Heat Transfer Mechanisms through a Masonry Element under Load

2 Heat Transfer Mechanisms through a Masonry Element under Load 

Masonry structures in heat transfer takes place by conduction in the solid parts, and by convection and radiation into the gaps. The proportion of void volume to total volume determines the thermal behavior of the materials against fire. Depending on the size of the cavity and the temperature of the walls thereof, the radiation transmission may become dominant. 

Masonry walls are formed by solid parts and cavities. In the solid parts, both coatings and masonry itself, heat transfer takes place by conduction and the mathematical model that governs it is the heat equation expressed as following  

In the cavities of masonry walls, the transmission of heat is by radiation and convection. The thermal inertia of the air is assumed to be neghgible, and the net heat flow into the cavity is given by the following expression  

In addition, the masonry wall is subjected to the boundary conditions that determine the stage of fire. The net heat flow into the fire exposed side is expressed as follows  

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