses due to fluid flow between the casing and the blade tip. In this study, we have evaluated the performance of several blade tip designs for different combinations of winglets and squealer geometries toward mitigating tip leakage losses. The effects of considering relative casing motion (RCM) on the aerothermal performance are also brought out. In particular, we have considered three different blade tip designs, which include winglets, top winglet bottom squealer (TWBS), and top squealer bottom winglet (TSBW). Inspired by the partial winglet configurations studied in the literature, we have also examined designs with partial squealers and winglets. The performance of all the designs and the dynamics within the tip gap is discussed through the distributions of total pressure loss within the tip gap and Nusselt number over the blade tips. Of all the blade tip designs, the aerothermal performance of a 100% TWBS design is demonstrated to be superior, both with and without relative casing motion. When compared to a flat tip design, a 100% TWBS design showed a 15% reduction in total pressure loss and a 22% reduction in the average Nusselt number over the blade tip. For this design, accounting for the relative casing motion showed a marked reduction in the total pressure loss and the heat transferred to the blade tip. In particular, RCM is shown to suppress the “hot spot” associated with a cavity vortex within the squealer cavity.

A parametric study of squealer tip geometries applied in a hydraulic axial turbine used in a rocket engine turbopump

Axial turbines are machines widely used in different engineering applications. Due to their constructive characteristics, they must have a space between the rotor blades and the turbine casing, called tip clearance. Unfortunately, this gap allows a part of the fluid to leak from the pressure side to the suction side of the rotor blades. This leakage is undesirable and represents an energy loss. A way to avoid part of this loss is through the use of desensitization techniques. Although the use of these techniques is widely known, no studies in the open literature have evaluated these techniques in hydraulic turbines. This work presents a numerical analysis of squealer desensitization techniques applied in a hydraulic axial turbine. The turbomachine under study is the first stage of the hydraulic axial turbine used in the Low Pressure Oxidizer Turbopump (LPOTP) of the Space Shuttle Main Engine (SSME). Numerical simulations were performed using CFX v.19.2 software, and computational meshes were generated in ICEM v.19.2 software. Initially, the computational model was validated, using the experimental results published by the National Aeronautics and Space Administration (NASA). A parametric analysis was performed considering the variation in squealer cavity depth and rim thickness. The study found that the squealer cavity depth has a greater influence on the stage performance than its rim thickness. The tendency is that the greater the cavity depth, the greater the stage efficiency. One of the squealer geometries analyzed allowed an average increased efficiency of 1.43%, over the entire turbine operational range. The results obtained also show that the application of the proposed geometries would enable the reduction in cavitation close to the trailing edge of the rotor blades. This result is extremely valuable, as it can impact the life cycle of the turbine.

Optimization of Turbine Cascade Squealer Tip Cooling Design by Combining Shaping and Flow Injection

Abstract In this study, a turbine cascade squealer tip is optimized by a multi-objective genetic algorithm (MOGA) with varying squealer heights and tip cooling configurations. The three objectives selected are the aerodynamic efficiency, the film cooling effectiveness, and the surface fluid temperature variance. The multi-scale methodology is implemented to reduce the computational cost and to skip the meshing of cooling holes. Two optimization approaches are compared: (a) a conventional method that optimizes an uncooled shape and the cooling configuration sequentially and (b) a method that optimizes shaping and cooling concurrently. The concurrent method is found to obtain better aerodynamic efficiency and heat transfer performance than the conventional optimization. Moreover, the aerodynamic efficiency ranking is changed by adding cooling to the uncooled blades. These observations are due to the strong interaction between the coolant and the tip leakage flow. They indicate that the coolant injected at the tip is not passive as expected in the conventional film cooling designs. By blocking the over tip leakage flow or forming a layer of air to level up the equivalent squealer cavity floor, the coolant can reduce the tip leakage loss, which contradicts the conventional wisdom that the added coolant should always lead to extra losses due to the extra mixing. The detailed observations of the flow field indicate that the influence of the squealer height towards the aerodynamic efficiency is caused by two competing effects: the blockage effect to reduce the tip leakage mass flowrate and the sudden expansion/contraction loss effect to generate additional loss.

An aerothermal analysis of the effects of tip gap height and cavity depth of a gas turbine blade

To investigate the effects of tip gap height and the squealer cavity depth on aerothermal performance of a gas turbine rotor, two tip gap height and eight squealer cavity depth are investigated under stage environment. For the aerodynamic performance of blade tips, when the tip gap height is increased from 1.0% span to 1.5% span, the tip leakage flow rate is increased by about 1.0% total mass flow and the total pressure loss coefficient is increased by about 0.03. With the increase of cavity depth, the tip leakage flow rate is reduced but the vortexes loss in the cavity and passage vortex is enlarged, so the total pressure loss coefficient is reduced at first and then is increased. For the film cooling effectiveness of blade tips, the average film cooling effectiveness of tip is improved at a large tip gap height and a small cavity depth. With the increase of cavity depth, the area needed cooling protection is increased; the relative location between the film hole and the separation line is changed, resulting in the change of film area of the squealer tip. Thus, the film cooling effectiveness of the squealer tip is reduced with fluctuation when the cavity depth is increased. In all, the influence of the tip gap height and cavity depth on tip leakage flow rate, total pressure loss coefficient and film cooling effectiveness is conflicting. Determining an optimal cavity depth in which more film holes located on the separation line for a fixed tip gap height is important to obtain better aerodynamic performance and film cooling performance.

Effects of the novel rib layouts on the tip leakage flow pattern and heat transfer performance of turbine blade

The steady simulation with flow field and heat transfer performance for different rib layouts is investigated. By referring the GE-E3 turbine blade tip (Case 1) profile as a prototype model, four kinds of rib layouts of full rib structure (Case 2), half rib structure connected with suction side (Case 3), half rib structure connected with pressure side (Case 4), and half rib structure in the rear squealer cavity (Case 5) was designed. The availability of k-ω turbulence model was validated through comparison of heat transfer coefficient distribution with the experimental data. The area-averaged heat transfer coefficients of Case 5 decreases up to 11.3%, 3.1%, 11.3%, and 2.8% in comparison to Cases 1, 2, 3, and 4. The total pressure coefficient of Case 5 reduces by 1.4%, 2.7%, and 4.0% compared to Cases 1, 2, and 3. The results reveal that, among the five novel rib layout cases, Case 5 obtains the optimal comprehensive performance under the performance of considering tip heat transfer and cascade loss. Further investigation on the influence of Cases 1 and 5 at different rotational speeds on the flow field and heat transfer characteristics of the tip clearance is also illustrated and discussed.

Heat Transfer Coefficient and Film Cooling Effectiveness on the Partial Cavity Tip of a Gas Turbine Blade

Leakage flow between the rotating turbine blade tip and the fixed casing causes high heat loads and thermal stress on the tip and near the tip region. For this study, new squealer tips called partial cavity tips, which combine the advantages of plane and squealer tips, were suggested, and the effects of the cavity shape on the tip heat transfer coefficient and film cooling effectiveness were investigated experimentally in a low-speed linear cascade. The suggested blade tips had a flat surface near the leading edge and a squealer cavity from the mid-chord to trailing edge region to achieve the advantages of both blade tip types. The heat transfer coefficient was measured via the 1-D transient heat transfer technique using an IR camera, and the film cooling effectiveness was obtained via the pressure-sensitive paint (PSP) technique. Results showed that the heat transfer coefficient and film cooling effectiveness on the partial cavity tips strongly depended on the cavity shape. Near the leading edge, the heat transfer coefficients for the partial cavity tip cases were lower than that for the squealer tip case. However, the heat transfer coefficient on the cavity surface was higher for the partial cavity tip cases. The D10 tip showed a similar distribution of film cooling effectiveness to that of the plane (PLN) tip near the leading edge and the double side squealer (DSS) tip near the mid-chord region. However, the overall average film cooling effectiveness of the DSS tip was higher than that of the D10 tip.

Performance of Partial and Cavity Type Squealer Tip of a HP Turbine Blade in a Linear Cascade

Three-dimensional highly complex flow structure in tip gap between blade tip and casing leads to inefficient turbine performance due to aerothermal loss. Interaction between leakage vortex and secondary flow structures is the substantial source of that loss. Different types of squealer tip geometries were tried in the past, in order to improve turbine efficiency. The current research deals with comparison of partial and cavity type squealer tip concepts for higher aerothermal performance. Effects of squealer tip have been examined comprehensively for an unshrouded HP turbine blade tip geometry in a linear cascade. In the present paper, flow structure through the tip gap was comprehensively investigated by computational fluid dynamic (CFD) methods. Numerical calculations were obtained by solving three-dimensional, incompressible, steady, and turbulent form of the Reynolds-averaged Navier-Stokes (RANS) equations using a general purpose and three-dimensional viscous flow solver. The two-equation turbulence model, shear stress transport (SST), has been used. The tip profile belonging to the Pennsylvania State University Axial Flow Turbine Research Facility (AFTRF) was used to create an extruded solid model of the axial turbine blade. For identifying optimal dimensions of squealer rim in terms of squealer height and squealer width, our previous studies about aerothermal investigation of cavity type squealer tip were utilized. In order to obtain the mesh, an effective parametric generation has been utilized using a multizone structured mesh. Numerical calculations indicate that partial and cavity squealer designs can be effective to reduce the aerodynamic loss and heat transfer to the blade tip. Future efforts will include novel squealer shapes for higher aerothermal performance.

Experimental study on axially non-uniform clearances in a linear turbine cascade with a cavity squealer tip

The effects of axially non-uniform clearances on the tip leakage flow and aerodynamic performance in a linear turbine cascade with a cavity squealer tip were investigated in this study with the objective of improving the flow loss and tip flow field structure. A calibrated five-hole probe was used for the measurement of three-dimensional flows downstream of the cascade. The method of oil-flow visualization was used to show the endwall flow field structure. The distribution of endwall static pressure was measured particularly by using the special moveable endwall. The axially non-uniform clearance, as a novel strategy that has a non-negligible influence on tip clearance flow and clearance leakage loss, may become a potential technology for improving aerodynamic performance in turbine cascades. By using the expanding clearance, the flow loss at the outlet is reduced effectively and an apparent improvement of aerodynamic performance in the turbine cascade is gained. Under the tip clearances of 0.75% H and 2% H, the maximum reduction of overall total pressure loss coefficient at the outlet is separately about 2.3% and 3.5% compared with the uniform clearance. The shrinkage of the buffer zone is considered to be able to weaken the interaction of the tip leakage vortex and passage vortex and thus reduce the loss of passage vortex. For the shrinking clearance, a noticeable decline in the aerodynamic performance of turbine cascade with cavity squealer tip is exhibited at both on and off design conditions in contrast to the uniform clearance. In addition, the effects of axially non-uniform clearances on the aerodynamic performance at off-design conditions have been investigated.

Investigations of film-cooling effectiveness on the squealer tip with various film-hole configurations in a linear cascade

An experimental investigation is performed to study the effect of film-hole configurations on the blade tip film cooling performance in a five-blade linear cascade. Six film-hole configurations are taken into consideration under four typical blowing ratios. In Type-A, Type-B and Type-C, the film holes are arranged in a single row along the middle-camber line with different hole-to-hole pitches. Type-D, Type-E and Type-F have the same film-hole number of 13 as Type-B. For Type-D, the film holes are also arranged in a row, but they are located close to the suction-side squealer. For Type-E, the film holes are arranged in two rows. For Type-F, two rows of 8 holes are concentrated at the leading edge, and the rest 5 holes are arranged in a line at the middle chord region of the blade tip. Besides, some numerical simulations are conducted to provide detailed coolant jet trajectory. The results show that film-hole arrangement has a great impact on the tip film-cooling effectiveness. The coolant issuing from middle-camber holes are suctioned by the vortex structures inside squealer cavity to flow towards the pressure side, producing good film coverage at the local zone between the middle-camber line and pressure-side squealer for Type-A, Type-B and Type-C. For Type-F, as more film holes are concentrated at the leading edge, the film coverage on the front tip surface is well improved compared to the other film-hole arrangements. Under the same coolant usage, Type-F obtains the most favorable film cooling performance except for the trailing edge, and Type-C produces wider film coverage extending to trailing edge.

Control of Tip Leakage in a High-Pressure Turbine Cascade Using Tip Blowing

Blowing from the tip of a turbine blade was studied experimentally to determine if total pressure loss could be reduced. Experiments were done with a linear cascade in a low-speed wind tunnel. Total pressure drop through the blade row and secondary velocity fields in the passage between two blades were measured. Cases were documented with various blowing hole configurations on flat and squealer tipped blades. Blowing normal to the tip was not helpful and in some cases increased losses. Blowing from the bottom of a squealer cavity provided little benefit. With a flat tip, blowing from holes located near and inclined toward the pressure side generally reduced total pressure drop by reducing the effect of the tip leakage vortex. Holes near the axial location of maximum loading were most helpful, while holes closer to the leading and trailing edges were not as effective. Higher jet velocity resulted in larger total pressure drop reduction. With a tip gap of 1.5% of axial chord, jets with a velocity 1.5 times the cascade inlet velocity had a significant effect. A total pressure drop reduction of the order 20% was possible using a jet mass flow of about 0.4% of the main flow. Jets were most effective with smaller tip gaps, as they were more able to counter the leakage flow.

Cooling Injection Effect on a Transonic Squealer Tip—Part I: Experimental Heat Transfer Results and CFD Validation

Recent studies have demonstrated that the aerothermal characteristics of turbine rotor blade tip under a transonic condition are qualitatively different from those under a low-speed subsonic condition. The cooling injection adds further complexity to the over-tip-leakage (OTL) transonic flow behavior and aerothermal performance, particularly for commonly studied shroudless tip configurations such as a squealer tip. However there has been no published experimental study of a cooled transonic squealer. The present study investigates the effect of cooling injection on a transonic squealer through a closely combined experimental and CFD effort. Part I of this two-part paper presents the first of the kind tip cooling experimental data obtained in a transonic linear cascade environment (exit Mach number 0.95). Transient thermal measurements are carried out for an uncooled squealer tip and six cooling configurations with different locations and numbers of discrete holes. High-resolution distributions of heat transfer coefficient and cooling effectiveness are obtained. ansysFluent is employed to perform numerical simulations for all the experimental cases. The mesh and turbulence modeling dependence is first evaluated before further computational studies are carried out. Both the experimental and computational results consistently illustrate strong interactions between the OTL flow and cooling injection. When the cooling injection (even with a relatively small amount) is introduced, distinctive series of stripes in surface heat transfer coefficient are observed with an opposite trend in the chordwise variations on the squealer cavity floor and on the suction surface rim. Both experimental and CFD results have also consistently shown interesting signatures of the strong OTL flow–cooling interactions in terms of the net heat flux reduction distribution in areas seemingly unreachable by the coolant. Further examinations and analyses of the related flow physics and underlining vortical flow structures will be presented in Part II.

Heat/mass transfer over the cavity squealer tip equipped with a full coverage winglet in a turbine cascade: Part 1 – Data on the winglet top surface

Heat/mass transfer characteristics on the winglet top surface for the cavity squealer tip equipped with a full coverage winglet has been investigated experimentally with the variation of h/s (tip gap height-to-span ratio) by employing the naphthalene sublimation technique. For a squealer rim height-to-span ratio of hst/s=3.75% and a winglet width-to-pitch ratio of w/p=10.55%, the tip gap is changed to be h/s=0.34%, 0.68%, 1.02%, 1.36%, and 1.70%. On the winglet top surface, high local heat/mass transfer rate is found (i) at the sites of the leading edge tip gap vortices, (ii) in the flow separation/reattachment area on the pressure-side winglet, (iii) in the upstream outflow area on the suction-side winglet, (iv) in the downstream outflow area on the suction-side winglet, and (v) in the area where the cavity fluid is discharged toward the trailing edge. On the other hand, very low local heat/mass transfer rate is observed in two separate areas on the suction-side winglet upstream of the mid-chord. In the case of very low h/s, the two separate low heat/mass transfer areas cannot be identified. Instead, a new high heat/mass transfer area is found near the mid-chord on the suction-side winglet. Average heat/mass transfer rate on the winglet top surface tends to increase consistently with increasing h/s, but it is always lower than that on the tip surface of the plane tip with no winglet.

Heat/mass transfer over the cavity squealer tip equipped with a full coverage winglet in a turbine cascade: Part 2 – Data on the cavity floor

Heat/mass transfer characteristics on the floor of the cavity squealer tip equipped with a full coverage (FC) winglet has been investigated experimentally with the variation of h/s (tip gap height-to-span ratio) by employing the naphthalene sublimation technique. For a squealer rim height-to-span ratio of hst/s=3.75% and a winglet width-to-pitch ratio of w/p=10.55%, the tip gap is changed to be h/s=0.34%, 0.68%, 1.02%, 1.36%, and 1.70%. In this study, the cavity floor is divided into four specific regions based on local heat/mass transfer rate, and the effects of h/s on local heat/mass transfer rate are fully discussed. The results show that average heat/mass transfer rate on the floor of the cavity squealer tip with the FC winglet increases steeply with the increment of h/s. This average value is found lower in the case of low h/s but higher in the case of high h/s, in comparison with that for the cavity squealer tip with no winglet. However, it is always lower than that on the plane tip surface with no winglet. For the cavity squealer tip with the FC winglet, average heat/mass transfer rate on the cavity floor is lower than that on the winglet top surface regardless of h/s, but the discrepancy between them tends to decrease rapidly as h/s increases.

Over-tip leakage flow and loss in a turbine cascade equipped with suction-side partial squealers

Over-tip leakage flow and loss in a turbine cascade equipped with suction-side partial squealer rims have been investigated for the squealer rim height-to-span ratios (hst/s) of 0.94%, 1.88%. 3.75%, and 5.63% in the case of a tip gap height-to-span ratio of h/s=1.36%. The casing wall and tip surface visualizations for hst/s=3.75% show that most of the incoming tip leakage flow tends to accelerate through a convergent (nozzle-like) tip gap flow channel and penetrates into the neighboring blade flow passage even upstream of the mid-chord in the form of a wall jet, whereas the rest of it is entrapped by the suction-side squealer rim, flows backward, and is separated from the tip surface along a backward flow separation line. Therefore, the tip surface can be divided into a separation bubble and a backward flow area by the backward flow separation line. A qualitative tip gap flow model for the suction-side squealer tip is suggested in this study. For the present suction-side squealer tip, the total pressure loss coefficient mass-averaged throughout the present measurement plane decreases consistently with increasing hst/s and is higher than that for the cavity squealer tip or the pressure-side squealer tip regardless of hst/s.

An Experimental and Numerical Study on the Aerothermal Characteristics of a Ribbed Transonic Squealer-Tip Turbine Blade With Purge Flow

Detailed heat transfer coefficient (HTC) and film cooling effectiveness (Eta) distribution on a squealer-tipped first stage rotor blade were measured using an infrared technique. The blade tip design, obtained from the Solar Turbines, Inc., gas turbine, consists of double purge hole exits and four ribs within the squealer cavity, with a bleeder exit port on the pressure side close to the trailing edge. The tests were carried out in a transient linear transonic wind tunnel facility under land-based engine representative Mach/Reynolds number. Measurements were taken at an inlet turbulent intensity of Tu = 12%, with exit Mach numbers of 0.85 (Reexit = 9.75 × 105) and 1.0 (Reexit = 1.15 × 106) with the Reynolds number based on the blade axial chord and the cascade exit velocity. The tip clearance was fixed at 1% (based on engine blade span) with a purge flow blowing ratio, BR = 1.0. At each test condition, an accompanying numerical study was performed using Reynolds-averaged Navier–Stokes (RANS) equations solver ansys fluent to further understand the tip flow characteristics. The results showed that the tip purge flow has a blocking effect on the leakage flow path. Furthermore, the ribs significantly altered the flow (and consequently heat transfer) characteristics within the squealer-tip cavity resulting in a significant reduction in film cooling effectiveness. This was attributed to increased coolant–leakage flow mixing due to increased recirculation within the squealer cavity. Overall, the peak HTC on the cavity floor increased with exit Mach/Reynolds number.