复合方腔顶盖驱动双扩散混合对流格子Boltzmann模拟

为本文基于热质耦合的Lattice Bhatnagar-Gross-Krook(CLBGK)模型,通过引入浓度分布函数,利用格子Boltzmann方法对顶盖驱动的复合方腔内的双扩散混合对流现象进行了研究,复合方腔由多孔介质区域和自由空间组成。分析了路易斯数Le=2.0,浮升力比N=1.0,格拉晓夫数Gr=10~4和普朗特数Pr=0.7时,孔隙率(ε=0.6/0.7/0.8)、方腔中多孔介质层位置及理查德森数Ri(10~(-3)≤Ri≤10~3)对内部混合对流及热质扩散的影响。给出了方腔内温度、浓度和流线分布,以及高温高浓度壁面的平均努塞尔数Nu_(av)和平均舍伍德数Sh_(av)。研究结果表明:多孔介质层对顶盖驱动方腔内热质双扩散影响显著,且方腔左壁壁面平均努塞尔数Nu_(av)与平均舍伍德数Sh_(av),在位置D_1~D_3之间随多孔介质层的右移而增大,在位置D_3上随Ri(10~(-3)≤Ri≤10~3)的增大而减小。     

浮升力对管内水过冷流动沸腾传热特性的影响研究

基于已知的2087组水的过冷流动沸腾传热实验数据,通过努塞尔数(Nu)和格拉晓夫数(Gr)的关系探讨了不同流动方向和加热方式下浮升力对过冷流动沸腾传热性能的影响。对上壁面单边加热水平矩形管内过冷流动沸腾传热进行了实验研究。实验结果表明,向上的浮升力阻碍了气泡向流体中的扩散,使得传热恶化。在增加流速、增大压力和减小过冷度的条件下,Nu均随Gr增加,使过冷流动沸腾传热得到强化。     

自然对流对横纹管内湍流对流传热的影响

以熔盐为传热工质,对考虑熔盐自然对流情况下横纹管内的流动传热进行了数值模拟。结果表明:同一Re数,横纹管内下侧Nu数大于上侧Nu数,随着Re数的增加,管内下侧Nu数与上侧Nu数的差值逐渐减小。考虑自然对流影响的横纹管内平均Nu数与不考虑自然对流影响的横纹管内平均Nu数基本相等。强制对流的Re数越低,自然对流对横纹管单侧传热影响越明显。横纹管槽宽越小,槽深越深,自然对流对单侧传热影响相对越小。通过非线性拟合,分别得出横纹管内下侧传热Nudown/Nuave-b、上侧传热Nuup/Nuave-b与浮升力参数Bo的关联式。     

布朗运动影响下方腔内纳米流体非稳态自然对流的数值研究

对方腔一侧壁面温度随时间按正弦规律变化、充满CuO-水纳米流体的二维方腔内的非稳态自然对流换热进行了数值研究。在相关参数一定的条件下,研究了Ra数、纳米颗粒体积份额φ和纳米颗粒布朗运动对自然对流换热特性的影响。数值模拟结果表明:考虑布朗运动时壁面时均Nu数均大于不考虑布朗运动时的相应值;不考虑布朗运动时,时均Nu数均随φ的增大而增大;而考虑布朗运动时,当Ra数较小时,时均Nu数均随φ的增大而增大,而Ra数较大时,存在有最佳的φ值,可使时均Nu数达到最大值。     

三维方腔内竖直平板混合对流换热的数值研究

本文建立了三维方腔内竖直板混合对流换热的数学模型,探讨Re数,进风口位置、竖直板位置对混合对流换热的影响.结果表明:随Re数增加,竖直板下部的平均Nu2下降,而总体平均Nu1增加.随送风口高度增加,竖直板对流换热量增加.在竖直板位置L1/H=0.5～1.5的范围内,竖直板处于中间位置L1/H=1.0时,竖直板混合对流换...     

多离散源驱动的双扩散自然对流传输结构

本文通过"矢量线,,可视化方法,解析了一个方腔内由双离散热源和污染源驱动的双扩散自然对流系统的传输结构.发现在不同浮升力比下,该系统存在着三种明显不同的宏观传输结构,即由热浮升力主导的传输结构,由热质浮升力共同支配的传输结构以及由质浮升力主导的传输结构.     

多组元化合物晶体生长过程中的双扩散对流现象

针对多组元化合物晶体的Bridgman生长过程,通过对液相区流场和浓度场的数值模拟,研究生长过程中容器内部双扩散对流与组分分布的情况.首先对比了三段场和梯度场炉壁温度设计情况下液相区的流体双扩散对流以及组分分布情况.在此基础上,针对梯度场炉壁温度条件,分析了不同参数的影响.结果表明:在溶质格拉晓夫数较大时,它的增大能够明显地削弱液相区的流体流动,从而使得界面附近的组分分布也产生变化;此外,拉晶速度的增大也能够使得液相区的双扩散对流受到抑制.     

亚临界与超临界压力水在管内的对流传热与传质

本文对含有金属腐蚀物杂质的亚临界与超临界压力水在竖直加热圆管内的受迫对流与混合对流传热与传质进行了数值模拟，分析了变物性、浮升力以及压力等因素对管内对流传热与传质的影响。结果表明：浮升力使自下而上流过竖直加热圆管的对流传热和传质增强；在不同的温度条件下，超临界压力水的热物性对传热传质的影响有很大不同。     

定向凝固宏微观热质传递及双扩散流作用

针对氯化铵水溶液定向凝固实验,从枝晶微尺度到宏观尺度耦合模拟了相变热质传递过程,并考察了双扩散流的影响.研究结果表明,NH4CI-H2O溶液的枝晶微界面浓度变化与非平衡态的谢尔方程趋于一致.两相区内液相率较高,易于流动的产生;热浮升力形成逆时针回流,溶质浮升力形成顺时针回流;热浮升力作用大于溶质浮升力.但双扩散流几乎不影响凝固沿着结晶室高度方向进行的一维定向特性.数值模拟中低估了两相区的渗透率,是造成两相区厚度模拟结果小于实验值的原因.     

竖直恒温面自然对流边界层不稳定性研究

在无扰动、随机式扰动以及正弦式扰动下,通过对竖直恒温面处状态Ra为1.328×10^9、Pr为6.24的自然对流进行模拟,探索了热边界层的不稳定性和共振强化自然对流换热。结果表明:(1)竖直自然对流边界层上游位置的随机式扰动对热边界层的影响主要体现在稳定阶段;(2)该状态下的竖直自然对流边界层的特征频率为15 067,且相比于无扰动状态,频率为15 067的正弦式扰动能在竖直恒温面处提高5.15%的换热量;(3)在竖直自然对流边界层上游位置加入特征频率的正弦式扰动,竖直恒温面处的局部努塞尔数Nu均出现明显波动,且波动随着边界层高度的增加而增大。     

天然气催化反应壁面传热传质的数值研究

应用二维数学模型研究天然气催化燃烧壁面反应及载体气固界面上热质传递与流动耦合的现象。通过改变载体通道入口混合气体(天然气与空气)的温度、质量分数及壁面上反应的条件来评价努塞尔数和舍伍德数。模拟结果表明,努塞尔数和舍伍德数并不单独取决于燃料质量分数及壁面条件,而取决于壁面的反应速率。模拟同时表明,进口的温度对通道壁面传质影响最大,其它影响较小。     

HEAT TRANSFER AND TEMPORAL BEHAVIOR OF THE LAMINAR MIXED-CONVECTION FLOW AROUND A DUCTED FLAT-PLATE THERMAL FLOW SENSOR

We present experimental results of heat transfer processes in mixed-convective flow from a ducted vertical hot-plate thermal flow sensor for aiding (upward) and opposing (downward) flows. The results are obtained for three different Grashof numbers, Gr = 289, 411, and 456, using air, in the Reynolds number range from 0 to 120. The Nusselt number for aiding flows can be adequately described by Nu tot – Nu 0 = (Nu n forced + Nu n free ) 1/n , with Nu 0 = 0.5, as originally proposed by Churchill for a free flat plate. For n, a value of 1.5–1.7 is found. For opposing flows in the mixed-flow region (0.1 h Gr/Re 2 S 10), flow visualization shows an oscillating buoyant plume around the flat plate. In the transition from free to mixed and to essentially forced convection, distinct sequences of instabilities of this plume are observed, leading to several local minima and maxima in the heat transfer from the plate. The results are summarized in a bifurcation diagram. Here, several windows with instabilities are found, both nonperiodic, with strong indications of chaotic behavior, and (quasi-) periodic. Typical fundamental frequencies of the instabilities range from 0.15 to 1 Hz.     

Magnetic impact on heat and mass transfer utilizing nonofluid in an annulus between a superellipse obstacle and a cavity with periodic side-wall temperature and concentration

The magnetic impacts upon the transport of heat and mass of an electrically conducting nanofluid within an annulus among an inner rhombus with convex and outer cavity with periodic temperature/concentration profiles on its left wall are assessed by the ISPH method. The right wall has ${T}_{c}$ and ${C}_{c},$ flat walls are adiabatic, and the temperature and concentration of the left wall are altered sinusoidally with time. The features of the heat and mass transfer and fluid flow through an annulus are assessed across a wide scale of Hartmann number $Ha,$ Soret number $Sr,$ oscillation amplitude $A,$ Dufour number $Du,$ nanoparticles parameter $\phi ,$ oscillation frequency $f,$ Rayleigh number $Ra,$ and radius of a superellipse $a$ at Lewis number $Le=20,$ magnetic field's angle $\gamma =45^\circ ,$ Prandtl number ${\Pr }=6.2,$ a superellipse coefficient $n=3/2,$ and buoyancy parameter $N=1.$ The results reveal that the velocity's maximum reduces by $70.93 \% $ as $Ha$ boosts from 0 to 50, and by $66.24 \% $ as coefficient $a$ boosts from $0.1$ to $0.4.$ Whilst the velocity's maximum augments by $83.04 \% $ as $Sr$ increases from 0.6 to 2 plus a decrease in $Du$ from 1 to 0.03. The oscillation amplitude $A,$ and frequency $f$ are significantly affecting the nanofluid speed, and heat and mass transfer inside an annulus. Increasing the parameters $A$ and $f$ is augmenting the values of mean Nusselt number $\overline{Nu}$ and mean Sherwood number $\overline{Sh}.$ Increasing the radius of a superellipse $a$ enhances the values of $\overline{Nu}$ and $\overline{Sh}.$     

Aspect-ratio dependence of charge transport in turbulent electroconvection

We present measurements of the normalized charge transport or Nusselt number Nu as a function of the aspect ratio Gamma for turbulent convection in an electrically driven film. In analogy with turbulent Rayleigh-Bénard convection, we develop the relevant theoretical framework in which we discuss the local power-law scaling of Nu with a dimensionless electrical forcing parameter R. For these experiments where 10(4) less, similar R less, similar 2 x 10(5) we find that Nu approximately F(Gamma)Rgamma with either gamma=0.26 (+/-0.02) or gamma=0.20 (+/-0.03), in excellent agreement with the theoretical predictions of gamma=1/4 and 1/5. Our measurements of the aspect-ratio dependence of Nu for 0.3相似文献   

不同磁致纵向涡形式对空气对流换热的影响

为揭示不同磁致纵向涡对通道内空气对流换热的影响规律,分别就两极和四极钕铁硼永磁体作用下的矩形通道内的对流换热进行了数值模拟。模拟以通道入口段的流动和换热为对象,得到了不同Re和不同壁温下的流场和温度场, 对流换热的Nu和阻力系数,以及场协同数Fc。结果表明,不同纵向涡形式下的流场和温度场的协同性不同,具有八纵向涡形式的对流换热的协同性优于四纵向涡形式,强化效果也优于四纵向涡。     

周期性矩形槽通道内对流换热的振荡

采用非稳态数学模型,通过数值模拟对周期性矩型槽通道内流动和换热的振荡特性进行了研究。结果表明:随着Re的增大,对流换热从入口后某一个几何周期开始振荡,振幅沿着通道逐渐增加,并经过若干周期后振幅基本不变;对于不同的Re开始振荡的几何周期数不同;当振幅基本不变时,各几何周期的平均Nu及相应点的速度按时间的平均值基本相等,表明振荡的流动和换热特征参数的时均值仍然具有周期性充分发展的特征;通道中突出物高度对振荡有显著影响,突出部分越高,开始振荡的几何周期数越小。     

Poiseuille-Rayleigh-Bénard流动中对流斑图的分区和成长

采用SIMPLE算法对二维流体力学基本方程组进行了数值模拟,研究了Poiseuille-Rayleigh-Bénard流动中对流斑图的分区、成长及水平流动对不同斑图特征物理量的影响.结果表明,上下临界雷诺数Re_u,Re_l将流动分成三个区域,即行波区、局部行波区、水平流区.Re_u和Re_l随着相对瑞利数r的增大而增大.在对流斑图的成长阶段,三种斑图随时间的成长过程是不同的,但对流圈都是从下游区开始成长;特征物理量随着时间的变化也是不同的,行波对流和局部行波对流的最大垂直流速wmax和努塞尔数Nu经过指数增长阶段后进入周期变化的稳定阶段;水平流斑图的w_(max)和Nu经过缓慢增长后又缓慢降到稳定值.三种斑图的w_(max)和Nu随雷诺数Re增大而减小,不同斑图区域有不同的变化规律.本文给出了Re_u和Re_l随r的变化关系式及不同斑图的w_(max)和Nu随着Re的变化关系式.     

不同精度格式的格子Boltzmann热模型的传热分析

通过在格子Boltzmann (LBM)热模型中添加参数项,使得在对应的宏观传热方程中,消除了一阶非线性误差项,具备二阶精度.通过Rayleigh-Benard对流数值试算,初步探索该二阶精度格式及其对应的一阶精度格式三个热模型的传热特征和适应性,并做出相应对比分析.针对一二阶精度模型在Ra数极高或热传导系数极大时,Nu数的计算与经验值相比出现较大偏差,分析LBM对应宏观热传导方程的截断误差后,在平衡分布函数中引进一个调节因子.通过调节对应宏观传热方程的截断误差项系数,校正Nu数的计算偏差,提高模拟精度,拓展模拟范围,增强了LBM作为一个数值方法在传热中的适应性.     

Mesoscopic analysis of heatline and massline during double-diffusive MHD natural convection in an inclined cavity

Double-diffusive natural convection in an inclined cavity with the presence of magnetic field is studied numerically via heatline and massline approach. The governing equations are discretized using the Lattice Boltzmann Method (LBM). In this investigation, the controlling parameters involved are Rayleigh number (10³ ≤ Ra ≤ 10⁵), buoyancy ratio (−5 ≤ N ≤ 5), cavity inclination angle (0° ≤ Ø ≤ 180°), Lewis number (2 ≤ Le ≤ 10), Prandtl number (Pr = 5.0) and Hartmann number (0 ≤ Ha ≤ 50). The numerical results obtained by the effect of parameters mentioned above are reported as contours of streamlines, isotherms, isoconcentrations, heatlines, and masslines. The obtained results are compared with the existing literature to validate the coding. The heat and mass transfer rate decrease with increasing the magnetic field and increase with an increase in Rayleigh number. The impact of Ø is maximum for higher Ra and negligible for lower Ra (10³). Increasing Le influences the mass transfer rate to increase and heat transfer to reduce for both opposing and aiding flow.     

Efficiency of heat transfer in turbulent Rayleigh-Bénard convection

We present an experimental study of turbulent Rayleigh-Bénard convection (RBC) in a cylindrical cell of height 0.3 m, diameter 0.3 m. It is designed to minimize the influence of its structure on the convective flow of cryogenic (4)He gas of Prandtl number Pr≈1, with the aim of resolving existing contradictions in Nusselt (Nu) versus Rayleigh number (Ra) scaling. For 7.2×10(6)≤Ra≤10(11) our data agree with suitably corrected data from similar cryogenic experiments and are consistent with Nu∝Ra(2/7). On approaching Ra≈10(11) our data display a crossover to Nu∝Ra(1/3) that approximately holds up to Ra=4.6×10(13); there is no sign of a transition to the ultimate Kraichnan regime. Differences in Nu(Ra) scaling observed in similar RBC experiments for Ra≥10(11) cannot be explained due to the difference in Pr, but seem to depend also on experimental details.