土壤高温储热热湿迁移过程的初步研究

初步建立了土壤高温储热热湿迁移过程的数学模型,并进行了数值求解.结果表明,土壤热湿迁移过程中的湿度场稳定时间明显滞后于温度场,且与土壤类型和水力传导特性等有关.对于水力传导性较差的土壤,在高温储热初期,靠近热源的地方容易产生湿份聚集,使得湿度曲线出现一个短期峰值.与低温储热相比,土壤高温储热时湿度迁移对温度场的影响较大,温度场呈整体降低趋势,降低幅度顺序为:砂土>壤土>粘土.在该文模拟条件下,粘土的热湿迁移过程对于初始湿度的变化不敏感,而砂土的热湿迁移过程则依赖于初始湿度,且影响程度随着初始湿度的增加而增大.     

Developing a temperature‐dependent Kersten function for soil thermal conductivity

This paper describes the development of the Kersten function (Ke), which depends on soil temperature and the degree of saturation. The new Kersten function enables the prediction of thermal conductivities of moist soils also including high temperatures. The eight soils used in this paper represent three distinct textural groups, each having a comparable shape of Kersten function. The soil thermal conductivity is obtained from a linear interpolation between the dryness and saturation states with the Ke as the slope. The new Kersten function is valid when the degree of saturation (S_r) is greater than 0.125 and soil temperature is between 30 and 90°C. At a lower degree of saturation (i.e. 0<S_r<0.125), a linear approximation is applied to Ke. The new Ke function gives particularly good agreement with experimental data for temperatures of 30 and 50°C and for low moisture contents at all temperatures. At higher temperatures (i.e. 70 and 90°C) and moisture contents above the permanent wilting point, generally good or acceptable results were obtained. Copyright © 2000 John Wiley & Sons, Ltd.     

The relative importance of moisture transfer,soil freezing and snow cover on ground temperature predictions

Predicting ground temperature is an important part of the analysis of geothermal resources assessment and use. Thus, we develop and validate one-dimensional numerical model for heat and mass transfer in partially frozen soils. The model is implemented in HVACSIM Plus and used to simulate the thermal regime of soil profile. In addition to modeling heat conduction, model variations also includes moisture transfer, snow accumulation and melting, and soil freezing and thawing. The results are compared against experimental measurements of ground temperature for three locations in Montana, USA. The differences between simulated depth temperature with and without snow cover and freezing and thawing of soil reveal that ground temperatures are predominantly influenced by these two factors. Considering moisture transfer slightly improves temperature predictions, although it increases computational time by one order of magnitude. To balance computational efficiency with prediction accuracy, we propose an equivalent moisture content of 40–60% saturation in predicting ground temperature.     

Experimental investigation of thermal and moisture behaviors of wet and dry soils with buried capillary heating system

An experimental study of thermal and moisture behaviors of dry and wet soils heated by buried capillary plaits was done. This study was carried out on a prototype similar to an agricultural tunnel greenhouse. The experimental procedure consisted on three different measuring phases distinguished by three different operational conditions of the capillary plaits: heating at 70 °C, heating at 40 °C and without heating in summer. During an experimental run, quantities measured are soil temperature, soil water content at various depths, soil surface heat flux, solar radiation under the plastic cover, internal relative humidity, internal and external air temperature. In unsaturated moist soils, the transport of heat is complicated by the fact that heat and mass transfer is a coupled process. During the daily soil temperature variation, it was found that the surface temperature amplitude was higher in wet soil than in dry soil. The water content increased during daytime and decreased during nighttime. The diurnal variation amplitude of water content was higher without underground heating and decreased with the buried heat source temperature.     

Spatial variability of soil temperature under greenhouse conditions

The spatial variability of surface temperature has been examined under greenhouse conditions at Al-Mada'in Research station. Soil temperatures were measured at 14:00 h on 3 consecutive days after a trickle irrigation. Measurements were made every 0.5 m along a 55-m transect with copper–Constantan thermocouples. In addition, soil samples were collected to determine the thermo-gravimetric water content between the soil surface and 0.05 m depth. Cross semi-variograms and cross-correlation functions were determined and the measurements were found to be correlated over space.     

Cooled soil as a cooling source for buildings

Two approaches have been tested by the author for cooling soil in a given location to temperatures well below the “normal” temperatures in that location. The first approach has been tested in Sde Boqer Campus, in the Israeli Negev desert. The soil was covered with a layer of pebbles, about 10 cm. thick, and watered in the mornings. Substantial cooling of the soil was achieved. This cooling system was tested in test cells in Sde Boqer, Israel, and in a full scale room in Riyadh, Saudi Arabia, and has provided effective cooling. The second approach was tested at A&M University in Tallahassee, Florida. Temperature measurements were taken of moist soil under a wooden shack on stilts raised about 60 cm above the ground. Thus the soil under it was permanently shaded. The cooled soil temperatures were below the outdoors’ minimum temperatures, even during the peak of the summer, providing a potential heat sink for cooling buildings. The thermal performance of an earth covered building in the Negev arid region of Israel is also summarized.     

Numerical investigation into mutual effects of soil thermal and isothermal properties on heat and moisture transfer in unsaturated soil applied as thermal storage system

In this article, a numerical model for simulation of the heat and mass transfer phenomena in unsaturated soil is presented. The system is provided to evaluate its behavior as a thermal storage system. The model includes a single pipe that is buried under unsaturated media which is connected to a source with the constant heat rate of 20 W/m³ in sandy soils during a specific time. All the numerical results show good agreement with previous results, thus excellent correlation was achieved. Additionally, sensitivity analysis is done to clear up the role of the thermal and isothermal properties respect to volumetric water content and temperature and to assess the effects of related parameters on heat transfer in the media. The results show that as getting far away from the source, in addition to reduce the collected energy, the intensity of increasing the temperature and decreasing the moisture diminished will be more. Generally over time, the cumulative stored energy for each point will tend to a constant quantity.     

Investigation of heat and moisture migration in unsaturated soil under natural boundary conditions

Theoretical and experimental investigations were conducted to determine the heat and moisture migration in unsaturated soil under natural surface boundary conditions. Theoretically, a new model of heat and moisture migration in unsaturated porous media was developed, in which the gradients of volume water content, temperature, and partial vapor pressure were considered as the main driving forces which influence the process of heat and moisture migration in unsaturated soil. A set of coupled, nonlinear, partial differential equations were developed, which are related dynamically to the surface boundary conditions. Heat and moisture migration in sandy soil under solar radiation and air convection were studied experimentally. Temperature, volume water content, and water table evaporation were measured under unsteady conditions. The predictions are in good agreement with experimental data from a fairly sandy soil. © 1999 Scripta Technica, Heat Trans Asian Res, 28(1): 3–17, 1999     

土壤热湿传递下的直埋电缆载流量数值计算

针对土壤热湿过程对直埋电缆载流量的影响,修正土壤热湿耦合模型,联合MAXWELL方程组,建立电磁-热湿三场耦合模型。利用三场耦合模型和COMSOL Multiphysics软件按正交法计算不同敷设土壤类型、干密度和粒径下的YJV22 0.6/1 3×6直埋电缆载流量。通过对比正交计算结果的最大、最小载流量组的磁通密度、发热量、温度分布、含水率分布等,发现大载流量组的缆芯发热量较高,但电缆外表皮温度较低,使得直埋电缆内部热流密度较大,同时电缆周围土壤的导热系数和含水率较高,发生的湿分运移和相变较为微弱,使得缆芯的热量及时被导出。因此,土壤中的湿分运移和相变对于直埋电缆的载流量有一定提升作用。     

Potential solar earth-water distillate yields in Africa

A model for predicting solar earth-water distillate yield for soil moisture contents up to saturation is presented. The model developed by join-point analysis for a 20 cm tall solar still with reflective interior siding is: Water Yield = C1(SR) + C2(T_MIN) + C3(T_MAX) − 30.65 + C4(T_MAX − 30.65) + C5(MC − 8.0 + C6(MC − 8.0 + I Where SR, T_MAX, T_MIN, and MC represent the total daily solar radiation, maximum and minimum daily temperatures, and soil moisture content, respectively. C1, C2, C3, C4, C5, and C6 are the regression coefficients in the predictive model, and I is the intercept. The model indicates that maximum yield can be obtained at 8% soil moisture. The equation is used to predict potential earth-water distillate yields for 8 locations in Africa. Four levels of soil moisture content (5, 10, 15, and 20% by dry weight), and 50% and 100% of clear-day solar radiation and appropriate temperature values are used. For the four tested soil moisture contents the predicted daily earth-water yields vary from a minimum of 0.56 1 m⁻²-day⁻¹ at 5% soil moisture and 50% solar radiation to a maximum of 2.12 1 m⁻²-day⁻¹ at 10% soil moisture and 100% solar radiation. Distillate yields increase as soil moisture content increases from 5 to 10%. Above 10% soil moisture, the earth-water yield decreases as the moisture content increases. Distillate yield varies with soil moisture in the following manner: Y_10% > Y_15% > Y_20% > Y_5%, where Y is the predicted yield.     

Enhancement of the cubic cell soil thermal conductivity model

An analysis of measured soil thermal conductivity (λ) data for temperatures (T) varying from 5 to 90°C, was conducted with respect to four soil moisture content domains, i.e. residual, transitory meniscus, micro/macro porous capillary, superfluous. It was shown that each domain has a specific behaviour of λ vs soil moisture content (θ). For example, λ varies insignificantly with θ and T at very low moisture contents (residual moisture domain). In the transitory meniscus and micro/macro porous capillary domains, the relation λ (θ) shows in general a nonlinear behaviour, which is difficult to model, particularly at high T. A sensitivity analysis applied to the Gori (1983) model for dry soil showed better predictions when the model was restricted to the use of the first term only (dependent on soil porosity and thermal conductivity of air). Two linear λ approximations have been tested, across the second domain (from a critical θ to the permanent wilting point) and across the second and third domains (from a critical θ to field capacity). The enhanced model has been tested against soil λ data measured at moderate and high T. The numerical results show considerably improved predictions in the first three soil moisture domains. The first linear λ interpolation shows better agreement with experimental data for T up to 65°C, while the second interpolation was much more beneficial at higher T. The original Gori model gives generally the best predictions in the superfluous domain. Copyright © 2002 John Wiley & Sons, Ltd.     

Identification of temperature and moisture content fields using a combined neural network and clustering method approach

Studies on the dynamics of temperature and moisture content distributions in porous soils have provided important insight on their effect on the building hygrothermal behavior, where the interaction between both building and soil can contribute to reduce building thermal gains or looses. Hygrothermal aspects can be related to many attributes such as energy consumption, occupants' thermal comfort and health, and material deterioration. Recently, a great variety of mathematical models to predict thermal and moisture content profiles in porous media have been presented in the literature. Most of those models are based on analysis of multilayer measurements or on Fourier analysis. The development and validation of such mathematical models facilitate the understanding of heat and moisture flows at different soil depths. In this research, a radial basis function neural network (RBF-NN) approach, combined with Gath–Geva clustering method in order to predict the temperature and moisture content profiles in soils, has been presented. A set of data obtained from the computation of the coupled heat and moisture transfer in porous soils for the Curitiba city (Paraná State, Brazil) weather data file has been used by the RBF-NN modeling method. Simulation results indicate the potentialities of the RBF-NNs to learn, for the one step ahead identification, the behavior of temperature and moisture content profiles in the media at various depths.     

Performance study on heat and moisture transfer in soil heat charging

In some cold areas, the system performance of the soil source heat pump system is reduced by the decreasing underground soil temperature, which is caused by the thermal imbalance between the heating demand in winter and the cooling demand in summer. Soil heat charging with solar energy in non-heating seasons is proposed for solving the problem. It has been found from previous studies that the effect of the moisture transfer on the heat transfer within porous media could not be neglected especially under higher temperature difference. Therefore, this paper provides an investigation on the heat and moisture transfer in soil during soil heat charging at high temperature. A numerical model is developed for the study. The simulation results are compared with the testing data from the authors' previous study for the model verification. Based on the verified model, the performance of the heat and moisture transfer in soil during soil heat charging in a longer time and a larger area is investigated in the paper. The results show that the testing data match very well with the simulation results within a relative error of ±9% and the mathematical model is reliable for the performance prediction of heat and moisture transfer in soil heat charging. The soil volumetric water content (VWC) distribution tends to be stable after soil heat charging for 13 days and the heat source has an effective influence on soil VWC distribution within 2.4?m. The effect of the heat source temperature and initial VWC on the soil temperature and VWC distribution and heat power is proved to be obvious. Loam has a better performance in soil heat charging than sand.     

Experimental Studies for Convective Drying of Potato

An experimental facility was built at the Indian Institute of Technology Delhi in order to examine the characteristics of convective drying of a moist object. The test facility consists of an inlet section, a divergent and convergent section, a settling chamber, a test section, and an outlet section. Initial moisture content and time-dependent moisture content of a rectangular shaped moist object (4 cm × 2 cm × 2 cm) are measured by this test facility. The potato slice was selected as a sample moist object. Moisture content was measured at different air temperatures of 40, 50, 60, and 70°C with an air velocity of 2 m/sec. The density of potato slice was determined for various drying temperatures. The volume shrinkage during drying decreased almost linearly with moisture content. The percentage air pores and porosity increased gradually with decreasing moisture content and increasing drying air temperature. Volumes of water, air, and solid content of potato were determined at different drying air temperatures. The results are validated with theoretical data.     

地源热泵间歇运行下土壤热湿迁移的实验研究

建立地埋管换热土壤热湿迁移过程的实验装置,对地源热泵间歇运行时不同进口流体温度及不同土壤体积含水率下土壤温湿度场的变化特性进行实验研究。实验结果表明∶间歇运行时,入口流体温度的升高会使土壤温度最大值升高,但不利于土壤温度的恢复,土壤体积含水率的增加在一定程度上有利于地下换热和土壤温度的恢复。系统开机后存在土壤温度上升的主上升区,此区温度增幅超过65%,关机后第18小时土壤温度基本恢复至初始温度;系统关停后在温湿度梯度的作用下会出现温度和含水率最大值后移的现象,热源对土壤温度和含水率的作用半径约为280和375 mm;开停比为1∶2时温湿度较1∶1能恢复得更低,合理设置停机时间有利于机组长期有效运行。     

Inter‐particle contact heat transfer in soil systems at moderate temperatures

An inter‐particle contact heat transfer model for evaluating soil thermal conductivity is analysed with respect to soils, representing different textural classes, exposed to moderate temperatures ranging from 15 to 30°C. This model is a combination of a self‐consistent approximation model, enhanced with an inter‐particle contact heat transfer correction coefficient. For dry and saturated soils, this coefficient is defined as a ratio of a soil harmonic mean thermal conductivity of solid and fluid (air or water) phases, to the average thermal conductivity of soil solid grains. For unsaturated soils, we assume a linear interpolation of the correction coefficient between absolutely dry and saturated states, with a Kersten function (Ke) as a proportional factor. The strongest impact of the correction coefficient (maximum reduction of heat transfer) is observed for coarse soils below a critical value of saturation degree (S_r‐cr–corresponds to Ke?0) followed by medium and fine soils. For S_r>S_rcr, the reduction of heat transfer gradually diminishes as S_r approaches 1 (i.e. saturated state). Soil texture, soil specific surface area, porosity and mineralogical composition (particularly quartz content) are important factors influencing the heat transfer correction coefficient. Their influence appears to be more substantial at the lower half of the wetness range (S_r<0.5). Simulation results from the new enhanced model closely follow experimental data.     

河北南部平原不同土壤质地下土壤墒情动态预报

土壤墒情预报是农田灌溉和区域水资源管理的重要基础性工作。为更加准确地预报土壤墒情,通过经验统计法,以河北南部邯郸市为例,基于该地区测站近十年的土壤墒情监测资料,以壤土、粘土两种土壤质地类型作为区分依据,分别建立土壤墒情动态预报模型,并将预测值与实测值进行对比验证。结果表明,模型预报具有较高精度,达到了甲级预报水平,能够准确预测两种土壤质地下土壤墒情情况。该成果对合理预测土壤墒情,指导农业生产,促进水资源合理利用具有现实意义。     

Effect of process parameters and raw material characteristics on physical and mechanical properties of wood pellets made from sugar maple particles

The aim of the current study was to investigate the influence of process parameters and raw material characteristics on physical and mechanical properties of wood pellets made from particles of sugar maple trees of different vigor. Pellets were made in a single pelletizer while controlling temperature (75, 100 and 125 °C), moisture content (8.1, 11.2 and 17.2%), compression force (1500, 2000 and 2500 N) and particle size (<0.25, 0.25–0.5 and 0.5–1.0 mm). Particle size was the most important factor influencing friction in the die, followed by moisture content, compression force and temperature. Moisture content was the most important factor affecting pellet density, followed by temperature, compression force and raw material particle size. Temperature was the most important factor for pellet compression strength, followed by compression force, particle size and moisture content. Friction in the die decreased with increasing particle size and moisture content of the material and increased with increasing compression force. It decreased initially with increasing temperature from 75 °C to 100 °C, and then increased with temperature. Density and strength of pellets increased with temperature and compression force, decreased with increasing particle size, and decreased with increasing moisture content. Pelletizing should be performed at 100 °C to minimize friction and a moisture content of 11.2% to maximize density and compression strength of the pellets. Wood particles from sugar maple trees of low vigor were more suitable for making wood pellets in terms of friction in the pelletizer and compression strength than those from vigorous trees.     

Effects of soil texture on germination and survival of non-toxic Jatropha curcas seeds

Toxic oil seeds of Jatropha curcas have been widely propagated in tropical and subtropical regions for biofuel production. However, very little is known about the non-toxic seeds of J. curcas and their germination. This paper describes the germination and survival of non-toxic J. curcas seeds over two consecutive years. Non-toxic seeds native from southeastern Mexico (600–800 mg weight) were sown in three soils with different texture (sandy; sandy-loam and clay-loam) in order to assess germination, speed of germination and survival rates of the emerged seedlings. Sandy soil had the lowest organic matter (OM) content with 1.68 g-kg⁻¹ of dry soil, followed by sandy-loam soil (39 g-kg⁻¹) and clay-loam soil with the highest OM (72.63 g-kg⁻¹). The highest germination rate was obtained in sandy-loam (76%), followed by sandy (75%) and clay-loam soil (24%). The highest survival rates were obtained in sandy (99%) and sandy-loam (99%) soils followed by clay-loam soil (87%). The highest average speed of germination index was recorded in sandy (155), followed by sandy-loam (125) and clay-loam soil (23). It can be concluded that sandy and sandy-loam soil textures, with bigger pore size and low organic matter content, were the more suitable substrates to germinate non-toxic J. curcas seeds; clay loam as substrate was not suitable for non-toxic J. curcas seeds due to the low germination rate and speed.     

Evaluation of the underground soil thermal storage properties in Libya

Experimental investigation was conducted of temperature distribution through the underground soil of Tripoli (Capital of Libya). The aim of the experiment is to monitor the temperature variation of the underground soil under a depth of 4 m and around the year, in order to know the thermal capacity ability of the soil to be used as a seasonal thermal storage. The measurements covered two types of systems: the first one is dry soil and the second is dry soil covered by a glass sheet. The measurements indicate that, at a depth of 4 m, the average temperatures for the dry and dry-glass covered systems are 21, 46 °C, with maximum temperatures of 21.5 and 47 °C during December and January, and the minimum temperatures occurred in May and June, are reached values of 19, 44 °C, respectively. The temperatures for the two systems were almost constant through the year and fluctuating with a monthly period of 2π/12. Results show that, the underground thermal capacity can be used as a source of heating and cooling of buildings leading to reduce the energy consumption in this application. Furthermore, for industrial and domestic heating processes, one can utilize the dry-glass covered system to cover a significant part of the heating load. Anyhow, the experimental study may not applicable everywhere, so an analytical presentation for the system will be necessary to save money and efforts. The first step to put the analytical model in reality is to get the thermal properties of the underground soil, and this is the aim of the present study.The paper described the followed procedure during theoretical-heat transfer approach. The thermal properties were presented as a function of the ground depth, furthermore, the paper presented the measured temperatures of the two systems for Tripoli underground soil.