Measurement and Observation of Elementary Transition Boiling Process after Sudden Contact of Liquid with Hot Surface

Abstract

A Fast response surface temperature measurement technique has been developed to investigate transient transition boiling phenomena after ethanol droplet impact on a nickel hot surface. Our motivation is to make clear the mechanism of boiling transition between nucleate and film boiling which is closely related the recovery of wetting on the hot surface being higher than the Leidenfrost temperature. In this study transient boiling phenomena during sudden contact of an ethanol sessile droplet on a hot solid surface has been investigated to understand what governs whether the solid surface keeps wet or not. The droplet impact boiling system is very simple and easier to observe but it includes the elementary transition boiling process. Since the transient transition took place within a couple ten milliseconds, observation with high speed video system and fast response surface temperature measurement technique are essential for better understanding of the transition boiling phenomena.The experiments were conducted for single and multiple droplet impacts. The multiple droplet impacts simulates liquid and solid contact situations on a hot surface during spray or laminar jet quench. The nickel disk of 50mm in diameter and 5mm in thickness was used as the hot surface. The hot surface was inclined from 0 deg (horizontal) to 40 deg. The fast-response film thermocouple with a typical minimum response time of 80 microseconds was developed and fabricated on the nickel disk at the depth of 3 micrometers from the surface. The temperature histories just beneath the surface were recorded at the sampling frequency of 200kHz. The boiling phenomena beneath a sessile droplet impacted on the hot surface were observed by using a microscope equipped with the high speed video camera at the maximum frame rate of 22.5 kfps. The surface temperature and surface heat flux were estimated with 1D inverse heat conduction analysis. Histories of local surface temperature and local heat flux were compared with the boiling video image. The experiments were done for different initial wall temperatures up to 250°C, different liquid subcoolings from 33 to 53K. The multiple impact frequency was changed from 280 to 840Hz.In case of the single droplet impact tests, the observation results showed that wetting situation was maintained for very short time even though the surface temperature was beyond the liquid superheat limit temperature. We defined the vapor film generation time as the characteristic boiling transition time scale from wetted nucleation boiling regime to dry film boiling regime. The film generation time were measured for the extensive initial surface temperature range, different subcoolings and different impact velocities. As the initial surface temperature increased, the film generation time decreased to order of microsecond and film boiling situation seemed to be established via spontaneous nucleation (vapor explosion) process. In case of the multiple droplets impact tests, film generation times were evaluated every droplet impact during continuous cooling from the initial solid temperature. As long as the generation time was less than 1 millisecond and much shorter than the impact period, stable film boiling was observed. Then the film generation time increased with the droplet impact times and approached to the droplet impact period as the cooling time elapsed. This region was recognized as the transition boiling region. After the generation time reached to the droplet impact period and then stable wetted situation, namely nucleate boiling was observed. As compared with the single impact tests, the generation time for the multiple impact became shorter than that for the single droplet impact at same initial surface temperature and the lower limit surface temperature observed vapor film generation was reduced. This fact may imply the effect of transient heat conduction on the wetting temperature shifts to higher wall superheat temperature beyond liquid superheat limit temperature during quenching with jets or sprays.