Flow visualization of silicon-based micro pulsating heat pipes

Two sets of silicon-based micro pulsating heat pipes (SMPHPs) with trapezoidal cross section having hydraulic diameters of 352 μm (#1) and 394 μm (#2) respectively were fabricated for the first time using MEMS technology. With FC-72 as the working fluid, the start-up, steady operation state, as well as flow patterns were investigated using a CCD camera. It was found that the start-up process of these two SMPHPs was rather rapid. At the start-up period, no nucleation was observed, and the vapor plugs at the evaporator U-bends were formed mainly due to the breakup of liquid slugs. At the steady operation state, self-sustained oscillation with large amplitudes dominated the flow behavior when the inclination angle varied from 10° to 90°, but the nucleate boiling and bulk circulation were observed only in SMPHP #2. While bubbly, slug/plug, annular/semi-annular, and wavy-annular flows were observed in both two SMPHPs, the injection flow only appeared in SMPHP #2.