Characterization of transition to turbulence in microchannels C. Rands, B.W. Webb * , D. Maynes Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602-4201, USA Received 28 September 2005; received in revised form 8 February 2006 Available online 18 April 2006 Abstract This paper reports on an experimental study characterizing the laminar-turbulent transition for water flow in circular microtubes. Microtubes with diameters in the range 16.6–32.2 lm of varying length were employed over the Reynolds number range 300–3400. The volume flowrate was measured for an imposed pressure differential using a timed displacement technique. Additionally, the viscous heating-induced mean fluid temperature rise was measured. Two independent approaches were used to identify transition from laminar to turbulent flow. Both methods showed transition to occur in the Reynolds number range 2100–2500, consistent with macroscale tube flow behavior. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Recent progress in development and application of mic- rodevices has provided motivation for improved under- standing of fluid transport at the microscale. Examples of typical microdevice uses include chemical detection and sep- aration processes and high performance electronics cooling techniques. An issue that has received much attention is whether conventional macroscale flow behavior can be uti- lized to predict flow in microchannels. More specifically, several studies have sought to determine whether effects which may become significant at the microscale influence the transition from laminar to turbulent flow. Sharp and Adrian [1] point out that such phenomena as acoustic dis- turbances, vibrations, inlet agitation, and molecular motion are not clearly understood in the context of transition to turbulence. Further, the impact of microtube surface rough- ness is not well characterized at the microscale. It is readily acknowledged that for macroscale flows the frictional pres- sure drop is considerably higher in the turbulent flow regime. Thus, while the required pressure to generate lami- nar flow at the microscale is very high, after the onset of tur- bulent flow the required driving pressure becomes even greater. Consequently, there is considerable incentive to characterize the transition from laminar to turbulent flow in microchannels of physical scales 100 lm or less. Although classical theory implies that scale has little effect on flow conditions, some prior work has concluded that transition to turbulence may occur at a critical Rey- nolds number (Re cr ) in microchannels lower than that commonly accepted for macroscale flows. While many investigations report deviations in laminar and turbulent friction factor in microchannels from conventional macro- scale data, the review of prior work presented here focuses exclusively on observations of transition to turbulence at such physical scales. The most common method of experi- mentally identifying transition to turbulence is the observed deviation of the friction factor-Reynolds number product from the constant value observed in laminar flow. Based on both frictional pressure drop [2] and heat transfer [3] measurements in rectangular microchannels microma- chined in stainless steel substrates of hydraulic diameter ranging from 133 to 367 lm, Peng et al. observed that tran- sition occurs at Reynolds numbers in the range 300–800. The experimental data of Harms et al. in deep rectangular microchannels of width 251 lm fabricated using chemical etching of silicon substrates suggest onset of turbulence at a critical Reynolds number Re cr = 1500 [4]. Hsieh et al. observed transition to turbulence at a Reynolds number 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.02.032 * Corresponding author. Tel.: +1 801 422 6543; fax: +1 801 422 0516. E-mail address: webb@byu.edu (B.W. Webb). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 2924–2930