Measurement of impinging and boiling droplet temperature by two-color/two-dye laser-induced fluorescence technique using continuous-wave laser

Abstract

The 2-color/2-dye laser-induced fluorescence technique is a non-intrusive method for measuring temperature in millimeter-scale fluids. This study determines optimal conditions for fluorescein disodium (FL) and Sulforhodamine 640 (SRh640) with a low-cost continuous-wave laser to reduce dye saturation and morphology-dependent resonances. The signal images of both dyes, captured by two high-speed cameras, were clearly distinguishable from the background with high contrast under specific concentration conditions. As a result, three concentration combinations were selected based on preliminary experiments, and the fluorescence signal ratio was analyzed to establish a temperature relationship between 32 and 71 °C. The intensities of FL and SRh640 increases and decrease with the temperature, respectively. Errors resulting from laser light attenuation in single-signal images were reduced using the ratiometric method. Solution 1 (CFL = 1.55 × 10−4 / CSRh640 = 3.2 × 10−6 M) exhibited the highest sensitivity at 0.718 (%/ °C) and a strong linear relationship (r2 = 0.997), so it was determined to be suitable concentration for measuring the droplet temperature. The proposed method estimated the temporal temperature variations of ethanol droplets impinging on a heated surface at TW = 80, 150 and 200 °C. At TW = 80 °C, contact boiling initiated without a recoil process, and the heat transfer elevated the droplet's temperature near the surface, leading to bubble formation and growth. As surface temperature increases, the film boiling behavior can be observed. The temporal evolution of the average heat transfer rate was analyzed.