Non-Fourier heat conduction in oil-in-water emulsions

Abstract

Emulsions have widely been used in various application, e.g. in energy system as a potential second refrigerant. However, there is a lack of comprehensive study on extraordinary effective thermal conductivity enhancement in the oil-in-water (O/W) emulsions, and effects of temperature and droplet size were not considered in the widely used models of effective thermal conductivity for the O/W emulsions. In this study, non-Fourier heat conduction characteristics in the O/W emulsions were investigated experimentally. The O/W emulsions were prepared with different droplet sizes by controlling the ultrasonic processing time. The O/W emulsions containing small droplets are stable. Effective thermal conductivity of O/W emulsions nonlinearly varies with droplet size, concentration, fluid properties and temperature. Small droplet size is beneficial for effective thermal conductivity enhancement of O/W emulsions. Thermal conductivity of fluids can be enhanced significantly especially at low concentration although thermal conductivity of oil is much lower than water conductivity, which could be due to non-Fourier heat conductions in O/W emulsions. Time lag ratio less than 1 indicates that no thermal waves exist in O/W emulsions, and diffusion-dominant non-Fourier heat conduction could exist in the O/W emulsions. A new model of effective thermal conductivity, which considers effects of fluid thermosphysical properties, oil concentration, droplet size and temperature, was developed for the O/W emulsions based on the measured data. This study could be helpful for exploring the mechanisms behind extraordinary heat conductivity enhancement phenomena of oil-in-water emulsions.