Thermal boundary layer

A thermal boundary layer is a thin region in which temperature changes rapidly and thermal conduction remains important. Its thickness can be estimated by balancing thermal diffusion against advection or by requiring a local Rayleigh number to be near its critical value.

In a renewal model, a diffusive thermal boundary layer grows until a convective instability overturns it, after which growth restarts. A layer reaching thickness $\delta \sim \sqrt{\kappa \tau }$ has mean heat flux of order $k\Delta T/\delta$. A diffusive $t^{-1/2}$ flux averaged over one cycle differs only by an order-one coefficient. Distinct thermal and material layers need not share a renewal time.