Measurement of local heat transfer coefficient during gas–liquid Taylor bubble train flow by infra-red thermography Balkrishna Mehta, Sameer Khandekar ⇑ Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, UP 208016, India article info Article history: Received 12 January 2013 Received in revised form 6 October 2013 Accepted 2 December 2013 Available online 25 December 2013 Keywords: Taylor bubble flow Local Nusselt number InfraRed Thermography Design of experiment Heat transfer enhancement abstract In mini/micro confined internal flow systems, Taylor bubble train flow takes place within specific range of respective volume flow ratios, wherein the liquid slugs get separated by elongated Taylor bubbles, result- ing in an intermittent flow situation. This unique flow characteristic requires understanding of transport phenomena on global, as well as on local spatio-temporal scales. In this context, an experimental design methodology and its validation are presented in this work, with an aim of measuring the local heat trans- fer coefficient by employing high-resolution InfraRed Thermography. The effect of conjugate heat transfer on the true estimate of local transport coefficients, and subsequent data reduction technique, is dis- cerned. Local heat transfer coefficient for (i) hydrodynamically fully developed and thermally developing single-phase flow in three-side heated channel and, (ii) non-boiling, air–water Taylor bubble train flow is measured and compared in a mini-channel of square cross-section (5 mm  5 mm; D h = 5 mm, Bo  3.4) machined on a stainless steel substrate (300 mm  25 mm  11 mm). The design of the setup ensures near uniform heat flux condition at the solid–fluid interface; the conjugate effects arising from the axial back conduction in the substrate are thus minimized. For benchmarking, the data from single-phase flow is also compared with three-dimensional computational simulations. Depending on the employed vol- ume flow ratio, it is concluded that enhancement of nearly 1.2–2.0 times in time-averaged local stream- wise Nusselt number can be obtained by Taylor bubble train flow, as compared to fully developed single- phase flow. This enhancement is attributed to the intermittent intrusion of Taylor bubbles in the liquid flow which drastically changes the local fluid temperature profiles. It is important to maintain proper boundary conditions during the experiment while estimating local heat transfer coefficient, especially in mini-micro systems. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Transport mechanisms of heat, momentum and species under two-phase flow conditions in mini/micro systems are greatly af- fected by local distribution of the phases or flow patterns in the channel. Taylor bubble train flow, a sub-set of slug flows occurring in mini/micro-systems, is typically characterized by a sequence of long bubbles which are trapped in between liquid slugs. Geometri- cal distribution of the liquid slugs and bubbles is fundamentally governed by the resultant of gravity, surface tension, inertia and viscous effects. For a given liquid–gas system, interplay between the gravity and surface tension forces mainly depends on the size of the channel, i.e. the applicable Bond number (Bo). When surface tension dominates over gravitational body force, Taylor bubbles adopt the characteristic capsular shape, with a liquid thin film sep- arating the gas/vapor phase with the wall. In horizontal flow con- ditions when Bo is high enough (Bo > Bo cr  1.835 (Bretherton, 1961)), gravity force dominates over surface tension and the liquid film essentially takes the lower part of the channel cross-section, whereas in the upper part, a negligibly thin liquid film may or may not exist. The existence of liquid film on the wall depends on Ca and surface energy characteristics (hydrophobic or hydro- philic) of the channel wall (Serizawa et al., 2002; Cubaud and Ho, 2004; Ajaev and Homsy, 2006). Taylor bubble train flow is expected to occur and, is employed in many new and upcoming systems and devices in diverse branches of engineering ranging from bio-medical, bio-chemical to thermal management of electronics, micro-two-phase heat exchangers and reactors, nuclear rod bundles, micro-fluidic de- vices, loop heat pipes, etc. (Triplett et al., 1999; Devesenathipathy et al., 2003; Spernjak et al., 2007; Moharana et al., 2011a). Quite frequently, due to the mini/micro fabrication techniques, such as laser machining, chemical etching, micro-milling, abrasive jet machining etc., emerging technological solutions employing inter- nal convective flows, make use of channels of non-circular cross sections. Rectangular micro-channels are of particular interest as they are used extensively in heat sinks of microelectronic devices, 0142-727X/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ijheatfluidflow.2013.12.001 ⇑ Corresponding author. Tel.: +91 512 259 7038; fax: +91 512 259 7408. E-mail address: samkhan@iitk.ac.in (S. Khandekar). International Journal of Heat and Fluid Flow 45 (2014) 41–52 Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff