Start-up, heat transfer and flow characteristics of silicon-based micro pulsating heat pipes Jian Qu a,b , Huiying Wu a,⇑ , Ping Cheng a a School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b School of Energy & Power Engineering, Jiangsu University, Zhenjiang 212013, China article info Article history: Received 9 March 2012 Received in revised form 9 June 2012 Accepted 11 June 2012 Available online 9 July 2012 Keywords: Micro pulsating heat pipe Silicon chip Start-up Thermal performance Flow pattern abstract A simultaneous visualization and measurement study has been carried out to investigate the start-up, heat transfer and flow characteristics of three silicon-based micro pulsating heat pipes (MPHPs) with the trapezoidal cross-section having hydraulic diameters of 251 lm (#1), 352 lm (#2) and 394 lm (#3), respectively. Experiments were performed under different working fluids, filling ratios, inclination angles (bottom heating mode) and heating power inputs. It is found that (1) the silicon-based MPHPs could start up within 200 s when charged with R113 or FC-72, but they failed to start up at all inclination angle when charged with water or ethanol having lower (dP/dT) sat , higher viscosity, higher latent heat and higher surface tension at the same temperature. During the start-up period, no obvious nucleation was observed. After the start-up period, MPHPs entered the operation period. The silicon-based MPHP could operate normally even at a Bond number of 0.26 and a hydraulic diameter of 251 lm, both smaller than the corresponding values in literatures; (2) the thermal performance of MPHPs depends greatly on the type of working fluid, filling ratio and inclination angle. At the lower power input, MPHPs charged with R113 showed better thermal performance than that charged with FC-72, however, the latter exceeded the former at the higher power input. For the same working fluid, there existed an optimal fill- ing ratio corresponding to the best thermal performance of MPHPs, which was about 52%, 55% and 47% for MPHPs #1, #2 and #3 at the vertical orientation (90°), respectively. When the MPHPs turned from the vertical to the horizontal orientation, the thermal performance tended to be decreased, indicating that the gravity effect cannot be ignored in these silicon-based MPHPs. In MPHP #3 at the inclination angle from 70° to 90°, there appeared a special thermal resistance curve with two local maximum points, which is absent in the traditional PHPs; (3) in the operation period of larger MPHP #3, nucleation boiling, bulk circulation and injection flow were all observed, while these flow patterns were absent in the smaller MPHPs #1 and #2. Intense liquid film evaporation, instead of bubbles’ generation and expansion which usually activated the oscillation flow in macro-PHPs, drove the two-phase flow in the smaller MPHPs #1 and #2. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction At the present time, thermal control of electronic chips remains to be a great challenge due to the continuous increasing heat flux to be dissipated with diminishing space associated with rapid ad- vances in the microelectronic integration technology [1,2]. Micro- cooling devices for the purposes of maintaining both chip temper- ature and temperature gradient at acceptable levels are in great de- mands [3,4]. Various efforts have been made in the past decades to develop novel technologies capable of removing larger amount of heat from chips, among which micro heat pipes are considered as one of the most promising options [5]. Compared with other cooling technologies, such as pump-assisted microchannel heat sinks, spray cooling, jet impingement cooling, thermoelectric cool- ing, etc., micro heat pipes seldom suffer from reliability, noise, effi- ciency, or cost issues, and moreover, they can easily be integrated into the chip itself. Micro heat pipes, originally proposed by Cotter [6], consisting of noncircular microchannels with hydraulic diameters of 10– 500 lm, are suitable for the direct heat removal from chip via func- tion themselves as integrated parts in a chip for the purposes of decreasing the maximum temperature and temperature gradient of the chip. In 1993, Peterson et al. [7] carried out the first pub- lished experiment on micro-grooved heat pipe arrays fabricated in silicon chips. As compared with a plain silicon wafer, their experiment demonstrated that the silicon chips of the same size integrated with rectangular and triangular micro heat pipe arrays 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.06.024 ⇑ Corresponding author. E-mail address: whysrj@sjtu.edu.cn (H. Wu). International Journal of Heat and Mass Transfer 55 (2012) 6109–6120 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt