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Multiphase Science and Technology

A Quarterly

ISSN for PRINT: 0276-1459

Institutional price:	$694.00
Issues per year:	4

Best Paper Award Selection - Editorial Board Site

2007, Volume19

103 pages

DOI: 10.1615/MultScienTechn.v19.i1

Issue price - $193.00

OPTICAL MEASUREMENTS TO CHARACTERIZE TWO-PHASE FLUID FLOW IN MICROCHANNELS

R. Revellin
Laboratory of Heat and Mass Transfer, Ecole Polytechique Fédérale de Lausanne, Station 9, CH-1015 Lausanne, Switzerland

Dr. John R. Thome
Laboratory of Heat and Mass Transfer, Ecole Polytechique Fédérale de Lausanne, Station 9, CH-1015 Lausanne, Switzerland

ABSTRACT

An extensive experimental two-phase flow study has been carried out on two single round tubes (D = 0.509 and 0.790 mm) for R-134a and R-245fa. The present paper summarizes our recently published results from this work. First of all, an optical measurement method for two-phase flow pattern characterization in microtubes was developed in order to determine the frequency of bubbles exiting a microevaporator, their coalescence rates, and lengths as well as their mean two-phase vapor velocity. Based on the bubble frequency results and CHF measurements made on the microevaporator (predicted with a new microchannel version of the Katto-Ohno correlation), a new type of flow pattern map for evaporating flow in microchannels was proposed with (i) an isolated bubble regime, where the bubble generation rate via nucleation in the microevaporator is much larger than the bubble coalescence rate and includes both bubbly and slug flows, (ii) a coalescing bubble regime, where the bubble coalescence rate dominates the bubble generation rate and exists up to the end of the coalescence process, (iii) an annular regime, which is a completely coalesced flow, and (iv) a mist flow regime, whose onset is indicated by the critical vapor quality corresponding to the critical heat flux (CHF). This flow pattern map may in the future be used as the starting point for the development of heat transfer and pressure drop models and the thermal design of microevaporators. In addition, an extensive new two-phase pressure drop database was obtained whose homogeneous friction factors versus Reynolds numbers fell into the well-recognizable laminar, transition, and turbulent flow regimes similar to those of the Moody diagram observed in single-phase flow.