The Biot number is a dimensionless number or parameter expressing a ratio of thermal internal resistance to surface film resistance. It is generally used in heat transfer calculations such as unsteady state flow. It is defined as:
where hT is a heat transfer coefficient, k the thermal conductivity, and Δx a mid–plane distance.
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Small Biot number case
Values of the Biot number below 0.1 indicate the heat conduction inside a material body is higher than the heat convection away from its surface into the atmosphere, and that the temperature gradient inside the material is negligible. A Biot number less than 0.1 normally implies a very small error will result from assuming a lumped-capacitance assumption for transient heat transfer. Normally this type of analysis leads to simple Newtonian exponential heating or cooling behavior, since the amount of thermal energy within the material is simply proportional to its temperature.
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- DeWitt Incropera and Lavine Bergman. 2007. Fundamentals of Heat and Mass Transfer (6th edition ed.) John Wiley & Sons. ISBN 978-0-471-45728-2.