9.6.- HEAT TRANSFER

The laws of thermodynamics deal with the energy transfer and interaction between heat and work, but only in equilibrium systems. Consequently, they allow us to determine the initial and final states, but they cannot be used to predict and quantify the intermediate process. Heat transfer provides methods to determine these processes.

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There are three heat transfer modes:

    • Conduction: occurs in a solid medium by the effect of a temperature gradient that causes the transfer of heat from the highest to lowest temperature zone. The heat transmitted by conduction per unit of time is proportional to the temperature gradient multiplied by the area A.
    • Convection: when a fluid comes into contact with a solid surface, an exchange between the particles of the fluid and the solid occurs. There are two convection processes:
      • Free convection: the driving force comes from the difference in density between particles of the fluid. The temperature and density of the particles in contact with the solid change, causing an upward flow.
      • Forced convection: the driving force comes from the outside and moves a fluid on the surface.
    • Radiation: in the absence of a medium, there is a net heat transfer by radiation between two surfaces at different temperatures due to changes in the electronic configurations of the atoms or molecules that form those surfaces. The heat emitted by radiation is described by the Stefan-Boltzmann law.

The heat transfer cannot be prevented, but the transfer rate can be modified. The heat transfer study is important to determine materials or configurations that optimize the heat exchange rate based on some parameters.

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HEAT TRANSFER UNITS
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