Boiling crisis dynamics: low gravity experiments at high pressure

TL;DR

This study investigates boiling crisis mechanisms near the critical point by conducting low gravity experiments with SF$_6$ and H$_2$, revealing dry spot growth regimes and bridging near-critical and low-pressure boiling behaviors.

Contribution

It presents the first comparative analysis of saturated boiling under high-pressure near-critical conditions using both space-based and ground-based magnetic gravity compensation experiments.

Findings

Identification of two dry spot growth regimes: circular and chain coalescence.

Visualization of dry spots with transparent heaters.

H$_2$ experiments connect near-critical and low-pressure boiling regimes.

Abstract

To understand the boiling crisis mechanism, one can take advantage of the slowing down of boiling at high pressures, in the close vicinity of the liquid-vapor critical point of the given fluid. To preserve conventional bubble geometry, such experiments need to be carried out in low gravity. We report here two kinds of saturated boiling experiments. First we discuss the spatial experiments with SF$_6$ at 46$^\circ$ C. Next we address two ground-based experiments under magnetic gravity compensation with H$_2$ at 33 K. We compare both kinds of experiments and show their complementarity. The dry spots under vapor bubbles are visualized by using transparent heaters made with metal oxide films. We evidence two regimes of the dry spots growth: the regime of circular dry spots and the regime of chain coalescence of dry spots that immediately precedes the heater dryout. A recent H$_2$ experiment is shown to bridge the gap between the near-critical and low pressure boiling experiments.