Study of heat transfer on concave and convex surfaces of an annular channel with flow swirling

Results of experimental studies of postcrisis heat transfer in annular steam-generating channels are presented. It is recognized that, in annular channels with flow swirling, the value of the vapor quality at which deteriorated postcrisis heat transfer takes place and the intensity of heat transfer on convex and concave surfaces are different. Correlations for determination of heat transfer coefficients on these surfaces in the annular channel with flow swirling are presented.     

Study of the heat-transfer crisis on heat-release surfaces of annular channels with swirl and transit flows

The results of the experimental study of the heat-transfer crisis on heat-release surfaces of annular channels with swirl and transit flow are presented. The experiments were carried out using electric heated annular channels with one and (or) two heat-release surfaces. For the organization of transit flow on a convex heat-release surface, four longitudinal ribs were installed uniformly at its perimeter. Swirl flow was realized using a capillary wound tightly (without gaps) on the ribs. The ratio between swirl and transit flows in the annular gap was varied by applying longitudinal ribs of different height. The experiments were carried out using a closed-type circulatory system. The experimental data were obtained in a wide range of regime parameters. Both water heated to the temperature less than the saturation temperature and water-steam mixture were fed at the inlet of the channels. For the measurement of the temperature of the heat-release surfaces, chromel-copel thermocouples were used. It was shown that the presence of swirl flow on a convex heatrelease surface led to a significant decrease in critical heat flows (CHF) compared to a smooth surface. To increase CHF, it was proposed to use the interaction of swirl flows of the heat carrier. The second swirl flow was transit flow, i.e., swirl flow with the step equal to infinity. It was shown that CHF values for a channel with swirl and transit flow in all the studied range of regime parameters was higher than CHF values for both a smooth annular channel and a channel with swirl. The empirical ratios describing the dependence of CHF on convex and concave heat-release surfaces of annular channels with swirl and transit flow on the geometrical characteristics of channels and the regime parameters were obtained. The experiments were carried out at the pressure p = 3.0–16.0 MPa and the mass velocity ρw = 250–3000 kg/(m²s).     

An experimental investigation into expedient enhancement of convective heat transfer in the rectangular ducts of tubular-plate heat-transfer surfaces having transverse projections and grooves

Results are presented from systematic experimental investigations into enhancement of convective heat transfer in air in short rectangular channels having transverse projections and grooves [with the Re numbers varying in the range (0.6–10) × 10³; the relative length of the channel’s smooth section, from 0.183 to 2.003; and the channel cross-section contracton ratio, from 0.748 to 0.953]. Conditions are determined under which convective heat transfer can be efficiently enhanced through artificially turbulizing the flow of coolant by means of periodical throttling, and the degree of this enhancement is evaluated.     

Burnout and distribution of liquid between the flow core and wall films in narrow slot channels

Previous works on studying distribution of liquid between the flow core and wall films in narrow slot channels are briefly reviewed. Interrelation between mass transfer processes and burnout is shown. A procedure for calculating burnout on convex and concave heat-transfer surfaces in narrow slot channels is presented.     

Thermal resistance network model for heat sinks with serpentine channels

Serpentine channels in liquid cooling have received considerable attention for their high heat transfer coefficient and potential applications in high heat flux electronics. In this paper, a simple thermal resistance network model is proposed to simulate the thermal performance of heat sinks with serpentine channels. The model comprises a series of thermal resistance units connected by flow networks, which display the temperature and pressure distribution between units. By varying the Reynolds number of the fluid and the aspect ratio of the channel, temperature and pressure results of a 10‐channel serpentine heat sink are obtained and subsequently compared with full three‐dimensional computational fluid dynamics simulations. The results demonstrate that the model can predict the heat transfer characteristics of serpentine channels with high accuracy and significantly less computing time. Copyright © 2013 John Wiley & Sons, Ltd.     

涡流发生器对直接空冷凝汽器换热的影响

通过数值模拟研究了矩形小翼、三角小翼、梯形小翼、柱面梯形翼、柱面三角翼和柱面矩形翼等6种涡流发生器分别安装于直接空冷凝汽器单排蛇形翅片扁管中的压降和换热性能。Re数在600~1800范围内,涡流发生器采用相同高宽比、尾高、攻角以及安装位置。结果表明:矩形小翼的换热及压降增大最多,Re=1729时,相比平直翅片分别增加了14.27%和18.32%。Re=1 729时,柱面梯形翼的压力损失比矩形小翼少4.7%,换热低1.5%。当以(j/j0)/(f/f0)1/3作为综合换热性能评价标准时,柱面梯形翼的综合换热性能最好。另外,考察了柱面梯形翼倾角的影响(0°、6°、12°、20°、24°和26°),相同工况下,倾角为12°的柱面梯形翼换热最好(较平直翅片增强4.29%~14.1%)、压降最小(增大7.5%~12.8%)且综合换热性能最好。柱面梯形翼因其迎流面积与斜边长度的匹配以及流线型柱面,具有良好的强化传热效果以及较小的压降。     

Local heat fluxes on the surfaces of dimples,ditches, and cavities

We present the results from investigating heat transfer at the surface of spherical and asymmetric dimples and in the trails downstream of these dimples on a plate, as well as on the surface of cylindrical and trapezoid ditches and on the plane downstream of them.     

LED模块用液冷基板的流道和换热性能研究

针对大功率LED模块用液冷基板,通过理论分析,提出中间入口的S型流道、双S型流道和中间入口的双S型流道3种改进流道结构,并进行数值模拟。模拟结果表明,在相同的入口流速（流量）和入口水温下,中间入口的S型流道具有最佳的流动和换热综合性能;双S型流道性能次之;中间入口的双S型流道压降最小、且随入口流速增长缓慢,具有最佳的流动性能,提高流量时其散热性能可得到显著改善,因而流量可变时,该流道的综合性能最好。     

The scientific basis of calculations of highly efficient compact heat-transfer apparatuses with judicious heat-transfer intensifiers

Results of classification of the existing data on hydrodynamics and heat transfer under forced convection in channels with spherical holes (SHs) are presented. The energy efficiency of the use of SHs under laminar and turbulent flow of a coolant is shown. The effects of regime and design parameters on heat transfer in such channels are studied. A flow-regime chart is developed.     

叶片安放角分布规律对液力透平性能影响研究

液力透平被广泛应用于高余压流体的能量回收。叶片安放角从叶片进口到出口的分布规律是关键部件叶轮设计的重要参数,具有不确定性和多样性。为研究叶片安放角分布规律对液力透平性能的影响,本文总结得到线性、下凹、上凸和3种形式多段线,共6种不同形式的叶片安放角分布规律。通过数值模拟和对比分析发现,6种方案的水头-流量曲线基本相同,下凹方案得到的液力透平整体效率较高,高效区较宽。不同方案对叶轮进口速度环量的影响较小。相比其他方案,下凹方案得到的叶轮的水力损失最小,叶轮流道内漩涡较小且分布不均匀,流场好于其他方案,叶片表面压力较大,从叶片进口到出口压力降低较为均匀,压力梯度较小,整体性能较好。     

Investigation of Hydrodynamics and Heat Transfer of an Upward Liquid Metal Flow in a Rectangular Channel under a Coplanar Magnetic Field

Hydrodynamics and heat transfer were investigated of an upward liquid metal flow through a rectangular channel having an aspect ratio of approximately 3 : 1 and one-sided heating under a coplanar magnetic field (MF). The flow in the cooling system’s cooling channel for a liquid metal blanket module of a Tokamak type thermonuclear reactor is simulated. Experiments were carried out in the mercury magnetohydrodynamic (MHD) test facility. Local heat transfer characteristics were measured using a probe technique. Two types of thermocouple probes were used: a lever-type pivoted probe for detailed measurements of velocity and temperature fields in a channel cross-section and a longitudinal probe for taking measurements along the heated zone in the channel. A correlation method was used for measuring local velocity. The distributions of averaged velocity and temperature, the distributions of dimensionless wall temperature along the cannel perimeter, and characteristics of flow temperature fluctuations are presented. The distributions of averaged and instantaneous wall temperatures along the channel were obtained. The effects caused by an increase in the intensity of temperature fluctuations in a coplanar magnetic field were revealed. It is the authors' opinion that natural convection is responsible for formation and separation of large-scale vortex structures, the axis of which is parallel to the magnetic field induction vector, at the heated wall. These vortices bring about temperature fluctuations that often exceed the level of turbulent fluctuations. The data on heat transfer should be considered in designing MHD cooling channels of a fusion reactor.     

直接空冷凝汽器翅片散热器流动传热特性

为了给直接空冷散热器的优化设计和直接空冷机组的运行提供一定的参考,采用CFD方法,研究单排扁平管在不同迎面风速和环境温度条件下翅片通道内流场和温度场分布,得到空冷凝汽器冷却空气对流换热的表面换热系数和压损随迎面风速的变化规律,并通过拟合得到对应的关联式。结果表明：随着迎面风速的增加,表面对流换热系数和压降均有显著增加;环境温度的变化对空冷单元的散热量影响较大。研究结果对于直接空冷的运行设计具有重要的意义。     

Study of pressure drop and heat transfer in a swirl flow with one-sided heating in a range of heat flowrates below boiling crisis

A representative mass of experimental data on pressure drop and heat transfer under forced convection and boiling in a swirl water flow under one-sided heating conditions is obtained. Recommendations are given for calculating pressure drop in a swirl flow. A method of calculating heat transfer in channels with onesided heating is proposed.     

矩形和圆形微通道内流体流动特性的实验研究

流体在微米级通道中的流动规律与常规通道不同,其区别主要体现在摩擦因子、转捩雷诺数和泊肃叶数等方面。针对上述问题,实验研究了水在水力直径为167~195 m矩形硅通道和水力直径为168~399 m圆形玻璃微管中的流动特性。结果表明,对水力直径为167~195 m的矩形通道,其转捩雷诺数发生在1 300~1 480;对水力直径为168~399 m的圆形通道,其转捩雷诺数位于2 600~2850,且二者的摩擦因子和泊肃叶数与理论值基本一致,这说明了微通道的形状对流动阻力特性没有明显影响。通道S-3的长径比为173,其入口段长度比常规管道入口段略长,说明微通道的入口效应较明显。实验测量了微通道的局部阻力压降随流量变化的关系。结果表明,突变微通道内水流动的局部阻力符合常规理论曲线。     

Local thermal and aerodynamic characteristics in intertube channels with attached vortices

The results of an experimental study of thermal and aerodynamic characteristics (static pressure, surface friction, and heat transfer) in diverging smooth-tube bundles are presented. The results obtained make it possible to develop vortex models of flow and heat transfer in intertube channels with attached vortices.     

An experimental investigation into rational enhancement of convective heat transfer in rectangular interrupted ducts of plate-fin heat-transfer surfaces

Results are presented from systematic experimental investigations of ten standard-size designs of plate-fin interrupted heat-transfer surfaces constituted by flat rectangular ducts carried out in air in the range of Reynolds numbers from 550 to 10 000. Conditions are determined for obtaining a rational enhancement of convective heat transfer through artificially turbulizing an air flow by interrupting the long smooth rectangular ducts of plate-fin heat-transfer surfaces, and assessments of the resultant effect are made.     

不同波纹通道综合性能的数值研究

利用数值模拟的方法,研究了在不同雷诺数下,反向和同向排列的各种周期性充分发展的层流流动波纹通道的综合性能,分析了雷诺数和通道型式对流动及换热综合性能的影响。结果表明:各通道的性能随雷诺数的增大均呈先下降后上升的趋势;反向和同向时,各通道的综合性能有所不同。     

CFD技术在平直和锯齿形板翅式换热器设计中的运用

传统的板翅式换热器设计一般是依靠简单的流动与传热理论分析来设计其结构形式,有周期长、投资大和研究结构有限的缺点。而通过计算流体力学(CFD)模拟计算可以在获得直观结果的同时大幅度地减少试验工作量。本文针对平直形和锯齿形两种不同翅片,通过对模型的合理简化,建立了翅片的物理模型,利用CFD软件FLUENT进行了模拟。计算得到了平直翅片和锯齿形片翅中流体的流场分布和传热特性,给出了两种情况下的局部换热系数和压力损失曲线。利用本文建立的模型,只需在计算时改变流体的材料和边界条件,就可以得到不同工质在不同工况时的换热情况。研究结果可以用于翅片结构的优化,为换热器设计提供指导。     

Heat transfer regimes for a flow of water at supercritcal conditions in vertical channels

Heat transfer regimes observed in experiments with water at supercritical conditions flowing in vertical channels of various cross-sections (such as round pipes, annulus, or rod bundles) are analyzed. In accordance with the established practice, the normal and the deteriorated heat transfer regimes were singled out as the basic regimes specific for heat carriers with highly variable properties. At the same time, it has been established that most published experimental data on supercritical pressure water heat transfer along the length of test sections demonstrate combined (or transient) heat transfer regimes. The features can be presented as a superposition of characteristics of the above-mentioned basic regimes. The combined regimes are not stable in certain ranges of water flow conditions in which sudden transitions between the basic regimes can occur. A system of similarity criteria governing heat transfer rate in the vicinity of the critical point is examined. As applicable to cores of water-cooled reactors, due to a small hydraulic diameter of cooling channels, buoyancy forces acting in these channels are negligible as compared with the inertia effects caused by thermal acceleration of the flow and viscous force. This concept yields two integrated criteria whose use in the correction factors for the basic heat transfer equation, which we proposed previously for the normal regimes, adequately (with an error of 20–25%) describes the specific of the heat transfer coefficient in the normal, deteriorated, and combined regimes. A system of equations is proposed for design calculation of heat transfer in channels of nuclear reactors cooled with supercritical pressure water.     

Analysis and generalization of experimental data on heat transfer to supercritical pressure water flow in annular channels and rod bundles

Experimental data on heat transfer to supercritical pressure water presented at ISSCWR-5, 6, and 7 international symposiums—which took place in 2011–2015 in Canada, China, and Finland—and data printed in recent periodical scientific publications were analyzed. Results of experiments with annular channels and three- and four-rod bundles of heating elements positioned in square or triangular grids were examined. Methodology used for round pipes was applied at generalization of experimental data and establishing of correlations suitable for engineering analysis of heat exchange coefficient in conditions of strongly changing water properties in the near-critical pressure region. Empiric formulas describing normal heat transfer to supercritical pressure water mowing in annular channels and rod bundles were obtained. As compared to existing recommendations, suggested correlations are distinguished by specified dependency of heat exchange coefficient on density of heat flux and mass flow velocity of water near pseudo-critical temperature. Differences between computed values of heat exchange coefficient and experimental data usually do not exceed ±25%. Detailed statistical analysis of deviations between computed and experimental results at different states of supercritical pressure water flow was carried out. Peculiarities of deteriorated heat exchange were considered and their existence boundaries were defined. Experimental results obtained for these regimes were generalized using criteria by J.D. Jackson that take the influence of thermal acceleration and Archimedes forces on heat exchange processes into account. Satisfactory agreement between experimental data on heat exchange at flowing of water in annular channels and rod bundles and data for round pipes was shown.