Cooling Hole Shape Research Articles

Particles deposition on plates with different film cooling hole shapes for different blowing ratios and angles of attack

Particles deposition on plates with different film cooling hole shapes for different blowing ratios and angles of attack

AN EXPERIMENTAL STUDY OF THE COOLING EFFICIENCY ON CURVED SURFACES WITH DIFFERENT SHAPED HOLES

Improving the film-cooling efficiency is one of the main goals of heat transfer in modern industrial applications. To follow the actual requirements to film-cooling systems, different hole geometries including fan-shaped, laidback, and complex shapes were earlier developed and studied. However, most appropriate studies were performed on flat surfaces whereas the actual geometry of protected walls has curved surfaces, e.g., gas turbine nozzle vanes. The present paper reports experimental results of comparative studies of the film-cooling efficiency for curved surfaces with five different hole geometries: cylindrical, short and long fan-shaped, laidback, and joint fan-shaped and laidback holes. Each experimental model had 11 holes located in a row perpendicular to the main stream direction and was studied using the IR-thermography technique. The experimental results show that the choice of the most efficient cooling hole shape also strongly depends on the blowing ratio value, which was changed in the range 0.5 ≤ m ≤ 1.5 during these studies. For cylindrical holes, the film separation takes place at m > 0.7. Meanwhile, short fan-shaped and laidback holes provide the film separation at m > 1. The highest values of the cooling efficiency were obtained for long fan-shaped holes in the blowing ratio range 0.5 ≤ m ≤ 1. Under mentioned conditions, long fan-shaped holes' efficiency is at least 35# higher compared to others. However, studies at m >1 showed that joint fanshaped and laidback holes provide maximal cooling efficiency due to the almost tangential-to-surface direction of the secondary air flow blown from these holes. The obtained results can be further applied in designing new cooling systems for the protection of the most thermally stressed elements of industrial applications.

Heat transfer and turbulent heat flux budgets in cooling films

Statistics from large-eddy-simulation (LES) of cooling films with different cooling holes are used to evaluate budgets in the transport equation of turbulent heat flux. The capability of LES has been assessed by comparing simulated results with experimental data, while the correctness of the procedure generating turbulent heat flux budgets has been examined on a turbulent boundary layer. The mechanism of heat transfer has been preliminarily studied throughout the three-dimensional flow field at different blowing ratios using a general outer scaling, to recognize specific regions with corresponding heat transfer patterns. A compressible version of budget terms in the transport equation of turbulent heat flux is then explored to show the thermal behavior of flow downstream from the cooling film holes. Characteristics of each budget term are presented in a defect scaling consistent with the scaling suitable for Reynolds stresses and Reynolds stress budgets. Furthermore, these budget terms are compared among an array of different cooling hole shapes combined with different blowing ratios to explore self-similarity. Results show that the dynamic balance of all budget terms is significantly influenced by the cooling hole shape, while each turbulent heat flux budget term may scale with the velocity defect, temperature deficit, and the normalized streamwise distance to the power of an exponent dependent on the hole shape.

Efficient Global Optimization of a laidback fan-shaped cooling hole using Large-Eddy Simulation

In the film cooling technique, cold fluid exiting from holes on the surface of the turbine creates a protective film which prevents the ruin of the solid material composing the blades. It is now known that the hole geometry is a key element of this approach and determines the adiabatic film cooling effectiveness. The design of optimal holes which can operate at conditions of high-pressure turbines is, however, a challenge for which several research groups and industrial companies have invested in intensive numerical and experimental campaigns. In the present study, a Large-Eddy Simulation based approach is adopted and demonstrated to be able to optimize the shape of a fan-shaped hole on a flat plate configuration in engine-representative conditions in static conditions, i.e. without rotation. The obtained results target high-pressure vanes and further studies are mandatory to investigate the effect of rotation for the application to turbine blades. This work is purely numerical as no experimental campaign has been done for such hot conditions. Nonetheless, the numerical setup used has been validated in a previously published studies. To do so, four main geometrical parameters defining such a cooling hole shape, namely the depth of the expanded section, the hole inclination angle and the forward as well as the lateral expansion angles are selected as design variables. The Bayesian based Efficient Global Optimization method along with the Expected Improvement as the acquisition function are used to find the maximum surface averaged film cooling effectiveness as the objective function. After nine database enrichment iterations needed to reduce the overall model error, an optimal shape for a given operating condition defined by the blowing and density ratios is obtained. Based on the response surface analysis, the effects of the design parameters on the adiabatic film cooling effectiveness are investigated. Finally, deeper analysis of the flow physics for four shapes is proposed to apprehend the role of the geometrical parameters on the fluid dynamics and on the adiabatic film cooling effectiveness.

Printability and Overall Cooling Performance of Additively Manufactured Holes With Inlet and Exit Rounding

Abstract To improve cooling effectiveness of gas turbine hardware, various film cooling hole shapes have previously been researched. Unique design modifications have recently been made possible through the design freedom allotted by additive manufacturing (AM). As one example, creating a rounded inlet for a film-cooling hole can mitigate separation at the inlet. This study explores various geometric features by exploiting the uses of additive manufacturing for shaped film cooling holes at engine scale. Both printability and cooling performance were evaluated. Resulting from this study, additively manufactured holes with hole inlet and exit rounding were printed with some variations from the design intent (DI). The largest deviations from the design intent occurred from dross roughness features located on the leeward side of the hole inlet. The measured overall effectiveness indicated that an as-built inlet fillet decreased in-hole convection as well as decreased jet mixing compared to the as-built sharp inlet. Including an exit fillet, which prevented an overbuilt diffuser exit, was also found to decrease jet mixing. A particular insight gained from this study is the importance of the convective cooling within the hole to the overall cooling performance. In-hole roughness, which is a result of additive manufacturing, increased convective cooling within the holes but also increased jet mixing as the coolant exited the hole. The increased jet mixing caused low overall effectiveness downstream of injection.

가스터빈 베인의 막냉각 홀 형상에 대한 냉각 효율의 수치 해석적 연구

가스터빈 베인의 막냉각 홀 형상에 대한 냉각 효율의 수치 해석적 연구

Multi-fidelity model based optimization of shaped film cooling hole and experimental validation

An optimization framework is developed for the film cooling hole shape design to achieve highest film cooling effectiveness with affordable cost. A key to this design method is the development and applications of a multi-fidelity model (MFM) to improve the efficiency of surrogate model construction. To construct multi-fidelity model, empirical correlations from existing literature are utilized as low-fidelity data and Reynolds-Averaged Navier-Stokes (RANS) simulations provide high-fidelity data. Compared with single-fidelity surrogate models, multi-fidelity model developed in this work combines vast existing knowledge about shaped film cooling hole and three dimensional numerical simulation. As a result, the computation cost has been substantially decreased by 64.5% while accuracy is maintained with similar level, which provides a firm basis for the optimization. Considering three geometric parameters as design variables, i.e. laidback angel, lateral angle, and hole length-diameter ratio, the shaped hole is optimized with genetic algorithm (GA) combined with sequential quadratic programming (SQP) algorithm. After the optimization, an experiment campaign is further carried out to validate the optimization result using pressure sensitive paint (PSP) technology. With current multi-fidelity model, the spatially averaged film cooling effectiveness has been improved by 39% compared to the baseline, which is an already well-designed shaped hole and the improvement is also verified by experiment. This work verifies the effectiveness of current multi-fidelity model for the design and optimization of shaped film cooling hole for better performance.

Robust design optimization of a turbine blade film cooling hole affected by roughness and blockage

High performance film cooling holes with complicated geometries have been regarded as impractical up to now because of manufacturability issues. However, recent advances in additive manufacturing (AM) technology have opened up new doors. Investigating characteristics of film holes built with AM, and finding the optimum shape considering these characteristics are now required to confirm their practical utility. In this paper, the performance of a high-efficiency film hole is numerically investigated. In-hole roughness and blade surface roughness are examined assuming an AM process, and contorted hole shape caused by partial blockage is also considered. A robust hole shape is obtained considering these uncertainties, utilizing a reference hole shape made by combining three cylindrical holes, which is meant to mitigate the detrimental effect of the counter-clockwise vortex pair. Main hole diameter, injection angle, and two angles for defining the two auxiliary holes are used as design variables to be optimized. For flow field and thermal analysis with roughness, compressible steady Reynolds averaged Navier-Stokes equations with a sand-grain roughness model are used. For the probabilistic assessment of each hole shape, Monte Carlo Simulations with the Kriging surrogate model is used, along with efficient global optimization (EGO) and a genetic algorithm. As a result, a high performance yet robust film cooling hole shape is obtained.

Construction and performance of a 38 T resistive magnet at the Nijmegen High Field Magnet Laboratory

In 2011, we have presented the design of a 38-T resistive magnet with a 32-mm bore, which has been designed to satisfy the users' demand for higher magnetic fields. Manufacturing of the parts started in 2012 and has been completed in early 2013. In this paper, we present the final design specifications and report on the construction, commissioning, and performance of the magnet. The five Florida–Bitter coils and the outer Bitter coil of this magnet consume 20.5 MW at 40 kA and generate a magnetic field of 37.5 T. The outer coil has a diameter of 1 m and weighs just over 3000 kg. Key ingredients in the design of this magnet are the modified Weggel stacking scheme, the modifications to the cooling hole shape, and a new compact housing.

Film-Cooling Performance of Multiple Arrays of Cylindrical and Fan-Shaped Holes

Experimental investigations are performed on the suction side of a cooled turbine guide vane. Transient infrared thermography is used to evaluate film-cooling performance of cylindrical and fan-shaped holes in a test facility representing engine conditions. Adiabatic film effectiveness and net heat flux reduction results due to coolant injection through double and multiple rows in the presence and absence of an upstream showerhead are presented. Two double staggered rows at different positions have been cross compared: one at a relatively high convex curvature region and the other close to the maximum throat velocity. A combination of the two double rows is considered to be multiple rows. The tested blowing ratios are in the interval of [0.6–1.2] and [0.3–1.2] for double and multiple rows, respectively. The showerhead cooling is maintained at a nominal blowing ratio. The findings suggest that the choice of best cooling hole shape for film-cooling design can be highly influenced by the number of cooling rows to be used and also the presence (or absence) of showerhead cooling. It is worth noting that the outcome may differ depending on the quantity of interest (i.e., adiabatic film effectiveness or net heat flux reduction).

Multi-Objective Optimization of a Row of Film Cooling Holes Using an Evolutionary Algorithm and Surrogate Modeling

Multi-objective shape optimization of a row of laidback fan-shaped film cooling holes has been performed using a hybrid multi-objective evolutionary approach in order to achieve an acceptable compromise between two competing objectives: the enhancement of film cooling effectiveness and the reduction of aerodynamic loss. In order to perform comprehensive optimization of a film cooling hole shape, the injection angle of the hole, lateral expansion angle of the diffuser, forward expansion angle of the hole, and pitch-to-hole diameter ratio are chosen as design variables. Forty experimental designs within the design spaces are selected using the Latin hypercube sampling method. The response surface approximation method is used to construct the surrogate using objective function values calculated at the experimental points using Reynolds-averaged Navier-Stokes analysis. The shear stress transport turbulence model is used as a turbulence closure. The optimization results are processed using the Pareto-optimal method. The Pareto-optimal solutions are obtained using a combination of a evolutionary algorithm and a local search method. The optimum designs are grouped using the k-means clustering technique, and the three optimal points selected in the Pareto-optimal solutions are evaluated by numerical analysis. The optimum designs give enhanced objective function values compared to the experimental designs.

Numerical simulation of film cooling effectiveness on a flat plate

AbstractNumerical simulation has been conducted to study film cooling effectiveness on a flat plate. Three‐dimensional geometry was generated and the effects of blowing ratio and geometrical shape were studied. A cylindrical round, simple angle (CYSA) and laterally diffused, simple angle (LDSA) hole with a streamwise angle of 30° and spanwise angle of 0° were used. Hole length to diameter ratio (L/D=4) is constant for all geometries. Also the diameter of film cooling hole for different cooling holes at the entrance surface (D=10 mm) is constant. The blowing ratio ranges from 0.5 to 1.67, and the mainstream Reynolds number based on the mainstream velocity and hole diameter (ReD) was 8563. Both local and lateral averaged values are presented. Results have a good correspondence with experimental data obtained by Yuen and Martinez‐Botas (Int. J. Heat Mass Transfer 2003; 46:221–235). The simulation results show that cooling hole shape affects film cooling effectiveness significantly. The LDSA hole decreases the momentum of jet flow at the exit area of the hole and avoids lift‐off phenomenon. Counter‐rotating vortex formed downstream of the hole is weaker for the LDSA hole and secondary flow is not powerful enough to disturb the jet flow structure next to the wall. Also the results show that the LDSA hole has a better lateral coverage due to the diffused shape of the hole and has a higher effectiveness value in a wider region on the wall. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of hole shape on the performance of a turbine vane endwall film-cooling scheme

Rising combustor exit temperatures in gas turbine engines necessitate active cooling for the downstream turbine section to avoid thermal failure. Film-cooling has long been an integral part of turbine cooling schemes. Cooling the endwall of a turbine airfoil is particularly difficult as much of the coolant is swept off the endwall by vortical flow patterns that develop in the passage. Although film-cooling has potential cooling benefits, this cooling method also leads to increased aerodynamic penalties in terms of total pressures losses in the turbine stage. This study investigated the trade-off between the cooling benefit and aerodynamic penalties associated with cooling the turbine endwall region for two different cooling hole shapes. Two commonly used film-cooling hole geometries were investigated; cylindrical holes and shaped holes. Compared to the case without any film-cooling, results showed that film-cooling with either hole geometry increased the aerodynamic losses through the turbine stage. Shaped film-cooling holes generated less total pressure losses through the turbine vane passage than cylindrical holes as a result of the separation from cylindrical hole injection having increased mixing losses. Shaped holes also provided better cooling to the endwall region than cylindrical holes, making them more effective both aerodynamically and thermally.

Effect of Hole Geometry on the Thermal Performance of Fan-Shaped Film Cooling Holes

This study evaluates the impact of typical cooling hole shape variations on the thermal performance of fan-shaped film holes. A comprehensive set of experimental test cases featuring 16 different film-cooling configurations with different hole shapes have been investigated. The shape variations investigated include hole inlet-to-outlet area ratio, hole coverage ratio, hole pitch ratio, hole length, and hole orientation (compound) angle. Flow conditions applied cover a wide range of film blowing ratios M=0.5 to 2.5 at an engine-representative density ratio DR=1.7. An infrared thermography data acquisition system is used for highly accurate and spatially resolved surface temperature mappings. Accurate local temperature data are achieved by an in situ calibration procedure with the help of thermocouples embedded in the test plate. Detailed film-cooling effectiveness distributions and discharge coefficients are used for evaluating the thermal performance of a row of fan-shaped film holes. An extensive variation of the main geometrical parameters describing a fan-shaped film-cooling hole is done to cover a wide range of typical film-cooling applications in current gas turbine engines. Within the range investigated, laterally averaged film-cooling effectiveness was found to show only limited sensitivity from variations of the hole geometry parameters. This offers the potential to tailor the hole geometry according to needs beyond pure cooling performance, e.g., manufacturing facilitations.

Effects of film cooling hole shape on heat transfer

AbstractThe effects of hole shapes, secondary injection Reynolds numbers, and blowing ratios on the heat transfer downstream of film cooling holes have been investigated by using a large‐scale low‐speed loop wind tunnel. The test model consists of five film cooling holes. Experiments on dustpan‐shaped holes, fan‐shaped holes, and round holes have been conducted with injection Reynolds number ranging from 10,000 to 25,000 and blowing ratio ranging from 0.3 to 2.0. Measurements are taken under 26 conditions. Results show that the critical blowing ratio is 1.3 for the dustpan‐ and fan‐shaped holes, 0.7 for the round holes. The turbulence generated by air injection through round holes is stronger than those through dustpan‐ and fan‐shaped holes. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 73–80, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20005

Film cooling from rows of holes--effect of cooling hole shape and row arrangement on adiabatic effectiveness.

In the present study the film cooling performance in terms of the adiabatic film cooling effectiveness on the scaled suction side model of an actual guide vane was investigated. An infrared thermography measurement system was used to determine highly resolved distribution of models surface temperature. Two different film cooling hole configurations were investigated: a single row of fanshaped holes and a double row of cylindrical holes in staggered arrangement. The influence of blowing rate and mainstream turbulence level on effectiveness was investigated in a wide range for both of the injection configurations.

The variation of heat transfer coefficient, adiabatic effectiveness and aerodynamic loss with film cooling hole shape.

The heat transfer coefficient and adiabatic effectiveness of cylindrical, fan shaped holes and a slot are presented for the region zero to 50 diameters downstream of the holes. Narrow-band liquid crystals were used on a heated flat plate with heated air coolant. These parameters have been measured in a steady state, low speed facility at engine representative Reynolds number based on hole diameter and pressure difference ratio (ideal momentum flux ratio). The aerodynamic loss due to each of the film cooling geometries has been measured using a traverse of the boundary layer far downstream of the film cooling holes. Compared to the cylindrical holes, the fan shaped hole case showed an improvement in the uniformity of cooling downstream of the holes and in the level of laterally averaged film cooling effectiveness. The fan effectiveness approached the slot level and both the fan and cylindrical hole cases show lower heat transfer coefficients than the slot and non film cooled cases based on the laterally averaged results. The drawback to the fan shaped hole was that the aerodynamic loss was significantly higher than both the slot and cylindrical hole values due to inefficient diffusion in the hole exit expansion.

A new concept in Bitter disk design

A new concept in cooling hole design in Bitter disks that allows for much higher power densities and results in considerably lower hoop stresses has been developed and successfully tested at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL. The new cooling hole shape allows for extreme power densities (up to 12 W/mm/sup 3/) at a moderate heat flux of only 5 W/mm/sup 2/. The new concept also reduces the hoop stress by about 30-50% by making a Bitter disk compliant in the radial direction through staggering small width and closely spaced elongated cooling holes. Finally, the design is optimized for equal temperature.