A MATHEMATICAL MODEL FOR HEAT AND MASS TRANSFER IN UNSATURATED WET POROUS MEDIA CONSIDERING EFFECT OF CAPILLARY HYSTERESIS

Based on the transport mechanism of heat and mass transfer and by using the minimum gradient theory for unsaturated flow in capillary porous media, a mathematical model is developed for heat and mass transfer in unsaturated wet porous media considering the effect of capillary hysteresis. The heat and mass transport properties in the derived differential equations are analyzed in detail. which opens new way to further develop practical methods for determining heat and mass transport properties in wet porous media.     

NATURAL CONVECTIVE HEAT AND MASS TRANSFER BY THE COUPLING OF TEMPERATURE CRADIENT AND CONCENTRATION GRADIENT IN A POROUS MEDIUM

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NUMERICAL SIMULATION OF FLOW AND HEAT TRANSFER CHARA-CTERISTICS OUTSIDE A PERIODICALLY VIBRATING TUBE

Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.     

一种基于实测混凝土温度梯度获取表面散热系数的新方法

混凝土表面散热系数计算的准确与否,会对温度仿真计算的可靠性产生很大影响。通过现场试验研究了不同边界条件下温凝土表面附近点温度和温度梯度随距临空面距离和时间的变化规律,利用现场温度实测值对混凝土表面散热系数进行反演分析,得到不同边界下混凝土表面散热系数的反演值。然后通过混凝土第三类边界条件下表面散热系数与温度梯度的关系,发现可以选取混凝土表面0.03 m~0.09 m范围内的中点温度和温度梯度计算混凝土表面散热系数。几种不同表面覆盖条件下的验证计算结果表明,该方法对于不同的边界具有普适性和较好的准确性,可供科研人员和设计人员参考。     

同轴套管间涡旋流动及强化传热数值模拟

相对旋转两同轴套管间的涡旋流动,能够带来二次流强化传热传质作用,在航空、水处理、生态保护、生物工程和膜分离等领域都具有广泛的应用价值。本文使用Fluent软件,对同轴套管间涡旋流动及传热特性进行数值模拟,考察了内管转速、内外管壁面温差等操作参数变化对同轴套管间流体传热性能的影响,分析了涡旋流动与传热效率之间的关联关系。模拟结果表明:内管转速增加在流场中形成泰勒涡,涡流扰动增大了高温壁面与流体间的热流密度,增强了流体传热效率。增大内外管壁面温差,也可加强流体传热性能,但其强化作用不及内管转速的强化作用显著。受流场中泰勒涡影响,流体速度、温度及热流密度沿轴向的分布都呈正弦状周期性波动,在相邻两涡交界面处,流体传热性能最好,在涡中心处的传热性能最差。     

小颗粒堆积多孔体传热特性实验研究

对平行平板间填充小颗粒后的换热实验结果进行了归纳和分析，发现板间距不变，平均Nu数不随颗粒直径单调变化，说明存在最佳填充层数，并对此进行了初步分析。实验表明，小颗粒填充可以得到较好的换热强化效果，而流动阻力却远小于微颗粒填充情况，因此有可能利用小颗粒填充方法在常规条件下强化平板间的换热。     

NUMERICAL SIMULATION OF ICE CONTROL EFFECT OF SOLAR ENERGY ON TIBETAN CHANNELS

With the Tanghe Diversion Channel in Tibet as an example, the theoretical study on the ice control effect of the solar sacks was conducted based on the previous study. The numerical simulation method was adopted and a one-dimensional numerical model for ice crystal in diversion channels in high-altitude cold regions was developed in this article. The heat transfer through the air-water interface and the mass transfer between ice and water were considered in the model. The model was validated by the field observation data on the diversion channel of the Tanghe Hydropower Station. The results show that the ice control effect of the solar sacks is obvious in the channel with large mass flow rate in the high-altitude cold regions.     

Condensation Heat Transfer of R-134A in Horizontal Straight and Helically Coiled Tube-in-Tube Heat Exchangers

This article presents an experimental investigation on condensation heat transfer of R-134a in horizontal straight and helically coiled tube-in-tube heat exchangers. The experiments were carried out at three saturation temperatures(35 ℃ , 40 ℃ and 45 ℃ ) with the refrigerant mass flux varying from 100 kg/m2 s to 400 kg/m2 s and the vapor quality ranging from 0.1 to 0.8. The effects of vapor quality and mass flux of R-134a on the condensation heat transfer coefficient were investigated. The results indicate that the condensation heat transfer coefficients of the helical section are 4%-13.8% higher than that of the straight section. The experimental results were compared with the data available in literature for helical and straight pipes.     

THE EXPERIMENTAL STUDY FOR NATURAL CONVECTIVE HEAT AND MASS TRANSFER IN POROUS MEDIA

An electrochemical method is employed to study experimentally natural convection driven by combined thermal and solutal buoyant forces in a fluid-saturated porous enclosure. The horizontal temperature and concentration gradients are imposed in such a way that their effects on the flow are either opposing or augmenting. The inside temperature profiles and heat and mass transfer characteristics on the vertical walls are experimentally determined. The effects of dimensionless parameter Ra, Le, N on flow, heat and mass transfer are discussed in details.     

冻结管引起的周围土体温度场演变规律及参数化分析

针对稳定线性冷源热传导问题,为研究人工冻结法冻结管周围土体温度场的瞬态变化,基于冻结锋面将土体分为冻结区及降温区,构建相应人工冻结相变温度场模型,并采用变量替换法得到冻结温度场解析解,利用指数积分函数解析式表达了冻结区和未冻结区的温度场分布。将理论解析计算结果与冻结槽试验结果进行比对,验证了理论封闭解的有效可靠性,并进一步通过参数化分析了与土体传热介质冻结温度场演变规律有关的初始温度、冻结温度、冻结温差、结冰潜热、盐水温度和冻结管吸热系数等影响因素的敏感性。结果表明:冻结锋面半径与冻结时间存在着平方根关系,冻结温度场呈对数型函数分布,且曲线曲率随冻结时间的增加而逐渐减小;土体初始温度、冻结温度、结冰潜能和盐水温度与冻结锋面半径均呈负相关,冻结管吸热系数与冻结锋面半径呈正相关;土体冻结温差、土体结冰潜能和冻结管吸热系数与土体冻结锋面半径的发展均呈现近似线性关系,传热介质初始温度和盐水温度对冻结锋面半径随冻结时间变化的对数型曲线曲率影响最大,结冰潜热和冻结管吸热系数次之,冻结温度最小。研究成果对认识稳定线性冷源作用下冻结管周围不良地基土温度场演变规律和进一步形成规范及指导冻结法施工技术具有重要意义。     

河道低温水对地表水-地下水交错带温度的影响

利用饱和多孔介质水热耦合运移数学模型,分析了流入河道的水库下泄低温水水位和水温对地表水-地下水交错带温度场的影响规律。研究表明:在低温水下渗过程中,水热运移以热对流作用占主导,热传导、热弥散等作用较弱;河道水位升降明显决定着受影响的温度场区域范围大小;河道水温高低主要影响温度场各点温度的变化幅度。     

高寒地区某RCC重力坝保温措施及效果分析

某RCC重力坝地处高寒地区,又属欧亚大陆腹地,冬季寒冷且历时较长,曾观测到的极端最低气温为-49.8 ℃,多年平均气温为2.7 ℃.在本地区修建RCC重力坝,温控设计是关系到大坝建设成败的关键,温度控制必须解决"冷"对大坝温度应力的影响,防止大坝表面裂缝及深层裂缝的出现.现阐述了此RCC重力坝采取的保温温控措施,并根据施工现场实际监测成果对保温措施的效果进行了分析评价,以资类似工程借鉴.     

ADJOINT METHOD OF PARAMETER IDENTIFICATION FOR SOME NON- LINEAR REACTION-DIFFUSION SYSTEMS

This paper deals with the problem of determining two unknown parameters of some nonlinear reaction-diffusion models. These reaction-diffusion models are derived from applications in the groundwater flow transport, environ mental sciences, gas dynamics, heat and mass transfer, industrial automatization and some other engineering technological fields. The adjoint method based on the variational principle is a relatively new optimal control method. It is used in the identification of the unknown diffusion coefficient, and some coefficients of the nonlinear sink or source terms in these systems.At first, the problem is transferred into an optimization problem of minimizing a functional, and the adjoint equations of the governing equations are derived from the adjoint method.Then, the formulas are given to calculate the gradient of the objective function with respect to the couple of unknown parameters. At last, an iterative gradient-based optimization algorithm is presented for solving the optimization problem. A numerical example is offered in the end. It shows the effectiveness of the proposed approach.     

The viscoelastic effects on thermal convection of an Oldroyd-B fluid in open-top porous media

The effects of two viscoelastic parameters on the thermal convection of a viscoelastic Oldroyd-B fluid in an open-top porous square box with constant heat flux are investigated. The results show that the increase of relaxation time is able to destabilize the fluid flow leading to a higher heat transfer rate, while the increase of retardation time tends to stabilize the flow and suppress the heat transfer. The flow bifurcation appears earlier with the increase of the relaxation time and the decrease of the retardation time, resulting in more complicated flow patterns in the porous medium.     

Kelvin-Helmholtz instability.with mass transfer through porous media： Effect of irrotational viscous pressure

This paper studies the effect of the irrotational viscous pressure on Kelvin-Helmholtz instability of the plane interface of two viscous and incompressible fluids in a fully saturated porous media with mass and heat transfers across the interface. In the earlier work, the instability of the plane interface of two viscous and streaming miscible fluids through porous media was studied by assuming that the motion and the pressure are irrotational and the viscosity enters the normal stress balance. This theory is called the viscous potential flow theory. Here, we use another irrotational theory in which the discontinuities in the irrotational tangential velocity and shear stress are eliminated in the global energy balance by considering viscous contributions of the irrotational pressure. The Darcy-Brinkman model is used in the investigation and the stability criterion is formulated in terms of a critical value of the relative velocity. It is observed that the heat and mass transfer has a destabilizing effect on the stability of the system while the irrotational shearing stresses stabilize the system.     

Numerical investigation of flow and heat transfer performances of horizontal spiral-coil pipes

The flow and heat transfer performances of horizontal spiral-coil pipes of circular and elliptical cross-sections are studied.The numerical results are compared with the experimental data,to verify the numerical method.The effects of the inlet water mass flow rate,the structural parameters,the helical pitch and the radius ratio on the heat transfer performances are investigated.Performances of the secondary fluid flow with different radius ratios are also investigated.Numerical results demonstrate that the heat transfer coefficient and the Nusselt number increase with the increase of the water mass flow rate or the helical pitch.The maximum heat transfer coefficient and the maximum Nusselt number are obtained when the radius ratio is equal to 1.00.In addition,the fluid particle moves spirally along the pipe and the velocity changes periodically.The particle flow intensity and the spiral movement frequency decrease significantly with the increase of the radius ratio.Besides,the secondary flow profile in the horizontal spiral-coil pipe contains two oppositely rotating eddies,and the eddy intensity decreases significantly along the pipe owing to the change of curvature.The decreasing tendency of the eddy intensity along the pipe increases with the increase of the radius ratio.     

SIMULTANEOUS MEASUREMENT OF THE HEAT AND MASS DIFFUSIVITIES IN UNSATURATED MOIST SOILS USING CONSTANT HEAT FLUX METHOD

The simultaneous measurement of the heat and mass diffusivities in unsa-turated moist soils is presented by use of constant heat flux method. By integrating the analytical solutions of the equations governing the heat and mass transfer process in a one-dimensional semi-infinite moist soil sample under condition of constant heat flux with the measurement data of the temperature and the moisture content fields in the soil sample, the heat and mass diffutivities of the soil sample are obtained. The experimental results show that the developed method is timesaving and can determine the heat and mass diffusivities in a moist soil through only one test, resulting in significant advantages in practical applications. This method has potential applications to the determination of heat and mass diffusivities in moist soils and may provide a new way for the fast and accurate measure-ment of the heat and mass diffusivities in moist soils and other unsaturated wet porous media.     

岩体单裂隙渗透性尺寸效应的数值模拟研究

针对渗透性尺寸效应涉及的影响因素众多、导致其演变规律及主控因素还不甚明了的问题,基于Comsol数值模拟软件,开展岩体单裂隙渗透性尺寸效应的数值模拟研究。建立开度服从正态分布的大尺寸岩体单裂隙模型(试样尺寸为5 m×5 m),在同一大尺寸试样上选取系列尺寸岩体单裂隙试样,并开展相应尺寸岩体单裂隙的渗透数值试验,观察岩体单裂隙渗透性随尺寸变化的演变规律。通过改变粗糙度、水力梯度和裂隙开度等参数,研究不同因素对岩体单裂隙渗透性尺寸效应的影响,确定其中的主控因素。数值模拟结果表明:岩体单裂隙的渗透性随试样尺寸的增大而增大,随后趋于一稳定值,其对应的试样尺寸即为临界尺寸;临界尺寸随粗糙度的增加、水力梯度的降低、裂隙开度的减小而增大;相较于粗糙度和水力梯度,裂隙开度对岩体单裂隙渗透性尺寸效应的影响更显著。     

双流程空冷散热器不同布置方式对散热系数影响分析

本文通过理论推导给出了散热器的冷热流体的温度分布计算公式,并计算分析了初始温差、顺逆流程及散热器高度对散热系数的影响。计算结果表明,初始温差(ITD)不会影响顺、逆流程的综合散热系数;逆流程散热系数总是大于顺流程;顺流程的散热系数随着散热器高度的增加不断减小,但散热器高度对逆流程的散热系数影响很小。此结果对工程设计具有重要的参考价值。     

自然通风湿式冷却塔热力特性数值研究

基于CFD软件和自然通风湿式冷却塔相关理论，对气流运动采用标准 湍流模型，喷淋区和雨区采用离散相模型计算，对填料区建立了基于Poppe理论的数值求解模型，并通过添加自定义源项，实现其在FLUENT中的求解。基于该模型，在不考虑自然风情况下，对某300MW机组自然通风冷却塔的气水流动与热质交换进行数值模拟，分析了淋水密度、进塔水温、喷嘴数和环境条件对冷却塔热力特性的影响。计算结果表明：基于CFD软件的冷却塔数值求解模型有很好的适用性，淋水密度和环境条件对出塔水温影响很大，为进一步完善冷却塔的设计与运行提供了理论依据。