Evaporation Of Liquid Droplets Research Articles

Optimisation design of short pitch spiral hybrid SCR structure for improving the exhaust performance

As exhaust emission standards become increasingly stringent, the requirements for diesel exhaust post-treatment systems become increasingly demanding. This research focuses on the optimisation of a new hybrid structure to improve the performance of exhaust post-treatment systems. First, a short pitch spiral mixing chamber with sufficient mixing space and low temperature loss was designed to improve the evaporation of liquid droplets. Additionally, a conical plate mixer was used to conduct secondary mixing of the gas to improve the flow characteristics further. The ammonia uniformity and crystallisation risk of the new hybrid structure were simulated using Computational Fluid Dynamics, and the optimal hybrid structure was verified by tests. The results showed that ammonia leakage, low temperature crystallisation and NOX conversion efficiency could effectively be improved by optimally designing the new hybrid structure. This research is of great significance for reducing exhaust pollution and optimising diesel exhaust post-treatment systems.

The transients in the evaporation of sessile liquid droplets and the applicability of the steady-state approximation

The initial transients in the droplet evaporation (i.e., the transient processes immediately after the droplet is placed on the substrate) has been investigated first both numerically and experimentally. The results show that, during the initial stage of droplet evaporation, the vapor concentration changes monotonically when approaching its steady-state while the temperature changes nonmonotonically. The initial transient times needed for the vapor concentration and for the temperature to change from their initial values to the steady-state values are almost the same, and the ratio between the initial transient time and the droplet drying time is mainly determined by the properties of the liquid, not by the underlying substrate or by the pressure of the surrounding atmosphere. The duration of the initial transients is in general longer than the duration of the process transients which denotes the adjustment of the vapor concentration and the temperature to the change in the droplet shape as the evaporation proceeds. Furthermore, a quantitative criterion for evaluating the applicability of the steady-state approximation in the droplet evaporation is obtained, indicating that the approximation is more reasonable for liquids with a lower saturated vapor concentration. The criterion is corroborated experimentally and may contribute to the body of knowledge concerning the droplet evaporation.

Direct numerical simulation of an evaporating turbulent diluted jet-spray at moderate reynolds number

The evaporation of liquid droplets dispersed in jet-sprays is involved in many industrial applications and natural phenomena. Despite the relevance of the problem, a satisfactory comprehension of the mechanisms that regulate the process has still not been achieved. Indeed, a wide range of turbulent scales and an enormous number of droplets are typically involved, causing the numerical and theoretical tackling of the problem to be challenging. In this context, we address the Direct Numerical Simulation (DNS) of a turbulent jet-spray at a bulk Reynolds number Re=10,000. We focus on the effect of the jet Reynolds number on the evaporation process and the preferential segregation of droplets. The analysis is conducted by comparing the outcomes of the present DNS with that from a lower Reynolds number one in corresponding conditions, Re=6,000. The problem is addressed by employing the point-droplet approximation in the hybrid Eulerian-Lagrangian framework. We present detailed statistical analyses of both the gas and the dispersed phases. We found that the mean droplet vaporization length grows as the bulk Reynolds number is increased from Re=6,000 to Re=10,000, while keeping all the remaining conditions fixed. We attribute this result to the complex interaction between the inertia of the droplets and the dynamics of the turbulent gaseous phase. In particular, at the higher Reynolds number, the effect of the faster turbulent fluctuations of the jet mixing layer, which tend to fasten the process, is compensated by the slower mass transfer from the liquid droplets to the surrounding environment. We also observe intensive preferential segregation of the dispersed phase that originates from the entrainment of dry environmental air into the mixing layer and is intensified by small-scale clustering in the far-field region. We show how the preferential segregation is responsible for a strongly heterogeneous Lagrangian evolution of the dispersed droplets. All these aspects contribute to the Reynolds-number dependence of the overall droplet evaporation rate. Finally, we discuss the accuracy of the d-square law, often used in spray modelling, for present cases. We found that using this law based on environmental conditions leads to a relevant overestimation of the droplet evaporation rate.

Controlling uniform patterns by evaporation of multi-component liquid droplets in a confined geometry.

Surface-coating technologies are important for a variety of applications, e.g. ink-jet printing, micro-electronic engineering and biological arrays. In this study, we introduce a novel idea to obtain uniform patterns with multi-component solution in a confined geometry. When a droplet of the multi-component liquid evaporates in the confined area, the evaporated vapors are stagnated inside the confined chamber where the evaporated liquid molecule is much heavier than the ambient air. These vapors change internal flow in the droplet by generating Marangoni effects during evaporation, which help to obtain uniform deposition. Finally, we show that a coffee-ring is totally suppressed and a uniformly dried pattern is achieved. For a potential application as display panels, we use quantum dots and create a uniform light-emitting layer.

Evaporation and breakup effects in the shock-driven multiphase instability

Abstract

Prediction of water droplet behavior on aluminum alloy surfaces modified by nanosecond laser pulses

Metal processing by laser radiation is shown to be a promising way to obtain surfaces with controllable wetting and evaporation characteristics. Such surfaces can be used in designing the modern cooling systems based on phase transitions. However, large-scale applications of metal processing by laser radiation are hampered by the lack of a theory that allows predicting the wetting and evaporation of liquid droplets on modified surfaces. In this work, we study a water droplet behavior during its wetting and evaporation on hydrophilic and hydrophobic aluminum alloy surfaces modified by single nanosecond laser pulses. To predict the wettability of the laser-processed surfaces, we proposed to use the empirical correlation of the contact angle on the surface free energy obtained in terms of dynamic contact angles. We analyzed the applicability of two well-known theoretical models to determine the evaporation rates of droplets on hydrophilic and hydrophobic laser-processed surfaces. The Spalding and diffusive evaporation models can be used for water droplet evaporating from the hydrophilic aluminum alloy surface at a temperature difference between the surface and air in the chamber of 0–6.5 K. At higher temperature difference from 6.5 K to 9 K, only the Spalding model is applicable. In the case of the hydrophobic laser-processed aluminum alloy surface, the mass loss rate of evaporating droplet at the temperature difference range of 0–9 K can be predicted by the diffusive model. In addition, this model was found to be appropriate for predicting the geometric dimensions of a droplet evaporating in a pinned contact line mode on hydrophilic surfaces without heating at any stage of its evaporation. The results of this study can be used to predict the wetting and evaporation characteristics of water droplets on aluminum alloy surfaces modified by single nanosecond laser pulses.

Effects of Entrainment and Mixing on the Wegener–Bergeron–Findeisen Process

Abstract The growth of ice crystals at the expense of water droplets, the Wegener–Bergeron–Findeisen (WBF) process, is of major importance for the production of precipitation in mixed-phase clouds. The effects of entrainment and mixing on WBF, however, are not well understood, and small-scale inhomogeneities in the thermodynamic and hydrometeor fields are typically neglected in current models. By applying the linear eddy model, a millimeter-resolution representation of turbulent deformation and molecular diffusion, we investigate these small-scale effects on WBF. While we show that entrainment is accelerating WBF by contributing to the evaporation of liquid droplets, entrainment may also cause aforementioned inhomogeneities, particularly regions filled with exclusively ice or liquid hydrometeors, which tend to decelerate WBF if the ice crystal concentration exceeds 100 L−1. At lower ice crystal concentrations, even weak turbulence can homogenize hydrometeor and thermodynamic fields sufficiently fast so as to not affect WBF. Independent of the ice crystal concentration, it is shown that a fully resolved entrainment and mixing process may delay the nucleation of entrained aerosols to ice crystals, thereby delaying the uptake of water vapor by the ice phase, further slowing down WBF. All in all, this study indicates that, under specific conditions, small-scale inhomogeneities associated with entrainment and mixing counteract the accelerated WBF in entraining clouds. However, further research is required to assess the importance of the newly discovered processes more broadly in fully coupled, evolving mixed-phase cloud systems.

Optimization the operation parameters of SDA desulfurization tower by flow coupling chemical reaction model

Abstract Spray Drying Absorber (SDA) has been widely used for large-scale desulfurization. However, it also has some limitations. For example, the liquid absorbent easily causes scaling, which impedes the contact between the serous fluid and the flue gas and reduces the chemical reaction rate and desulfurization efficiency. This paper establishes the mathematical and physical model of gas and liquid two-phase flow and droplet evaporation and heat transfer in rotary spray desulfurization tower. To study the accumulation and distribution of chemical reaction precipitates in the desulfurization tower and analyze the removal efficiency of sulfur dioxide (SO2) in different atomization diameters, this paper establishes a simulation model concerning the coupling of desulfurization reaction and flow field calculation based on the absorption and reaction mechanism of SO2. Baffle in different widths are set to optimize the internal flow field and balance the distribution of flue gas. By setting baffles of different widths to optimize the flow field in the tower and changing the distribution of flue gas, this model reduces the scaling while ensuring the desulfurization efficiency. The results of the simulation experiment have verified that the droplet with a diameter of 50 μm is the optimal option, which can effectively remove the scaling and ensure that the desulfurizing tower runs in high efficiency and stability. When the width of baffles is 2250 mm, the efficiency of desulfurization exceeds 95%, and the amount of scaling on the desulfurization tower main wall is controlled at the minimum level, which is the optimal option for production.

Machine Learning Analysis for Quantitative Discrimination of Dried Blood Droplets

One of the most interesting and everyday natural phenomenon is the formation of different patterns after the evaporation of liquid droplets on a solid surface. The analysis of dried patterns from blood droplets has recently gained a lot of attention, experimentally and theoretically, due to its potential application in diagnostic medicine and forensic science. This paper presents evidence that images of dried blood droplets have a signature revealing the exhaustion level of the person, and discloses an entirely novel approach to studying human dried blood droplet patterns. We took blood samples from 30 healthy young male volunteers before and after exhaustive exercise, which is well known to cause large changes to blood chemistry. We objectively and quantitatively analysed 1800 images of dried blood droplets, developing sophisticated image processing analysis routines and optimising a multivariate statistical machine learning algorithm. We looked for statistically relevant correlations between the patterns in the dried blood droplets and exercise-induced changes in blood chemistry. An analysis of the various measured physiological parameters was also investigated. We found that when our machine learning algorithm, which optimises a statistical model combining Principal Component Analysis (PCA) as an unsupervised learning method and Linear Discriminant Analysis (LDA) as a supervised learning method, is applied on the logarithmic power spectrum of the images, it can provide up to 95% prediction accuracy, in discriminating the physiological conditions, i.e., before or after physical exercise. This correlation is strongest when all ten images taken per volunteer per condition are averaged, rather than treated individually. Having demonstrated proof-of-principle, this method can be applied to identify diseases.

Nonlinear dynamics of polydisperse assemblages of particles evolving in metastable media

The article addresses theoretical approaches in describing the evolution of particulate assemblages in metastable liquids. The evolutionary model to deal with such processes and phenomena is put forward, which is based on the Fokker–Planck equation for the particle-size distribution function and on the mass/heat balance equation for the degree of metastability. The model is generalized in describing various phase transformation phenomena met in materials science and condensed matter physics. Two general analytical approaches in solving the generalized integro-differential model are considered with special attention to crystallization processes in supercooled and supersaturated liquids, evaporation of liquid droplets and dissolution of dispersed solids. These analytical theories are based on the saddle-point method in solving the integro-differential equation for the degree of system metastability and on the separation of variables in the kinetic and balance equations with subsequent summation or integration over different elementary solutions. The main focus here is to determine the analytical expressions for the particle-size distribution function and the degree of metastability dependent on the evolutionary kinetics of particulate assemblages in metastable media.

Methodological Characteristics of an Experimental Investigation of the Process of Evaporation of Suspended Liquid Droplets

A comparative analysis of the components of a heat flux occurring in evaporation of various liquid droplets suspended on fibers from different materials (polyvinylchloride, asbestos, optical fiber, and horsehair) blown with dry air is presented. The contribution of free convection, radiated emission, and heat transfer through a fiber supporting an evaporating liquid droplet to an integral heat flux on its surface has been determined. The effect of the droplet size, the temperature and velocity of air flow, and the dimensions of suspension and its material on the relationships of the heat flux components on the droplet′s surface has been studied.

Evaporation of liquid nitrogen droplets in superheated immiscible liquids

Liquid nitrogen or other cryogenic liquids have the potential to replace or augment current energy sources in cooling and power applications. This can be done by the rapid evaporation and expansion processes that occur when liquid nitrogen is injected into hotter fluids in mechanical expander systems. In this study, the evaporation process of single liquid nitrogen droplets when submerged into n-propanol, methanol, n-hexane, and n-pentane maintained at 294 K has been investigated experimentally and numerically. The evaporation process is quantified by tracking the growth rate of the resulting nitrogen vapour bubble that has an interface with the bulk liquid. The experimental data suggest that the bubble volume growth is proportional to the time and the bubble growth rate is mainly determined by the initial droplet size. A comparison between the four different bulk liquids indicates that the evaporation rate in n-pentane is the highest, possibly due to its low surface tension. A scaling law based on the pure diffusion-controlled evaporation of droplet in open air environment has been successfully implemented to scale the experimental data. The deviation between the scaling law predictions and the experimental data for 2-propanol, methanol and n-hexane vary between 4 and 30% and the deviation for n-pentane was between 24 and 65%. The more detailed bubble growth rates have been modelled by a heuristic one-dimensional, spherically symmetric quasi-steady-state confined model, which can predict the growth trend well but consistently underestimate the growth rate. A fixed effective thermal conductivity is then introduced to account for the complex dynamics of the droplet inside the bubble and the subsequent convective processes in the surrounding vapour, which leads to a satisfactory quantitative prediction of the growth rate.

Electrical surface charge patterns induced by droplets sliding over polymer and photoresist surfaces

We have studied the generation of surface charge distributions on polymer surfaces due to the deposition, motion and evaporation of liquid droplets. Using liquids with different static dielectric constants, we found that the magnitude of the surface charge conforms well to the Boltzmann relation for the dissociation probability of ions in solution, indicating that the phenomenon is electrokinetic in nature. The origin of the sensitive dependence of the surface charge on the substrate thickness was identified to be an image charge effect. Experiments on polycarbonate and chemically-amplified photoresist yield surface charges of opposite polarity. By varying the photoresist composition, we identified the leaching of negative quencher ions as the reason for the charge inversion.

Vaporization of liquid droplet with large deformation and high mass transfer rate, II: Variable-density, variable-property case

A novel approach for treating vaporization of liquid droplets with large deformation and high mass transfer rate is developed. The formulation is based on decomposed vaporization mechanisms and accommodates the effects of density and property variations in both the gas and liquid phases. In this model, the mass and energy transfer across the interface are realized by a source layer in the gas phase and a sink layer in the liquid phase around the interface. The flow field in each phase is described by a set of unified conservation equations in the low Mach number limit. The equations are then solved by a modified projection method, in which the nonzero velocity divergence is treated as a source term in the pressure Poisson equation. The vaporization model is implemented in a volume-of-fluid (VOF) based code “Gerris” featuring adaptive mesh refinement. The vaporization of a single n-decane droplet in both quiescent and convective air environments of 1000 K at 1 atm is investigated to verify and validate the present model in terms of interface localization and mass conservation. The results demonstrate the accuracy and robustness of the present work. The vaporization of an impulsively started n-decane droplet with large deformation and breakup is also studied. The droplet surface dynamics are captured well, as are the vaporization behaviors.

Vaporization of liquid droplet with large deformation and high mass transfer rate, I: Constant-density, constant-property case

A numerical model based on decomposed vaporization mechanisms has been developed for the study of vaporization of liquid droplets with large deformation and high mass transfer rate. In this model, the vaporization-induced volume, energy, and species source terms in the gas phase are modeled using a body-fitted layer placed around the interface on the gas side. In this layer, continuous distributions of volume, energy, and species sources are specified by the conservation laws. A corresponding body-fitted sink layer is placed on the liquid side of the interface to account for mass, energy and species loss in the liquid phase due to vaporization. The formulation retains the advantages of interface localization methods and mitigates the error and complexity caused by the combined treatment of mass/heat transfer and interface localization. The new model is versatile and does not depend on the specific interface localization method, and can thus be implemented in any of the existing methods. As a specific example, the present approach is implemented in a volume-of-fluid (VOF)-based code “Gerris”. A number of test cases concerning liquid vaporization in both quiescent and convective environments are presented, with the constant-density assumption, to verify and validate this model in terms of interface localization. In particular, the vaporization of a single n-decane droplet in a uniform air flow of 1000 K at 1 atm is investigated. The predicted liquid volume evolution demonstrates the robustness and accuracy of interface localization for cases with large density ratio and high vaporization rate. Vaporization of n-decane droplets with large deformation and high mass transfer is also investigated systematically.

Heating, evaporation, fragmentation, and breakup of multi-component liquid droplets when heated in air flow

Current technologies of thermal and flame liquid treatment involve high energy consumption, repeated supply of untreated liquid to the chamber, long-term heating and additive evaporation, hence quite low efficiency. These aspects are the main reason why high-temperature evaporation and burnout of impurities has limited technological implementation. The efficiency of such technologies may be improved by means of explosive breakup of untreated droplets in heating chambers. As a result, one parent droplet breaks up to form several dozens to several hundreds of the so-called child-droplets. The development of such technologies requires fundamental understanding of heating, evaporation, boiling, and explosive breakup of one-, two-, and multi-component droplets. The same processes can be used to improve the environmental and energy performance indicators of burning liquid, emulsion, and slurry fuels in combustion chambers of power plants and internal combustion engines. In this paper, we present the experimental research into the main heat and mass transfer processes occurring during the heating, evaporation, boiling, and explosive breakup of two- and multi-component droplets in a heated air flow. We identify the principal differences of two dispersion behaviors – puffing and micro-explosion – and indicate the conditions, in which this or that behavior dominates. There is also a transient parameter domain, in which both behaviors can occur. The curves are plotted showing droplet heating times until explosive breakup versus temperature, component concentration, and droplet radii. Finally, we single out the conditions for fragmentation and atomization of two-, three-, and multi-component droplets.

Numerical investigation of the liquid-fueled pulse detonation engine for different operating conditions

In this study, an intensive simulation platform is developed and implemented to simulate the three stages in the operational cycle of the liquid-fueled pulse detonation engine. The three stages encompass the liquid fuel injection and evaporation process, deflagration-to-detonation transition process, and detonation propagation process. The Lagrangian–Eulerian approaches are employed to model the discrete liquid fuel droplets and the continuous vapor phase, respectively. The breakup and evaporation of liquid droplets are modeled using sub-models, while the interactions between the liquid droplets and the vapor phase are expressed through the two-way interaction models. The Jet-A liquid fuel is injected into the detonation chamber as the fuel for the engine, while the air flow is used as the oxidizer. A reduced chemical kinetic model of fuel/air is used to model the combustion process. The simulation platform is systematically validated against the experimental data for every stage of the operating cycle. To study the influence of the inlet and operating conditions, the numerical simulations are performed for three different operating conditions, which are the change in inlet air temperature, the change in inlet air flow velocity, and the change in liquid fuel mass flow rate. The obtained results indicate that the mass fraction of pre-vaporization of fuel plays an important role in the successful DDT process and/or detonation onset. The deflagration can successfully transit to detonation for both the cases of complete and incomplete vaporization of the liquid droplets inside the detonation chamber. The deflagration cannot successfully transit to detonation for the case of too lean or too rich fuel vapor in the mixture. The calculated burning temperature and Chapman–Jouguet (C–J) detonation velocity are slightly lower in the cases of the incomplete vaporization when compared to the complete vaporization cases. In addition, our numerical results show that the burning process occurs in two stages in the incomplete vaporization case: The first burning stage plays a main role in the successful DDT process, while the second burning stage only plays the role of augmentation.

The critical nanofluid concentration as the crossover between changed and unchanged solar-driven droplet evaporation rates

The challenge of the nanofluid solar energy harvesting lies in the strong coupling between nanoparticles generated heat and flow/temperature fields in various length and time scales. In this paper, the solar-driven evaporation of nanofluid droplet is investigated. The droplet evaporation rates behave a sharply increased regime and a constant regime when crossing a critical nanofluid concentration of ci,cr= 10 ppm. We decouple the total solar energy absorption into two parts in terms of the optical bands, and the whole droplet into two spatial domains. Our study reveals a small length scale starting from contact line to behave higher temperature and larger temperature gradient there, newly recognized as a boundary layer effect. Correspondingly, a contact line region CLR is defined, while the remain is the bulk volume region BVR. The observed coffee-ring behaves a self-assembly of nanoparticles in CLR. We found that even though CLR is two magnitudes smaller compared to the whole droplet, it dominates almost all of the electromagnetic energy absorption, while BVR contributes almost all of the infrared energy absorption. Our finding successfully explains the two regimes of droplet evaporations. With increase of initial nanofluid concentrations, the droplet evaporation rates are raised due to more heat generation induced by enhanced nanoparticles deposition in CLR. The increased evaporation rates stop at a critical concentration of 10 ppm, corresponding to sufficient nanoparticles deposition. Beyond the critical condition, extra nanoparticles are hidden beneath the effective nanoparticles layers to cause unchanged evaporation rate. The present work is useful to develop novel solar steam generation device. The evaporation induced coffee-ring can be regarded as a novel “fabrication” method of nanostructure for plasmonic heating. The pure water evaporation of liquid film or droplet on a prepared coffee-ring array offers great convenience for solar energy utilization.

Density-Driven Flows in Evaporating Binary Liquid Droplets.

In the evaporation of microlitre liquid droplets, the accepted view is that surface tension dominates and the effect of gravity is negligible. We report, through the first use of rotating optical coherence tomography, that a change in the flow pattern and speed occurs when evaporating binary liquid droplets are tilted, conclusively showing that gravitational effects dominate the flow. We use gas chromatography to show that these flows are solutal in nature, and we establish a flow phase diagram demonstrating the conditions under which different flow mechanisms occur.

Effect of the surface wettability on water droplet evaporation

Experimental studies of evaporation of water droplets lying on hydrophobic surfaces were carried out using non-contact measurement methods. The behavior of a contact angle and contact spot size was investigated using high-speed micro photography. The dynamics of the change in an average temperature of the surface of evaporating droplets was studied by the method of infrared thermography. For a theoretical study of the evaporation of liquid droplets, the previously developed emission-diffusion model, which takes into account the influence of the contact angle on the droplet evaporation rate, was used. The results of computations and experiments were compared and satisfactory agreement was obtained.