Thin Film Annular Flow: Nucleate Boiling and Obstruction Wake Dryout

Abstract

Nucleate boiling in thin film annular flow is a complex phenomenon utilized in many energy systems and is influenced by a variety of heat transfer mechanisms. While the fundamentals are rooted in the comparatively well-understood fields of pool and flow boiling, thin film flow boiling introduces additional challenges that make the heat transfer behavior hard to predict. Considering this, the foundational aspects of nucleate boiling and the relationship between flow parameters – such as liquid film thickness, system pressure, surface tension, channel geometry, mass flow rate, and applied heat flux – and the intensity of nucleate boiling in thin annular films are analyzed and their effects demonstrated. Additionally, recent research has identified disturbance waves as a significant factor in enhancing bubble nucleation, yet the underlying mechanisms driving this phenomenon remain undefined. Therefore, the insights and deficiencies of three theories seeking to explain wave-based nucleation are extensively considered. Furthermore, this work investigates the effect of obstructions under annular two-phase flow conditions flowing through a vertical, upward channel. The flow around a cylindrical obstruction in direct contact with the heated wall, tested with two different diameters, is characterized using high-speed video, temperature measurements, and optically based liquid film thickness measurements. The experiments investigate the flow phenomena over a range of operating conditions from those conducive to nucleate boiling and to those where nucleate boiling is fully suppressed. The results indicate that the obstruction induces sustained film dryout with the size, shape, and number of dryout areas dependent on flow conditions. Additionally, dryout in the wake does not always yield a severe reduction in heat transfer coefficient.