Conference Agenda

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Session Overview
Poster introductions 05: Heat and moisture transfer - numerical
Wednesday, 25/Aug/2021:
11:40am - 12:00pm

Session Chair: Prof. Targo Kalamees, Tallinn University of Technology
Location: Room 1 - Room 082, Building: 116

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11:40am - 11:43am

Hygrothermal simulation on effective dehumidification methods in a museum storage room

Kazuki Ishikawa, Chiemi Iba, Daisuke Ogura, Shuichi Hokoi, Misao Yokoyama

Kyoto University, Japan

Museums and galleries collect and display cultural properties made of various materials. Any deterioration can be caused and accelerated by an inappropriate hygrothermal conditions of surrounding air. The environment in which the collection is placed requires appropriate temperature and humidity control to prevent deterioration of the collection.

The authors have conducted a field survey in some storage rooms in a middle-scale museum in Kyoto, which was built in 1986. From the results, high relative humidity was found in the storage room for metal artifacts. The humidity in the room was higher than 45%RH during the year, and especially in summer season, with an existing air conditioner in operation, it was higher than 70%RH, even though according to ICOM and IIC, an environment of less than 45% RH is recommended for the preservation of metallic materials.

In this study, the possibility of methods to suppress high humidity in the storage room with low-capacity dehumidifier is examined by using numerical model of the room. Initially, the reduction of relative humidity in the storage room was evaluated when a dehumidifier was operated throughout a year. Based on one year of numerical analysis from March 2018, the relative humidity in the storage room was kept below 50% RH. Next, it was examined how dehumidification load could be reduced by simple architectural renovation to improve the airtightness of the room and by changing the operation method of the ventilation system in the museum. Dehumidification load was reduced by making the windows airtight and by stopping the ventilation fans that indirectly contributed to the inflow of outdoor air into the storage room. Finally, the reduction of the operating time of the dehumidifier was discussed, and a method of environmental control that saves energy and requires less staff effort was proposed.

11:43am - 11:46am

Investigation of the transport properties for saline water in porous materials - Modeling of the permeability coefficient for saline water-

Kotaro Sakai1, Nobumitsu Takatori1, Daisuke Ogura1, Soichiro Wakiya2, Masaru Abuku3

1Kyoto University, Japan; 2Nara National Research Institute for Cultural Properties, Japan; 3Kindai University, Japan

Most of buildings and cultural properties are consist of porous materials and there are moisture and salt inside the pore. With the wetting and drying cycle, materials may be broken down and these phenomena are known as wet-dry weathering or salt weathering. For suppressing these deteriorations, it is important to predict the amount of saline water transfer in porous materials.

However, we don’t fully understand how salt influences on the transfer of saline water in porous materials. For example, the driving force and the viscosity of saline water change when salt dissolves in water, and also as salt precipitates the path for the saline water migration changes. These phenomena are well known, but we don’t understand how they effect on each other and how to fit into the saline water transport theory.

Our final aim is to predict the amount of saline water transfer in porous materials and develop the method for suppressing salt weathering. For the first step to achieve this aim, we aim to understand how the saline water transports in porous materials and investigate the modeling method for saline water permeability.

In this study, we will measure the saline water permeability by two experiments, the falling-head method and the saline water absorption test. For the falling-head method, we measure the saline water permeability on the condition that the driving force of saline water is constant and salt dose not precipitate. On the other hand, the saline water absorption test is similar to the field condition in which the salt weathering occurs, so we can observe the change of the driving force and the salt precipitation in the porous materials. Comparing these results, we will investigate the modeling method for saline water permeability.

11:46am - 11:49am

A picture is worth 1000 words: Validation of radiant and convective heat transfer models of photonic membrane using non-invasive imaging of condensation pattern

Eric Teitelbaum1, Dorit Aviv2, Miamiao Hou2,3, Jiewei Li2, Adam Rysanek4, Forrest Meggers1

1Princeton University, Princeton, NJ, United States; 2University of Pennsylvania,Philadelphia, PA, United States of America; 3Tongji University, Shanghai, China; 4University of British Columbia, Vancouver, BC V6T 1Z4, Canada

Cooling a sample of a material until condensation is observed is a standard technique for accurately measuring the dewpoint and associated relative humidity in a volume. It is often deployed when highly accurate measurements are required since humidity can be difficult to accurately measure, and latent responses (condensation) are long -time constant signals.

When conducting an experiment with a membrane-assisted radiant cooling panel, we found that membrane surface temperatures were difficult to measure directly, since the membranes were transparent to thermal radiation, and affixing a surface temperature sensor would locally distort the membrane’s temperature. Instead, the onset of condensation was used to infer the membrane’s surface temperature. However, the radiant cooling panels displayed variations of membrane surface temperature, and thus a resulting condensation contour was observed. The condensation contour represented a curve on which the membrane surface temperature was accurately known and constant - the dewpoint. The curve was in equilibrium between the internal panel temperature driven by internal free convection in the air gap and the view factor to surrounding surfaces, which can be evaluated at each point along the curve. In this paper, we both analytically assess the convective and radiative heat transfer balances, and match this to a simulation-driven approach to radiative heat transfer and buoyancy airflow. Our methods expand the “sensing” of condensation to provide information about view factor and convection equilibrium, both of which are quantities that are difficult to measure adequately in the field.

11:49am - 11:52am

What affects the performance of POD for the simulation of (hygro)-thermal performance

Tianfeng Hou1,2,3,4, Staf Roels1, Hans Janssen1

1KU Leuven, Department of Civil Engineering, Building Physics Section, 3001 Leuven, Belgium; 2Data Assimilation Research Team, RIKEN Center for Computational Science, Kobe, Japan; 3RIKEN Prediction Science Laboratory, Kobe, Japan; 4RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program, Kobe, Japan

Numerical simulations are widely used for building (hygro-)thermal performance assessment. These tools are often based on the standard methods for space and time discretization though, and can thus be very time-consuming. To reduce the computational complexity of the system, recently one common model order reduction technique - proper orthogonal decomposition (POD) has been investigated via different applications. From the assessment of linear thermal performances through a building component, over the analysis of air transfer through a light-weight wall, to the simulation of the highly nonlinear hygrothermal performance of a composite wall. It is shown that POD reveals to perform very well for the linear thermal case, as it allows to obtain a very good accuracy with a relatively low number of construction modes. However, relative to the linear thermal cases, (much) more construction modes and snapshots are required in order to obtain an accurate result for the nonlinear hygrothermal cases.

To illustrate what may affect the performance of POD for the simulation of (hygro)-thermal performance, case studies with different levels of nonlinearity and complexity are firstly introduced and for each case study the accuracy of POD is studied as a function of its construction modes. It is shown that comparing to the linear thermal case study, a (much) larger number of construction modes is needed to simulate the nonlinear hygrothermal performance. To further investigate the reason for this degrading performance of POD, the decay progress of the singular values from the different snapshots is addressed. Compared to the linear case study, the singular values’ decay of the nonlinear hygrothermal snapshots tends to be a lot slower. Therefore, we confirm that the interrelation of the snapshots is key, and a fast decay of the singular values implies a high interrelation of the snapshots and a better performance of the POD method.

11:52am - 11:55am

Hygrothermal simulations of timber-framed walls with air leakages

Anssi Laukkarinen, Teemu Jokela, Topi Moisio, Juha Vinha

Tampere University, Finland

Air leakages can create substantial excess moisture loads into envelope structures and degrade their hygrothermal performance. Multiple previous research projects have studied the behaviour and modelling of air leakages in building physics applications, but it is still quite rare to see air leakages being considered in practical building design simulations. The purpose of this paper is to present the selection of input parameters for air leakage simulations, utilisation of a factorial design to manage simulation cases and the results for a timber-frame wall with and without air leakages. According to the results, the air permeability of mineral wool and the air pressure difference over the envelope were the two most important factors for the dry air mass flow through the structure, as opposed to gap width and leakage route. An ideally airtight structure had a better hygrothermal performance compared to leaky structure. However, when leakages were present, the exact yearly average air flow rate in the range 70…420 dm3/(m2h) did not have a strong correlation to the performance indicators. For the other studied variables, the existence of a 50 mm thick mineral wool insulation on the exterior side of the gypsum board wind barrier and the impacts from climate change had the biggest effect on the moisture performance of the structure.

11:55am - 11:58am

Transient numerical simulation of coupled heat and moisture transfer through wall-to-floor thermal bridges

Yucong XUE, Jian GE, Yifan FAN

College of Civil Engineering and Architecture, Zhejiang University, China, People's Republic of

The moisture modifies the characteristics of heat transfer in building envelopes. Multiple factors, including the distinct hygric properties of various material, gravity and wind-driven rain, etc., affect the moisture content, resulting in a non-uniform distribution of water vapour in different parts of the envelope (e.g. column, beam, the main part of exterior walls). Usually, the more the water vapour in a material, the higher the thermal conductivity, resulting in more heat transfers here. Moreover, condensation easily occurs where there is humid, marking such parts have risks both on structural safety and mould growth. The wall-to-floor thermal bridge (WFTB) occupies the largest area among all kinds of thermal bridges that formed by frame structures. In this study, we aimed to (1) investigate the moisture distribution in the WFTB and its surrounding area, and (2) quantify the influence on the heat loss through WFTB when the moisture transfer in the envelope is considered. The annual average apparent thermal resistance of WFTB (RTB,ave), which is affected by the hygrothermal properties of materials, meteorological parameters, and indoor conditions, was defined to access the thermal performance of WFTB when the thermal and moisture transfer are both considered. As the thermal conductivity varies when other parameters change, the apparent thermal resistance of WFTB (RTB) is not fixed. RTB,ave is thus defined as the average of RTB to reflect the insulation situation of WFTB for a whole year. The results of transient numerical simulation indicated that moisture condensation occurs easily at the interface of WFTB and the main part of exterior walls, and the RTB,ave is significantly higher than the theoretical thermal resistance when moisture transfer is neglected. Therefore, condensation inside the building envelope should attract enough attention as it occurs between not only different layers but also thermal bridge and its surrounding areas.

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