阵列射流冲击冷却换热特性的数值研究

运用数值计算的方法对不同流动取向的多排孔冲击射流冷却特性进行了三维模拟,并对有初始横向流的多排孔冲击射流冷却特性进行了数值研究,揭示出射流雷诺数、流动方向、初始横向流对冲击冷却传热特性的影响规律。结果表明:研究范围内,射流雷诺数越大,冲击靶面换热效果越好;冲击腔室两端都设为出口时努赛尔数峰值所对应的射流驻点区向下游偏移最小且换热效果最好;当横流雷诺数与射流雷诺数之比大于0.5之后,有横流时的冲击射流冷却局部努赛尔数比无横流时有较为显著下降。     

阵列射流冲击冷却传热特性的数值研究

以涡轮叶片冷却技术为背景,采用带转捩的剪切应力输运(Transition SST)模型对阵列射流冲击冷却的传热特性进行数值模拟,分析了冲击Re、冲击间距、初始横向流和冲击孔排列方式的影响规律。结果表明:冲击间距对靶面平均Nu的影响存在最优值,在所计算的范围内,Zn/d=2时平均Nu最大;在冲击孔排列方式影响中,当冲击间距Zn/d≤2时,顺排孔冲击冷却传热效果优于错排,而当Zn/d≥3时,错排孔冷却传热效果优于顺排。     

“冲击-气膜”复合式结构冷却效果数值研究

分析了六种不同的"冲击-气膜"复合式冷却结构,将其应用在燃气轮机涡轮导向器叶片中弦区并对其内部流体的流动和换热进行了数值模拟.计算条件采用某燃气轮机的典型工况,流体物性参数随温度变化.将不同"冲击-气膜"复合式冷却结构的计算结果进行对比得出:冲击孔与气膜孔在展向的排列形式对冷却效果有较大影响,叉排明显优于顺排;随着冲击孔的后移,冷却气体对腔内壁的覆盖面积逐渐减小,冷却效果逐渐降低,流阻逐渐增大;在来自冲击冷却和气膜冷却多种影响因素的共同作用下,气膜孔角度和所在面曲率对冷却效果和流阻的影响被大幅度削弱.     

阵列射流冲击冷却流场与温度场的数值模拟

采用数值模拟方法对冲击冷却的流动和传热过程进行了三维数值研究。特别研究了在冲击孔叉排方式下,相邻孔间距、冲击距离以及射流入口雷诺数对冲击表面冷却流动传热特性的影响规律。     

横流对振荡射流冲击冷却特性影响的数值研究

为了分析横流条件下振荡射流冲击靶面的换热特性,采用数值计算方法研究了横向排列和纵向排列方式下振荡射流冲击靶面的冷却性能,并对纵向排列下的射流振荡器进行了排布角度修正.结果 表明:横流对冲击换热的作用主要受到两方面影响,即冲击点附近的马蹄涡和冲击点向后偏移程度;对于靶面上的冲击核心区,总体上纵向排列的高换热区域后移程度小...     

关于蒸发换热系数he的证明

从湿空气状态方程出发,依据Ｌｅｗｉｓ关系证明了蒸发换热系数和对流换热系数的一般关系,推导了湿表面上水分蒸发换热系数ｈｅ的一般式,对其影响因素进行系统分析,证明了空气湿度对ｈｅ的影响远没有风速大。     

平表面垂直圆射流冷却换热模拟的k-ε模型研究

针对气体圆射流垂直冲击平板表现换热问题,通过理论分析与实验提出了一种新的不依赖特定壁面函数的k-ε模型修正方案,提高了模拟分析中数值格式的结构性和便利性。通过实例的模拟运算,发现随边界层风格精度的提高,模拟结果渐趋稳定,并与相应的实验数据进行比较,结果吻合较好,说明了修正模型的合理性。     

涡轮叶片冷却数值模拟进展

本文概括总结了近年涡轮叶片冷却数值模拟的研究成果：对通道内部对流，总结了哥氏力和浮升力对Nu数和温度分布规律的影响；对于气膜冷却，总结了不同紊流模型情况下，叶片表面的Nu数分布以及各种紊流模型在模拟流动和换热方面的优劣；对于冲击冷却，总结了冲击各种表面情况下，滞止和平均Nu数分布规律和冷却效率对冲击距离设计的影响。了解了涡轮空冷叶片的数值模拟的现状，为今后空冷叶片的数值模拟提供一定的指导。     

涡轮叶片尾缘端部冲击流动与换热特性研究

针对涡轮叶片尾缘吸力面热应力集中,容易造成叶片尾部烧毁的现象,提出端部冲击扰流柱结构,采用Realizable k-ε湍流模型和增强壁面函数分析涡轮叶片尾缘内部流场和吸力面换热特性,研究不同冲击孔与扰流柱排列方式的影响,揭示端部冲击扰流柱结构的流场与换热机理。研究表明,端部冲击扰流柱结构对于改善吸力面的换热效果要优于中间冲击扰流柱结构,对端壁的换热有显著提高;各表面平均换热系数均随着压比的增大而增大,顺排结构时,冲击孔换热最强,扰流柱换热次之;叉排结构时,冲击孔换热最强,隔板迎风换热次之;近距离冲击,顺排的综合效果优于其它几种结构,而远距离冲击,叉排的综合效果最好,其吸力面温度分布较均匀。     

基于换热系数的水电机组冷却器冷却效率分析方法

水电厂水轮发电机组运行过程中会产生大量的热量,须在热量集中的重要部位安装冷却器,对其及时进行冷却散热,否则会严重影响设备使用性能和寿命。水电厂一般通过监视冷却体与被冷却体的温度,来判断冷却器是否工作正常,但温度通常是一个长期的缓变过程,很难从单一温度反映冷却器工作效率,因此如何有效判断冷却器工作效率是否良好显得尤为重要。本文利用水电厂冷却器温度采集监视特点,提出一种很好的基于换热系数的冷却器效率计算监视方法,指导设备运行,通过结合水电厂冷却器实际运行数据进行案例分析,表明此分析方法正确有效。     

Analytical Study on Impingement Heat Transfer with Single—Phase Free—Surface Circular Liquid Jets

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Effects of outlet port positions on the jet impingement heat transfer characteristics in the mini-fin heat sink

Effects of outlet port positions on the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink are numerically investigated. The three-dimensional governing equations for fluid flow and heat transfer characteristics are solved using finite volume scheme. The standard k-ε turbulent model is employed to solve the model for describing the heat transfer behaviors. The predicted results obtained from the model are verified by the measured data. The predicted results are reasonable agreement with the measured data. The outlet port positions have significant effect on the uniformities in velocity and temperature. Based on the results from this study, it is expected to lead to guidelines that will allow the design of the cooling system to ensure the electronic devices at the safe operating temperature.     

受限空气射流冲击矩形柱鳍热沉换热实验研究

采用实验方法研究了受限空气冲击射流与矩形柱鳍热沉相结合的散热方式应用于芯片冷却的换热规律,采用最小二乘法对实验数据进行了拟合,并最终获得平均努塞尔数关于雷诺数、喷口高度-孔径比及普朗特数的实验准则方程。在此基础上将这种散热方式与其他空冷方式进行了换热能力的比较,结果表明此种散热方式的换热能力大大超过其他空冷方式。最后,对实验系统误差进行了分析,根据误差传递理论求得的平均努塞尔数的实验相对误差不超过6%。     

Liquid Jet Impingement Heat Transfer with or without Boiling

The purpose of this paper is to summarize the important studies in the area of impingement heat transfer with or without phase change, with emphasis on the research conducted at Beijing Polytechnic University mainly with circular jets. Heat transfer characteristics of single phase jets are discussed in detail. Comment is presented on boiling heat transfer of impinging jets for steady and transient states. Some special colling configurations of two-phase jets are also introduced.     

Flow and heat transfer studies of multijet impingement cooling for different configurations: A review

Increasing the gas turbine engine's turbine intake temperature has long been a potential strategy for increasing the specific work output of the engine. However, the melting temperature of the turbine blades and vane material limits the maximum intake temperature. As a result, internal and external cooling techniques are commonly used to maintain the vane material in a safe condition. This study provided an overview of internal impingement cooling to highlight the significance of geometrical variations, such as flat plate, curve plate, and actual vanes. It was observed that flat and curved plate impingement heat transfer studies were reported extensively, whereas limited studies were found on the conjugate effects on airfoil surfaces. The importance of conjugate heat transfer studies and their impact has recently been described in the literature. In most of the literature, a wide range of instruments, such as Laser Doppler Velocimeter, Particle Image Velocimeter, liquid crystal sheets, and so forth, were used for experimental investigations. According to most studies, the local value of internal surface temperature and heat transfer coefficient are vital factors of local flow behavior. Jet-to-jet spacing, jet-to-plate spacing, jet hole diameter, and jet Reynolds numbers played a crucial role in both numerical and experimental analyses. Different geometric variations strongly influence flow behavior. Therefore, the usual method for determining interior temperature distributions and heat transfer coefficients by considering generalized geometries like the flat and curved plate may not produce accurate conjugate solutions. Most of the computational studies on the flat and curved plate indicate the usage of κ−ω shear stress transport and κ–ε realizable model to predict the heat transfer coefficient.     

Study on flow and heat transfer of multiple impingement jets

Multiple jets are often used to obtain high heat transfer in a wide area. Heat and flow behavior of multiple jets show more complicated characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer coefficients over the impingement plate were measured using an infrared radiometer when four jets impinged to the impingement plate for various jets arrangements and separating distances from jet exit to the impingement plate. The flow fields were also visualized by means of a water table. The heat transfer characteristics are made clear from the results of the instantaneous heat transfer contour or temperature fluctuation values. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 419–431, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20073     

Thermal characteristics of arrays of swirling impinging jets: Effect of Reynolds number,impingement distance,and jet-to-jet separation

This paper numerically investigates both the inline and staggered arrays of circular multiple swirling air jets that impinge perpendicularly onto a smooth flat surface. The simulations were conducted for various flow and geometric parameters, such as Reynolds number (Re = 11,600, 24,600, and 35,000), jet-to-surface distance (H/D = 1, 2, 3, and 4), jet-to-jet separation distance (Z/D = 1.5, 3.0, and 4.5) and swirl numbers (S = 0, 0.3, and 0.75), where D is the nozzle diameter. For S = 0.75, a strong recirculation develops due to the vortex breakdown, depends on Z/D, around the axis and near the wall. The extent of the recirculation is larger for the staggered array. Intense heat transfer is anticipated at strong swirl and close impingement cases with the expense of uniformity; whereas a relatively even heat transfer is observed for larger impingements. The increase in jet-to-jet separation enhances the overall cooling effect and the staggered configuration of nozzles gives better performance. It appears that both the crossflow and turbulence around the periphery of each jet predominantly govern the heat transfer characteristics. The enhancement of heat transfer occurs for increasing Re, and the overall Nusselt number (Nu) prediction is scaled by Reⁿ, with n dependent on S. Finally a correlation is developed for the average Nusselt number to relate different control parameters.     

Theoretical study on impingement heat transfer with single-phase free-surface slot jets

A theoretical analysis was conducted to characterize heat transfers from horizontal surfaces to normally impinging slot jets under arbitrary-heat-flux condition. The thermal and hydraulic boundary layers of laminar flow were divided into four regions of flow along heat transfer surfaces including a stagnation zone and three wall jet zones, from which general expressions of heat transfer coefficients were obtained. Furthermore, these results were compared with experimental and analytical data available in published literature. Good agreements were observed from the comparisons.