The effect of thermal mass is becoming increasingly important under the scope of low energy use building construction and energy conservation. In this paper the dynamic behaviour of the effect of the thermal mass of external walls on the development of solar-assisted buoyancy-driven ventilation flow rates, air flow patterns and temperature distributions in a simple three-storied atrium building was investigated using unsteady Reynolds Averaged Navier-Stokes equations (RANS) modelling approach. The SST-k-omega turbulence model and the Discrete Transfer Radiation Model (DTRM) were used in the CFD simulations. The unsteady-state governing equations were solved using a commercial CFD solver FLUENT©. The numerical results were obtained for indoor transient thermal conditions by assuming the external concrete walls of the building under three situations: (i) walls with internal insulation covering (ii) walls with external insulation covering, and (iii) walls without insulation covering. The buoyancy-driven ventilation volume flow rates and internal temperature variations from 8:00 to 15:00hrs on July 15, 2011 were calculated using CFD simulations. Numerical results obtained indicate that the transient CFD simulations provide important information about the effect of the thermal mass on the thermal behaviour of the building. It was found that the thermal mass of external walls with external insulation covering was a better option which helped to control the diurnal temperature swings making the conditions within the building more comfortable. Heat stored in the walls can be useful for the night-time buoyancy-driven ventilation when there is no solar radiation.

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