Microflow-Enhanced Bubble Dynamics Along with Gradient Porous Surfaces

Boiling heat transfer has been a popular topic for decades because of its ability to remove a significant amount of thermal energy while maintaining a low wall superheat during the liquid phase change. Such boiling mechanisms can be tailored by engineering new boiling substrates through surface wettability modification and/or microscale feature installation. Here, we create new types of heterogeneous boiling surfaces that integrate vertical gradient micropores on macroscale fins by using a template-free electrodeposition method. The gradient morphology and corresponding gradient wettability simultaneously enable bubble nucleation on the top pores and capillary wicking through the bottom pores. With these unique wetting characteristics, we find that the gradient pores installed at the trench bottom demonstrate the most significant boiling enhancement in critical heat flux and heat transfer coefficients by 160% and 600%, respectively. This enhancement can be attributed to the microflow-enhanced nature of bubble departures around the fins while isolating bubble nucleation and liquid supply through gradient pores. These results provide fundamental insights into boiling mechanisms using porous media and the potential for future works that can optimize the design of multi-dimensional heterogeneous surfaces to engineer flow patterns and boiling mechanisms accordingly.

Lasered Roughness to Increase Wicking Rates in Pin-Fin Microstructure

In this study, we demonstrate an inexpensive and fast method of creating hybrid microstructures for enhancement of capillary wicking by using UV laser ablation. We have also experimentally observed that capillary wicking rates are greatly increased by laser-machining induced surface roughness in pin-fin microstructures, and in one case the amount of enhancement was as large as 116%. The capillary wicking enhancement is a strong function of the geometry of the largescale pin-fin microstructures as well as the surface roughness of the flat areas. This study will help lead to the better understanding of capillary wicking in hybrid structures that in turn can assist with more informed designing and optimization processes of evaporator wicks in vapor chambers.

Design and Fabrication of Graded Copper Inverse Opals (g-CIOs) for Capillary-Fed Boiling in High Heat Flux Cooling Applications

Capillary-fed boiling in microporous copper inverse opals (CIOs) is capable of removing an excess of 1 kW/cm2 at 10–15 °C superheat over small wicking distances ∼ 200 μm. In order to remove heat from large area chips (> 1 cm2), longer capillary wicking distance is desired to reduce the manufacturing complexity of the 3D manifold for liquid delivery and vapor extraction. In this study, we propose graded copper inverse opals (g-CIOs) where smaller pores at the bottom provide high capillary pressure for liquid delivery, while larger pores at the top reduce viscous pressure drop for vapor extraction. This nonhomogeneous wicking material decouples the permeability and capillary pressure in the vertical and lateral directions, resulting in greater CHFs and capillary wicking distances. In this study, we demonstrate the feasibility of fabricating g-CIOs material with up to three different pore diameters (2 μm, 5 μm, and 10 μm) using a multi-step template sintering and copper electrodeposition process. We then leverage and expand upon a well-calibrated experimental model for the prediction of CHF in monoporous CIOs to map the performance metrics for g-CIOs. The model combines a hydraulic resistance network with Darcy’s law and accounts for the nonhomogeneous permeabilities in lateral and vertical directions. Using this model, we study the impact of total wick thickness and graded pore-size combinations on the critical heat fluxes and wicking distances. Our modeling results conclude that a two-layer g-CIOs can potentially reach ∼70% enhancement in the critical heat flux or ∼30% enhancement in the wicking length compared to monoporous CIOs of the same thickness. Our fabrication capability and preliminary modeling results offer the opportunity to design boiling tests with optimized g-CIOs and exploring the potential of dissipating high heat flux for large area cooling applications.

Effects of capillary wicking irrigation on soil moisture, plant growth and surface temperature of green roof with rain storage

In this study, the authors designed and applied a new irrigation method called capillary wicking irrigation (CWI), which used microfiber fabric as the source material of irrigation. At present, the effects of CWI on soil moisture, plant growth and surface temperature of a green roof with rain storage are not clear. An experiment was conducted on a green roof in Guangzhou. The authors set three transparent Plexiglas containers (A, B and C) with a side length of 1.5 m as an experimental frame on the roof. The authors put ‘steering wheel’ microfiber CWI in containers A and C, which were planted with Sedum lineare Thunb and Fittonia verschaffeltii, respectively. Container B with no CWI was planted with Sedum lineare Thunb. Results indicated that CWI could increase soil water content and make the variation of soil water content gentle in the containers on the roof. The green roof with rain storage had the function of heat preservation in winter and cooling effect in summer, especially for the green roof with CWI. Compared with container B, container A gave better plant growth, for ‘steering wheel’ microfiber CWI can basically provide automatic and suitable water supply for the plant. Therefore, CWI is an effective infiltration irrigation technique for roof greening.