Bubbles in capillaries: Relaxing traditional assumptions

Abstract

<h2>Abstract</h2> The dynamics of long gas bubbles and thin liquid films in confined channels is a classical problem in fluid mechanics and has been studied extensively under the assumptions of two-dimensional (Hele-Shaw cells or axisymmetric tubes) geometries, very viscous flows, negligible inertial and gravitational effects, and isothermal conditions. However, recent engineering applications such as microfluidics, where channels present angular cross-sections, or two-phase cooling via microchannel flow boiling, where low-viscosity fluids are utilised, require relaxation of these assumptions to deliver robust design principles. This work presents new insights into bubbles and thin films dynamics that emerge when studying this flow in unconventional settings. When bubbles propagate in noncircular capillaries, liquid films are much thinner than what observed in pipes and cross-stream film drainage induces a monotonic film thinning towards the bubble rear. Inertial forces cause the appearance of undulations at the bubble rear, which are described well by augmenting the traditional lubrication theory. Gravitational forces are impactful already when the Bond number is much smaller than unity, despite Bo<1 being a popular criterion to disregard the impact of gravity. Finally, these insights are exploited to explain heat transfer trends emerging from numerical simulations of flow boiling in microchannels for thermal management applications.