Modeling of Mixed Convection in a Lid Driven Wavy Enclosure with Two Square Blocks Placed at Different Positions

The purpose of this paper is to investigate the simulation of mixed convection in a lid-driven wavy enclosure with blocks positioned at various positions. This study also examined the impact of the longitudinal position of the heated block on heat transfer enhancement. The Galerkin weighted residual finite element method is employed to computationally solve the governing equations of Navier-Stokes, thermal energy, and mass conservation. The enclosure consists of two square heated blocks strategically placed at different heights—firstly, one set is closer to the bottom surface;secondly, one set is nearer to the middle area and finally, one set is closer to the upper undulating surface of the enclosure. The wavy top wall’s thermal insulation, along with active heating of the bottom wall and blocks, generates a dynamic convective atmosphere. In addition, the left wall ascends as the right wall falls, causing the flow formed by the lid. The study investigates the impact of the Richardson number on many factors, such as streamlines, isotherms, dimensionless temperature, velocity profiles, and average Nusselt numbers. These impacts are depicted through graphical illustrations. In all instances, two counter-rotating eddies were generated within the cage. Higher rotating speed consistently leads to improved performance, irrespective of other characteristics. Furthermore, an ideal amalgamation of the regulating factors would lead to increased heat transmission.

Mixed convection in gravity-driven nano-liquid film containing both nanoparticles and gyrotactic microorganisms

Analysis of a gravity-induced film flow of a fluid containing both nanoparticles and gyrotactic microorganisms along a convectively heated vertical surface is presented.The Buongiorno model is applied. Two kinds of boundary conditions, the passive and the active boundary conditions, are considered to investigate this film flow phenomenon.Through a set of similarity variables, the ordinary differential equations that describe the conservation of the momentum, the thermal energy, the nanoparticles, and the microorganisms are derived and then solved numerically by an efficient finite difference technique.The effects of various physical parameters on the profiles of momentum, thermal energy,nanoparticles, microorganisms, local skin friction, local Nusselt number, local wall mass flux, and local wall motile microorganisms flux are investigated. It is expected that the passively controlled nanofluid model can be much more easily achieved and applied in real circumstances than the actively controlled model.

Numerical Investigation of a Mixed Convection Flow in a Lid-Driven Cavity

This study is devoted to the computational fluid dynamics (CFD) modeling of steady laminar mixed convection flow and heat transfer in lid driven cavity (10 ≤ Re ≤ 1000). The ratio of the height to the width of the cavity is ranged over H/L = 0.5 to 1.5. The governing equations are solved using commercial finite volume package FLUENT to visualize the nature of the flow and estimate the heat transfer inside the cavity for different aspect ratio. The simulation results are presented in terms of average Nusselt number of the hot wall, velocity profile, and temperature contours. It was found that the average Nusselt number inside the cavity is strongly governed by the aspect ratio as well as the Reynolds number. A parametric study is conducted to demonstrate the effect of aspect ratio on the flow and heat transfer characteristics. It is found that heat transfer enhancement was obtained by decreasing the aspect ratio and/or increasing the Reynolds number.

Hydrodynamic Mixed Convection in a Lid-Driven Hexagonal Cavity with Corner Heater

Hydrodynamic mixed convection in a lid-driven hexagonal cavity with corner heater is numerically simulated in this paper by employing finite element method. The working fluid is assigned as air with a Prandtl num-ber of 0.71 throughout the simulation. The left and right walls of the hex-agonal cavity are kept thermally insulated and the lid moves top to bottom at a constant speed U0. The top left and right walls of the enclosure are maintained at cold temperature Tc. The bottom right wall is considered with a corner heater whereas the bottom remaining part is adiabatic and inside the cavity a square shape heated block Th. The focus of the work is to investigate the effect of Hartmann number, Richardson number, Grashof number and Reynolds number on the fluid flow and heat transfer characteristics inside the enclosure. A set of graphical results is presented in terms of streamlines, isotherms, local Nusselt number, velocity profiles, temperature profiles and average Nusselt numbers. The results reveal that heat transfer rate increases with increasing Richardson number and Hartmann number. It is also observed that, Hartmann number is a good control parameter for heat transfer in fluid flow in hexagonal cavity.

Onset and Stability of Thermally-Driven Fluid Convection in a Vertical Rock Crack and Their Implication for Hydrothermal Ore-Forming Processes

Dynamic equations controlling the thermally-driven fluid convection in a single rock crack are established in the paper . The critical criteria for the onset of convection in such a configuration are found by linear dynamic analysis of the equations and the stability of the convection discussed by nonlinear analysis method . The research demonstrates that the critical Rayleigh number has a magnitude 103. While the Rayleigh number R of real ore-forming fluids exceeds this value , the convection happens , and as R becomes larger , the fluid convection pattern develops from nonequilibrium steady states to double-periodically produced limit cycles and eventually to chaos (turbulences ).The implication of these dynamic analyses for the ore-forming processes of late-magma tic hydrothermal deposits is also discussed in the paper .

Numerical Study of Forced Convection Lid-Driven Cavity Flows Using LES （Large Eddy Simulation）

This study presents the LES （large eddy simulation） of forced convection in laminar and two dimensional turbulent flows when the flow reaches the steady state. The main purpose is the evaluation of a developed numerical methodology for the simulation of forced convection flows at various Reynolds numbers （100 _〈 Rex 〈_ 10,000） and for a fixed Prandtl number （Pr = 1.0）. The hexahedral eight-node FEM （finite element method） with an explicit Taylor-Galerkin scheme is used to obtain the numerical solutions of the conservation equations of mass, momentum and energy. The Smagorinsky model is employed for the sub-grid treatment. The time-averaged velocity and temperature profiles are compared with results of literature and a CFD （computational fluid dynamics） package based on finite volume method, leading to a highest deviation of nearly 6%. Moreover, characteristics of the forced convection flows are properly obtained, e.g., the effect of the Reynolds number over the multiplicity of scales.

岩石圈地幔分层性对克拉通稳定性的影响

克拉通是大陆岩石圈中长期稳定存在的古老构造单元,通常被相对年轻的活动带所包围,二者之间岩石圈厚度的横向差异会诱发边界驱动地幔对流.近年来,越来越多的证据显示克拉通岩石圈地幔内部存在成分和结构的垂向分层性.在边界驱动地幔对流的环境中,岩石圈地幔分层性如何影响克拉通的稳定性仍然不清楚.本文利用二维热-力学数值模拟方法,系统探讨了在边界驱动对流作用下,克拉通岩石圈地幔密度和黏度的垂向分层性以及中岩石圈不连续面(MLD)的分布样式对克拉通稳定性的影响.模拟结果显示:(1)当克拉通岩石圈内部不存在MLD时,即使克拉通具有密度分层的岩石圈地幔,克拉通岩石圈减薄的水平范围也是有限的(<100 km),且主要发生在克拉通边缘处;(2)当克拉通岩石圈内部存在一层连续的MLD时,克拉通岩石圈减薄的水平范围随着克拉通下层岩石圈地幔密度的增加而增大,岩石圈地幔的减薄方式随着其强度的增加从缓慢丝状剥离转变为快速块状拆沉;(3)当克拉通岩石圈内部存在一层不连续分布的MLD时,分段MLD之间的间隙可以有效延缓岩石圈地幔的减薄,间隙越稀岩石圈减薄越快,反之亦然,减薄方式表现为:早期缓慢丝状剥离,中后期快速块状拆沉.华北克拉通破坏持续时间长(>100 Ma),破坏的峰期集中在早白垩世(130~120 Ma),且现今东部陆块岩石圈破坏的前缘位置与华北中部造山带下方MLD的东端对应.根据模拟结果,我们推测华北克拉通破坏的时空范围主要是由岩石圈地幔分层性和MLD不连续分布引发的岩石圈地幔逐段拆沉所导致.

密度驱动条件下饱和介质盐分迁移规律

为探究一维饱和介质中考虑密度驱动条件下的盐分迁移规律,在室内开展了从土柱顶端释放NaCl盐溶液的密度驱动下盐分迁移试验,结果表明:浓度为100 g/L的盐溶液从顶部释放后仅720 min土柱底部浓度即达到最大值;土柱顶部释放的盐分浓度越高密度驱动效果越明显,迁移到土柱底部(深度70~100 cm)的盐分含量占比越高;释放盐分浓度为6、12、40、60、100、150 g/L时底部盐分含量占比分别为33.3%、36.7%、38.0%、40.0%、40.2%、47.1%。为对比密度驱动和浓度弥散作用下的盐分迁移规律差异,开展了从土柱底部释放高浓度盐溶液的反向弥散试验,结果表明:试验期内密度驱动对盐分迁移的影响效果远大于浓度弥散作用,试验进行30 d后弥散作用仅对土柱底部20 cm范围内的浓度分布产生影响;底部释放盐分浓度越高,向上的弥散通量越大,但对浓度的重分布影响不明显,浓度为6、40、60、100 g/L的NaCl溶液试验进行15 d后土柱底部20 cm范围内含盐量占比依次增大,分别为37.5%、62.4%、72.7%、82.7%。该研究量化分析了在密度驱动和弥散作用下盐分的迁移规律,可为干旱灌区盐分平衡规律的研究提供一定的理论依据。

Edge-driven convection and thinning of craton lithosphere:Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts.Lateral variations in temperature and density between the two can trigger small-scale mantle convection(edge-driven convection,EDC).Here,we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt,and its role in the thinning of the craton lithosphere.The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt,given the same initial condition.When the craton lithospheric density is relatively large,a high-strength craton has strong ability to resist EDC,and craton lithospheric thinning is limited to the edge.In contrast,the ability of a low-strength craton to resist EDC is weak,and the craton lithosphere is gradually eroded by the downward flow,eventually leading to large-scale thinning of the cratonic lithosphere.When the craton lithospheric density is relatively small,regardless of the strength of the cratonic lithosphere,the craton can well withstand the impacts of EDC.In this case,upwelling flow drives cratonic lithosphere materials to the base of the mobile belt,and lithospheric thinning only occurs at the edge of the craton lithosphere.The Archean North China Craton(NCC)was surrounded by Phanerozoic mobile belts,and its lithospheric thinning first occurred on the northern and eastern margins,a process that lasted for a long time.We suggest that EDC has played an important role in lithospheric thinning of the NCC,in particular the initiation of lithospheric thinning,but we cannot rule out the additional contributions from other mechanisms.

Combined effect of Reynolds and Grashof numbers on mixed convection in a lid-driven T-shaped cavity filled with water-Al_2O_3 nanofluid

Lid-driven mixed convection has been given immense importance due to its wide range of applications. A T-shaped cavity is introduced and pertinent parameters controlling mixed convection phenomenon are analyzed in this paper. Water-Al2O3 nanofluid is considered inside the cavity to augment heat transfer rate. Galerkin weighted residual method of finite element analysis is applied for the numerical simulations. Numerical solution is obtained for different solid volume fractions of nanofluid（？= 0- 0.15）, Grashof numbers（Gr= 0.1 ？5 000） and Reynolds numbers（Re= 0.31 ？1 000） in laminar flow regime. Special attention is given on the analysis of flow at the pure mixed convection regime. It is found that Grashof, Reynolds and Richardson numbers along with solid volume fraction of nanofluid have significant effect on heat transfer characteristics inside the cavity. Results are presented using streamline and isotherm contours along with related variation of average Nusselt numbers of the heated wall and average fluid temperature inside the cavity.

内蒙古天和永新生代玄武岩成因及其地质意义

天和永玄武岩为碧玄岩,至少可以划分出3种矿物共生组合类型。天和永玄武岩总体具有贫硅(w(SiO2)=43.97%～45.68%)、富碱(w(K2O+Na2O)=5.91%～7.65%)、富钾(w(K2O)=2.04%～2.89%)、高钛(w(TiO2)=2.18%～2.37%)、高Mg值(Mg#=68～76)的特征;稀土元素含量高(∑REE=(246.62～329.82)×10-6),稀土配分曲线呈右倾平滑直线,强烈富集轻稀土,轻重稀土强烈分馏((La/Yb)N>30),无明显的Eu(δEu=0.90～1.02)和Ce异常(δCe=0.96～1.00);强烈富集不相容元素,其中高场强元素(HFSE)Nb、Ta和Th出现峰值,具有近似OIB配分型式的特征;玄武岩富含相容元素Co((39.1～48.9)×10-6)、Ni((130～257)×10-6)、Cr((138～320)×10-6)。上述所有特征以及岩石结晶程度低、富含橄榄岩包体和少量捕虏晶、元素变异关系等均表明,天和永玄武岩是原生玄武岩质岩浆固结的产物。微量元素比值Ba/Rb(12～35)和碱金属的变化暗示源区可能遭受过流体的交代作用,源岩可能是富集的二辉橄榄岩。岩石成因模拟表明,形成天和永玄武岩的原生岩浆是在变压、部分熔融的条件下富集地幔源区岩石非实比熔融的产物,变压熔融柱穿切了Sp/Gt二辉橄榄岩相边界。岩浆形成于源区岩石的低度(约<5%)部分熔融,其中石榴石二辉橄榄岩部分熔融约为1%,尖晶石二辉橄榄岩部分熔融2%～5%。综合分析显示,源区部分熔融的触发机制是边际驱动的地幔对流,因而其形成深度大于东部的集宁玄武岩和汉诺坝玄武岩。

表面张力对流对BaB_2O_4晶体生长界面边界层的作用

利用高温光学实时观察方法,实时地观察了BaB_2O_4(BBO)高温熔体的表面张力对流效应以及BBO单晶的旋转生长过程,计算了固液界面附近的浓度、温度以及动量边界层厚度δ_C,δ_T和δ_V,并研究了热毛细对流对边界层厚度的影响.结果发现,浓度边界层厚度远远小于温度以及动量边界层厚度,说明晶体生长过程中,质量扩散在界面输运过程中起着主导性作用,同时发现,边界层厚度随体系无量纲Marangoni数的增大而线性地减小.

半导体材料的微重力生长

空间微重力环境提供了一个独特平台,以改进地面材料性能、深入理解被地面重力掩盖的晶体生长现象。半导体空间材料科学的主要进展有:(1)基于对组分均匀的完美半导体的追求,人们对于晶体生长机理,特别是对流、溶质传输及组分分凝的相互作用,有了更加深入的理解;(2)基于空间实验结果,人们澄清了非接触Bridgman生长的内在机理,并将之用于指导空间及地面实验;(3)提出了新的微重力晶体生长技术并成功用于组分均匀半导体合金材料的制备。回顾了以上方面的研究进展,并对半导体空间材料科学的未来挑战进行了展望。

凝固界面前沿自然对流的实验研究

本文在水平Bridgman定向凝固装置中,对超薄试样中固液界面前沿的自然对流进行了实验研究。发现在厚度仅为200μm—700μm的试样中,由于水平温度梯度的存在,凝固界面附近仍然存在着不可忽视的自然对流,并系统研究了各种不同因素对界面前沿热对流的影响规律,为界而过程的微观分析和凝固理论的研究提供了丰富的信息,为采用该装置进行透明有机物模拟研究提供了设计依据。

太阳能空气隙膜蒸馏系统实验研究

空气隙膜蒸馏是一种新的膜分离技术,它可以使用太阳能等低位热能作驱动力,用于海水、苦咸水的淡化处理,在淡化水制备方面具有极强的竞争力。文章介绍的空气隙膜蒸馏实验装置,采用太阳集热装置来加热进液,通过膜蒸馏获得纯净水。结合实验数据,文章给出了料液进口温度、流量、空气隙厚度等参数对于膜通量的影响和空气隙膜蒸馏系统热容腔内对流换热传热系数的经验公式。

重力对流效应与晶体生长

本文应用高温实时观察法显示了二种对流状态：扩散一平流状态和扩散-重力对流状态，并测出它们的特征扩散长度为0.01～0.1cm．通过晶体旋转生长，计算了质量、热量和动量变换区厚度，数值分别为7．5×10-3Cm；86×10－2cm和44×10－1本文还应用高温实时观察法进一步研究了上述二种不同对流状态下的KNbO3（001）晶面的生长动力学，实验结果指出：（1）在同一过冷度条件下，扩散一平流状态下的生长速度大于扩散一重力对流状态下的生长速度；（2）在过冷度小于20℃时，二种对流状态下的KNbO3（001）晶面生长机理都服从二维成核生长，但台阶自由能ε和二维晶核的形成热力学势能△G等生长参数不一致，从而表明这些参数是实验研究熔体对流对晶体生长界面动力学的重要依据，并指出熔体对流效应增强了界面的奇异性．

辐射对流传热对不可逆太阳能Braysson热机性能的影响

建立高温端同时辐射对流传热、低温端对流传热的不可逆太阳能Braysson热机循环新模型,探索有限速率热传导和绝热过程不可逆性等对热机循环性能的影响,基于热力学分析方法,导出热机输出功率和效率的解析表达式,数值计算结果揭示了等压温比、内不可逆参量及与热传导系数和面积有关的匹配参量等对热机性能特性的影响,所得结论不仅能导出其它不可逆太阳能热机的优化性能,且可为太阳能热机的参数设计和性能评价提供新理论参考。

雷暴大风与龙卷的预报预警和灾害现场调查

近年来强雷暴大风与龙卷导致的灾害事件引发广泛关注。在总结雷暴大风和龙卷机理、短期预报技术和天气形势特征基础上,给出了这两类天气的监测和短时临近预报预警技术,最后简要总结了灾后现场调查工作和风灾强度评估方法。雷暴大风多数是由对流风暴内强烈下沉气流产生;而龙卷一类由中气旋产生,另一类由辐合线上的中小尺度涡旋和快速发展对流风暴中的强上升气流共同作用形成。但归根结底,巨大的静力不稳定能量是产生强雷暴大风与龙卷(热带气旋龙卷除外)的必要条件。新一代天气雷达观测是雷暴大风与龙卷的监测和临近预警主要手段。自动气象站观测风场能够相当程度上监测大风天气,地球静止气象卫星和自动气象站变压、变温等观测资料也能够辅助监测雷暴大风天气,但龙卷监测尚无法直接使用这两类资料。对这两类天气所致灾害的现场调查工作依然必不可少。快速更新或者集合的高时空分辨率数值模式预报及其后处理是这两类天气短时预报的主要途径。

垂直裂隙中热驱动流体对流的发生和稳定性及其热液成矿作用意义

本文建立了控制垂直裂隙中热驱动对流的动力学方程。通过对方程进行线性动力学分析,揭示了裂隙中对流发生的临界条件,而非线性动力学分析则揭示了裂隙中对流的稳定性。研究表明,垂直裂隙中发生热驱动对流的临界瑞利常数R为10~3数量级,当实际瑞利常数超过这个临界值时,裂隙中将产生对流,并随着瑞利常数的增大裂隙中的流体运动从非平衡定态、倍周期极限环发展到混沌(紊流)。最后作者讨论了垂直裂隙中热驱动对流的发生及稳定性行为对岩浆期后热液矿床成矿作用的意义。

竖直和水平圆管换热器在内热源液池内的两相换热特性

针对溶液堆整体模拟试验模型的结构尺寸和运行工况,建立了内热源液池内自然对流和鼓泡驱动同时作用下竖直和水平圆管换热器的两相流动和换热模型,进行了相应的数值模拟计算。数值计算得到不同加热强度和鼓泡流量下的对流换热性能,与相应试验结果进行了比较分析。利用相似理论和量纲分析,推导了内热源液池内两相流动换热的无因次准则关系式,根据数值计算结果总结出换热Nu数的经验关系式。最后,分析比较了竖直和水平圆管换热器的两相对流换热特性,为溶液堆内换热器设计和布置提供理论参考。