Dynamics of the Liquid Microlayer Underneath a Vapor Bubble Growing at a Heated Wall

Abstract

Evaporation of the liquid microlayer developing underneath a bubble in the initial (inertia controlled) phase of its growth can be a significant vapor source in the later (heat-diffusion controlled) phase of bubble growth. In the literature, representation of this microlayer is typically limited to a very short (order of microns) region near the apparent Triple Phase Line (TPL) between the bubble and the wall. However, experimental observations show that the microlayer may actually extend hundreds of microns beyond the TPL region. Guided by this observation, we develop a simple model to predict the time evolution of the extended microlayer, and the associated corresponding evaporation rate and heat flux underneath a bubble. The model is derived as a special case of the complete governing equations, which account for the complicated effects of disjoining pressure, capillarity, vapor recoil and interfacial resistance. The predictions of the model are in reasonable quantitative agreement with the experimentally observed behavior of the microlayer.

Simulations of microlayer formation in nucleate boiling

“…Such coupling would require to solve for the heat equation in the substrate, while using Eq. 4.6 as a flux boundary condition at the wall surface, as shown in previous work by Guion et al (2013). The initial evaporation of the microlayer would locally reduce the wall superheat and therefore slow down the rate of evaporation of the microlayer.…”

“…We include this uncertainty in initial bubble growth rates in our validation below through a sensitivity study on the initial bubble growth rate: in blue, we assume the bubble growth rate to be constant during the first 20µs, and in red we assume its acceleration to be constant during the first 20µs. We compare our numerical results against experimental results in Figure 11 (b): black squares represent the earliest experimental measurement of the microlayer thickness, which took place at time t = 410µs; and grey triangles are obtained by deducing the microlayer thickness at t = 20µs, assuming that the heat transfer within the microlayer is purely by conduction, see Guion et al (2013):…”

“…Depending on the fluid, flow and nucleation temperature, the contribution from microlayer evaporation to overall heat transfer and bubble growth can be large in the case of water, see Gerardi (2009), or relatively small in the case of refrigerants, see Kim (2009). Typical microlayer models focus on evaporation at the Triple-Phase-Line (TPL) region where the three phases (liquid, solid, and vapor) are in apparent contact, see Dhir (2009) and Kunkelmann & Stephan (2010), or in the extended microlayer region, see Guion et al (2013), Jiang et al (2013), and Sato & Niceno (2015). In practice, numerical simulations of boiling resolve the macroscopic liquid vapor interface of boiling bubbles, but typically resort to subgrid models to include contributions at the microscopic scale such as from evaporation of the microlayer, which requires initialization of the microlayer shape and extension.…”

“…Predicting boiling heat transfer has therefore garnered significant attention, but remains complicated by the need to consider phenomena occurring over multiple scalessee Figure 1, from the adsorbed liquid layer at the wall at the nanometer scale -see Churaev (1975) and Chung et al (2011), up to the bubble diameter at the millimeter scale, see Dhir (1998), Stephan & Kern (2004), Kim (2009), Kunkelmann & Stephan (2010), Guion et al (2013), Jiang et al (2013), Jung & Kim (2014), Sato & Niceno (2015), Giustini et al (2016), Zou et al (2016a), and Zou et al (2016b).…”

Evaporative thermal resistance and its influence on microscopic bubble growth

“…Being such a thin layer, the flow of heat through it is large. This is because heat flow is driven by the temperature difference between the substrate-liquid interface and the liquid-vapour interface, and this difference exists over a very small distance [7]. Consequently, the micro-layer is believed to be responsible for a significant fraction of the vapour generation under many circumstances, such as the boiling of atmospheric water at a superheat around 10 K [8].…”