Theoretical model for alternate-channel induced capillary pressure difference in flat-plate oscillating heat pipes

Startup failure or operation stagnation at the horizontal orientation or against gravity conditions is a tough challenge for safe and reliable applications of oscillating heat pipes (OHPs). The alternate channel design provides a simple and feasible way to address this problem, however its physical mechanism is still not fully understood. In this study, we developed a theoretical model capable of quantitatively predicting the capillary pressure difference produced by alternate channels in flat-plate OHPs, providing the extra driving power for OHP operation at unfavorable orientations. To determine contact angles of different working fluid mediums and then capillary pressure differences, the surface wetting properties of different fluid-medium/substrate-material combinations were measured. The capillary pressure difference is normally of the order of magnitude of several to tens of Pascal, and it is much smaller than the gravitational potential in terms of the order-of-magnitude analysis. However, it could suppress the Marangoni effect and support circulation motions of slugs/plugs at unfavorable orientations, indicating the high instability of OHP system. This study provides an insight into the physical mechanism of OHP operation using alternate channels, and it will broaden their application fields at both terrestrial and microgravity conditions.