Micro pulsating heat pipes with alternate microchannel widths

The thermal performance of two silicon-based micro pulsating heat pipes (μPHPs) that contained microchannels of alternate widths were fabricated and tested to investigate the effect of turn and the thermophysical properties of the working fluid on the μPHP start-up. The overall size of the μPHPs was 35 mm × 40 mm × 1.25 mm and exhibited 64 parallel, rectangular channels. Every neighboring microchannel in the μPHPs featured two different widths of 0.5 mm and 0.3 mm and an identical depth of 0.25 mm. The working fluid of the μPHPs was either deionized water or methanol. The results showed that the water-charged μPHPs always failed to create a pulsating two-phase flow and thus developed higher thermal resistance, whereas the vertically oriented methanol-charged μPHP at a filling ratio of 80% exhibited a pulsating two-phase flow at an input power of higher than 6 W. Failure to create the pulsating two-phase flow in the water-charged μPHPs was likely due to its unfavorable fluid properties, such as high surface tension, high latent heat of vaporization, and, as well, low Bond number. The methanol-charged μPHPs, of which the Bond number was approximately 0.2, functioned because of the particular microchannel design and its superior fluid properties.