Indoor air quality : combining air humidity with construction moisture
The project aims to improve the modelling of moisture transfers at internal surfaces by linking the finite volumes representing the Heat, Air and Moisture (HAM) and Computational Fluid Dynamics (CFD) domains. Conflation of both models facilitates the detailed study of moisture flow as it impacts on indoor air quality and occupant health. The thesis lays down the conceptual framework for the subsequent development of an indoor air quality analytical tool. The work thus improves the modelling of construction feature risk assessment, for example, moisture absorption and desorption at the internal fabric surfaces in as much as it relates to indoor air quality. Through such an improvement, an indoor air quality analytical tool for the prediction of time-varying temperature/humidity conditions at specific locations within the building is enabled and subsequently these conditions may be related to the likely occurrence of mould. Humidity in indoor spaces is one of the most important factors in the determination of indoor air quality. High indoor humidity is a major contributor to the accumulation of moisture in the building envelope. This often results in dampness within the building envelope and subsequent health-related problems for the occcupants. Moderation of the indoor relative humidity, temperature and moisture content of the indoor air amongst others is a pre-requisite for a healthy building because it affects the perception of indoor air quality, thermal comfort, occupant health (asthma, respiratory illness, etc), building durability, material emission and energy consumption. Excessively high relative humidity promotes the growth of moulds and mildew on building surfaces. The basis for the envisaged conflation evolves around the boundary layer theory as it pertains to the velocity, thermal and concentration profiles associated with flow parallel to a flat surface, a phenomenon which is recognised as being similar in nature to buoyancy-driven convective heat transfer within building enclosures (White 1988). Within the framework of modelling of indoor air flows, the conflated modelling approach is very much dependent upon the treatment of the internal surface convection, for example, in the conflation of HAM and CFO models. This is referred to as the pivot point for the handshaking between HAM and CFO modelling domains. Within the framework of this project, the pivot point refers to the treatment of surface convection mass transfer at the internal surface to facilitate the hand shaking between HAM and CFO modelling domains. The two-time step coupling approach based on the loose coupling algorithm is adapted for the conflation. The technique involves a process whereby the HAM and CFO models are processed independently but exchange information at the interface at every time-step. The numerical method for the solution of the Navier-Stokes equations is based on the co-located grid arrangement, whereby all flow variables are defined in the centre of the grid cells. The transport equations are integrated for each grid cell and the Gauss Theorem applied to yield an integral over the cell face. These cell face integrals are then approximated using interpolation of the cell centred data. For the resolution of flow in the near-wall regime, the Low-Reynolds number k-ε turbulence model is used. A configuration mechanism with a rules-based moisture control algorithm to facilitate the handshaking of the HAM and CFO domains is presented. Methods for the solution of problems due to moisture migration across the interface, which are effected through variation of the convective mass transfer coefficient, hm, through variation of the standard k-ε turbulence model, namely the lowReynolds number model with its embedded wall damping functions and through adjustment of the source terms of governing transport equations of the CFO and HAM models are also discussed.