Heat , Air And Moisture Research Articles

Validation of a coupled heat, vapour and liquid moisture transport model for porous materials implemented in CFD

Moisture-related damage is an important issue when looking at the performance of building envelopes. In order to accurately predict the moisture behaviour of building components, building designers can resort to Heat, Air and Moisture (HAM) models. In this paper a newly developed heat and mass transfer model that is implemented in a 3D finite volume solver, Fluent®, is presented. This allows a simultaneous modelling approach of both the convective conditions surrounding a porous material and the heat and moisture transport in the porous material governed by diffusion. Unlike most HAM models that often confine to constant convective transport coefficients it is now possible to better predict these convective boundary conditions. An important application of the model is the convective drying of porous building materials. Especially during the first drying stage, the drying rate is determined by the convective boundary conditions. The model was validated against a convective drying experiment from literature, in which a saturated ceramic brick sample is dried by flowing dry air over one side of the sample surface. Temperature and relative humidity measurements at different depths in the sample, moisture distribution profiles and mass loss measurements were compared with simulation results. An overall good agreement between the coupled model and the experiments was found, however, the model predicted the constant drying rate period better than the falling rate period. This was improved by adjusting the material properties. The adjustment of the material properties was supported by neutron radiography measurements.

Drying behaviour of calcium silicate

Nowadays, the hygrothermal performance of the building envelope is often evaluated using HAM (heat, air and moisture) models. These models can be used to predict the hygrothermal response of the building envelope and can assist in reducing the risk of any moisture-related damage (e.g. decrease of thermal insulation value due to wetting, interstitial condensation, etc.). At the same time it is important to understand the physical mechanisms of wetting and drying of building materials. Experimental research can contribute to a better understanding of these mechanisms.In this paper the focus lies on the wetting and drying phenomena occurring in building materials. One specific material is highlighted: calcium silicate. Calcium silicate is an inorganic, hygroscopic and capillary active insulation material, which is often used in interior thermal insulations systems. The paper describes a drying experiment in which a calcium silicate sample dries out starting from saturation. The experiments showed that calcium silicate has an atypical drying behaviour: during the second drying phase an intermediate plateau was observed in the temperature course. Numerical simulations performed with a recently developed CFD–HAM model were compared with the experimental results and were used to explain the experimental observations.

Transient Pulse Method for Measuring of Heat Conductivity of Autoclaved Aerated Concrete

The heat conductivity, specific heat and bulk density are the basic material parameters and thus indispensable. There are numbers of methods used for measurement of heat conductivity. However, the non-stationary (transient pulse method) measurements methods are preferred for description of heat conductivity moisture dependence. This article discusses on the establishment dependence of heat conductivity on water content for selected porous building materials by using non stationary measurement methods. Dependence of the thermal conductivity on the water content is necessary for coupled heat, air and moisture (HAM) simulation in building construction.

Numerical and experimental investigation of the hygrothermal response of timber frame walls with an exterior air barrier

Laboratory tests and numerical simulations to study the hygrothermal consequences of using an exterior air barrier in light-weight building envelopes are discussed. Full-scale test walls are placed between a vertical hot and coldbox, operating at controlled temperatures, humidity levels and total air pressures. The test walls are exposed to different boundary conditions during five consecutive measuring stages. Each test wall is provided with 18 thermocouples, 15 relative humidity, 3 heat flux and 1 pressure gauge sensor, covering the most important positions of the wall. In this way, the obtained results can be used as detailed validation data for heat, air and moisture (HAM)-models with a strong focus on natural and forced air convection. The main emphasis of this article is the evaluation of a numerical model with the current dataset. The model used is a modified version of DELPHIN 5 including a quasi-steady state airflow model with the ability to capture natural convection.

Predicting the microbial exposure risks in urban floods using GIS, building simulation, and microbial models

London is expected to experience more frequent periods of intense rainfall and tidal surges, leading to an increase in the risk of flooding. Damp and flooded dwellings can support microbial growth, including mould, bacteria, and protozoa, as well as persistence of flood-borne microorganisms. The amount of time flooded dwellings remain damp will depend on the duration and height of the flood, the contents of the flood water, the drying conditions, and the building construction, leading to particular properties and property types being prone to lingering damp and human pathogen growth or persistence. The impact of flooding on buildings can be simulated using Heat Air and Moisture (HAM) models of varying complexity in order to understand how water can be absorbed and dry out of the building structure. This paper describes the simulation of the drying of building archetypes representative of the English building stock using the EnergyPlus based tool ‘UCL-HAMT’ in order to determine the drying rates of different abandoned structures flooded to different heights and during different seasons. The results are mapped out using GIS in order to estimate the spatial risk across London in terms of comparative flood vulnerability, as well as for specific flood events. Areas of South and East London were found to be particularly vulnerable to long-term microbial exposure following major flood events.

Conjugate modelling of convective drying phenomena in porous building materials

Moisture storage and the associated heat and moisture transport in buildings have a large impact on the building envelope durability, the energy consumption in buildings and the indoor climate. Nowadays HAM (Heat, Air and Moisture transport) models are widely used to simulate and predict the effect of these transport phenomena in detail. Recently these HAM models are being coupled to CFD (Computational Fluid Dynamics) to study the moisture exchange between air and porous materials on a local scale (microclimates). The objective of this research is to develop such a model to study drying phenomena. In this paper the emphasis lies on the modelling of convective drying of porous building materials. An important aspect for the correct modelling of convective drying is the way the air boundary is implemented. A short literature review reveals that different modelling approaches can be used. This paper gives a short overview of the state of the art in conjugate heat and mass transport modelling for convective drying. In this review shortcomings of currently applied modelling approaches are highlighted. Finally the newly developed model is used to simulate the convective drying of a sample of ceramic brick. These simulations were then compared with measurements from literature. A good agreement was found.

Numerical investigation of the influence of room factors on HAM transport in a full-scale experimental room

Abstract To model the indoor environment influenced by room factors including moisture loads and ventilation conditions, the governing equations of heat, air and moisture (HAM) transport inside the room and through the envelope, including the coupling process of the interaction of heat and moisture between indoor air and envelope, need to be solved. This paper presents a CFD model of the indoor environment that takes into account the heat and moisture transfer through the envelope in a single simulation environment. The comparisons between experimental data and simulation results show close agreement. This model is then implemented in 12 cases to study the influence of moisture loads, ventilation rates, and positions of the air inlet and outlet on the indoor environment and on the heat and moisture transport through the envelope. Simulation results show that the humidity responses of the test room vary significantly for different moisture loads from 42 g/h to 91 g/h and for different ventilation rates from 0.5 air change per hour (ach) to 1 ach. Changing the positions of the ventilation vents alters the air flow pattern significantly, thus affecting the ventilation efficiency. With a heat and moisture source inside the test room, the changed ventilation efficiency influences the indoor environment significantly, especially under low ventilation rates. The increased ventilation rate carries away more heat and moisture, which reduces both temperature and moisture levels inside the room. Under the mixed convection condition, the indoor temperature and moisture levels dominate the heat and moisture transport through the wall system.

Multiphysics modeling of building physical constructions

The paper presents an overview of Multiphysics applications using a Multiphysics modeling package for building physical constructions simulation. The overview includes three main basic transport phenomena for building physical constructions: (1) heat transfer, (2) heat and moisture transfer and (3) heat, air and moisture (HAM) transfer. It is concluded that full 3D transient coupled HAM models for building physical constructions can be build using a Multiphysics modeling package. Regarding the heat transport, neither difficulties nor limitations are expected. Concerning the combined heat and moisture transport the main difficulties are related with the material properties but this seems to be no limitation. Regarding the HAM modeling inside solid constructions, there is at least one limitation: the validation is almost impossible due to limitation of measuring ultra low air velocities of order μm/s.

Experimental data set for validation of heat, air and moisture transport models of building envelopes

This paper reports experimental studies on heat, air and moisture (HAM) transfer through a full scale light weight building envelope wall under real atmospheric boundary conditions. The main objective of the article is to generate informative data so that it can be used for numerical validation of HAM models. The considered wall is a multilayered structure built up from outside to inside of external board, vented cavity, fibreboard sheathing, mineral wool between wooden studs and interior finishing. The global wall has a surface area of (1.80 × 2.68) m 2; and is subdivided into three vertical parts. The parts differ from each other by the applied interior finishing. Between the different layers of each part and on the surfaces of the wall humidity, temperature and heat flux sensors are placed in a 3D matrix. At the outer surface of the wall, the applied sheathing is a bituminous wood board. In the board nine removable specimens are included. By regularly weighing the fibreboard samples, their moisture content could be quantified. Using data collected over a total time span of about two years, insight about the hygrothermal behaviour of the different envelope parts is obtained and at the same time a well-documented data set is generated that can be used for hygrothermal envelope model validation purposes.

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. This paper describes the design of a new test facility for humidity experiments. A dedicated AHU system is used to provide well-controlled air (constant temperature and RH) to an airtight and well-insulated room-size test chamber. In one of the walls of the chamber a calcium silicate sample is installed. A step in RH of the supply air is imposed. Temperature and RH of the supply air, the room air and on various depths inside the sample are continuously registered during the experiments. Two types of experiments were carried out to validate a coupled CFD–HAM model and a coupled BES–HAM model. The temperature outside the test chamber was controlled and there was no temperature difference imposed across the chamber walls. Comparing the models with the measured data gave satisfactory agreement.

Reliability of material data measurements for hygroscopic buffering

Numerical models of heat, air and moisture (HAM) transport in building components continue to advance, but the reliability of the measured material properties, which serve as input for the HAM-models, remains very uncertain. In the framework of IEA/ECBCS Annex 41 a round robin experiment on the determination of the hygric properties of porous building materials was performed to generate a dataset for benchmarking numerical models. In the round robin experiment, 14 laboratories measured the vapour permeability and sorption isotherm of coated and uncoated gypsum board. Apart from a zero off-set due to different conditions during oven-drying, the measured sorption isotherms showed acceptable agreement. But for the vapour transmission tests, though performed according to an international standard, a wide spread in the measured data was found. The differences are most pronounced for vapour tight specimen. The large number of participants in the round robin experiment, allowed to investigate possible causes for the observed deviations. To show the importance of correct material data, the consequences of using this kind of contradictive results was investigated by calculating the impact on the moisture penetration depth and equivalent moisture capacity.

Integrated analysis of whole building heat, air and moisture transfer

There is a continuous dynamic heat, air and moisture (HAM) interaction between the indoor environment, building envelope and mechanical systems. In spite of these interdependences, the current indoor, building envelope and energy analysis tools are used independently. In this paper a holistic HAM model that integrates building envelope enclosures, indoor environment, HVAC systems, and indoor heat and moisture generation mechanisms, and solves simultaneously for the respective design parameters is developed. The model is benchmarked with internationally published test cases that require simultaneous prediction of indoor environmental conditions, building envelope moisture performance and energy efficiency of a building.

Transient model for coupled heat, air and moisture transfer through multilayered porous media

Most building materials are porous, composed of solid matrix and pores. The time varying indoor and outdoor climatic conditions result heat, air and moisture (HAM) transfer across building enclosures. In this paper, a transient model that solves the coupled heat, air and moisture transfer through multilayered porous media is developed and benchmarked using internationally published analytical, numerical and experimental test cases. The good agreements obtained with the respective test cases suggest that the model can be used to assess the hygrothermal performance of building envelope components as well as to simulate the dynamic moisture absorption and release of moisture buffering materials.

Evaluation of the Climate Control Performance and Reliability of Active Display Cases / Bewertung der Qualität einer Klimaregelung und der Zuverlässigkeit aktiver Ausstellungsschränke

The study concerns the reliability of active display cases to be used at the Dutch Maritime Museum located in Amsterdam. The paper presents heat, air and moisture (HAM) models of display cases and the indoor climate of the zone surrounding them. Furthermore, data from measurements are provided for validation purposes. It is concluded that in case of failure of the climate system, responsible for the indoor climate surrounding the active display case, the climate inside the active display case will stay within the required limits for more than five hours during the winter and for more than half an hour during the summer. However, some undetected failure scenarios could be responsible for exceeding the required limits of the indoor climate inside the display case within five minutes. The latter shows the importance of the presence of reliable failure detection and handling system.

On coupling 1D non-isothermal heat and mass transfer in porous materials with a multizone building energy simulation model

Hygroscopic materials available in the interior of buildings such as wood, gypsum, paper etc, are able to absorb moisture if the relative humidity of the room increases and release it again if the relative humidity decreases. This moisture buffering phenomenon is often accounted for in a simplified way in Building Energy Simulation programs (BES) e.g. TRNSYS, which limits their applicability. Nevertheless several building applications require an accurate prediction of the indoor relative humidity already from the design stage. This paper presents the development and possibilities of a coupled TRNSYS-HAM model which allows to account for the response of a multizone building on moisture buffering effects in a more detailed way. The HAM (Heat, Air and Moisture) model describes one dimensional transient coupled heat and mass transfer in porous materials. The coupled model agrees well with the analytical solution of two verification exercises. The model is applicable for the design and evaluation of relative humidity controlled HVAC (Heating, Ventilation and Air Conditioning) systems and can be employed in numerous building applications. To illustrate this an evaluation of gypsum cooled ceilings was made using a simplified model and the coupled model. The results show that when non-isothermal vapour transfer in the gypsum layer is neglected, the buffering effect is largely underestimated.

Thermal Benefits of Tiled Roofs with Above-sheathing Ventilation

Computer simulations are conducted to demonstrate the thermal benefits of good roofing design in hot climates. The work is aimed to quantify the thermal benefits from batten-space ventilation, commonly referred to as above sheathing ventilation (ASV), and thermal mass. Both roof features are effective in reducing the heat transmission into the habitable space. Nevertheless, the use of roof ventilation is often misunderstood and most procedures for energy assessment of building structures do not recognize the contribution from ASV and thermal mass. As part of this work, a computer algorithm is developed to predict the heat, air and moisture (HAM) transport in the building envelope. The algorithm predicts the transient temperature and moisture behavior in the building envelope and takes account of the air movement from bouyancy and wind driven airflows.

Combined Heat, Air and Moisture (HAM) Transfer Model for Porous Building Materials

In the building science area, mathematical models are developed to provide better indoor thermal comfort with lower energy consumption. Although the fact moisture and air transfer can strongly affect the temperature distribution within constructions, whole-building simulation codes do not take into account the convective air transport in porous materials. In this way, this article presents a heat, air, and moisture (HAM) transfer model based on driving potentials of temperature, air pressure, and water vapor pressure gradients for consolidated porous material in both pendular and funicular states. The solution of the set of governing equations has been simultaneously obtained using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three potentials. To conclude, results are presented showing the impact of convective terms on the HAM transfer through a two-layer porous building envelope.

Application of an integrated indoor climate, HVAC and showcase model for the indoor climate performance of a museum

This paper presents a case study on the performance based design of a HVAC system and controller of a museum. A famous museum in The Netherlands has reported possible damage to important preserved wallpaper fragments. The paper provides an evaluation of the current indoor climate by measurements, showing that the indoor climate performance does not satisfy the requirements for the preservation of old paper. To solve this problem we developed an integrated heat air and moisture (HAM) model consisting of models for respectively: the indoor climate, the HVAC system and controller and a showcase. The presented models are validated by a comparison of simulation and measurement results. The integrated model is used for the evaluation of a new HVAC controller design and the use of a showcase. It is concluded that it is not possible to satisfy the indoor climate within the recommended limits, exclusively by the use of a new control strategy. Furthermore in order to meet the recommendations, the wallpaper fragments should be placed in a showcase and a similar control strategy as presented in the paper, has to be implemented in order to limit the room air temperature change.

Assessment Method of Numerical Prediction Models for Combined Heat, Air and Moisture Transfer in Building Components: Benchmarks for One-dimensional Cases

The standardised Glaser method for calculation, prediction and evaluation of moisture performance is considered as rarely applicable. The present state of knowledge, analytical as well as experimental, concerning heat, air and moisture demands updating of standards. This paper presents five numerical benchmark cases for the quality assessment of simulation models for one-dimensional heat, air and moisture (HAM) transfer. In one case, the analytical solution is known and excellent agreement between several solutions from different universities and institutes is obtained. In the remaining four cases, consensus solutions have been found, with good agreement between different HAM models. The work presented here is an outcome of the EU-initiated project for standardisation of HAM calculation methods (HAMSTAD WP2).

Towards an Engineering Model of Material Characteristics for Input to Ham Transport Simulations - Part 1: An Approach

Heat, Air and Moisture (HAM) modelling of building performance is a quite young research subject but the experimental determination of material properties is often based on classical methods. One should review the manner in which we define characteristic material parameters and there is a need to develop an approximation used to generate the required material functions for input to HAM-transport simulations. The paper presents such an approach, called an engineering model for hygrothermal material characterisation. The paper poses the question, how to arrive at input data that can be used for a model based on thermodynamically defined potentials (Only such a model allows introduction of new potential components (freezing depression, osmotic pressure, air pressure, overburden envelope pressure)) (e.g., Grunewald, J. (1997) and Grunewald, J. (1999)) and yet the respective functions used to describe changes in the material response as a function of the variables of state. Such functions should have a reasonable precision and goodness of fit while the number of measured points must be reduced to a minimum. Those measurements should be relatively easy to perform (i.e., they would not require determination of temporal and spatial profiles of moisture). This discussion paper highlights steps already taken (Part 1), and lists issues that need to be resolved before reaching this goal (Part 2).