Temperature-Dependent Rheological Behaviors of Binary Eutectic Mixtures of Sugar Alcohols for Latent Heat Storage: A Comparative Study with Pure Sugar Alcohols

Abstract

The temperature-dependent rheological behaviors of five selected binary eutectic mixture sugar alcohols, with great potential for latent heat storage in the range of 353.15 K to 523.15 K, were investigated. It was found that the rheological behaviors of the mixture sugar alcohols depend on those of the pure compounds as well as their molar ratios. The two mixtures of xylitol (75 mol%)+erythritol and erythritol (84 mol%)+d-mannitol behave like pseudoplastic fluids with typical non-Newtonian shear-thinning behaviors, as indicated by the power law index of 0.99 (<1). The mixture of d-mannitol (70 mol%)+d-dulcitol is a nonlinear Bingham fluid, exhibiting a slight yield stress (0.001 Pa to 0.01 Pa) at low shear rates. The rest two mixtures containing the cyclic-structured inositol behave like Herschel-Bulkley fluids. The infinite shear viscosities of the eutectic mixtures over the entire temperature range appear to be higher than those of their respective pure compounds, except for inositol. The mixture of xylitol (75 mol%)+erythritol at its melting point shows higher dynamic viscosity of about 0.546 Pa·s than the values of about 0.396 Pa·s and 0.035 Pa·s for xylitol and erythritol, respectively. In addition, the activation energies of viscous flow of the mixtures, as determined by fitting the dynamic viscosity-temperature curves using the Arrhenius model, also exhibit higher values than those of their pure compounds. The activation energy of viscous flow of the mixture xylitol (75 mol%)+erythritol was determined to be about 92 400 J/mol in the supercooled liquid state, while the supercooled liquid xylitol and erythritol have much lower values of 83 500 J/mol and 51 900 J/mol, respectively. Both the increased dynamic viscosities and activation energies of viscous flow can result in deteriorated crystallization performance during latent heat retrieval.