CFD Modeling Of An Indoor Space By Including The External Wall

When applying computational fluid dynamics (CFD) to model indoor environments, it is usually hard to accurately specify the thermal boundary conditions of the walls. As for the interior wall surfaces, if there is good thermal insulation or there is no meaningful temperature difference across the walls, the interior wall surfaces may be treated as adiabatic. However, for the interior surface of the external wall, the thermal boundary condition on it is very complex, because the thermal condition of the wall surface is subject to the wall heat conduction, the heat convection with the indoor air and the heat radiation to the indoor space. Knowing the exact thermal information on walls is of great importance in predicting the indoor air temperature, velocity distribution, as well as energy consumption of the building.This thesis proposes a simple method to specify the thermal boundary conditions on the external wall by including the external wall and/or window into the solution domain. In this way, one can specify the thermal boundary information to the exterior surface of the external wall. The heat conduction through the wall, heat convection between the interior wall surface and the room air, heat radiation between the interior wall surface and the indoor space, and thermo-flow of the indoor air must be coupled together for solution. Owing to relatively uniform condition on the exterior wall surface due to wind interruption, the temperature or heat flux on the exterior wall surface is more uniform. It is hence easier to specify the thermal boundary condition on the exterior wall surface. Meanwhile, the outdoor climatic parameters, such as wind speed, outdoor dry-bulb temperature, background radiation temperature as well as the solar radiation can be taken into account. If the window is excluded, the efficient S2S (surface to surface) radiation model can be used; however, considering the solar radiation penetrating the window, the more powerful DO (discrete ordinates) radiation model should be used. The DO model can not only handle the radiation heat exchange between the surfaces, but also the heat absorption in the semi-transparent window glass. To test the above modeling strategies, an office environment was solved with turbulence modeled by the RNG k-t; model. The Fluent enhanced wall treatment was adopted to handle the near-wall effect. For model validation, another indoor environment case from the literature was also solved and the computed air velocity and air temperature profiles and the heat sources’surface temperatures were compared with the measurement data.It finds that the coupled modeling using the RNG k-εturbulence model and the S2S or DO radiation model is able to obtain results in consisting with the measurement. The temperature and heat flux distribution on the interior surface of the external wall is not uniform, implying that the traditional method by treating the interior surface of the external wall as uniform may lead errors. Such temperature or heat flux distribution is not only related to the indoor airflow distribution and heat source positioning but also the outdoor climate. The proposed modeling strategies can not only compute the temperature and heat flux on the interior surface, but also can provide the heat transfer information through the exterior wall or window. This type of information is very useful for indoor air environment analysis and energy consumption evaluation.