Observation of the mechanisms of boiling heat transfer enhancement through the addition of a nonionic surfactant using high-speed infrared thermometry

•Mechanisms of boiling heat transfer enhancement by a surfactant were investigated.•Wall temperature was visualized at 3000 fps using a high-speed infrared camera.•Increase in microlayer area resulted in boiling heat transfer enhancement.•Microlayer evaporation contributed most to wall heat transfer.•Depletion of macrolayer beneath mushroom bubble led to a large dry surface. It is well known that the addition of a surfactant to water as a boiling medium enhances its boiling heat transfer coefficient (HTC). The mechanisms of boiling heat transfer enhancement by adding a nonionic surfactant, which remain unclear, were explored in this study by visualizing the distributions of the temperature and heat transfer on the wall surface using a high-speed infrared camera. Heat flux partitioning, which partitions the heat transfer distribution into various fundamental heat transfer processes, demonstrated that the increase in the area of the microlayer region having the highest HTC enhances the heat transfer. The addition of surfactant softens the bubbles (reduces the surface tension) and suppresses their coalescence, thereby increasing the area occupied by the microlayer on the wall. Furthermore, the formation of the microlayer via the sliding motion of primary bubbles suppressed the formation of a dry surface. The contribution of microlayer evaporation to the overall wall heat transfer increased as the area coverage ratio of the microlayer increased, and unlike in pure water boiling, microlayer evaporation dominated the wall heat transfer over a wide range of heat flux conditions during surfactant solution boiling. In the high heat flux region, bubble coalescence was frequently observed even in the surfactant solution, and a macrolayer was formed at the bottom of mushroom bubbles. In the heat flux region above 0.9 MW/m2, a large dry surface was generated at the bottom of the mushroom bubbles due to the depletion of the macrolayer that covered a wide area of the wall surface.