Influence of floating bubbles in a liquid layer on temperature measurements under acoustic-vacuum exposure

Abstract

Investigations were made on how bubbles formed in a liquid influence the readings of a temperature sensor – spherical junction of the thermocouple (SJT) under acoustic-vacuum exposure. According to the proposed hypothesis, the sharp change in liquid temperature can be explained by the fact that under acoustic-vacuum exposure, liquid steam bubbles form on the SJT surface or floating bubbles stick (slip) to it. The temperature of the liquid steam inside the bubbles is higher than that of the surrounding liquid. When the bubbles break away or collapse, the surface temperature of the spherical junction of the thermocouple decreases due to heat exchange with the liquid. A mathematical model was developed for the process of bubbles generation in a liquid under acoustic-vacuum exposure and numerical experiments were carried out. They demonstrated that changes in the temperature of thermocouple readings are influenced mostly by the bubbles forming at the bottom of the experimental container with their subsequent ascent and collapse on the SJT surface. A sharp change in the temperature of the liquid is by 1.5 K–7 K at the bubble collapse on the surface of the thermocouple; by 0.7 K at a bubble sliding over the thermocouple surface; and by 3.5 K–5.5 K at bubble formation on the thermocouple. The results of comparing the liquid temperature values during physical and numerical modeling confirm the hypothesis put forward about the influence of bubbles on the thermocouple readings.