Prediction and Characterization of Dry-out Heat Flux in Micropillar Wick Structures

Abstract

Thin-film evaporation in wick structures for cooling high-performance electronic devices is attractive because it harnesses the latent heat of vaporization and does not require external pumping. However, optimizing the wick structures to increase the dry-out heat flux is challenging due to the complexities in modeling the liquid-vapor interface and the flow through the wick structures. In this work, we developed a model for thin-film evaporation from micropillar array wick structures and validated the model with experiments. The model numerically simulates liquid velocity, pressure, and meniscus curvature along the wicking direction by conservation of mass, momentum, and energy based on a finite volume approach. Specifically, the three-dimensional meniscus shape, which varies along the wicking direction with the local liquid pressure, is accurately captured by a force balance using the Young-Laplace equation. The dry-out condition is determined when the minimum contact angle on the pillar surface reaches the receding contact angle as the applied heat flux increases. With this model, we predict the dry-out heat flux on various micropillar structure geometries (diameter, pitch, and height) in the length scale range of 1-100 μm and discuss the optimal geometries to maximize the dry-out heat flux. We also performed detailed experiments to validate the model predictions, which show good agreement. This work provides insights into the role of surface structures in thin-film evaporation and offers important design guidelines for enhanced thermal management of high-performance electronic devices.

Surface Structure Enhanced Microchannel Flow Boiling

“…Therefore the liquid pressure is the lowest at the dry-out point based on the Young-Laplace equation (P cap = P vapor -P liquid = 2σ/r); (2) at the dry-out location, the contact angle of water on silicon dioxide (pillar surface) is the receding contact angle θ r (θ r ≈ 15º) [45]; (3) the pressure gradient is approximated as dP/dx = (P max -P min )/L w , where P max is the maximum pressure along the wicking path which is at the sidewalls (P max ≈ P vapor ) since the curvature is approximately zero, P min is the pressure at the dry-out location (P min = P vapor -2σ/r), and L w is the wicking distance. From above, dP/dx = P cap /L w; (4) P cap is derived using a force balance on the liquid-vapor interface, [46,47]; (5) …”

“…A more detailed numerical model which takes these two factors into consideration can be found in [46]. In addition, this model cannot predict the liquid velocity along the channel direction due to the existence of vapor shear stress (i.e., an axially varying vapor pressure).…”

Tuning Superhydrophilic Nanostructured Surfaces to Maximize Water Droplet Evaporation Heat Transfer Performance

“…Previous studies of droplet spread on surfaces mentioned above have not considered this specific type of surface (e.g., see Refs. [7] and [20][21][22]).…”

“…In this regard, this investigation differs from earlier studies that have considered liquid spreading on deep porous layers or larger microstructured surface morphologies (e.g., see Refs. [7] and [20][21][22]). In addition, we specifically focused on nonordered, nanostructured surfaces that can be thermally grown on metal substrates because this type of process is scalable and adaptable to complex substrates, making it an ideal approach to putting nanostructured, superhydrophilic coatings on heat exchanger surfaces.…”

Characterization of thin film evaporation in micropillar wicks using micro-Raman spectroscopy

“…It plays an increasingly important role in thermal management 1 , water purification 2 , humidification 3 and vapor generation 2,4 . It has been widely that reported the heat transfer of evaporation can be significantly enhanced by micro/nanostructures [5][6][7][8][9] . For this reason, thin film evaporation on microstructured surfaces has attracted particular interest for high heat flux thermal management, especially for cooling high-performance electronic devices [6][7][8][9] (e.g., micro-processors 1 and highpower radio-frequency amplifiers 10,11 with highly concentrated heat generation > 100 W/cm 2 ).…”