Film cooling effectiveness from trenched shaped and compound holes

This paper presents a comparative-numerical investigation on film cooling from a row of simple and compound-angle holes injected at 35° on a flat plate with four film cooling configurations: (1) cylindrical film hole; (2) 15° forward diffused film hole; (3) trenched cylindrical film hole; (4) trenched 15° forward-diffused film hole. All simulations are at fixed density ratio of 1.6, blowing ratio of 1.25, length-to-diameter L/D = 4 and pitch-to-diameter ratio of 3.0. The effect of length-to-diameter ratio on film cooling has been also investigated using L/D in the range of 1–8. Computational solutions of the steady, Reynolds-averaged Navier–Stokes equations have been obtained using a finite volume method. It has been found that the shape of the hole and the trenched holes can significantly affect the film cooling flow over the protected surface. Further, it has been shown that the film cooling effectiveness by trenched shaped holes is higher than all other configurations both in spanwise and streamwise specially downstream of the injection. Also, a trenched compound angle injection shaped hole produces much higher film cooling protection than the other configurations investigated in the present paper. The length-to-diameter ratio of trenched holes was found to have a significant effect on film cooling effectiveness and the spread of the coolant jets.     

A numerical evaluation on the effect of sister holes on film cooling effectiveness and the surrounding flow field

A numerical study was performed to evaluate the effectiveness of the novel sister hole film cooling technique. Two secondary coolant holes bound the primary coolant hole slightly downstream of its midpoint, intended to minimize the primary vortex pair and improve cooling performance. An unstructured hexahedral mesh was generated and the realizable k–ε turbulence model with near-wall modeling was used in these simulations. Blowing ratios of 0.2, 0.5, 1.0, and 1.5 were simulated to evaluate the applicability of sister holes in practical applications. It was found that sister holes significantly improved cooling performance over the entire computational domain, particularly at high blowing ratios. These results arose by countering the primary vortex pair with a secondary pair from these sister holes, ultimately maintaining flow adhesion where the coolant stream would have otherwise separated.     

Heat transfer in film-cooled turbulent boundary layers at different blowing ratios as affected by longitudinal vortices

Effects of embedded longitudinal vortices on heat transfer in film-cooled turbulent boundary layers at different blowing ratios are discussed. These results were obtained in boundary layers at free-stream velocities of 10 and 15 m/s. Film coolant was injected from a single row of holes at blowing ratios of 0.47–1.26. A single longitudinal vortex was induced upstream of the film-cooling holes using a half-delta wing attached to the wind tunnel floor. Heat transfer measurements were made downstream of injection using a constant heat flux surface with 126 thermocouples for surface temperature measurements. For all blowing ratios examined, the embedded vortices cause significant alterations to wall heat transfer and to film cooling distributions. Measurrments of mean temperatures and mean velocities in spanwise planes show that high wall heat transfer regions are associated with regions of high near-wall longitudinal velocity where very little film coolant is present. In addition to high heat transfer regions associated with the vortex downwash, there are also secondary heat transfer peaks. These secondary peaks develop due to shear layer mixing and interaction between the vortex and cooling jets and become higher in magnitude and more persistent with downstream distance as the blowing ratio increases from 0.47 to 1.26.     

Numerical study on film cooling effectiveness from shaped and crescent holes

This paper presents a comparative numerical investigation on film cooling from a row of holes injected at 35° on a flat plate with three film cooling configurations, including cylindrical hole, 15° forward diffused shaped hole, and new crescent hole. All simulations are conducted at blowing ratio of 0.6 and 1.25, length-to-diameter ratio of four and pitch-to-diameter ratio of three. Computational solutions of the steady, Reynolds averaged Navier–Stokes equations are obtained using a finite volume method. Previous successful application of a two-layer turbulence model to cylindrical hole is extended to predict film cooling for the different hole geometries. It has been found that the film cooling effectiveness of cylindrical holes obviously declined along with increasing the blowing ratio. While the forward diffused shaped hole presents a marked improvement, with a higher effectiveness at the lateral area between adjacent holes. By comparison, the crescent hole exhibits the highest film cooling effectiveness among the three configurations both in spanwise and streamwise especially downstream of the intersection of the two holes. Also, the crescent hole can restrain the vortex intensity, and then enhance the film cooling effectiveness.     

Film cooling on a convex surface: influence of external pressure gradient and Mach number on film cooling performance

 The film cooling performance on a convex surface subjected to zero and favourable pressure gradient free-stream flow was investigated. Adiabatic film cooling effectiveness values were obtained for five different injection geometries, three with cylindrical holes and two with shaped holes. Heat transfer coefficients were derived for selected injection configurations. CO₂ was used as coolant to simulate density ratios between coolant and free-stream close to gas turbine engine conditions. The film cooling effectiveness results indicate a strong dependency on the free-stream Mach number level. Results obtained at the higher free-stream Mach number show for cylindrical holes generally and for shaped holes at moderate blowing rates significant higher film cooling effectiveness values compared to the lower free-stream Mach number data. Free-stream acceleration generally reduced adiabatic film cooling effectiveness relative to constant free-stream flow conditions. The different free-stream conditions investigated indicate no significant effects on the corresponding heat transfer increase due to film injection. The determined heat flux ratios or film cooling performance indicated that coolant injection with shaped film cooling holes is much more efficient than with cylindrical holes especially at higher blowing rates. Heat flux penalties can occur at high blowing rates when using cylindrical holes. Received on 29 May 2000     

高超声速溢流冷却实验研究

高超声速溢流冷却是一种新型的飞行器热防护方法,基本思想为:在高热流区布置溢流孔,控制冷却液以溢流方式流出,之后通过飞行器表面摩阻作用展布为液膜,形成热缓冲层以降低飞行器表面热流. 目前,溢流冷却技术还处于探索阶段,实现工程应用前还需开展大量的实验验证和机理研究工作. 本文首次开展溢流冷却的实验研究工作,采用热流测量、液膜厚度测量及液膜流动特性观测技术,搭建了完善的溢流冷却风洞实验平台,对溢流冷却热防护性能和高超声速条件下液膜流动规律进行了初步研究. 研究表明:(1) 高超声速流场中通过溢流能够在飞行器表面形成液膜并有效隔离外部高温气流,可降低飞行器表面热流率;(2) 楔面上的液膜前缘流动是一个逐渐减速的过程,增加冷却液流量液膜厚度变化不明显,但液膜前缘运动速度增大;(3) 液膜层存在表面波,在时间和空间方向发生演化,导致液膜厚度的微弱扰动;(4) 液膜层存在横向展宽现象,即液膜层宽度大于溢流缝宽度. 原因是液膜层与流场边界层条件不匹配,存在压力梯度,迫使冷却液向低压区流动,从而展宽液膜层,并且流量越高,横向展宽现象越明显.      

高超声速边界层液膜演化过程和冷却机理研究

高超声速液膜冷却技术是通过一系列狭缝或孔洞压出冷却工质,在飞行器表面边界层形成一层低温冷却膜,阻止高超声速气流对飞行器的气动加热.其作为一种主动冷却方式在高超声速飞行器表面热防护有着巨大的应用潜力.文章采用数值方法,结合VOF模型,研究25 km飞行高度和Ma=5气流条件下的液膜铺展情况,并通过不同冷却工质的入射速度、角度、表面张力和黏性系数条件,讨论了液膜在平板上的演化过程和冷却机理.结果表明,在气流作用下,液膜向壁面下游发展,液膜的存在导致边界层分离,连续液膜会在一定位置断裂为液块,然后进一步破碎为液滴.入射条件和液体性质的改变,会影响液膜沿流向的发展,具体表现在连续液膜断裂点的位置和连续液膜的厚度.在所设定的计算域内,壁面热流降低了80%～95%,液膜对壁面的冷却效率随着液膜形态的变化而变化.     

Improvement of film cooling effectiveness with a small downstream block body

The aim of this study is to predict the improvement in film cooling performance over a flat plate through a single row of cylindrical holes with different streamwise angles by using the Ansys CFX software package. In order to improve the film cooling effectiveness, a short crescent-shaped block is placed downstream of a cylindrical cooling hole. The numerical results of the cylindrical hole without the downstream short crescent-shaped block are compared with experimental data.     

A predictive model based on a 3-D computational approach for film cooling effectiveness over a flat plate using GMDH-type neural networks

A 3-D numerical solution is implemented for investigating incompressible turbulent flow and thermal fields of film cooling through a single row of inclined cylindrical holes over a flat plate model. The effects of parameters of interest on the film cooling performance have been simulated. The group method of data handling (GMDH)-type neural networks successfully employed for modeling and presenting a correlation for area-weighted average adiabatic film cooling effectiveness.     

Effects of varying Reynolds number, blowing ratio, and internal geometry on trailing edge cutback film cooling

Three-dimensional mean velocity and concentration fields have been measured for a water flow in a pressure side cutback trailing edge film cooling geometry consisting of rectangular film cooling slots separated by tapered lands. Three-component mean velocities were measured with conventional magnetic resonance velocimetry, while time-averaged concentration distributions were measured with a magnetic resonance concentration technique for flow at two Reynolds numbers (Re) differing by a factor of 2, three blowing ratios, and with and without an internal pin fin array in the coolant feed channel. The results show that the flows are essentially independent of Re for the regime tested in terms of the film cooling surface effectiveness, normalized velocity profiles, and normalized mean streamwise vorticity. Blowing ratio changes had a larger effect, with higher blowing ratios resulting in surface effectiveness improvements at downstream locations. The addition of a pin fin array within the slot feed channel made the spanwise distribution of coolant at the surface more uniform. Results are compared with transonic experiments in air at realistic density ratios described by Holloway et al. (2002a).     

Numerical modelling of film cooling with and without mist injection

Two-phase CFD calculations, using a Lagrangian model and commercial code Fluent 6.2.16, were employed to calculate the gas and droplet flows and film cooling effectiveness with and without mist on a flat plate. Two different three dimensional geometries are generated and the effects of the geometrical shape, size of droplets, mist concentration in the coolant flow and temperature of mainstream flow for different blowing ratios are studied. A cylindrical and laterally diffused hole with a streamwise angle of 30° and spanwise angle of 0° are used. The diameter of film cooling (d) hole, and the hole length to diameter ratio (L/d) for both of geometries are 10 mm and 4, respectively. Also the blowing ratio ranges from 1.0 to 2.0, and the mainstream Reynolds number based on the mainstream velocity and hole diameter (Re _d) is 6,219. The results are shown for different droplets diameters (1–10 μm), concentrations (1–5%) and mainstream temperatures (350–500 K). The centreline effectiveness and distribution of effectiveness on the surface of cooling wall are presented.     

Measurement of the temperature field downstream of simulated leading-edge film-cooling holes

Experiments to measure the temperature field downstream of simulated leading-edge-region film-cooling holes were performed in an 11 m/s wind tunnel flow. Heated air was passed to a hollow 140 mm diameter cylinder in which three 10.5 mm diameter, spanwise-inclined, film-cooling holes had been machined. A fine nylon mesh, coated with encapsulated thermochromic liquid crystals, was used to measure temperature contours downstream of the holes by moving the mesh relative to the holes and adjusting the power to the air heater. The measurements indicate the extent of the lateral spreading of the coolant gas and show the influence of hole location and coolant mass flow rate on film trajectory and spreading. Received: 5 February 1998/ Accepted: 22 October 1998     

Influence of coolant injector configuration on film cooling effectiveness for gaseous and liquid film coolants

An experimental investigation is conducted to bring out the effects of coolant injector configuration on film cooling effectiveness, film cooled length and film uniformity associated with gaseous and liquid coolants. A series of measurements are performed using hot air as the core gas and gaseous nitrogen and water as the film coolants in a cylindrical test section simulating a thrust chamber. Straight and compound angle injection at two different configurations of 30°–10° and 45°–10° are investigated for the gaseous coolant. Tangential injection at 30° and compound angle injection at 30°–10° are examined for the liquid coolant. The analysis is based on measurements of the film-cooling effectiveness and film uniformity downstream of the injection location at different blowing ratios. Measured results showed that compound angle configuration leads to lower far-field effectiveness and shorter film length compared to tangential injection in the case of liquid film cooling. For similar injector configurations, effectiveness along the stream wise direction showed flat characteristics initially for the liquid coolant, while it was continuously dropping for the gaseous coolant. For liquid coolant, deviations in temperature around the circumference are very low near the injection point, but increases to higher values for regions away from the coolant injection locations. The study brings out the existance of an optimum gaseous film coolant injector configuration for which the effectiveness is maximum.     

Evolution of jets effusing from inclined holes into crossflow

The turbulent flow structure and vortex dynamics of a jet-in-a-crossflow (JICF) problem, which is related to gas turbine blade film cooling, is investigated using the particle-image velocimetry (PIV) technique. A cooling jet emanating from a pipe interacts with a turbulent flat plate boundary layer at a Reynolds number Re_∞ = 400,000. The streamwise inclination of the coolant jet is 30° and two velocity ratios (VR = 0.28, VR = 0.48) and two mass flux ratios (MR = 0.28, MR = 0.48) are considered. Jets of air and CO₂ are injected separately into a boundary layer to examine the effects of the density ratio between coolant and mainstream on the mixing behavior and consequently, the cooling efficiency. The results show a higher mass flux ratio to enlarge the size of the recirculation region leading to a more pronounced entrainment of hot outer fluid into the wake of the jet. Furthermore, the lateral spreading of the coolant is strongly increased at a higher density ratio. The results of the experimental measurements are used to validate numerical findings. This comparison shows an excellent agreement for mean velocity and higher moment velocity distributions.     

Experimental investigation of the effects of blowing conditions and Mach number on the unsteady behavior of coolant ejection through a trailing edge cutback

The present paper shows the results of an experimental investigation into the unsteadiness of coolant ejection at the trailing edge of a highly loaded nozzle vane cascade. The trailing edge cooling scheme features a pressure side cutback with film cooling slots, stiffened by evenly spaced ribs in an inline configuration. Cooling air is also ejected through two rows of cylindrical holes placed upstream of the cutback. Tests were performed with a low inlet turbulence intensity level (Tu₁ = 1.6%), changing the cascade operating conditions from low speed (M_2is = 0.2) up to high subsonic regime (M_2is = 0.6), and with coolant to main stream mass flow ratio varied within the 0.5–2.0% range. Particle Image Velocimetry (PIV) and flow visualizations were used to investigate the unsteady mixing process taking place between coolant and main flow downstream of the cutback, up to the trailing edge. For all the tested conditions, the results show the presence of large coherent structures, which presence is still evident up to the trailing edge. Their shape and direction of rotation change with injection conditions, as a function of coolant to mainstream velocity ratio, strongly influencing the thermal protection capability of the injected coolant flow. The Mach number increase is only responsible for a positioning of such vortical structures closer to the wall, while the Strouhal number almost remains unchanged.     

Film cooling and heat transfer from a combination of two rows of simple and/or compound angle holes in inline and/or staggered configuration

This paper describes the results of an experimental investigation into the film cooling effectiveness and heat transfer characteristics of two staggered injection rows of either a combination of one row of simple angle holes with another row of compound angle holes or with both rows of compound angle holes. The effect of using various injections holes arrangements as well as the relative location of the compound angle holes row to the simple angle holes row have been investigated for different blowing rates. Using combination of one row of downstream compound angle holes with another upstream simple injection holes row provides a significant increase in the film cooling protection over a flat plate surface, over that obtained from either two rows of only simple injections holes or compound angle holes. Received on 17 July 1998     

Numerical modelling of film cooling from converging slot-hole

This paper presents a numerical prediction of a new 3D film cooling hole geometry, the converging slot-hole or console. The console geometry is designed in order to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems without loosing the mechanical strength of a row of discrete holes. The cross section of the console changes from a circular shape at the inlet to a slot at the exit. Previous successful application of a new anisotropic DNS based two-layer turbulence model to cylindrical and shaped hole injections is extended to predict film cooling for the new console geometry. The suitability of the proposed turbulence model for film cooling flow is validated by comparing the computed and the measured wall-temperature distributions of cylindrical hole injections. The result shows that the anisotropic eddy-viscosity/diffusivity model can correctly predict the spanwise spreading of the temperature field and reduce the strength of the secondary vortices. Comparative computations of the adiabatic film cooling effectiveness associated with the three geometries tested in the present study (cylindrical, shaped, and console) show that the new console film-cooling hole geometry is definitely superior to the other geometries as shown by the uniform lateral spreading of the effectiveness with a slight enhancement downstream of the intersection of the two consoles.     

High resolution PIV measurements around a model turbine blade trailing edge film-cooling breakout

Film cooling downstream of a model turbine blade trailing edge has been studied experimentally. High resolution particle image velocimetry was used to obtain spatially resolved mean velocity and turbulence measurements in the immediate vicinity of the trailing edge breakout. The mean velocity measurements imply the presence of a pair of counter-rotating longitudinal vortices shed from the sides of the breakout lands. The turbulent shear stress measurements above the breakout are significantly intensified as blowing ratio is increased. These results suggest that there is a strong mixing between the film cooling slot jets and the mainstream flow which degrades the film cooling effectiveness.     

FLOW STRUCTURE IN THE TURBULENT BOUNDARY LAYER OVER DIRECTIONAL RIBLETS SURFACES

本文采用时间解析的二维粒子图像测速技术,对零压力梯度光滑以及汇聚和发散沟槽表面平板湍流边界层统计特性和流动结构进行了研究.结果表明在垂直于汇聚和发散沟槽表面的对称平面内,相对于光滑壁面,发散沟槽壁面使当地边界层厚度、壁面摩擦阻力、湍流脉动、雷诺应力等明显减小;而汇聚沟槽壁面对湍流边界层特性和流动结构的影响正好相反,汇聚沟槽使壁面流体有远离壁面向上运动的趋势,因而导致边界层厚度增加了约43%;同时,在汇聚沟槽表面情况下流向大尺度相干结构更容易形成,这对减阻是不利的.此外,顺向涡数量在湍流边界层的对数区均存在一个极大值,发散沟槽表面所对应的极大值位置更靠近沟槽壁面,而在汇聚沟槽表面则有远离壁面的趋势,由顺向涡诱导产生的较强的喷射和扫掠运动会在湍流边界层中产生较强的剪切作用,顺向涡数量的减少是发散沟槽壁面当地摩擦阻力降低的主要原因.     

A broad reconsideration of anti-vortex film cooling method using numerical optimization and an improved heat-flux model

This paper represents the detailed results of an evolutionary optimization framework towards the exploration of vortex mechanisms leading to effective anti-vortex film cooling. In this regards, several arrangements of triple cooling holes were studied on flat and curved geometries using differential-evolution optimization algorithm and a modified Reynolds-stress based flow solver. Depending on the flow and geometric parameters, four distinct types of vortex interaction with different cooling mechanisms were identified. The vortex-trapping mechanism, observed in the optimized upstream arrangement acts through imposing a mild downwash over the main counter-rotating vortex pair and provides the best cooling effectiveness for the low injection angle (less than 30°) cases. The vortex-suppression and -balancing are the optimal possible solutions of the adjacent arrangement. The latter is the classic well-known type of anti-vortex cooling, while the former provides a sudden strong controlling potential for high blowing ratios (higher than 1.0) and high injection angle film cooling. For the non-flat surfaces the triple holes effectively perform up to blowing-ratio of 2.0. However, the reverse-vortex-trapping mechanism occurring in the downstream arrangement is recommended for convex surfaces, while the adjacent arrangement is the choice for concave regions. In general, there is a possibility of reducing the coolant consumption about 30% through increasing the pitch-to-diameter ratio, while the values of cooling-effectiveness still remain in an acceptable range.