Heat Andmoisture Research Articles

Assessment of Deterioration Risk of the Wooden Columns of Historical Buildings in Southern China Based on HAM Transfer Model

Chinese historical buildings are mainly made of wood structure system, however, as an organic material, wood is prone to decay under the fluctuation of hygrothermal environment. This study aims to assess the risk of decay of two types of wooden columns (exposed column under eaves and semi-exposed column embedded in walls) and further propose reasonable protection measures for Nanjing area where with a hot and humid summer and cold winter and with a high groundwater level. A two-dimensional heat and moisture (HAM) transfer model is established to simulate the fluctuations of temperature and moisture content at the bottom of columns in a historical Chinese building. Then, the risk of decay of columns was evaluated. The results show that because the stone column base isolates the groundwater and the amount of air condensation is limited, the risk of decay at the bottom of the exposed column under the eaves is lower than the semi-exposed column embedded in wall which is more prone to be affected by the high level of groundwater.

DEVELOPMENT OF A CELL AUTOMATA MODEL FOR SIMULATION OF THE DRYING PROCESS OF CAPILLARY-POROUS MATERIALS IN PERIODIC ACTION CHAMBERS

In this work, the main emphasis is placed on the creation of a cellular automaton model for simulating the drying process of capillary-porous materials. The possibility of creating a cell-automatic field for the studied 3D model of a periodic drying chamber is considered. Appropriate algorithms are developed and their software implementation is carried out, since the cellular automata field is one of the key requirements for using the cellular automata method. Another key requirement is the availability of adequate transition rules. For their development, the analysis of mathematical models of heat and moisture transfer processes in periodic drying chambers is carried out. Transition rules are being developed, which make it possible to determine changes in temperature and moisture content of the material, as well as changes in the main parameters of the drying agent, which include temperature and relative humidity. The paper also simulates the drying process of capillary-porous materials based on the developed cellular automaton model, which includes the cellular automaton field and transition rules. The input data for modeling are taken from the parameters of technological conditions that are used in real drying chambers. Pine with a thickness of 45 mm was chosen as the studied capillary-porous material. with an initial moisture content of 65%. As a result of the simulation, graphical dependences of the desired parameters of the material and the drying agent with respect to time were obtained. Verification of the obtained results is carried out by comparing them with data collected by sensors in a real drying chamber. For comparison, a relative error is determined, the average values of which do not exceed 10%. This result confirms the adequacy of the developed model of cellular automata.

Comparison of thermal bridge calculation methods

We need to consider linear heat losses due to thermal bridges for the accurate calculation of building heat losses. Our research examined a whole building, and different thermal bridge calculation methods were compared. The following techniques were included in the study: a simplified method according to the Hungarian energy performance regulation where the effect of thermal bridges can be taken into account by multiplication factors applicable to thermal transmittances; a simplified thermal bridge catalogue of ISO 14683; a recent national thermal bridge catalogue; two-dimensional thermal modelling as well as a conjugated heat and moisture (HAM) simulation considering steady-state and dynamic conditions. Overall, we created eight different numerical modelling approaches depending on the type of simulation and boundary condition. The modelling and simulations were carried out using multiphysics software based on the finite element method according to ISO 10211 and EN 15026. All the relevant details of the building were analysed to get a complete picture. Based on the results, we analysed each method's relative proportions of surface and linear heat losses. The evaluation showed that the Hungarian simplified method generated the lowest heat losses for thermal bridges, while ISO 14683 produced the highest results, with the numerically simulated results in the middle. The overall heat losses varied by 30 %, depending on the thermal bridge calculation method. Linear heat losses were between 12 % to 32 % of the surface heat losses. Our study helps to choose the adequate method to perform thermal bridge simulations.

Assessment of Deterioration Risk of Maijishan Grotto under the Radiation Difference Based on Heat and Moisture Transfer Model

The ambient environment of architectural heritage is an important factor affecting its conservation. Two adjacent rows of Buddha statues in Grottoes No. 3 (semi-open) of Maijishan Grotto in Gansu, China, show apparent differences in the degree of deterioration. This study made a monitoring scheme of grottoes microenvironments such as air temperature, relative humidity, radiation, and surface temperature to explore the cause of the difference. A two-dimensional heat and moisture (HAM) transfer model was established and verified to simulate the temperature and humidity on the surface and inside of the Buddha statues. Then, temperature and water content fluctuation and the risks of thermal stress destruction on the surface and near the surface of the Buddha statues were evaluated. The results show that the radiation difference causes thermal stress and water content differences both in heights and in depths. This impact brought by the direct sunlight may contribute to the different deterioration on the two rows of Buddha statues. The eaves shaded the upper row of the Buddha statues much longer than the lower ones. Less severe fluctuation and differences in temperature and water content occur at the middle and upper points. This study evaluates the degradation of Grottoes No. 3 and has guiding significance for its preservation methods.

Case study of a humidity layer above Arctic stratocumulus and potential turbulent coupling with the cloud top

Abstract. Specific humidity inversions (SHIs) above low-level cloud layers have been frequently observed in the Arctic. The formation of these SHIs is usually associated with large-scale advection of humid air masses. However, the potential coupling of SHIs with cloud layers by turbulent processes is not fully understood. In this study, we analyze a 3 d period of a persistent layer of increased specific humidity above a stratocumulus cloud observed during an Arctic field campaign in June 2017. The tethered balloon system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) recorded vertical profile data of meteorological, turbulence, and radiation parameters in the atmospheric boundary layer. An in-depth discussion of the problems associated with humidity measurements in cloudy environments leads to the conclusion that the observed SHIs do not result from measurement artifacts. We analyze two different scenarios for the SHI in relation to the cloud top capped by a temperature inversion: (i) the SHI coincides with the cloud top, and (ii) the SHI is vertically separated from the lowered cloud top. In the first case, the SHI and the cloud layer are coupled by turbulence that extends over the cloud top and connects the two layers by turbulent mixing. Several profiles reveal downward virtual sensible and latent heat fluxes at the cloud top, indicating entrainment of humid air supplied by the SHI into the cloud layer. For the second case, a downward moisture transport at the base of the SHI and an upward moisture flux at the cloud top is observed. Therefore, the area between the cloud top and SHI is supplied with moisture from both sides. Finally, large-eddy simulations (LESs) complement the observations by modeling a case of the first scenario. The simulations reproduce the observed downward turbulent fluxes of heat and moisture at the cloud top. The LES realizations suggest that in the presence of a SHI, the cloud layer remains thicker and the temperature inversion height is elevated.

Influence of the Meteorological Record Length on the Generation of Representative Weather Files

Heat and moisture (HM) transfer simulations of building envelopes and whole building energy simulations require adequate weather files. The common approach is to use weather data of reference years constructed from meteorological records. The weather record affects the capability of representing the real weather of the resulting reference years. In this paper the problem of the influence of the length of the records on the representativeness of the reference years is addressed and its effects are evaluated also for the applicative case of the moisture accumulation risk analysis with the Glaser Method and with DELPHIN 6, confirming that records shorter than 10 years could lead to less representative reference years. On the other hand, it is shown that reference years obtained from longer periods are not representative of the most recent years, which present higher dry-bulb air temperatures due to a short-term climate change effect observed in all the considered weather records. An alternative representative year (Moisture Representative Year) to be used in building energy simulations with a strong dependence on moisture is presented.

Impact of poleward heat and moisture transports on Arctic clouds and climate simulation

Abstract. Many general circulation models (GCMs) have difficulty simulating Arctic clouds and climate, causing substantial inter-model spread. To address this issue, two Atmospheric Model Intercomparison Project (AMIP) simulations from the Community Atmosphere Model version 5 (CAM5) and Seoul National University (SNU) Atmosphere Model version 0 (SAM0) with a unified convection scheme (UNICON) are employed to identify an effective mechanism for improving Arctic cloud and climate simulations. Over the Arctic, SAM0 produced a larger cloud fraction and cloud liquid mass than CAM5, reducing the negative Arctic cloud biases in CAM5. The analysis of cloud water condensation rates indicates that this improvement is associated with an enhanced net condensation rate of water vapor into the liquid condensate of Arctic low-level clouds, which in turn is driven by enhanced poleward transports of heat and moisture by the mean meridional circulation and transient eddies. The reduced Arctic cloud biases lead to improved simulations of surface radiation fluxes and near-surface air temperature over the Arctic throughout the year. The association between the enhanced poleward transports of heat and moisture and increase in liquid clouds over the Arctic is also evident not only in both models, but also in the multi-model analysis. Our study demonstrates that enhanced poleward heat and moisture transport in a model can improve simulations of Arctic clouds and climate.

Dehumidification performance investigation of run-around membrane energy exchanger system

Liquid-to-air membrane energy exchanger is a novel membrane base energy exchanger, which allows both heat and moisture transfer between air and a salt solution. It uses semi-permeable membrane to eliminate entrainment of liquid desiccant as aerosols in air stream and allow simultaneous heat and moisture transfer between salt solution flow and airflow. The heat and mass transfer performance of a single liquid-to-air membrane energy exchanger is significantly dependent on two dimensionless parameters. They are the number of heat transfer units (NTU) and the ratio of heat capacity rates between solution flow and air flow (Cr*). The liquid-to-air membrane energy exchangers can also be applied in a run-around membrane energy exchanger system, which is mainly comprised of two liquid-to-air membrane energy exchangers and a closed loop of aqueous desiccant solution and used as a passive energy recovery system to recover the energy (both heat and moisture) from the exhaust air to precondition the supply air in air conditioning systems. In this study the dehumidification capacity of a run-around membrane energy exchanger is investigated numerically at different exhaust air temperatures and Cr* values. Increasing the exhaust air temperature or the Cr* would enhance the dehumidification capacity of the a run-around membrane energy exchanger system under Cr*?1, but the improvement is limited. The dehumidification capacity at low Cr* is much lower than that under the optimal Cr* value (Cr*=3.2) where the maximum latent effectiveness is obtained.

The Role of Transient Wetting on Mould Growth on Wooden Claddings

Mold growth on the surface of wooden façades is usually dealt with by using surface treatment such as paint with fungicides. However, new developments in architecture are moving towards less use of coating, and more use of untreated wooden claddings. Mould growth is well understood and described, and several models for predicting mould growth on building materials exist. It is commonly known that mould growth is directly controlled by the climate which the wood is exposed to. Several authors identify humidity, temperature and time as the main drivers of mould growth. However, most of the current growth models developed are based on laboratory measurements at stable climatic conditions. Consequently, these models are less suitable for prediction of mould growth on exterior surfaces exposed to rapidly changing weather conditions. This paper analyses the effect of variations of meteorological data on the mould growth on wooden claddings. An experimental setup of wood samples was exposed to outdoor conditions and hourly weather conditions as well as the mould growth at different intervals were measured. The measurements were supplied with 1-D Heat And Moisture (HAM) simulations to provide a more accurate estimate of the conditions on the surface of the samples. The purpose of the analysis was to evaluate if an existing mould growth model might be applicable also for predicting outdoor mould growth. Several profiles of temperature and moisture were continuously monitored on different locations of an eight-story building made from Cross Laminated Timber (CLT). The results from the analysis of the samples of wood cladding were used to model the mould growth on different locations of the building. Also the drying effect of wind around the building was studied. The study shows that there is large variation of potential mould growth on the façade of the building.

HAM and mould growth analysis of a wooden wall

Abstract The aim of engineers is to design structures, while minimising their energy dependence and create a suitable environment for living. Some of the most important factors for maintaining the indoor environment are temperature and humidity. Unsuitable combinations of heat and moisture (HAM) could indicate a danger of mould growing. The mould’s existence influences the reliability and lifespan of a structure. Some materials, for example wood, are very sensitive to mould growth. To analyse a structure for mould growth risk it is necessary to know the values of temperature and relative humidity on the surfaces and inside of structures. There are two possibilities to acquire these quantities; taking measurements at a construction site or in a laboratory, or applying numerical solutions using an accessible programme. One such programme is Wufi, which has been developed especially for HAM analysis. An advantage of measuring at the construction site is the reality, which is not affected by numerical errors. On the other hand, numerical analysis usually needs lower costs and does not demand so much time to simulate long term periods. From the point of view of accuracy, it is suitable to verify the numerical solution with measurements. The aim of this paper is threefold. First, to analyse the risk of mould growth. Second, it presents a verification of numerically calculated data using Wufi 2D against actual measurement data acquired from a real structure, which is presented by a low-energy house located in Oulu/Finland. Third, to prove that the form of low-energy building structures do not increase mould growth risk. After carrying out the verification, the received outcomes are utilised for mould growth analysis. For an expression of reality in the numerical solution, a transient simulation was needed. The received outcomes were then used for mould growth analyses. There is a possibility to predict real mould growth risk inside the structure and on its surfaces, corresponding to the critical relation between temperature and relative humidity, which separates favourable and unfavourable areas for mould growth.

Editorial - New Trends in the Numerical Analysis of Masonry Structures

The Special Issue of The Open Civil Engineering Journal entitled “New trends in the numerical analysis of masonry structures” provides an insight into the most up-to-date nu-merical techniques used at academic and professional level to perform advanced structuralanalyses on masonry struc-tures. Masonry is a building material that has been used for more than ten thousand years. In many countries, masonry structures still amount to 30–50%of the new housing devel-opments. Also, most structures built before the 19th century and still surviving are built with masonry. Masonry is usu-ally described as a heterogeneous material formed by units and joints, with or without mortar, and different bond ar-rangements. Units are such as bricks, blocks, ashlars, adobes, irregular stones and others. Mortar can be clay, bitumen, chalk, lime/cement based mortar, glue or other. The almost infinite possible combinations generated by the geometry, nature and arrangement of units as well as the characteristics of mortars raise doubts about the accuracy of the term “ma-sonry”. Still, much information can be gained from the study of regular masonry structures, in which a periodic repetition of the microstructure occurs due to a constant arrangement of the units (or constant bond). The difficulties in performing advanced testing and pro-viding sufficiently general numerical models for this kind of structures are basically due to the innumerable variations of masonry typologies, the large scatter of in situ material prop-erties and the impossibility of reproducing all in a specimen. Therefore, most of the advanced numerical research carried out in the last decades concentrated in brick / block masonry and its relevance for design. Accurate modelling requires a comprehensive experimental description of the material, which seems mostly available at the present state of knowl-edge. From a numerical point of view, masonry behaviour is quite complex to model, exhibiting non-linearity very early during the loading process, with softening in both tension and compression, low ductility and differed deformations under sustained loads. In addition, masonry is the result of the assemblage of bricks or stones, where mortar is laid, with common geometric irregularities adding further complexity to the problem. The special issue collects ninepapers from experts in the field, including contributions of researchers from six differ-ent countries (Czech Republic, Iran, Italy, Portugal, Spain, Switzerland), either devoted to the utilization of non-standard numerical models for case-studies or presenting new approaches for the interpretation of masonry behaviour in presence of different kinds ofnon-linearity. The effort is always to put the knowledge beyond the existing state-of-the art. Karbassi and Lestuzzi [1]present a fragility analysis per-formed on unreinforced masonry buildings, conducted by means of the so called Applied Element Method (AEM), to define fragility curves of typical masonry buildings which may be regarded as representative of building classes. A se-ries of nonlinear dynamic analyses using AEM are per-formed for a 6-storey stone masonry and a 4-storey brick masonry building using more than 50 ground motion re-cords. The distribution of the structural responses and inter-storey drifts are finally used to develop spectral-based fragil-ity curves for the five European Macro-seismic Scale dam-age grades. In the second paper, Milani et al. [2]perform a detailed non-linear analysis (both pushover and limit analysis) on the San Pietro di Coppito bell tower in L’Aquila, Italy, trying to have an insight into the causes of the collapse occurred dur-ing the devastating 2009 earthquake. Sykora et al. [2]review several topics related to the ho-mogenization of transport processes occurring in historical masonry structures. Particular attention is paid to variations of temperature and moisture fields, whose contribution to structural damage usually far exceeds the effects of me-chanical loadings. The concept of Statistically Equivalent Periodic Unit Cell (SEPUC) is reviewed and utilized to deal with historic masonry and random patterns. Accepting SEPUC as a reliable representative volume element, a Fast Fourier Transform to both the SEPUC and large binary sam-ples of real masonry is used to tackle effective thermal con-ductivities problems. Fully coupled non-stationary heat and moisture transport problems are addressed next in the framework of a two-scale first-order homogenization, with emphases on the application of boundary and initial condi-tions at the meso-scale.

On modelling the drying of porous materials: analytical solutionsto coupled partial differential equations governing heat andmoisture transfer

Luikov′s theory of heat and mass transfer provides a framework to model drying porous materials. Coupled partial differential equations governing the moisture and heat transfer can be solved using numerical techniques, and in this paper we solve them analytically in a setting suitable for industrial drying situations. We discuss the nature of the solutions using the physical properties of Pinus radiata. It is shown that the temperature gradients play a significant role in deciding the moisture profiles within the material when thickness is large and that models based only on moisture potential gradients may not be sufficient to explain the drying phenomena in moist porous materials.

Charcoal as an airway isoflurane reflection filter

The isoflurane-saving and CO2-retaining effects of a charcoal filter were compared with a Siemens standard heat and moisture (HME) exchanger and an emptied specimen (dummy). Isoflurane was delivered during the inspiratory phase and consumption investigated at 10, 15 and 25 cycles min-1. The investigation was performed by ventilation with humidified air with a constant end-tidal CO2 and temperature. For a comparison, isoflurane was delivered in a conventional manner via the ventilator. The arrangement with a charcoal filter reduced the isoflurane consumption by a factor of 2.0-2.6, depending on ventilatory rate. Most of the saving was a result of the method of isoflurane delivery (factor 1.4-2.0), while adding the reflector gave a further reduction (factor 1.3-1.5). One circumstance that reduced the net efficiency of the charcoal filter was that it also reflected CO2; consequently, total minute ventilation had to be increased to maintain constant end-tidal CO2.

Charcoal as an airway isoflurane reflection filter

The isoflurane-saving and CO2-retaining effects of a charcoal filter were compared with a Siemens standard heat and moisture (HME) exchanger and an emptied specimen (dummy). Isoflurane was delivered during the inspiratory phase and consumption investigated at 10, 15 and 25 cycles min−1. The investigation was performed by ventilation with humidified air with a constant end-tidal CO2 and temperature. For a comparison, isoflurane was delivered in a conventional manner via the ventilator. The arrangement with a charcoal filter reduced the isoflurane consumption by a factor of 2.0−2.6, depending on ventilatory rate. Most of the saving was a result of the method of isoflurane delivery (factor 1.4−2.0), while adding the reflector gave a further reduction (factor 1.3−1.5). One circumstance that reduced the net efficiency of the charcoal filter was that it also reflected CO2; consequently, total minute ventilation had to be increased to maintain constant end-tidal CO2.