•A unified relationship between bubble departure frequency and diameter during saturated nucleate pool boiling.•Two dominant heat transfer processes for the recovery of bubble base temperature due to ...different regimes of bubble sizes.•Two characteristic timescales associated with two dominant heat transfer processes.•Prediction of bubble departure behaviors for various combinations of heating substrates and working fluids.
The relationship between bubble departure frequency and diameter is fundamental to the boiling process and needs to be fully understood for prediction of overall boiling heat transfer performance. Hydrodynamic models for bubble departure were developed in previous studies. However, these models could not explain the dependence of bubble frequency on properties of the heating substrate and surrounding liquid, which was observed in many experiments. In this work, we develop a unified bubble departure theory for saturated nucleate pool boiling. The heat transfer in the bubble base region after bubble departure is taken into consideration. Two characteristic timescales, representing two dominant heat transfer processes for different regimes of bubble sizes, are extracted. These timescales, which depend on substrate and liquid properties, are used to determine bubble departure frequency. The results from our theory show reasonably good agreement with existing experimental data. The proposed model provides a unified relationship between bubble departure frequency and diameter for various combinations of heating substrates and working fluids in the saturated nucleate pool boiling regime.
This study developed a numerical model for an ammonia/diesel dual direct-injection two-stroke engine and validated it based on engine bench test results. The engine combustion and emission ...characteristics were discussed under the cases of different injection parameters, intake temperatures, and exhaust gas re-circulation (EGR) rates. The findings indicate that altering the injection angle of ammonia and diesel sprays impacts the in-cylinder swirl and the ignition timing of the ammonia spray, consequently affecting the heat release phase of ammonia combustion. A suitable arrangement spray plumes can effectively reduce the emissions of unburned ammonia and greenhouse gas (GHG), while increase the emissions of NOx. On the other hand, higher intake temperatures can increase the average in-cylinder temperature, reducing unburned ammonia, N2O and GHG emissions. However, it also results in greater NO emissions, which can affect fuel economy negatively. Furthermore, the use of EGR technology can reduce the combustion temperature, thereby lowering the emission of nitrogen-based pollutants, but it also reduces ammonia combustion efficiency. By simultaneously applying the three aforementioned technologies within a certain range of conditions, it is possible to effectively synergize and control nitrogenous emissions and GHG while maintaining high thermal efficiency of ammonia/diesel dual direct injection engines.
•Utilize experimental and numerical simulation methods to analyze low emission technology in ammonia-diesel engines.•Revealed the effects of spray layout on the combustion and emissions of ammonia-diesel dual direct injection engines.•The influence law of intake temperature on the combustion and emissions of ammonia-diesel engines is reasonably analyzed.•The study evaluated the effects of EGR on NOx and unburned ammonia emissions in ammonia-diesel engines.•By integrating above technologies, synergistic control of nitrogen emissions and GHG in ammonia-diesel engines was achieved.
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•The evaporation of emulsified HFO droplet is divided into three phases.•The heat equilibrium sub-phase is the distinguishing feature of emulsified HFO.•The first two phases can be ...prolonged with the increase of water content.•Compared with HFO the evaporation of emulsified HFO is promoted significantly.•The duration of fluctuation evaporation phase is decisive for droplet lifetime.
In this study, the evaporation characteristics of emulsified heavy fuel oil (HFO) with water content from 0 to 30 % at the ambient temperatures of 673, 773, and 873 K are experimentally studied. Three distinguishing phases can be clarified and summarized, including initial heating, fluctuation evaporation, and steady evaporation phases. The fluctuation evaporation phase involves the two sub-phases of heat equilibrium and secondary heating, which are first discovered in the evaporation of emulsified HFO. And the heat equilibrium sub-phase is the distinguishing feature of emulsified HFO from other blended fuels. The duration of the initial heating phase and the fluctuation evaporation phase decreases with the elevated ambient temperature but increases with rising water content. However, the variation of micro-explosion intensity with ambient temperature and water content is the opposite. The time consumed in the fluctuation evaporation phase determines the droplet lifetime. The difference between the evaporation characteristics of HFO and emulsified HFO is that the fluctuation evaporation phase of HFO is not distinct, which is due to the violent and frequent micro-explosion occurring in the emulsified HFO droplet. Consequently, the droplet lifetime of emulsified HFO is significantly shorter than that of HFO. Eventually, a regression formula for the dependences of droplet lifetime from temperature and water content is proposed, which can provide a reference for the practical application of emulsified HFO.
Recent work has demonstrated adsorption-based solar-thermal-driven atmospheric water harvesting (AWH) in arid regions, but the daily water productivity (L/m2/day) of devices remains low. We developed ...and tested a dual-stage AWH device with optimized transport. By recovering the latent heat of condensation of the top stage and maintaining the required temperature difference between stages, the design enables higher daily water productivity than a single-stage device without auxiliary units for heating or vapor transport. In outdoor experiments, we demonstrated a dual-stage water harvesting device using commercial zeolite (AQSOA Z01) and regeneration under natural, unconcentrated sunlight where ∼0.77 L/m2/day of water was harvested. Our modeling showed that by further increasing top-stage temperatures via design modifications, approximately twice the daily productivity of the single-stage configuration can be achieved. This dual-stage device configuration is a promising design approach to achieve high performance, scalable, and low-cost solar-thermal AWH.
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•Developed dual-stage device with daily water harvesting productivity of ∼0.77 L/m2/day•Demonstrated productivity can be twice the single-stage device by heat loss reduction•Provided model framework and guidelines to utilize higher performing adsorbents
Atmospheric water harvesting (AWH) using adsorbents is a source of decentralized drinking water, particularly where the moisture content of the air is low. Beyond adsorbent development, device-level improvements are needed to realize practical water productivity (L/m2/day) for solar-driven AWH. We developed a dual-stage AWH device using commercial zeolite (AQSOA Z01) where the two stages increased the productivity, and the latent heat of condensation was recycled from the top stage to assist in desorption of the bottom stage. In outdoor experiments using unconcentrated sunlight, we demonstrated that our dual-stage device had greater productivity than its single-stage counterpart. The dual-stage framework can be used with high-performance adsorbent materials and in different AWH systems to improve thermal efficiency. This work highlights opportunities for higher capacity and more efficient water production and opens new pathways toward scalable, lower-cost solar-thermal AWH systems.
Solar-thermal adsorption-based AWH devices have suffered from low daily water productivity per solar absorber area due to heat and mass-transfer limitations. We developed a dual-stage device to improve productivity by recycling the latent heat of condensation. In outdoor experiments under unconcentrated sunlight and using a commercially available adsorbent, we demonstrated a water harvesting productivity of ∼0.77 L/m2/day, an 18% increase over the single-stage device. Our model that incorporates device characteristics and material properties shows further opportunities to enhance performance.
Wetting states for droplets have been extensively investigated in the past. As the counter phase of the droplets, bubbles’ wetting states have rarely been systematically explored. The wetting state ...of a bubble is closely related to its departure diameter, which plays significant roles in bubble-generated processes in boiling heat transfer and gas-evolving reactions. Based on the principle of minimum surface energy, we explicitly define three equilibrium wetting states (hemi-wicking state, Wenzel state, and Cassie–Baxter state) for bubbles on micro-/nanostructured surfaces in this paper. We analyze the three-phase contact line profiles for bubbles under these wetting states and propose theoretical models for predicting departure diameters of hemi-wicking-state bubble and Wenzel-state bubble on micro-/nanostructured surfaces. We identify competing effects of bubble departure in Wenzel state: the augmentation of contact line length due to the roughness, which would delay bubble departure, and the decrease of contact line length due to the reduced apparent contact angle, which would facilitate bubble departure. We demonstrate that hemi-wicking-state bubble exhibits a much smaller departure diameter on the textured surfaces. These findings are supported by numerical simulations by the three-dimensional (3D) multiple-relaxation-time lattice Boltzmann method. It is found that the length of the outermost contact lines instead of all contact lines determines the departure diameter of hemi-wicking-state bubble based on bubble detachment processes captured by our 3D numerical simulations. This work offers an avenue for the accurate prediction and control of bubble departure behaviors from micro-/nanostructured surfaces, and therefore can guide optimal designs of micro-/nanostructured surfaces in a variety of applications in boiling, desalination, and hydrogen production by electrolysis.
•The difference between the triggering mechanisms of critical heat flux (CHF) and film boiling is clarified.•It is demonstrated that the heat flux of the wet region on the heater surface increases ...while the wet area fraction decreases with superheat, leading to the CHF.•It is shown that a vapor recoil force plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF.
Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulations of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs.
Bubble evolution plays a fundamental role in boiling and gas-evolving electrochemical systems. One key stage is bubble departure, which is traditionally considered to be buoyancy-driven. However, ...conventional understanding cannot provide the full physical picture, especially for departure events with small bubble sizes commonly observed in water splitting and high heat flux boiling experiments. Here, we report a new regime of bubble departure owing to the coalescence of two bubbles, where the departure diameter can be much smaller than the conventional buoyancy limit. We show the significant reduction of the bubble base area due to the dynamics of the three-phase contact line during coalescence, which promotes bubble departure. More importantly, combining buoyancy-driven and coalescence-induced bubble departure modes, we demonstrate a unified relationship between the departure diameter and nucleation site density. By elucidating how coalescing bubbles depart from a wall, our work provides design guidelines for energy systems which can largely benefit from efficient bubble departure.
Hygroscopic hydrogels hold significant promise for high-performance atmospheric water harvesting, passive cooling, and thermal management. However, a mechanistic understanding of the sorption ...kinetics of hygroscopic hydrogels remains elusive, impeding an optimized design and broad adoption. Here, we develop a generalized two-concentration model (TCM) to describe the sorption kinetics of hygroscopic hydrogels, where vapor transport in hydrogel micropores and liquid transport in polymer nanopores are coupled through the sorption at the interface. We show that the liquid transport due to the chemical potential gradient in the hydrogel plays an important role in the fast kinetics. The high water uptake is attributed to the expansion of hydrogel during liquid transport. Moreover, we identify key design parameters governing the kinetics, including the initial porosity, hydrogel thickness, and shear modulus. This work provides a generic framework of sorption kinetics, which bridges the knowledge gap between the fundamental transport and practical design of hygroscopic hydrogels.
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•The nucleation modes in emulsified HFO droplets are clarified.•Dual-mode micro-explosion accompanied by a honeycomb-like structure is identified.•The micro-explosion intensity is ...quantified and a regression formula was proposed.•The bubble-containing secondary droplet is the distinguishing phenomenon.•A plateau for the evolution of droplet temperature appears during evaporation.
The application of emulsified fuels can improve atomization quality which mainly benefits from the micro-explosion phenomenon. To explore bubble behaviors and droplet micro-explosion characteristics of emulsified heavy fuel oil (HFO) droplets during evaporation, the suspended droplet method was adopted. A water content of 0–30 % was used at atmospheric pressure and ambient temperatures of 573–873 K. Three trigger conditions for heterogeneous nucleation in the emulsified HFO droplet were proposed based on the properties of emulsified HFO. The dual-mode micro-explosion of emulsified HFO was defined as local and global micro-explosion. It is of great interest to mention that dual-mode micro-explosion accompanied by a honeycomb-like structure was clarified for the first time. Based on the experimental results, the working conditions of the droplet breakup modes and the appearance of a honeycomb-like structure were summarized. A regression formula was introduced to express the dependence of micro-explosion intensity on ambient temperature and water content. The temperature characteristic curve of the evaporation process presented an obvious plateau, which was a typical feature of emulsified HFO droplets in the evaporation process. Secondary droplets that entrapped bubble nuclei inside were also captured with an occurrence of droplet micro-explosion. This experimental investigation on bubble behaviors and micro-explosion characteristics during evaporation is expected to provide guidance for the application of emulsified HFO.
Atomically thin two-dimensional (2D) materials have shown great potential for applications in nanoscale electronic and optical devices. A fundamental property of these 2D flakes that needs to be ...well-characterized is the thermal expansion coefficient (TEC), which is instrumental to the dry transfer process and thermal management of 2D material-based devices. However, most of the current studies of 2D materials’ TEC extensively rely on simulations due to the difficulty of performing experimental measurements on an atomically thin, micron-sized, and optically transparent 2D flake. In this work, we present a three-substrate approach to characterize the TEC of monolayer molybdenum disulfide (MoS2) using micro-Raman spectroscopy. The temperature dependence of the Raman peak shift was characterized with three different substrate conditions, from which the in-plane TEC of monolayer MoS2 was extracted on the basis of lattice symmetries. Independently from two different phonon modes of MoS2, we measured the in-plane TECs as (7.6 ± 0.9) × 10–6 K–1 and (7.4 ± 0.5) × 10–6 K–1, respectively, which are in good agreement with previously reported values based on first-principle calculations. Our work is not only useful for thermal mismatch reduction during material transfer or device operation but also provides a general experimental method that does not rely on simulations to study key properties of 2D materials.
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