Hydrodynamics and heat transfer characteristics of oil-in-water emulsion droplets impinging on hot stainless steel foil

Abstract

The hydrodynamics and heat transfer characteristics of oil-in-water (O/W) emulsion droplets impinging on a hot stainless steel foil were investigated experimentally. A two-directional flash-photography technique was adopted to track the time evolution of the droplet shapes. The temperature history of the foil during the collision with the droplets was also measured using a high-speed infrared thermometer. The main objective was to investigate the effects of varying the solid temperature and oil concentration on the heat transfer characteristics. The foil temperature was varied from 140 to 470 °C, and the oil concentration in the O/W emulsion was varied to be 1, 5, and 15 wt%. The impact velocity of the droplets was 1.0 m/s, and the pre-impact diameter of the emulsion droplets was approximately 2.5 mm for oil concentrations of 1 and 5 wt% and 2.4 mm for the oil concentration of 15 wt%. Water and base oil were also used as test liquids for reference. Because the boiling temperature of the oil (∼300 °C) is considerably higher than that of water and the thermal conductivity of the base oil is appreciably smaller than that of water, the hydrodynamics and boiling phenomena of droplets are strongly dependent on not only the solid temperature but also the oil concentration. In the nucleate boiling regime, the heat removal increases with the solid temperature, reaches a peak, and then decreases; the peak heat removal depends on the oil concentration. The heat transfer characteristics are discussed in detail in terms of the liquid motion, flow boiling, and local concentration of the oil phase.