Effect of reentrant cavities on the thermal performance of a pulsating heat pipe

Highlights: • Flow characteristics of the MPHP with reentrant cavities are investigated. • Reentrant cavities in the MPHPs are shown to promote nucleation and early startup. • The thermal resistance of the MPHP with reentrant cavities is decreased by up to 57%. • A single MPHP incorporating various sizes of reentrant cavities is proposed. - Abstract: This study is performed to investigate the effect of the size of reentrant cavities on the thermal performance of a micro pulsating heat pipe (MPHP). The flow and thermal characteristics of the MPHPs with each MPHP having a different size of reentrant cavities, along with a MPHP without reentrant cavities are experimentally obtained and compared. Silicon-based MPHPs with and without reentrant-type artificial cavities inside the channels are fabricated using MEMS techniques. The MPHPs have rectangular channels which are engraved on a silicon wafer with a hydraulic diameter of 667 $\mu \text{m}$. Ethanol is used as the working fluid. To allow for flow visualization, the etched micro-channels are covered with a transparent glass. Each MPHP with reentrant cavities has reentrant cavities of one size, which are either 10, 20, 30, or 40 $\mu \text{m}$, respectively. Reentrant cavities in the MPHPs are shown to promote nucleation and early startup. Furthermore, the thermal resistance of the MPHP with reentrant cavities is decreased by up to 57%. As the size of the reentrant cavities increases, lower input power is required for startup, and the MPHP with the largest cavities (40 $\mu \text{m}$) shows the earliest startup. On the contrary, as the size of the reentrant cavities decreases, a reduction of the thermal resistance of the MPHPs is maintained to higher input power, and the MPHP with the smallest cavities (10 $\mu \text{m}$) shows the lowest thermal resistance at high input power (>12 W). Finally, a single MPHP incorporating various sizes of reentrant cavities (10, 20, 30, and 40 $\mu \text{m}$) is shown to exhibit extended operating range and enhanced thermal performance simultaneously.