Investigation of thermo-hydrodynamic characteristics in micro-oscillating heat pipe by alternatively-arranged ratchet microchannels

Abstract

In this study, three flat-plate micro-oscillating heat pipes (FPMOHPs) with different alternatively-arranged ratchet microchannels were designed and fabricated. Then, a combined visualization and thermal experiment was conducted to examine the regulation of thermo-hydrodynamic characteristics in the FPMOHPs by the alternatively-arranged ratchet microchannels, via the comparison with a traditional FPMOHP with completely uniform microchannel. The detailed fluid flow motions and corresponding temperature oscillations as well as thermal performance of the FPMOHPs were illustrated and analyzed to clarify the advantages of this microchannel design. It is indicated that the oscillating frequency and velocity of the fluid are diminished by the ratchet microchannels because of the higher flowing resistance. However, the ratchet microchannels can trigger the stronger flow disturbance, leading to the ascending “wave-like” annular flow broken into the dispersive slugs or bubbles with irregular shape. Among the three different ratchet microchannels, the asymmetric ratchet structure with the larger contraction-expansion ratio near the inlet of the fluid can trigger more intense boiling in the evaporator, which also energizes the downward flow from the condenser. Furthermore, the additional actions on the bubbles/slugs caused by the Laplace pressure jump in three different ratchet microchannels play distinct roles in upward and downward bubble/slug flow, i.e., driving force or resistance force or little role, which determine the most favorable ratchet structure for the fluid flow and heat transfer in the FPMOHPs. In general, the positive effects of the enhanced boiling and the stronger fluid flow disturbance caused by the ratchet microchannel dominates the negative effect of growing flow resistance, leading to the enhanced convective heat transfer. Therefore, the equivalent thermal conductivity can be improved up to 33.8 % on average as compared to the traditional FPMOHP with completely uniform rectangular channel.