Start-up, heat transfer and flow characteristics of silicon-based micro pulsating heat pipes

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 μm (#1), 352 μm (#2) and 394 μm (#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 μm, 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 filling 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.