= Stagnant mushy-layer model
{title2=$E=c_pT+L\phi,\ B=\phi C$}
A stagnant mushy-layer model has no bulk advection, equal phase <mass densities>, pure solid, and local equilibrium on the <liquidus>. With <liquid fraction> $\phi$, <specific enthalpy> $E=c_pT+L\phi$ and bulk solute <concentration> $B=\phi C$, neglecting solute <diffusion> inside the <mushy layer> gives $T_t+(L/c_p)\phi_t=\kappa T_{zz}$ and $(\phi C)_t=0$. The latter fixes composition at each stationary material level; it does not automatically set every level to the initial liquid <concentration> when solute can diffuse ahead of the growing mush.
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