RESEARCH ARTICLE Non-invasive measurement of void fraction and liquid temperature in microchannel flow boiling David Fogg Æ Milnes David Æ Kenneth Goodson Received: 30 January 2008 / Revised: 12 November 2008 / Accepted: 21 November 2008 / Published online: 27 December 2008 Ó Springer-Verlag 2008 Abstract Past thermometry research for two-phase microfluidic systems made much progress regarding wall temperature distributions, yet the direct measurement of fluid temperature has received little attention. This paper uses a non-invasive two-dye/two-color fluorescent tech- nique to capture fluid temperature along with local liquid fraction in a two-phase microflow generated by injecting air into a heated microchannel. The fluorescent emission of Rhodamine 110 and Rhodamine B, measured with photo- diodes, is used to obtain local liquid temperature (±3°C) and void fraction (±2% full-scale) over a temperature range from 45 to 100°C. Arrays of these sensors can sig- nificantly expand the set of measurable flow parameters to include bubble/slug frequency, size, velocity, and growth rates in addition to mapping the local liquid temperature and void fraction. List of symbols a void fraction b liquid fraction e perturbation in signal j extinction coefficient of H 2 O (m -1 ) n intensity constant group (W m) r surface tension (N m -1 ) / quantum efficiency U quantum efficiency constant group (m -1 ) a molar extinction coefficient (L mol -1 m -1 ) A area (m 2 ) c concentration (mol L -1 ) d diameter (m) H channel depth (m) E error I incident intensity (W m -2 ) p pressure (N m -2 ) P power (W) ^ P normalized Power T temperature (°C) T transmittance t time (s) V voltage (V) Subscripts k,inc incident wavelength a absorbed d detected dye property of a dye e emitted fo all liquid g glass go all vapor m measurement ref reference value PD photodiode PRED predicted TC thermocouple 1 Introduction The study of two-phase microfluidics has received signif- icant attention over the past two decades. Micro-sized heat D. Fogg (&) Creare Inc.,16 Great Hollow Rd, Hanover, NH 03755, USA e-mail: dwf@creare.com M. David  K. Goodson Department of Mechanical Engineering, Stanford University, Building 530, Room 224, Stanford, CA 94305-3030, USA 123 Exp Fluids (2009) 46:725–736 DOI 10.1007/s00348-008-0604-3