Experimental study on the dynamic foaming properties and heat transfer characteristics of aqueous film-forming foam (AFFF) droplets impacting heated surfaces

The impact and evaporation of foaming droplets on heated surfaces are critical to applications such as fire suppression and spray cooling, yet the coupled heat transfer and foaming dynamics remain poorly understood. This study experimentally investigates the impact dynamics, foaming behavior, and heat transfer characteristics of aqueous film-forming foam (AFFF) droplets on heated surfaces. Droplet concentration (0, 3, and 6% AFFF), surface temperature (110–150 °C), and Weber number (62.5–337.1) are varied to examine the effects on droplet morphology, dimensionless height factor, bubble volume fraction, average bubble area, and heat transfer performance. Four distinct impact patterns are identified: adhesion, fully foamed, foaming-rebound, and rebound. Intense foaming occurs only in AFFF droplets and is absent in distilled water. During the evaporation phase, the dimensionless height factor of 6% AFFF droplets exceeds that of 3% AFFF. Increasing the AFFF concentration significantly enhances foaming while suppressing evaporation. Compared with 6% AFFF, 3% AFFF exhibits more pronounced fluctuations in both bubble area fraction and average area of bubbles. Moreover, complete evaporation of 6% AFFF requires a larger amount of heat than that of 3% AFFF. Finally, a predictive model for the evaporation heat of AFFF droplets on hot surfaces is established, incorporating the expansion effect, Weber number, and surface temperature. This work provides theoretical insights into the dynamics and heat transfer mechanisms of foaming droplets on heated surfaces, providing a theoretical foundation for the optimizing the use of AFFF in fire suppression scenarios.