Macro-to-microchannel transition in two-phase flow: Part 1 – Two-phase flow patterns and film thickness measurements

• Published in: Experimental thermal and fluid science Vol. 35; no. 1; pp. 37 - 47
• Main Authors: Ong, C.L., Thome, J.R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.01.2011; Elsevier
• Subjects: Applied sciences; APPROXIMATIONS; Asymptotic properties; BOILING; Channels; CLASSIFICATION; CONFINEMENT; Energy; Energy. Thermal use of fuels; ENGINEERING; Exact sciences and technology; Flows in ducts, channels, nozzles, and conduits; Fluid dynamics; Fundamental areas of phenomenology (including applications); Heat transfer; Liquid films; Microchannels; Minichannels; Multiphase and particle-laden flows; Nonhomogeneous flows; Physics; REFRIGERANTS; Refrigerating engineering; Refrigerating engineering. Cryogenics. Food conservation; SATURATION; SHEAR; STRATIFICATION; SURFACE TENSION; SYMMETRY; Techniques. Materials; THICKNESS; Thresholds; Transition; TWO-PHASE FLOW; Two-phase flow patterns; Two-phase flow patterns; Transition; Minichannels; Microchannels; Test facilities; Pipe flow; Experimental study; Boiling; Layer thickness; Flow regime; Two-phase flow; Microchannel; Liquid vapor interface; Liquid films; Microstructure; Miniaturized design; Refrigerant fluid; Heat transfer
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2010.08.004

The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. The main objective of this study focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of three different sizes with three different refrigerants over a range of saturation conditions to investigate the effects of channel confinement on two-phase flow patterns and liquid film stratification in a single circular horizontal channel (Part 2 covers the flow boiling heat transfer and critical heat flux). This paper presents the experimental two-phase flow pattern transition data together with a top/bottom liquid film thickness comparison for refrigerants R134a, R236fa and R245fa during flow boiling in small channels of 1.03, 2.20 and 3.04 mm diameter. Based on this work, an improved flow pattern map has been proposed by determining the flow patterns transitions existing under different conditions including the transition to macroscale slug/plug flow at a confinement number of Co ≈ 0.3–0.4. From the top/bottom liquid film thickness comparison results, it was observed that the gravity forces are fully suppressed and overcome by the surface tension and shear forces when the confinement number approaches 1, Co ≈ 1. Thus, as a new approximate rule, the lower threshold of macroscale flow is Co = 0.3–0.4 while the upper threshold of symmetric microscale flow is Co ≈ 1 with a transition (or mesoscale) region in-between.