Squealer Tip Research Articles

An experimental and numerical investigation of film cooling effectiveness on a gas turbine blade tip region

In this paper, the effects of coolant flow rate, location of film holes, tip clearance height, and density ratio on blade tip film cooling effectiveness were experimentally investigated using pressure-sensitive paint. And numerical simulation was used to analyze the flow characteristic. Three different turbine blade tips were studied under a mainstream Reynolds number of 6.7 × 105. The pressure ratio was 1.7 and the blowing ratio ranged from 0.4 to 2.2. The results indicated that the cooling effectiveness gradually increased as the blowing ratio increased. The film tends to lift off under a larger blowing ratio, resulting in a decrease in the cooling effectiveness. The squealer tip with an inclined pressure side rim exhibited better film cooling effectiveness under the design conditions. The film cooling effectiveness on the pressure-side rim was lower than that on the suction-side rim under the same operating conditions. The analyses also indicated that an increase in the density ratio effectively enhances film cooling. Compared with the most published investigations conducted in a low-speed wind tunnel, these studies can be better applied to turbine blade cooling design.

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.

Experimental and numerical study of novel tip configurations in a turbine cascade

Two novel tip modelling methods are proposed in this paper. One novel tip is established by the mathematical method and the other is generated by the B-spline surface. And the genetic algorithm, the orthogonal design and the Kriging model are applied to optimize two kinds of novel tips numerically. The optimal mathematical tip and the optimal B-spline tip are acquired, respectively. The aerodynamic tests were investigated at the wind tunnel test section, including the flat tip, the squealer tip, the optimal mathematical tip and the optimal B-spline tip. The static pressure distributions on the casing and the blade surface were measured by the pressure taps. Meanwhile, the secondary flow and the loss were obtained by the five-hole probe at the exit section. A comparison of the simulation analysis results with the experimental results shows good agreement. Compared with the flat tip, the measured loss on the squealer tip, the optimal mathematical tip and the optimal B-spline tip is decreased by 7.2%, 10.5% and 12.6%, respectively.

Comparisons of blade tip phantom cooling effectiveness for two tip structures with three tip clearances

The blade tip region of the gas turbine is exposed to an extreme operating condition with a high heat load. The conventional cooling concept that places cooling holes on the tip would consume a lot of cooling air and lead to a reduction in thermal efficiency. Whereas phantom cooling was supposed to help cool the tip region without using additional cooling air. In this paper, computational comparisons were conducted to forecast the tip phantom cooling effects caused by the blade ejections by employing the standard k-ω model. Then, phantom cooling performance was presented under the flat tip (FT) and the squealer tip (ST), with three tip clearances. Results show that the FT shows a better phantom cooling performance compared to the ST. The phantom cooling effects on the FT are distributed at the tip forepart and near the pressure side (PS). Whereas, for the ST, traces of phantom cooling are barely detected on the PS rim. Increasing the tip clearance would weaken the phantom cooling performance on the FT forepart but enhance it on the rear part. For the ST, its phantom cooling effectiveness values decrease as the tip clearance increases. A lower aerodynamic loss is obtained for the ST under any coolant MFR or tip clearance.

Numerical Study on Vortex Structures and Loss Characteristics in a Transonic Turbine with Various Squealer Tips

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.

Very Large Eddy Simulation of Aero-Thermal Performance in Squealer Tip Gap

Abstract To improve the resolution accuracy and get deep insight into the flow structures in squealer tip gap, the very large Eddy simulation (VLES) method was implemented into the commercial computational fluid dynamics (CFD) solver with the user-defined function (UDF). Based on the published experimental data, the numerical accuracy of VLES method was validated. With the VLES method, the unsteady heat transfer coefficient distributions on the squealer tip and total pressure loss in the blade passage were computed. The influences of coherent vortex structures on aero-thermal performance in the squealer tip gap were analyzed. The results show that the Brown-Roshko vortices are the main driver for the formation of cavity vortex system. The direct impingement of pass-over leakage into the cavity is the main cause of high heat transfer area on the cavity floor near leading edge. The unsteady fluctuations of leakage rate through the tip gap reach about ±8% of the time-averaged value. The development of leakage vortex accounts for the major contribution of total pressure loss in the squealer tipped blade. Due to flow unsteadiness, the fluctuation of pitch-averaged total pressure loss coefficient induced by leakage vortex system reaches about ±30% of the time-averaged value. The unsteady fluctuation of pitch-averaged heat transfer coefficient on the cavity floor reaches about ±35% of the time-averaged value, while on the shroud surface it is only fluctuated by about ±10%.

Effects of tip gap on transonic turbine blade heat transfer characteristics with pressure side film cooling

Investigated are local, spatially-resolved and line-averaged distributions of surface adiabatic film cooling effectiveness and surface heat transfer coefficient ratios for a transonic turbine blade tip. The tip contains a squealer rim, and a single row of film cooling holes is located on the pressure-side of the blade very near to the blade tip. A two-dimensional linear cascade, with four flow passages and five complete blades, is utilized for the investigation. Associated data are provided for different film cooling flow conditions for two different tip gaps, which correspond to 1.16 and 0.66% of true blade span. Experimentally-measured, spatially-resolved surface data demonstrate that improved surface thermal protection on the squealer tip surface, from pressure side film cooling, is often present with a smaller tip gap. Such a conclusion is illustrated by heat transfer coefficient values at many surface locations which are generally lower, and by adiabatic film cooling effectiveness magnitudes which are often substantially larger for the smaller tip gap, relative to values associated with the larger tip gap. Associated effectiveness values, along the upper pressure side, vary in a significant manner as blowing ratio changes. Along the tip region of the blade, for both tip gap values, line-averaged adiabatic film cooling effectiveness generally increases with increasing blowing ratio for the pressure side rim. Effectiveness data for the suction side rim with the smaller tip gap, show more complicated variations as blowing ratio changes. Heat transfer coefficient data along the squealer tip surface and on the upper pressure side of the blade surface generally have variations which are similar to the baseline blade with no film cooling for many surface locations. The local deviations, which are present, are due to locally augmented mixing and shear, and increased turbulent transport from the advective presence of the film coolant.

Uncertainty quantification and sensitivity analysis of aerothermal performance for the turbine blade squealer tip

In this paper, an uncertainty quantification (UQ) method is proposed using the non-intrusive polynomial chaos expansion method and Smolyak sparse grids. Then coupled with three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions, an uncertainty quantification procedure is carried out for aerodynamic and heat transfer performance of GE-E3 rotor blade squealer tip. A parameter sensitivity analysis using the Sobol Indice method is carried out to identify the key parameters for aerothermal performance of the squealer tip. Wherein, the inlet total pressure and the inlet flow angle are considered as flow condition uncertainty parameters and cavity depth is considered as geometrical uncertainty parameters. The uncertainty analysis results showed that under the influence of the uncertain geometry and operating conditions, the heat flux of squealer tip basically conforms to the normal distribution and the statistical mean value of it increased by 28.30% relative to the design value and the probability of it deviating from the design value by 10% is as high as 86.59%. The statistical average of the squealer tip film cooling effectiveness is reduced by 23.89% compared to the design value, and the probability of it deviating from the design value by 10% is as high as 85.08%. The result of sensitivity analysis reveals that the uncertainty of the aerodynamic characteristics of the squealer tip is almost entirely caused by the inlet flow angle which accounts for 94.01% of the variance of the leakage flow rate. And it is also the dominant variable for the uncertainty of the heat transfer performance considering that its variance indexes for tip heat flux QTip and film cooling effectiveness η are 67.71% and 54.42% respectively. Compared with the main effects, the influence of the interaction effects among the variables on the squealer tip aerothermal performance is almost negligible.

Effect of Multiple Ribs and Tip Injection on the Aerothermal Characteristics of the Squealer Tip in Turbine Stage

Effect of Multiple Ribs and Tip Injection on the Aerothermal Characteristics of the Squealer Tip in Turbine Stage

Efficient Uncertainty Quantification on the Aerodynamic Performance of the Transonic Multi-Cavity Squealer Tip

Efficient Uncertainty Quantification on the Aerodynamic Performance of the Transonic Multi-Cavity Squealer Tip

An Artificial Neural Network (Ann) Based Aerothermal Optimization of Film Cooling Hole Locations on the Squealer Tip of an Hp Turbine Blade

An Artificial Neural Network (Ann) Based Aerothermal Optimization of Film Cooling Hole Locations on the Squealer Tip of an Hp Turbine Blade

Effects of the casing step size on the tip leakage flow pattern and heat transfer characteristic of turbine blade squealer tip

In this paper, under the GE-E3 turbine stage condition, the effect of the casing step on the heat transfer performance is obtained by solving three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The heat transfer characteristics predicted by standard k-ω model are consistent with the experimental data. The simulation results reveal that, the tip heat transfer coefficient decreases and the area of the high heat transfer region is reduced by introducing the casing step structure. Compare to the baseline case, when the casing step height is 1%, 2% and 3% of the blade height, the area-average heat transfer coefficient at the blade tip decreases by 6.1%, 6.7% and 7.0%, respectively. The introduction of casing step structure can eliminate the swirling strength of Scraping Vortex (SV), thus weaken the reattachment of tip leakage flow on the cavity bottom. Under the step vortex effect, the mainstream from the tip pressure side entering the tip cavity is reduced and replaced by entering from the tip suction side. Because of the lower pressure difference between the tip suction side and the tip clearance, the swirling strength of SV is decreased. Arranging a casing step with the same size as the tip clearance upstream the blade can reduce the tip heat transfer performance to the greatest level. The introduction of casing step structure also has a negative effect on the output power and efficiency of the turbine, it is necessary to balance the aerodynamic and heat transfer of the turbine in order to achieve the optimal results.

Effect of Mainstream Velocity on the Heat Transfer Coefficient of Gas Turbine Blade Tips

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.

Effect of casing purge flow on heat transfer and cooling performance of blade squealer tip for a gas turbine stage

The first stage of GE-E3 turbine is employed to investigate effect of casing purge flow upstream rotor blade tip. Three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations and standard k-ω model are solved to obtain tip heat transfer simulations. The results reveal that: heat transfer coefficient of blade tip surface can be significantly reduced when casing purge flow is set. Tip averaged heat transfer coefficient of cases with and without swirly velocity casing purge flow decrease 3.5% and 3.4% compared with the case without casing purge flow. Compared with case which blowing ratio equals to 0.5, it can be found that averaged tip heat transfer coefficient of cases which blowing ratio equals to 1.0 and 1.5 decrease 2.3% and 1.8%, respectively. Setting blowing ratio as 1.0 can best cool tip surface without wasting cold air resources. Increasing rotating speed can induce cold air entering tip trailing region and improve local cooling effect. Flow structure inside the tip clearance are also revealed and discussed.

Near Tip Loss Control With a Winglet Baffle Cavity Tip in a Turbine Cascade

Abstract The aerodynamic performance of a winglet baffle cavity tip is investigated at different inlet incidences from −12.5 to +12.5 deg. This blade tip shows a geometry feature with a pressure side winglet and a baffle within the tip cavity. The experimental studies were carried out in a large-scale linear cascade. Numerical methods were also used to obtain the detail flow physics. The baffle on the tip divides the cavity vortex into two main parts, and enhances flow mixing over the tip. As the tip leakage flow exits the tip near the baffle and the cavity corner, flow separation occurs over the suction side and its blockage effect reduces the local tip leakage mass flow rate. The additional pressure side winglet reduces the contraction coefficient on the pressure side squealer. Compared with the squealer tip, the winglet baffle cavity tip reduces the tip leakage mass flow by 12.1% and the near tip loss by 4.2%. Removing the baffle on the tip also results in an increase of the loss. As the incidence of the incoming flow decreases, the loss near the tip reduces mainly due to a reduction of the strength of the passage vortex, which develops from the casing endwall. At all incidences studied, the winglet baffle cavity tip shows better aerodynamic performance than the squealer tip.

Aerothermal Effect of Cavity Welding Beads on a Transonic Squealer Tip

Abstract High-pressure turbine blade tips are critical for gas turbine performance and are sensitive to small geometric variations. For this reason, it is increasingly important for experiments and simulations to consider real geometry features. One commonly absent detail is the presence of welding beads on the cavity of the blade tip, which are an inherent by-product of the blade manufacturing process. This paper therefore investigates how such welds affect the Nusselt number, film cooling effectiveness and aerodynamic performance. Measurements are performed on a linear cascade of high-pressure turbine blades at engine realistic Mach and Reynolds numbers. Two cooled blade tip geometries were tested: a baseline squealer geometry without welding beads, and a case with representative welding beads added to the tip cavity. Combinations of two tip gaps and several coolant mass flow rates were analyzed. Pressure sensitive paint was used to measure the adiabatic film cooling effectiveness on the tip, which is supplemented by heat transfer coefficient measurements obtained via infrared thermography. Drawing from all of this data, it is shown that the weld beads have a generally detrimental impact on thermal performance, but with local variations. Aerodynamic loss measured downstream of the cascade is shown to be largely insensitive to the weld beads.

Aerothermal Optimization of Turbine Squealer Tip Geometries With Arbitrary Cooling Injection

Abstract The optimization of the turbine rotor tip geometry remains a vital opportunity to create more efficient and durable engines. Balancing the aerodynamic and thermal aspects, while maintaining the mechanical integrity, is key to reshape one of the most vulnerable and life-determining parts of the entire turbine. The ever-increasing turbine gas temperatures, combined with the difficulty of cooling the tip and the aerodynamically penalizing nature of the overtip leakage vortex, make the design of the tip a truly multidisciplinary challenge. While many earlier efforts focused on uncooled geometries or studied the aerothermal impact with a fixed cooling configuration, the current paper presents the outcome of a multi-objective optimization where both the squealer rim geometry and the cooling injection pattern were allowed to vary simultaneously. This study explores a significantly wider design space, seeking a further synergistic aerothermal benefit through the combination of a quasi-fully arbitrary cooling arrangement, with mutating squealer rim structures. Specifically, the current manuscript presents the results of over 330 cooled and uncooled squealer tip geometries. The high-pressure turbine tip was automatically altered using a novel parametrization strategy adopting a maximum of 40 design variables to vary the squealer rim structures, as well as the size and location of the various cooling holes. The aerodynamic and thermal characteristics of every design were evaluated through Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) simulations with the k–ω shear-stress transport (SST) model for the turbulence closure, adopting an unstructured hexahedral grid typically containing more than 8 × 106 cells. A multi-objective differential evolution algorithm was used to obtain a Pareto front of designs which maximize the aerodynamic efficiency, while minimizing the overtip thermal loads. Eventually, a detailed investigation and robustness study was performed on a set of prime squealer geometries, to further investigate the aerodynamic flow topology and the effect of various cooling injection schemes on the heat transfer patterns.

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.

Investigation on aerothermal performance of a rib-slot scheme on the multi-cavity tip of a gas turbine blade

A rib-slot blade tip is proposed to improve the film cooling performance of an unshrouded gas turbine blade tip in the current paper. The rib-slot blade tip is formed by arranging the slot into ribs of the multi-cavity squealer tip. The film cooling performance of the rib-slot blade tip is experimentally studied by using the Pressure Sensitive Paint (PSP) technique in transonic flows. The inlet Reynolds number is 370,000 based on the blade axial chord length. The exit Mach numbers is 1.05. Effects of density ratios and mass flow ratios on the film cooling characteristics are investigated, and the experiments are carried out at four mass flow ratios and three density ratios. The numerical simulation is used to predict the flow field and analyze the film cooling characteristics. The multi-cavity squealer tip consists of cavity 1-4. The results indicate that at the high mass flow ratio, the film can cover the whole squealer tip bottom surface downstream of rib-slots at three density ratios. Increasing the mass flow ratio has a slight effect on the film cooling effectiveness (η) near the tip pressure side upstream of the cavity 3. There is a low η region near the suction side of the cavity 2 and cavity 3 at low mass flow ratios, particularly for the high-density jet due to the leakage flow reattachment. A low η region is formed in the middle region of the cavity 4 at the low mass flow ratio. The blade tip with the jet shrinks the high aerodynamic loss coefficient region formed by the counter vortex, whereas, it expands the high aerodynamic loss coefficient region formed by the leakage vortex in the blade span-wise direction. The pitch-wise averaged aerodynamic loss coefficient of the blade tip with the jet is lower than that of the no film case near the shroud.

Effect of shelf squealer tip configuration on aerothermal performance of gas turbine blade

The tip leakage flow between rotating turbine blades and the stationary turbine casing causes aerodynamic losses and high thermal loads. Numerous novel tip geometries have been proposed for reducing both of these effects, one of them is a shelf squealer tip. The objective of this study is to investigate the aerothermal performance of three squealer blade tip configurations: conventional, vertical shelf and inclined shelf. Heat transfer was measured to analyze heat transfer features on the blade tip using transient IR thermography method. Also, computational fluid dynamics simulation and particle image velocimetry were conducted to visualize the flow characteristics around the blade tip. The experiment was conducted under conditions of a Reynolds number of 140,000 which was based on the inlet velocity in a wind tunnel. Thermal loads with vertical and inclined shelf squealer tip configurations are lower than with the conventional squealer tip. Due to the recessed pressure side rim of the vertical shelf squealer tip, the tip leakage flow was less reattached comparing to conventional squealer tips. This resulted in reduced thermal load on the tip surface and a larger tip leakage vortex around the blade suction side, which increased aerodynamic losses. Especially in the inclined shelf squealer tip, a separation bubble was generated on its inclined pressure side rim, which reduced the tip leakage flow due to the vena contracta effect. Consequentially, the reduced tip leakage flow decreased both heat loads and aerodynamic losses for the inclined shelf squealer tip. Therefore, the inclined shelf squealer tip demonstrated a good performance with respect to both aerodynamic loss and heat transfer.