Proceedings of the 4 th European Conference on Microfluidics - Microfluidics 2014 - Limerick, December 10-12, 2014 © SHF 2014 1 μFLU14-73 LOCAL THERMO-HYDRODYNAMICS OF A LIQUID PLUG PULSATING INSIDE A DRY CAPILLARY TUBE Sushant Kumar 1 , Balkrishna Mehta 2 , Ashish Bajpai 2 , Sameer Khandekar *2 1 Birla Institute of Technology and Science Pilani - Goa Campus, Zuarinagar 403726 (Goa) India sushant08kr@gmail.com 2 Indian Institute of Technology Kanpur, Kanpur 208016 (UP) India baluk@iitk.ac.in, ashishkb007@gmail.com, samkhan@iitk.ac.in KEY WORDS Liquid slug flow, Pulsating heat pipe, Capillary tube, Time-averaged Nusselt Number ABSTRACT Taylor-bubble flow is one of the most important two-phase flow patterns, which belongs to a class of intermittent flows. Because of their unique flow features, such intermittent flows have proved to be promising for augmenting local heat transfer compared to single component flows. One application of such flows is the pulsating heat pipe (PHP), wherein thermally induced self-sustained oscillations of two-phase capillary slug flow takes place; the slugs having a net translational motion coupled with local flow oscillations. In this work, the numerical simulations of the flow of a single isolated liquid slug (of water), pulsating inside a capillary tube of inner diameter = 2.0 mm, is reported. Simulations have been carried out for four different Womersley Numbers ( Wo R    ) i.e., Wo = 2.35, 3.32, 4.7 and 6.65 which covers the frequency range of 0.44-7 Hz (which, incidentally, is typical for the operation of PHPs). The Capillary number (Ca = µU/σ) is 0.001 for all the cases, with the length-to-diameter (L/D) ratio as 2. The effect of pulsations on the thermo-hydrodynamics is discussed and the importance of correctly defining the Nusselt number is highlighted. It is found that there is a marginal enhancement in the thermal transport due to the imposed pulsations, as compared to steady flow; rather, there can be an overall reduction in Nusselt number too, which can happen at low imposed frequencies. An interesting result is observed wherein the estimated value of the local instantaneous Nu as per the classical definition may come out to be ‘negative’, due to the crossover of the wall temperature and mean fluid temperature. This happens due to the unique flow characteristics coupled with the thermal boundary layer development in the plug translating with imposed flow pulsations. 1. INTRODUCTION The last five decades have seen a drastic evolution in the techniques used for efficient thermal management of electronics. Apart from the existing two-phase cooling technologies which comprise pool boiling, spray/mist cooling, flow boiling, liquid jet impingement etc., special technologies like thermoelectric devices and various kinds of heat pipes have gained more prominence over the years. The capability of heat pipes to remove large quantities of heat with a minimal temperature drop has made them highly popular in electronics thermal management. Of these, the Pulsating Heat Pipe (PHP), due to some inherent advantages over the conventional heat pipes has carved a niche for itself. The usage of wicks in conventional heat pipes engenders its own performance limitation in the form of the capillary limit. Other limitations include the viscous, the sonic, the entrainment and the boiling limit. The simple design which obviates the need for a wick makes PHPs more reliable, cost effective and easy to manufacture. In the last decade, active research has been done in understanding the complex internal two-phase thermo-hydrodynamics of PHPs [1-4]. In a PHP, self-sustained thermally driven oscillations of bubbles and slugs (Taylor slug-bubble flow) take place. * Corresponding author: samkhan@iitk.ac.in