The regimes of droplet vaporization in saturated conditions under low pressure

The study focuses on the vaporization of water droplets at saturated temperature (i.e. in the absence of non-condensable gases) from a superheated surface under low-pressure conditions, ranging between 2 and 7 kPa. Two primary types of vaporization were identified: evaporation and explosive vaporization. It also reveals key characteristics of droplet vaporization regimes, mapping them based on saturation pressure, superheat, and the presence or absence of bubbles, which determine the vaporization type. Thermal signatures from experiments exhibit distinct features. In evaporation, the process starts with a sharp increase in heat flux, followed by a steady phase and a secondary surge during the depinning phase, forming a saddle-like curve. In contrast, explosive vaporization leads to an immediate and notable heat flux surge, primarily due to the nucleation, growth, and subsequent burst of a bubble. This process also entails heat flux fluctuations, influenced by the evaporation of secondary droplets after the primary bubble bursts. The experiments confirm that explosive vaporization increases the overall heat transfer coefficient and reduces vaporization times compared to evaporation, primarily due to the rapid removal of secondary droplets from the superheated surface. To quantify this improvement, the energy transfer enhancement factor (ETE) was introduced. Evaporation generally increases the coefficient by approximately 250%, with respect to single phase convection from the wall to the water vapor, whereas explosive vaporization can elevate it by more than 900%. Overall, the study presents experimental evidences supporting the potential of explosive vaporization to enhance heat transfer efficiency in low-pressure spray evaporators. •Evaporation and explosive vaporization are distinguished, revealing their distinct thermal behaviors.•Distinct regimes for evaporation and explosive vaporization at low pressure are mapped through video and thermal signature analysis.•Superior heat transfer efficiency and shorter vaporization times are demonstrated by explosive vaporization.