Search

• A laboratory experiment on the energy pile in unsaturated soils is conducted. • A model considering the coupled heat and moisture transfer is established. • The impacts of thermal imbalance on long-term performance are discussed. • A complete case study is carried out for practical engineering. • The economic and technical feasibility is analyzed for design optimization. The operation of energy piles in unsaturated soils involves coupled heat and moisture transfer, demanding further detailed investigation. In this paper, laboratory tests were conducted to validate a numerical model that incorporated vapor and liquid migration, convection, and latent heat transfer in unsaturated soils. Moreover, this model captured the dynamic variation in characteristic parameters of unsaturated soils. Building upon this observation, this study delved into the thermal performance of the energy pile under ten-year cooling and heating cycles, specifically focusing on the temperature, heat exchange rate, volumetric water content, and thermal conductivity. It is demonstrated that the imbalanced cooling and heating load adversely affects the long-term operation. In particular, the cooling power experiences a reduction of 15.1% after ten years of operation. The results indicate that higher soil temperatures positively impact the heat exchange rate in heating mode, while rainfall facilitates heat transfer in both seasons. Furthermore, a case study of an energy pile system was carried out, encompassing load calculation, pile group simulation, economic analysis, and design optimization. A comprehensive evaluation of the technical and economic feasibility demonstrates that the system exhibits excellent performance, particularly when the ratio of cooling and heating loads ranges from 1.1 to 1.3. [ABSTRACT FROM AUTHOR]

The paper addresses the problem of coupled heat and moisture transfer in porous materials with the time-periodic boundary conditions applied. The solution of Luikov equations [1] , which describe coupled heat and moisture transfer, is presented. Laplace transform is used, where some terms of the inverse Laplace transform ought to be solved by Gaussian quadrature, meaning that the solution is semi-analytical. The time-periodic boundary conditions are applied to simulate the humidity and temperature oscillations of natural environment. Therefore, the proposed solution is appropriate to evaluate the distribution of moisture and temperature within the porous material exposed to everyday natural cycles. The paper presents convergence tests, validation of semi-analytical solution and application to different building materials are presented in the paper. [ABSTRACT FROM AUTHOR]

The objective of this paper is to present a design of an experimental apparatus for studying coupled heat and moisture transport phenomena in soils at high temperatures up to 90°C and to present some preliminary testing results from the apparatus for understanding the extent of its measurement uncertainties. A soil cell was designed and constructed for enclosing a vertical soil column for experimental studies of one-dimensional heat and moisture transfer in the soil column. The design of the experimental apparatus was realized based on the worst condition of achieving one-dimensional heat flow within a soil cell filled with dry soil. The soil cell, which is of cylindrical shape and contains five thermo-time domain reflectometry (T-TDR) probes, was sandwiched between a hot plate on the top and a cold plate at the bottom for studying coupled heat and moisture transfer in soils. The existence of one-dimensional heat flow from the top to the bottom of the soil column can be indicated by two conditions. In an ideal steady-state condition, first, the amounts of inflow and outflow rates of heat transfer at the top and bottom of the soil column should be the same, and secondly, the heat flux distribution at any cross-section of the soil column should be uniform. In the present design, these conditions were met within ±5% of their variations, as a design criterion. With respect to the design criterion, the general integrity and reliability of the design models were numerically assessed using COMSOL, based on realistic geometries and boundary conditions. Once the apparatus was built, a preliminary test of the apparatus was performed to check the existence of one-dimensional heat flow through the soil column in the soil cell. [ABSTRACT FROM AUTHOR]

A performance based two step numerical model to determine the behaviour of timber beam exposed to fire is presented. The first step is based on the coupled heat and moisture transfer model that has been recently presented in Refs. [ 1 , 2 ] and is labelled as Heat & Moisture model. In the second step, the newly developed mechanical model is given which enables to determine the mechanical response of timber beam simultaneously exposed to mechanical and fire load. Based on the model validation for standard fire exposure it was concluded, that a two phase numerical model appropriately simulates the behaviour of timber beam in fire. Furthermore, parametric study revealed that different levels of initial moisture content have a considerable influence on the temperature distribution, charring depth progression and mechanical response of timber beam in fire. In addition, it was presented that the use of simple Heat model based on the Fourier partial differential equation is limited since only one moisture state of timber can be accounted for. At the end of the paper, also average charring rates for standard fire exposure and at different initial moisture contents are given. The possible range of charring rates for bottom side varies from 0.646 to 0.550 mm/min, while for lateral side it varies from 0.571 to 0.517 mm/min. These rates were determined at initial moisture content of 10 and 20%, respectively. • New two-step computational model for the analysis of timber beam exposed to fire. • Implementation of coupled hygro-thermal model into the fire analysis of timber beam. • Influence of moisture content on the mechanical response of timber beam in fire. • Charring rates at different moisture contents. [ABSTRACT FROM AUTHOR]

In this paper, a new method for precise evaluation of the effective thermal conductivity (ETC) caused by moisture transfer inside porous building materials was proposed. Moreover, the mathematical models for coupled heat and moisture transfer without and with moisture phase change were established. Based on the interactions among the moisture phase change, temperature field, and moisture field inside the porous materials, a loop iteration calculation method was proposed to solve for the heat flux caused by moisture migration and phase change. By combining the heat flux with the classical evaporation and condensation theory, the additional thermal conductivities (ATCs) caused by the moisture migration and phase change were obtained. In order to obtain the quantitative relationships between the main factors and ATC caused by moisture transfer, different temperature and humidity boundary conditions were used to analyze the ATC. The results show that the ATC caused by moisture migration increases when the temperature difference decreases and the water vapor pressure difference between the indoor and outdoor air increases. The ATC is greatly influenced by outdoor air temperature fluctuations, while the influence of water vapor pressure fluctuations is relatively small. The value for the ATC caused by moisture phase change is mainly determined by the temperature gradient. [ABSTRACT FROM AUTHOR]

The degradation of concrete materials under freezing and thawing cycles is an important issue for structures in cold and wet regions and concrete material degradation in real environment is to be predicted. This paper proposed a mesoscale simulation program to investigate the material deterioration considering the non-uniform environmental condition, which causes non-uniform distribution of humidity and temperature inside concrete. Theoretical coupled heat and moisture transfer model, internal pressure model and degradation constitutive model are applied in a discrete numerical method (Rigid Body Spring Model) to simulate the crack propagation as well as the strength degradation of concrete with non-uniform frost damage. In addition to simulation studies, experimental work is conducted to verify the reliability of the simulation program, where satisfactory agreement is found. With this program, further research related to prediction of structure performance is planned to be conducted. [ABSTRACT FROM AUTHOR]

This paper aims to identify the transient temperature and humidity distribution during the coupled heat and moisture transfer in porous building materials. To this end, the Laplace transform and finite-difference method were combined into a hybrid numerical method, after the preliminary discussion on the coupled heat and moisture transfer in building materials. Then, the governing equations of heat and moisture transfer in porous materials were solved by the hybrid numerical method. Under the boundary conditions, the general solution of the governing equation was obtained considering the coupling of temperature and humidity on the inner and outer surfaces of building materials. The inversion of the real domain was accomplished by matrix operation and Fourier sequence. The research provides practical methods and empirical data for further research and engineering practice, especially for buildings in humid regions. [ABSTRACT FROM AUTHOR]

Coupled heat and moisture transfer has a severe influence on building envelop durability. This phenomenon has more concerns as the rapid development of new moisture buffering materials. In this paper, a detailed description of hollow concrete block filled with compressed straw bricks is presented. In order to reveal heat and moisture coupling mechanism of the multilayer energy-saving wall, a series of tests were developed based on our designed controllable coupled heat and moisture test. The experiments were divided into 3 groups according to directions of heat and moisture transfer and the variations of boundary conditions. The temperature and humidity profiles within the wall thickness through two different directions are monitored during the test. The experimental results highlight the important influence of temperature and relative humidity gradient on the hygrothermal properties of the multilayer wall. It also illustrates that filling compressed straw bricks into hollow concrete block can hinder heat transfer and improve moisture buffering performance of multilayer wall. Based on our developed model, a piece of software (HMCT 1.0) to simulate the hygrothermal performance in multilayer wall was proposed for deep understanding of the experimental results. A good agreement was found between simulation results and tested values. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a mathematical model is proposed for predicting air temperature and humidity in a two-room region. The model contains a coupled relationship between temperature and humidity within the constructions and can be solved by using the numerical method. However, the two-room region can be reduced to a single region when the region with no ventilation is considered, and then the room temperature and relative humidity can be obtained analytically. The solution obtained in this paper is verified by comparing with the result of the analytical method. It shows that the two results are in agreement. In addition, the proposed model can also be applied to simultaneously obtain the transient temperature and humidity of a two-room region for different porous construction materials. [Copyright &y& Elsevier]

• Building integrated earth-to-air heat exchanger for global footprint reduction. • Offering similar energy gains than traditional earth-to-air heat exchanger. • Two time-dependent 3-dimensional finite volume models are developed and coupled. • Coupled heat and moisture transfers and complex boundary conditions are considered. • Successful validation against in-situ temperature and water content measurements. This paper investigates an innovative earth-to-air heat exchanger (EAHE), namely the geothermal foundation Fondatherm®. This system solves numerous technical problems inherent to a traditional EAHE. However, an accurate understanding of its thermal behavior is crucial to ensure the best energy gain and its economic viability. This current paper focuses on a numerical study of the ventilated foundation. A time-dependent three-dimensional finite volume numerical model is developed. This model considers coupled heat and moisture transfers as well as complex boundary conditions such as the weather, the building and the water table simultaneous influences. A matric-head based system of equations is used to represent the heat, vapor and liquid transfers within the soil. It is combined to a capillary pressure-based formulation of the heat and vapor balance equation for the concrete foundation. Furthermore, a numerical method built on a switching from a potential- to a flux-based system of equations is proposed. It is added to a variable time-step method of resolution and allow to handle the greatest hygric loads variations at the ground surface. The numerical code is successfully validated against analytical solutions for simple cases, and against experimental and in situ measurements. [ABSTRACT FROM AUTHOR]

In recent years, the ground source heat pump (GSHP) system has been widely used due to its high unit efficiency, considerable energy conservation and low operating cost. However, the heat transfer efficiency of ground heat exchangers (GHE) in some projects decreases year by year. This results in the decrease of performance of the GHSP system. This is mainly because of lacking of the deep research about the heat and moisture transfer influence on the GHE in unsaturated soil. In this paper, a new model for predicting the soil thermal conductivity under different temperatures was developed. By comparing with the Campbell and de V-1 models, the new model showed better performance on predicting the soil thermal conductivity. Thereafter, a coupled heat and moisture transfer model in unsaturated soil was established for the GHE. In addition, the mode1 was verified by comparing its results against those established by other researchers. By the results, it was found that the model calculation results agreed well with the experimental data in the literature. Finally, the effects of different factors, including the soil types, soil porosity on the characteristics of the GHE were studied. [ABSTRACT FROM AUTHOR]

The buildings are exposed to the hot-humid climate with high temperature and humidity in hot summer and cold winter zone of China. Moisture transfer and accumulation within exterior walls have notable effect on the cooling and heating transmission load. Finally, it will influence the thickness of insulation. In this paper, a coupled heat and moisture transfer model which considers the effect of the moisture transfer on heat transfer is presented to calculate the cooling and heating transmission load. Then, the optimum insulation thickness of exterior walls is determined by using the P 1 – P 2 economic model. Three representative cities, Changsha, Chengdu and Shaoguan, are chosen as the sample cites. The results show that the optimum thickness of extruded polystyrene (XPS) is between 0.053 and 0.069 m and the optimum thickness of expanded polystyrene (EPS) is between 0.081 and 0.105 m. The maximum lifecycle saving varies from 16.60 to 28.50 $/m 2 and the payback period varies from 1.89 to 2.56 years. EPS is more economical than XPS as insulation because of its lower lifecycle total cost. The comparisons are made between the results based on the coupled heat and moisture transfer model and the transient heat transfer model without considering the influence of moisture transfer. [ABSTRACT FROM AUTHOR]

The building envelopes are exposed to the hot-humid climate with high humidity in hot summer and cold winter zone (HSCW) of China. The moisture transfer may severely influence the conduction loads through exterior walls. In this paper, a coupled heat and moisture transfer model is developed and validated to investigate the thermal performance of exterior walls. The conduction loads through a typical exterior wall are used to evaluate the effect of moisture transfer on the thermal performance of exterior walls in HSCW zone of China. The results show that the peak cooling and heating loads are overestimated by 2.1–3.9% and 4.2–10.1%, respectively, when ignoring moisture transfer. In cooling season, the sum of the latent load accounts for 14.3–52.2% of the sum of the total load and the yearly latent load accounts for 4.9–6.1% of the yearly total load when considering moisture transfer. The total cooling, heating and the yearly load are underestimated by 9.9–34.4%, 1.7–4.0%, and 5.2–6.8%, respectively, when ignoring moisture transfer. The results indicate that ignoring moisture transfer causes significant discrepancy in predicting the conduction loads. A detailed model considering moisture transfer in building envelope is essential to accurately evaluate the building energy performance in HSCW zone of China. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a dynamic model of transient heat and mass transfer across a green roof component. The thermal behavior of the green roof layers is modeled and coupled to the water balance in the substrate that is determined accounting for evapotranspiration. The water balance variations over time directly impact the physical properties of the substrate and the evapotranspiration intensity. This thermal and hydric model incorporates wind speed effects within the foliage through a new calculation of the resistance to heat and mass transfer within the leaf canopy. The developed model is validated with experimental data from a one-tenth-scale green roof located at the University of La Rochelle. A comparison between the numerical and the experimental results demonstrates the accuracy of the model for predicting the substrate temperature and water content variations. The heat and mass transfer mechanisms through green roofs are analyzed and explained using the modeled energy balances, and parametric studies of green roof behavior are presented. A surface temperature difference of up to 25°C was found among green roofs with a dry growing medium or a saturated growing medium. Furthermore, the thermal inertia effects, which are usually simplified or neglected, are taken into account and shown to affect the temperature and flux results. This study highlights the importance of a coupled evapotranspiration process model for the accurate assessment of the passive cooling effect of green roofs. [Copyright &y& Elsevier]

In this paper a finite-difference method is proposed for the first time for the solution of coupled heat and moisture transfer problems of transient nature in 1-D multilayer building elements. Transient boundary conditions are specified for both heat and moisture transfer at external surfaces, while constant values are assumed for material properties of each layer of the structure. The calculations for a typical three layers wall are conducted to obtain the dynamic distributions of the relative humidity at the interfaces of every layer during 24 h by use of the mentioned method, and the results agree with the transfer function method in (ASHRAE Trans. 1999; 105(2) : 954–961). The method can be extended to 2-D and 3-D coupled heat and moisture transfer problems. Copyright © 2003 John Wiley Sons, Ltd. [ABSTRACT FROM AUTHOR]