Heat and Mass Transfer in Built Structures - Numerical Analyses

Doktorsavhandling, 2003

Heat and mass transport processes are often present simultaneously in built structures in building applications. The emphasis of this thesis is to contribute to knowledge about predicting the effects of combined heat and mass transport in built structures. In the study on thermal aging of gas-filled insulation forms, a simplified model is developed for predicting the aging behavior and long-term thermal performance of gas-filled insulating foams. Diffusion is the transport mechanism for heat and gases on the macroscopic scale. The heat and gases diffusion processes are coupled through the dependence of foam thermal conductivity on gas composition and the temperature dependency of gas permeability. The comparison studies show that the predicted results agree fairly well with the measured ones. A numerical procedure aimed at accelerating the calculation process is developed for the simulations of combined heat and gases transport in gas-filled foams, based on an explicit difference scheme and a relaxation scheme. The procedure treats the heat transfer as a steady-state process and the gas diffusion processes are treated as transient ones. Calculation examples demonstrate that the algorithm gives quite acceptable accuracy of the results while reasonably enlarging the time step. In the study on combined heat, air and moisture transport in building envelope components, a simplified model aimed at evaluating the thermal and moisture effects of air convection on the hygrothermal performance of building envelopes is developed. It treats air as an incompressible fluid, except its density variation with temperature that gives rise to buoyancy force. Heat and moisture are transported through diffusion and convection. The latent heat effect induced by the phase transition of moisture between liquid water and water vapor is taken into account in the energy balance equation. An experimental investigation for validating the model is performed. A comparison between the simulated results and the measured ones shows that the model can predict the thermal and moisture effects of air convection reasonably well. Another numerical procedure aimed at ensuring the numerical stability for simulations of combined heat, air and moisture transport has been developed. The algorithm combines an explicit difference scheme and a relaxation scheme. The former is used for heat and moisture processes, and the later for air process. The calculation time step is determined in terms of the stability criteria for heat and moisture processes, and a criterion for controlling the latent heat effect. A comparison between the simulated results and the analytical solutions demonstrates that the algorithm gives fairly accurate results while ensuring numerical stability.