This paper shows the performance enhancement of heat pipes by tailoring the density distribution of micropillar wicks to balance capillary pumping with viscous pressure loss. In a heat pipe, the capillarity and permeability are two linked parameters, since, for example, small pores create higher capillary pumping while unfortunately inducing more pressure drops along the heat pipe. This pressure loss accumulates along the heat pipe, leading to a pressure distribution where the lowest liquid pressure is in the evaporator region. Similarly, the vapor pressure drops as it travels from the evaporator to the condenser, leads to a maximum pressure in the evaporation region. The results are a non-uniform pressure difference along the heat pipe, thereby we do not need constant capillary pressure to withstand this difference.

This provides the opportunity to spatially tailor the wick structure, aiming for a high capillarity to pump the liquid, but a low permeability to induce less pressure loss. Our study offers a compromise between capillarity and permeability by designing a density distribution for the pillars wick structure. We propose an evaporator with high-density pillars to increase the capillarity and a condenser with low-density pillars to minimize the pressure loss, leading to a gradient of density along the heat pipe. This density distribution, which was not studied before, will be shown to enhance the heat pipe performance.

The methodology consists of building theoretical models that specify a range of pillar dimensions that maximize power without reaching the capillary limit, followed by experimental characterization to validate the modeling observation. We reach this limit when the capillary pressure generated by the porous media does not withstand the pressure drops due to liquid and vapor flow, as well as gravity. The model is used to design and compare two cases: uniform pillar density and distributed pillar density. Optimal pillar density was first calculated for the uniform case, leading to 50µm spacing for a 25 cm long and 1 mm thick heat pipe with 0.01 W/cm^2^ over the 20 cm long evaporator. We compared this to density-distributed pillars with a gradient spacing of 250µm to 50µm. The results show that wicks with a density distribution offer 50% more capillary pumping than the optimal uniform case. As a reference, uniform density pillars with 250µm spacing were not able to meet the power density.

To demonstrate the benefits of tailoring wick structures, we also measured the rate-of-rise of water in uniform and distributed density wicks. We fabricated the wicks from polymer-based materials since it offers the possibility of using molding (for batch fabrication) or 3D printing processes (for rapid prototyping). We made our pillar wicks by stereolithography, which is a 3D printing process that reaches small dimensions, down to 100 µm. The surfaces were plasma treated to make them hydrophilic. We compared density-distributed pillars with a spacing of 250µm to 50µm to the constant density pillars of 250µm spacing, with a capillary height of 2.3cm and 0.8cm respectively. Results, therefore, show that pillars with density distribution offer higher pumping capacity than pillars with constant density.

The theoretical models and the rate of rise measurements demonstrate that a density distribution for pillar wick structures promises a higher thermal and fluidic performance, especially for the long heat pipes considered here.


Impact of Micropillar Density Distribution on the Capillary Limits of Heat Pipes

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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