Drag reduction characteristics of heated spheres falling into water

We experimentally investigate the drag reduction characteristics of heated spheres falling into water by using a high-speed camera. In 25-°C water, with the increase of the sphere temperature the average velocity increases to a maximum value at a temperature of 400 °C and then decreases until the temperature reaches 700 °C, the average velocity will increase while the sphere temperature continually rises until the temperature reaches 900 °C. The average and the maximum velocity of the heated sphere are larger than those of the room-temperature sphere. The flow separates at the rear of the heated sphere, leading to low pressure drag. The drag reduction effect of the stable film boiling is lower than that of the nucleate boiling. In the nucleate boiling regime, the average velocity decreases with the increase of water temperature, the drag of the sphere with gentle boiling intensity is smaller. The vapor layer formed in the stable film boiling regime can improve the stability of the fall trajectory. The intense turbulence caused by the nucleate boiling can make the sphere largely deviate from rectilinear motion. (paper)