Determination of moisture diffusivity in porous materials using gamma-method

In the present paper, results of an experimental gamma ray study of moisture transfer processes in porous material are reported. Evolution of moisture profiles in porous concrete samples during sorption and capillary moistening has been examined. Measured moisture profiles were used to determine, by the Boltzmann–Matano method, the coefficient of moisture diffusion versus moisture content of the material under various moistening conditions.     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

单面加热竖直窄通道内环状流沸腾传热特性

基于汽芯的动量方程和液膜的质量和动量方程,建立了单面均匀热流竖直窄通道内环状流沸腾传热模型,利用数值法对方程组进行求解,得出了环状流区域的液膜厚度,并进一步预测了环状流两相沸腾传热系数。研究表明:模型预测的两相沸腾传热系数比Mahmound关联式计算值偏小;将不同工况下的291组环状流两相沸腾传热系数实验值与模型预测值进行对比,平均绝对误差为12.7%。     

Direct numerical simulation of mass transfer from Taylor bubble flow through a circular capillary

In this work mass transfer during a Taylor bubble flow regime has been investigated by a volume of fluid (VOF) based numerical method. The hydrodynamics of Taylor bubble flow through a circular capillary has been simulated in a single unit cell by a moving reference frame. The validity of Taylor bubble hydrodynamics simulation has been checked by comparing the liquid film thickness and the relative bubble velocity obtained from computational fluid dynamics (CFD) simulations with reported empirical correlations and experimental results. The conservation equation of tracer has been solved in whole of flow domain for simulating mass transfer from Taylor bubble to surrounding liquid. The tracer concentrations in the cells that are either completely or partially filled with the gas phase are assigned the equilibrium concentration by employing the concept of internal boundary condition. By this concept an artificial diffusion of tracer has been appeared in the simulations. In order to eliminate this artificial diffusion, the advection scheme in the tracer conservation equation has been modified by using the two film mass transfer model. The simulation of mass transfer from a single bubble has been validated by comparing the CFD results with reported experimental data. Afterwards, the effects of capillary number, unit cell length and capillary diameter variation on mass transfer from Taylor bubble has been investigated.     

Water pool boiling heat transfer enhancement for modified surface tubes

This paper deals with the method of decreasing the size of heat exchanger surfaces by increasing the heat transfer coefficients and the importance of heat transfer enhancement for vaporization. We report an experimental study on surfaces modified by passive methods applied to heat transfer surfaces mechanically processed, covered with sleeves made by metallic tissues or covered with metallic porous layers performed using welding procedures. Experiments are made to investigate the heat transfer coefficient on copper tubes with a 22 mm external diameter using heat from inner source to outer vaporizing liquid. There are developed specific heat transfer correlations for each group of enhanced surfaces. The experimental data and new proposed correlations are compared with well known correlations. The results are in best agreement with the Cornwell–Houston correlation.     

Heat transfer in simulated boiling

An experimental study was undertaken to determine the variations of heat-transfer coefficient on a submerged heating surface while air bubbles were injected into the liquid through an orifice in the plate. The results indicated that heat transfer is most intensive during the time that the bubble detaches from the surface. This casts doubts on boiling heat-transfer correlations based on bubble growth or rising phase considerations. In conclusion, it is suggested that the “agitation” and “latent heat” views of boiling heat transfer may be combined in a unified model.     

Conjugate numerical analysis of flow and heat transfer with phase change in a miniature flat plate CPL evaporator

Two-dimensional numerical model for the global evaporator of miniature flat plate capillary pumped loop (CPL) is developed to describe heat and mass transfer with phase change in the porous wick, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The governing equations for different zones are solved as a conjugate problem. The side wall effect heat transfer limit is introduced to estimate the heat transport capability of evaporator. The influences of liquid subcooling, wick material, metallic wall material and non-uniform heat flux on the evaporator performance are discussed in detail.     

垂直窄缝通道内环状流沸腾传热特性

本研究基于液膜和蒸汽的质量、动量和能量方程,建立了均匀热流垂直窄缝通道内环状流沸腾传热模型,通过相关文献估算环状流起始点处液膜厚度,利用有限差分法对环状流模型方程组进行数值求解,得到沿流道环状流区域的液膜厚度,并进一步预测了局部沸腾传热系数,结果表明:环状流区域的局部沸腾传热系数随质量流量和干度的增加而增加,与Kenning关联式对比,模型预测沸腾传热系数较关联式计算值偏低。将不同工况下的226组两相环状流实验数据与模型预测结果进行对比,平均绝对误差为18.2%。     

Extending the validity of lumped capacitance method for large Biot number in thermal storage application

In a typical thermal energy storage system, a heat transfer fluid is usually used to deposit/extract heat when it flows through a packed bed of solid thermal storage material. A one-dimensional model of the heat transfer and energy storage/extraction for a packed-bed thermal storage system has been developed previously by the authors. The model treats the transient heat conduction in the thermal storage material by using the lumped capacitance method, which is not valid when the Biot number is large. The current work presents an effective heat transfer coefficient between the solid and fluid for large Biot numbers. With the corrected heat transfer coefficient, the lumped capacitance method can be applied to model the thermal storage in a wide range of Biot numbers. Four typical structures for the solid thermal storage material are considered. Formulas for the effective heat transfer coefficient (and effective Biot number) are presented. To verify the prediction by the lumped capacitance method using the effective heat transfer coefficient, we compare the results to the corresponding analytical solutions. The results are in very good agreement. The effective heat transfer coefficient extended the validity of the lumped capacitance method to large Biot numbers, which is of significance to the analysis of thermal energy storage systems.     

Heat transfer in a high temperature region of spray cooling interacting with liquid film flow

We present experimental results on heat transfer distribution in the high temperature region of spray cooling interacting with subcooled liquid film flow. The results show that the flow field can be divided into the interacting and film flow regions by the heat transfer distribution. In the interacting region, the heat transfer coefficient can be correlated to the liquid-film-flow heat transfer by using a heat-transfer enhancement coefficient defined as the ratio of the droplet flow rate to liquid film velocity. In the wall region, it can be predicted from the equation obtained from a previous study, which is very similar to that of turbulent heat transfer of single-phase flow. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 236–248, 1997     

Capillary column enhanced CHF microgravity flow boiling

Flow boiling heat transfer under microgravity conditions can be extended and enhanced by means of using porous stacks, or capillary columns, arranged on top of a flat heated surface. Under these conditions, body forces are negligible to remove the generated vapor away from the hot surface, which eventually hinders liquid from reaching it. It is possible to increase the critical heat flux (CHF) by having porous stacks symmetrically arranged on this surface; which draws the liquid phase towards it by means of capillary forces. Various flow regimes in the capillary enhanced surface flow boiling can be identified. These include: the regime where the liquid is supplied between the columns, the regime where the liquid flow is controlled by liquid capturing and the viscous drag-capillarity in the columns, and the critical heat flux. For the theoretical model, the expression for the interfacial lift-off model critical heat flux was interpreted based on customizable parameters instead of those imposed by the physics of the flow. This study indicates a potential improvement in CHF by having an inter-column spacing smaller than the critical wavelength for a plain surface. There is also a potential benefit of having the wetting contact to wavelength ratio to be larger than the constant of 0.2 found in experimental studies. The CHF regime can occur by a limitation of the stacks to have access to the liquid phase, as it happens when they are completely submerged in a vapor phase, or by reaching the maximum capillary pressure drop in the stack (as per the Darcy–Ergun momentum equation), or by reaching an entrainment limit of the vapor flow passed the capillary columns. Therefore the critical heat flux can also be extended as long as the capillary columns protrude over the vapor layer and their viscous capillary and entrainment limits are not reached.     

Evaporation heat transfer of liquid nitrogen on microstructured surface at high superheat level

Liquid nitrogen, as a coolant, is generally applied in cell vitrification cryopreservation. It takes heat from the carrier with cell samples through its violent evaporation on the carrier surface. As a result, the temperature of the carrier plunges dramatically. This article focuses on the unsteady evaporation heat transfer characteristics of liquid nitrogen on a microstructured surface etched into the frozen carrier surface at a high superheat level. The heat flux and evaporation heat transfer coefficient of liquid nitrogen were investigated using a lumped capacitance method. The experimental results showed that the cooling rate of the thin film evaporation on the microstructured surface is obviously higher than that of pool boiling, which is currently being used for cell cryopreservation. The heat flux and the evaporation heat transfer coefficient work together to present a parabolic trend with the superheat decreasing during this heat transfer process. Besides, the microstructure of the surface has an important effect on the evaporation heat transfer of liquid nitrogen. The larger the thin film evaporation zone is, the higher the heat transfer coefficient is. The current investigation results in a cell cryopreservation method through vitrification with relatively low concentrations of cryoprotectants.     

Augmentation of boiling heat transfer from horizontal cylinder to liquid by movable particles

This paper presents a series of experimental results on a passive augmentation technique of boiling heat transfer by supplying solid particles in liquid. A cylindrical heater 0.88 mm in diameter is placed in saturated water, in which a lot of mobile particles exist, and the nucleate and film boiling heat transfer characteristics are measured. Particle materials used were alumina, glass, and porous alumina, and the diameter ranged from 0.3 mm to 2.5 mm. Particles are fluidized by the occurrence of boiling without any additive power, and the heat transfer is augmented. The maximum augmentation ratio obtained in this experiment reaches about ten times the heat transfer coefficient obtained in liquid alone. The augmentation ratio is mainly affected by the particle material, diameter, and the height of the particle bed set at no boiling condition. The augmentation mechanism is discussed on the basis of the experimental results. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 28–41, 2002     

An experimental study on the quantitative interpretation of local convective heat transfer for a plate fin and tube heat exchanger using the lumped capacitance method

An experimental study has been performed to investigate the heat transfer characteristics of a plate fin and tube heat exchanger. Existing transient and steady methods are inappropriate for the measurement of heat transfer coefficients of the thin heat transfer model. In this study, the lumped capacitance method based on liquid crystal thermography was adopted. The method is validated through impinging jet and plate flow experiments. The two experiments showed very good agreements with those of the well-known transient method with the thick acryl model. And the lumped capacitance method showed similar results regardless of the thickness of the polycarbonate model if the Bi of the fin is small enough. The method was also applied for the heat transfer coefficient measurements of a fin and tube heat exchanger. Quantitative heat transfer coefficients of the plate fin were successfully obtained.     

Mass Transfer during gas absorption from a linear cluster of slugs in the presence of inert gases

A model of mass transfer during isothermal gas absorption in the presence of inert gas from a slug rising in a channel filled with liquid is suggested. The work studies the case of a small amount of soluble gas inside a slug and employs the approximation of a thin concentration boundary layer. Theoretical results of mass transfer analysis for a single gas slug are applied for determination of mass transfer rate in gas liquid slug flow. In the assumption of a perfect mixing of the dissolved gas in liquid plugs, recurrent relations for the dissolved gas concentration in the n-th liquid plug and mass flux from the n-th gas slug are derived. The mass transfer coefficient in gas-liquid slug flow is determined. In the limiting case the derived formulas for mass transfer in the presence of inert admixtures recover the obtained expressions for mass transfer during absorption of a pure gas. Theoretical results are compared with available experimental data.     

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

This paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ).DOI 10.1002/htj.10050     

A MONTE CARLO (MC) METHOD APPLIED TO THE MEDIUM WITH NONGRAY ABSORBING-EMITTING-ANISOTROPIC SCATTERING PARTICLES AND GRAY APPROXIMATION

A Monte Carlo (MC) method is applied to calculate radiative transfer in a nongray medium using spectral radiative exchange factor RD ij u . The creditability of the present MC model has been validated by comparing it with the results using other research methods. Meanwhile, the radiative transfer in an isothermal and nonisothermal medium with nongray absorbing-emitting-anisotropic ash particles is calculated by a nongray model and several gray approximation methods. A simplification from a nongray problem to a gray one by Rosseland's mean extinction coefficient, mean albedo y bar 2 , and Planck mean phase function is suggested.     

Liquid metal material genome: Initiation of a new research track towards discovery of advanced energy materials

As the basis of modern industry, the roles materials play are becoming increasingly vital in this day and age. With many superior physical properties over conventional fluids, the low melting point liquid metal material, especially room-temperature liquid metal, is recently found to be uniquely useful in a wide variety of emerging areas from energy, electronics to medical sciences. However, with the coming enormous utilization of such materials, serious issues also arise which urgently need to be addressed. A biggest concern to impede the large scale application of room-temperature liquid metal technologies is that there is currently a strong shortage of the materials and species available to meet the tough requirements such as cost, melting point, electrical and thermal conductivity, etc. Inspired by the Material Genome Initiative as issued in 2011 by the United States of America, a more specific and focused project initiative was proposed in this paper—the liquid metal material genome aimed to discover advanced new functional alloys with low melting point so as to fulfill various increasing needs. The basic schemes and road map for this new research program, which is expected to have a worldwide significance, were outlined. The theoretical strategies and experimental methods in the research and development of liquid metal material genome were introduced. Particularly, the calculation of phase diagram (CALPHAD) approach as a highly effective way for material design was discussed. Further, the first-principles (FP) calculation was suggested to combine with the statistical thermodynamics to calculate the thermodynamic functions so as to enrich the CALPHAD database of liquid metals. When the experimental data are too scarce to perform a regular treatment, the combination of FP calculation, cluster variation method (CVM) or molecular dynamics (MD), and CALPHAD, referred to as the mixed FP-CVM-CALPHAD method can be a promising way to solve the problem. Except for the theoretical strategies, several parallel processing experimental methods were also analyzed, which can help improve the efficiency of finding new liquid metal materials and reducing the cost. The liquid metal material genome proposal as initiated in this paper will accelerate the process of finding and utilization of new functional materials.     

Numerical analysis on heat transfer enhancement by waves on falling liquid film

IntroductionLiquid films flowing on a vertical or inclined wall bythe gravitational force are encountered in the wideindustrial and engineering fields['1, such as condensatefilm, evaporating falling film, gas absorb by liquid film,etc. In the case of Indnar film flow with smooth surface,heat transfer through the liquid film is mainly carried outby conduction, and it is sufficiently explained by theNusselt's theory. On the other hand, the heat transfer isfairly enhanced for films generating su…     

Heat transfer with flow and phase change in an evaporator of miniature flat plate capillary pumped loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop (CPL) evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.