Heterogeneous Nucleation With Artificial Cavities

Abstract

Bubble incipience in artificial cavities manufactured from silicon has been studied using gas nucleation and pool boiling. Moderately wetting water and highly wetting ethanol have both been used as the bulk fluid with cylindrical cavities, as well as those with a triangle, square, and rectangle shape cross section. Nominal cavity sizes range from 8 to 60 μm. The incipience conditions observed for water using both gas nucleation and pool boiling suggest that bubble initiation originates from a concave meniscus. Cornwell’s contact angle hysteresis theory for vapor trapping cavities is used to explain the gas nucleation results. The pool boiling results are more difficult to explain. Using ethanol, cavities appeared to be completely flooded and were not activated using either gas nucleation or pool boiling. Using water and gas nucleation cavities were almost always activated, provided the incipience criterion was satisfied; in contrast cavities in pool boiling with water activated with different superheats during different experiments. The difference in incipience behavior between gas nucleation and pool boiling with water is explained based on vapor trapping and thermal suppression considerations. Based on limited experimental results, it appears that the backpressure does not influence gas bubble incipience, provided the pressure difference is the same. The experimental results presented affirm the theory of heterogeneous nucleation from vapor trapping cavities provided contact angle hysteresis and vapor trapping are fully accounted for. However, the results also suggest that the theoretical considerations required for a deterministic model for incipience from vapor trapping cavities during boiling is more complex than previously hypothesized.

Subcooled boiling incipience on a highly smooth microheater

“…It is concluded that boiling incipience on the microheater is a homogeneous liquid-vapor phase change process. This is in contrast to recent observations of lowsuperheat heterogeneous nucleation on metallic surfaces of rms roughness ranging from 4 to 28 nm [1,2,3]. Following the explosive bubble incipience, the boiling process on the microheater can be maintained at much lower superheats.…”

“…In contrast, Theofanous et al [1] observed low-superheat heterogeneous nucleation on a smooth titanium heater with 4 nm rms roughess submerged in water. Qi and Klausner [2,3] also observed low-superheat heterogeneous nucleation more recently on smooth brass (18 nm rms roughness) and stainless steel (28 nm rms roughness) surfaces submerged in ethanol. In contrast, Qi and…”

Heat transfer mechanisms of a vapour bubble growing at a wall in saturated upward flow

“…In order to increase predictability, many researchers employed predefined artificial nucleation sites (Bonjour, Clausse & Lallemand 2000;Shoji & Takagi 2001;Zhang & Shoji 2003;Qi & Klausner 2005;Moghaddam & Kiger 2009a, amongst many others) by creating artificial cavities in the boiling substrate. The shape of these cavities differs between studies, examples are cylindrical, conical, rectangular and even triangular cavities.…”

Pool Boiling Performance Comparison of Smooth and Sintered Copper Surfaces with and Without Carbon Nanotubes

“…Previous authors [31][32][33][34][35] considered mechanisms by which noncondensable gas could be trapped on a surface, thereby providing two-phase interfaces for heterogeneous nucleation.…”