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• Bubble dynamics inside droplet in the nucleate boiling regime on two-tier hierarchical micro-pillar array surfaces are systematically studied. • Secondary pillars are most effective at improving heat transfer performance when configured on side of primary pillars and least effective when configured on top of primary pillars. • Configuration of secondary pillars on side of primary pillars enhances heat transfer significantly when triple-phase contact line on pillars dominates evaporation. • Droplet evaporation rate decreases anomalously because evaporation is hindered near primary pillars by wrapping of droplet when secondary pillars are configured on substrate. • Geometrical effects of primary pillar including pillar side length, spacing and height and mechanisms are analyzed. Although hierarchical structured surfaces have shown great potential in improving heat transfer performance of boiling, the mechanisms associated with structure configuration and structure size remain elusive. In this work, the nucleate boiling regime of droplet on single-tier micro-pillar array (SM) surfaces and three types of two-tier hierarchical micro-pillar array (THM) surfaces is investigated comprehensively using the lattice Boltzmann model. Effects brought by the primary pillar size and the secondary pillar configuration on the development of vapor confined in pillar gaps are emphatically discussed from bubble nucleation through vapor coalescence to vapor expansion. Boiling dynamic characteristics concerning droplet morphological evolution, liquid-vapor interfacial deformation and triple-phase contact line (TPCL) motion are elucidated by the distributions of vapor pressure, fluid temperature and substrate heat flux. It is most efficient for the secondary pillar to enhance boiling heat transfer performance when configured on the side of primary pillar and least efficient when configured on the top of primary pillar. [Display omitted] [ABSTRACT FROM AUTHOR]

• Breakup regime of flashing jet under thermodynamic and mechanical effects is revealed. • Relationship between void fraction (α) and jet angle (k) is revealed. • The inadequacy of the existing criterion (Ja-We, χ*) are identified. • Relationship between flashing regimes and void fraction (α) is revealed. Accident release of superheated liquid could produce a violent phase change and trigger a highly destructive flashing jet. In this work, release experiments are performed using a 20 L tank and transparent nozzles, with water as the fluid throughout. The experimental conditions included storage temperature (T st = 100-160 °C), storage pressure (P st = 6-16 bar), and nozzle sizes (D = 1-6 mm). High-speed image processing and capacitive void fraction sensor are used to quantify the two-phase flow of superheated liquid inside the nozzle and the downstream jet break-up regimes. The experimental results show that void fraction (α) goes through three phases during the release process: growth, stabilization and decay. It is also demonstrated that bubble nucleation, growth and burst determine the breakup of the downstream jet. The T st and D are the main factors affecting the α , while the P st only change the release rate. Moreover, it is verified that none of the existing flashing criteria (Ja-We, χ*) can accurately determine the boundary conditions of the flashing regimes. Instead, the parameters characterizing bubbles nucleation and growth, α, can unify the effects of (T st , P st , and D) to determine the physical mechanism of the flashing process. In this work, a novel method to distinguish between external flashing (α = 0), internal flashing (0 < α ≤ 25) and full flashing (α > 25), and a steady-state model k = f (Ja, We, α) is developed for the flashing jet. [Display omitted] [ABSTRACT FROM AUTHOR]

• Boiling of water droplet in glass-oil-water multiphase system was investigated. • Two transition temperatures are determined experimentally and theoretically. • Boiling bifurcation is found due to nucleation sites at solid-water or oil-water interface. • The mode of boiling in oil and water is more violent than that for boiling in oil side only. Boiling a volatile droplet in non-volatile liquid is complex because of various interface interactions in a solid–liquid–liquid system. Previous studies have reported explosive boiling and attributed this phenomenon to heterogeneous nucleation due to solid–liquid contact. In this study, we investigated the boiling of an approximately 2 mm-diameter water droplet in a silicon oil pool inside a glass container. Two transition temperatures were observed: 122.5 °C (for the transition from evaporation to boiling) and 160.0 °C (for the transition from the bubble growth on the oil side to that on both the oil and water sides). Moreover, four boiling modes were identified: evaporation (E), boiling in water (BIW: bubble nucleation at the glass–water interface and growth inside a drop), boiling in oil (BIO: bubble nucleation at the oil–water interface and growth in oil), and boiling in oil and water (BIOW: bubble nucleation at the oil–water interface and growth in oil and water). For the first time, boiling bifurcation was observed for drop boiling in a three-phase system. For the oil temperature (T b) in the range 122.5–160.0 °C, the boiling mode switches between BIW and BIO. In contrast, for T b exceeding 160.0 °C, either BIW or BIOW occurs. This bifurcation occurs because droplets prefer minimum activation energy between that for the bubble nucleation at the solid–water interface and that for the bubble nucleation at the oil–water interface. Interfacial instabilities were analyzed to explain why BIOW was more violent than BIW. This work presents a comprehensive understanding of droplet boiling in a three-phase system. [ABSTRACT FROM AUTHOR]

Highlights: [•] We performed fully flashing experiments in both standard and retrogrades fluids. [•] Bubble nucleation is the rate-controlling process for the onset of fully flashing. [•] The initial spray opening is directly linked to the population of bubble clusters. [•] Far from the critical point, retrograde phase transitions play no role. [•] At high superheat, we observe shock-systems in both standard and retrograde fluids. [Copyright &y& Elsevier]

Abstract: Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (Ra ) are 0.03 and 5.89μm for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, active nucleation site density, and bubble terminal velocity are measured from the monochrome movies, which have been recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported. [Copyright &y& Elsevier]