叶型表面曲率对离散孔气膜冷却性能的影响

由于型面曲率的影响,涡轮叶片前缘和吸力面的冷却气膜易于脱离型面,气膜冷却效果下降。本研究将叶片型线分段拟合,建立了多个单一曲率的曲面模型(R/D=-30、-75、120、∞),研究涡轮叶片表面曲率对于气膜冷却的影响。流动与传热的数值模拟采用Fluent软件,湍流模型选择RNGk-ε模型,模拟方法经平板流动进行的结果验证是可靠的。在不同吹风比(M=0.5、1.2、2.0)条件下,计算比较了不同曲率曲面上气膜单孔下游的壁面传热系数以及局部平均气膜冷却效率。结果表明:涡轮叶片型面曲率对气膜冷却效果的影响与吹风比有关。不同曲率的型线部分,应该设计采用不同的吹风比,气膜冷却效果可能取得最佳。低吹风比M<1时,凹面曲率对气膜换热系数是强化,凸面基本没有作用。高吹风比M>1时,曲率不影响换热能力,冷却效果则取决与气膜相对于型面的流动状态和与主流的掺混能力。     

复合角度对称射流气膜冷却效率的数值研究

采用 realizable k-ε模型,通过数值模拟分析了各种孔排结构在不同吹风比下对气膜冷却效率的影响.结果表明:边界层等温线反映了上、下游射流孔所产生的射流之间的相互作用;当吹风比M=O.5时,由于射流紧贴壁面,各种孔排结构都有较好的冷却效果,随着吹风比的增大,射流逐渐脱离壁面;当顺排排列时,由于上游射流孔的作用,使得下游射流能很好地贴附壁面,在M=2.0时,冷却效果仍然较好;孔排 3 在M=0.5 时有较高的冷却效率;而对于孔排1,当M=1时,气膜在x/d=2区域以后有很好的保护作用.     

流向涡对扇形孔气膜冷却效果影响的实验研究

在平板气膜实验台上安装涡流发生器(VG)来模拟叶栅中的流向涡,利用红外相机测量吹风比M=0.5～2.5时的气膜冷却效率和传热系数,分析二次流对扇形孔气膜冷却效果的影响.结果 表明:流向涡增强了主流与气膜射流的掺混,导致气膜绝热冷却效率明显下降以及覆盖面积减小,气膜面平均冷却效率最高降低了63％;当吹风比达到2.5时,流向涡能抑制扇形孔射流在高吹风比时的吹离趋势,抵消了部分主流与冷气掺混导致的气膜横向平均冷却效率降低的影响;流向涡使气膜与壁面的横向传热系数比增大了3.5％,壁面的热通量比最高上升了20％,在低吹风比时气膜失去了对壁面的保护作用.     

气膜孔形状对冷却效率影响的数值研究

采用控制容积法对三维定常不可压缩雷诺时均紊流方程(N-S方程)进行了离散,并在吹风比M为0.6和1.2的情况下,利用非结构化网格及两层k-e湍流模型,对气膜孔几何形状对涡轮叶片气膜冷却效率的影响进行了数值模拟,得到气膜孔附近的流场分布.结果表明:圆柱形孔的冷却效率随吹风比的增大而明显降低.前向扩张孔的冷却效率优于圆柱形孔,射流在叶高方向上扩展较广,在侧向孔间区域的气膜冷却效率较高.缩放槽缝孔在不同吹风比下的冷却效率均高于圆柱形孔和前向扩张孔,而且在孔下游较远区域,2个孔之间沿叶高方向的气膜覆盖性较好.缩放槽缝孔和前向扩张孔不同程度地抑制了反向涡旋对的产生,因而提高了射流对壁面的贴附性,增强了壁面的冷却效果.     

双叉排孔脉动射流气膜冷却效率的实验与数值模拟研究

采用高精度红外热像仪测量了平板绝热气膜冷却效率,比较了双叉排孔和单排孔气膜冷却效率,分析了吹风比(M=0.65,1.0,1.5)和脉动频率(St=0,0.01,0.015,0.025)以及孔间作用对气膜冷却效率的影响,结合数值计算得到的瞬态流场和温度场分析了脉动射流气膜冷却下的流动传热机理。结果表明：在稳态射流工况下,单排孔的气膜冷却效率随着吹风比的增加而减小,双叉排孔的气膜冷却效率却随着吹风比的增加而增大;在脉动射流时,单排和双叉排孔的气膜冷却效率在低吹风比下低于稳态射流,在高吹风比下,脉动射流对气膜冷却效率的影响减小,且低频脉动射流气膜冷却效率略高于稳态射流。     

气膜孔布置对旋流双层壁结构流动传热特性影响研究

为了研究气膜孔复合角和孔排布方式对冲击诱导旋流气膜双层壁冷却流动与耦合传热的影响,采用流固耦合的数值计算方法,对不同吹风比下4种气膜孔复合角的综合冷却效率和射流流动结构进行了详细研究,同时研究了两种气膜孔排布方式对旋流双层壁结构流动和传热的影响,揭示了4种气膜孔复合角θ下射流空间旋涡的形成、发展和演化过程,并对比分析了不同吹风比下4种气膜孔复合角对面积平均综合冷却效率和总压损失系数的影响。结果表明：气膜孔排布方式不同时,气膜孔复合角对综合冷却效率的影响相差较大。吹风比为1.0时,单旋流交错排布方式下增大气膜孔复合角可以有效提高射流的展向覆盖范围,从而提高综合冷却效率,θ=30°时的面积平均综合冷却效率比θ=0°时高约14.22%;双旋流交错排布时θ=30°比θ=0°时的面积综合冷却效率提高约4.58%。吹风比增加到2.0时,由于射流穿透主流吹离了冷却壁面,降低了气膜保护效果,使得冷却壁面均匀性变差,并且双旋流交错排布时尤为明显。随着吹风比的增加,两种气膜孔排布方式的总压损失系数均呈指数形式增加。     

圆形孔排的气膜冷却曲率实验研究

在低速大尺度叶栅风洞中对放大五倍的叶片模型进行实验,采用红外热像仪进行温度场的测量,研究在叶背表面不同曲率位置、不同主流Re、不同吹风比对气膜冷却效果的影响.结果表明:叶背处气膜冷却效果随着吹风比的增大而降低;主流Re对冷却效果有不同程度的影响,在吹风比不变时,冷却效果随Re增大呈现出先增大后减小的趋势;凸面曲率特征的气膜冷却容易造成射流脱壁,沿程冷却效果下降幅度较大,在附着区域相对较小;在研究范围内,气膜冷却效果随着曲率值的增加先增加再减小.     

气膜孔形状对涡轮叶片气膜冷却效果的影响

基于控制容积法对三维定常不可压缩N-S方程进行离散,采用非结构化网格及两层k-ε湍流模型,在吹风比M为0.6和1.2的情况下,数值模拟了气膜孔几何形状对涡轮叶片气膜冷却效果的影响,得到了气膜孔附近的流场分布.所选孔形为圆柱孔、前向扩张孔、开槽前向扩张孔及新型缩放槽缝孔.结果表明:圆柱孔的冷却效率随着吹风比的增加而显著地降低;开槽前向扩张孔的冷却效率优于圆柱孔和前向扩张孔;缩放槽缝孔在不同吹风比下的冷却效率均高于其它3种孔形,缩放槽缝孔和开槽前向扩张孔不同程度地抑制了反向涡旋对的产生,提高了射流对壁面的贴附性,增强了壁面的冷却效果.     

不同吹风比下双出口孔射流气膜冷却数值模拟计算

为了获得吹风比对新型气膜冷却孔冷却效率的影响规律,利用Fluent软件求解Navier-Stokes方程,对吹风比分别为0.5、1.0、1.5和2.0时单入口-双出口孔射流冷却效率进行了数值模拟计算,得到了不同吹风比下的流场和冷却效率.结果表明：吹风比对冷却效率有很大影响;随着吹风比的提高,不同次孔方位角下的冷却效率变化规律也不相同;当次孔方位角γ=30°时,吹风比为1.0时的冷却效率最高;当γ=45°时,冷却效率随着吹风比提高而提高;当γ=60°时,冷却效率随着吹风比提高而降低;在研究高吹风比对气膜冷却效率的影响时,γ=45°最佳.     

后台阶三维缝隙冷却效率的数值模拟

针对涡轮叶片尾缘冷却结构特点,建立了后台阶三维缝隙结构气膜冷却特性计算模型,计算了冷却效率在出口壁面的分布,研究了不同雷诺数(5 000~15 000)与吹风比(0.5~2.0)影响,计算结果表明:在缝后壁面冷却效率是单调递减的,而肋后冷却效率是先增大后减少的分布规律;二次流出口壁面冷却效率受吹风比影响较大,冷却效率随吹风比增大而减小;壁面缝后冷却效率受雷诺数的影响较小。     

300MW循环流化床锅炉屏式受热面传热系数计算及其变化规律

根据300MW循环流化床（CFB）锅炉现场测试数据并结合以往CFB锅炉传热系数的研究成果,建立了屏式受热面烟气侧的传热模型,包括辐射传热模型和对流传热半经验公式．利用该模型对某300MWCFB锅炉在94％锅炉最大连续蒸发量（BMCR）工况下炉膛内屏式受热面的传热系数进行了计算,分析了屏式受热面管间节距、炉膛温度、工质温度、壁面黑度及烟气速度等因素对传热系数的影响．结果表明：烟气速度、炉膛温度和壁面黑度对传热系数的影响较大,所建立的传热模型能够合理地反映主要因素对CFB锅炉屏式受热面传热的影响．     

Endwall heat transfer and film cooling measurements in a turbine cascade with injection upstream of leading edge

Detailed heat transfer measurements were conducted on the endwall surface of a large‐scale low‐speed turbine cascade with single and double row injection on the endwall upstream of leading edge. Local film cooling effectiveness and the heat transfer coefficient with coolant injection were determined at blowing ratios 1.0, 2.0, and 3.0. In conjunction with the previously measured flow field data, the behaviors of endwall film cooling and heat transfer were studied. The results show that endwall film cooling is influenced to a great extent by the secondary flow and the coverage of coolant on the endwall is mainly determined by the blowing ratio. An uncovered triangle‐shaped area with low effectiveness close to pressure side could be observed at a low blowing ratio injection. The averaged effectiveness increases significantly when injecting at medium and high blowing ratios, and uniform coverage of coolant on the endwall could be achieved. The averaged effectiveness could be doubled in the case of double row injection. It was also observed that coolant injection made the overall averaged heat transfer coefficient increase remarkably with blowing ratio. It was proven that film cooling could reduce endwall heat flux markedly. The results illustrate the need to take such facts into account in the design process as the three‐dimensional flow patterns in the vicinity of the endwall, the interactions between the secondary flow and coolant, and the augmentation of heat transfer rate in the case of endwall injection. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 141–152, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20007     

Film cooling performance of a row of dual-fanned holes at various injection angles

Film cooling performance about a row of dual-fanned holes with injection angles of 30°, 60 ° and 90° were experimentally investigated at blowing ratios of 1.0 and 2.0. Dual-fanned hole is a novel shaped hole which has both inlet expansion and outlet expansion. A transient thermochromic liquid crystal technique was used to reveal the local values of film cooling effectiveness and heat transfer coefficient. The results show that injection angles have strong influence on the two dimensional distributions of film cooling effectiveness and heat transfer coefficient. For the small injection angle of 30 degree and small blowing ratio of 1.0, there is only a narrow spanwise region covered with film. The increase of injection angle and blowing ratio both leads to the enhanced spanwise film diffusion, but reduced local cooling ability far away from the hole. Injection angles have comprehensive influence on the averaged film cooling effectiveness for various x/d locations. As injection angles are 30 and 60 degree, two bands of high heat transfer coefficients are found in mixing region of the gas and coolant. As injection angle increases to 90 degree, the mixing leads to the enhanced heat transfer region near the film hole. The averaged heat transfer coefficient increases with the increase of injection angle.     

Parametric study of turbulent slot-jet impingement heat transfer from concave cylindrical surfaces

Numerical investigation of convective heat transfer process from concave cylindrical surfaces due to turbulent slot-jet impingement is performed. Constant heat flux condition is specified at the concave surfaces. The flow and thermal fields in the vicinity of the surfaces are computed using the RNG k–? turbulence model with a two-layer near wall treatment. Parametric studies are carried out for various jet-exit Reynolds numbers, surface curvature, and nozzle-to-surface spacing. Results presented include streamlines, isotherms, velocity and temperature profiles in the wall-jet region, and the local Nusselt number distribution on the impingement concave wall for various parameter values in the study. The results indicate that while the jet-exit Reynolds number and the surface curvature have a significant effect on the heat transfer process, it is relatively insensitive to the jet-to-target spacing. A correlation for the average Nusselt number at the concave surface as a function of the parameters considered in the study is also derived.     

凹凸变化壁面强化传热机理与传热性能的研究

介绍了一种近年来开发的板式外流降膜蒸发器中的凹凸变化壁面的传热元件;分析讨论了其强化传热的机理及其传热性能;建立了凹凸变化壁面上降落液膜的传热系数准数关联式,为其应用于工业生产提供了参考数据.     

Numerical simulation of film-cooling concave plate as coolant jet passes through two rows of holes with various orientations of coolant flow

Computations are performed to predict the three-dimensional flow and heat transfer of concave plate that is cooled by two staggered rows of film-cooling jets. This investigation considers two coolant flow orientations: (1) the coolant jets were ejected from a straight-blow supply plenum, so the coolant supply plane is parallel to the entrance plane of the coolant jets; (2) the coolant jets were ejected from the cross-blow supply plenum so the coolant supply plane was normal to the entrance plane of the coolant jets. The effects of numerous film-cooling parameters were investigated, including the mainstream Reynolds number, the angular locations of the two-row injections and the blowing ratio. The mainstream Reynolds number, determined by the diameter of the injection hole as the characteristic length, varied from 3440 to 13,760. The blowing ratio ranged from 0.5 to 2.0 with a fixed density ratio of 1.14. Additionally, two angles of injections, 40° and 42°, from the exit plane of the entrance duct are considered. Results are presented as the surface adiabatic film-cooling effectiveness, the temperature distribution and the velocity vector profile. The formation and trace of counter-rotating vortex pairs that result from the interaction between the mainstream hot gas and the cooling jets was clearly exhibited. The laterally averaged film-cooling effectiveness over the concave surface with a straight-blow plenum is slightly higher than that of a cross-blow plenum at all test blowing ratios. Results of this study demonstrate that the blowing ratio is one of the most significant film-cooling parameters over a concave surface.     

Improvement of film cooling performance by trenched holes on turbine leading-edge models

The present study aims to improve cooling performance over the leading edge surface with the high temperature and high thermal stress by the introduction of trenched holes. Three staggered rows of leading-edge film cooling holes with different trench arrangements and hole orientations are included under blowing ratios of 0.5, 1.0, 1.5, and 2.0, compared with round-hole cases. Under the conditions of leading-edge flow patterns and convex curvature, the trenched hole with 2D width plays a role of “protection” of coolant against the impinging hot gas at a large range of blowing ratios. This contributes to the better lateral spread of coolant and cooling performance. Besides, the trenched holes narrow the regions with a high heat transfer coefficient and reduce the detrimental heating on the surface. Compared with round holes, the trenched holes guarantee the downstream coolant coverage and higher cooling performance at a larger inclined angle, in spite of the changed compound angles.     

Flow and heat transfer in curved wall jets on circular surfaces

Consideration is given to curved wall jets (i.e. flow along a wall which is curved in the streamwise direction) and, in particular, to flow along either a convex or a concave circular surface. The laminar flow and heat-transfer characteristics are investigated by making use of the method of inner and outer expansions. The Navier-Stokes and energy equations are expanded in series, with as the expansion parameter. The first-order equations are identical to the conventional boundary-layer equations, whereas the second-order equations are corrections for curváture and displacement effects. The latter equations were solved by a difference-differential method, with Pr = 0·72 for the energy equation. The second-order correction increases the wall shear, the extent of the increase being greater for flow over a concave surface than for flow over a convex surface. On the other hand, the second-order correction either increases or decreases the Nusselt number, depending on whether the surface is convex or concave. The Coanda effect, whereby an induced transverse pressure difference inhibits flow separation, was demonstrated by the analysis.     

Analysis of Convective Heat Flow Visualization within Porous Right Angled Triangular Enclosures with a Concave/Convex Hypotenuse

Analysis of natural convection in porous right angled triangular enclosures with a concave/convex hypotenuse has been carried out using the Bejan's heatlines approach. A generalized non-Darcy model without Forchheimer term is employed for fluid flow in a porous matrix and the governing equations are solved by the Galerkin finite element method. The cavity is subjected to a thermal boundary condition of an isothermal cold left wall, isothermal hot curved right wall, and adiabatic bottom wall. Due to intense closed loop heatlines, thermal mixing is higher in convex cases compared to the concave case for all parameters. Average heat transfer rate is found to be largest in the concave hypotenuse case.