  In this paper we develop and significantly extend the thermal phase change
model, introduced in [12], describing the process of paraffinic wax layer
formation on the interior wall of a circular pipe transporting heated oil, when
subject to external cooling. In particular we allow for the natural dependence
of the solidifying paraffinic wax conductivity on local temperature. We are
able to develop a complete theory, and provide efficient numerical
computations, for this extended model. Comparison with recent experimental
observations is made, and this, together with recent reviews of the physical
mechanisms associated with wax layer formation, provide significant support for
the thermal model considered here.
