Water-entry cavity of heated spheres

Abstract

The present study aims to address the effect of sphere temperature on water-entry cavity. For this purpose, an experiment on vertical water-entry cavity of a heated sphere is conducted by utilizing a high-speed video camera. The temperature of the sphere ranges from 17℃ to 800℃. The complex flow phenomena of water entry, produced by a change in temperature of a sphere, is obtained for the first time. According to the finding, cavity is not formed around the room temperature sphere under the condition of the impact velocity of 1.5 m/s. When the temperature of the sphere is 300℃, the cavity appears, while it disappears when the temperature reaches up to 400℃. Interestingly, cavity appears again as the sphere is heated to a temperature of 700℃. The degrees of drag reduction of the sphere are different in various temperature conditions. Based on the theory of heat transfer and fluid dynamics, we analyze the mechanism for the influences of temperature and velocity on the forming of cavitation. The results show that the heat-transfer efficiency and heat-transfer mode between sphere and water change with the increase of temperature. Meanwhile the turbulent characteristic around the sphere, the surface roughness and hydrophobicity of the sphere are affected by the bubbles and vapor layer. In consequence, these characteristics influence the formation of cavity. The results of the effect of impact velocity on water-entry cavity reveal that the heat transfer performance plays a significant role in the forming of cavity, while the heat transfer efficiency is improved by the increase of impact velocity. The water-entry characteristics are similar to those in flow field under high temperature at low impact velocity as well as under low temperature at high impact velocity. The flow field of water entry looks similar under 330℃ at high impact velocity as well as under 400℃ at low impact velocity. Thus, an abnormal phenomenon appears. That is to say, the cavity size first decreases, and then disappears with the increase of impact velocity for the sphere at 330℃. The heat transfer performance can determine whether a cavity forms under the conditions of the impact velocity ranging from 1.5 m/s to 3.8 m/s. Meanwhile, the impact velocity itself can merely affect the cavity shape. The pitch-off time of the 300℃ sphere is irrelevant to impact velocity, which shows a good consistency with the literature result. Also, this research will be conductive to gaining an insight into the complex flow of water-entry with a heated sphere.