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We reveal and justify, both theoretically and experimentally, the existence of a unifying criterion of the boiling crisis. This criterion emerges from an instability in the near-wall interactions of bubbles, which can be described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble footprint radius and product of average bubble growth time and detachment frequency. Our analysis suggests that the boiling crisis occurs on a well-defined critical surface in the multidimensional space of these parameters. Our experiments confirm the existence of this unifying criterion for a wide variety of boiling surface geometries and textures, two boiling regimes (pool and flow boiling) and two fluids (water and liquid nitrogen). This criterion constitutes a simple mechanistic rule to predict the boiling crisis, also providing a guiding principle for designing boiling surfaces that would maximize the nucleate boiling performance. Boiling crisis is a physical phenomenon limiting the operation of many technologies cooled by boiling. Zhang et al. reveal theoretically and experimentally the existence of a unifying criterion to explain and predict the boiling crisis.

The power conversion efficiencies (PCEs) of laboratory-sized organic solar cells (OSCs), usually processed from low-boiling-point and toxic solvents, have reached high values of over 18%. However, there is usually a notable drop of the PCEs when green solvents are used, limiting practical development of OSCs. Herein, we obtain certificated PCEs over 17% in OSCs processed from a green solvent paraxylene (PX) by a guest-assisted assembly strategy, where a third component (guest) is employed to manipulate the molecular interaction of the binary blend. In addition, the high-boiling-point green solvent PX also enables us to deposit a uniform large-area module (36 cm 2 ) with a high efficiency of over 14%. The strong molecular interaction between the host and guest molecules also enhances the operational stability of the devices. Our guest-assisted assembly strategy provides a unique approach to develop large-area and high-efficiency OSCs processed from green solvents, paving the way for industrial development of OSCs. Organic solar cells processed from green solvents are easier to implement in manufacturing yet their efficiency is low. Chen et al. devise a guest molecule to improve the molecular packing, enabling devices with over 17% efficiency.

Steam generation using solar energy provides the basis for many sustainable desalination, sanitization, and process heating technologies. Recently, interest has arisen for low-cost floating structures that absorb solar radiation and transfer energy to water via thermal conduction, driving evaporation. However, contact between water and the structure leads to fouling and pins the vapour temperature near the boiling point. Here we demonstrate solar-driven evaporation using a structure not in contact with water. The structure absorbs solar radiation and re-radiates infrared photons, which are directly absorbed by the water within a sub-100 μm penetration depth. Due to the physical separation from the water, fouling is entirely avoided. Due to the thermal separation, the structure is no longer pinned at the boiling point, and is used to superheat the generated steam. We generate steam with temperatures up to 133 °C, demonstrating superheated steam in a non-pressurized system under one sun illumination. Solar steam generation is limited by fouling of solar converters, and the steam temperature is usually pinned to 100 °C. Here, both limitations are overcome in a system utilizing a solar absorber and light down-converter to achieve radiative heating, which does not require physical contact between absorber and water.

Boiling is considered an important mode of heat transfer (HT) enhancement and has several industrial cooling applications. Boiling has the potential to minimize energy losses from HT devices, compared with other convection or conduction modes of HT enhancement. The purpose of this review article was to analyze, discuss, and compare existing research on boiling heat transfer enhancement techniques from the last few decades. We sought to understand the effect of nucleation sites on plain and curved surfaces and on HT enhancement, to suggest future guidelines for researchers to consider. This would help both research and industry communities to determine the best surface structure and surface manufacturing technique for a particular fluid. We discuss pool boiling HT enhancement, and present conclusions and recommendations for future research.

In this study, pool boiling experiments were conducted using carbon nanotubes (CNTs)-Cu nanoparticle-coated porous surfaces. Applying a layer of CNTs-Cu nanoparticles on boiling surfaces resulted in the formation of porous structures, which led to a rise in the quantity of active nucleation sites. We fabricated three coated boiling test sections, namely 0.1CNTs-99.9Cu, 0.2CNTs-99.8Cu, and 0.3CNTs-99.7Cu in weight percentages. A bare copper surface and a surface coated only with Cu particles were tested as benchmarks. Distilled water was used in the pool boiling experiments. To investigate the bubble dynamics, a high-speed camera was employed, while scanning electron microscopy was utilized to analyze the morphologies of the surfaces that were coated. The objective of this research was to elucidate the impact of varying CNTs content on boiling inversion and pool boiling efficiency. Findings indicated that an increased content of CNTs improved the boiling heat transfer coefficient, with boiling inversions observed on the porous coated surfaces.

The increasing demand for energy-efficient and environmentally sustainable cooling technologies has led to a renewed focus on ammonia and lithium nitrate-based absorption refrigeration systems, particularly those utilizing Plate Heat Exchangers (PHEs). Despite their importance, reliable predictive models for boiling heat transfer and frictional pressure drop in PHEs using ammonia and lithium nitrate mixtures, such as NH 3 –LiNO 3 and NH 3 –LiNO 3 –H 2 O, remain limited and often suffer from structural deficiencies. This study provides a comprehensive evaluation of existing correlations for boiling heat transfer and friction factor in PHEs, specifically focusing on ammonia-based mixtures (NH 3 –H 2 O, NH 3 –LiNO 3 , and NH 3 –LiNO 3 –H 2 O). More than 20 correlations for boiling heat transfer coefficient and friction factor were critically analysed and adjusted to account for the unique thermophysical behaviors of multi-component salt mixtures. The study reveals that many correlations fail to accurately predict boiling heat transfer in NH 3 –H 2 O mixtures due to inadequate sensitivity to heat flux. Scaling these correlations led to notable improvements in prediction accuracy, underlining the significance of appropriate scaling for different PHE configurations. Additionally, the study validates the assumption that lithium nitrate remains in the liquid phase in NH 3 –LiNO 3 and NH 3 –LiNO 3 –H 2 O mixtures, supporting its exclusion from latent heat calculations. Friction factor correlations that include positive exponents for Reynolds and Weber numbers were found to be structurally inconsistent, resulting in inaccurate predictions. The analysis further highlights that many correlations are overly empirical or based on narrow experimental conditions, limiting their applicability to diverse heat exchanger geometries. A key contribution of this work is the unique visual comparison of the correlations, providing a detailed depiction of their structural characteristics and offering more precise insights than those available in previous studies.

Layer-by-layer (LbL) strategy has been developed to form bulk heterojunction (BHJ) structure for processing efficient organic solar cells (OSCs). Herein, LbL slot-die coating with twin boiling point solvents (TBPS) strategy was developed to fabricate highly efficient OSCs, which matches with large-scale, high throughput roll-to-roll (R2R) industrialized mass process. The TBPS strategy could produce high-quality thin film without any additive, leading to the optimized vertical phase separation with interpenetrating nanostructures, as well as the enhanced charge transport and extraction. Thus, the power conversion efficiency up to 14.42% was achieved for [(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo [1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)]:2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4″,5″]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene)) bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (PM6:Y6) OSCs fabricated via sequentially LbL slot-die coating using the TBPS strategy under ambient condition. The research provides a potential route for industrialized production of high-efficiency and large-area OSC devices.

Spray cooling of a hot target is characterized by strong heat flux and fast change of the temperature of the wall interface. The heat flux during spray cooling is determined by the instantaneous substrate temperature, which is illustrated by boiling curves. The variation of the heat flux is especially notable during different thermodynamic regimes: film, transitional and nucleate boiling. In this study transient boiling curves are obtained by measurement of the local and instantaneous heat flux produced by sprays of variable mass flux, drop diameter and impact velocity. These spray parameters are accurately characterized using a phase Doppler instrument and a patternator. The hydrodynamic phenomena of spray impact during various thermodynamic regimes are observed using a high-speed video system. A theoretical model has been developed for heat conduction in the thin expanding thermal boundary layer in the substrate. The theory is able to predict the evolution of the target temperature in time in the film boiling regime. Moreover, a remote asymptotic solution for the heat flux during the fully developed nucleate boiling regime is developed. The theoretical predictions agree very well with the experimental data for a wide range of impact parameters.

Objective and MethodsThe reliable estimation of phase transition physicochemical properties such as boiling and melting points can be valuable when designing compounds with desired physicochemical properties. This study explores the role of external rotational symmetry in determining boiling and melting points of select organic compounds. Using experimental data from the literature, the entropies of boiling and fusion were obtained for 541 compounds. The statistical significance of external rotational symmetry number on entropies of phase change was determined by using multiple linear regression. In addition, a series of aliphatic hydrocarbons, polysubstituted benzenes, and di-substituted napthalenes are used as examples to demonstrate the role of external symmetry on transition temperature.ResultsThe results reveal that symmetry is not well correlated with boiling point but is statistically significant in melting point.ConclusionThe lack of correlation between the boiling point and the symmetry number reflects the fact that molecules have a high degree of rotational freedom in both the liquid and the vapor. On the other hand, the strong relationship between symmetry and melting point reflects the fact that molecules are rotationally restricted in the crystal but not in the liquid. Since the symmetry number is equal to the number of ways that the molecule can be properly oriented for incorporation into the crystal lattice, it is a significant determinant of the melting point.

Adding solid particles of nanometer scale to fluids is one of the most important passive methods of enhancing heat transfer performance. However, this gives numerous chances to investigate new frontiers, but also raises remarkable difficulties. Nanofluids act as suspension that can be obtained by dispersing nanometer-sized nanoparticles (1-100nm) in host fluids with the aim of enhancing thermal properties. This paper is a review of recent studies on boiling heat transfer of nanofluids for pool and convective flow boiling of nanofluids. The research results, collected since 2012 to present of the recent survey are reviewed and briefly outlined. An emphasis is put on the enhancement and the deterioration of the boiling heat transfer coefficient and critical heat flux of pool and convective flow boiling of nanofluids. Other important parameters affecting the boiling of nanofluids are identified and discussed in this review. While preparing future studies is greatly encouraged in order make this phenomenon well understood. nema