A metal matrix is inserted into a phase-change material to enhance the heat transfer during phase-change processes. The metal matrix tends to act like a heat transfer fin from the heated to cooled surface and to increase the effective thermal conductivity of the phase-change fluid. The scope of the study involves numerically modelling the heat transfer to and from constant-temperature cylindrical tubes surrounded by water during the freezing and melting processes. Comparisons of heat transfer with and without the metal matrix are made, with different porosities and densities of the metal matrix being examined. The results are discussed in terms of the effectiveness of the metal matrix as a heat transfer enhancement device.KEYWORDS: heat flow, metals, changing physical state, materials, fins, thermal conductivity, comparing, performance, calculating, models