Hygrothermal Transfer Research Articles

Modeling and numerical simulation of heat and mass transfer in the cave wall of the Mogao Grottoes in China

In the Mogao Grottoes in Dunhuang, China, temperature and relative humidity (RH) fluctuations are the main causes of the observed deterioration. In this paper, we experimentally and numerically analyze hygrothermal transfer in the cave wall of the Mogao Grottoes. A coupled heat and moisture transfer 1D model is developed to simulate the hygrothermal behavior of the cave wall of the Mogao Grottoes, which adopts the temperature and water vapor pressure as driving potentials. The model is implemented in a programming. By constructing the numerical model of the wall under boundary conditions and comparing the measured data and simulation results, we validate the numerical model and analyze the hygrothermal transfer characteristics of the wall. The temperature and moisture profiles of the cave wall are experimentally measured and numerically determined. The simulation results are compared to experimental temperature and RH values, with good agreement despite the simplifying hypotheses adopted during modeling. By analyzing the heat and moisture distributions, we find that the heat and moisture transfer process in the cave wall changes in space and time. The complicated heat and moisture transfer directions vary cyclically. Additionally, moisture transfer in the cave wall reaches the steady state more rapidly than heat transfer. Hygrothermal cycling plays an essential role in the development of mural deterioration. The developed model provides theoretical support and a scientific method for the conservation of cave sites and a quantitative analysis approach to study mural relics.

Experimental and numerical validation of hygrothermal transfer in brick wall

AbstractIn this paper, a mathematical model dealing with a coupled heat, air, and moisture transfer in a building envelope was developed. Based on the three‐following driving potential: vapor pressure, dry air pressure, and temperature, an application on a hygrothermal behavior of a real wall was carried out for different climatic conditions. For this purpose, a characterization of the heat and moisture properties of the materials constituting the wall made with red brick and cement mortar was carried out in the laboratory. This was used to evaluate experimentally the input parameters of the model as a function of relative humidity. To validate the numerical model, an experimental platform was improved. The wall was set up in a double‐climatic chamber with different boundary conditions, and then the temperature and humidity evolutions were recorded using several sensors within the wall thickness. The results have highlighted a good agreement between numerical simulation results and experimental ones.

Thermal and hygroscopic study of hemp concrete in real ambient conditions

Most building materials, and more particularly bio-based materials, are subject to hygrothermal transfers in the environment in which they are disposed. These transfers depend on their thermophysical characteristics as well as the ambient humidity and temperature conditions. In such environment, and despite these variations, the material must be able to ensure in a sustainable manner, the functions for which it was implemented (thermal, mechanical, acoustic …). Among these materials, hemp concrete, which is a bio-based material, is widely considered in building construction for its superior thermal and hygroscopic performance. The hygrothermal modelling of such materials in real conditions is essential for a better understanding of buildings’ energy performances. Several works targeted the numerical hygrothermal modelling of hemp concrete; however most of them are done in controlled laboratory conditions, which may be different from the real buildings scenarios. In this paper, heat and mass transfer are investigated both numerically and experimentally in real conditions. The hygrothermal properties of hemp concrete were first determined through laboratory experiments; then, an experimental wall segment made of hemp concrete was instrumented in real ambient conditions in order to validate the Philip and De Vries model describing heat and mass transfer. The comparison of the numerical results to the experimental data leads to interesting results regarding local temperature and relative humidity variations. Moreover, numerical investigation of the moisture buffer values of hemp concrete was performed and the results were validated by bibliographic data. The hemp concrete was found to be a very interesting potential hygrothermal regulation material in terms of thermal conductivity, decrement factor, time lag, and humidity regulation.

Energy Consumption Assessment and Energy-Saving Management in Tourist Resorts

For tourism industry, the ever-increasing energy consumption and the high carbon emissions are requiring close attention and prompt solution, thusly endowing the research on the energy-saving of buildings in tourist resorts very important and practical significance. For this reason, this study carefully considered the actual situations such the energy utilization method and the hygrothermal environment of tourist resorts and constructed a hygrothermal transfer model and the corresponding hygrothermal balance equation for buildings in tourist resorts; then, the paper proposed a few effective strategies for the energy-saving management of buildings in tourist resorts, and studied the annual energy consumption of tourist resorts and gave a building energy consumption analysis. At last, experimental results verified the rationality and effectiveness of the proposed energy consumption assessment and energy-saving management methods for tourist resort buildings in hygrothermal environment. This study provided a useful reference for the energy-saving methods of buildings in tourist resorts.

Experimental and numerical highlighting of water vapor sorption hysteresis in the coupled heat and moisture transfers

Heat, air and moisture transfers have a significant influence on indoor ambience and affect both temperature and hygrometry values and energy performance of buildings. Furthermore, high levels of humidity may lead to severe pathologies in building walls in addition to tamper indoor air quality and hygrothermal comfort. Sorption hysteresis phenomenon is responsible for the moisture accumulation in porous building materials, and leaving it out in hygrothermal modelling would result in an incorrect water content value.The present work aims to highlight the influence of the sorption hysteresis phenomenon on heat and moisture transfer in expanded polystyrene concrete. In order to achieve this, a coupled heat and mass transfer model will be elaborated, taking consideration of sorption hysteresis phenomenon. Experimental protocols will be conducted to highlight the hysteresis in the materials, and the results will be compared to those of the numerical simulations. A simple hygrothermal transfer model without including hysteresis will also be used in the numerical simulations so that it is compared to the developed model. Experimental results show that considering the hysteresis phenomenon allows to better apprehend the hygrothermal behavior of building materials, and neglecting it leads to important differences sometimes reaching 28%.

Modeling, numerical simulation and validation of the hygrothermal transfer through a wooden building wall in Nancy, France

The present work aims to model heat and mass transfer through wooden building walls in Nancy. Adsorption and desorption isotherms of three exotic wood and one temperate wood using a dynamic vapor sorption apparatus at 35 °C were experimentally determined then modelled using Nelson sorption isotherm model. Sorption hysteresis of these woods was deduced. Heat and mass transfer model validation shows that the thickness of wooden wall higher than 0.15 m permits to cancel the influences of ambient air temperature on the temperature of the air in the building. Also, the results show that influence of the four wood species is not observed and can be neglected. To reduce the quantity of water absorbed by the wooden wall, it is important to dry the plank to a moisture content of 0.14 kg/kg dry basis. It is observed that heat and mass transfer gradients are highly influenced by the seasonal variations and characteristics of indoor air. During the day, the wooden wall gives a low variation of climatic conditions of air inside the house.

Modeling and numerical investigation for hygrothermal behavior of porous building envelope subjected to the wind driven rain

Wind driven rain (WDR) may have serious effect on the heat transfer through exterior wall, optimal design of envelope, and building energy consumption simulation. Accurate evaluation of the impact of WDR on the hygrothermal performance of building envelope is important for energy-saving design of buildings. This work presented the numerical model and analysis method for the hygrothermal behavior of building wall exposed to the WDR. A capillary pressure based coupled heat and moisture (CHM) transfer model applicable to the high humidity condition was established and validated. The ISO15927-3 method of WDR was improved based on the previous experimental results. WDR on four orientation walls were investigated use the improved ISO method and weather station data between November 2017 and October 2018 in the representative city of Changsha. Dynamic simulation of hygrothermal performance for external wall is conducted under three calculation conditions: the transient heat conduction (TH), CHM (no rain) and CHM (WDR) models. The results show that total cooling load transmitted through the east, south, west and north orientation walls calculated with CHM (no rain) model is predicted to increase 8.5%, 8.6%, 5.3% and 10.3% compared to the TH model. But total cooling load transmitted through the four orientation walls calculated with CHM (WDR) model separately increases −2.5%, 1.8%, −7.9% and −46.1% in contrast with TH model. Total heating load transmitted through the four orientation walls calculated with CHM (WDR) model is predicted to separately increase 3.2%, 3.2%, 63.3% and 71.5% compared to the TH model. The results indicate that WDR has significant influence on the hygrothermal performance of building wall. It is essential to incorporate the WDR into the hygrothermal transfer model for the validity of optimal design and the accuracy of building energy simulation in rainy areas.

Modeling of hygrothermal transfers through a bio-based multilayered wall tested in a bi-climatic room

A bio-based multilayered wall has been developed in the framework of the European ISOBIO project. A key point was to be able to perform proper simulations of the hygrothermal transfers occurring through the wall: local predictions are of first importance to characterize the behavior of the wall and thereafter its ability to ensure comfortable hygrothermal conditions inside buildings. A previous study proved that the conventional assumption of an instantaneous equilibrium between local relative humidity and water content according to the sorption isotherm is not relevant for bio-based porous materials, where in practice slow sorption kinetics occur. In the present study, an improved expression of the local kinetics is proposed and validated by sorption experiments. Then, Moisture Buffer Value tests are performed (Nordtest project's protocol). The simulations are adjusted to these measurements by using the inverse method in order to refine the knowledge of some critical parameters. Depending on the studied materials, the local kinetics constants are between 0.15 and 14 day-1/(kg.m-3). Finally, the ISOBIO wall is studied in a bi-climate room under a wide range of operating conditions. Simulations carried out with the conventional approach (TMC) and the local kinetics approach (TMCKIN) are compared to measurements: this clearly shows that the latter is able to predict well the relative humidity dynamics while the former underestimates it by a factor up to 66%. These results highlight the relevance of the new expression of the local kinetics and its ability to describe well the local hygric dynamics is certainly an achievement.

Thermal and energy performance of green roof and cool roof: A comparison study in Shanghai area

Green roofs and cool roofs are commonly used to improve indoor thermal environment, reduce air-conditioning load and mitigate the Urban Heat Island (UHI) effect. This study aimed to quantify the differences in thermal and energy performance between the two different roof types under the climate of Shanghai. Firstly, field experiment was conducted in the Shanghai area. Thermal performance of green roof, cool roof and common roof in summer and winter were measured. Results showed that, compared to common roof, the cool roof had an average cooling effect of 3.3 °C on the outer roof deck surface in summer, while the green roof only had a cooling effect of 2.9 °C. In winter, the green roof provided good insulation, and could improve outer surface temperature of the roof deck by an average of 3.3 °C compared to the cool roof. A hygrothermal transfer model for green roof was coupled with a dynamic building thermal performance simulation software (THERB) and validated using measured data. The coupled model was used to predict the effect of both roof types on energy performance of a public building. Simulation results showed that green roof could reduce the cooling and heating loads of the top floor by 3.6% and 6.2%, respectively. The cool roof could reduce cooling load by 3.6% and increase heating load by 10.4%. Finally, a parametric analysis was implemented. The functional mechanism of the main parameters of green and cool roof as well as their impacts on thermal and energy performance of public buildings were analyzed in detail. Conclusions drawn from this paper could provide guidance for the design optimization and application of green roof and cool roof in Shanghai area.

Optimization and Assessment of the Protective Shed of the Eastern Wu Tomb

The Eastern Wu tomb in Shangfang Town, Nanjing City is a brick tomb of the Six Dynasties in China, which is very famous for its big scale and complex structure. After being excavated, biodeterioration occurred on the interior wall of the tomb chambers due to the fluctuation of environmental factors, which threatens the cultural value of this architectural heritage. Biodeterioration is highly related to the mild temperature and the high humidity in the tomb chamber and condensation on the wall surface. To reduce biodeterioration in the Eastern Wu tomb, environment monitoring was carried out and the effect of the current protective shed on the Eastern Wu tomb was examined. The hygrothermal transfer model of the protective shed was developed to evaluate the effects of the optimization of the protective shed for reducing the condensation on the wall surface. The results show that condensation on the wall surface of the site was reduced by 53% in a year after the functional space utilizing solar energy was added to the protective shed.

Framework for model predictive control (MPC)-based surface condensation prevention for thermo-active building systems (TABS)

The potential risk of developing surface condensation keeps thermo-active building systems (TABS) from being applied in buildings located in partly warm and humid climate regions. This study presents a framework for model predictive control (MPC)-based surface condensation prevention that can avoid the surface condensation during the cooling periods when the TABS is in operation. Because MPC determines the input signal for the system not only on the basis of the current states but also on the impact that the actions will have on the future states, it is suitable for anticipatory surface condensation control that must respond to both dynamic indoor condition changes and the time-delay in hygrothermal transfer in advance. Heat and moisture transfer dynamic models were developed for prediction of future states and these dynamic models were calibrated with the measured data to improve the surface condensation prediction accuracy. On the basis of future states predicted by the calibrated dynamic models, the MPC-based condensation prevention framework adjusts the surface temperature for the TABS in ways that ensure indoor thermal comfort and energy efficiency without the development of surface condensation. The results demonstrated that the MPC-based TABS operation achieved 21.0 − 29.6% site cooling distribution energy savings compared to the conventional forced air-based cooling mode depending on the climatic regions; in comparison between the TABS governed by the MPC framework and by the on/off control, 2.5 − 10.0% more site cooling distribution energy savings were achieved through use of the MPC framework than through use of the on/off control.

Hygrothermal effects and moisture kinetics in a bio-based multi-layered wall: Experimental and numerical studies

A bio-based multi-layered reference wall has been developed within the framework of the European ISOBIO project. One of the key points of this project was to be able to perform proper simulations of the hygrothermal transfers occurring inside such walls. Previous published investigations, also performed in the framework of this project, have demonstrated that the classic assumption of instantaneous equilibrium between local relative humidity and water content according to the sorption isotherm is not relevant for bio-based porous materials, where, in practice, a rather slow kinetics of sorption occurs. The theoretical background developed in this previous study is used here to determine the kinetic constants of the bio-based construction materials and to perform 1D hygrothermal simulations. The kinetics constants are determined thanks to a 1D cylindrical tool based on the local kinetics approach, validated against several experiments of sorption. Then, heat and hygric transfers recorded on a demonstrator building (The HIVE, Wroughton, UK) are analyzed and are simulated using two modeling tools: TMC based on the Künzel approach and TMCKIN based on the local kinetic approach. From the simulations, the local kinetics improves the small timescale RH dynamics. The comparison with measurements performed in the demonstrator confirms the relevance of the local kinetics approach.

Numerical sensitivity analyses for identifying rate-limiting factors influencing total energy exchange efficiency in energy recovery ventilator

Energy recovery ventilators (ERVs) are used to recover sensible and latent heat to reduce the heating and cooling load caused by outdoor air intake into indoor environments. The efficiency of the heat exchanger, comprising flow channels and constituent materials, determines the heat exchange performance of the ERV. Understanding the heat and mass transfer mechanisms in the ERV and optimizing the geometry and materials of the flow channel are essential for improving the ERV performance. Herein, numerical methods for predicting and clarifying the hygrothermal transfer mechanism in the heat exchange element of an ERV were developed, and their prediction accuracy was confirmed by analysing the exchange efficiencies in the scaled-down ERV unit model. The verified numerical model was applied to sensitivity analyses to clarify the rate-limiting factors influencing the total heat exchange efficiency of the ERV. Our findings have clarified that under Japan Industrial Standard cooling conditions, a 50-fold increase in the thermal conductivity of the spacer plate, the total heat recovery performance was enhanced by 13%, as for a 2-fold increase in the moisture conductivity, the performance was enhanced by 20%. The findings of this research can be expected to contribute to the energy saving effect in buildings.

A simplified numerical model for evaporative cooling by water spray over roof surfaces

Evaporative cooling is a well-known passive technique adopted especially in hot and dry climatic regions. This paper presents a simplified numerical model to assess the impact of water spray over roof surfaces on the building thermal load. First, we performed a series of long-term outdoor measurements of heat fluxes from an asphalt pavement and roof top of a laboratory building waterproofed by asphalt sheet. Based on the measured data, a correlation is proposed to estimate the evaporation rate of the concerned surfaces after rain showers. A simplified numerical model is then developed using this correlation based on the unsteady heat transfer to analyze the effect of precipitation and artificial water spray on the roof of the laboratory building. Simulation is performed with the help MATLAB using the hourly weather data from Fukuoka, Japan, and the simulated results are found to be in correspondence with the measured data. Finally, the cooling effect achieved by varying the amount and rate of water spray over the roof is compared to find out the optimum spraying conditions.The novelty of this study is that, in this model, the evaporation rate can be expressed in terms of the evaporation ratio, which is defined as a function of the water content of the surface layer of the roof. The water content of the surface layer can be obtained by the water balance equation, which is coupled dynamically with conductive equations. This procedure can drastically reduce the calculation load compared to the conventionally applied simultaneous hygrothermal transfer equation.

Development of numerical heat and mass transfer model for predicting total heat exchange performance in Energy Recovery Ventilator

An energy recovery ventilator (ERV) is a widely used equipment that can recover sensible and latent heat. To improve its performance, it is essential to understand the heat and moisture transfer mechanism in the ERV. Against this background, overarching objectives of this study are to develop mathematical/numerical models for predicting and clarifying the hygro-thermal (i.e. heat and moisture) transfer mechanism in the heat exchange element of the ERV in terms of (i) simplified model for sensitivity analysis, and (ii) comprehensive model to integrate hygro-thermal transfer equations with computational fluid dynamics (CFD) analysis. Toward this end, we conducted fundamental experiments to measure temperature, humidity, and enthalpy exchange efficiencies in the scale-down ERV unit model, and then, numerical analyses were conducted according to the experimental scenario. As results of this study, we confirmed the reasonable prediction accuracy of our proposed numerical models for predicting total heat exchange performances.

Modelling and numerical simulation of hygrothermal transfer through a building wall for locations subjected to outdoor conditions in Sub-Saharan Africa

In the building sector, the improvement of the heat and mass insulation of the walls can reduce the values of the global energy demand and the CO2 emission. A coupled one-dimensional heat and mass transfer model through a building wall was proposed in this study. The governing equations integrate the influences of Dufour and Soret effects and are numerically resolved using FORTRAN 90 and finite difference method in Crank-Nicolson scheme. Gauss-Seidel relaxation iteration method is used in order to generate the numerical solutions. The accuracy of the model was evaluated with experimental data on wooden wall and concrete wall from literature. A very satisfactory agreement is noted between the numerical predictions, experimental observations and physical phenomenon obtained. The model was further used to simulate the hygrothermal transfer through five tropical woods used as building walls in sub-Saharan African region after integration of outdoor conditions. These five woods have densities ranging from 430 to 800 kg/m3 and the conditions of the interior air were fixed to maintain a comfortable state for the occupants. Twelve towns located within latitudes 4.33°S to 12.37°N in Sub – Saharan Africa were used to discuss the results given by the model. At the end, influences of anatomical direction, tropical wood types, wall thickness, latitudes of the locations and solar radiation on evolutions of temperature and moisture content in the wall are studied. Our results showed that these parameters have significant effects on temperature and moisture content propagation within the wall. Using a wooden wall of 5 cm thickness, it is possible to reduce significantly the effect of the exterior heat.

Study of the Hygrothermal Behavior of Wood Fiber Insulation Subjected to Non-Isothermal Loading

Building envelopes are constantly subjected to temperature and moisture gradients. This loading induces a complex response, particularly for highly hygroscopic insulating materials. Latent effects can no longer be neglected for these materials in which heat and moisture transfers are strongly coupled. The purpose of this article is to analyze the behavior of a wood fiber insulation subjected to non-isothermal loading under a vapor concentration gradient. An experimental setup and a mathematical model of hygrothermal transfer were developed to analyze the behavior of the wall. The mathematical model describes the main physical phenomena involved, notably water vapor adsorption and the dependence of thermophysical properties in state variables. The experimental setup developed allows studying a wall under controlled conditions. The temperature and relative humidity profiles within the wall were measured. The evolution of the profiles with time suggests that the adsorption of the water vapor occurs together with the redistribution of liquid water within the envelope. The comparison of the experimental results with the numerical model shows good agreement although the prediction can be improved during the transient phase. The comparisons of these results with a purely diffusive thermal transfer model show the limits of the latter and permit quantifying the latent effects on the total heat transfer.

Hygromorphic characterization of softwood under high resolution X-ray tomography for hygrothermal simulation

Because of their complex hygromorphic shape, microstructural study of wooden materials behavior has recently been the point of interest of researchers. The purpose of this study, in a first part, consists in characterizing by high resolution X-ray tomography the microstructural properties of spruce wood. In a second part, the subresulting geometrical parameters will be incorporated when evaluating the wooden hygrothermal transfers behavior. To do so, volume reconstructions of 3 Dimensional images (3D), obtained with a voxel size of 0.5 μm were achieved. The post-treatment of the corresponding volumes has given access to averages and standard deviations of lumens’ diameters and cell walls’ thicknesses. These results were performed for both early wood and latewood. Further, a segmentation approach for individualizing wood lumens was developed, which presents an important challenge in understanding localized physical properties. In this context, 3D heat and mass transfers within the real reconstructed geometries took place in order to highlight the effect of wood directions on the equivalent conductivity and moisture diffusion coefficients. Results confirm that the softwood cellular structure has a critical impact on the reliability of the studied physical parameters.

Hygrothermal behavior for a clay brick wall

In Egypt, the clay brick is the common building materials which are used. By studying clay brick walls behavior for the heat and moisture transfer, the efficient use of the clay brick can be reached. So, this research studies the hygrothermal transfer in this material by measuring the hygrothermal properties and performing experimental tests for a constructed clay brick wall. We present the model for the hygrothermal transfer in the clay brick which takes the temperature and the vapor pressure as driving potentials. In addition, this research compares the presented model with previous models. By constructing the clay brick wall between two climates chambers with different boundary conditions, we can validate the numerical model and analyze the hygrothermal transfer in the wall. The temperature and relative humidity profiles within the material are measured experimentally and determined numerically. The numerical and experimental results have a good convergence with 3.5% difference. The surface boundary conditions, the ground effect, the infiltration from the closed chambers and the material heterogeneity affects the results. Thermal transfer of the clay brick walls reaches the steady state very rapidly than the moisture transfer. That means the effect of using only the external brick wall in the building in hot climate without increase the thermal resistance for the wall, will add more energy losses in the clay brick walls buildings. Also, the behavior of the wall at the heat and mass transfer calls the three-dimensional analysis for the whole building to reach the real behavior.

Characterization of hygrothermal transfers in the unfired earth

In front of the building energy issues and environmental impact bound, it appears that the earth envelopes are a promising track in terms of improving thermal comfort, indoor air quality, energy consumption and indoor humidity regulation. For this purpose, a characterization of hygrothermal transfers in the unfired earth was been conducted with experimental and numerical approaches. Thus, the studies at the material and wall scales were necessary to check dissociate and coupled hygrothermal behaviour. The material scale has been used to characterize properties related to heat and mass transfers. The hygrothermal coupling has been the subject of a specific study via a dual-climate test chamber at an earth wall scale. Finite Differences (FD) and Finite Elements (FE) implementations have resulted in a detailed analysis of transfers across the tested earth wall with a reduction work of order required to limit the calculation time. Emphasis is placed on the effects of moisture in the hygrothermal transfer in order to validate a digital tool developed in this work.