Soil Thermal Diffusivity Research Articles

Temperature variations in unfrozen soils with variable hydrothermal properties

SummaryA comprehensive understanding of the hydrothermal properties of soil is required to model heat distribution in unsaturated soils. In this study, we aim to model heat distribution throughout the profile of unfrozen soil while its thermal diffusivity varies with time and depth. The proposed model is based on the fundamental solution of the one‐dimensional transient heat conduction equation using the decomposition method. We calibrate our model using experimental data from soils of different textures in the literature. The new model can estimate soil thermal diffusivity at different depths and times and uses easily accessible characteristics such as the degree of saturation and the texture of the soil. In this study, the performance of the new model is compared to the performance of the simplified model in which constant thermal diffusivity is considered throughout the profile. Moreover, the model is validated by comparing it with in‐situ temperature measurements within depth of soil profiles with different textures. The results show a very good agreement between the predicted and the measured temperature throughout the soil profiles. Such a validation shows that with increasing degree of soil saturation, depletions in temperature for fine‐textured soils are more significant than those for coarse‐textured soils. Finally, the new model is applied to a double‐layer soil in the Alsace region to define temperature variation in the profile of soil with different characteristics in each layer. For a double‐layer profile, the continuity of the temperature as well as the heat flux is verified at the interface between the two layers.

Transient effect of soil thermal diffusivity on performance of EATHE system

Abstract This paper presents effect of thermo-physical properties of soil on performance of an Earth Air Tunnel Heat Exchanger (EATHE). The analysis has been carried out using a validated three-dimensional, transient numerical model for three different types of soil. The governing equations, based on the k – e model and energy equation were used to describe the turbulence and heat transfer phenomena, are solved by using finite volume method. Comparisons were made in terms of temperature drop, heat transfer rate and COP of the EATHE system by operating it continuously for 12 h duration. The study reveals that each soil exhibits different rate of heat dissipation and thermal saturation over a period of continuous operation, which adversely affects the performance of EATHE. Dissipation of heat from the EATHE pipes to its surrounding soil and subsequently to the outer subsoil region is mainly found to be depending upon the thermal conductivity of soil; even of their thermal diffusivity is of different order.

植被对土壤热扩散特征的影响研究——以长白山阔叶红松林为例

PDF HTML阅读 XML下载 导出引用 引用提醒 植被对土壤热扩散特征的影响——以长白山阔叶红松林为例 DOI: 10.5846/stxb201308172098 作者: 作者单位: 南京信息工程大学,中国科学院沈阳应用生态研究所,中国科学院沈阳应用生态研究所,中国科学院沈阳应用生态研究所,沈阳市水利建筑勘测设计院,南京信息工程大学 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目(41105112, 41105077); 南京信息工程大学生态气象环境研究中心科研基金; 江苏省自然科学基金面上项目(BK2012859); 江苏高校优势学科建设工程(PAPD) The influence of vegetation on soil thermal properties: a case study of broadleaved Korean pine forest in Changbai Mountain Author: Affiliation: International Center for Ecology,Meteorology and Environment,School of Applied Meteorology,Nanjing University of Information Science and Technology,Institute of Applied Ecology,Chinese Academy of Sciences,Institute of Applied Ecology,Chinese Academy of Sciences,Institute of Applied Ecology,Chinese Academy of Sciences,Shenyang Hydraulic Construction Survey and Design Institute,International Center for Ecology,Meteorology and Environment,School of Applied Meteorology,Nanjing University of Information Science and Technology Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤温度变化及热传递是影响土壤和大气水热交换的重要过程,而植被是决定这种变化和影响的环境因子之一。通过比较林地与裸地土壤热特性的差异分析植被对土壤热扩散的影响。 研究内容包括观测2007年长白山阔叶红松林区不同深度的土壤温度以及同期土壤含水量和叶面积指数。分析不同深度林地与裸地土壤温度年周期特征,根据热传导方程估算各土壤层温度的阻尼深度和热扩散率,并探讨引起林地与裸地土壤热特性差异的可能原因。结果表明,林地与裸地的土壤温度有明显的时空变化规律。随着深度的增加,土壤温度年周期的振幅逐渐减小、相位逐渐增大、平均值逐渐升高。林地土壤温度年平均值低于裸地,表层年平均温度相差约0.8℃;地表以下相同深度处,林地土壤温度年周期的振幅约低于裸地2.6-2.9℃,相位约小于裸地0.2-0.24 rad(角速度),这表明林地土壤温度极大值和极小值出现的时间比裸地滞后约11-14d。土壤温度阻尼深度和热扩散率随深度的增加而逐渐增大,而在1.6-3.2m则略有降低的趋势。林地与裸地土壤温度和热特性的时空特征和差异可能与土壤含水量和叶面积指数有关。 Abstract:Vegetation is one of the determining factors for soil temperature variation and thermal transfer, which are important processes for the energy and water exchange on the land-atmosphere interface. This study was to understand the impacts of vegetation on soil thermal diffusivity through comparing observations on forested and bared lands. The observations were made in broadleaved Korean pine forest over Changbai Mountain during 2007. The annual cycle of soil temperature and thermal properties at five depths were measured, together with soil volumetric water content and leaf area index.. The soil thermal diffusivity and damping depth of the soil temperature fluctuations were estimated using the heat conduction equation and the sinusoidal-fluctuating temperature at the ground surface. The potential environmental factors for the thermal properties were explored. The results show that soil temperature in both bared land and forest had similar spatial and temporal variations but with different magnitudes. With increasing soil depth, the amplitude of annual temperature variation decreased gradually, while both its phase and annual average temperature itself increased. In comparison with the based soil, the annual average temperature of the surface soil layer in the forest was 0.8℃ lower, the annual amplitude of soil temperature was 2.6-2.9℃ lower, and the temperature phase was 0.2-0.24 rad lower, suggesting that the maximum and minimum temperatures of the forest lagged by 11-14 days behind those of the bare land. The damping depth and thermal diffusivity increased with the soil depth, with an exception for the 1.6-3.2m layers where slightly decrease with depth was observed. It is indicated that soil moisture and leaf area index are probably responsible for the different characteristics in soil temperature and thermal properties between the forest and bared land. 参考文献 相似文献 引证文献

Intermittent experimental study of a vertical ground source heat pump system

In this paper, the intermittent experiment of a vertical ground source heat pump (GSHP) system is investigated and the corresponding geo-temperature variations are studied. The performance of the GSHP system under intermittent operation and the comparisons of different intermittent modes are presented in the paper. The parameters of soil backfill material, air temperature and inlet volume flow rate are also investigated. Experimental results suggest that, due to the recovery in ground thermal energy in intermittent time, the heat exchange rate and the operation performance coefficient (COP) of the heat pump increases, and the compressor power decreases in the successive working. But an insufficient soil recovery time leads to a rapid decline of the performance parameters and the soil temperature. The temperature transports faster under large soil thermal conductivity conditions and the soil temperature decreases more quickly and recovers more slowly with larger inlet flow rate and lower weather temperature for different soil thermal diffusivities. Through multiple nonlinear regression analysis, a curve formula can be fitted to predict the soil temperature variations under intermittent operation of the ground source heat pump in winter, Dalian. It can be found that the soil temperature increases at an exponential function with each factor.

Numerical evaluation on the effects of soil freezing on underground temperature variations of soil around ground heat exchangers

Freezing of soil around ground heat exchangers (GHE) is a common problem for a ground coupled heat pump (GCHP) operated in the cold district, which will affect the underground soil temperature variation and its heat transfer performance. In this paper, a two-dimensional model for heat transfer with freezing-thawing phase change in soil around GHE is developed and numerically analyzed by apparent heat capacity method. The influences of soil freezing, soil water content, and soil type on soil temperature variations and long term underground thermal imbalance of GCHP with GHE array are investigated. The results indicate that the soil freezing can lessen the soil temperature drop and thus increase the temperature difference between the fluid inside GHE and far-field soil. This helps to shorten the GHE design length and cut down the initial system cost. The soil freezing characteristics are mostly affected by the soil thermal diffusivity. From the view of the ability in delaying soil temperature drop, the sandstone is preferable to sand, and sand is better than clay. Additionally, increasing water content can reduce the drop degree and speed of the soil temperature, as well as, accordingly alleviate the underground thermal imbalance of GCHP.

APPLICABILITY OF COLUMN DEVICES TO MEASURE THERMAL PROPERTIES IN POROUS MEDIA

Soil thermal property data, especially as a function of water content, are currently not readily available. Demand for these data is, however, on the increase because of improvements in wider applications of soil heat and water transport models. Most of these investigations were focused in sandy soils, clayed soils or peat horticultural substrates, due to the different properties and applications of each ones. Otherwise, in order to partly fill the thermal gap soil properties studies into other types of soil, we focused this work in the relationship between thermal and hydraulic soil properties for a silt soil under laboratory conditions. Samples were obtained from Can Sol Road located in the Llobregat delta plain (NE of Spain). Small dual-needle sensors, employing the heat pulse methodology were used to measure the soil thermal diffusivity, volumetric specific heat capacity and thermal conductivity. A experimental soil column device with a specific design was used. Column device was monitorized to determine the volumetric water content and matric potential, as well as thermal properties. A continuous dataset was collected from data-logger which connected frequency domain probes and micro-tensiometers were used. Preliminary results allowed a rather complete understanding of the relation between thermal and hydraulic properties under laboratory scale. Also, dataset contributed to evaluate the column device as a well-constructed device for these types of experiments.

Influence of soil moisture content on surface albedo and soil thermal parameters at a tropical station

Half hourly data of soil moisture content, soil temperature, solar irradiance, and reflectance are measured during April 2010 to March 2011 at a tropical station, viz., Astronomical Observatory, Thiruvananthapuram, Kerala, India (76°59’E longitude and 8°29’N latitude). The monthly, seasonal and seasonal mean diurnal variation of soil moisture content is analyzed in detail and is correlated with the rainfall measured at the same site during the period of study. The large variability in the soil moisture content is attributed to the rainfall during all the seasons and also to the evaporation/movement of water to deeper layers. The relationship of surface albedo on soil moisture content on different time scales are studied and the influence of solar elevation angle and cloud cover are also investigated. Surface albedo is found to fall exponentially with increase in soil moisture content. Soil thermal diffusivity and soil thermal conductivity are also estimated from the subsoil temperature profile. Log normal dependence of thermal diffusivity and power law dependence of thermal conductivity on soil moisture content are confirmed.

Preliminary Studies on Subsoil Temperatures at Jodhpur

Diurnal temperature profiles of Jodhpur soil in relation to depth on a few selected days of four different months have been presented. Thermal diffusivity of the soil has been estimated by three different methods and the values, obtained by one of these methods, have been utilized for evaluating the depth of penetration of diurnal temperature wave, which is found to be 50.1 cm on the average. Certain other aspects of surface temperature in relation to ambient temperature, have been discussed.

Active layer characterization by instrumented dynamic cone penetrometer in Ny-Alesund, Svalbard

Global warming may induce an increase of active layer thickness in the Arctic region. The freezing and thawing of the active layer can damage infrastructures such as roads, railways, and embedded pipe lines in cold regions. A few methods, however, have been proposed to characterize the active layer. The objective of this study is to evaluate the characteristics of the active layer by laboratory and field tests, especially using the instrumented dynamic cone penetrometer (IDCP). Geographical and geological characteristics of Ny-Alesund, Svalbard are introduced, and the geotechnical properties, microstructure observations, and thermal properties of the Ny-Alesund soils are investigated. In addition, subsurface temperatures monitored for a year are discussed. The IDCP, which is able to measure the energies transferred into the rod head and the cone tip, is applied to the evaluation of the strength variation and the thickness of the active layer in Ny-Alesund. During dynamic penetration tests, the IDCP can produce profiles of the corrected cone tip resistance as well as the dynamic cone penetration index (DCPI). The results show that the active layer thickness estimated from the DCPI and corrected cone tip resistance profiles is approximately 1700mm. Furthermore, the bottom of the active layer significantly corresponds to that estimated by the maximum ground temperature profile with a soil thermal diffusivity of 5.5·10−7m2·s−1. This study represents the characteristics of Ny-Alesund soils investigated with a variety of laboratory tests, and suggests that the IDCP may be effectively used for active layer characterization.

Estimation of Soil Thermal Properties in Two and Three Dimensions

Better methods of estimating soil thermal conductivity and diffusivity from temperature measurements taken in practical situations where the thermal properties vary spatially will aid both soil science and engineering. In recently published work, I presented a method for estimating these properties when they vary vertically and the system may not be in the periodic steady state. This study extended that analysis to two and three dimensions. Using numerical solutions of the heat equation for conduction in inhomogeneous solids, synthetic temperatures were generated on a fine spatial grid and used to verify that the methods were accurate for daily periods, even in the absence of the periodic steady state. Errors due to increasing the spacing of the generated temperatures and to ignoring lateral variations were investigated. Temperatures on coarser grids led to errors that ranged up to a maximum of 20% at spacings of 0.25 m laterally and 0.1 m vertically when thermal conductivity varied fourfold in one lateral direction and ninefold vertically. In contrast, one‐dimensional analyses on a fine grid with 0.01‐m spacing produced errors of up to 54% that decreased to 7% when conductivity changed laterally by a factor of 1.2 instead of four. This error could be reduced by inclusion of a few lateral temperatures. Analysis of synthetic data that included a sharp spatial transition in thermal properties showed that the method would still be applicable where abrupt changes of soil material or water content occur.

Estudo acerca da variabilidade térmica do solo

The knowledge of temperatures and thermal properties of the soil of a given region is of fundamental importance for the understanding of the various physical processes that occurs in the soil. Although it is known that soil temperature is an environmental factor of great importance to agriculture, few studies have been performed by the scientific community about this variable. Therefore, this work aims the study of thermal characteristics of the soil using data collected in National Institute of Semiarid, during the months from july to december of the year 2012. By analyzing the results it was found that the highest daily thermal amplitudes are associated to portions closest to the surface. Also observed similarity significant of the estimates of soil thermal diffusivity obtained through different methods.

An Experimental Study of Coupled Heat and Water Transfer in Wettable and Artificially Hydrophobized Soils

The effect of soil wettability on coupled heat and water transfer in soil is not well understood. The objective of this work was to determine the effect of soil wettability on coupled heat and water transfer for two wettable soils and their artificially hydrophobized counterparts. Closed soil cells instrumented with heat-pulse sensors provided in situ measurements of soil temperature, soil volumetric water content (q), soil thermal conductivity (l), and soil thermal diffusivity (a). Nonlinear temperature distributions developed in response to the 150°C m −1 temperature gradients applied to the soil cells for both the wettable and hydrophobized soils due to soil moisture redistribution. Hydrophobized sand and silt loam soils had different responses relative to their wettable counterparts. Soil moisture redistribution was similar in the wettable and hydrophobized sand soils. Net water transfer was reduced by 56% in the hydrophobized silt loam compared with the wettable silt loam. Reduced net water transfer in the hydrophobized silt loam indicated that water vapor transfer must have been lower in the hydrophobized silt loam than in the wettable silt loam. Diffusive pathways for water vapor may increase due to hydrophobicity. In both the wettable and hydrophobized soils, l and a decreased in the warm regions and increased in the cold regions of the soil cells due to soil moisture redistribution. Wettability is a function of water content, and relative to wettable soil, water redistribution in hydrophobized soil is reduced only when the water content is small enough for the soil to behave as hydrophobic. If the water content is large enough for a hydrophobized soil to behave as wettable, water redistribution is similar to that in a wettable soil.

Application of numerical method in the estimation of soil thermal diffusivity and soil temperature prediction under different textures and moisture contents

Numerical solution of heat conduction equation was used to estimate the soil thermal diffusivity () under different texture and moisture contents. The estimated value of was applied to soil temperature prediction at several depths and times. The results showed that the values of α at the beginning increased with increased moisture up to a critical point and then decreased. The maximum values for α occurred at 15 and 10% moisture contents for silty clay and sandy soils, respectively. Results of soil temperature prediction showed both overestimated and underestimated trends. The range of errors in most cases was ±1°C. Key words: Thermal diffusivity, numerical method, moisture, soil temperature, texture.

Estimation of Nonuniform Soil Thermal Properties by Harmonic Analysis

Existing methods of estimating spatially varying soil thermal conductivity and diffusivity using harmonic analysis of soil temperatures are applicable only at periodic steady state. This study adopted a more general Fourier analysis which is applicable when transients are present. Two new methods of estimation were compared with existing methods using synthetic data generated by solution of the one‐dimensional heat conduction equation in a soil whose water content varied with depth. All methods gave accurate results for the logarithmic derivative of conductivity and the diffusivity at steady state, but only the new methods were accurate when initial temperatures led to transients. Sensitivity to depth spacings and to noise were also investigated. Spatially interpolating the amplitude and phase of the temperature data with filtering for estimating derivatives allowed good results to be obtained in the presence of noise, especially for the diffusivity. Although the methods work best for temperatures at several depths and spacings of 0.05 m or less, errors in diffusivity were less than 8% with only three depths at 0.1 m spacing.

Using Late Time Data Improves the Heat‐Pulse Method for Estimating Soil Thermal Properties with the Pulsed Infinite Line Source Theory

Soil‐probe contact resistance and finite radius and heat capacity of the heat pulse (HP) probe produce significant errors in thermal property estimates. In this study, we demonstrated that estimating soil thermal properties from late‐time data of the temperature change‐by‐time (ΔT(t)) curve reduces these errors effectively. The weighted nonlinear curve fitting method was applied to estimate soil thermal properties following the pulsed infinite line source (PILS) theory using ΔT(t) data from the complete (PILS‐Complete), peak‐time (PILS‐Peak), and late‐time (PILS‐Late) ranges. Three experiments on specific heat of soil solids (cs), soil thermal properties, and soil water content (θHP) were conducted to examine the performance of these approaches. The results showed that the PILS‐Complete and PILS‐Peak methods overestimatedcsby 16.6% and 13.0% respectively, and the error from the PILS‐Late method reduced to 3.2%. Soil thermal conductivity measurements from the PILS‐Late method agreed well with those from the identical‐cylindrical‐perfect‐conductors theory and with the estimates from the heat flux plate data. The PILS‐Late method also effectively reduced the overestimation of soil heat capacity and underestimation of soil thermal diffusivity. In comparing to the PILS‐Complete method, the PILS‐Late method reduced the root mean square error (RMSE) of θHPfrom 0.039 to 0.021 m3m−3on a sand soil, and from 0.032 to 0.018 m3m−3on a clay loam soil. Thus, using late‐time data improved the accuracy of HP method for measuring soil thermal properties.

Analysis the Influence of the Thermal Diffusivity of the Soil around Buried Pipe on Soil Physical Property

In chilliness area, the temperature drop of oil in buried pipeline is affected by soil temperature field, and the thermal diffusivity is one of the main of physical property the soil, which affects the temperature drop of oil directly. This paper introduced the test principle of the thermal diffusivity of soil, and researched the influence of thermal diffusivity of soil on the soil physical property, such as soil natural temperature field, soil frozen days, depth of freezing and temperature delay, which can offer theory support for the calculation of hot oil temperature drop in buried pipeline.

Assessments about soil temperature variation under censored data and importance for geothermal energy applications. Illustration with Romanian data

The study presents an experimental research carried out to study the natural soil temperature variations and to determine the soil thermal diffusivity based solely on temperature measurements. The research was conducted for two locations in Romania: Cluj-Napoca on bare clay soil and Reghin on grass covered clay soil. It was proved that the design and methodology of the research can be successfully applied in any location. Based on a statistical analysis of the experimental data, a simple and precise mathematical model for natural temperature variation was identified for each location. An important particularity of the models is that for each parameter are presented specific ranges of variation and confidence intervals. To the best knowledge of the authors, this approach is unique in the scientific literature. Another particularity of the models is that it was obtained under censored data. The presented models are generally suitable for any depth up to 10 m. For both locations the soil thermal diffusivity was calculated with specific ranges of variation and confidence interval. The obtained results are of utmost importance for the evaluation of local soil thermal potential for geothermal energy applications.

Numerical investigation on the underground thermal imbalance of ground-coupled heat pump operated in cooling-dominated district

Underground thermal imbalance is a common problem in ground-coupled heat pump (GCHP) system operated in cooling-dominated district, which will cause the average underground soil temperature to rise, and inevitably deteriorate the operation performance of GCHP system. In this paper, a two-dimensional heat transfer model with groundwater advection was developed for modeling the heat transfer of underground GHE array. The influences of underground thermal imbalance rate, soil type, borehole layout and groundwater advection on the underground soil temperature distribution were analyzed. The results indicate that the soil temperature increase with the increase of the ratio of annual heat released into the ground to that extracted from the ground, and thus the underground thermal imbalance becomes worse. The better the soil thermal conductivity and diffusivity are, the faster the soil heat diffusion is, and thus the underground heat buildup can be alleviated. At the same time, the underground thermal imbalance can be controlled or eliminated by decreasing the intensive degree of borehole layout through increasing the borehole spacing or using strip type and block layout. Additionally, the underground heat buildup can be efficiently removed by the groundwater advection.

Investigating soil thermodynamic parameters of the active layer on the northern Qinghai-Tibetan Plateau

The soil thermodynamic parameters, including thermal conductivity, diffusivity and volumetric capacity within the active layer on the northern Tibetan Plateau, were calculated using the measured data of soil temperature gradient, heat flux, and moisture at four stations from October 2003 to September 2004. The results showed that the soil thermodynamic parameters exhibited clear seasonal fluctuation. The thermal conductivity and diffusivity in summer and autumn at Beiluhe, Kexinling, and Tongtianhe were larger than those in winter. The volumetric thermal capacity causes an opposite change; it was larger in autumn and winter than in summer. In spring, the soil thermal conductivity at the Kekexili station was larger than that in summer. Generally, fine-grained soils and lower saturation degrees in the topsoil might be a reason for the lower soil thermal conductivity in winter. For a given soil, soil moisture was the main factor influencing the thermodynamic parameters. The unfrozen water content that existed in frozen soils greatly affected the soil thermal conductivity, whose contribution rate was estimated to be 55 %. The thermodynamic parameters of frozen soils could be expressed as a function of soil temperature, volumetric ice content and soil salinity, while for the unfrozen ground the soil moisture content is the dominant factor for those thermal parameters. As for the soil thermal diffusivity, there exists a critical value of soil moisture content. When the soil moisture content becomes less than a critical value, the soil thermal diffusivity increases as the soil moisture content rises.

Diversity of thermal conditions within the paleocryogenic soil complexes of the East European Plain: The discussion of key factors and mathematical modeling

Spatial distribution of soil temperature was modeled for paleocryogenic complexes of the East European Plain. The purpose of the research was to answer the following question: Can lateral temperature differentiation within studied complexes be explained by lateral differentiation of soil properties? Lateral distribution of soil temperature was modeled for the case of laterally uniform surface conditions and laterally variable soil properties. Simulations were performed for two plots with bare soils. In both cases the dynamics of upper boundary conditions was set by weather data and was the same for the whole plot. The spatial distribution of soil thermal diffusivity was set in two ways. For the first plot the input data on spatial arrangement of soil horizons and laboratory data on thermal diffusivity vs. moisture content dependencies for different horizons were used. For the second plot field data on spatial distribution of soil bulk density, organic carbon content and soil moisture content were used as input data for earlier derived pedotransfer functions, allowing to estimate soil thermal diffusivity. For both plots modeled temperature was underestimated as compared to field data with RMSE of 1.0–1.5°C, but the pattern of temperature spatial differentiation was similar to that observed in the field. Lowest temperatures corresponded to areas with low bulk density and high organic carbon content, that is, areas occupied by soils with the second humus horizon. Thus, mathematical modeling has confirmed that the observed heterogeneity of soil properties is sufficient to explain the formation of a laterally heterogeneous thermal field within the studied soils. It also confirmed that we may use suggested pedotransfer functions to estimate lateral temperature variability in loamy soils from data on bulk density, organic carbon content and soil moisture content.