Benchmark experiments for moisture transfer modelling in air and porous materials

The presence of hygroscopic materials has a large impact on the moisture balance of buildings. Nowadays, HAM (Heat, Air and Moisture) models are widely used to investigate the role of hygroscopic materials on the performance of buildings, i.e. on the building envelope, the indoor climate and valuable objects stored within the building. Recently, these HAM models are being coupled to CFD (Computational Fluid Dynamics) models to study the moisture exchange between air and porous materials on a local scale (microclimates), or to BES (Building Energy Simulation) models which focus on the interaction between air and porous materials at building level. Validation of these numerical codes is essential to gain confidence in the codes. However, available experimental data are rather scarce.     

多孔建筑材料热湿传递过程的研究

从理论计算和实验研究等方面对国内外多孔建筑材料的传热、传湿及热湿耦合传递研究方面的进展进行了综合分析。指出了目前多孔建筑材料热湿传递过程研究中存在的问题，并对未来的研究方向进行了展望。     

Microclimatic coupling as a solution to improve building energy simulation in an urban context

To streamline the design of the energy efficient buildings, appropriate tools are needed to assess their energy performance taking into account the microclimatic context.Numerical simulation seems to be the most suitable issue, but none tool is dedicate to the direct evaluation of the microclimate influence on the building energy consumption. A complete solution could be to use both CFD and thermoradiative simulation tools complementary with the coupling technique perspective. This paper presents both a developed CFD-thermoradiative coupled simulation tool and a typical application on an urban fragment.The results lead to two kind of observations:

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Integration of the thermal model of a building in the microclimatic simulation platform enable a quantitative evaluation of the building energy demand regarding different urban design scenarii (e.g. mineralized vs vegetated).

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Different physical phenomena do not contribute as much in the energy balance and it is important to compute precisely each one to obtain the small scale microclimatic influence.

     

Effects of combined heat and mass transfer on heating load in building drying period

In initial used building with EPS walls, the wall still contains a considerable amount of water that evaporates over time. In order to analyze the effect of moisture levels on a drying building energy performance, the heat and moisture coupling transfer of building wall with multilayer composition in the cold serious area Harbin, China, was simulated. The freezing of liquid moisture in exterior envelope was considered and the moisture content gradient was used as mass transfer driving forces. The effect of some impact factors, as initial moisture content, vapor retarder, interior wallpaper and exterior glazed brick on the wall energy consumption were also analyzed. It is concluded that the effect of drying are most significant during the first year of building initial use. The drying rate of the new building was significantly high in the first year, especially in the first few months. These changes during initial drying often account for a significant fraction of the overall load and have notable effect on the building energy performance.     

A study of coupled heat and mass transfer across a porous building component in intertropical conditions

In this paper, one-dimensional coupled heat and mass transfer through a plane geometry porous building component submitted to intertropical climatic conditions is studied. An analytical method using the periodic solution approach has been proposed to evaluate the heat and moisture transfer process in building materials. Results are discussed using the three climatic regions of Cameroon (2-13 °N). With latitude of 11°, it offers examples of practically whole range of intertropical climates. Influence of latitude on various temperature and moisture content through the building component has been presented. The results are in good agreement with the experimental data and other analytical solutions published in the literature.     

双流体模型气液传质数值模拟初探

以实验测量和数值模拟相结合的方式对气液传质的流体力学行为进行了系统的研究,从更深层次上揭示了气液体系的流动机制,得出了气含率径向分布规律,对气液传质研究具有重要意义。     

Coupled simulation of heat and moisture transport in air and porous materials for the assessment of moisture related damage

This paper describes the coupling of a model for heat and moisture transport in porous materials to a commercial Computational Fluid Dynamics (CFD) package. The combination of CFD and the material model makes it possible to assess the risk of moisture related damage in valuable objects for cases with large temperature or humidity gradients in the air. To couple both models the choice was made to integrate the porous material model into the CFD package. This requires the heat and moisture transport equations in the air and the porous material to be written down in function of the same transported variables. Validation with benchmark experiments proved the good functionality of the coupled model. A simulation study of a microclimate vitrine for paintings shows that phenomena observed in these vitrines are well predicted by the model and that data generated by the model provides additional insights in the physical mechanisms behind these phenomena.     

Overview of pressure coefficient data in building energy simulation and airflow network programs

Wind pressure coefficients (C_p) are influenced by a wide range of parameters, including building geometry, facade detailing, position on the facade, the degree of exposure/sheltering, wind speed and wind direction. As it is practically impossible to take into account the full complexity of pressure coefficient variation, building energy simulation (BES) and Airflow network (AFN) programs generally incorporate it in a simplified way. This paper provides an overview of pressure coefficient data and the extent to which they are currently implemented in BES–AFN programs. A distinction is made between primary sources of C_p data, such as full-scale measurements, reduced-scale measurements in wind tunnels and computational fluid dynamics (CFD) simulations, and secondary sources, such as databases and analytical models. The comparison between data from secondary sources implemented in BES–AFN programs shows that the C_p values are quite different depending on the source adopted. The two influencing parameters for which these differences are most pronounced are the position on the facade and the degree of exposure/sheltering. The comparison of C_p data from different sources for sheltered buildings shows the largest differences, and data from different sources even present different trends. The paper concludes that quantification of the uncertainty related to such data sources is required to guide future improvements in C_p implementation in BES–AFN programs.     

Diagnostic test cases for verifying surface heat transfer algorithms and boundary conditions in building energy simulation programs

Verification and validation are crucial in developing and implementing models. Although there are standards to test energy simulation software, this article describes an additional set of eight test cases that are a combination of analytical cases and numerical cases for solid conduction heat transfer. These tests focus on diagnosing and verifying conductive heat transfer algorithms and boundary conditions in building envelopes or fabrics. As an example, EnergyPlus versions 5, 6 and 7 are tested using these eight test cases. The test cases were useful for detecting several bugs in the code. The authors recommend these test cases as useful complements to existing verification test cases for building envelopes.     

Heat and mass coupled transfer combined with freezing process in building materials: Modeling and experimental verification

Newly completed building envelope is always characterized by high initial moisture content, and so the liquid moisture permeability is the main feature of mass transferring on its initial use. The high initial moisture content has strong impact on indoor condition and energy consumption especially in severe cold area where the moisture freezing in building envelope would occur in winter. Therefore, accurately predicting the hygrothermal states of building envelope to obtain useful envelope parameters is very important. In order to analyze the moisture transferring performance of enclosure on building initial use in severe cold area, the paper studied the coupling transfer of heat and moisture in building envelope. The permeability and freezing of the liquid water in porous building material were considered. The moisture content gradient was used as mass transfer driving force, and the temperature gradient was used as heat transfer driving forces. Heat and moisture coupled transfer conservation equations on different transferring conditions were built. An experimental set-up was built to verify the model, and good agreements were obtained, which suggests that the model can be used to simulate the heat and moisture coupled transfer in newly completed building envelope of severe cold area.     

Modeling coupled heat transfer and air flow in a partitioned building with a zonal model: Application to the winter thermal comfort

The assessment of building thermal comfort quality in the Mediterranean context necessitates detailed information concerning local air speed and temperature inside the space. We have extended the three-dimensional zonal model ZAER (Zonal AERial model) to enable predictions of air flow pattern and thermal distributions between and within rooms. Numerical simulations from the new program have been compared with data obtained from measurements on the experimental cell Minibat (CETHIL, INSA Lyon laboratory) and with the prediction of another zonal model as well as a computational fluid dynamics (CFD) tool. The comparison indicates that this new program is an effective model for predicting air flow and temperature distribution in a partitioned building. By coupling ZAER with a thermal comfort model, we study the influence of a passive solar component belonging to a south-oriented room upon the winter thermal comfort of an unconditioned Tunisian dwelling. The obtained results show that this simulation tool has the potential to describe realistically the thermal comfort within a dwelling, and that a Trombe wall can be a useful heating component to improve thermal winter comfort in the Tunisian context, even in another room.     

Predicting psychrometric conditions in biocontaminant microenvironments with a microclimate heat and moisture transfer model -- description and field comparison

A numerical model is described that is designed to model psychrometric conditions in biocontaminant microenvironments, such as in bedding and the base of carpets for dust-mites, and on the surface of linings for molds. The model is very general and can include room air, other room components, other zones including the outdoors, other rooms and any subfloor space. Mechanical plant can be modeled. Good agreement between modeled and field results are reported for the complex case of an occupied bed and for the microclimate in the base of a carpet, before and after its timber floor above a crawl space was retrofitted with insulation. PRACTICAL IMPLICATIONS: Biocontaminants such as dust-mites and molds can pose serious health problems. Understanding microclimates in biocontaminant microhabitats, when coupled with biologic models, will make it possible to predict how the life cycles of these biocontaminants are affected as these conditions change. In turn, this will suggest which interventions that modify indoor climate and microclimate are likely to control these biocontaminants. Furthermore such interventions might include indoor humidity control, changing building insulation and ventilation levels, covering mattresses, use of electric blankets, use of carpet heating, etc. Such models will provide a fast way for screening for interventions that are likely to be effective in the control of biocontaminants.     

A cellular automaton based model simulating HVAC fluid and heat transport in a building. Modeling approach and comparison with experimental results

A discrete model characterizing heat and fluid flow in connection with thermal fluxes in a building is described and tested against experiment in this contribution. The model, based on a cellular automaton approach, relies on a set of a few quite simple rules and parameters in order to simulate the dynamic evolution of temperatures and energy flows in any water or brine based thermal energy distribution network in a building or system. Using an easy-to-record input, such as the instantaneous electrical power demand of the heating or cooling system, our model predicts time varying temperatures in characteristic spots and the related enthalpy flows whose simulation usually requires heavy computational tools and detailed knowledge of the network elements. As a particular example, we have applied our model to simulate an existing fan coil based hydronic heating system driven by a geothermal heat pump. When compared to the experimental temperature and thermal energy records, the outcome of the model coincides.     

Efficient 1D model for blind assessment of existing bridges: simulation of a full-scale loading test and comparison with higher order continuum models

Assessing the structural performance of existing concrete bridges is nowadays a major task. Nonlinear finite element (FE) analysis can quantify their capacity, evaluate strengthening interventions and prevent premature dismantle. However, this technique, mainly performed with 2D/3D FE, is seldom used at true scale due to the great complexity and computational costs involved. In this paper, the loading test of a strengthened concrete bridge in Sweden (Örnsköldsvik) is simulated using a 1D model. The bridge failed in combination of shear–bending–torsion triggered by fibre-reinforced polymer bond failure. Consecutive levels of refinement of the 1D model are presented and available results from higher order models are compared. The study of the structural response involved comparing displacements, strains, cracking patterns and failure mechanisms. The demonstrated robustness and efficiency of the proposed model makes it adequate for blind assessments of existing bridges.     

Developing a one-dimensional heat and mass transfer algorithm for describing the effect of green roofs on the built environment: Comparison with experimental results

This paper investigates the mathematical modelling of the effect of green roofs on mitigating raised urban temperatures. A dynamic, one-dimensional model is developed, describing heat and mass transfer in building materials, considered as capillary-porous bodies, the vegetated canopy, modelled as a combined plant–air canopy layer, the soil and the air. The model is validated with an experiment, conducted in the Welsh School of Architecture, in Cardiff, in summer 2004. The right choice of parameters that affect the accuracy of the model (such as the expression of the convective heat transfer coefficient and stomatal resistance) is discussed. Special attention is given to the comparison between the experimental results and the outputs of only heat transfer algorithms and heat and mass transfer expressions. Taking these comparisons into consideration, conclusions are drawn about developing an accurate algorithm describing the thermal effect of green roofs on the built microclimate.     

A study on a porous residential building model in hot and humid regions: Part 1—the natural ventilation performance and the cooling load reduction effect of the building model

This study targets environmental load reduction in hot and humid regions. It reveals the effects that porous residential buildings have on the natural ventilation performance and, consequently, the cooling load reduction. Two residential building models, namely a model with a void ratio of 0% and a model with a void ratio of 50%, are evaluated using computational fluid dynamics (CFD) analysis and thermal and airflow network analysis. The analysis on components of the heat load indicates that improvements in the natural ventilation performance would significantly reduce the cooling load.     

Inflow turbulence generation techniques for large eddy simulation of flow and dispersion around a model building in a turbulent atmospheric boundary layer

The present paper investigates the performance of various inflow turbulence generation techniques (ITGT) for large eddy simulation (LES) of flow and dispersion around a model building in a turbulent atmospheric boundary layer. Four different ITGT comprising 1 – no fluctuations, 2 – spectral method, 3 – vortex method and 4 – internal mapping, based on two basic methodologies (i.e. precursor and synthetic turbulence methods), are employed. These techniques are evaluated by considering their prediction accuracy, computational costs, complexity of implementation, inflow information required to operate and impacts on the flow downstream of the inlet, particularly in the wake region of the model building. Results indicate that the accuracy of LES predictions is greatly reliant on ITGT. It is shown that ITGT not only have significant effects on flow field vortical structures, but also influence frequency contents of velocity fluctuations, recirculation regions and plume shapes in the wake region.     

Assessing the total energy impact of manual and optimized blind control in combination with different lighting schedules in a building simulation environment

In this work, we investigated the interplay and the influence of lighting and blind control models on the heating, cooling, and lighting energy loads of an office room. By including different stochastic models for occupancy and appliances, we built a complete simulation environment based on the building simulation program IDA ICE. For control models, we implemented different strategies, including a simple on/off scheme, a realistic model of occupants, and an optimized control. In literature, the results are often compared with simple on/off schemes, which are not hard to beat in terms of performance. With an optimal control, the real saving potential is assessed, which can be used as benchmark case for comparison with other control models. Results based on annual simulations show that active occupants can reduce energy consumption by up to 50% from a worst-case scenario, whereas advanced controllers can further reduce the consumption by another 60%.     

The application of heat pipe heat exchangers to improve the air quality and reduce the energy consumption of the air conditioning system in a hospital ward—A full year model simulation

Research was conducted to study the effect of heat pipe heat exchangers on the existing air conditioning system of a hospital ward located in Malaysia, a tropical region. The present research employs the transient system simulation software (TRNSYS) to study the hour-by-hour performance of the system in terms of supply duct air and indoor air conditions in the ward space. Fieldwork study showed that the existing air conditioning system operating in the Orthopaedic Ward, University of Malaya, Malaysia is not capable of providing the desired supply duct air and indoor air to the space. Therefore, the possibility of improving the air conditioning system by adding heat pipe heat exchangers was investigated. The impact on energy consumption, power savings, supply duct air and indoor air with heat pipe heat exchangers incorporated in the air conditioning system were simulated and the results were compared with the existing system.Based on the present research, the system with the added eight-row heat pipe heat exchangers is recommended to provide convenient and healthy air into the ward space according to the ASHRAE recommendations. Moreover, it was found that by applying the new design, a considerable amount of energy and power could be saved.