Analysis of the Moisture and Heat Transport in a Wet Porous Sand Layer with Water Sucking Ability

The moisture content and heat transfer in a wet porous sand layer was influenced by the weather conditions while the layer was exposed outdoors. The changes in water content and temperature in the wet porous bed with water supplied due to the weather conditions differed from the case without water supplied via the bottom in the tests. A one‐dimensional mathematical model describing the heat and mass transfer in the unsaturated porous layer was used to analyze the change of the water content, temperature and rate of water evaporation or vapor condensation in the wet porous layer. As the ambient temperature, relative humidity and solar irradiation changed periodically and the gravity and capillary affected the water transport greatly in the wet porous media, variations of water content and temperature occurred cyclically in the wet porous layer that was exposed to outdoor conditions. In the wet porous bed, the rate of the water evaporation or vapor condensation was closely related to the temperature, gradient of the temperature along the depth and the rate of temperature variation over time. The particle size and porosity associated with the permeability had great impact on the water content and its variation range in the wet porous media with water sucking ability while the weather conditions changed periodically. The simulations of the water content and temperature variation in the sand bed agreed with the test data. All these results can be used to analyze the behavior of heat and moisture in the unsaturated porous layer under weather conditions.     

Experimental study on the impact of mass moisture content on the evaporative cooling effect of porous face brick

Porous face bricks on the outside of a building wall exhibit evaporative cooling after absorbing water. Thus, these bricks are promising for use as a passive energy-saving building technology. Artificial watering is an effective method to ensure a sufficient water supply for effective evaporative cooling. However, improper watering measures may result in the waste of water resources while failing to achieve the evaporative cooling effect. In this paper, studies were conducted on a composite wall composed of a facing layer and a base layer as follows. (1) The mass moisture content variations in the porous face brick with respect to soaking time were measured, and the feasibility of using this type of face brick as an evaporative cooling carrier was investigated. (2) The relationship between mass moisture content and the evaporation capacity of the porous face brick was determined under stable conditions. A critical mass moisture content for the porous face brick was determined through analysis of the measurement data. (3) A field measurement was performed to verify the feasibility of using the critical mass moisture content as a criterion for watering. The results indicate that the temperature and heat flows at the inner and outer surfaces of the composite wall were reduced significantly. However, when the mass moisture content exceeded the critical mass moisture content, the evaporative cooling capability tended to stabilize with further watering. Therefore, the critical mass moisture content of the porous face brick can be used as a criterion for watering to conserve water while facilitating the evaporative cooling effect.     

太阳辐照下湿份分层土壤中热湿传输的数值模拟

考虑温度对土壤湿分迁移的影响，建立描述存在干饱和层时的土壤热湿传递的数学模型，并就自然环境和恒定太阳辐照下两种情况进行数值模拟，获得不同环境条件下土壤中温度和湿分分布以及水分蒸发的动态特性，分析干饱和土壤层对土壤热湿迁移与水分蒸发以及温度对土壤湿分传输的影响。     

Cooling enhancement in an air-cooled finned heat exchanger by thin water film evaporation

A theoretical analysis on the cooling enhancement by applying evaporative cooling to an air-cooled finned heat exchanger is presented in this work. A two-dimensional model on the heat and mass transfer in a finned channel is developed adopting a porous medium approach. Based on this model, the characteristics of the heat and mass transfer are investigated in a plate-fin heat exchanger with the interstitial surface fully covered by thin water film. Assuming that the Lewis number is unity and the water vapor saturation curve is linear, exact solutions to the energy and vapor concentration equations are obtained. The cooling effect with application of evaporative cooling was found to be improved considerably compared with that in the sensible cooler. This is because the thermal conductance between the fin and the air increases due to the latent heat transfer caused by the water evaporation from the fin surface. It is also found that the cooling enhancement depends greatly on the fin thickness. If the fin is not sufficiently thick, the cooling enhancement by the evaporative cooling decreases since the fin efficiency drops considerably due to the water evaporation from the fin surface. The fin thickness in the evaporative cooler should be increased larger than that in the sensible cooler to take full advantage of the cooling enhancement by the water evaporation.     

自然环境下湿分分层土壤中热湿迁移规律的研究

建立描述存在干饱和层时的土壤热湿传递的数学模型并进行数值模拟，获得自然环境下土壤中温度、湿分分布以及水分蒸发的动态特性，分析干饱和土壤层对土壤热湿迁移及水分蒸发的影响。数值模拟获得实验支持。     

Study of heat and mass transfer in a dehumidifying desiccant bed with macro-encapsulated phase change materials

The present article reports on the feasibility of using encapsulated phase change materials (EPCMs) in the dehumidifying bed of a desiccant cooling system. The mathematical model used to simulate the coupled non-equilibrium heat and moisture transfer processes in the porous composite structure containing the EPCM and desiccant particles is presented. Numerical investigations of heat and mass transfer in a desiccant dehumidifying bed composed of silica gel and EPCM particles have been carried out for different values of process parameters. Careful choices of EPCM volume fraction and thermo physical characteristics have been found to increase the overall effectiveness of the desiccant dehumidifier with negligible loss in the dehumidification efficiency. The air stream exits the desiccant/EPCM bed at relatively lower temperature and slightly higher moisture content than from purely desiccant bed. Desiccant cooling systems with less sensible heating and higher cooling capacity can be obtained by employing EPCM in the dehumidifier.     

多孔介质蒸发制冷板应用于建筑墙体的性能分析

把含湿多孔介质置于建筑墙体结构中,在室外环境的作用下,多孔介质外表面和内部的水分蒸发吸热,为房间提供部分冷量。文章表述将含湿多孔介质置于建筑北墙中,在建立多孔介质热湿平衡的基础上,采用描述非饱和多孔介质热质迁移的数学模型,分析了室外环境参数及多孔床层结构对床层内温度分布、蒸发量场及水蒸气迁移的影响,为多孔介质应用于建筑结构的推广和应用提供理论指导。     

高水分燃料的沸腾层烟气热平衡方程

以树皮、褐煤、I类烟煤为例,分析了沸腾层内未燃烧燃料中的水分蒸发吸热对埋管吸热份额和沸腾层温度的影响得到了高水分燃料的沸腾层烟气热平衡方程。计算和分析表明,对高水分燃料,沸腾层内未燃烧燃料中的水分蒸发吸热所需热量较大,在沸腾层烟气热平衡方程中应予考虑。     

Study of heat and moisture transfer in soil with a dry surface layer

Mathematical model for describing simultaneous heat and moisture transfer in the porous soil with a dry surface layer was developed by using the volume-averaging method. Numerical simulation was conducted to investigate water evaporation, transient distributions of temperature and moisture in the porous soil at environmental conditions, which might be useful for agricultural application. In order to validate the mathematical model and numerical method, an experiment was conducted under natural environmental conditions. An additional experiment was conducted in a closed-loop wind tunnel to investigate the temperature effect on soil moisture transport. Theoretical and experimental results indicate that the dry surface layer has an important effect on heat and moisture migration in soil and the influence of temperature on moisture transport in unsaturated soil is significant.     

Influence of moisture content on measurement accuracy of porous media thermal conductivity

The thermal conductivity measurement accuracy of sand was experimentally studied with a hot disk thermal constant analyzer and water morphologies, distribution, and evolution at the pore scale were observed with a charge coupled device (CCD) combined with a microscope. It was found that thermal conductivities of samples with low moisture content (<25%) could not be accurately measured. For samples with low moisture content, the analysis showed that the water in the region adjacent to the analyzer sensor mainly existed as isolated liquid bridges between/among sand particles and would evaporate and diffuse to relatively far regions because of being heated by the sensor during measurement. Water evaporation and diffusion caused the sample constitution in the region adjacent to the sensor to vary throughout the whole measurement process, and accordingly induced low accuracy of the obtained thermal conductivities. Due to high water connectivity in pores, the rate of water evaporation and diffusion in porous media of high moisture content was relatively slow when compared with that of low moisture content. Meanwhile, water in the relatively far regions flowed back to the region adjacent to the sensor by capillary force. Therefore, samples consisting of the region adjacent to the sensor maintained the constant and thermal conductivities of porous media with relatively high moisture content and could be measured with high accuracy. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20272     

Numerical study of the thermal–hydraulic performance of evaporative natural draft cooling towers

In this paper, the mathematical and physical models governing the flow, mass and heat energy of moist have been set up for an evaporative natural draft cooling tower. The models consider the effect of non-spherical shape of water drops on the flow, heat and mass transfer. Experimental data has been adopted to validate the numerical scheme. Average difference between the measured and the predicted outlet water temperature is 0.26°C. Distributions of the velocity components of the moist air, density, pressure, enthalpy and moisture content, the water temperature and its mass flux have been predicted. The simulation shows that some recirculation exits under the lower edge of the shell, where the air enthalpy, temperature, humidity and moisture content are higher, but the density is lower. The simulation also proves that the main transfer processes take place in the fill region where the percentage of latent heat transfer is predicted as 83%. However, about 90% of the heat energy is transferred via evaporation in the rain region although the total heat transfer rate there is very small compared to the fill region. Hourly performance of a natural draft cooling tower under the meteorological condition of Singapore has also been predicted.     

Thermal analysis on the cooling performance of a porous evaporative plate for building

In this paper, a wet porous cooling plate has been used for a building wall. Cooling can be achieved due to the evaporation in the porous layer. A mathematical model on the heat and mass transfer in the unsaturated porous media is developed to analyze the influences of ambient conditions and the porous layer thickness on the cooling performance of the porous evaporative plate. With a decrease in ambient relative humidity and an increase in ambient temperature, more cooling of the porous evaporative plate can be supplied for the inside of the room. The heat exchange between the inside surface of the porous plate and the air in the room should be intensified to achieve a higher cooling efficiency of the porous plate. The ambient wind speed and the thickness of the porous plate also have significant influence on the average temperature of the porous plate. All these results should be taken into account for the utilization of the porous evaporative cooling plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20284     

Moisture transport in silica gel packed beds—I.Theoretical study

Diffusion mechanisms of moisture within silica gel particles are investigated. It is found that for microporous silica gel surface diffusion is the dominant mechanism of moisture transport, while for macroporous silica gel both Knudsen and surface diffusions are important. A model is proposed for simultaneous heat and mass transfer in a thin packed bed of desiccant particles, which accounts for diffusion of moisture into the particles by both Knudsen and surface diffusions. Using finite difference methods to solve the resulting partial differential equations, predictions are made for the response of thin beds of silica gel particles to a step change in air inlet conditions, and compared to a pseudo-gas-side controlled model commonly used for the design of desiccant dehumidifiers for solar desiccant cooling applications.     

1 kWe sodium borohydride hydrogen generation system: Part II: Reactor modeling

Sodium borohydride (NaBH₄) hydrogen storage systems offer many advantages for hydrogen storage applications. The physical processes inside a NaBH₄ packed bed reactor involve multi-component and multi-phase flow and multi-mode heat and mass transfer. These processes are also coupled with reaction kinetics. To guide reactor design and optimization, a reactor model involving all of these processes is desired. A one-dimensional numerical model in conjunction with the assumption of homogeneous catalysis is developed in this study. Two submodels have been created to simulate non-isothermal water evaporation processes and pressure drop of two-phase flow through the porous medium. The diffusion coefficient of liquid inside the porous catalyst pellets and the mass transfer coefficient of water vapor are estimated by fitting experimental data at one specified condition and have been verified at other conditions. The predicted temperature profiles, fuel conversion, relative humidity and pressure drops match experimental data reasonably well.     

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.     

植物布置模式对光伏组件和屋面被动降温及水分蒸发的影响

绿色光伏屋顶是一种光伏组件遮阳与植被绿化相结合的隔热屋顶,为了揭示植物布置模式对光伏组件和屋面降温的影响,提出了2种植物布置模式并建立了2个混凝土屋顶试验小室。分别对光伏组件背面和屋面上表面温度、植物叶片温度和蒸发量进行测试和对比分析。结果表明：1）相对于植物架空布置模式,植物直接实铺在屋面上可有效降低屋面上表温度平均值和波幅值;2）植物与光伏组件背面距离增加时会减弱其对光伏组件的冷却效果,间距为10 cm时冷却效果较为明显;3）植物与光伏组件背面距离变小时,叶片温度会变高,水分蒸发量变多。因此,植物直接实铺模式有利于绿植光伏屋面节水和节能。     

Experimental study on 75 kWth downdraft (biomass) gasifier system

Experimental study on 75 kW_th, downdraft (biomass) gasifier system has been carried out to obtain temperature profile, gas composition, calorific value and trends for pressure drop across the porous gasifier bed, cooling–cleaning train and across the system as a whole in both firing as well as non-firing mode. Some issues related to re-fabrication of damaged components/parts have been discussed in order to avoid any kind of leakage. In firing mode, the pressure drop across the porous bed, cooling–cleaning train, bed temperature profile, gas composition and gas calorific value are found to be sensitive to the gas flow rate. The rise in the bed temperature due to chemical reactions strongly influences the pressure drop through the porous gasifier bed. In non-firing mode, the extinguished gasifier bed arrangement (progressively decreasing particle size distribution) gives much higher resistance to flow as compared to a freshly charged gasifier bed (uniformly distributed particle size). The influence of ash deposition in fired-gasifier bed and tar deposition in sand filters is also examined on the pressure drop through them. The experimental data generated in this article may be useful for validation of any simulation codes for gasifiers and the pressure drop characteristics may be useful towards the coupling of a gasifier to the gas engine for motive power generation or decentralized electrification applications.     

Experimental method for determining Forchheimer equation coefficients related to flow of air through the bales of soy straw

The paper presents results of experimental investigation conducted to determine permeability coefficient of a porous bed formed by biomass bales, whereby the said coefficient represents one of the most important parameters of the bed material specified when considering its combustion in the pusher-type furnaces (i.e. combustion of biomass bales in cigar burners). Correlations obtained are deemed very important for optimization of biomass bale combustion, as well as for modeling transport phenomena occurring in the porous bed formed by biomass bales during their combustion in cigar burners.In order to determine permeability coefficient of air flow through soy straw bales, appropriate experimental method was developed and suitable experimental apparatus designed and constructed. The relevant coefficients were determined in such manner as to be representative of the viscous and inertial terms of the Forchheimer equation. Experimental investigation conducted examined the effects of relevant biomass bale parameters i.e. porosity and the moisture content on the abovementioned Forchheimer coefficients. Measurements conducted, as well as analysis of the results obtained, defined functional dependencies of the permeability coefficients on the porosity (which was varied in the range 0.62–0.78, for moisture content of 0.0948) and the sample’s moisture content (which was varied in the range 0.0948–0.6394, for the selected value of porosity of 0.68) to be established. The correlations obtained are deemed suitable to be used in a broad spectrum of operating conditions that usually occur during real operation.Results obtained indicate that reduction in biomass bed porosity results in reduced permeability, with Forchheimer coefficients exhibiting a hyperbolic dependence on the bed porosity. Since it has been observed that increased moisture content of the sample (porosity value of 0.68) caused a decrease in permeability, a functional dependence of the sample moisture content on the porosity has been established. The dependence established is valid for the range of the sample moisture content between 0.0948 and 0.6394.Permeability coefficients, as well as their dependencies on the relevant factors (porosity and moisture content), represent basic data used in mathematical modeling of biomass bale combustion in cigar burners. In addition, the ones may also be very important when considering modifications of furnaces utilizing the combustion principle mentioned.     

Experimental investigation of combustion in porous heating burners

The combustion characteristics of liquefied petroleum gas inside porous heating burners have been investigated experimentally under steady-state and transient conditions. Cooling tubes were embedded in the postflame region of the packed bed of a porous heating burner. The flame speed, temperature profile, and [NO_x] and [CO] in the product gases were monitored during an experiment. Due to the heat removal by the cooling tubes, a phenomenon termed metastable combustion was observed; this is that only one flame speed exists at a particular equivalence ratio for maintaining stable combustion within the porous bed of the porous heating burner. This behavior is quite different from that of porous burners without cooling tubes, in which an extended range of flame speeds usually is found for maintaining stable combustion. After metastable combustion has been established in a porous heating burner, a change in the equivalence ratio will stop the metastable combustion and drive the flame out of the packed bed. From the steady-state results, the porous heating burner was shown to maintain stable combustion under fuel-lean conditions with an equivalence ratio lower than the flammability limit of a normal free-burning system. The flame speed in a porous heating burner was found to decrease with an increase in the length of the porous bed. Combustion within a porous heating burner has the features of low flame temperature, extended reaction zone, high preheating temperature and low emissions of NO_x and CO. The flame temperature ranged from 1050 to 1250 °C, which is ∼200 °C lower than the adiabatic flame temperature at the corresponding equivalence ratio. The length of the reaction zone could be more than 70 mm and the preheating temperature ranged from 950 to 1000 °C. Both [NO_x] and [CO] were low, typically below 10 ppm.     

多孔介质中的相变传热特性及其在建筑物节能中的应用

将非饱和多孔介质应用于建筑节能，使低品位，低密度的太阳能通过工质的相变得以利用，对实现建筑物的采暖具有应用价值。通过对非饱和多孔材料内的二维流场，温度场以及蒸发量场的数值计算，分析了两端开口的圆柱环型多孔腔内非饱和多孔介质的传热传质特性，研究了流体雷利数及边界条件的变化对多孔床热质迁移的影响。