Experimental study on a single droplet and droplet-droplet boiling phenomena

An experimental, phenomenological study was conducted in order to explore single droplet and droplet-droplet nucleation, transition, and film boiling characteristics and fundamental behaviors. Essential differences were identified and described for the different boiling processes. In the case of single droplet impact and nucleation boiling, we identified three stages of evolution: Temperature increase, nucleation boiling incipience, and bubble growth. For droplet-droplet collision prior to nucleation boiling, we examined cases of off-centered droplet-droplet collisions on a heated surface, where collision occurred prior to boiling incipience of the first deposited droplet. Such cases reveal two impact mechanisms in one system: Dry impact and wet collision areas with different characteristics. Different temperature areas develop in these cases, resulting in different boiling incipience and bubble growth time periods. However, in the case of centered droplet-droplet collision during nucleation boiling (of the first deposited droplet), there is a different outcome. No crown forms in such a case. Instead, boiling inhibition occurs, followed by temporary boiling elimination. After some time, nucleation boiling of the whole liquid system returns. In the case of off-centered droplet-droplet collision during nucleation boiling (of the first deposited droplet), similar behavior takes place, with some directional differences. Boiling inhibition and temporary elimination occur along with the spreading movement of the colliding droplet and the subsequent boiling incipience and bubble growth return in the opposite order. The higher temperature area reaches nucleation boiling faster than the colder temperature area. In the case of droplet-droplet collision during transition boiling a similar, somewhat reduced boiling mechanism occurs before the system returns to the transition boiling process. In the case of single droplet impact followed by the droplet's film boiling, we identified the droplet bounce as a key parameter that greatly affects the heat transfer regime of the system. Dependence was found between the ratio of film boiling bounce duration and bounce start time, along with the We number and the ratio of surface temperature to the droplet's initial temperature.