Heat transfer and rheological performance of a phase change dispersion during crystallisation

Abstract

A phase change dispersion consists of a phase change material dispersed within a continuous fluid, generally water. The phase change material changes from liquid to solid or vice versa whilst releasing or absorbing heat at a nearly constant temperature. Due to the phase change of the dispersed phase, phase change dispersions can be classified as high potential heat transfer fluids that can transport and store heat isothermally. Despite the promise of phase change dispersions the physical mechanisms governing their fundamental heat transfer and rheological behaviour is poorly documented, especially during the crystallisation process. Therefore, this paper reports experimental results on the heat transfer and rheological behaviour of a 16 wt% paraffin-in-water dispersion during crystallisation. Additionally, thermophysical properties and emulsion stability properties are documented. The Nusselt numbers of the phase change dispersion during cooling in low Reynolds number flow conditions were determined to be higher than water at all investigated Reynolds numbers. Furthermore, the observation of a temperature-dependent flow-regime transition at a Reynolds number much below the classical transition for Newtonian fluids was observed and a discussion on the physical mechanisms generating this phenomenon is presented. Additionally, rheological analysis showed that the phase change dispersion is a non-Newtonian fluid, with shear-thinning behaviour, and dynamic viscosities higher than water. As a result an experimental Reynolds number was calculated to account for the variations of the dynamic viscosity with shear-rate and temperature. Figure of merit analysis was also performed, taking into consideration the Nusselt numbers and friction factors of the developed phase change dispersion and water where it was found that at Reynolds numbers above 650 it was more favourable to use the investigated phase change dispersion than water.