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Abstract – Condensation risk assessment of window‐wall facades under the effect of different heating systems using 2‐D Finite Element and CFD simulations

Derek Kin Fung Yan, M.Eng. 2012
Supervisor: Dr. Rodrigo Mora

PDF-download Research Poster Derek Yan

Keywords: condensation, window, heating system, simulation

In northern coastal climates windows are the coldest surfaces, causing thermal discomfort, heat losses, and moisture condensation. This paper investigates the interactions between various heating systems and window‐wall systems through convection and radiation heat exchanges, and their effects on surface condensation. The three most common heating systems for multi‐unit residential buildings are evaluated: electric baseboard, radiant floor and forced air system. Each heating system provides vastly different indoor conditions due to differences in thermal stratification, room air distribution and location of the heater. These differences have direct impacts on the risk of condensation in a window. A literature review shows a knowledge gap in understanding the effects of the type of heating system on window condensation. This paper attempts to address this gap.

In the first phase of this project, two typical window wall details were studied. Two‐dimensional finite element heat transfer simulation software, THERM was used to model these details under different boundary scenarios. Convective heat transfer coefficients were drawn from the literature to attempt to represent the air flow and radiation induced by each heating system. A total of eight THERM models were built. In order to attempt to represent convection heat transfer more accurately, 2D computational Fluid Dynamic (CFD) models were employed. CFD models are able to model local flow conditions more accurately than the correlation‐based convective coefficients. A total of eight CFD models were built. Surface temperature data were collected from all models to predict window condensation. The analysis showed that THERM models provided results with consistent general trend as the CFD model, but they attained as much as 30% margin of difference comparatively. In general, a forced air system was considerably more susceptible to window condensation risk due to the typical location of supply inlet and uneven room thermal stratification. The radiant floor system also resulted in significant condensation risk when indoor relative humidity was above 55%.

In the second phase of this project, various window‐wall construction details will be explored to reduce the risk of window condensation. These will be evaluated using THERM 2D FEM and improved 3D CFD models to more accurately represent the air flow at room corners connected to the window. However, CFD results can be completely inaccurate if not calibrated properly. Therefore, field monitoring and thermal measurements will also be attempted to calibrate the models.

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