Preferential Evaporation Research Articles

Numerical investigation of multi-component droplet evaporation and autoignition for aero-engine applications

The droplet evaporation and autoignition behaviour of a three-component kerosene surrogate is numerically investigated for a wide range of ambient pressures and temperatures representing realistic aero-engine conditions. Vitiated air with different levels of dilution is considered to represent mixing of air with combustion products. Particular attention is given to the analysis of multi-component fuel effects at the various operating conditions. The investigation also considers the impact of fuel preheating and multiple initial droplet diameters, and extends to the quantification of NOx emissions at the droplet scale level. Considering pure evaporation, results show that preferential evaporation and variation of the droplet composition are mainly affected by the gas temperature. An increase of the pressure generally increases the duration of the droplet heat-up period and reduces the effects of preferential evaporation, especially when high temperatures are considered. Autoignition in vitiated air is strongly influenced by both the level of dilution and ambient pressure, with the latter playing an important role in determining the value of the initial droplet diameter below which no autoignition occurs. Lower pressures generally make the kerosene droplet more resistant to autoignition for the same level of dilution or gas temperature. Droplet preheating mainly affects the heat-up period and can be used as a design parameter to control the autoignition delay time. NOx levels are substantially related to the gas-phase temperature and the existence of a flame at the droplet scale. Potential implications for the design of future low-emission combustor technologies are discussed with an emphasis on the fuel preparation.

Evaluations of the preferential evaporation of binary droplet by rainbow technique and simulation

The evaporation behavior of the ethanol and heptane binary droplets was investigated using both, experimental and simulation methods. Droplets with three compositions of ethanol/heptane and three initial diameters were tested at ambient temperatures of 373 and 473 K, respectively. The preferential evaporation behavior of the individual components inside the binary droplet was obtained and confirmed by both, the numerical and experimental results. The heavier component undergoes prior evaporation when the lighter component dominates the liquid mixture. A systematic comparison between the experimental measurements and simulation results was made with respect to the normalized diameter and refractive index. The spatial and temporal dependences of the refractive index on the temperature and composition were obtained. The composition of the liquid phase has a decisive effect on the variation of refractive index near the droplet surface. Temperature controls the refractive index during the initial stage. Furthermore, the experimental results validate the accuracy of the non-ideal equilibrium model for predicting the diameter, temperature, and concentration of binary droplets for the first time. A linear fitting between the refractive index gradient in the liquid phase and the deviations of the equivalent refractive index from the refractive index at the droplet center has been proposed.

Droplet evaporation of water and fuel jets in low and high temperature subsonic crossflows

Water and Jet A fuel were injected into low and high temperature crossflows of 65 °C and 425 °C, respectively, at a fixed crossflow pressure of 100 kPa, a liquid jet to gaseous crossflow momentum flux ratio of 7.5, and a crossflow Mach number of 0.175. Liquid jet injection temperature was also varied between 20 °C and 90 °C. The orifice had a diameter of 1 mm and a length of 20 mm. Visualization of the spray primary breakup processes was achieved using high-speed imaging. Droplet characteristics were measured at cross-sectional planes 300 mm, 400 mm, and 500 mm downstream of the orifice using a 2-component phase Doppler particle anemometry system. Results discussed included the variation of liquid volume flux, droplet size, and droplet velocities with increasing distance downstream of the injector. Analysis of the results revealed distinct changes in local and global droplet characteristics for both test liquids when injected into the high temperature crossflow compared to the low temperature crossflow. These changes, including increases in measured droplet size, were more significant at the bottom of the spray plume, especially for Jet A, due to the preferential heat-up and evaporation of droplets based on size and liquid thermal properties. For Jet A droplets smaller than 5 μm evaporated completely, leaving behind larger droplets that resisted evaporation. Increasing jet injection temperature was observed to reduce droplet size at a fixed crossflow velocity by reducing liquid surface tension which enhanced atomization. Higher liquid jet injection temperatures also resulted in increased droplet evaporation that initiated closer to the orifice exit.

Effects of atom probe analysis parameters on composition measurement of precipitates in an Al-Mg-Si-Cu alloy

Defining analysis parameter space for reliable composition measurement of materials is of significance for atom probe tomography applications. This research carefully explores the influence of specimen temperature and ultraviolet laser energy on measured compositions of precipitates and the matrix in an Al-Mg-Si-Cu alloy with atom probe tomography using voltage pulsing and laser pulsing. Low specimen temperature and high laser energy are beneficial to reduce background noise and improve mass resolution. Under both voltage pulsing and laser pulsing, the detected compositions and Mg/Si ratios of precipitates are highly sensitive to specimen temperature in the range of 20 - 80 K. In contrast, at a fixed temperature of 20 K, laser energy variation from 40 to 80 pJ provides consistent measurements in composition and Mg/Si ratio of precipitates. Related field-induced preferential evaporation and surface migration of certain elements have been discussed.

Quantitative determination of 137Cs and 90Sr in dissolver solutions without pre-separation by isotope dilution thermal ionization mass spectrometry

Schematic representation of the measurement of isotopes of Cs and Sr present in a dissolver solution based on preferential evaporation.

Deuterium retention of liquid Sn and SnLi in a CPS target under plasma exposure at high temperatures

In the present work, the results of the exposure of in laboratory manufactured SnLi and Sn CPS targets to Deuterium plasmas with a flux of 4 × 1019 D m−2s for different fluences and temperatures are presented. For both SnLi and Sn CPS targets a saturation of the deuterium retention is observed for fluences over 1.2 × 1022 D m−2 in liquid state, while no saturation is observed up to 1.9 × 1023 D m−2 for the solid state. In both cases a decreasing Deuterium retention with increasing temperature is observed and a 100 times higher retention for SnLi than for Sn is obtained. The SnLi CPS target presents a preferential evaporation of Li, showing an almost complete depletion of the Li content after 100 min heating at 770 °C. Bubble formation and bursting was also visually observed and recorded during plasma exposure for the case of the SnLi CPS target but not in the case of the Sn CPS target for the same conditions. This behaviour for Sn is different from what has been observed in previous works but the system geometry and surface type, as well as the difference in the flux to the target, could explain this difference. The results stress the relevance of the design of the containing system in the D retention and Sn behaviour under D exposure.

Compositional control and high-temperature phase stability of plasma-sprayed Ba(Mg1/3Ta2/3)O3 coatings

The Ba(Mg 1/3 Ta 2/3 )O 3 (BMT) ceramic is considered a promising material for high-performance thermal barrier coating (TBC), due to the ultra-high melting point and excellent thermophysical properties . However, the preferential evaporation of Mg from BMT during plasma spraying causes the composition in deposited coatings to deviate from the standard stoichiometry , resulting in the formation of undesirable barium tantalite. These impurities would accelerate the cracking and failure of BMT with low fracture toughness during thermal shock , which greatly limits the application of BMT coatings. In this study, the evaporation of Mg, and the evolutionary characteristics of the composition and microstructure of BMT particle in the plasma spray process were investigated. The relationship between the melting conditions of BMT particles and the composition and microstructure of the coatings was analyzed. The compositional controls, and the high-temperature phase stability of plasma-sprayed BMT coatings were studied and characterized. The results indicate that the evaporation intensity of Mg was strongly affected by the melting conditions of the BMT particles in the plasma jet . The particles with a higher degree of melting had a greater evaporation intensity of Mg, owing to the higher heat transfer and element diffusion rate, and more impurity phases formed in the deposited coating. The nonuniformity of Mg evaporation in BMT particle could lead to composition segregation, promoting the formation of the impurity phases during particle solidification, and ultimately affect the composition uniformity of the coating. By employing the powder with large particle sizes and adjusting the spray parameters to reduce the energy of plasma jet under the premise of ensuring the coating deposition quality, the evaporation intensity of Mg and the content of impurity phase could be significantly restrained, and the high-temperature phase stability of the coatings could be greatly improved compared to the original unmodified BMT coatings. • Evaporation behavior of Mg in plasma sprayed Ba(Mg 1/3 Ta 2/3 )O 3 (BMT) was investigated. • Relationship between the melting conditions of BMT particles and the characteristics of the coating was analyzed. • The composition of BMT coating was controlled by adjusting the powder size and spraying parameters. • Significant improvement in high-temperature phase stability of the BMT coatings was achieved.

Influence of laser power on precipitate formation and multiple step transformation kinetics in NiTi shape memory alloy weld joints

NiTi shape memory alloys are considered to be one of the fastest-growing materials with vast potential. However, the broader applications in intricately designed components are limited due to difficulties faced during welding processes such as degradation of functional and mechanical properties, precipitate formation, and selective vaporization of elements. In this study, the effect of laser power on precipitates formation and subsequent transformation temperatures on NiTi shape memory alloy was investigated by testing 0.8 kW, 1.0 kW and 1.2 kW welded specimens. The microstructures revealed the formation of Ni4Ti3 precipitates in the fusion zone and energy dispersive X-ray spectroscopy assessment exhibited preferential Ni evaporation. Differential scanning calorimetry analysis revealed the evolution of multiple step transformations (3-step / 3-step) after laser welding. Furthermore, enthalpy change in peak transformations was evaluated and increasing order of enthalpy change was recorded for weld joints with increasing laser power. The kinetics of martensite transformation was evaluated by placing emphasis on average activation energy. High laser power welded joints showed easy martensitic transformation compared to low laser power welded joints. X-ray diffraction assessment also validated the existence of Ni4Ti3 precipitates and martensitic phase in NiTi alloy after laser welding. The electron backscattered diffraction maps of welded joints revealed the grain growth in the fusion zone and heat affected zone, and the unindexed black region emphasized the formation of distorted interphase of austenite/martensite and additional martensite phase at the fusion zone/heat affected zone interface.

Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings

This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.

Behavior of the ε-Ga2O3:Sn Evaporation During Laser-Assisted Atom Probe Tomography.

The measurement of the composition of ε-Ga2O3 and the quantification of Sn doping in ε-Ga2O3:Sn by laser-assisted atom probe tomography (APT) may be inaccurate depending on the experimental conditions. Both the role of the laser energy and surface electric field were investigated, and the results clearly indicate that deviations from stoichiometry are observed changing the electric field conditions during APT. The measured atomic fraction of Ga can change from 0.45 at low field to 0.38 at high field, to be compared with the expected 0.4. This was interpreted in terms of preferential evaporation of Ga at high field and deficit of O at low field, which was caused by the formation of neutrals. The quantification of Sn-doping is accurate at low-field conditions, with an overestimation of the detected Sn-metallic fraction at high field. This suggests that Sn has a higher evaporation field compared to Ga. Finally, multiple detection events were in-depth studied, revealing that three dissociation reactions occur during APT: GaO2+ → Ga+ + O+; Ga2O22+ → Ga+ + GaO2+; Ga3O22+ → Ga+ + Ga2O2+. Nevertheless, only 2% of the detected events are related to such dissociation reactions, too small a fraction to fully explain the observed deviation from the stoichiometric composition in ε-Ga2O3.

Characterizing the Occurrence and Spatial Heterogeneity of Liquid, Ice, and Mixed Phase Low‐Level Clouds Over the Southern Ocean Using in Situ Observations Acquired During SOCRATES

AbstractSupercooled liquid water (SLW) and mixed phase clouds containing SLW and ice over the Southern Ocean (SO) are poorly represented in global climate and numerical weather prediction models. Observed SLW exists at lower temperatures than threshold values used to characterize its detrainment from convection in model parameterizations, and processes controlling its formation and removal are poorly understood. High‐resolution observations are needed to better characterize SLW over the SO. This study characterizes the frequency and spatial distribution of different cloud phases (liquid, ice, and mixed) using in situ observations acquired during the Southern Ocean Clouds, Radiation, Aerosol Transport Experiment Study. Cloud particle phase is identified using multiple cloud probes. Results show occurrence frequencies of liquid phase samples up to 70% between −20°C and 0°C and of ice phase samples up to 10% between −5°C and 0°C. Cloud phase spatial heterogeneity is determined by relating the total number of 1 s samples from a given cloud to the number of segments whose neighboring samples are the same phase. Mixed phase conditions are the most spatially heterogeneous from −20°C to 0°C, whereas liquid phase conditions from −10°C to 0°C and ice phase conditions from −20°C to −10°C are the least spatially heterogeneous. Greater spatial heterogeneity is associated with broader distributions of vertical velocity. Decreasing droplet concentrations and increasing number‐weighted mean liquid diameters occur within mixed phase clouds as the liquid water fraction decreases, possibly suggesting preferential evaporation of smaller drops during the Wegener‐Bergeron‐Findeisen process.

Controlling martensitic transformation characteristics in defect-free NiTi shape memory alloys fabricated using laser powder bed fusion and a process optimization framework

Laser Powder Bed Fusion (L-PBF) was utilized to fabricate fully dense, near-equiatomic (Ni50.1Ti49.9) and Ni-rich NiTi (Ni50.8Ti49.2) shape memory alloy (SMA) parts which exhibited tensile ductility up to 16%, shape memory strain of 6%, and tensile superelasticity up to 4%. Annealing heat treatments marginally improved the superelasticity of the as-fabricated Ni-rich NiTi parts due to the formation of a small volume fraction of Ni4Ti3 precipitates. The selection of optimum processing parameters that yielded fully dense parts was guided by a process optimization framework based on a computationally inexpensive analytical model used to predict the melt pool dimensions. The framework also included single-track experiments to validate the model predictions and a criterion for the maximum allowable hatch spacing to prevent the formation of lack of fusion porosity. This framework allowed for constructing L-PBF processing maps for the present NiTi SMAs and revealed that fully dense parts could be printed over a wide range of process parameters. By controlling the L-PBF process parameters, in particular laser power, laser scan speed, and volumetric energy density, in the processing space that result in fully dense parts, it was demonstrated systematically that the composition of the printed parts could be precisely changed by controlling the evaporation of Ni. The flexibility of parameter selection to print defect-free NiTi SMAs and composition control by preferential evaporation of Ni opens the possibility to print functional NiTi SMA parts or devices without post-processing.

Quartz Transformation into Opal at the Water–Vapor Interface

The kinetics of reactions between minerals and water vapor or two water phases simultaneously (liquid + vapor) is still understood inadequately poorly, and the absence of these data disable realistic and comprehensive quantitative description of the evolution of hydrothermal system after its fluid heterogenizes. To bridge this gap, we have conducted quenching experiments with quartz crystals and water at 300°C and Psat = 86 bar. In addition to usually applied analytical techniques (ICP-AES, SEM, XRD, and optical microscopy), we used 3d scanning of the crystals, measured their surface areas, and plotted the changes in the crystal sizes because of dissolution and deposition reactions. In the experimental runs with the crystal occurring in the vapor phase, the first ever data were acquired on the rate constant of quartz dissolution for saturated water vapor (Psat = 86 bar) at 300°C (2.7 nmol m–2 s–1). The constant turned out to be 630 times lower than that for pure water. Calculations indicate that equilibrium between quartz and water and vapor is established during comparable time spans, but quartz recrystallization in vapor due to the temperature gradient proceeds two orders of magnitude more slowly than in water. In the runs with the crystal occurring in both water and vapor, not only the stable quartz dissolved, but also metastable cristobalite-tridymite opal was formed. The opal was deposited on the autoclave walls and even on the quartz itself above the water surface, and the silica concentration in the water remained remarkably lower that the quartz solubility. The rate of opal formation (10–7.5 mol m–2 s–1) was 3.5 orders of magnitude higher than the quartz recrystallization rate (which is the only process possible in this system according to the traditional geochemical approach). This disagreement is explained within the framework of the distillation hypothesis, which is based on the preferential evaporation of the thin (<100 nm) solution layer at the meniscus edge. The system is found out to be able to evolve according to two scenarios, which result in the scattered and compact opal deposition because of the different ratios of the ascent velocity of the solution film and the evaporation rate. This phenomenon may explain the asymmetry of naturally occurring crystallization cavities, whose lower parts dissolved, and minerals were deposited in the upper parts.

Real-time characterization of particles produced by laser ablation for analysis by inductively coupled plasma mass spectrometry

Laser ablation (LA) inductively coupled plasma mass spectrometry (ICPMS) is a powerful technique used for trace element analysis of solid samples. While quantitative analysis can be achieved for many samples with careful use of matrix-matched standards, the complex relationship between the properties of the LA-generated particles and their dependence on the operating conditions, the vagaries of particle transport in the cell and transfer system, and the preferential or incomplete particle evaporation, atomization and ionization in the ICP limits this technique from being fully quantitative over a wide range of sample types and or analytical conditions. The characterization of LA-generated particles represents a particular challenge because of their complex shapes, morphologies, and large and size-dependent void fractions. Improved understanding of how laser settings affect the detailed properties of the LA-generated particles and how particle properties relate to ICPMS signal can potentially lead to ways to effectively tailor or modify inherent sample particles or to add engineered particles as standards, tracers, modifiers, carriers, or concentrators. This work investigates changes in physicochemical properties of LA-generated particles in response to altered laser ablation conditions, including laser power, repetition rate, flow rate, scan rate, and spot size. This is achieved using a multidimensional particle characterization approach that measures in real-time number concentrations, mobility and vacuum aerodynamic diameter distributions, mass, chemical composition, and effective density of individual particles as a function of LA conditions, using a glass standard and a pitchblende ore as samples. These measurements yield fractal dimension, average diameter of primary spherules, number of spherules, void fraction, and dynamic shape factors as a function of particle mass or size. This approach can be extended to other samples and laser wavelengths to comprehensively characterize LA-generated particles and their effects on corresponding ICPMS signals.

Development of a Multiphase Flamelet Generated Manifold for Spray Combustion Simulations

Abstract In this study, a model flame of quasi-one-dimensional (1D) counterflow spray flame has been developed. The two-dimensional (2D) multiphase convection-diffusion-reaction equations have been simplified to one dimension using similarity reduction under the Eulerian framework. This model flame is able to directly account for nonadiabatic heat loss, preferential evaporation, as well as multiple combustion regimes present in realistic spray combustion processes. A spray flamelet library was generated based on the model flame. To retrieve data from the spray flamelet library, the enthalpy was used as an additional controlling variable to represent the interphase heat transfer, while the mixing and chemical reaction processes were mapped to the mixture fraction and the progress variable. The spray flamelet generated manifolds (SFGM) approach was validated against the results from the direct integration of finite rate chemistry as a benchmark. The SFGM approach was found to give a better performance in terms of predictions of temperature and species mass fractions.

On explosive boiling of a multicomponent Leidenfrost drop

The gasification of multicomponent fuel drops is relevant in various energy-related technologies. An interesting phenomenon associated with this process is the self-induced explosion of the drop, producing a multitude of smaller secondary droplets, which promotes overall fuel atomization and, consequently, improves the combustion efficiency and reduces emissions of liquid-fueled engines. Here, we study a unique explosive gasification process of a tricomponent droplet consisting of water, ethanol, and oil ("ouzo"), by high-speed monitoring of the entire gasification event taking place in the well-controlled, levitated Leidenfrost state over a superheated plate. It is observed that the preferential evaporation of the most volatile component, ethanol, triggers nucleation of the oil microdroplets/nanodroplets in the remaining drop, which, consequently, becomes an opaque oil-in-water microemulsion. The tiny oil droplets subsequently coalesce into a large one, which, in turn, wraps around the remnant water. Because of the encapsulating oil layer, the droplet can no longer produce enough vapor for its levitation, and, thus, falls and contacts the superheated surface. The direct thermal contact leads to vapor bubble formation inside the drop and consequently drop explosion in the final stage.

CVD growth of high-quality graphene over Ge (100) by annihilation of thermal pits

The physical and chemical state of the underlying germanium (Ge) substrate is crucial for the CVD synthesis of high-quality graphene. Here, we investigate the main causes responsible for formation of pit-like defects on Ge (100) and (111) surfaces, their elimination, and their influence on the quality of deposited graphene. Our data provides evidence of pit-like defects formation on Ge surfaces during thermal annealing at T ≥ 800 °C. We establish that the pits are due to preferential evaporation of residual Ge suboxides, and that their efficient removal would result in a pit-free surface. Oxide removal on HF, HCl and HBr treated surfaces is investigated using X-ray photoemission spectroscopy (XPS). The results confirm that in contrast to HF and HCl, HBr effectively eliminates GeO2 and Ge suboxides, providing pit-free surfaces, which enable CVD growth of high-quality graphene. Hence, we demonstrate that effective elimination of Ge suboxides enables the synthesis of graphene monolayers with excellent quality (ID/IG ∼ 0.2) as confirmed by Raman spectroscopy measurements. The experimental data provides clear corroboration for the role played by the Ge suboxides in the formation of thermal pits and the control of the graphene quality. Our findings lay the foundation for Ge substrates cleaning and the synthesis of high-quality graphene films for advanced applications.

Spreading and retraction dynamics of sessile evaporating droplets comprising volatile binary mixtures

Abstract

A thermal fluid dynamics framework applied to multi-component substrates experiencing fusion and vaporisation state transitions

The fluid dynamics of multi-component alloy systems subjected to high energy density sources of heat largely determines the local composition, microstructure, and material properties. In this work a multi-component thermal fluid dynamics framework is presented for the prediction of alloy system development due to melting, vaporisation, condensation and solidification phenomena. A volume dilation term is introduced into the continuity equation to account for the density jump between liquid and vapour species, conserving mass through vaporisation and condensation state changes. Mass diffusion, surface tension, the temperature dependence of surface tension, buoyancy terms and latent heat effects are incorporated. The framework is applied to describe binary vapour collapse into a heterogeneous binary liquid, and a high energy density power beam joining application; where a rigorous mathematical description of preferential element evaporation is presented.

Stability of slowly evaporating thin liquid films of binary mixtures

We consider the evaporation of a thin liquid layer which consists of a binary mixture of volatile liquids. The mixture is on top of a heated substrate and in contact with the gas phase that consists of the same vapour as the binary mixture. The effect of thermocapillarity, solutocapillarity and the van der Waals interactions are considered. We derive the long-wave evolution equations for the free interface and the volume fraction that govern the two-dimensional stability of the layer subject to the above coupled mechanisms and perform a linear stability analysis. Our results demonstrate two modes of instabilities, a monotonic instability mode and an oscillatory instability mode. We supplement our results from stability analysis with transient simulations to examine the dynamics in the nonlinear regime and analyse how these instabilities evolve with time. More precisely we discuss how the effect of relative volatility along with the competition between thermal and solutal Marangoni effect defines the mode of instability that develops during the evaporation of the liquid layer due to preferential evaporation of one of the components.