Local heat transfer measurement and thermo-fluid characterization of a pulsating heat pipe Mauro Mameli a, * , Marco Marengo a , Sameer Khandekar b a University of Bergamo, Department of Industrial Engineering, viale Marconi 5, 24044 Dalmine, Italy b Indian Institute of Technology Kanpur, 208016 Kanpur, UP, India article info Article history: Received 21 February 2013 Received in revised form 18 July 2013 Accepted 29 July 2013 Available online Keywords: Pulsating Heat Pipes Local heat transfer Pressure variation Flow patterns abstract A compact Closed Loop Pulsating Heat Pipe (CLPHP), filled with ethanol (65% v/v), made of four trans- parent glass tubes forming the adiabatic section and connected with copper U-turns in the evaporator and condenser sections respectively, is designed in order to perform comprehensive thermal-hydraulic performance investigation. Local heat transfer coefficient is estimated by measurement of tube wall and internal fluid temperatures in the evaporator section. Simultaneously, fluid pressure oscillations are recorded together with the corresponding flow patterns. The thermal performances are measured for different heat input levels and global orientation of the device with respect to gravity. One exploratory test is also done with azeotropic mixture of ethanol and water. Results show that a stable device operation is achieved (i.e. evaporator wall temperatures can reach a pseudo-steady-state) only when a circulating flow mode is established superimposed on local pulsating flow. The heat transfer perfor- mance strongly depends on the heat input level and the inclination angle, which, in turn, also affect the ensuing flow pattern. The spectral analysis of the pressure signal reveals that even during the stable performance regimes, characteristic fluid oscillation frequencies are not uniquely recognizable. Equiva- lent thermal conductivities of the order of 10e15 times that of pure copper are achieved. Due to small number of turns horizontal mode operation is not feasible. Preliminary results indicate that filling azeotropic mixture of ethanol and water as working fluid does not alter the thermal performance as compared to pure ethanol case. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction The present industry demand of high heat transfer capability coupled with relatively cheaper component costs catalyses evolu- tion of novel two-phase passive devices. A conceptually similar to the Pulsating Heat Pipe (PHP), was introduced by Smyrnov and Savchenkov in 1971 [1]. More practical design variation of this concept, from an engineering stand-point, was proposed in the early 90s by Akachi [2,3], which subsequent fuelled several in- vestigations to better understand this device (as summarized in Refs. [4,5]). In contemporary times too, the qualitative, as well as quantitative investigations of several design variants of Pulsating or Oscillating Heat Pipes, for potential passive thermal management applications in nuclear, defense and space are emerging at a rapid pace; this has indeed become one of the most interesting and vibrant fields of investigation. The PHP design variants being proposed and studied have the potential to meet all the present and possibly future specific requirements from electronics cooling [6,7], heat recovery [8,9] and passive cooling of reactor containments, to name a few. A PHP usually consists of a copper capillary tube bended in a serpentine-shape closed loop (CLPHP), evacuated from within and partially filled with a working fluid, typically in its liquid-phase. As the filling volume of the fluid is less than the total internal volume of the PHP tube, the liquid-phase and vapor-phase co-exist inside the tube in the form of alternating liquid plugs and vapor bubbles, typically as a Taylor bubble train. The capillary tube diameter is chosen in such a manner that surface tension dominates over gravity forces, resulting in no bulk stratification of the phases. During heat transfer operation, one end of the serpentine tube bundle receives heat (acting as an evaporator) while the other end is kept at a lower temperature (acting as a condenser). Heating causes the thin liquid film surrounding the vapor bubbles to evaporate; bubbles thus expand and push the adjacent Taylor bubble train towards the condenser zone, where heat gets rejected to the cold source. The shrinking of vapor bubbles in the condenser * Corresponding author. Tel.: þ39 (0)352052068; fax: þ39 (0)352052077. E-mail addresses: mauro.mameli@unibg.it, mcjmameli@gmail.com (M. Mameli). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2013.07.025 International Journal of Thermal Sciences 75 (2014) 140e152