EXPERIMENTAL INVESTIGATION ON DYNAMIC CHARACTERISTICS OF LIQUID NITROGEN SINGLE BUBBLE IN THE FREE FIELD

The objective of this paper is to investigate the transient evolution and dynamic characteristic of liquid nitrogen single bubble. In the experiment, electric spark transient discharge (EDM) was used to stimulate the evaporation of liquid nitrogen to form a single bubble, and the evolution process of the single bubble was captured by a high-speed camera with high resolution. In order to further reveal the unique physical properties of low-temperature media and the strong thermodynamic effects on the evolution of the single bubble, the unsteady evolution process and dynamic characteristics of single bubble in liquid nitrogen at 77.41 K and water at 298.36 K under the same ambient pressure were analyzed. And quantitative data such as the radius of bubble and interfacial velocity were obtained experimentally to elucidate the unsteady characteristics of the spherical and non-spherical evolution of liquid nitrogen single bubble. The results show that (1) the size of a single bubble in liquid nitrogen is smaller than that of ambient water at the same input voltage. The maximum radius of the liquid nitrogen bubble is about 0.69 times that of the ambient water bubble, when the input voltage is 400. The evolution of a single bubble in liquid nitrogen experiences an expansion stage, a contraction stage, an oscillation stage, and a up phase, respectively. (2) The shrinkage stage of liquid nitrogen vacuoles is mainly dominated by the heat conduction at the phase interface, and there is no obvious collapse phenomenon. The minimum radius of liquid nitrogen bubble is about 5.5 times bigger than that of the ambient water bubble during the shrinkage stage. (3) The heat transfer at the phase interface is enhanced during the early stage of the oscillation stage, the surface roughening effects is amplified over the bubble surface resulting from Rayleigh-Taylor instability coupled with the thermal effects. And small broken bubbles exist near the bubble surface during the oscillation stage. When the input voltage is higher, the number of small bubbles at the bottom of the vacuole increases significantly. (4) Due to the large buoyancy coefficient of the liquid nitrogen bubble, the overall upward migration of liquid nitrogen bubble is significant in the late stage of liquid nitrogen. The bottom of the liquid nitrogen vacuole shrinks more quickly to create a depression, driving the vacuole to into a ring shape.