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Oscillating heat pipes (OHPs) represent a promising advancement over traditional heat pipes, yet their operational boundaries, especially for long OHPs, remain insufficiently understood. This study investigates the impact of varying adiabatic length, channel diameter, and fill ratio on thermal performance, crucial for assessing their suitability for engineering applications like spacecraft thermal management. Three long OHPs, ranging from 451 mm to 770 mm in total length, were subjected to multiple performance tests, employing channel diameters of 1.1 mm and 1.9 mm, along with adiabatic lengths of 305 mm and 610 mm. The experimental setup involved mounting the OHPs onto a testbed, monitored by nine K-type thermocouples. The tests, conducted horizontally to eliminate gravity-assistance, revealed that thermal performance is significantly influenced by channel diameter, adiabatic length, and fill ratio. Notably, optimal performance was observed at a 50% fill ratio, while reductions in diameter hindered start-up at a 70% fill ratio and failed to start-up at 30% fill ratio. These findings highlight the limitations of long OHPs, which is crucial to determine the limits of their applicability and dimensional constraints.
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Electrowetting-based microfluidic operations on rapid- manufactured devices for heat pipe applications
The heat transport capacity of traditional heat pipes is limited by the capillary pressure generated in the internal wick that pumps condensate to the evaporator. Recently, the authors conceptualized a novel heat pipe architecture, wherein wick-based pumping is replaced by electrowetting (EW)-based pumping of microliter droplets in the adiabatic section. An electrowetting heat pipe (EHP) can overcome the capillary limit to heat transport capacity and enable compact, planar, gravity-insensitive, and ultralow power consumption heat pipes that transport kiloWatt heat loads over extended distances. This work develops a novel technique for rapid, scalable fabrication of EW-based devices and studies critical microfluidic operations underlying the EHP, with the objective of predicting the key performance parameters of the EHP. Devices are fabricated on a printed circuit board (PCB) substrate with mechanically-milled electrodes, and a removable polyimide dielectric film. The first set of experiments uncovers the maximum channel gap (1 mm) for reliable EW-based pumping; this parameter determines the heat transport capacity of the EHP, which scales linearly with the channel gap. The second set of experiments uncovers the maximum channel gap (375 microns) at which EW voltages can successfully split droplets. This is an important consideration which ensures EHP operability in the event of unintentional droplet merging. The third set of experiments demonstrate and study EW-induced droplet generation from an open-to-air reservoir, which mimics the interface between the condenser and adiabatic sections of the EHP. The experimental findings predict that planar, water-based EHPs with a (10 cm by 4 mm) cross section can transport 1.6 kW over extended distances (>1 m), with a thermal resistance of 0.01 K W−1.
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- Award ID(s):
- 1653412
- PAR ID:
- 10062016
- Date Published:
- Journal Name:
- Journal of micromechanics and microengineering
- Volume:
- 27
- ISSN:
- 0960-1317
- Page Range / eLocation ID:
- 075004
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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