Phase Transition Of Droplets Research Articles

Investigation of thermodynamic state evolution, phase transition and mixing of the hydrocarbon droplet in convective supercritical environments

Liquid alkane droplets transit to supercritical fluids under supercritical conditions. However, the subsequent thermodynamic state evolution, phase transition, and mixing in this case, which directly influences combustion in actual diesel engines, need further investigation. We studied the thermodynamic state evolution, phase transition, and mixing of n-heptane droplets in supercritical laminar nitrogen streams through a set of conservation equations. Non-ideal thermodynamic and transport properties were calculated to incorporate the dramatic variation of physical properties in the supercritical regime. We focus on the effects of Reynolds number, ambient pressure, and droplet size on the thermodynamic state evolution, phase transition and mixing of droplets. Results showed that increasing the Reynolds number can accelerate the evolution of thermodynamic states, which leads to the faster transition of droplets from the liquid phase and supercritical fluids to gas-phase mixtures. With the increased Reynolds number, the phase transition rate is increased and the mixing homogeneity is improved due to the enhanced mass transfer. Caused by the negative correlation between mass transfer and ambient pressure, in higher pressure, droplets after the supercritical transition require more time to transition from supercritical fluids to vapor-phase mixtures. Increasing ambient pressure inhibited the droplet phase transition process and deteriorated the mixing homogeneity, but no significant influence was shown on the droplet dynamic evolution process. Smaller droplets transition faster from liquid phase and supercritical fluids to vapor-phase mixtures. The faster thermodynamic state evolution and faster dynamic evolution process of the smaller droplet leads to its more rapid homogeneous mixing with ambient gasses, but the mode of dynamics is independent of the droplet size.

Study on the Dependence between Surface Topography and Icephobic Behavior of Ni–Cu–P Ternary Coatings

Unpredictable ice accretion and accumulation phenomena could lead to serious problems and even disastrous accidents. Applying icephobic coatings onto the engineering surfaces could be an effective ice protection method. The icephobicity could be mainly affected by several material strategies, such as wettability, surface topography and surface toughness, etc. Herein, several topographical surfaces (Ni–Cu–P coatings prepared on 304 stainless steel using electroless deposition) are critically investigated to understand the role of surface topography in affecting icephobic performances, including anti‐icing and deicing aspects. The freezing delay tests show that rougher surfaces contribute to longer icing delay in anti‐icing. The ice adhesion strength results indicate that rougher surfaces lead to a more mechanical interlock ice and increase the ice adhesion strength between the ice–solid interfaces. The electrothermal deicing tests indicate that the rough voids introduce large thermal resistance and impede heat conduction, resulting in the increase in detachment duration and energy input in deicing. The rough asperity could introduce air pockets at the surface interfaces, which leads to the difficulty of droplet phase transition. The rough structure also increases the mechanical interlock as well as the thermal resistance and impedes the heat conduction, which deteriorates the deicing ability.

Interfacial and intrinsic molecular effects on the phase separation/transition of heteroprotein condensates

Liquid–liquid phase separation (LLPS) and droplet formation by LLPS are key concepts used to explain compartmentalization in living cells. Protein contact to a membrane surface is considered an important process for protein organization in a liquid phase or during transition to a solid or liquid dispersion state. The direct experimental comprehensive investigation is; however, not performed on the surface–droplet interaction and phase transition. In the present study, we constructed simple and reproducible experiments to analyze the structural transition of aggregates and droplets in an ovalbumin (OVA) and lysozyme (LYZ) complex on glass slides with various coatings. The difference in droplet-surface interaction may only be important in the boundary region between aggregates and droplets of a protein mixture, as shown in the phase diagram. Co-aggregates of OVA-LYZ changed to droplet-like circular forms during incubation. In contrast, free l-lysine resulted in the uniform droplet-to-solid phase separation at lower concentrations and dissolved any structures at higher concentrations. These results represent the first phase-diagram-based analysis of the phase transition of droplets in a protein mixture and a comparison of surface-surface and small molecular-droplet structure interactions.

Near-infrared light-responsive functionalised surfaces with shape memory microstructure for droplet manipulation and anti-icing/de-icing

Preparation techniques of functional surfaces by studying the correlations between the macroscopic properties and microscopic morphologies of biological surfaces and by replicating their microstructural features have been widely discussed. However, surfaces with intelligently switched microstructural morphologies, according to the application scenario, have rarely been reported. Herein, laser etching techniques are employed to construct micropillar arrays on a photothermally responsive shape memory polymer (SMP) surface. In situ reversible conversion of surface microstructure morphology is achieved with near-infrared (NIR) light and external forces, further promoting surface wettability conversion. Cycling experiments show that this reversible wettability transition can be repeated many times, demonstrating its applicability as a microdroplet manipulation platform for droplet capture and remote release. By combining the microstructural morphologies and photothermal response properties of the reversible surface transition, the passive anti-icing and active de-icing performance characteristics of different surfaces at various stages are compared by analysing the droplet phase transition processes and variations in ice shear strengths. These functionalized surfaces with NIR light-controlled reversible wettability may allow new possibilities for droplet selection and transfer processes, chemical microreactions and full-process dynamic anti-icing/de-icing applications.

VISUALIZATION RESEARCH OF DROPLET FREEZING ON AN INCLINED TITANIUM SURFACE

Freezing of droplets on inclined cold surfaces was investigated through their visualization. A DSA100 Droplet Surface Analyzer was employed to study the effects of surface tilt angle and temperature on the phase transition time of droplets of various sizes. The post-freezing contact diameter and contact angle between the droplets and the titanium surface were measured and analyzed. From these experimental studies and the related analysis it appears that the freezing time of droplets on cold surfaces at different tilt angles (15°, 30°, 45°, 60°, 75°, and 90°) is the longest when the bottom plate is tilted at 45°. As the tilted surface temperature is reduced, the droplet shape as it froze hardly changes with increasing tilt angle, and at the same time the freezing time of the droplets at each inclination angle is further reduced with decreasing bottom plate temperature. The experiments show that the deformation of a freezing 20-μL droplet on an inclined cold surface is more pronounced than that of a 1-μL droplet. The phase transition time of a large-volume droplet also decreases as the cold plate inclination increases. Thermodynamic equations are also employed to explain the longest droplet phase transition time which occurs at an inclination of 45°.

Study of a new method for the instant preparation of ice particles in ice abrasive air jet

The ice abrasive air jet is a clean surface treatment technology, which currently has limitations such as high energy consumption, uncontrollable particle size and hardness. Realizing the instant preparation and utilization of ice particles are crucial for solving the energy consumption problem. This paper based on the icing principle of heterogeneous nucleation, proposed a new method of ice making, the heat transfer mechanism of low temperature droplets was studied, and the method was proved to be feasible. Using the FLUENT solidification and melting model combined with the VOF model to calculate the freezing process of droplets, the effects of droplet particle size, initial temperature, and wall temperature on the freezing time were analyzed, and the calculation equation of the freezing time was determined, which was corrected by the icing test results. The results showed that the outside of the droplet freezes first, the liquid–solid boundary is parabolic, and the parabolic concavity increases with time and droplet size. In the freezing process, the larger the droplet size, the longer the droplet phase transition time; the higher the droplet initial temperature, the longer it took to reach the phase transition; the higher the wall temperature, the longer the ice formation time.

Research on the Phase Transition Process of Sessile Droplet on Carbon Fiber Cold Surface

Abstract The droplet phase transition process on the cold surface of a T300 carbon fiber substrate was studied by observing the droplet freezing process. Through the construction of visualized experimental device, the change in the droplet phase transition time under different experimental conditions, the progression of the solid–liquid interface during the phase transition process, the droplet deformation rate, and the ratio of growth of the interface height after the phase interface appears were experimentally obtained. The influence of different surface temperatures and different droplet volumes on the phase transition process was investigated. The experimental results show that the phase interface shows an irregular profile during the phase transition of the sessile droplet on the cold surface of the carbon fiber substrate; it presents a wave shape early and a smooth concave shape later. The influence of droplet volume on the phase transition time is not a proportional relationship. The height of the solid–liquid phase interface during the droplet phase transition process first grows rapidly, then slowly, and then fast once again. In other words, the growth rate of the phase interface is relatively fast when the phase transition has just occurred and then when the bulged tip is formed. At different cold surface temperatures, the droplet deformation rate with a volume of 10 µL on the carbon fiber substrate is basically the same, which is about 32.4%, within an uncertainty of about 1%, and it is higher than the contrast substrate. However, the influence of gravity factor is important in determining the droplet deformation rate for different droplet volumes.

Large eddy simulation of an ignition sequence and the resulting steady combustion in a swirl-stabilized combustor using FGM-based tabulated chemistry

PurposeThis study aims to deal with the large eddy simulation (LES) of an ignition sequence and the resulting steady combustion in a swirl-stabilized liquid-fueled combustor. Particular attention is paid to the ease of handling the numerical tool, the accuracy of the results and the reasonable computational cost involved. The primary aim of the study is to appraise the ability of the newly developed computational fluid dynamics (CFD) methodology to retrieve the spark-based flame kernel initiation, its propagation until the full ignition of the combustion chamber, the flame stabilization and the combustion processes governing the steady combustion regime.Design/methodology/approachThe CFD model consists of an LES-based spray module coupled to a subgrid-scale ignition model to capture the flame kernel initiation and the early stage of the flame kernel growth, and a combustion model based on the mixture fraction-progress variable formulation in the line of the flamelet generated manifold (FGM) method to retrieve the subsequent flame propagation and combustion properties. The LES-spray module is based on an Eulerian-Lagrangian approach and includes a fully two-way coupling at each time step to account for the interactions between the liquid and the gaseous phases. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for the flow field while the eddy diffusivity model is used for the scalar fluxes. The fuel is liquid kerosene, injected in the form of a polydisperse spray of droplets. The spray dynamics are tracked using the Lagrangian procedure, and the phase transition of droplets is calculated using a non-equilibrium evaporation model. The oxidation mechanism of the Jet A-1 surrogate is described through a reduced reaction mechanism derived from a detailed mechanism using a species sensitivity method.FindingsBy comparing the numerical results with a set of published data for a swirl-stabilized spray flame, the proposed CFD methodology is found capable of capturing the whole spark-based ignition sequence in a liquid-fueled combustion chamber and the main flame characteristics in the steady combustion regime with reasonable computing costs.Research limitations/implicationsThe proposed CFD methodology simulates the whole ignition sequence, namely, the flame kernel initiation, its propagation to fully ignite the combustion chamber, and the global flame stabilization. Due to the lack of experimental ignition data on this liquid-fueled configuration, the ability of the proposed CFD methodology to accurately predict ignition timing was not quantitatively assessed. It would, therefore, be interesting to apply this CFD methodology to other configurations that have experimental ignition data, to quantitatively assess its ability to predict the ignition timing and the flame characteristics during the ignition sequence. Such further investigations will not only provide further validation of the proposed methodology but also will potentially identify its shortfalls for better improvement.Practical implicationsThis CFD methodology is developed by customizing a commercial CFD code widely used in the industry. It is, therefore, directly applicable to practical configurations, and provides not only a relatively straightforward approach to predict an ignition sequence in liquid-fueled combustion chambers but also a robust way to predict the flame characteristics in the steady combustion regime as significant improvements are noticed on the prediction of slow species.Originality/valueThe incorporation of the subgrid ignition model paired with a combustion model based on tabulated chemistry allows reducing computational costs involved in the simulation of the ignition phase. The incorporation of the FGM-based tabulated chemistry provides a drastic reduction of computing resources with reasonable accuracy. The CFD methodology is developed using the platform of a commercial CFD code widely used in the industry for relatively straightforward applicability.

A SIMPLE review

The Superheated Instrument for Massive ParticLe searches (SIMPLE) consists of an astroparticle dark matter search experiment with large active mass superheated droplet detectors (SDDs), that are installed at the underground low noise laboratory in France (LSBB). Several factors made the use of SDDs an attractive approach for the detection of weakly interacting massive particles (WIMPs), namely their intrinsic insensitivity to minimally ionizing particles, high fluorine content, low cost and operation at near ambient pressure and temperature. The signal that arises from the droplet phase transition generates a millimetric-sized gas bubble that is recorded by acoustic means. I describe the SIMPLE detectors, their acoustic instrumentation and signal analysis, which generated several science measurements executed through several phases over the last 20 years: the Pilot Phase with an exposure of 0.19 kgd, Phase I with an exposure of 0.42 kgd, Phase II with an exposure of 18.24 kgd and, an intended Phase III with an expected exposure increasing from 100 to 2500 kgd to be executed with a bubble chamber.

Lipid Droplet Phase Transition in Freezing Cat Embryos and Oocytes Probed by Raman Spectroscopy

Embryo and oocyte cryopreservation is a widely used technology for cryopreservation of genetic resources. One limitation of cryopreservation is the low tolerance to freezing observed for oocytes and embryos rich in lipid droplets. We apply Raman spectroscopy to investigate freezing of lipid droplets inside cumulus-oocyte complexes, mature oocytes, and early embryos of a domestic cat. Raman spectroscopy allows one to characterize the degree of lipid unsaturation, the lipid phase transition from the liquid-like disordered to solid-like ordered state, and the triglyceride polymorphic state. For all cells examined, the average degree of lipid unsaturation is estimated as ∼1.3 (with ±20% deviation) double bonds per acyl chain. The onset of the lipid phase transition occurs in a temperature range from −10 to +4°C and does not depend on the cell type. Lipid droplets in cumulus-oocyte complexes are found to undergo abrupt lipid crystallization shifted in temperature from the ordering of the lipid conformational state. In the case of mature oocytes and early embryos obtained in vitro, the lipid crystallization is broadened. In the frozen state, lipid droplets inside cumulus-oocyte complexes have a higher content of triglyceride polymorphic β and β′ phases than estimated for mature oocytes and early embryos. For the first time, to our knowledge, the temperature evolution of the phase state of lipid droplets is examined. Raman spectroscopy is proved to be a promising tool for in situ monitoring of the lipid phase state in a single embryo/oocyte during its freezing.

Study on water droplet flash evaporation in vacuum spray cooling

Droplet flash evaporation plays an important role in determining the performance of the vacuum spray cooling, but the properties of the droplet flash evaporation on the micrometer scale are less reported. In this article, a one-dimensional droplet flash evaporation model has been proposed by introducing a simple heat balance correlation and a phase transition model to Fick’s law. Both the performance and applicability of the present model are validated against the published experimental data. Based on the current model, the thermal effects including temperature, heat transfer coefficient, phase transition and droplet lifetime are analyzed in order to offer a guidance for future numerical models of the droplet impingement cooling in vacuum condition. The effects of various parameters including ambient pressure and temperature, droplet size and the fraction of vapor concentration on the droplet phase transition are also addressed from the standpoint of vacuum spray cooling. The numerical results obtained will provide deep insight into the heat transfer mechanisms of the vacuum spray cooling and contribute to advanced industrial applications of the vacuum spray cooling technique.

The SIMPLE Phase II dark matter search

Phase II of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) searched for astroparticle dark matter using superheated liquid C$_{2}$ClF$_{5}$ droplet detectors. Each droplet generally requires an energy deposition with linear energy transfer (LET) $\gtrsim$ 150 keV/$\mu$m for a liquid-to-gas phase transition, providing an intrinsic rejection against minimum ionizing particles of order 10$^{-10}$, and reducing the backgrounds to primarily $\alpha$ and neutron-induced recoil events. The droplet phase transition generates a millimetric-sized gas bubble which is recorded by acoustic means. We describe the SIMPLE detectors, their acoustic instrumentation, and the characterizations, signal analysis and data selection which yield a particle-induced, "true nucleation" event detection efficiency of better than 97% at a 95% C.L. The recoil-$\alpha$ event discrimination, determined using detectors first irradiated with neutrons and then doped with alpha emitters, provides a recoil identification of better than 99%; it differs from those of COUPP and PICASSO primarily as a result of their different liquids with lower critical LETs. The science measurements, comprising two shielded arrays of fifteen detectors each and a total exposure of 27.77 kgd, are detailed. Removal of the 1.94 kgd Stage 1 installation period data, which had previously been mistakenly included in the data, reduces the science exposure from 20.18 to 18.24 kgd and provides new contour minima of $\sigma_{p}$ = 4.3 $\times$ 10$^{-3}$ pb at 35 GeV/c$^{2}$ in the spin-dependent sector of WIMP-proton interactions and $\sigma_{N}$ = 3.6 $\times$ 10$^{-6}$ pb at 35 GeV/c$^{2}$ in the spin-independent sector. These results are examined with respect to the fluorine spin and halo parameters used in the previous data analysis.

Partially premixed reacting acetone spray using LES and FGM tabulated chemistry

Large Eddy Simulations of dilute spray-air reacting two phase flow are performed following an Eulerian–Lagrangian approach. The method includes a full two-way coupling in which phase properties and spray source terms are interchanging between the two phases within each coupling time step. To achieve sub-grid scale closures in the filtered equations used for LES of the carrier phase, the Smagorinsky model with dynamic procedure is applied for the flow field while an eddy diffusivity approach is used for the scalar fluxes. The phase transition of droplets is captured using a non-equilibrium evaporation model. The evaporating droplets are tracked using a Langragian procedure. They are injected in a polydisperse manner and generated in time dependent boundary conditions. The combustion is described by a reaction progress variable and a mixture fraction as well as their variances in the line of the Flamelet Generated Manifold method. The ultimate objective of this work is to appraise the ability of this LES-based spray module developed here to retrieve the spray flow and combustion properties of the configuration under study. The configuration consists of a spray jet issued into a pilot flame and a co-flowing atmospheric air in which droplets traverse a pre-evaporation distance and release part of their mass before reaching the combustion zone. Thereby the spray pre-evaporation turns the combustion regime from diffusion to partially premixed. The fuel used is liquid acetone, which is modeled by a detailed reaction mechanism including 84 species and 409 elementary reactions. A series of spray established at different operating conditions are investigated, analyzed and compared. Analysis allows to gain a deep insight into the process ongoing. Comparisons include exhaust gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit planes. An overall good agreement is reported.

On the acoustic vaporization of micrometer-sized droplets

This paper examines the vaporization of individual dodecafluoropentane droplets by the application of single ultrasonic tone bursts. High speed video microscopy was used to monitor droplets in a flow tube, while a focused, single element transducer operating at 3, 4, or 10 MHz was aimed at the intersection of the acoustical and optical beams. A highly dilute droplet emulsion was injected, and individual droplets were positioned in the two foci. Phase transitions of droplets were produced by rarefactional pressures as low as 4 MPa at 3 MHz using single, 3.25 micros tone bursts. During acoustic irradiation, droplets showed dipole-type oscillations along the acoustic axis (average amplitude 1.3 microm, independent of droplet diameter which ranged from 5 to 27 microm). The onset of vaporization was monitored as either spot-like, within the droplet, or homogeneous, throughout the droplet's imaged cross section. Spot-like centers of nucleation were observed solely along the axis lying parallel to the direction of oscillation and centered on the droplet. Smaller droplets required more acoustic intensity for vaporization than larger droplets, which is consistent with other experiments on emulsions.

Quantum statistics of an electron–hole plasma

AbstractThermodynamic functions of an electron–hole plasma in an optically excited semiconductor are derived in the nondegenerate region. The existence of excitons and the ionization equilibrium are discussed in the ne–T (electron density–temperature) plane. Above the Mott curve (r0 = 0.84 a0, r0 screening length, a0 Bohr's radius) excitons do not exist. Besides the well known electron–hole droplet phase transition the critical point of which lies above the Mott curve, a region of instability is found below the Mott curve which might correspond to a plasma phase transition (weakly ionized plasma → strongly ionized plasma). Outside of this instability region at the Mott curve a diffuse phase transition (Mott‐transition) exists.