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 mu m (#1) and 394 mu m (#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 10A degrees to 90A degrees, 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.