Suppression of bubbles in subcooled liquid nitrogen under heat impulse

This paper describes an experimental investigation to verify that subcooling of liquid nitrogen can suppress bubbles when an impulsive heat is applied. A heater is attached on a surface of substrate in liquid nitrogen which simulates the quench state of the superconducting coil in high temperature superconductor (HTS) system. A pulse of power input, whose period is around 100 ms, is applied to the heater in liquid nitrogen bath at temperatures between 77 K and 65 K. Two kinds of experiments are performed to scrutinize the thermal behavior of liquid nitrogen under the heat impulse. First, the temperature near the heater is directly measured during the heating and the recovery periods for different surface orientations (vertical, horizontal up and down). It is observed that the temperature history is strongly dependent on the orientation in saturated liquid, but nearly independent of the orientation in subcooled liquid. The main reason for this difference is that the generation of bubbles is less active in subcooled state than in saturated state. The subcooling of liquid nitrogen below 70 K at atmospheric pressure is found to be very effective in suppressing bubbles. Second, bubbles generated from the heater are recorded by a high-speed camera for different degrees of subcooling. The detached bubbles from the heater surface are quickly diminished in the liquid region under the subcooled condition of nitrogen. In the case of the subcooled condition at 65 K and 101 kPa, only vapor film is formed on the heater surface during the heating period when the heating power is within approximately 150 W/cm2.