Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers

Abstract

Porous hydrophilic surfaces have been shown to enhance the critical heat flux (CHF) in boiling heat transfer. In this work, the separate effects of pore size and porous layer thickness on the CHF of saturated water at atmospheric pressure were experimentally investigated using carefully engineered surfaces. It was shown that, for a fixed pore diameter (∼20 nm), there is an optimum layer thickness (∼2 μm), for which the CHF value is maximum, corresponding to ∼115% enhancement over the value for uncoated surfaces. Similarly, a maximum CHF value (∼100% above the uncoated surface CHF) was observed while changing the pore size at a constant layer thickness (∼1 μm). To explain these CHF maxima, we propose a mechanistic model that can capture the effect of pore size and pore thickness on CHF. The good agreement found between the model and experimental data supports the hypothesis that CHF is governed by the competition between capillary wicking, viscous pressure drop and evaporation, as well as conduction heat transfer within the porous layer. The model can be used to guide the development of engineered surfaces with superior boiling performance.

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

“…The Buongiorno group at MIT investigated the separate effects of surface characteristics such as hydrophilicity, porosity and roughness on CHF, and found that large CHF enhancement can be obtained with textured surfaces that produce strong wicking of liquid to the surface [14]. They also observed that CHF maxima can be obtained depending on the geometric characteristics of the surface structures, such as thickness of the porous layer and size of the pores which determined the competition between conduction heat transfer within the porous layer, as well as capillary wicking, viscous pressure drop and evaporation [15], or spacing and size of the micro-pillars which influenced characteristic dry spot heating and rewetting timescales during boiling crisis [16]. Mechanistic models were also developed to explain those CHF maxima capturing the geometrical parameters while provided guidelines for further optima [15,16].…”

“…They also observed that CHF maxima can be obtained depending on the geometric characteristics of the surface structures, such as thickness of the porous layer and size of the pores which determined the competition between conduction heat transfer within the porous layer, as well as capillary wicking, viscous pressure drop and evaporation [15], or spacing and size of the micro-pillars which influenced characteristic dry spot heating and rewetting timescales during boiling crisis [16]. Mechanistic models were also developed to explain those CHF maxima capturing the geometrical parameters while provided guidelines for further optima [15,16]. While in macroscale channels inertia and buoyancy forces have determined effect on the flow structures and hydraulic and thermal transport process, the surface tension and surface characteristic parameters such as surface porosity, roughness, and wettability are just of the same importance in the microchannel.…”

“…In the meantime micro/nano-scale surface modification techniques which structure the heat transfer surface with higher surface area to volume ratio and nucleation sites, can effectively change all the surface characteristic parameters including surface roughness, wettability and porosity simultaneously. Design and optimization of the microchannel heat sinks and surface modification methods have been emerging in recent years, both of which intend to pursue low boiling incipience superheat, high heat transfer coefficient under practically applied heat fluxes, and high critical heat flux [14][15][16]17]. Thus the combination of surface micro/nano-structures and microchannel heat sinks may promise a much better heat transfer performance at cost of minimal pressure drop increment compared to untreated surfaces without apparent changes in the surface topography.…”

Effect of grooves on nucleate boiling heat transfer from downward facing hemispherical surface

“…Various methods have been proposed for enhancing ERVC performance, including using structured surface, nanofluids (Pham et al, 2012;Angayarkanni and Philip, 2015) and thermal insulation (Yang et al, 2005;Noh and Suh, 2013). The structured surfaces include porous coating (Yang et al, 2006;Jun et al, 2016;Tetreault-Friend et al, 2016;Sohag et al, 2017;Wang et al, 2018), pin fins (Chu et al, 2013;Zhong et al, 2015), micro channels (Bai et al, 2016;Hou et al, 2017;Zhong et al, 2018aZhong et al, , 2018b, honeycomb porous plates (Mt Aznam et al, 2016;Fogaça et al, 2018), and spherical porous bodies (Mori et al, 2018), and so on. Yang et al (2006) fabricated a downward facing hemispherical surface with micro-porous coatings by sintering, while Sohag et al (2017) developed a Cold Spray technique to coat the same scale surface, both the coating surfaces were tested in the SBLB (Sub-scaled Boundary Layer Boiling) facility to investigate the CHF.…”

Study of Super Hydrophilic Surface Modification and Visualization of Evaporation

“…The Peyki et al [10]2014 has a more hydrophilic surface on the reverse osmosis membrane, and the SiO2 Nano-layer method is used to find that hydrophilicity can enhance the decontamination ability and improve the efficiency of seawater desalination, but no quantitative results are presented. 2015 Wu et al [11] using the method of literature [8], the best contact angle is 7⁰, and it is pointed out that the modified surface can enhance the evaporation efficiency, but it does not show the reason of the layer number and contact angle change.2016 Tetreault-Friene [12] using the Nano-layer method to improve the quality of sapphire, the best nano-layer thickness, boiling test, lifting critical heat flux 115%. From the literature review, it is found that the surface modification method of the nano-laminated layer has been reported in many literatures.…”