Influence of Convection on the Piston Effect

This paper answers some recent questions posed by heat propagation mechanisms in near-supercritical pure fluids under normal gravity conditions. The role of microgravity experiments in the discovery of a fourth temperature equilibration mode in hypercompressible fluids, called the piston effect, is described, as well as its basic mechanisms that are responsible for temperature equilibration on a much shorter time scale than heat diffusion. The question whether this mechanism still exists on the ground is then approached. The results of the numerical calculations which answer this question and the basic references are presented. It is emphasized, in particular, that, although the piston effect has been demonstrated by microgravity experiments, this effect appears to be the temperature equilibrating mechanism on ground, also leading to the striking evidence of a quasi-isothermal convection. Recent modeling in connection with experiments that suggested that the piston effect could be killed by convection shows, on the contrary, that a cooling piston effect, triggered by thermal plumes, prevents the bulk increase in temperature.     

Convective heat transfer enhancement produced by secondary heat sources in a liquid nitrogen pool

This paper studies the influence of secondary heat sources on the convective heat transfer from a vertical cylindrical heater immersed in a LIN pool. Two types of secondary heat source have been used. In the first case, the heat leak through the vessel walls into the pool was varied without causing nucleation, so as to retain convective heat transfer. It was found that the convective flow loops induced in the pool produced a small enhancement of the heat transfer from the cylindrical heater. In the second case, a small horizontal heater loop was introduced into the pool below the cylindrical heater. The secondary heat flux used was such that boiling occurred at the loop to give rising bubbles surrounding the cylindrical heater. This produced a large enhancement of the convective heat transfer from the cylindrical heater. The enhancement ratio is presented as a function of the power supplied to the loop heater.     

Thermal Response of a Two-Phase Near-Critical Fluid in Low Gravity: Strong Gas Overheating as Due to a Particular Phase Distribution

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF₆ in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase ³He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.     

物料与窑壁间歇接触对回转窑传热过程的强化效应

根据回转窑内物料颗粒的运动特点，推导了颗粒团在贴壁运动过程中的非稳态导热系数及界面处的接触传热系数，进而得出了物料与封盖窑壁间的换热系数；结合已有研究成果，建立了回转窑的传热数学模型。计算表明，未考虑物料与窑壁间歇接触对回转窑传热过程的强化效应时，物料温度偏低；温度越高，强化效应对物料温度的影响越大；考虑物料与窑壁间歇接触对回转窑传热过程的强化效应，有利于提高回转窑煅烧熟料的质量和热效率。     

顶棚辐射供冷房间人体与环境换热的实验研究

辐射供冷空调系统具有舒适、节能等优点,备受国内外工程设计和研究者的关注。在此通过实验研究了顶棚辐射供冷房间在冷吊顶辐射作用下,室内空气温度、房间壁面温度、人体表面温度的变化规律。在此基础上,分析了人体与环境的对流换热及辐射换热的变化规律。研究结果表明,在辐射供冷系统开启后约60min内,辐射板表面温度、室内空气温度有显著变化,人体服装外表面温度变化比较缓慢,人体和环境的对流及辐射换热量逐步增大,系统开启60min后,上述各项趋于稳定。     

空气声高声压的激光活塞法溯源技术研究

目前空气声高声压的溯源存在空白,基于激光-活塞发声器法,采用激光直接测量高声压下活塞振速的方法进行研究,实现了180 dB下高声压的溯源,并通过试验验证了理论公式和结果的一致性,为高声压的绝对溯源提供了技术方法,保障了高声压量值的准确可靠.     

一种新型工艺冷却系统

论述了目前已有的各种冷却形式以及它们的机理和特点，并根据作者所承担项目的需要，提出了一种新型工艺冷却方式，即：利用汽化潜热进行吸热而冷却的方式，并对其优缺点进行了比较，提出了其应用的场合。     

应用二氧化钛纳米流体提高冷却壁换热能力的实验研究

着重研究了纳米流体在的小型冷却壁内流动时强化换热能力,分别测试了不同浓度的纳米流体在热交换装置内Nu准数,以及纳米流体随着Re数的变化情况,结果显示加入纳米颗粒后,流体的换热系数得到了明显的提高,且纳米流体的强化对流换热系数随着颗粒浓度的增加以及在管内流动的Re数增加而增加。     

A New Mechanism of Light‐Induced Bubble Growth to Propel Microbubble Piston Engine

Radiation pressure refers to the momentum transfer of photons during light “particles” impacting a surface. The force is too small to drive microengines. Different from the classical radiation pressure, the indirect radiation pressure (F_m) is introduced, coming from the momentum change of light‐induced bubble expansion. F_m is shown to obey F_m ～ (I·r_b)², behaving faster growth of indirect radiation pressure versus light intensity I and bubble radius r_b. An effective bubble size range is identified for F_m to suppress other forces for bubble in liquid. The top laser irradiation on nanofluid is used in this experiment. A well‐defined bubble pulsating flow, being a new principle of bubble piston engine, is demonstrated. During pulse on (≈ns scale), F_m exceeding other forces generates an extremely large acceleration, which is three to four orders larger than the gravity acceleration, propelling the bubble traveling downward. During pulse off, the bubble is floating upward due to the nonexistence of F_m. In such a way, the piston engine sustains the oscillating ranges of 38–347 µm for bubble diameters and 2.7–457.9 µm for traveling distances of piston. This work is useful to manipulate bubble dynamics in solar energy systems, and can find various applications in optofluidics.     

Heat Transfer of Heat-Releasing Fluid in the Top Portion of a Closed Volume

The method of analytic estimates is used to determine the characteristics of steady-state free-convection heat transfer of a fluid with internal heat sources in the top part of a closed volume with different conditions of heat removal on the top horizontal boundary at the Prandtl number value on the order of unity. It is demonstrated that, in the case of adiabatic condition on the top boundary of the volume, the maximal heat flux q _max attained in the region of intersection of the top horizontal and vertical boundaries depends only on the maximal temperature in the volume T _max and on the thermal characteristics of the fluid. The correction to the bulk temperature (outside of the boundary layers) T _b z ^1/4, which is a function of the vertical coordinate z, significantly prevails over perturbations in the horizontal section. When the turbulent Rayleigh-Benard (RB) convection arises, the heat removal through the top boundary is defined only by the energy release in the RB-layer. Given a fixed power of heat release Q, the RB-layer thickness increases by the linear law h=q/Q with increasing heat flux q through the top horizontal boundary.     

多孔泡沫材料强化传热特性及场协同分析

实际生产生活中使用到的多孔泡沫材料通常都是非均质的,文章建立了多孔泡沫材料均质与非均质模型,结合场协同理论,从速度与温度梯度矢量的协同关系出发,分析了多孔泡沫材料内部单相流体对流强化换热的物理机制,研究了孔隙率、孔密度以及空气流速对流体顺流方向协同性能的影响。研究表明:场协同原理适用于分析多孔泡沫材料的强化传热机制;多孔泡沫材料孔隙中心与骨架后缘处的协同程度最好,骨架侧缘协同程度最差(协同角接近90°);非均质多孔泡沫材料孔壁附近协同程度较差,相同条件下全场平均场协同角比均质泡沫大;多孔泡沫材料越均匀全场协同情况越好,在相同流速、孔隙率和孔密度下,均质泡沫材料全场平均协同角余弦值可达非均质泡沫的1.2倍。计算结果表明,空气流速为3m/s时,孔隙率为0.8、0.85和0.9的多孔泡沫材料强化传热强度分别是普通平直翅片的3.3倍、1.9倍和1.2倍。该研究对新型散热器设计具有指导意义。     

活塞阳极氧化表面气泡形成的机理及影响

对铝合金阳极氧化区气泡形成的机理、影响和防止措施进行了探讨.在氧化表面常产生并聚集大量气泡(气泡主要是阳极热量集中处形成的水蒸气气泡和氧离子形成的氧气气泡),导致电压较快地增大而易出现"打火"现象,严重影响活塞氧化膜的质量和厚度的均匀性.利用自制脉冲恒流电源等装置,对ZL109活塞顶面硬质阳极氧化过程中气泡的形成、生长进行了研究.结果表明,快速排除活塞顶面氧化过程中的气泡,可以有效地防止"打火"现象,提高氧化膜质量.     

耦合对流传热的Stokes流体形状优化设计

本文研究了耦合对流传热的Stokes流体中的形状优化问题.利用不可压缩的定常Stokes方程耦合对流传热的模型来描述流体的特性,运用形状导数方法分析依赖于区域的状态方程解的极小化问题.通过引入共轭状态方程,计算出目标函数的微分形式,并构造求解该形状优化问题的梯度型算法.数值实验的结果验证了所用方法的有效性和可行性.     

地热源热泵冬夏暖冷联供试验研究

介绍了自行建设的10kW地热源热泵系统的试验装置及测试仪表,测试数据的处理方法,冬季供暖及夏季供冷的效果测试,夏季变水量运行的试验结果.采用能量平衡结合热传导方程,并辅以管群修正系数,建立了浅埋竖管地热源热泵系统传热模型,按径向和管长方向建立了二维传热方程用于计算过渡季大地温度场的恢复情况,并将数学模型计算值与实测值进行了比较.     

穿圆形孔翅片对自然对流传热的强化研究

研究了一种水平矩形翅片开圆形孔时对自然对流换热的强化作用。通过实验和模拟比较了实心翅片和开不同孔数翅片的散热功率。除开孔数外,还考虑了翅片开孔的位置、开孔大小和开孔形状对传热强化的影响。结果表明:开孔翅片要优于实心翅片,并且随着翅片开孔数的增加,翅片的散热功率先增大后减小,当开孔数为7时最优,其散热功率与实心翅片相比,增加了13.25%。开孔的位置在底部时,翅片的散热效果最好,中间次之,顶部最差。同时在研究范围之内,翅片的散热功率随开孔半径的增大先增加后减小。开孔半径为7mm时,翅片的散热效果最好,与实心翅片相比,其散热功率增加了27.14%。此外,还比较了同等面积的三角孔、方孔和圆孔对翅片散热的影响,其中开圆孔的散热效果最好。     

不同冷却方式对冷鲜鸡肉品质的影响

目的 探究不同冷却方式对鸡肉水分分布、蛋白质变性等理化性质的影响.方法 分析风冷、真空冷却和浸没式真空冷却对鸡肉的冷却速率、pH、色差、质构等指标的影响,通过低场核磁共振和差式扫描量热技术分析冷却后鸡肉中水分分布和肌蛋白的变化.结果 风冷的冷却速率最慢,肉色暗沉,且质量损失率高,硬度和剪切力也较大;真空冷却的冷却速率最快,肉色较为鲜艳,其质量损失率、硬度和剪切力也最大;浸没式真空冷却能有效减小鸡肉的质量损失率(P<0.05),肉色最鲜艳,弹性和回复性也最好.通过差式扫描量热法分析可知,3种冷却方式下肌蛋白的变性焓均减小,风冷组的肌球蛋白、肌浆蛋白和肌动蛋白降解最多,真空冷却组的蛋白降解较风冷组更好,浸没式真空冷却组的蛋白降解最小,与对照组无显著差异;由低场核磁共振分析可知,风冷组不易流动水组分显著降低(P<0.05),自由水显著增加(P<0.05),真空冷却自由水组分显著降低(P<0.05),浸没式真空冷却后鸡肉中水分分布与对照组无显著差异.结论 浸没式真空冷却弥补了真空冷却不足,在一定程度上提高了鸡肉品质.     

脉冲电压波形和冷却条件对发动机铝活塞硬质氧化的影响

采用自制宽频脉冲电源和氧化装置对发动机铝活塞在不同脉冲电压波形和冷却条件下的硬质阳极氧化进行了研究.结果表明,脉冲电压波形及冷却速度对氧化膜的生长和致密度有很大的影响,梯形脉冲电压和冷却方式的较好匹配可使氧化铝单胞和其间的微孔垂直于铝基底面生长,有利于氧化热量的传递,氧化膜生成速度、致密度和硬度较高.用自制的纳米封闭剂对这种氧化膜的封闭更加有效.     

空冷/水冷混合冷却竖管内LiBr溶液降膜吸收过程数值解

针对风冷和水冷联合冷却的竖管降膜吸收器,考虑汽液界面的阻力、变膜厚、横向对流和冷却水的冷却作用的影响,建立了降膜吸收过程中热质耦合数学模型和同心管环空内冷却水换热数学模型.计算了沿竖管内表面的液膜厚度、温度、浓度以及冷却水在混合冷却条件下的温度分布等参数.分析了冷却水进口温度、LiBr溶液Re数和PE数等参数对传热系数和吸收速率的影响.数学模型的计算结果与实验数据吻合较好.得出的结论对联合冷却吸收器的设计和优化具有指导意义.     

Enhancement of thermal performance in a sintered miniature heat pipe

Abstract

The performance of a sintered miniature heat pipe is enhanced. With the capillary limitation, porosity takes priority over the wick structure parameters that would affect the heat transfer capacity. Since sintered dendritic copper powder has higher porosity, it is used to mix with pore former (Na₂CO₃) in experiments for increasing porosity, and hence enhancing the thermal performance. The results show that, for a heat pipe with a 3mm outer diameter and 200 mm effective length, the heat transfer rate is up to 16.5W and the thermal resistance is 0.9°C/W. In comparison with the unmixed case, the performance increases about 40%.     

Numerical analysis of conjugate natural and mixed convection heat transfer of nanofluids in a square cavity using the two-phase method

In the present study, the problem of conjugate natural and mixed convection of nanofluid in a square cavity containing several pairs of hot and cold cylinders is visualized using non-homogenous two-phase Buongiorno's model. Such configuration is considered as a model of heat exchangers in order to prevent the fluids contained in the pipelines from freezing or condensing. Water-based nanofluids with Cu, Al₂O₃, and TiO₂ nanoparticles at different diameters ($25\text{nm}?d_p?145\text{nm}$) are chosen for investigation. The governing equations together with the specified boundary conditions are solved numerically using the finite volume method based on the SIMPLE algorithm over a wide range of Rayleigh number (${10}^4?\mathit{Ra}?{10}^7$), Richardson number (${10}^{-2}?\mathit{Ri}?{10}^2$) and nanoparticle volume fractions ($0?\phi ?5\%$). Furthermore, the effects of three types of influential factors such as: orientation of conductive wall, thermal conductivity ratio ($0.2?K_r?25$) and conductive obstacles on the fluid flow and heat transfer rate are also investigated. It is found that the heat transfer rate is significantly enhanced by incrementing Rayleigh number and thermal conductivity ratio. It is also observed that at all Rayleigh numbers, the total Nusselt number rises and then reduces with increasing the nanoparticle volume fractions so that there is an optimal volume fraction of the nanoparticles where the heat transfer rate within the enclosure has a maximum value. Finally, the results reveal that by increasing the thermal conductivity of the nanoparticles and Rayleigh number, distribution of solid particles becomes uniform.