No abstract

No abstract

No abstract

Manipulating surface topography is one of the most promising strategies for increasing the efficiency of numerous industrial processes involving droplet contact with superheated surfaces. In such scenarios, the droplets may immediately boil upon contact, splash and boil, or could levitate on their own vapor in the Leidenfrost state. In this work, we report the outcomes of water droplets coming in gentle contact with designed nano/ microtextured surfaces at a wide range of temperatures as observed using high-speed optical and X-ray imaging. We report a paradoxical increase in the Leidenfrost temperature (T LFP ) as the texture spacing is reduced below a critical value (∼10 μm) that represents a minima in T LFP . Although droplets on such textured solids appear to boil upon contact, our studies suggest that their behavior is dominated by hydrodynamic instabilities implying that the increase in T LFP may not necessarily lead to enhanced heat transfer. On such surfaces, the droplets display a new regime characterized by splashing accompanied by a vapor jet penetrating through the droplets before they transition to the Leidenfrost state. We provide a comprehensive map of boiling behavior of droplets over a wide range of texture spacings that may have significant implications toward applications such as electronics cooling, spray cooling, nuclear reactor safety, and containment of fire calamities.

We numerically investigated a novel galinstan-based microfluidic heat-sink. Galinstan is an eutectic alloys of gallium, indium, and tin. The thermal conductivity of galinstan is ∼27 times that of water, while the dynamic viscosity is only twice of water. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the specific heat of galinstan is significantly lower than that of water, which will inevitably undermine the cooling capability by increasing fluid outlet temperature (i.e., increase of caloric thermal management) and/or flow rate. As an alternative, therefore, galinstan/water heterogeneous mixture was proposed as a working fluid and the cooling performance was numerically explored with varying volume composition of galinstan. Effective medium theory for heterogeneous medium was used to evaluate the thermal conductivity of the mixture. The viscosity change with respect to the volume composition was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Classical models were used for the mixture density and specific heat calculations. Heat transfer and pressure drop characteristics of laminar flow through a silicon microchannel heat-sink was simulated using Fluent. The length and width of the channel array are 10 mm and 9.5 mm, respectively. The cross-sectional area of each channel is 300 μm × 300 μm and the spacing between channels is 100 μm. The heat dissipation was 50 W and the pumping power was fixed at 5 mW for the comparison between the varying galinstan/water compositions. The results showed that more than 30% of the thermal resistance enhancement was attainable using the novel working fluid. Due to the compromise between the convective thermal resistance (effect of thermal conductivity) and the caloric thermal resistance (effect of viscosity and specific heat), the lowest junction temperature was marked at the galinstan composition of ∼35% by volume.