Droplet Evaporation on Porous Nanochannels for High Heat Flux Dissipation

Abstract

Droplet wicking and evaporation in porous nanochannels is experimentally studied on a heated surface at temperatures ranging from 35 o C to 90 o C. The fabricated geometry consists of cross-connected nanochannels of height 728 nm with micropores of diameter 2 µm present at every channel intersection; the pores allow water from a droplet placed on the top surface to wick into the channels. Droplet volume is also varied and a total of 16 experimental cases are conducted. Wicking characteristics such as wicked distance, capillary pressure, viscous resistance and propagation coefficient are obtained at the high surface temperatures. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments, but is also independent confirmed via a new set of experiments where water is continuously fed to the sample through a microtube such that it matches the evaporation rate. High heat flux as high as ~294 W/cm 2 is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannels geometry in spray cooling application, and is found to be capable of dissipating high heat fluxes upto ~77 W/cm 2 at temperatures below nucleation, thus highlighting the thermal management potential of fabricated geometry.

Disjoining Pressure of Water in Nanochannels

“…The most common mechanism of wicking in nanochannels is attributed to capillary pressure. − If wicking is only driven by capillary pressure i.e., Σ P = P c ≈ 2σ cos θ/ h where σ is the surface tension, and θ is the contact angle, then eq can be simplified to the widely used Washburn equation for capillary filling: Although the linear dependence of wicking distance on t 1/2 holds at nanoscale, Washburn equation is inconsistent in predicting the experimental wicking rate. Other published literature on wicking in rectangular cross section nanochannels include a similar observation ,− and explained this inconsistency, primarily due to electro-viscous effect − or geometrical effect. , However, these effects do not explain our observed deviation (please refer to the Supporting Information for a detailed explanation).…”

Thermal Management of Photovoltaics Using Porous Nanochannels

“…Similarly, where ε l is the emissivity of the PV panel surface (ε l ∼ 0.7) , and q tm is the heat flux removal achieved through thermal management, which is obtained by employing the porous nanochannels device (henceforth denoted as nanochannels). The nanochannels utilized in the present study offer a passive dissipation of a high heat flux through thin-film evaporation of the spray droplets dispersed over it, thus eliminating the need for a continuous supply of a coolant.…”

“…In line with that, a device with micro/nanoscale structures that can (1) continuously supply the required liquid coolant through wicking − and (2) simultaneously allow a high heat removal rate at the evaporating menisci − is anticipated as the best candidate to offer a promising solution to the overheating issue of the PV panel. Accordingly, we utilize the porous nanochannels device, which has demonstrated excellent wicking characteristic , as well as high heat flux dissipation through nanoscale thin-film evaporation, and numerically integrate it on the back face of a commercial PV panel to evaluate the extent of cooling. The cross-connected geometry of the buried nanochannels (height = 728 nm) offer a potential solution for a high rate of wicking, while the micropores (diameter = 2.1 μm) provided at each intersection host the sites for evaporating menisci. ,, Hence, the finding of the heat flux removal attained in such a porous nanochannels device is utilized as a technique of thermal management, and the numerical investigation of PV cooling employing an energy balance model is reported here.…”

“…Accordingly, we utilize the porous nanochannels device, which has demonstrated excellent wicking characteristic , as well as high heat flux dissipation through nanoscale thin-film evaporation, and numerically integrate it on the back face of a commercial PV panel to evaluate the extent of cooling. The cross-connected geometry of the buried nanochannels (height = 728 nm) offer a potential solution for a high rate of wicking, while the micropores (diameter = 2.1 μm) provided at each intersection host the sites for evaporating menisci. ,, Hence, the finding of the heat flux removal attained in such a porous nanochannels device is utilized as a technique of thermal management, and the numerical investigation of PV cooling employing an energy balance model is reported here. Finally, in the current state of the literature where the majority of the research in PV panel cooling is accomplished using a single-phase coolant, − by integration of a novel porous nanochannel on the back face of a commercial PV panel, we induce multiphase cooling accompanied with a large extent of heat transfer through phase change.…”

“…The cross-connected geometry of the buried nanochannels (height = 728 nm) offer a potential solution for a high rate of wicking, while the micropores (diameter = 2.1 μm) provided at each intersection host the sites for evaporating menisci. ,, Hence, the finding of the heat flux removal attained in such a porous nanochannels device is utilized as a technique of thermal management, and the numerical investigation of PV cooling employing an energy balance model is reported here. Finally, in the current state of the literature where the majority of the research in PV panel cooling is accomplished using a single-phase coolant, − by integration of a novel porous nanochannel on the back face of a commercial PV panel, we induce multiphase cooling accompanied with a large extent of heat transfer through phase change. We project an enhanced performance of the PV panel by such integration as compared to the typically reported values in the literature: the reduction in the temperature of the PV panel is in the range of 3–20 °C ,,, and the enhancement in the PV electrical power output is by 2–12%. ,− …”