This paper explores the method of internal variables, which is used to
describe the nonequilibrium state between a liquid and its vapour in
two-phase flows. The actual dryness fraction, chosen as an internal va
riable, is introduced to the model by means of a relaxation equation.
A one-dimensional system of conservation equations supplemented by the
relaxation equation is applied in order to predict the pressure distr
ibution during flashing flow along a divergent channel. A second evolu
tion equation containing the diffusive term is developed to predict th
e propagation of a pressure pulse in a two-phase flow mixture.