### Basic constitutive equations

*Convection from a fluid to a wall*

Heat flows from a fluid to a wall as illustrated to the right: | |

The rate of heat flow Q [W] over the area A [m ^{2}] and
through a laminar film of thickness x [m] may be written as: | |

where λ [W/m·K] is the wall thermal conductivity. | |

Since the thickness of the film is unknown and almost impossible to estimate this equation is usually rewritten as: | |

where λ is called the individual
heat transfer coefficient [W/m^{2}·K]. | |

In English literature the heat transfer coefficient is normally denoted h
[Btu/hr·ft^{2}·°F]. |

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