Development of the theory and methods for calculating the dynamics of sorption and desorption

A system of equations of heat and mass transfer, phase transformations, and deformation of porous bodies in sorption and desorption are derived and substantiated. Formulas for the area of contact between a liquid and gas in a unit volume of a porous body, for the intensity of phase transition, heat of phase transformations, equilibrium partial vapor pressure, and for the thickness of adsorbate layer on the surfaces of capillaries with allowance for the influence of adsorption forces are presented. The results of a comparison between calculated and experimental data are given.     

Numerical Modelling of Flow Boiling Heat Transfer in Horizontal Metal‐Foam Tubes

The flow boiling heat transfer performance in horizontal metal‐foam tubes is numerically investigated based on the flow pattern map retrieved from experimental investigations. The flow pattern and velocity profile are generally governed by vapour quality and mass flow rate of the fluid. The porous media non‐equilibrium heat transfer model is employed for modelling both vapour and liquid phase zones. The modelling predictions have been compared with experimental results. The effects of metal‐foam morphological parameters, heat flux and mass flux on heat transfer have been examined. The numerical predictions show that the overall heat transfer coefficient of the metal‐foam filled tube increases with the relative density (1‐porosity), pore density (ppi), mass and heat flux.     

Superfluid flow in porous stainless steel

This Paper shows that porous plug material can serve as a control device during liquid transfer. The application of heat flux directly relates to the temperature and pressure gradient across the plug in the flow regime 1. For a given porous material an empirical relation exists that predicts the first critical state which is the upper limit for low pressure drop liquid transfer. Thus, the porous plug can serve a dual purpose for liquid helium transfer in space: the plug can serve as a phase separator during zero gravity storage: and the plug can serve as a low pressure drop control device during liquid transfer.     

Unsteady natural convection flow of a suspension comprising Nano-Encapsulated Phase Change Materials (NEPCMs) in a porous medium

The free convection phase change heat transfer of a suspension comprising Nano-Encapsulated Phase Change Materials (NEPCMs) in a porous space is theoretically addressed. The core of the nanoparticles is made of a phase change material and encapsulated in a thin shell. Hence, the core of the nanoparticles of the suspension undergoes a phase change at its fusion temperature and release/store large amounts of latent heat. The phase change of nanoparticles is modeled using a sine shape temperature-dependent heat capacity function. Darcy-Brinkman model is used to model the flow in the porous medium. The governing equations including the conservation of mass, momentum, and heat are transformed into a non-dimensional form before being solved by the finite element method in a structured non-uniform mesh. The influence of the porosity, Darcy number, Rayleigh number, fusion temperature of nanoparticles, and the unsteady time-periodic boundary conditions on the thermal behavior of the porous medium in the presence of NEPCM particles is investigated. The results show that the presence of NEPCM particles improves the heat transfer. The increase of porosity improves the heat transfer when the volumetric concentrations of NEPCM particles are higher than 3%. There exists an optimal dimensionless fusion temperature of NEPCM nanoparticles for the interval [0.25; 0.75].     

Development of a theory and methods for calculating the heat and mass transfer in drying a porous body with multicomponent vapor and liquid phases

Theoretical fundamentals and a numerical method for calculating the heat and mass transfer and phase transformations in drying porous bodies with multicomponent vapor and liquid phases have been developed. We have obtained expressions for the evaporation intensity and the phase transition heat of liquid mixture components and a formula for the equilibrium partial pressure of vapor phase components, from which, as limiting cases, empirical Raoult and Henry laws follow. The results of comparison between the calculated and experimental data are presented. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 6, pp. 1111–1124, November–December, 2008.     

On the Problem of Nonisothermal Mass Transfer in Porous Media

We propose a model of nonstationary processes of interrelated heat and mass transfer in porous media that takes into account the mutual influence of vapor and liquid pressures determined by the contribution of the capillary and surface forces and temperature on the interphase masstransfer intensity, the thermocapillary flows, and the mechanical and dynamic equilibrium conditions of thin layers of liquids on curved interfaces. The results of the numerical solution of the system of equations of heat and mass transfer in capillaryporous bodies in discrete and continuous heating as well as with allowance for the effect of nonequilibrium of phase transformations have been analyzed.     

Evaluation of heat and mass transfer coefficients in a gauze-type structured packing air dehumidifier operating with liquid desiccant

In this paper, high performance packing, namely, structured packing that has good heat and mass transfer characteristics, is proposed for dehumidification of air using liquid desiccants and for regeneration of liquid desiccants. In order to design a structured packing tower for liquid desiccant — air contacting operations, heat and mass transfer coefficients for each phase are required. This paper is concerned with the interface transfer of heat and mass when air is brought into contact with the liquid desiccant solution. A theoretical study of evaluating heat and mass coefficients in an air-desiccant contact system employing three liquid desiccants, namely calcium chloride, lithium chloride, and a mixture of 50% calcium chloride and 50% lithium chloride (called cost effective liquid desiccant, CELD) is investigated. Moreover, air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid. The findings for the three liquid desiccants are compared and discussed.     

A comparison of the finite element and control volume numerical solution techniques applied to timber drying problems below the boiling point

Drying is a process which involves heat and mass transfer both inside the porous material, where a phase change in moisture occurs from the liquid to the gaseous state, and in the external boundary layer of the convected hot dry air, which heats the porous medium. The equations which govern this process consist of three tightly coupled, highly non-linear partial differential equations for the unknown system variables of moisture content, temperature and pressure. Due to the inherently complex boundary conditions and intricate physical geometries in any practical drying problem, an analytical solution is not possible. In order to obtain a transient drying solution it is necessary to resort to a numerical technique. The numerical solution techniques which were employed in this research were the finite element method and the control volume method. The transient numerical results were compared and contrasted for two timber drying problems, first, at a dry bulb temperature of 50°C, and secondly, at 80°C, both cases being below the boiling point of water.     

Thermal Response of a Two-Phase Near-Critical Fluid in Low Gravity: Strong Gas Overheating as Due to a Particular Phase Distribution

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF₆ in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase ³He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.     

Liquid vapour phase change on a single nucleation site: Heat and mass transfer

In this paper experimental results of heat and mass transfer variations during a single vapour bubble growth are carried out. The vapour bubble was created in a FC-72 liquid, on a downward facing heating element maintained at constant heating power. Heat flux and temperature measurements combined with image processing enable us to study the influence, on heat transfer, of the level of liquid subcooling, the heating power applied on the nucleation surface and the nucleation surface’s inclination. It was found that the phase change heat flux (i.e. the difference between the flux of evaporation and the flux of condensation) and the total heat flux (measured with fluxmeter and due to convection, conduction and phase change) decreased in the same order of magnitude when the level of subcooling was increased. When the heating power increased, the total heat flux increased whereas the phase change heat flux varied in a non-significant way. It was also found that the total heat flux depends on the nucleation surface inclination, whereas, for small angles, the phase change heat flux didn’t change. The analysis of those results lead to the conclusion that during boiling the liquid motions due to the bubble growth and departure play an important role in the heat transfer enhancement.     

Transport and fate of volatile organic chemicals in unsaturated, nonisothermal, salty porous media: 1. Theoretical development.

A wide variety of volatile organic chemicals (VOC) have been applied to agricultural land or buried in chemical waste sites. The fate of these chemicals depends upon several mechanisms such as sorption, degradation, and transport in liquid and gaseous phases. Understanding the transport mechanisms affecting the volatile chemicals can lead to better management strategies. A theory describing inorganic solute transport, water and heat transfer, and the fate and transport of VOC in porous media has been developed. This theory includes matric water pressure head, solution osmotic pressure head, gravity pressure head, temperature, inorganic solute concentration, and VOC concentration gradients as driving forces for heat and mass transfer. The effect of surface tension, as a function of VOC concentration and temperature, on the matric water pressure head is included. The VOC can be associated with gas, liquid, and solid phases of the porous media. The gas and liquid phases are mobile, but the solid phase is immobile. The transfer of VOC across the gas/liquid, liquid/solid, and gas/solid interfaces is included using sorption-equilibrium assumptions at the interfaces. The VOC can degrade. This degradation is described by a first-order decay rate. The theory can be used to predict spatial and temporal variations of water content, temperature, inorganic concentration and the total concentration of VOC within a porous medium. The concentration of VOC in each phase can be predicted also.     

Numerical investigation on the natural convection effects in the melting process of PCM in a finned container using lattice Boltzmann method

This work presents a numerical study to investigate the melting process inside a finned rectangular container of phase change material. This issue is interested to know what will happen in the presence of fins, which increase the rate of heat transfer and immerse the solid body through the liquid phase. Momentum exchange between the liquid and solid phases necessitates to consider the immersed boundary condition on the solid–liquid interface. This procedure is done by considering the direct-forcing scheme in the lattice Boltzmann framework. In this regard, velocity and temperature fields are obtained using the multi relaxation time model. To track the solid–liquid interface, a set of Lagrangian points is intended on the boundary of the solid phase. Also, the phase change process is modeled by applying the energy conversion on the finite control volume around these points. Results include melting process for three states: assumption of pure conduction, considering the effects of natural convection around the fixed solid body and natural convection through the free solid phase. Comparing results of melting history and solid–liquid interface position in those states specifies the importance of momentum exchange between solid and liquid phases, in which the forced exerted by the fluid flow accelerates the melting process.     

多角度肋片强化相变材料换热仿真分析

目的 针对太阳能热利用领域中相变材料的封装结构提出圆柱体相变蓄热棒,并设计多角度肋片以加快相变材料融化速率。方法 采用CFD仿真技术,分析不同形状肋片对蓄热棒中相变材料融化特性的影响,计算各模型相变材料的融化时间、温度响应速率和平均传热系数。结果 在800 W/m²的热流边界条件下,无肋片蓄热棒的相变材料完全融化需要2 813 s,设计的12组肋片中Tra–45模型性能最优,相变材料的融化时间比无肋片对照组的缩短了5.4%;Tra–45模型中相变材料温度分布集中,且最高温度上升了6 ℃,Tra–45模型的温度响应速率较对照组的提升了5%;Tra–45模型的平均换热系数达到9.97 W/(m².K),较对照组的提升了2.8%。结论 蓄热棒内增加梯形45°肋片后,相变材料融化速率加快,蓄热棒内温度分布均匀。同时,相变材料的温度响应速率提高,平均换热系数显著增加,可满足频繁充放热的需求。     

A spectral method for solving heat and moisture transfer through consolidated porous media

This work presents an efficient numerical method based on spectral expansions for simulation of heat and moisture diffusive transfers through multilayered porous materials. Traditionally, by using the finite-difference approach, the problem is discretized in time and space domains (method of lines) to obtain a large system of coupled ordinary differential equations (ODEs), which is computationally expensive. To avoid such a cost, this paper proposes a reduced-order method that is faster and accurate, using a much smaller system of ODEs. To demonstrate the benefits of this approach, three case studies are presented. The first one considers nonlinear heat and moisture transfer through one material layer. The second case, ie, highly nonlinear, imposes a high moisture content gradient, simulating a rain-like condition, over a two-layered domain, whereas the last one compares the numerical prediction against experimental data for validation purposes. Results show how the nonlinearities and the interface between materials are easily and naturally treated with the spectral reduced-order method. Concerning the reliability part, predictions show a good agreement with experimental results, which confirm robustness, calculation efficiency, and high accuracy of the proposed approach for predicting the coupled heat and moisture transfer through porous materials.     

Modeling of ice formation in porous solids with regard to the description of frost damage

The freezing and thawing of liquid in porous media in connection with the question concerning the frost durability of solid materials is an important subject for discussion in civil engineering. Each construction or body which is in contact with liquid and frozen water is criticized by its resistance to the environment. The durability concerning frost attacks of a building material is affected by its porosity and the pore size distribution. The ice formation is a phenomenon of coupled heat and mass transport in freezing porous media, and is primarily caused by the expansion of ice in connection with hydraulic pressure. The volume increases due to the freezing front inside the porous solid. Taking into account the aforementioned effects in porous materials, a simplified macroscopic model within the framework of the Theory of Porous Media (TPM) for the numerical simulation of initial and boundary value problems of freezing and thawing processes of super saturated porous solids will be presented. The phase change between the ice and the liquid phase is modeled by different real densities of the phases.     

制冷系统热回收装置相变材料的分析研究

分析了相变材料特性,并找出了适用于制冷系统热回收装置的相变材料,通过对热物性和工作性能的研究,选取Fecl3·6H2O为本装置的相变蓄能材料,同时建立蓄热体的物理模型及模拟简化模型,在自然对流的影响下,模拟相变材料的蓄、放热特性,找出固——液界面、温度场、速度场、液相比例、传热系数、热流密度、监测点的温度等随时间的变化规律,为制冷系统热回收装置的设计提供了理论依据。     

制冷系统热回收装置相变材料的分析研究

分析了相变材料特性,并找出了适用子制冷系统热回收装置的相变材料,通过对热物性和工作性能的研究,选取Fecl3·6H2O为本装置的相交蓄能材料,同时建立蓄热体的物理模型及模拟简化模型,在自然对流的影响下,模拟相变材料的蓄、放热特性,找出固——液界面、温度场、速度场、液相比例、传热系数、热流密度、监测点的温度等随时间的变化规律,为制冷系统热回收装置的设计提供了理论依据。     

Superfluid plug as a control device for helium coolant

New results have been obtained on the characteristics of the superfluid plug as a nonmechanical control device for supplying cold helium vapour on demand from a container of superfluid helium. The superfluid plug is a device which has been proposed for space applications to serve as a phase separator for liquid helium in the superfluid state. Typical plugs are made of a porous material having pores of one to ten μm in diameter. The experimental arrangement is such that one side of the plug is in contact with the superfluid liquid helium while vapour at a low pressure (of order 1 to 10 torr) is maintained on the other side.The data reported here are for a plug with approximately 5 μm diameter pores. Temperatures, pressures, and flow rate were monitored during the experiment. A theoretical background and steady state data are presented on mass flow rates and pressures as a function of liquid temperature.The typical response of the flow rate to a change in heat input from a heater is an exponential rise or fall to an equilibrium flow rate which is proportional to the amount of heat input. The time constants of the exponential changes were measured for two heater control modes under study. The study has included an investigation of the important parameters effecting the dynamic response of the plug including the superfluid properties, plug material properties, plug pore size and plug permeability. Operating temperatures from 1.5 K to the lambda point were investigated and heating rates up to two watts were applied. These tests serve to demonstrate that the superfluid plug can be employed as a flow control device in a control system designed to provide coolant on demand.     

Heat and mass transfer in thermal protection composite materials upon high temperature loading

Heat and mass transfer in thermal protection composite materials, used as heat protection of the structural elements of hypersonic aircrafts, is simulated in this work. Two phases arise upon high-temperature loading of the composite materials: one, unaffected by the decomposition of the binder of the composite material, and the other, a porous residue in which phase transitions are completed. These two phases are separated by a narrow zone of the binder decomposition, limited by moving boundaries of the beginning and end of the phase transformations with gas formation and a variable density of the composite materials. Analytical solutions of the problems of heat and mass transfer are obtained for the first two phases; a transcendental equation for determining the coordinates and velocity of the pyrolysis zone is derived based on these solutions and the balance of heat flows in this zone. The found mass velocity of the pyrolysis zone made it possible to determine the mass generation rate, density, and stagnation pressure of the pyrolysis gases in the decomposition zone, as well as the mass filtration rate in the porous coke residue. The validity of the proposed mathematical model is confirmed by many numerical experiments. The results of some experiments are given as functions of time, temperature, the thermal characteristics, the mass and linear velocities of the pyrolysis zone, the density and stagnation pressure of gases in this area, and the pressure distribution and the mass filtration rate in the resulting porous residue.     

石墨烯改性相变聚氨酯发泡材料传热特性研究及仿真

目的 提升无源冷链物流中保温容器的保温性能,减少或取消短途冷链运输中蓄冷剂的使用量.方法 将石墨烯改性相变大胶囊与聚氨酯发泡材料共混,制备石墨烯改性相变聚氨酯发泡材料,研究石墨烯的添加对其导热系数和保温性能的影响,并基于COMSOL Multiphysics对石墨烯改性相变大胶囊和石墨烯改性相变聚氨酯发泡材料的传热过程进行有限元模拟.结果 在常温状态下,石墨烯的添加会增加石墨烯改性相变聚氨酯发泡材料的导热系数;在冷藏工况下,石墨烯改性相变聚氨酯发泡材料的导热系数随石墨烯添加量的增加而降低,石墨烯改性相变聚氨酯发泡材料制备的保温容器的系统热阻随石墨烯的增加而升高.通过有限元模拟可发现,石墨烯的加入能够提升相变大胶囊的导热系数,同时能够减缓升温平台阶段的升温速率;在跨越平台区后,即内部相变大胶囊完全融化后,石墨烯会提升保温材料的导热系数,从而降低保温容器的保温效果.结论 制备的石墨烯改性相变聚氨酯发泡材料能够在设计使用时间内提升保温容器的保温性能,该材料在需要冷藏的生鲜果蔬短途无源冷链物流领域具有广阔的应用前景.