  The high degree of uncertainty and conflicting literature data on the value
of the permeability coefficient (also known as the mushy zone constant), which
aims to dampen fluid velocities in the mushy zone and suppress them in solid
regions, is a critical drawback when using the fixed-grid enthalpy-porosity
technique for modelling non-isothermal phase-change processes. In the present
study, the sensitivity of numerical predictions to the value of this
coefficient was scrutinised. Using finite-volume based numerical simulations of
isothermal and non-isothermal melting and solidification problems, the causes
of increased sensitivity were identified. It was found that depending on the
mushy-zone thickness and the velocity field, the solid-liquid interface
morphology and the rate of phase-change are sensitive to the permeability
coefficient. It is demonstrated that numerical predictions of an isothermal
phase-change problem are independent of the permeability coefficient for
sufficiently fine meshes. It is also shown that sensitivity to the choice of
permeability coefficient can be assessed by means of an appropriately defined
P\'eclet number.
