Ca(OH)2/CaO热化学储能过程中变孔隙率影响的多物理场耦合数值模拟研究

基于Ca(OH)₂/CaO的热化学储能体系在实验中均表现出孔隙结构的显著变化，而该现象在已有的大多数数值模拟研究中并未被考虑。本文针对该现象建立了变孔隙率反应动力学模型及非稳态“流动–传热–化学反应”耦合模型，并基于此对直接、间接传热式固定床反应器中Ca(OH)₂/CaO的定/变孔隙率脱水反应过程特征参数(反应渗透率、压力、温度、反应速率及转化率等)变化进行了对比模拟研究。结果表明：定孔隙率模型与变孔隙率模型之间有明显的差异，且变孔隙率模型更能体现实际反应过程的特征。以初始孔隙率为0.8的工况为例，定孔隙率假设在直接传热式反应器中的转化速率比变孔隙率模型快19.8%，而在间接传热式反应器中转化速率比变孔隙率慢6.1%。说明以往研究中使用的定孔隙率假设高估了直接传热式反应器的性能，而低估了间接传热式反应器的性能。     

直通道内两相流动的格子玻尔兹曼方法模拟

本文改进了多弛豫时间多组分伪势格子玻尔兹曼方法,使其可以适用于极大黏度比的情况.研究了二维直通道内的两相流动,分析了饱和度、黏度比、润湿性和毛细管数对相对渗透率和相渗曲线的影响.结果表明,大黏度相的相对渗透率随着黏度比的增大而增大,维持高流动能力的饱和度范围也随之扩大,小黏度相与之相反;黏性耦合效应对两相相对渗透率的影响存在极限,即不会导致小黏度相的相对渗透率低于最小值,也不会使大黏度相的相对渗透率高于最大值;非润湿条件会使大黏度相的流动能力增强,相对渗透率增加,并强化相渗曲线的非单调性;润湿条件会减弱大黏度相的流动能力,相对渗透率降低;毛细管数的增加会导致更明显的接触角滞后效应,使两相相对渗透率降低.     

厚翅片管内流体流动和传热的数值分析

本文应用Ｐａｔａｎｋａｒ等人［１］研究薄翅片管的湍流模型，对一种工业化的厚翅片管内的流体流动和传热进行了数值分析。计算范围包括了层流和湍流（Ｒｅ＝１０１～１０６），所得计算结果与较窄范围内实验所测的传热与阻力数据相当符合，本计算结果具有较大的推广价值。     

含内热源多孔介质–自由流通道内非达西强制对流

基于多孔介质局部非热平衡模型,对考虑内热源条件下的多孔介质–自由流耦合通道内非达西对流换热特性进行研究。多孔介质区内流体运动方程采用Darcy-Brinkman-Forchheimer模型,利用有限差分法获得通道内各区域流体运动速度、流固相温度分布及努塞尔数,并进一步分析了相关参数对流体流动传热的影响。结果表明：在文中研究参数下,惯性参数F_f对通道内各区域流体速度及温度分布的影响仅在达西数Da大于10^-3时需要考虑;增加F_f或降低固相内热源W_s绝对值会使流固两相温差减小,且改变固相内热源换热方向会使多孔介质区内流固相发生温度分岔现象;固相内热源对通道换热效果影响较大且更为复杂,不同惯性参数F_f下,考虑W_s时可能会使Nu出现奇异点。     

气体动理学格式模拟多孔介质流动的可行性

将气体动理学格式（GKS）拓展到模拟多孔介质内的低速渗流,并检验在孔隙尺度上模拟不可压缩低速流动的可行性与有效性.结果表明：GKS具有二阶空间精度,能够较精确地计算多孔介质的渗透率;相比于单松弛格子玻尔兹曼方法,GKS能够精确实现壁面无滑移边界条件,从而正确反映渗透率与黏性无关的特性;对于Berea砂岩切片结构中的复杂流动,模拟结果与实验吻合较好,能较精确地计算渗透率.给出GKS模拟达西渗流的马赫数选取准则,为研究多孔介质流动提供新的工具.     

超临界压力CO2在非均匀加热竖直管内的换热特性研究

基于工程中存在的非均匀热流问题,针对四种非均匀热流条件下超临界压力CO₂在竖直管内的流动换热特性进行了数值研究,分析了热流密度、质量流量、浮力效应和排布方式对流动换热性能以及圆管表面温度分布的影响。超临界压力CO₂在非均匀热流条件下表现更为复杂的流动换热特性,轴向热流密度分布不均匀会使传热恶化,增大热流密度和减小雷诺数可以弱化传热恶化效应;热流分布不均时,Bo^*比■/Re2.7更能准确地预测浮力效应;在非均匀热流条件下,竖直向下流动比竖直向上流动表现出更好的传热性能,径向速度和湍流动能分布可以较好地解释传热恶化的产生机理。本文对于光热、锅炉等非均匀热流条件下的工程应用具有一定的指导意义。     

微通道内沸腾流动与传热特性耦合机理研究

以水为工质，模拟研究不同条件下水平矩形微通道沸腾流动过程中气泡发生发展及流型演变与温度、压力、传热系数的耦合关系。结果表明：持续吸热的弹状流会占据通道大部分流动区域，易造成堵塞和局部高温；泡状流区域压力波动幅度较小，较长弹状气泡和大雷诺数均会导致较大的局部压降；升高热流密度减小了单相区长度，强化了核态沸腾，提高了通道整体传热性能；增大雷诺数使得通道内气泡尺寸减小，减少了两相流动的不稳定与堵塞现象，通道整体传热性能得以提升。     

利用多松弛格子Boltzmann方法预测多孔介质的渗透率

多孔介质内的流动问题在工程热物理领域有着重要的研究价值和应用背景。本文利用多松弛格子Boltzmann方法详细预测了两种二维多孔介质中的渗透率。研究结果表明:一方面,多松弛模型可以用来克服由单松弛模型带来的一些不足;另一方面,借助于达西定律,多松弛模型可以准确预测多孔介质的渗透率,并将计算结果与已有文献做了对比。     

格子Boltzmann方法用于模拟超临界CO2自然对流的研究

本文提出一种能够体现流体物性变化的格子Boltzmann模型。首先,通过模拟常压下空腔内的自然对流,检验了模型的可靠性。其次,模拟了超临界压力下CO2的自然对流,讨论了伪临界温度附近急剧的物性变化对流动及传热的影响。模型的提出为深入探讨CO2的流动和传热特性提供了有力工具。本文结果为有效利用超临界CO2提高换热系统的性...     

基于气液耦合的中等尺度甲醇池火数值模拟

本文提出了一种模拟池火的气液双向耦合的三维数值模型。该模型通过直接求解油池内部对流流动和气液共轭耦合传热确定池火的蒸发速率。气相区域利用大涡模拟方法求解经滤波过滤的三维可压缩N-S方程组,液相区域使用三维不可压缩层流控制方程求解流动,两相交界面处通过基于“薄膜理论”的蒸发模型及共轭传热方法求解传质传热过程。通过甲醇池火实验验证模型的准确性。计算结果表明该模型能准确计算燃油蒸发速率,由马兰戈尼效应诱导的液相内部对流运动对池火蒸发速率有一定影响。     

MHD natural convective flow through a porous medium in a square cavity filled with liquid gallium

This article contains a computational study of free convective flow through a square enclosure filled with liquid gallium saturated porous medium in the presence of a uniform inclined magnetic field. Lower boundary of enclosure is considered to be heated uniformly, upper horizontal boundary is taken insulated, left wall of the cavity is heated linearly, and right wall is heated linearly or taken cold. Navier–Stokes equations governing the flow problem are first exposed to penalty method to eliminate the pressure terms and then Galerkin FEM is employed to solve reduced equations. Grid independent results are achieved and shown in tabular form for numerous ranges of physical flow parameters. To ensure the accuracy of developed code, computed results are compared with those available in earlier studies through figures. It is found that the strength of streamlines circulation is increased due to increase in Darcy number while imposition of vertical magnetic field instead of horizontal magnetic field causes slow rate of increase in strength of streamlines circulation. Whereas, in the case of linearly heated right wall, the average Nusselt number is an increasing function of the Darcy number, and vertical magnetic field causes higher values for average Nusselt number as compared to horizontal magnetic field along bottom and side walls of cavity. Contrarily, in the case of cold right wall, the horizontal magnetic field results in higher values of average Nusselt number as compared to the vertical magnetic field case, and the average Nusselt number reduces as we move along lower and right boundary while increases along left wall with increase in distance.     

不连续冷源布局多孔介质内热流耦合LBM数值模拟

为研究不连续冷源边界对内置发热体多孔介质方腔内传热及流动的影响,采用格子Boltzmann方法对REV尺度下多孔介质方腔内的自然对流进行计算,并研究瑞利数(Ra)、达西数(Da)、孔隙度对多孔介质方腔内传热流动的影响.发现Da对方腔内的流体流型影响很大,Da为10^-4时,多孔介质方腔内只有一个涡流,而Da为10^-2时,方腔内有两个涡流.增大Ra、Da、孔隙度可以提高冷源壁面的平均努赛尔数(Nu),增强散热效果,孔隙率对平均Nu影响程度和Da的大小有关.当冷源布置在壁面上方,壁面的平均Nu随Ra的增加剧烈变化,方腔处于高Ra条件下时,将冷源布置在边界的上方可以提高散热效果.6种布置方案中Case 6的散热效果最好.     

THERMOSOLUTAL CONVECTION WITH COUPLED HEAT AND MASS TRANSFER PROCESS IN A SQUARE ENCLOSURE

Thermosolutal convection flow and its effect on the heat and the mass transfer in a square enclosure is studied experimentally. Both thermal and solute diffusion are induced from the sides, and natural convection is initiated by the combined thermal and solutal buoyancies, which either augment or oppose to each other. The solute diffusion is initiated in an electrochemical system that uses copper sulfate-sulfuric acid solution as an electrolyte. Depending on the magnitude of buoyancy ratio, three different kinds of flow regimes and structures can occur, which lead to different distributions of concentration in the enclosure. The formation and growth of layered flow structure is attributed to the solutal boundary-layer flow that can intrude and accumulate along the horizontal wall. The nearly stagnant layer that occurs can reduce the heat transfer rate. The Nusselt numbers at different flow regimes are measured and correlated in terms of relevant nondimensional parameters. This suggests the correlation of Sherwood number in different ranges of buoyancy ratio. The visualization of flow structures and measurements of both heat and mass transfer allow better understanding of the complicated system.     

Thermal Lattice Boltzmann Flux Solver for Natural Convection of Nanofluid in a Square Enclosure

In the present study, mathematical modeling was performed to simulate natural convection of a nanofluid in a square enclosure using the thermal lattice Boltzmann flux solver (TLBFS). Firstly, natural convection in a square enclosure, filled with pure fluid (air and water), was investigated to validate the accuracy and performance of the method. Then, influences of the Rayleigh number, of nanoparticle volume fraction on streamlines, isotherms and average Nusselt number were studied. The numerical results illustrated that heat transfer was enhanced with the augmentation of Rayleigh number and nanoparticle volume fraction. There was a linear relationship between the average Nusselt number and solid volume fraction. and there was an exponential relationship between the average Nusselt number and Ra. In view of the Cartesian grid used by the immersed boundary method and lattice model, the immersed boundary method was chosen to treat the no-slip boundary condition of the flow field, and the Dirichlet boundary condition of the temperature field, to facilitate natural convection around a bluff body in a square enclosure. The presented numerical algorithm and code implementation were validated by means of numerical examples of natural convection between a concentric circular cylinder and a square enclosure at different aspect ratios. Numerical simulations were conducted for natural convection around a cylinder and square in an enclosure. The results illustrated that nanoparticles enhance heat transfer in higher Rayleigh number, and the heat transfer of the inner cylinder is stronger than that of the square at the same perimeter.     

Lattice Boltzmann simulation of 3-dimensional natural convection heat transfer of CuO/water nanofluids

The present study investigated fluid flow and natural convection heat transfer in an enclosure embedded with isothermal cylinder. The purpose was to simulate the three-dimensional natural convection by thermal lattice Boltzmann method based on the D3Q19 model. The effects of suspended nanoparticles on the fluid flow and heat transfer analysis have been investigated for different parameters such as particle volume fraction, particle diameters, and geometry aspect ratio. It is seen that flow behaviors and the average rate of heat transfer in terms of the Nusselt number (Nu) are effectively changed with different controlling parameters such as particle volume fraction (5 % ≤ φ ≤ 10 %), particle diameter (d _p = 10 nm to 30 nm) and aspect ratio (0.5 ≤ AR ≤ 2) with fixed Rayleigh number, Ra = 10⁵. The present results give a good approximation for choosing an effective parameter to design a thermal system.     

Experimental studies on natural convection of water in a closed-loop vertical annulus

Experimental studies on heat transfer and fluid flow of water in a vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon, have been carried out in this article. The annulus has a radius ratio (outer radius to inner radius) of 1.184 and aspect ratio (length to annular gap) equal to 352. The experiments were conducted for constant heat fluxes of 1, 2.5, 5, 7.5, 10, 12.5, and 15 kW/m². Transient behavior during the heat-up period of the system until the steady-state condition is attained and discussed. Variation in the heat transfer coefficient and Nusselt number along the annulus height represent the developing boundary layer at the entrance and fully developed flow in the remaining length. A large drop in the differential pressure is experienced when the liquid is circulated through the flow meters, which restrict the flow due to their very small passages. Flow restriction causes mass accumulation and rise of pressure at the exit of the annulus. It also causes a decrease in liquid head in the cooling leg. An increase in the heat flux leads to an increase in the heat transfer coefficient and Nusselt number. As a result of the data analysis correlations for the average Nusselt number, Reynolds number and circulation rate have been developed in terms of the heat flux.     

Numerical investigation of natural convection heat transfer of nanofluids in a Γ shaped cavity

This paper analyzes the heat transfer and fluid flow of natural convection in a Γ shaped enclosure filled with Al₂O₃/Water nanofluid that operates under differentially heated walls. The Navier–Stokes and energy equations are solved numerically. Heat transfer and fluid flow are examined for parameters of non-uniform nanoparticle size, mean nanoparticle diameter, nanoparticle volume fraction, Grashof number and different geometry of enclosure. Finite volume method is used for discretizating positional expressions, and the forth order Rung-Kuta is used for discretizating time expressions. Also an artificial compressibility technique was applied to couple continuity to momentum equations. Results indicate that using nanofluid causes an increase in the heat transfer and the Nusselt number so that for R = 0.001 in Gr = 10³, the Nusselt number 25%, in Gr = 10⁴ 26%, and in Gr = 10⁵ 28% increases. Furthermore; by decreasing the mean diameters of nanoparticles, Nusselt number increases. By increasing R parameter (d_p,min/d_p,max) and nano particle volume fraction, Nusselt number increases.     

多孔介质方腔内混合对流格子Boltzmann模拟

采用格子Boltzmann方法,对多孔介质方腔内的混合对流现象进行研究.方腔内部中心有一发热圆,径宽比D/L=0.4,冷流体从方腔左下角入口流进,从方腔左上角出口流出,四周壁面绝热.在普朗特数Pr=0.71和格拉晓夫数Gr=1.4×10⁴时,分析理查德森数Ri和达西数Da对发热圆表面平均努赛尔数Nu的影响.结果表明：Ri数位于10^-3~10范围内,Nu随Ri的增大而减小.Da越大,Ri对Nu的影响越显著;Da数位于10^-5~10^-2范围内,强制对流占主导的情况下（Ri≤0.1）,Nu随着多孔介质的Da的增大而增大.自然对流占主导的情况下（Ri=10）,Nu对Da的变化不敏感.     

NATURAL CONVECTION FROM DISCRETE HEAT SOURCES IN A VERTICALLY VENTED RECTANGULAR ENCLOSURE

Natural convection in a discretely heated, vertically vented enclosure has been investigated experimentally. A vertically vented enclosure is one whose top and bottom boundaries are partially open, allowing ambient fluid to be drawn in by buoyancy. Mach-Zehnder interferometry was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the discrete heat sources. A smoke-generation technique was used to visualize the flow structure. The experimental parameters investigated include Grashof number, vent gap width, and heater locations for a dual heater configuration and a single-enclosure aspect ratio. The vent gap width was varied between a fully open condition (discretely healed parallel plates) and nearly closed. The Mach-Zehnder interferograms and local heat transfer results suggest a complex buoyancy-driven flow field that depends intimately on the rent gap width. For some geometric configurations, the heat transfer coefficient was found to be nearly uniform across the length of the heater, while for others traditional boundary-layer-type heat transfer characteristics were found. In general, the heat transfer coefficient increased with increased vent gap spacing. However, a maximum in average Nusselt number was observed for a dimensionless gap spacing of G/W – 0.67 for one of the two dual-heater placement configurations studied. The heat transfer characteristics were compared to those of a single isolated plate and unobstructed parallel-plate channels.     

复杂多孔介质腔体内自然对流换热的数值研究

采用曲线坐标系下压力与速度耦合的SIMPLEC算法,数值研究复杂多孔介质腔体内的自然对流换热问题.腔体的曲面温度分别保持恒定,上下表面绝热.在曲线坐标系中用有限容积法离散方程,并采用Brinkman扩展达西模型及局部非热平衡模型求解,综合研究Rayleigh数,Darcy数、孔隙率等参数对腔体内自然对流换热的影响.计算结果表明:Rayleigh数和Darcy数的影响最大而孔隙率的影响很小,同时存在使得腔体内换热达到最强的最佳纵横比.