Heat Transfer In Soil Research Articles

Optimization of feedstock and insulating strategies to enhance biogas production of solar-assisted biodigester system

Assisted by solar greenhouse, the temperature of the digestion liquid can be increased by solar radiation to promote the biogas production rate in cold climate. Due to the dynamic characteristics of solar radiation, air temperature and other variable parameters jointly affect the digestion process. A novel dynamic biogas production prediction model for solar-assisted biogas digestion is developed in this study. The model integrates soil heat transfer, feeding process and covering/uncovering of greenhouse insulation quilt, and coupled with biogas anaerobic digestion kinetic model. After validated by experimental results of an actual biogas project in rural area of Xuzhou, China, the feedstock process and insulating strategies for biogas plants are optimized based on prediction model. The results show a significant increase of 55% average volumetric biogas production rate could be achieved, it indicates that optimizing operation strategy of solar assisted biogas plants has a great potential for enhancing gas production in cold winter climates with sufficient solar radiation resources, which can promote biogas applications in rural North China. The models could be used for predicting biogas production rate of passive solar-assisted biogas digester, and could also help for energy savings and renewable energy utilization in China and other countries.

A whole-process simulation for energy piles in unsaturated soils considering the coupled heat and moisture transfer

Compared with traditional borehole heat exchangers, energy geostructures are more common to be embedded in unsaturated soils. The coupled heat and moisture transfer in unsaturated soils makes the thermal performance of energy geostructures more complicated. In this study, a three-dimensional heat-water–vapor mathematical model has been developed through theoretical derivation, considering the changes in thermodynamic properties. The spatially and temporally varying moisture and temperature profiles were generated with a finite element method. Then, a comprehensive simulation of the whole operation process for energy piles was carried out by coupling the validated mathematical model with the non-isothermal pipe flow and solid heat transfer physical fields. The results showed that after operation at constant power for one month, the liquid content of the unsaturated soil surrounding the pile decreased by 16.9 %, and the thermal conductivity dropped from 1.4 W∙m−1∙K−1 to 1.31 W∙m−1∙K−1. In addition, after three months of operation at a given inlet temperature, the heat exchange rate of the fully-coupled model was 9.6 % lower than that of the model considering heat transfer only. The intermittent operation mode could effectively reduce the heat accumulation and alleviate the decrease of liquid content, helping improve the long-term heat transfer performance. During a three-year operation period, the cooling power generally presented a downward trend while the heating power was the opposite. Therefore, it can be concluded that the application of energy piles in unsaturated soil areas requires more accurate evaluations and long-term monitoring.

More Accurate Predictions of Daily Soil Temperature

More Accurate Predictions of Daily Soil Temperature

A multiscale finite element method for coupled heat and water transfer in heterogeneous soils

Large-scale modeling of coupled heat and water transfer (CHWT) is challenging due to the spatial variabilities of soil hydraulic and thermal properties. A multi-scale finite element method (MsFEM) has been designed for simulating liquid water fluxes in unsaturated soils. In this study, the MsFEM approach is expanded as a new scheme that can handle CHWT in soils. Two groups of MsFEM basis functions are constructed to incorporate the heterogeneities in soil hydraulic conductivity and thermal conductivity, and a Petrov-Galerkin formulation is adopted to implement the proposed MsFEM scheme. The MsFEM scheme is also exploited as a sequential solver when heat transfer and water transfer are expressed in a partially coupled formulation (Wang et al., 2022a). Numerical examples illustrate that, without essentially increasing the computing load, the MsFEM scheme can improve the accuracy by up-to 30% compared to the standard finite element method (FEM), especially for thermally driven water transfer. Some fine scale spatial variations in soil temperature can only be revealed with the MsFEM scheme. If the MsFEM sequential solver is applied, the coupled heat and water transfer model can be rewritten into a sequence of modules, where liquid water transfer, heat transfer and vapor transfer are solved step-by-step, but the thermally driven liquid water is omitted due to its relatively small value (Wang et al., 2022a). With the MsFEM sequential solver, a flexible modeling architecture can be achieved at the cost of a relatively small increase of error (<5%). Therefore, the MsFEM scheme presented in this study is an effective numerical approach to simulating CHWT in heterogeneous soils.

Thermo-Hydro-Mechanical Coupling Model of Unsaturated Soil Based on Modified VG Model and Numerical Analysis

A modified VG model considering both pore ratio and temperature effect is constructed. Based on the wet-thermal elasticity theory and mixture theory, coupled thermo-hydro-mechanical (THM) equations for moisture migration, heat transfer, and deformation in unsaturated soil are derived. The numerical implementation of the coupled theory was implemented by secondary development in the finite element platform. The validity of the established theoretical equation was verified by comparing it with the experimental data. Then the THM coupling response characteristics and internal mechanism of axisymmetric soil samples were numerically analyzed. Study shows that the water in unsaturated soil migrates from high-temperature zone to low-temperature zone under temperature load, and the permeability will change during the process of moisture absorption and dehydration. Particular attention should pay to the reasonable determination of the parameters in the modified VG model for the warming and cooling process of different measurement points during the numerical calculation. The higher the heat source temperature, the greater the saturation at the specific measurement point of the soil sample at thermal stability. The lower the heat source temperature, the shorter the time required for the saturation to stabilize at the specific measuring point. The volumetric strain of unsaturated soil results from a combination of wet expansion and thermal expansion, which is dominated by wet expansion near the heat source and mainly by thermal expansion far from the heat source. The change of the total displacement with time is dominated by the z-direction displacement, and its change trend along the radial direction is similar to that in the r-direction. The peak of the total displacement curve keeps moving backward with time.

Three-Dimensional Model of Soil Water and Heat Transfer in Orchard Root Zone under Water Storage Pit Irrigation

To reveal the water and heat transfer characteristics of the orchard soil under water storage pit irrigation and to regulate the distribution of soil water and heat for improving apple quality and increasing yield, a 3D soil water and heat transfer model of orchards under water storage pit irrigation was established. The model not only considered the influences of root water uptake, precipitation, evaporation, and irrigation, but also simulated the infiltration process of the variable water head in the pit according to the principle of mass conservation and introduced the pit coefficient to simulate the difference in radiation in the pit to describe the influence of the pit on the model. Verify and analyze the simulation results. Results showed that the variation trend of simulated soil moisture and heat was consistent with that of measured data. The mean absolute percentage error, root mean square error, and mean absolute deviation were 3.23%, 0.9460, and 0.6984 for soil temperature and 10.05%, 0.0269, and 0.0214 for water content after irrigation, respectively. The simulation results have high accuracy and show that the soil moisture content centers on the pit with an ellipsoid distribution and tends to be uniform over time. The soil temperature was higher in the 4–5 cm area near the soil surface and the wall of pit, and it remarkably changed with time. The intraday variation of soil temperature was mainly affected by atmospheric temperature, but a certain lag was observed compared with the change of atmospheric temperature. With the increase of the irrigation amount, the distribution range of soil moisture content and water high value area increased, while the average and maximum soil temperature decreased. With the increase of irrigation water temperature to 18–24 h after irrigation, the soil temperature in the ellipsoidal area around the pit remarkably increased. The model established in this paper can be used to simulate the hydrothermal status of the soil in the field under water storage pit irrigation. The results prove that the water storage pit irrigation can effectively improve the hydrothermal status of the middle-deep soil and promote the root system of fruit trees to absorb water.

A method for leak detection in buried pipelines based on soil heat and moisture

Leakages in urban heating pipeline networks are mostly caused by underground corrosion, external environment interference and construction. The fluid leaking from pipes accumulates in surrounding soil for a long time, resulting in changes in soil machinery and hydraulic structure, and potential accidents threaten people's daily life and property safety. A preliminary judgment was made to determine whether the pipeline failed according to the infrared images of underground heating pipeline taken from the actual engineering. By comparing the soil moisture around the faulty pipe, we subsequently proposed a fault detection method for buried pipelines which coupled infrared thermography and the variation characteristics of surrounding soil moisture. In order to verify the effectiveness of this method, the numerical simulation was carried out on a leaking buried pipe, and the variation characteristics of soil heat and moisture in the direction of the failure point and its driving mechanism were analyzed. The results showed that the difference between the internal and external pressures at the failure point of the leaking buried pipeline enhanced the effect of soil pressure potential, resulting in significant heat and moisture transfer in soil, which contributed to detecting the buried pipeline failure and identifying the failure types of buried pipelines.

COMSOL Modeling of Heat Transfer in SVE Process

Non aqueous phase liquid (NAPL) pollution exists in porous media such as soil. SVE technology can be used to remove this pollution in soil. However, few domestic and international studies have paid attention to the changes of soil temperature in the field, which we believe can be useful information to optimize the layout of heating wells. In this research we established partial differential equations of soil heat transfer using the COMSOL multi-field coupling tool to simulate the field distribution of the change in soil internal temperature in the process of SVE to obtain the change of effective heating area with time under certain initial heating conditions. At the same time, we used liquid ethylbenzene to represent NAPL pollutants, and designed a simulation of soil temperature field distribution under the movement of liquid ethylbenzene under external pressure. It was found that under the action of Darcy’s velocity field, the utilization efficiency of the SVE system for the heat source was significantly improved, that is, the temperature distribution of the soil was more uniform. However, the temperature of the heated area increased slowly because the extraction well took away the heat energy. The heat source power should be increased or prolonged to improve the effect of Darcy’s field. Through a coupled simulation, we obtained a variation relationship of the soil temperature field in 1800 min under the action of one extraction well and four heating wells. These data will provide the basis for our next step in designing an algorithm to optimize the distribution of heating wells.

Numerical Model for Heat Transfer in Layered Soil with Local Thermal Nonequilibrium

A numerical model, called HT2, is presented for one-dimensional heat transfer in saturated incompressible layered soil with steady fluid flow, effective porosity, and possible local thermal nonequilibrium (LTNE) between solid and fluid phases. The model uses a series-parallel approach and accounts for advection, conduction, linear and nonlinear thermal dispersion, and interstitial heat transfer. The key to HT2 is the definition of separate columns for the solid matrix (SM) and mobile pore fluid (MPF). The SM column includes the solid phase and immobile pore fluid and consists of fixed elements. The MPF column consists of moving elements and uses Lagrangian element-tracking to follow the fluid motion, which reduces numerical dispersion and simplifies heat transfer within the column to that of dispersive flux between contiguous elements. The model uses an interstitial heat transfer coefficient to calculate kinetic heat transfer between SM and MPF elements. The development of HT2 is first presented, followed by verification checks and a limited parametric study. Numerical simulations indicate that larger particle size and higher fluid discharge velocity yields greater local temperature disequilibrium between solid and fluid phases during heat transfer through saturated soil.

Experimental study on coupled heat and moisture transfer in soils during heat absorption and release processes

Heat and mass migration during the process of heat absorption and release in soil, as a randomly distributed porous medium, is different. The main objective of this work was to comparatively analyze the difference in heat and moisture transfer during the process of heat absorption and release in different soil. In this paper, an experimental study on heat and moisture migration in soils (coarse sand, gravel, and loam) was carried out, during heat absorption and release processes under the same excess temperature. Results showed that moisture migration nonlinearly increases with the excess temperature and initial moisture content, temperature variation linearly increases with the excess temperature, and hardly changes with the initial moisture content. There would be obvious deviation in temperature distribution during the process of heat absorption and release in gravel, loam and coarse sand with an initial moisture content of 15.4%, 20.4%\\23.3% and 23.3%, under the excess temperature of ±20 °C. The temperature variation and moisture migration during heat absorption process are all greater than those during heat release process. The final temperature distribution deviation of loam is the largest, reaches 15.7%, the final moisture migration difference of gravel is the largest, reaches 31.7%, and the moisture migration difference increases with soil porosity.

Experimental study and performance evaluation of a ground heat exchanger with different intermittent ratios and soil thermal properties

The temperature behavior as well as the heat transfer evaluation of a ground heat exchanger (GHE) are investigated under different intermittent operation strategies, different amount of water added in soil at the initial moment of heating and recovery phase. For this purpose, small-scale experiments are respectively carried out in a soil-box system and a soil-column system. The results from the soil-box experiment showed that the dimensionless temperature rise rate and recovery rate of the soli represent a downtrend with an increasing on-off ratio. The effect of heat source on its nearby soil temperature rise would fade away after the pump operation time exceeds a certain value. In addition, the larger heat transfer mainly occurs in the initial 4 h during the heating and recovery phase. Thus, the further experiment performed in the 4h-4 h mode indicates that the better temperature recovery and heat transfer performance compared with the 12h-12 h mode. Compared with soil humidification at the recovery phase, soil humidification at the heating phase has the better heat transfer of soil, and the values of the temperature rise and recovery rate at 13 cm are respectively increased by 141.9% and 63.6% when 150 ml water added in soil. Moreover, its advantage becomes more noticeable as the increasing distance of the testing position from the GHE. Additionally, it is found that there is a critical soil moisture content for the temperature recovery improvement. This research on the GHE give the idea to elevate its energy performance and may be helpful to validate the future theoretical study.

Economic and environmental analysis of ground source heat pump system according to operation methods

A ground source heat pump (GSHP) system can achieve high energy performance than conventional air source heat pump system by using annual stable underground temperature. However, the GSHP system can occurred soil thermal imbalance when the system is used for a long-term operation. Therefore, in order to prevent soil thermal imbalance, increasing the spacing and depth of boreholes and combining GSHP system with auxiliary heat sources are used. However, these methods require large sites and high initial investment. This study proposes an intermittent operation as an alternative to minimize the imbalance by altering the GSHP system. A numerical simulation model that can simultaneously analyze mass and heat transfer in soils was used to accurately analyze the performance of the GSHP system. In addition, the economic analysis and life cycle climate performance (LCCP) were compared based on the numerical analysis results of the GSHP system. Consequently, an appropriate operation method for the system is proposed by comprehensively considering the analysis results for the performance, economics, and environmental impact of the GSHP system according to the operation method. The numerical analysis of the GSHP system showed that the intermittent operation method increased the heat exchange rate of the GSHP system by 15%, compared to the continuous operation method, and the borehole center temperature was higher by 0.8 °C when the system operation concluded. Moreover, the economic analysis results showed that the intermittent operation method could reduce the total cost by up to 18.7% (1,921 USD) compared to the continuous operation method because it can dramatically reduce the initial investment cost. However, as the result of the LCCP analysis for three months, the total emissions increased by up to 11.8% (793 kg CO2e) because the intermittent operation method has a higher energy consumption of the heat pump than the continuous operation method.

Thermo-Hydromechanical Coupling Responses Driven by a Central Heat Source in Unsaturated Soils

Based on the thermodynamic and thermoelastic theory, the coupled governing equations of deformation, heat transfer, and moisture migration in unsaturated soils were given. The coupled calculation process was realized by the finite element method. The hydrothermal coupling characteristics of two cases of unsaturated kaolin clay were studied by using a self-developed test device, and the test results were compared with the numerical results. The results showed that when the initial saturation of the soil is high, the volumetric water content of the measured point increases, and then it is in a stable state during the heating process. When the initial saturation is low, the volumetric water content near the heat source increases firstly and then decreases during the heating process. The rise and fall of temperature make the volumetric water content of the soil irreversible. The volumetric water content of each measured point is lower than the initial state. The closer it is to the internal measured heat source, the more obvious this phenomenon is; the lower the initial saturation is, the more obvious it is.

Comparison of Three Energy-Saving Strategies Based on Models Established for In Situ Gas Thermal Remediation of Contaminated Soil

Soil pollution has become an issue of concern with the development of industrialization. In situ gas thermal remediation is a suitable remediation technology for heavily organic contaminated soil, yet its high energy consumption limits the application. In this study, three energy-saving strategies, off-gas burn-back mode, heat-returning mode, and air-preheating mode, were proposed, and their natural gas consumption and energy utilization ratio were analysed. A mathematical model was established for the heat and mass transfer in unsaturated soil by employing conjugate heat transfer. The temperature of the soil and flue gas, and the concentration of components in the soil and off-gas were simulated, which indicated the thermal behavior of the contaminated soil and the timing for operation control. Models of burners were also developed under different energy-saving modes for energy analysis. By comparing the basic method, adopting the off-gas burn-back mode obtained an energy-saving effect of 3.37%, which relied on the content and calorific value of the pollutant in the off-gas. Under combustion air-preheating ratios of 0.5 and 1.0, natural gas could only be saved by 13.56% and 18.88%, respectively. In general, the most effective energy-saving measure was the heat-returning mode, by which 21.44% of natural gas could be saved when the reflux ratio of the flue gas was 0.5.

THERMOPHYSICAL CHARACTERISTICS OF THE HEAT AND MASS TRANSFER EQUATION IN SOIL

The paper presents the coefficient inverse problem of heat and mass transfer. It is known that the process of moisture and heat transfer in soil is described by a system of partial differential equations. The inverse coefficient problem is solved for finding the thermophysical characteristics of the heat and mass transfer equations. The initial-boundary conditions for the heat and mass transfer system are set. To solve the problem, the measured values of temperature and moisture are additionally set at the accessible boundary of the considered area. To solve the inverse problem, the Dufort- Frankel scheme and Matlab Optimization Toolbox are used. The Jacobian is used to reduce the number of iterations. The results of numerical calculations with and without Jacobian are presented.

Experimental study of the freeze thaw characteristics of expansive soil slope models with different initial moisture contents

This paper presents an experimental investigation on the effect of freeze–thaw cycling on expansive soil slopes with different initial moisture contents. Clay soil from Weifang, China, was remolded and selected to build the expansive soil slope for the indoor slope model tests. A total of five freeze–thaw cycles were applied to the three expansive soil slopes with different moisture contents ranging from 20 to 40%. Variations of the crack developments, displacements, soil pressures and moisture contents of the expansive soil slope with different initial moisture contents during the freeze–thaw cycling were reported and discussed. The results indicate that higher moisture contents can slow the development of cracks and that the soil pressure increases with decreasing temperature. The soil pressure of slope decreases after freeze–thaw cycle, and the change amplitude of soil pressure after freeze–thaw is proportional to water content. The slopes with a moisture content of 20% and 30% shrinks during freezing and expands during thawing, which was named ES-FSTE Model, while the slope with a 40% moisture content shows the opposite behavior. During freeze–thaw cycles, moisture migrates to slope surface. As initial moisture contents increase, the soil heat transfer rate and bearing capacity decreases after five freeze–thaw cycling.

Enhanced Heat Transfer Performance of a New Horizontal Buried Tube Heat Exchanger

Horizontal buried heat exchangers cost less than vertical buried heat exchangers, but they need more land area. In order to reduce the required land area of horizontal buried tube heat exchanger, a finned strengthened heat exchanger pipe was designed to enhance the heat transfer performance, and a three-dimensional research model of shallow geothermal utilization system was established in Chengdu area . In this paper, the optimal shape and arrangement of fins are investigated, and the effects of inlet velocity, soil type, ambient temperature and intermittent operation on the enhanced heat transfer performance are studied. The results show that the enhanced heat transfer performance of rectangular fins is the best in the case of horizontal arrangement. The working fluid flow rate should not exceed 0.3m/s.Soil type has a great influence on heat transfer performance. The heat transfer of rock and soil is the largest, the heat transfer of clay is the smallest, but the strengthening effect of clay is the best. The longer the system runs, the obvious the influence of ambient temperature is, and the heat transfer efficiency increases by about 4% when the ambient temperature drops by 5K.

An evaluation of soil thermal conductivity models based on the porosity and degree of saturation and a proposal of a new improved model

Soil thermal conductivity is a key factor for the soil heat transfer analysis, and is widely applied in many fields. However, it is difficult to measure the thermal conductivities of various soils with different porosities and degrees of saturation. Therefore, an accurate soil thermal conductivity model is essential for practical engineering. In this research, the comparisons and evaluations of eight soil thermal conductivity models (including seven from other scholars and a new improved model) are conducted based on the measured data from published literature. Additionally, these models are categorized into two groups: theoretical models and empirical models. The results show that the predictive capabilities of the empirical models are higher than those of the theoretical models because the first use fitted values of the empirical parameters better fit the experimental data. However, the theoretical models have definite physical meanings. The new proposed model shows better performance, and its Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) values when the thermal conductivities of seven soils are calculated are less than 0.1 Wm−1K−1. This work provides references for the selection and application of soil thermal conductivity models

Impact and Sensitivity Analysis of Soil Water and Heat Transfer Parameterizations in Community Land Surface Model on the Tibetan Plateau

AbstractSoil water and heat transfer is especially complicated during the freezing and thawing processes over the high‐altitude cold regions. In this study, four sensitivity tests of soil water and heat transfer parameterizations including replacing soil property data (SP1), soil resistance scheme modification (SP2), soil thermal conductivity scheme (SP3) and virtual temperature scheme (SP4), and four combination experiments (SP1+SP2+SP3/SP5, SP1+SP2+SP4/SP6, SP1+SP3+SP4/SP7, and SP1+SP2+SP3+SP4/SP8) were done using Community Land Model (CLM5.0) to examine its performances for soil water and heat transfer modeling on the Tibetan Plateau (TP) both in single‐point and regional simulations. The observed data from five eddy covariance sites, four soil moisture and temperature networks and 60 sites of soil temperature observations on the TP were used to evaluate the results. Single‐point simulations show that SP2 experiment reduced the wet biases of soil moisture in semiarid area, but enhanced the error of soil temperature. SP6 shows the best performances in simulating soil moisture, and SP3 in soil temperature. Regional simulations show that the SP7 experiment had the best performances for soil water and heat transfer simulation on the TP, and improved the simulation of soil freezing‐thawing processes. Compared to CLM5.0 default simulation, SP7 shows the best performances. For soil moisture, it reduced average Bias by 23%, Root Mean Square Error (RMSE) by 18%, and increased the Correlation Coefficient (Corr) by2%. For soil temperature, it reduced the Bias by 9%, 10%, 23%, and 13% at four soil depths on the TP, respectively.

Physical connection of sensible and ground heat flux

Characterizing physical relationships between the sensible, latent and ground heat flux at the Earth surface is of crucial importance in studying the global energy, water, and carbon cycles. Here we demonstrate an analogy between the daily mean turbulent heat transfer in the atmospheric surface layer and conductive-advective heat transfer in soil. This finding, in conjunction with temperature continuity at the land–atmosphere interface, uncovers a physical linkage between sensible and ground heat flux. The proposed relationship and its ramification for estimation of latent heat flux are validated based on daily averaged data from six AmeriFlux eddy covariance stations in North America over a wide range of land covers and climate regimes.