Boiling flow characteristics in microchannels with very hydrophobic surface to super-hydrophilic surface

An experimental investigation of saturated flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel was conducted with deionized water as the working fluid. The bare silicon wafer bottom surface of the microchannel was hydrophilic with a contact angle of 65° ± 3°, compared with the super-hydrophilic surface deposited by a thin film of 100-nm-thickness silicon dioxide through PECVD with a contact angle less than 5°. In experimental runs the mass fluxes were in the range of 120 kg/m 2 s-360 kg/m 2 s, the wall heat fluxes were spanned from 4 W/cm 2 to 20 W/cm 2 and the inlet vapor qualities were varied from 0.03 to 0.1. Parametric study and flow visualization on pressure drop, local heat transfer coefficient, and flow pattern for surfaces of different surface wettability characteristics were carried out. Measured total pressure drops in single phase and two phase flow experiments agreed well with predicted values. The experimental data points were almost all located in the annular flow regime, and the local heat transfer coefficients approached a constant value and then increased towards the exit along the flow direction. According to flow visualization, the local dryout phenomenon occurred on the untreated hydrophilic surface at high heat fluxes for low mass fluxes, accompanied with deteriorative heat transfer performance, while it was not observed on the super-hydrophilic surface at the identical condition. Meanwhile severe heat transfer deterioration was obtained on the hydrophilic surface with increased inlet vapor quality, while the heat transfer coefficient of the super-hydrophilic surface was relatively constant which outperformed the untreated silicon wafer surface without increased pressure drop penalty.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

Zhou

Coyle

et al. 2017

“…The silicon nanowire surface was thereby produced in smooth silicon surface by electroless electrochemical etching technique with silver nanoparticles (AgNPs) catalyst. These SiNWs were then naturally oxidized to make super hydrophilic surface (approximate contact angle 0°) using the Wenzel effect [32,37]. The Scanning Electron Microscope (SEM) images of SiNW surfaces and plainwall surfaces are shown in Fig.…”

Section: Experimental Studymentioning

confidence: 99%

“…Artificial nucleation cavities were formed on the boiling surfaces to enhance nucleate boiling by various methods, such as micromachining [18][19][20][21], nanostructured surfaces [22][23][24][25][26], porous metal coating [27][28][29], and chemical etching [30,31]. Recently, nanowires (NWs) [32,33] and carbon nanotubes (CNTs) [34][35][36] were used to enhance nucleate pool boiling and convective boiling in microchannels [23,25,26,34,37,38] coefficient and critical heat flux were reported owing to the higher nucleation site density and enhanced wettability. However, optimization of surface properties to control bubble size, forces acting on bubble and flow patterns/ regimes has not yet been well resolved.…”

Section: Introductionmentioning

confidence: 99%

Force analysis and bubble dynamics during flow boiling in silicon nanowire microchannels

Alam

Yang

et al. 2016

a b s t r a c tIn microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this flow boiling mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid-vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of nucleating bubble and bubble dynamics in flow boiling silicon nanowire microchannels have been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regimes, instabilities and heat transfer performances of flow boiling in silicon nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid-vapor interface as discussed earlier by other researchers. However, no comparative study has been done for different surface properties till date. Detail analyses of these forces including contact angle effect, channel dimension effect, heat flux effect and mass flux effect in flow boiling microchannels have been conducted in this study. A comparative study between silicon nanowire and plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. Compared to plainwall microchannels, enhanced surface rewetting and CHF are owing to higher surface tension force at liquid-vapor interface and Capillary dominance resulting from silicon nanowires. Whereas, low Weber number in silicon nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, results from these studies are compared with the literatures and great agreements have been observed.

“…These studies experimentally measured the onset of nucleate boiling [4], pressure drop [5,6], and heat transfer coefficient [5,7,8], and also developed models to predict the critical heat flux (CHF) [9,10]. In addition, flow regime maps have been developed under a variety of operating conditions [11,12]. While these studies have provided a thorough understanding of microchannel flow boiling under ideal heating conditions, realistic applications may impose highly nonuniform heat fluxes due to chip-and system-level variations [13].…”

Section: Introductionmentioning

confidence: 99%

Local measurement of flow boiling heat transfer in an array of non-uniformly heated microchannels

Ritchey

Weibel

Garimella

2014

As electronics packages become increasingly thinner and more compact due to size, weight, and performance demands, the use of large intermediate heat spreaders to mitigate heat generation nonuniformities are no longer a viable option. Instead, non-uniform heat flux profiles produced from chipscale variations or from multiple discrete devices are experienced directly by the ultimate heat sink. In order to address these thermal packaging trends, a better understanding of the impacts of non-uniform heating on two-phase flow characteristics and thermal performance limits for microchannel heat sinks is needed. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse peak-heating conditions spanning across the entire heat sink area. The investigation is conducted using a silicon microchannel heat sink with a 5 × 5 array of individually controllable heaters attached to a 12.7 mm × 12.7 mm square base.