Numerical Modeling of Composite Thermal Walls

Juliana Avila, Kamal A. R. Ismail, and Fatima A. M. Lino

Energy Department, Faculty of Mechanical Engineering, State University of Campinas, Brazil

Abstract: Thermal comfort in buildings, offices and residences is of extreme importance especially in very cold and very hot regions. Usually this mismatch between the human necessities and the ambient conditions was solved by artificial heating and cooling where fossil based energy is used to accomplish this desired comfort. This practice led serious problems of energy generation, distribution, ambient degradation where the heating and cooling shares reached unsustainable levels. It is for these reasons, in last few decades, more technical and political attentions were devoted to keep this problem under tight control. The correct modeling of walls used in buildings and their construction elements promote greater energy efficiency and thermal comfort to users.

Many numerical, theoretical and experimental studies were realized with the objective of reducing the use of air conditioning systems and heating systems or preferably eliminating this necessity by adequate design of the building and using thermally efficient elements in the construction. The main objective of this work is to investigate composite thermal walls and their possible impact on thermal comfort during summer and winter seasons in Campinas, São Paulo, Brazil. The investigation includes analyzing the effects of varying the wall thickness and thermo-physical properties of building materials. The mathematical model is based on one dimensional transient formulation, both the model and the boundary conditions were discretized using finite difference scheme. Numerical tests were realized to make the results independent of the grid size and the time step. Simulations were then performed on a simple wall considering conventional materials locally used in construction of residences, the geographical and climatic aspect of the place. The results indicated that materials with insulating characteristics, low thermal conductivity, low solar absorptivity and thicker walls can retard the heat flow into the buildings and reduce and shift the maximum temperature in the interior of the building. Three models of different thermal walls analyzed. The model is composed of a flat surface subjected to solar radiation, convection on external surface and conduction through the walls and convection on the internal wall. From the numerical simulations, it was possible todetermine the temperature distribution curves through the wall, the internal and external temperature variation, the internal peak temperature and its time lag behind the external peak temperature. The results confirm that beneficial effects can be achieved by using composite walls.

Key words: thermal walls, passive thermal comfort, low energy dwellings, modeling of thermal walls