A three-dimensional transient numerical model incorporating coupled mass, momentum, heat, and species transport was developed to investigate the evaporation dynamics of binary sessile droplets. The evaporation behavior and the temporal evolution of the internal flow structures were systematically examined, revealing four distinct stages based on internal flow pattern: (i) the initial stage of coexistence of solutal and thermal Marangoni convection, (ii) solutal Marangoni-dominated stage with complex multi-vortex structures, (iii) solutal Marangoni-dominated stage with imbalanced macroscopic vortex flow, and (iv) stable low-velocity convection stage. The flow characteristics at each stage were delineated, and the underlying physical mechanisms driving the transitions were elucidated. Particular attention was given to the interplay between thermal and solutal effects and their influence on the evolving flow field, temperature and concentration distributions, and local evaporation rates. These results provide new insight into the coupled fluid dynamics governing the evaporation of multi-component droplets.

Bubble flow in confined geometries is a problem of fundamental and technological significance. Among all the forms, bubble breakup in bifurcated microchannels is one of the most commonly encountered scenarios, where an in-depth understanding is necessary for better leveraging the process. This study numerically investigates the non-uniform breakup of a bubble slug in Y-shaped microchannels under different flow ratios, Reynolds numbers, and initial bubble volumes. Overall, the bubble can either breakup or non-breakup when passing through the bifurcation and shows different forms depending on flow regimes. The flow ratio-Reynolds number phase diagrams indicate a power–law transition line of breakup and non-breakup. The bubble takes longer to break up with rising flow ratios yet breaks earlier with higher Reynolds numbers and volumes. Non-breakup takes less time than the breakup patterns. Flow ratio is the origin of non-uniform breakup. Both the Reynolds number and initial volume influence the bubble states when reaching the bifurcation and thus affect subsequent processes. Bubble neck dynamics are analyzed to describe the breakup further. The volume distribution after breaking up is found to have a quadratic relation with the flow ratio. Our study is hoped to provide insights for practical applications related to non-uniform bubble breakups.

Understanding droplet evaporation has broad implications for science and industry. Over the past decades, theoretical models concerning this topic have been extensively studied for better prediction of this process. Although the existing models have shown reliable accuracy both in qualitative and quantitative analysis, most focus on specific physical scenarios and therefore consider only part of the transport mechanisms in the process, e.g., a diffusion-based model. In this study, we numerically investigated the evaporation of vertical and pendant ethanol droplets in an open space and developed a novel comprehensive model for simulation. The model incorporated vapor diffusion, thermal conduction, evaporative cooling, natural convection, and Marangoni stress, showing excellent accuracy in three-dimensional scenes. A sessile droplet evaporating on top of a horizontal substrate was compared. Our analysis started with the temperature distribution surrounding and inside the droplet, from which a complex multi-mechanism coupling behind the concerned issues was implied. We then studied the vapor diffusion and flow features in the gas domain and revealed geometric restriction as an important influencing factor. Next, we focused on the internal flows, demonstrating the dominance of Marangoni convections within the droplet and the characteristics of flow behaviors. The local evaporation flux was identified. Finally, we compared lifetimes in different cases and discussed the effects of droplet volume during evaporation. Overall, a vertical droplet had higher evaporation intensity and internal convection, while pendant and sessile droplets were similar. Our study hopes to provide a reference for ethanol droplet evaporation and possible applications based on such a phenomenon.

The flow disturbance technique is an effective method to enhance the heat and mass transfer efficiency in microchannels. This paper numerically investigates the influence of two side-by-side freely rotatable square cylinders on microchannel flow, heat transfer, and mass transfer under a Reynolds number (Re ) range of 5-300. Results indicate that the cylinders show five different vortex-induced rotation modes, namely the rotation mode ( Re <~10), oscillation mode (~10 < Re < ~70), stationary mode (~70 < Re < ~90), random mode (~90 < Re < ~250), and reversal rotation mode ( Re > ~250). The vortices generated by rotations of the cylinders disturb the flow field significantly, thus promoting heat and mass transfer in the microchannel. As Re increases, the average Nusselt number ( Nu ) of the channel first rises slowly then sharply, specifically, from 3.50 when Re = 5 to 8.35 when Re = 75, and then to 42.45 when Re = 300. Under a small Re , the rotating cylinders only affect the local heat transfer near them, while only when Re > 100 can the cylinders significantly promote the heat transfer in a relatively long distance in the microchannel. This behavior should be attributed to the insufficient vortex disturbance under small Reynolds numbers. Mass transfer is scrutinized by releasing fluorescein at the upper half of the channel entrance, and mixing efficiency ( eta) is introduced to characterize the mixing situation. We show that when Re = 100, the substances in the channel are almost completely mixed (eta & nbsp;= 0.98) compared with eta & nbsp;= 0.84 for stationary cylinders; beside, as Re increases, the position where the mixture reaches a steady-state gradually moves forward in the channel, and when R > 100, the distribution of eta & nbsp;along the channel seldom changes with Re . Considering fluid pressure loss, heat transfer enhancement index phi and mass transfer enhancement index phi are introduced to compare heat and mass transfer capability between microchannels set with rotatable cylinders and stationary cylinders, and phase diagrams relative to Re and x * are drawn. Analysis reveals that the freely rotatable cylinders better improve the heat and mass transfer with a maximum phi of 1.6 at Re & AP; 25, x* & AP; 12 or Re & AP; 100, and maximum phi of 1400 at Re & AP; 50, x* & AP; 10-25. (c) 2022 Elsevier Ltd. All rights reserved.

Precise determination of microfluidic behaviors is theoretically significant and has shown remarkable application prospects. This work numerically studies the self-ordering and organization of an in-line particle chain flowing through a square microchannel. The immersed boundary-lattice Boltzmann method is employed, and effects of particle Reynolds number (Rep), length fraction (⟨Lf⟩, characterizes particle concentration), and particle size are focused. Results imply a relatively complex migration of small-particle chains. Three typical states are observed, that is, the equilibrium position finally in a stabilized, fluctuated, or chaotic condition. The corresponding dynamic processes are presented. Interestingly, how interparticle spacing evolves with time shows similar regularity with the three states, corresponding to a particle chain either being evenly distributed, moving like a bouncing spring, or continuously in disordered motions. The flow field and force conditions are analyzed to clarify the mechanisms, suggesting the subtle interaction among vortex-induced repulsive force, wall-induced lift force, and shear gradient lift force is the reason behind. Based on different states, migratory patterns are categorized as Stable Pattern, Spring Pattern, and Chaotic Pattern, and an overall classification is also obtained. Moreover, effects of Rep and ⟨Lf⟩ are identified, where a rising Rep leads to an equilibrium position toward the wall and larger volatility of interparticle spacings. The dynamic characteristics are characterized by lagging, translational, and angular velocities of particles in the chain. Finally, a contrastive study of large particles is performed. The present investigation is expected to provide insight into regularities of in-line particle chains and possible applications.

An in-depth understanding of inertial-focusing mechanism is significant to developing high-throughput microfluidic devices. This paper numerically studies the forming and ordering of a staggered particle train in a square microchannel using the immersed boundary-lattice Boltzmann method. Effects of the particle Reynolds number (Rep) and average length fraction (⟨Lf⟩) are mainly concerned, where ⟨Lf⟩ describes the initial particle concentration. Results reveal that the staggered particle train has two distribution patterns depending on ⟨Lf⟩, namely, Continuous Pattern that particles uniformly distributed in the channel and Discontinuous Pattern that an interruption occurs in the train. A detailed train-forming process is provided. Particles within the train are approximately uniformly distributed in both patterns; thus, influencing factors of this uniform interparticle spacing [(L/D)uni] are investigated. A critical ⟨Lf⟩ (⟨Lf⟩*) is defined, dividing determinants of (L/D)uni into Rep-dependent and ⟨Lf⟩-dependent areas. The flow fields and forces acting on the particles were analyzed for further investigation. Four forces are considered: shear gradient lift force, wall-induced lift force, attractive forces, and repulsive forces. Analysis shows that the latter two forces play an essential role in forming a train and the vortex or counterflow is crucial in determining interparticle spacing. Finally, the lagging, translational, and angular velocities were employed to describe particle dynamic characteristics. These parameters are decisively affected by Rep and slightly by ⟨Lf⟩. Inertial-focusing behaviors of a single particle are also compared. The present study is expected to help understand the inertial-focusing behaviors of staggered particle trains and provide a reference for practical applications of microfluidics devices.

We studied the effects of temperature (180-260 degrees C) and catalytic methods (ultrafine grinding and phosphoric acid treatment) on the microwave-assisted hydrothermal carbonization of crop residues by taking rice straw, corn stover and rape stalk as the research materials. The physicochemical and structural properties of hydrochars and liquid products generated under different conditions were characterized. The results showed that with increasing temperature, the hydrochar yield of crop residues decreased from 62.97-83.09% to 42.73-57.38%; the liquid yield rose from 15.03-33.38% to 37.94-47.58%; and the gas yield increased from 1.88-4.92% to 4.68-10.90%. The liquid products were all acidic (pH = 2.53-4.40), and the NH4+-N and PO43-P concentrations respectively showed an upward trend and downward trend with increasing temperature. Higher temperature led to enhanced carbonization degree, causing decreases in H/C and O/C atomic ratios but increases in higher heating value (HHV). Hydrochars prepared at 240 degrees C had the most intensified oxygenated functional groups, the largest specific surface area (5.5652-15.5058 m(2)/g), total pore volume (0.0403-0.1116 cm(3)/g) and mesopore volume (0.0395-0.1101 cm(3)/g), and the most carbon microspheres. However, temperature had a rather limited effect on the micropore volume of hydrochars derived from the three types of crop residues. Both ultrafine grinding and phosphoric acid catalysis could enhance hydrochar properties, with the latter being the more low-cost and efficient method. Hydrochars from ultrafine grinding crop residues had the highest peak strength of oxygenated functional groups, while those from phosphoric acid catalysis had higher fixed carbon (23.58-35.91%), C (45.03-65.50%) and HHV (17.45-27.18 MJ/kg), and lower ash content (4.21-15.67%), as well as the largest specific surface area (6.9140-15.5058 m(2)/g), total pore volume (0.0638-0.1116 cm(3)/g), micropore volume (0.0007-0.0015 cm(3)/g) and mesopore volume (0.0631-0.1101 cm(3)/g). Besides, crop residue type was also found to be an inherent factor determining hydrochar property. (c) 2020 Elsevier Ltd. All rights reserved.

In order to explore the important influence of pyrolysis conditions on the pyrolysis mechanism and product formation, the compositions, energy conversions and structural characteristics of pyrolysis products from crop residues with various particle sizes at different temperatures were investigated. The results showed that increasing temperature (300-600 degrees C) resulted in decreases in the yields (75-35%), 0 (40-6%) and energy conversion rate (88-50%) of biochars, and increases in EC (2.3-6 ms/cm), pH (6.3 -10), fixed carbon (16-60%), C (45-70%) and HHV (18-26 MJ/kg) of biochars as well as the percentages of CH4, CnHm, H-2, HHV (2.5-13 MJ/kg) and energy conversion rate (1-14%) of gases. Besides, higher temperature also led to increases in specific surface area (1-12 m(2)/g) and pore volume (0.004 -0.028 cm(3)/g) and decreases in pore diameter (28-8 nm) and functional group intensity of biochars. Among different particle sizes, coarse grinding sample had the highest yield, H, O and carbon conversion rate of biochars, while fine grinding sample had the highest percentages of CO, CH4 and HHV and energy conversion rate of gases. However, ultrafine grinding sample had the lowest biochar and gas yields and the highest pH, C, specific surface area, pore volume and functional group strengths while the smallest pore size. (C) 2019 Elsevier Ltd. All rights reserved.

Efficient utilization of crop residues can help to solve the problem of global resource scarcity. The pyrolysis and combustion characteristics of 40 crop residues and 3 lignocellulosic components in different atmospheres by thermogravimetric analysis were analyzed. The results showed that with the increase of heating rate, the pyrolysis and combustion of cellulose, hemicelluloses, lignin and crop residues shifted to higher temperature zones. Meanwhile, the comprehensive combustibility index increased significantly, indicating that a higher heating rate could intensify and improve the pyrolysis and combustion performances. At the heating rate of 20 degrees C/min, the variation ranges of ignition temperatures of crop residues in N-2, air and O-2 were 253-295 degrees C, 247-275 degrees C and 245-268 degrees C, and the variation ranges of burnout temperatures were 555-682 degrees C, 466-596 degrees C and 323-357 degrees C, respectively. The comprehensive combustibility index in O-2 was 144.24 x 10(-7), which were much higher than those in air and N-2. The variation ranges of activation energies of crop residues were 25.52-36.84, 27.96-43.35 and 41.07 -69.35 kJ/mol in N-2, air and O-2 at 20 degrees C/min, respectively, and the combustion of crop residues in O-2 is the most intense. Overall, rape stalk showed the best comprehensive pyrolysis and combustion performance, and rice straw was the poorest. (C) 2019 Elsevier Ltd. All rights reserved.

研究不同热解温度、粒度、种类的秸秆生物炭的燃烧特性,并进行动力学分析.结果 表明,随着热解温度升高,秸秆生物炭的固定碳、C和高位热值均增加,综合燃烧指数减小,燃烧向高温区移动,活化能增大.随着升温速率增高,生物炭综合燃烧指数增大,活化能降低.生物炭着火温度为260～395℃,燃尽温度为480～555℃,500℃制备的生物炭燃烧特性最好,活化能为48～65 k J/mol.超微生物炭的着火温度、燃尽温度和活化能最低,综合燃烧指数最高,棉花秸秆生物炭更适合作固体燃料.

热解炭化技术的开发对秸秆的能源化利用具有重要意义.试验研究了保温时间与粒度对水稻和棉花秸秆热解产物理化特性及能源转化的影响.结果表明,保温时间从0到120 min中,秸秆生物炭产率先降低后略增加,热解气中CH4、CnHm和H2百分含量增加,其高位热值和能量转化率增加,而生物炭的pH值、电导率、灰分、固定碳、C、高位热值增加,保温时间为90 min的生物炭的炭化程度最好.秸秆中能量有1.5％～5.4％保留在热解气中,有50％～57％保留在生物炭中.不同粒度相比,粗粉秸秆的生物炭的炭产率、挥发分、H、O、N及碳转化率最高,细粉秸秆热解气中CO和CH4百分含量、高位热值和能量转化率最高,而超微秸秆生物炭的pH值、灰分、C最高.棉花秸秆生物炭的挥发分、固定碳、C、H、碳转化率、高位热值和能量转化率高于水稻秸秆生物炭.