Effects of wall superheat and thermophysical properties on single bubble dynamics during nucleate boiling at different saturation pressures

Nucleate boiling is known for its high heat transfer coefficients and is widely implemented from large-scale, such as flooded evaporators, to small-scale, such as electronics cooling, systems. As the bubbles are responsible for removing heat from the heated surface, a comprehensive understanding of the bubble dynamics is required to grasp the fundamental mechanisms behind this heat transfer regime. However, little experimental data is available on the bubble dynamics of dielectric fluids and refrigerants due to the challenges associated with their small bubbles in comparison to water, optical properties, and material compatibility. In addition, previous studies were generally conducted with unpressurised setups at a saturation pressure of 1 bar. In this study, the effects of wall superheat and thermophysical properties on the bubble growth rate, bubble growth period and departure diameter of R245fa, R1336mzz(Z) and Novec 649 are presented. To enable a direct comparison across a wide range of saturation pressures and superheats, the bubble dynamics were experimentally investigated nucleating from a single conical cavity using shadowgraphy. An increase in superheat generally led to increased bubble growth and a shorter bubble growth period, while the effect on the departure diameter became more prominent at lower saturation pressures. When increasing the saturation pressure, the bubble growth rate decreased, bubble growth time increased, and smaller spherical bubbles departed from the heated surface. At low saturation pressures (0.44 bar and 0.62bar) and superheats (3.7 K and 5.2 K) the bubble dynamics of Novec 649 differed from R1336mzz(Z) and R245fa. A negligible waiting period suggested sufficient vapour entrapment to enable increased growth rates and the departure diameter increased with increasing pressure, while an increase in superheat further increased the departure diameter but not the bubble growth period. It was found that irrespective of fluid, saturation conditions, or superheat, the normalised bubble growth could generally be described by a power law with an exponent of approximately 0.42, while the bubble growth was more sensitive to variations in pressure than in thermophysical fluid properties, particularly at very low pressures and superheats.