### Basic constitutive equations

*Heat transfer between fluids through a wall*

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

The rate of heat flow Q [W] over the area A [m ^{2}] and
through a laminar films of thickness x_{1} and x_{2} [m]
respectively may be written as: | |

where λ [ W/m·K] is the wall thermal conductivity, α_{1}
and α_{2} [W/m^{2}·K] are the respective
individual heat transfer coefficients and k [W/m^{2}·K]
is the overall heat transfer coefficient. | |

In English literature the individual heat transfer coefficient is normally denoted h and
the overall heat transfer coefficient U,
both with units [Btu/hr·ft^{2}·°F]. |

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