On the transition from bubbly to elongated bubbles flow regime: A physics-based framework

Accurate identification of flow regime transitions is a foundational step toward developing high-fidelity predictive models for two-phase flow in microchannels. This study presents a comprehensive experimental investigation of the transition from bubbly to elongated bubbles flow regimes in machined metal microchannels. Recognizing that regime boundaries are governed by a combination of interfacial dynamics, vapor generation, and inertial transport, the objective of this work is to establish a generalized transition model applicable across various channel dimensions and working fluids. To contextualize the need for such a model, we conducted a critical evaluation of existing regime transition models and identified key gaps in their predictive capability. The influence of operating and geometric parameters – including heat flux, mass flux, channel size, and bubble coalescence dynamics – on flow regime transition was systematically examined. Key nondimensional parameters – Reynolds number (Re), Boiling number (Bo), Weber number (We), and Confinement number (Co) – were identified as governing variables. In addition, we introduce the Bubble Confinement Ratio (BCR $=D_b/D_h$). This parameter exhibits a threshold beyond which the flow transitions from bubbly to elongated bubbles regime. A new empirical model was developed by correlating these parameters through a unified power-law expression, capable of predicting transition boundaries with accuracy across all tested conditions. These findings provide a foundation for a generalized transition framework that can be further refined through additional experiments and validations across a broader range of conditions.