09 Heat transfers

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L.D.D Advanced Building Physics - Heat Transfers 1 - TRANSPORT MECHANISM Thermodynamic deals with systems in equilibrium. It can be used to predict how much energy is needed to move a system to one equilibrium state to another. Transport phenomena are defined as phenomena in which there is a transfer of extensive physical quantities (mass, energy, momentum) between different parts of a system or through its boundaries. We consider for the general case systems for which it is possible the movement of macroscopic portions of matter. The transport mechanism can be classified into two groups: 1 - Transport by molecular collision (or interaction): exchange of extensive physical quantities (mass, energy, momentum) is due to collisions or interactions between molecules, without any displacement of macroscopic portions of the system in the direction of displacement of the physical quantities 2 - Transport by turbolent collision: exchange of extensive physical quantities is due to the movement of macroscopic parts of matter in the direction of the displacement of the physical quantities L.D.D Examples of transfer by molecular collision: - Mass diffusion: a drop of ink falls in a motionless fluid and by molecular collision it is diffused into the entire mass of the fluid - Diffusion of heat in a solid or a motionless fluid: vibration of electrons (in metals) or collision between molecules (in fluids or gases) enable the transfer of kinetic energy from an area to another without any movement of macroscopic portions of matter. - Laminar flow in a fluid: flow of a fluid where macroscopic portions of the fluid are moving in x direction and no macroscopic mass movement occurs in y direction. The exchange of energy and momentum in the y direction is due only to molecular collisions.…