湍流热对流近底板流态与温度边界层特性

本文采用湍流热对流的并行直接数值模拟(PDM-DNS),计算了系列Ra数的二维方腔和三维扁方腔的Rayleigh-Bénard热对流.针对平均场计算结果,选取Ra=10~9,10~(10),5×10~(10),讨论了二维方腔和展向平均三维扁方腔热对流流动特性.发现二维方腔和三维扁方腔流动中都存在大尺度环流和角涡,而且随着Ra数的增加,大尺度环流的形状变圆,角涡尺寸变小.在二维方腔流动中,大尺度环流呈椭圆形,有四个角涡,而展向平均三维扁方腔流动中,大尺度环流呈梭形,只有两个角涡.由于角涡特性的不同,二维方腔流动中羽流向上运动的范围比展向平均三维扁方腔流动更广,造成二维流动局部区域温度分布高温层厚度变大.温度边界层厚度λ_θ与Ra数之间存在标度关系,二维方腔和三维扁方腔热对流温度边界层变化的标度指数基本一致,标度关系的系数稍有不同.     

湍流热对流中的若干问题

对流是自然界中的一种常见现象,与人们的日常生产、生活息息相关。作为湍流和非线性系统的一个简单模型,在20世纪90年代以后,人们对热对流进行了系统而深入的研究。然而,直到现在,人们对湍流热对流的规律和本质仍然所知有限。文章主要从湍流传热、相干结构、大尺度环流和湍流中脉动量的小尺度统计等四个方面,简要地介绍了近年来湍流热对流的一些新进展。     

封面说明

左图:用粒子成像测速法测到的湍流热对流系统的瞬时速度场,图中箭头的颜色和长短表示该点的速度大小,方向表示该点的流体速度方向,在图中我们可以清晰的观察到一个尺度为对流槽高度的所谓大尺度环流;右边中间的小图是与左图对应的流动模式卡通图,其中红色的椭圆圈表示大尺度环流,对角上两个蓝色小圆圈表示反向小环流,褐色箭头表示大尺度环流的角向运动方向;最右边的小图是长时间平均后的流动模式,由于大尺度环流的角向运动,中间的流场抵消,只留下了上下底板附近的两个如螺线管般的流动模式.在对流系统中,无论从时间还是空间上看,流场都显示…     

湍流热对流大尺度环流反转时的角涡特性

本文采用DNS方法计算二维方腔Rayleigh–Bénard热对流.在软湍流区热对流场呈现大尺度环流和两个反向转动的角涡,并出现了大尺度环流的反转现象.连续的温度等值线和流线图清晰地描述了反转现象的全过程.在反转过程中,角涡的大小尺度变化起到重要的作用.对角涡大小尺度变化的分析发现,在反转现象中其角涡尺度随时间的变化出现剧烈的振荡,而没有反转现象的热对流场中角涡尺度变化只有小幅的脉动.对反转过程前后的角涡大小尺度、典型位置速度及角点附近温度等流动特性进行了探讨和分析,发现反转是在瞬间完成的,角涡内速度脉动较小、温度较高,反转前角涡尺度与角涡侧壁垂向速度变化具有同步性.     

二维湍流热对流羽流运动路径对传热特性的影响

本文采用并行直接求解方法计算了0.7和4.3两个Pr数的二维湍流热对流,最高Ra=10~(13).高和极高Ra数的二维湍流热对流传热Nu数与反映羽流运动的大尺度环流路径随Ra数的变化具有很好的相关性,并具有两个Ra数转折点.第一转折点出现在大尺度环流由椭圆变为圆形,其周长C_(LSC)随Ra数变化突然减小,第二转折点出现在大尺度环流周长C_(LSC)最小值处,随后羽流变为旋涡团状而周长C_(LSC)随Ra数增高而增大.Pr数较小对应的转折Ra数较低.传热Nu数的变化规律用Ra~(0.3)补偿后发现, Nu的局部标度律在大尺度环流周长C_(LSC)较小的情况下减小,出现偏离GL理论预测倍数线的现象.当Ra数大于第二转折点后,极高Ra数二维湍流热对流Nu数随Ra数的变化与GL理论预测倍数线符合良好,走向保持一致.     

湍流热对流Prandtl数效应的数值研究

本文采用直接数值模拟的并行直接求解方法,计算了Ra=10~(10),0.05≤Pr≤20的系列Prandtl(Pr)数二维湍流热对流.通过流动显示技术,讨论了Pr数对羽流形态和大尺度环流结构的影响.在Ra=10~(10)时,随着Pr数减小,羽流的运动和分布表现出更强的湍流性质,较高Pr数的羽流则表现出较强的规律性,当Pr4.3时,流场中存在明显的大尺度环流和角涡结构.不同Pr数的温度边界层厚度差异不大,并随Pr数存在标度率变化关系.当Pr数较低时,系统的传热Nusselt(Nu)数随着Pr数增加而增加,当Pr数较高时,Nu数随Pr数的变化不敏感.靠近底板处速度脉动随Pr数有显著的变化,Pr数越低速度波动越剧烈.通过底板中心位置水平脉动速度和平均场水平速度最大值给出的雷诺数Re_((u))和Re_(U_(max)),两种Re数随Pr数的变化满足同一标度律,为Re～Pr~(-0.81).     

圆柱腔体内水凝固过程热对流实验研究

本文通过匹配折射率和标定技术,应用PIV测量了圆柱腔体内水在底部冷却过程中热对流的变化规律．实验发现了主流场的涡结构;另外在底部拐角处出现新的涡,并且不断向上推移;冷却后期整个液相区出现流动反转现象,靠近壁面处水向上流动,中心区域水向下流动。因为存在着复杂的热交换和密度变化,水在凝固过程中存在流动结构失稳,使得液相区内发生强烈的热对流．最后流动强度减弱,重新形成稳定结构。结合泰勒不稳定性分析,本文对这一过程给出了合理解释．     

湍流热对流的高效并行直接求解方法

结合泊松方程直接求解的PDD算法,创建二维湍流热对流DNS模拟的并行直接求解方法（PDM-DNS）.在"天河二号"超级计算机上完成高Ra数湍流热对流大规模DNS计算,计算效果令人惊喜,实现了一年前难以达到的计算规模.高分辨率的湍流热对流计算结果显示,不同高Ra数的湍流热对流流动状态完全不同.为高Ra数湍流热对流大规模并行计算和数值模拟研究提供了有价值的计算方法.     

滑移边界对二维Rayleigh-Bénard热对流流态和传热特性的影响

采用并行直接数值模拟（PDM-DNS）计算无滑移和滑移边界二维Rayleigh-Bénard热对流.与无滑移边界形成的随机羽流运动的湍流热对流不同,滑移边界热对流最终形成湍流特征消失,且温度仅分布于四壁的一个大尺度环流的流动形态.平均场近底板的温度分布特性,无滑移边界逐渐变化而滑移边界出现过冲现象.宽高比Γ=1时,Nusselt数（Nu）随Rayleigh数（Ra）的变化具有相同标度指数,Nu~Ra^0.3.滑移边界热对流具有传热增强作用.滑移边界热对流Nu随Γ变化明显,并分为两个阶段,在Γ=0.5时出现Nu_max≈250,是无滑移边界热流Nu的5倍.     

湍流热对流温度剖面双参数拟合及其变化特性

采用高效并行的直接数值模拟方法计算了三个Pr数的系列Ra数的二维热对流.对所有计算结果的平均场温度边界层特性进行研究,采用考虑脉动作用的双参数温度边界层理论对温度边界层剖面进行拟合,得到了拟合参数a和参数c的分布.参数a决定了温度剖面的基本特性,而参数c起到了对温度剖面外区的修正作用,使得在5个边界层厚度温度边界层剖面的计算值与理论解符合良好.参数c的变化特性与参数a的相反, a值增大则c值减小.不同的Pr数拟合参数a随Ra数变化具有不同的分布特性,但都存在突然减小的间断.参数a的间断变化是由湍流热对流流动状态由椭圆形大尺度环流突变到圆形大尺度环流造成的,且随着Pr数变大间断点的特征Ra数增高.相同Ra数时Pr数越大温度剖面的拟合参数a值越小,表明温度边界层中脉动的影响越小.传热特性N u/Ra0.3、表征羽流运动特性的大尺度环流路径周长,及温度边界层拟合参数a三者随Ra数的变化都存在转折或间断,且对应相同的特征Ra数,显示三者具有很好的相关性并与流动形态变化直接关联.     

Azimuthal symmetry, flow dynamics, and heat transport in turbulent thermal convection in a cylinder with an aspect ratio of 0.5

We report an experimental study of flow dynamics and structure in turbulent thermal convection. Flow visualization, together with particle image velocimetry (PIV) measurement, reveal that the instantaneous flow structure consists of an elliptical circulatory roll and two smaller counterrotating rolls, and that the azimuthal motion of the quasi-2D instantaneous flow structure produces a time-averaged 3D flow pattern featuring two toroidal rings near the top and bottom plates, respectively. The apparently stochastic azimuthal motion of the flow structure, which generates a net rotation on average, is found to possess the characters of a Brownian ratchet. Using an artificially generated flow mode, we are able to produce a bimodal-Nu behavior and thus demonstrate that different flow states can indeed produce different global heat transport in a turbulent convection system.     

管内对流换热的场协同分析及换热强化

导出管内湍流换热Nu与局域时均参数的关系式,将对流换热的场协同理论扩展至湍流换热。分析了管内对流换热的特点,并根据场协同理论提出强化湍流换热的方法,发展了一种新型强化换热管一交叉椭圆管,既适合于层流换热强化也适合于湍流换热强化,其强化传热效果显著而流阻较小。     

Convection and correlation of coherent structure in turbulent boundary layer using tomographic particle image velocimetry

The present experimental work focuses on a new model for space–time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wall. A turbulent boundary layer flow at Reθ= 2460 is measured by tomographic particle image velocimetry(tomographic PIV). It is demonstrated that arch, cane,and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space–time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space–time correlation instead of Taylor hypothesis. The convection velocities derived from the space–time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition(FD) shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition(POD) has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.     

Natural and forced convective heat transfer on slender cylinders

This paper presents an experimental study of free and forced convective heat transfer along vertical slender cylinders. The local heat transfer coefficient is determined from the measurement of the surface temperature distribution performed by quantitative infrared thermography. It is found that the convective heat transfer is strongly dependent on the cylinder curvature and misalignment with the flow. The effect of proximity of two cylinders is emphasized in the case of forced convection. Correlations are proposed for the two types of convection. It is worth noting that circumstances exist where the turbulent heat transfer in free convection can be of the same order of magnitude as for laminar forced convection. The outcome of the study demonstrates the suitability of quantitative infrared thermography to solve complex problems and to provide a deeper understanding of the heat transfer on slender cylinders.     

黑腔冷冻靶传热与自然对流的数值模拟研究

惯性约束聚变的设计要求在靶丸内形成均匀光滑的氘氚冰层, 靶丸周围的热环境对冰层的质量特别是低阶粗糙度有很大的影响. 本文对自主研发的黑腔冷冻靶实验装置中的热物理问题展开了数值模拟, 重点考察了黑腔冷冻靶的传热和流体力学特性. 通过参数分析得到了自然对流对靶丸温度均匀性产生影响的临界条件. 比较了黑腔不同布置朝向时的流场和温度分布, 结果显示黑腔水平布置时自然对流更加强烈, 造成的靶丸温度不均匀性也更大. 在此基础上, 讨论了消除自然对流影响的可能性, 结果发现仅当黑腔垂直布置时利用黑腔分区方法能够消除对流效应对靶丸温度不均匀性的影响而黑腔水平布置时不能消除. 研究结论对于实验中冷冻靶结构的设计、改进和实验的开展等具有指导意义.     

湍流热对流DNS多分辨率并行直接求解方法

依据温度标量场与动量计算的空间和时间计算分辨率不同的特点,采用两套网格,建立多分辨率双网格并行直接求解方法,用以解决极高Ra数湍流热对流DNS模拟巨大计算工作量的难题.在两套网格的数据交换上,根据每个细网格都满足连续方程,设计了速度的守恒平移插值方法.二维极高Ra数湍流热对流的计算结果表明,采用多分辨率双网格并行直接求解方法的DNS计算,可以使计算工作量降低近一个量级.瞬时温度场显示,双网格方法的计算结果可以很好地描述极高Ra数下快速运动的小尺寸漩涡团状羽流,得到的结果与原网格一致,不同方法计算得到的传热Nu数误差不超过1%.     

Measured local heat transport in turbulent Rayleigh-Bénard convection

Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one convection cell filled with water. It is found that fluctuations of the vertical heat flux are highly intermittent and are determined primarily by the thermal plumes in the system. The experiment reveals a unique mechanism for the heat transport in turbulent convection.     

Heat transfer analysis of MHD rotating flow of Fe_3O_4 nanoparticles through a stretchable surface

The flow of a magnetite-H_2O nanofluid has been considered among two rotating surfaces,assuming porosity in the upper plate. Furthermore, the lower surface is considered to move with variable speed to induce the forced convection. Centripetal as well as Coriolis forces impacting on the rotating fluid are likewise taken into account. Adequate conversions are employed for the transformation of the governing partial-differential equations into a group of non-dimensional ordinary-differential formulas. Numerical solution of the converted expressions is gained by means of the shooting technique. It is theoretically found that the nanofluid has less skin friction and advanced heat transport rate when compared with the base fluid. The effect of rotation causes the drag force to elevate and reduces the heat transport rate. Streamlines are portrayed to reveal the impact of injection/suction.     

温度敏感型磁流体热磁对流特性研究

建立了描述外磁场作用下温度敏感型磁流体热磁对流特性的数学模型,数值模拟了回路中热磁对流的流动与传热特性.搭建了磁流体热磁对流回路装置,采用粒子示踪测速技术(PIV)测量了磁流体的流速,用热电偶测量了磁流体的温度分布.通过实验值和数值模拟结果,分析了不同磁场大小、不同热负荷,以及不同冷却温度下回路的运行特性.     

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

Abstract

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.