Yoji Okita

Film Effectiveness Performance of an Arrowhead-Shaped Film-Cooling Hole Geometry

This paper presents the first experimental and numerical work of film effectiveness performance for a novel film-cooling method with an arrowhead-shaped hole geometry. Experimental results demonstrate that the proposed hole geometry improves the film effectiveness on both suction and pressure surface of a generic turbine airfoil. Film effectiveness data for a row of the holes are compared to that of fan-shaped holes at the same inclination angle of 35 deg to the surface on a large-scale airfoil model at engine representative Reynolds number and Mach number in a high-speed tunnel with moderately elevated temperature mainstream flow. The film effectiveness data are collected using pressure-sensitive paint. Numerical results show that the coolant film with the proposed hole geometry remains well attached to the surface and diffuses in the lateral direction in comparison with the conventional laidback fan-shaped holes for coolant to mainstream blowing ratios of 0.6-3.5.

Comparisons of High-Reynolds-Number EVM and DSM Models in the Prediction of Heat and Fluid Flow of Turbine Blade Cooling Passages

This paper presents computations of flow and heat transfer through passages relevant to those used to internally cool gas-turbine blades, using high-Reynolds-number models of turbulence. Three types of internal flows are first examined, which between them contain all the main elements found in blade cooling passages; developing flow through a heated straight duct rotating orthogonally, repeating flow and heat transfer through a straight ribbed duct and flow and heat transfer through a round-ended U-bend of strong curvature square and of cross-section. Next, flows influenced by a combination of these elements are computed. The main objective is to establish how reliably, industry-standard high- Reynolds-number models can predict flow and wall-heat transfer in blade-cooling passages. Two high-Reynolds-number models have been used, the standard version of the high Re k- (EVM) model and the basic high-Re model of stress transport (DSM). In all the cases the second-moment closure (DSM) consistently produced flow and thermal predictions that are closer to available measurements than those of the EVM model. Even the high-Re DSM predictions, however, are not in complete agreement with the experimental data. Comparisons with predictions of earlier studies that use low-Re models of turbulence show that at least some of the remaining differences between the current predictions and experimental data are due to the use of the wall-function approach.

Film Cooling Hole Shape Optimization Using Proper Orthogonal Decomposition

Film cooling is a very effective cooling method for protecting the turbine blades exposed to hot gas from the heat. Since its cooling effectiveness is highly dependent on the shape of the hole, a wide variety of concepts and design parameters regarding hole shapes have been researched. However, there are no well-defined ways to determine the optimum shape of a film cooling hole.The CFD is a powerful tool for film cooling hole optimization. But with the number of parameters that define the film cooling hole shapes being so numerous, analytical optimization with CFD often requires computational resources that are unrealistic for the average design environment. Accordingly, for CFD to be effective in the optimization process, it is necessary to reduce the number of computations or shorten the calculation time per computation.In order to solve this problem, this paper presents a novel approach of applying 3D-POD (3D-Proper Orthogonal Decomposition) to the optimization of film cooling holes. POD is one of the most important component analysis methods and has the potential to reduce the number of parameters.From the computation results, a solution group was made by the RSM (Response Surface Method) and assessment functions, i.e., film cooling effectiveness, heat transfer coefficient, mixing loss, concentration of stress and robustness were considered first. In the end, however, considering the sensitivity of each objective function, the optimal hole shapes were obtained with only the film effectiveness being evaluated.In the following sections, this method and its results are described in detail.Copyright

Computational Predictions of Endwall Film Cooling for a Turbine Nozzle Vane With an Asymmetric Contoured Passage

This paper presents results of a computational study for the endwall film cooling of an annular nozzle cascade employing a circumferentially asymmetric contoured passage. The investigated geometrical parameters and the flow conditions are set consistent with a generic modern HP-turbine nozzle. Rows of cylindrical film cooling holes on the contoured endwall are arranged with a design practice for the ordinary axisymmetric endwall. The solution domain, which includes the mainflow, cooling hole paths, and the coolant plenum, is discretized in the RANS equations with the realizable k-epsilon model. The calculated flow field shows that the pressure gradients across the passage between the pressure and the suction side are reduced with the asymmetric endwall, and consequently, the rolling up of the inlet boundary layer into the passage vortex is delayed and the separation line has moved further downstream. With the asymmetric endwall, because of the effective suppression of the secondary flow, more uniform film coverage is achieved especially in the rear part of the passage and the laterally averaged effectiveness is also significantly improved in this region. The closer inspection of the calculated thermal field reveals that, with the asymmetric passage, the coolant ejected from the holes are less deflected by the secondary vortices, and it attaches better to the endwall in this rear part.Copyright

Experimental and Numerical Studies on Leading Edge Film Cooling Performance: Effects of Hole Exit Shape and Freestream Turbulence

This study deals with the experimental and numerical studies of the effect of hole exit shape and free-stream turbulence on turbine blade leading edge film cooling. The study examines several test cases with two blowing ratios (BR = 1.0 and 2.0) and three mainstream turbulence intensities (1.0, 3.3 and 12.0%) using two types of leading edge models with cylindrical holes and diffuser holes [1]. The leading edge model consists of a semi-circular part of 80mm diameter and a flat after-body. In this study total pressure loss coefficient is measured by total pressure probe. Film effectiveness and heat transfer coefficient on the model surface are measured by the transient method using thermochromatic liquid crystal with video camera. In addition, detailed investigation of the film cooling is carried out using CFD simulations. RANS approach using Shear Stress Transport turbulence model and Detached Eddy Simulation (DES) approach are employed to solve the flow field. In the case of diffuser hole, the effect of mainstream turbulence intensity appears significant, and its spanwise averaged film effectiveness is decreased.Copyright

Experimental Study on Racetrack-Shaped Holes Impingement Cooling With Bump Type Roughening Element

Cooling effectiveness of an impingement cooling with array of racetrack-shaped impingement holes is investigated. Two types of specimens are investigated. One is a plain target plate and the other is a plate roughened with bump type elements. Sensitivity of relative location of bump to impingement hole on the cooling effectiveness is also investigated.Experiments are conducted under three different mainflow Reynolds numbers ranging from 2.6×105 to 4.7×105, with four different cooling air Reynolds numbers for each main flow condition. The cooling air Reynolds numbers are in the range from 1.2×103 to 1.3×104.Copyright

Film Cooling in a Separated Flow Field on a Novel Lightweight Turbine Blade

The primary contribution of this research is to clarify the feasibility of a novel lightweight turbine blade with internal and external cooling, which is invented, aiming at drastic reduction in weight. With a considerably thinner airfoil, an extensive separation bubble is formed on the pressure side, and film cooling performance in such a flow field has to be investigated. Experimental results with a curved duct setup, which simulates the flow field around the proposed airfoil, show that a film cooling is still an effective measure of cooling even in the vastly separated region, and it behaves quite similarly to the conventional correlation, except for lower blowing ratios, where the thermal field is strongly affected by the intense recirculation flow. Comparisons between the experimental and numerical results verify that an affordable Reynolds-averaged Navier-Stokes simulation is useful to investigate the detailed physics of this flow field. With the numerical modeling, a cooling performance of the proposed blade under a typical engine operating condition is simulated, and the metal temperatures of the blade are also predicted with a fluid-solid conjugate calculation. The resultant thermal distribution in the airfoil suggests that the trailing edge portion is inevitably most critical in the temperature, and also a considerable thermal gradient across the blade is induced. Thermal profile, however, is partly recovered with some of the film coolant being bypassed from the pressure side to the suction side.

Film Effectiveness Performance of an Arrowhead-Shaped Film Cooling Hole Geometry

This paper presents the first experimental and numerical work of film effectiveness performance for a novel film cooling method with an arrowhead-shaped hole geometry. Experimental results demonstrate that the proposed hole geometry improves the film effectiveness on both suction and pressure surface of a generic turbine airfoil. Film effectiveness data for a row of the holes are compared with that of fan-shaped holes at the same inclination angle of 35° to the surface on a large-scale airfoil model at engine representative Reynolds number and Mach number in a high speed tunnel with moderately elevated temperature mainstream flow. The film effectiveness data are collected using pressure sensitive paint (PSP). Numerical results show that the coolant film with the proposed hole geometry remains well attached to the surface and diffuses in the lateral direction in comparison with the conventional laidback fan-shaped holes for coolant to mainstream blowing ratios of 0.6 to 3.5.Copyright

Effects of Shape and Arrangement of Dimples on Film Cooling Performance Over Cutback Surface at Airfoil Trailing Edge

Trailing edge of a gas turbine blade is under very high thermal load because both sides are exposed to hot mainstream. The cooling film ejected from slots has to protect the cutback surface from the hot mainstream, and remove the heat from the surface. In this study, the film cooling performance of cutback surfaces with two types of dimples, spherical and teardrop-shaped dimples, were experimentally investigated with a transient infrared thermography method. Also, to examine the effects of arrangements, two different arrangements of the teardrop-shaped dimples, which are parallel and inclined to mainstream, were investigated. The dimples were arranged in two rows on the cutback surfaces. The Reynolds number of mainstream defined by the mean velocity and hydraulic diameter was 20,000, and profiles of local heat transfer coefficient and film cooling effectiveness on the cutback surface were measured for blowing ratios of 0.5–2.0. With the parallel teardrop-shaped dimples, reduction of the heat transfer in the upstream portion was less than that of the spherical dimples, and the heat transfer at downstream rims was higher. In the case of the inclined teardrop-shaped dimples, heat transfer enhancement at the downstream rims was higher than that of parallel one, and overall heat transfer coefficient was also higher. The film cooling effectiveness of all cases are almost equal values, namely, the dimpled surfaces could enhance heat transfer without reduction of the film cooling effectiveness; consequently significant cooling performance improvement was obtained for the teardrop-shaped dimple cases, especially with the introduction of inclined arrangement.Copyright

Simulations of Multi-Phase Particle Deposition on a Showerhead With Staggered Film-Cooling Holes

The demand for cleaner, more efficient energy has driven the motivation for improving the performance standards for gas turbines. Increasing the combustion temperature is one way to get the best possible performance from a gas turbine. One problem associated with increased combustion temperatures is that particles ingested in the fuel and air become more prone to deposition with an increase in turbine inlet temperature. Deposition on aero-engine turbine components caused by sand particle ingestion can impair turbine cooling methods and lead to reduced component life. It is necessary to understand the extent to which particle deposition affects turbine cooling in the leading edge region of the nozzle guide vane where intricate showerhead cooling geometries are utilized. For the current study, wax was used to dynamically simulate multi-phase particle deposition on a large scale showerhead cooling geometry. The effects of deposition development, coolant blowing ratio, and particle temperature were tested. Infrared thermography was used to quantify the effects of deposition on cooling effectiveness. Although deposition decreased with an increase in coolant blowing ratio, results showed that reductions in cooling effectiveness caused by deposition increased with an increase in blowing ratio. Results also showed that effectiveness reduction increased with an increase in particle temperature. Reductions in cooling effectiveness reached as high as 36% at M = 1.0.Copyright