Ratiometric laser-induced fluorescence for liquid-phase thermometry around boiling bubbles at extended temperatures above 70 °C

Local and temporal temperature mapping of the liquid phase around boiling bubbles and above the heat-transfer surface reveals that convective heat transfer plays a significant role in boiling heat transfer. To investigate this, we employ one of the most accurate methods for detecting temperature: the laser-induced fluorescence (LIF) technique. Previously, ratiometric LIF has been conducted mainly at temperatures well below 70 °C, which is too low for our purposes. We used Rhodamine B (RhB) and Rhodamine 110 (Rh110) as fluorescent dye pairs. We found that Rh110 excited by a 532-nm laser (rather than the blue laser used in some earlier studies) exhibited a temperature dependence: the combination of RhB and Rh110 could be used as a temperature indicator up to 88 °C. The calibration curve was extended beyond the superheating temperature range, permitting liquid-phase temperature visualization of boiling. A thin film heater sandwiched between two transparent plates was used to gain optical access to the liquid–vapor structure. This method successfully captured characteristic phenomena such as superheated liquid layer formation and thermal plumes beneath the rising boiling bubbles.