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

Abstract

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.

Transient force analysis and bubble dynamics during flow boiling in silicon nanowire microchannels

“…Hence, due to the difficulty in measuring dynamic contact angle in flow boiling microchannels, static contact angles were measured and used in force analysis. Static contact angle over flat silicon is 45.13 ± 0.67°and SiNW is 10.21 ± 2.74°as described in our earlier study [18]. Experiments were conducted in a forced convection loop with deionized (DI) water over a range of mass fluxes, 100-600 kg/m 2 s and heat fluxes, up to 400 W/cm 2 .…”

“…Inertia, surface tension, shear, buoyancy, and evaporation momentum are the five major forces. Understanding the effects of these forces on nucleating bubble in microchannels, the normalized forces with respect to channel cross-sectional area were used in our previous study [18]. In this study, forces acting on L-V interface of elongating bubble have been studied.…”

“…Definition and influences of forces in flow boiling microchannels are briefly discussed in our previous study [18] and also by Kandlikar [11]. Properties of elongating bubble are captured from flow visualization image processing as described earlier.…”

Experimental studies of flow boiling heat transfer by using nanofluids

“…In addition, the CHF of the coating surface was increased with Fe doping increasing for both hybrid coatings with and without surfactant. A maximum enhancement in CHF was 44.6% and 29.7% in Alam et al [62] performed a force analysis study of bubbles dynamic and nucleation on flow boiling heat transfer of silicon nanowire and plain-wall microchannel. They focused on investigating the relative effects of different forces on flow boiling regimes, instabilities and CHF of flow boiling in silicon nanowire microchannel.…”

Physics of microstructures enhancement of thin film evaporation heat transfer in microchannels flow boiling

“…As a result, the unsaturated nanowires in the downstream instantly act as wicking structures to activate capillary flows along the channels walls. In a more recent study, through an analysis of different forces (inertia, surface tension, shear, buoyancy and evaporation momentum forces), Alam et al 25. argued that the enhanced surface rewetting and CHF in microchannels with nanowires are owing to a higher surface tension force at the liquid–vapor interface.…”