Physics of Transition to Annular Flow in Microchannel Flow Boiling Process

Transition to annular flow regime in microchannels is arguably one of the most complex phenomena in the flow boiling process. The instability of the vapor-liquid interface in this interstitial regime presents an intricate situation in which the interface pattern rapidly changes with the mass flow rate and surface heat flux. Although a few past studies have reported observing this regime, thermohydraulics of the process and flow and boundary conditions under which this transition occurs have remained largely unknown. The main obstacle in deciphering the physics of this process has been the lack of measurement tools to characterize hydrodynamics and thermal characteristics of this flow regime. The present study benefits from a novel test device that enables measuring the liquid film thickness and velocity with unprecedented spatial and temporal resolutions. Our experimental results show that each flow regime has a unique thermal signature associated with the onset of the liquid film flow. This can be considered as the most fundamental mechanistic-based transition criterion.