Numerical investigation of an axi-symmetric laminar jet impinging on a dimpled surface under uniform heat flux using a finite element method

The purpose of this study is to numerically investigate heat transfer and flow field in a semi-confined axi-symmetric laminar air jet impinging on a concave surface, or dimple, with uniform heat flux. A commercial software package relying on the finite element method was used for the simulation, and mesh convergence was examined in order to minimize computational cost. Jet impingement on a flat plate was used as a baseline reference case, and flat plate results were validated against previously published experimental data with good agreement. The effects of various parameters involved in dimple impingement -such as Reynolds number (Re) between 100–1,400; jet-to-plate spacing (H/D_j) ranging from 2 to 6 jet diameters; dimple depths (d/D_d) of 0.1, 0.15, and 0.2; and the ratio of jet diameter and dimple projected diameter (D_j/D_d) from 0.25 to 1—were all studied. Comparisons show that heat transfer reduction occurs in the presence of dimples because of the larger impingement area, which results in less momentum flux. The dimple curvature lifts the post-impinging fluid and creates a backflow, instead of allowing it to maintain contact with the surface, as is the case with flat plate impingement.     

A numerical study on the transient impulsive pressure of a water jet impacting nonplanar solid surfaces

This paper presents an in-depth investigation into the transient impulsive pressure of an arc-curved water jet impacting a solid surface. The emphasis of this study is on the variations of the surface shape, which are classified into four types: The flat surface, the concave surface, the convex surface and the inclined surface. The numerical tool of arbitrary Lagrangian-Eulerian formulations is used to model the arc-curved jet impacting these different solid surface types. Elaborately designed experiments were conducted to test the impulsive pressure profile; the experimental results are found to be in approximate agreement with the numerical results. The impulsive pressure profiles of water jet impacting the flat and inclined solid surface are observed to exhibit two quintessential stages, in line with the traditional pressure profile; however, a double/multiple-peaked pressure structure is observed for the cases of the water jet impacting the concave and convex solid surfaces. Additionally, the value of the peak pressure is found to be a quadratic representation with the jet velocity, and the duration of the peak pressure is found to be an exponential representation with the jet velocity. The compression degrees of the liquid jet impacting the different surfaces are validated to be the root cause for the discrepancy of the impulsive pressure.     

Effects of combustor-level high inlet turbulence on the endwall flow and heat/mass transfer of a high-turning turbine rotor cascade

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The endwall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the endwall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the endwall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suctionside corner are influenced most strongly by the high free-stream turbulence. In this study, the endwall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.     

Cooling by sub-zero cold air jet in the grinding of a cylindrical component

This paper investigates the cooling effect when using an air jet at sub-zero temperature of ?15 °C in the plunge grinding of a cylindrical component made of high strength steel EN 26. A three-dimensional finite element heat transfer model with a moving heat source was developed to reveal the complexity of the heat transfer mechanism involved. It was found that the use of cold air does not significantly reduce the temperature rise in grinding and that the cooling effectiveness is mainly limited by the following facts: (a) the air jet is difficult to penetrate into the grinding zone, (b) the heat transfer coefficient provided by an air jet is small and (c) cooling is limited by the time which the rotating workpiece surface can be exposed to the jet impingement. The study also showed that the present modelling method can be used as a first tool to assess the feasibility of a new cooling medium for grinding operations.     

Boundary layer flow over a finite flat plate with a constant slip velocity

Two-dimensional incompressible laminar flow induced by a constant slip velocity on the surface of a finite flat plate is studied both analytically and numerically for large Reynolds numbers. It turned out that the thickness of the thin layer downstream of the trailing edge increases in square root of the distance from the trailing edge. Numerical integration of the boundary layer equations for the whole flow field confirmed two asymptotic natures of the flow field; near the trailing edge the analytic result is approached, and far downstream of the plate the jet flow solution is attained.     

An experimental study on the flow and heat transfer characteristics of an impinging jet

The flow and heat transfer characteristics of an impinging jet is investigated in two major stages. The first stage is about the investigation of the three dimensional mean flow and the turbulent flow quantities in free jet, stagnation and wall jet region. After a complete documentation of the flow field, the convective heat transfer coefficient distributions on the impingement plate are presented, during the second stage of the study. Heat transfer experiments using the new hue-capturing technique result in high resolution wall heating rate distributions. The technique is fully automated using a true color image processing system. The present heat transfer results are discussed in detail in terms of the flow characteristics. The measurements from the new method are compared with conventional heat flux sensors located on the same model. These heat transfer distributions are also compared with other studies available from the literature. The new non-intrusive heat transfer method is highly effective in obtaining high resolution heat transfer maps with good accuracy.     

A naphthalene sublimation study on heat/mass transfer for flow over a flat plate

It is important to completely understand heat/mass transfer from a flat plate because it is a basic element of heat/mass transfer. In the present study, local heat/mass transfer coefficient is obtained for two flow conditions to investigate the effect of boundary layer using the naphthalene sublimation technique. Obtained local heat/mass transfer coefficient is converted to dimensionless parameters such as Sherwood number, Stanton number and Colburnj-factor. These also are compared with correlations of laminar and turbulent heat/mass transfer from a flat plate. According to experimental results, local Sherwood number and local Stanton number are in much better agreement with the correlation of turbulent region rather than laminar region, which means analogy between heat/mass transfer and momentum transfer is more suitable for turbulent boundary layer. But average Sherwood number and average Colburnj-factor representing analogy between heat/mass transfer and momentum transfer are consistent with the correlation of laminar boundary layer as well as turbulent boundary layer.     

红外探测中潜艇冷热尾流的传热传质特性

本文采用有限体积法建立了1/72龙鲨Ⅱ号核潜艇的三维计算模型,结合动参考系、用户自定义函数和物性多项式函数等实现了高速旋转螺旋桨和海水温度密度分层的仿真。基于该模型,探讨了螺旋桨高速旋转、海水温度密度分层和高温热尾流喷射等因素对潜艇冷热尾流传热传质特性的影响,所得结论如下:高速旋转螺旋桨促使热尾流后向延迟距离增大、海表温差减小,忽略旋转时海表温差的绝对误差和相对误差分别为3.23mK和52.7%;水下航行潜艇扰动温度密度分层海水浮升形成冷尾流温差信号,与温度密度均匀海水相比,海表温变区域显著增大、尾流温差由6.13mK增大到84mK;通过海表上游冷尾流特征判断是否存在水下航行潜艇,若存在,再结合海表下游热尾流特征实现潜艇位置的精确反演。上述结论可为优化潜艇冷热尾流的数值仿真精度提供参考与借鉴。     

不锈钢表面裂纹方向电磁检测方法

针对传统交流电磁场检测表面不定方向裂纹容易出现漏检的问题,研究了旋转磁场下感应电流方向、裂纹方向变化对平板表面裂纹检测幅值的影响,推导了裂纹走向角度与平面磁场分量波谷幅值的关系表达式,从检测机理角度提出了裂纹方向判定方法及影响因素.建立了奥氏体不锈钢表面裂纹交流电磁场检测有限元仿真模型,开发了基于双U形激励和高分辨率隧...     

新型单圆锥体热沉单孔射流散热数值模拟*

射流冷却是高热流密度换热的有效方法之一,其热沉形状是影响换热的关键因素。提出一种新型圆锥体热沉,应用RNG k-ε湍流模型,对新型单圆锥体热沉进行单孔水冷散热数值模拟,并进行试验验证。结果表明：单圆锥体热沉的散热效果明显强于常规平板热沉,前者的射流流体域比后者多一个转折区,转折区内存在二次冲击,使换热得到强化。雷诺数越大,圆锥体热沉散热性能越优异;在不同锥角(15°~60°)的单圆锥体热沉传热模拟中,当锥角在45°左右时,热沉表面的平均努塞尔数数最高;圆锥体底面直径d₁和射流孔直径d比值在1~3范围内,数在比值为2.5左右达到最大值;射流高度H与射流孔直径d比值在5左右时,数趋于最大。     

An investigation of local heat transfer characteristics in a ventilated disc brake with helically fluted surfaces

Local Nusselt numbers in the cooling flow passage of the automobile disc brake with helically fluted surfaces are presented. The flat surface in the flow passage is modified to the helically fluted surface for the purpose of enhancing the heat transfer rate, thereby reducing the thermal stress and deformation in the disc brake. Thermochromic liquid crystals and shroud-transient technique are used to measure spatially-resolved surface temperature distributions, which are used to deduce local Nusselt numbers. The Reynolds number Re ranges from 30,000 to 70,000, the helix angle θ is fixed at 45° and the dimensionless streamwise distance z/d ranges from 1.5 to 4.5. The results show that in general, local Nusselt numbers monotonically decrease with a distance away from both windward and leeward crests of the helical flute and reach a minimum value near its valley for all Re’s and z/d’s tested. The local Nusselt numbers on the helically fluted grooves are maximum 51.6 to 93.7% higher than values measured on the flat surface. The heat transfer enhancement magnitudes become more pronounced with smaller Re and z/d. The largest enhancement occurs at the windward side of the helical flute at z/d=1.5 and Re=30,000. It is also found that at Re=30,000 the average Nusselt numbers on the helically fluted surface are maximum 37% higher than those on the flat surface. The numerical results show that with 10 braking cycles, the temperatures with helically fluted surface are maximum 44.3%, 36.8%, and 36.6% lower than those with the flat surface in the inlet, the center, and the outlet, respectively.     

Experimental investigation of aerodynamic effects of probe on heat transfer from hot-wire sensors at vertical and horizontal orientations

Aerodynamic effects due to hot-wire anemometer (HWA) probe directly influence heat transfer from the probe sensor and result in reduced accuracy in two-dimensional measurements. This experimental research investigates the aerodynamic effects for hot-wire sensors through the study of some important factors such as probe geometry, flow scheme (velocity and direction) and orientation of the probe relative to the flow direction. In addition, flow velocity field between the prongs of a 10:1 model of a single normal probe is explored at different velocities and yaw angles, both at vertical and horizontal orientations of the probe. Results indicate that in vertical orientation, heat transfer from the sensors is always higher than horizontal orientation. Moreover, the aerodynamic effects cause a velocity increase of up to 6% in the vicinity of the sensor. In addition, the presence of the sensor in the flow, generates low-velocity field in the flow wake and a minor rotation of the flow in the vicinity of the sensor, which result in reduced heat transfer from the sensor in horizontal orientation compared to the vertical orientation.     

Optimum cut orientation of piezoelectric substrate and propagation direction of SAW for rotation rate sensor

The effect of Coriolis and centrifugal forces on the propagation characteristics of surface acoustic waves (SAW) on a rotating piezoelectric substrate can be utilized for sensing rotation rate. In this paper, the relationship between SAW phase velocity and the rotation rate of the substrate is analyzed theoretically and numerically based on the coupling wave equations on anisotropic piezoelectric substrate with rotation effect. Partial wave theory and surface effective permittivity method are employed. Using LiNbO₃ as an example, some quantitative results of SAW rotation rate sensor, including free and metalized interfaces between substrate and free space in X-cuts, Y-cuts, and Z-cuts, are obtained. Furthermore, by considering comprehensively SAW rotation rate sensitivity with SAW excitation efficiency and beam steering, the optimum cut orientation of piezoelectric substrate and propagation direction of SAW for rotation rate sensor are presented. The research findings can provide theoretical guidance and simulation basis for the experimental research on SAW rotation rate sensor.     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

Application of temperature-sensitive paint for surface temperature measurement in heat transfer enhancement applications

This paper introduces the use of temperature-sensitive paint (TSP) to obtain the whole-field temperature distributions in heat transfer enhancement applications. Knowing the surface temperatures in such applications is important to quantify the heat transfer, as well as to obtain insight into the flow physics. Applying the TSP to a flat plate with and without a vortex generator showed that the TSP Nusselt number distribution over the flat plate alone matched the Blasius solution with less than 10% error. The TSP temperature distribution of the flat plate with the delta wing vortex generator showed a local heat transfer enhancement of about 30% and the locations of temperature extremes. This paper also provides a detailed guideline for the application, image acquisition, and image processing procedures for TSP along with its calibration surfaces.     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

燃气轮机燃烧室冲击射流冷却火焰筒壁的结构参数研究（英文）

The flame temperature in the combustor of a gas turbine is usually as high as 2000 K, while the maximum temperature that can be endured by metal materials is less than 1200 K at present. Therefore, various protective and cooling measures are needed to ensure the operation life of the liner wall which wraps the flame. The lean premixed combustor can meet the increasingly stringent emission requirements, but it requires more air for premixed combustion and then less air for cooling and dilution. In order to obtain a better impingement jet cooling structure, this paper studied the impingement jet cooling structure with vertical circular holes of equal diameter under single outlet condition. The structural variables studied include the jet hole diameter D, the impinging distance Z, the jet hole length(jet plate thickness) t, and the jet-to-jet spacing X is ignored. Among them, X/D(the ratio of the jet-to-jet spacing to the jet diameter) is inversely correlated with the mass flow rate. Within the constant X/D being equal to 10, the influence of D, Z and t on the average heat transfer coefficient h of the target surface under same mass flow was determined by means of conjugate numerical heat transfer analysis and orthogonal test. The results show that Z has significant influence on h, D has moderate influence on h, and t has negligible influence on h. Further, by means of regression orthogonal test, the influence trend of parameters Z and D on h at X/D=10 was studied. The optimal values of Z and D within the research scope were found.     

Effect of guide wall on jet impingement cooling in blade leading edge channel

The characteristics of fluid flow and heat transfer, which are affected by the guide wall in a jet impinged leading edge channel, have been investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes analysis via the shear stress transport turbulence model and gamma theta transitional turbulence model. A constant wall heat flux condition has been applied to the leading edge surface. The jet-to-surface distance is constant, which is three times that of the jet diameter. The arrangement of the guide wall near the jet hole is set as a variable. Results presented in this study include the Nusselt number contour, velocity vector, streamline with velocity, and local Nusselt number distribution along the central line on the leading edge surface. The average Nusselt number and average pressure loss between jet nozzle and channel exit are calculated to assess the thermal performance. The application of the guide wall is aimed at improving heat transfer uniformity on the leading edge surface. Results indicated that the streamwise guide wall ensures the vertical jet impingement flow intensity and prevents the flow after impingement to reflux into jet flow. Thus, a combined rectangular guide wall benefits the average heat transfer, thermal performance and heat transfer distribution uniformity.     

等离子射流与喷涂粒子微观交互作用研究现状

从微观角度看,等离子喷涂层实质上是由大量喷涂粒子在等离子射流中经过一系列复杂的理化变换之后,撞击基体并迅速铺展凝固所形成的,因而熔滴撞击基体前的理化特性对涂层的组织结构、缺陷密度、力学性能等指标具有重要影响。通过对喷涂粒子基本特征参数、射流中的传热机理、传质机理与粒子加速行为四个方面的总结,详细综述等离子射流与喷涂粒子的交互作用过程。总体来说,温度、速度是粒子的基本特征参数,而采用一些综合温度与速度的复合参数（如熔融指数、雷诺数、韦伯数等）对熔滴的理化特性具有更好的表征效果;粒子的加热过程由表及里,受到热导率、比表面积、热容量、飞行路径及射流特性等多种因素影响,部分熔滴容易由于温度过高而发生汽化现象;处于熔融状态的粒子具有较高的活性,因而容易在射流中与气体介质发生反应,包括O₂、N₂、H₂等,同时粉体内部也会发生一定的元素迁移或化学反应;粒子在射流中由于受到气流拖拽力、重力、热泳力及气压梯度力的综合作用而不断加速,同时会由于射流特性及熔化状态的差异而发生不同程度的破碎或细化现象。     

Heat/mass transfer characteristics in the near-tip region on a turbine blade surface under combustor-level high inlet turbulence

Heat/mass transfer characteristics on the near-tip blade surface under combustor-level high inlet turbulence have been investigated within a high-turning turbine rotor passage by using the naphthalene sublimation technique. The inlet turbulence intensity and length scale are 14.7% and 80 mm, respectively. The tip gap-to-chord ratio is changed to beh/c = 0.74, 1.47, and 2.94 percents. Increasingh/c results not only in higher heat/mass transfer in the pressure-side tip region but also in more convective transport on the pressure surface even far away from the tip edge. Severe heat/mass transfer is always observed in the suction-side tip-leakage flow region which can be divided into two distinct high transport regions. There is a local maximum of heat/mass transfer along the trailing-edge centerline. This arises from the interaction of a tip-leakage vortex with a trailing-edge vortex shedding. Comparisons of the present data forh/c = 2.94 percents with the previous low turbulence one show that there is a large discrepancy of heat/mass transfer in the pressure-side near-tip area, which diminishes with departing from the tip edge. The suction-side heat/mass transfer in the tip-leakage flow region is less influenced by the high inlet turbulence than that at the mid-span. The leading-edge heat/mass transfer under the high inlet turbulence is always higher than that in the low turbulence case, while there is no big difference in the trailing-edge heat/mass transfer between the two cases.