Melting process of frozen sessile droplets on superhydrophobic surfaces

Superhydrophobic surfaces can exhibit icephobicity in many ways due to their large contact angles and small rolling angles. The melting process of frozen droplets on superhydrophobic surfaces is still unclear, hindering the understanding of surface icephobicity. In this experimental study of the melting process of frozen sessile droplets on superhydrophobic surfaces, we find two types of melting morphologies with opposite vortex directions on a single-scale nano-structured (SN) superhydrophobic substrate and a hierarchical-scale micro-nano-structured (HMN) superhydrophobic substrate. Melting pattern visualizations and flow field measurements showwed Marangoni convection and natural convection occuring in the melting sessile droplets. For the HMN superhydrophobic substrate, the internal flow was found to be dominated by Marangoni convection due to the temperature gradient along the surface of the droplet. For the SN superhydrophobic substrate, Marangoni convection was inhibited by the superhydrophobic particles at the surface of the droplet, which were shed from the fragile superhydrophobic substrate during the freezing--melting process, as confirmed by surface characterizations of the substrate and flow measurements of a water pool. These results will help researchers better understand the melting process of frozen droplets and in designing novel icephobic surfaces for numerous applications.Comment: 31 pages, 12 figure

Breakup of particle-laden droplets in airflow

The atomisation of suspension containing liquid and dispersed particles is prevalent in many applications. Previous studies of droplet breakup mainly focused on homogeneous fluids, and the heterogeneous effect of particles on the breakup progress is unclear. In this study, the breakup of particle-laden droplets in airflow is investigated experimentally. Combining synchronised high-speed images from the side view and the 45$^\circ$ view, we compare the morphology of particle-laden droplets with that of homogeneous fluids in different breakup modes. The results show that the higher effective viscosity of particle-laden droplets affects the initial deformation, and the heterogeneous effect of particles appears in the later breakup stage. To evaluate the heterogeneous effect of particles quantitatively, we eliminate the effect of the higher effective viscosity of particle-laden droplets by comparing cases corresponding to the same inviscid Weber number. The quantitative comparison reveals that the heterogeneous effect of particles accelerates the fragmentation of liquid film and promotes localised rapid piercing. A correlation length that depends on the particle diameter and the volume fraction is proposed to characterise the length scale of the concentration fluctuation under the combined effect of the initial flattening and later stretching during the droplet breakup process. Based on this correlation length, the fragment size distributions are analysed, and the scaling results agree well with the experimental data.Comment: 29 pages, 19 figure

Benefits and Cost-effectiveness Analysis of Exhaust Energy Recovery System Using Low and High Boiling Temperature Working Fluids in Rankine Cycle

AbstractIn this paper, six attactive working fluids, including low boiling refrigerants such as R123, R141b and R245fa (Group L) and high boiling substances such as cyclohexane, ethanal and water (Group H), are applied on Rankine cycle, in order to examine the potential of these two categories of working fluids in high temperature exhaust energy recovery system (EERs) from a gasoline engine. The influences of engine speed at full load and evaporating pressure on the EERs performances are analyzed. The results reveal that water in Group H and R141b in Group L contribute the peak improvement in system benefits, while fluids in Group H show better cost-effectiveness. The EERs performances would be influenced strongly by evaporating pressure at high engine speed, while it also requires high pressure to enhance the performances at low speed. Besides, when the evaporating pressure is low, selection of working fluid should be emphasized