Evaporation Period Research Articles

Droplet combustion behavior of oxidatively degraded methyl laurate and methyl oleate in microgravity

Droplet combustion behavior of degraded and non-degraded fatty acid methyl ester (FAME) were studied under microgravity condition. Accelerated oxidation tests were conducted for methyl laurate (LME) and methyl oleate (OME) as representatives of FAME, and the changes in viscosities and compositions were evaluated. The Rancimat method was employed for oxidizing fuels at 100 °C for 6 or 24 h. The viscosities and compositions were measured with an Ubbelohde viscometer and a gas chromatography-mass spectrometer (GC–MS), respectively. Results showed that various oxidation products were formed only for OME during the oxidation test, and the amounts increased with an increase in the oxidation time. In addition, viscosities of degraded OME were higher than those of non-degraded OME, which would be attributable to the formation of oxidation products. Subsequently, the droplet combustion experiment was performed at 0.1 MPa, 750 °C in CO2-rich atmosphere under microgravity. Droplet combustion behavior was analyzed in terms of evolutions of droplet diameter squared and the instantaneous burning rate. As a result, the instantaneous burning rate of LME maintained almost constant during its combustion period as well as n-decane, which indicates that the behavior of instantaneous burning rate was not influenced by the difference in fuel properties between hydrocarbon and unsaturated FAME. In contrast, the droplet of degraded OME exhibited puffing during the evaporation and combustion periods. This indicates that oxidation products with various volatilities were formed in the degraded fuels, which resulted in puffing. As oxidation proceeded, the release rate of liquid and vapor by puffing decreased owing to the increased viscosity, which causes puffing more frequently. Furthermore, the time of bubble formation increased with an increase in the initial droplet diameter owing to an increase of the heat capacity of the droplet. On the contrary, the time of bubble formation did not vary drastically with the oxidation time, which was probably owing to a limited change in the heat capacity during the oxidation test. This study addresses the droplet burning behavior of LMEs and OMEs with an emphasis on the effects of oxidation.

Droplet evaporation on a structured surface: The role of near wall vortexes in heat and mass transfer

Experimental studies on the evaporation of a drop located on a horizontal hot wall with cavities of different diameters of 0.5–2.5 mm were carried out. The wall temperature Tw was constant (74 °C and 83 °C). The evaporation behavior on a structured surface was compared with that on a smooth wall. Instantaneous velocity profiles have been obtained over a single cavity and in the vicinity of several cavities using the Micro Particle Image Velocity method (Micro PIV). It has been established that a hotter liquid is periodically ejected from the cavity, which increases convection inside the drop. The strongest intensification of mass transfer is specific for the largest cavities with a diameter of 2.5 mm. The behavior of the droplet evaporation on a smooth wall coincides with that on a structured surface with a cavity diameter of 0.5 mm. Until now, there have been no data that would link the convection in the drop with the vortexes in the cavity at non-isothermal evaporation and at high heat fluxes. The strongest influence of cavities is manifested in the initial period of evaporation, when a cold drop is placed on a hot wall. Over time, the evaporation rate on a structured wall approaches evaporation on a smooth (unstructured) surface. The article considers the influence of several key factors on the convection in a drop.

On the oscillatory nature of heat transfer in steady annular flow

Using a combination of high-speed infrared thermography and electrical-conductance sensors, we observe the surprisingly large effect of surface waves on the instantaneous heat transfer response at the heated wall in nominally-steady upward annular flow of steam-water mixtures. While the heat flux applied at the wall is constant, the passage of the waves causes significant periodic oscillations of the local heat transfer coefficient and wall superheat in the forced convective evaporation regime, with amplitudes and periods much higher than the fluctuations due to turbulent eddies. We discuss the physical mechanisms potentially responsible for the experimental observations.

Effect of Surface Wettability on the Sessile Droplet Evaporation

In this study, crystallized Polycarbonate (PC) surfaces are fabricated by the immersion durations in liquid acetone. Droplet evaporation on the surfaces was studied using a contact angle meter at room temperature, and the evolution of contact angle, contact diameter, droplet volume and droplet height against evaporation time are analyzed. The results showed crystallized PC surfaces (CPCS) have hydrophobic. CCR mode is appeared in the most of evaporation period as sessile droplet evaporation occurred on the crystallized PC wafer surface. For untreated PC wafer surface (UPCS), sessile droplet evaporation showed three evaporation modes. The temporal variation of the droplet volume versus time makes a difference on the crystallized and untreated PC wafer surfaces. The droplet height decreased almost linearly with time for both surfaces.

Green preparation process using methyl lactate for cellulose-acetate-based nanofiltration membranes

Cellulose acetate (CA) membranes were prepared via phase inversion using methyl lactate as a green solvent and 2-methyltetrahydrofuran as a green co-solvent with or without evaporation period prior to immersion of the cast polymer solution in the coagulation bath. The membrane morphologies were characterized using scanning electron microscopy and the membrane performance was screened using aqueous rose bengal (RB) or MgSO4 feed solutions. With CA-concentration in the casting solution increasing from 8 to 20 wt%, RB-rejection increased from 31.1% to 99.5%, while permeance decreased from 32.0 to 2.4 L/m2·h·bar. CA-membranes cast from a 10 wt% polymer solution had a RB-rejection higher than 92.0% with good permeance around 3.5 L/m2·h·bar at low co-solvent concentration (10 wt%). However, with increased co-solvent concentration (i.e. 30–50 wt%), both rejection and permeance surprisingly decreased, regardless evaporation time. For membranes cast from a 20 wt% polymer solution, RB-rejection remained around 99% and permeance decreased with increasing co-solvent concentration in the casting solution. MgSO4 rejection was around 93.0% for membranes cast from a 20 wt% polymer solution with 10 wt% co-solvent without evaporation (at a flux of 1.0 L/m2·h·bar) and 96.5% with evaporation at a flux of 1.3 L/m2·h·bar.

The study of evaporation characteristics of the desulfurization wastewater (electrolyte solution) droplet

In this study, the evaporation behavior of a desulfurization wastewater (electrolyte solution) droplet with different solute concentration at different ambient gas temperature, was investigated by hanging on a glass fiber in a hot constant chamber. A high speed camera was used to record the evaporation process of the droplet. The radius change of the droplet in different conditions were given. A fine thermocouple was inserted in the distilled water droplet to record the temperature history during evaporation. A mathematical model was built with consideration of the effects on the concentration of the water activity and the density change. The results showed that concentration in the surface region was higher than that in the uniform concentration model. The uniform concentration was an assumption of less precision for this droplet evaporation calculation. Additionally, for the wastewater solution whose density changed greatly with concentration, the results showed the density change gave the dominated factor affecting the change of the droplet evaporation rate. Finally, the evaporation and crystallization process gave the effect on the surface region of the droplet keeping the solute concentration relatively high. This lead to the lower evaporation rate when adding non-evaporating dissolved solutes at the beginning period of evaporation.

ATR-FTIR spectroscopy and thermogravimetry characterization of water in polyelectrolyte-surfactant hydrogels

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and thermogravimetric analysis (TGA) were evaluated as useful techniques to study bulk and hydration water in hydrogels. Hydrogels formed by cationized dextran (DEAE-dextran) and anionic surfactants were used as model materials. The IR technique was applied both on fresh samples and on dehydrating samples during the evaporation process. Not only overall IR spectra but especially the deconvoluted OH band was used in the analysis. IR spectroscopy was shown to provide especially structural information which is nicely complemented by the mainly quantitative data provided by TGA. Significant differences between the dehydration processes for the model gels with different surfactants were revealed. The DEAE-dextran-based hydrogels were characterized by a continuous step dehydration with different drying rates in the initial evaporation period. Three different water subpopulations were identified in all model hydrogel samples. Water molecules in these hydrogels containing a higher concentration of surfactants demonstrated a more ordered hydrogen network, which was formed by subpopulations of pentamers, tetramers and/or distorted pentamers.

Simulations on effects of rated ullage pressure on the evaporation rate of liquid hydrogen tank

The self-pressurization process of a liquid hydrogen (LH2) tank in normal gravity due to the heat leakage through the insulation layer and the exposed supports is investigated using the computational fluid dynamic (CFD) approach. The ringlike supports are welded to the upper and lower ends of the cylindrical tank body. The Lee model is used to calculate the interfacial mass transfer, in which the accommodation coefficient is modified by trial test method and compared with the measured liquid level drop rate. In consideration of the possibility of ice formation on the foam and support surfaces exposed to the environment, the thermal boundary conditions are theoretically determined. A numerical strategy for controlling the ullage pressure is also presented to simulate the safety valve. Emphases are put on analyzing the effects of the rated ullage pressure on the evaporation rate. The results show that the whole evaporation process experiences three periods, which are pressure rise, liquid level invariant, and steady evaporation. The rated pressure has almost no effect on the duration of the pressure rise and the steady evaporation period. However, the duration of the liquid level invariant period is linearly related to the rated pressure. All of the mechanisms are explained in detail. Effects of the rated pressure on the temperature and flow distribution are also investigated. The calculation can be used to optimize the cryogenic tank operation for long-time storage.

Experimental study on evaporation characteristics of lubricating oil/gasoline blended droplet

The auto-ignition of lubricating oil/gasoline blended droplet is one of super-knock origins in gasoline direct injection engine. This paper investigates the effect of ambient temperature and blended ratio on evaporation characteristics of lubricating oil/gasoline blended droplet. The experimental results indicate that the evaporation process of blended droplets can be divided into three periods. The second evaporation period may be rapid evaporation or fluctuation evaporation, which is depended on ambient temperature and gasoline content. Gasoline is the main reason of bubble generation inside blended droplets. The bubble generation, puffing and micro-explosion becomes more violent with the increase of gasoline content. The blended droplet lifetime decreases with the increase of ambient temperature. The evaporation residue is a colloidal substance at low ambient temperature and it is a hard solid material at high ambient temperature.

Surface modification of polymeric flat sheet membranes by adding oligomeric fluoroalcohol

Abstract The present study attempted to modify the surface of polymeric membranes by mixing 2-(perfluoroalkyl)ethanol with polyethersulfone to improve the hydrophobicity of the membrane. The characteristics of the surface-modified membrane were examined by using the contact angle, liquid entry pressure (LEPw), X-ray photoelectron spectroscopy spectra, and scanning electron microscopy measurements. The modified membrane was tested for SO2 absorption. The surface contact angle and LEPw values increased by the addition of fluoroalcohol. The fluorine/carbon atomic ratio decreased with increasing evaporation period. The modified membrane was utilized for the absorption of SO2 from the simulated flue gas in alkaline medium (0.5 m NaOH solution). The SO2 removal efficiency slightly decreased probably due to the transformation of the finger-like structure into a sponge-like structure, and the increase in the membrane resistance. The SO2 absorption rate also increased as the gas flow rate increased.

Free convection in a drop at liquid evaporation

This paper is devoted to the study of convection inside a droplet and its effect on heat transfer. A drop of water and a water-salt solution of the LiBr salt was located on a horizontal cylindrical surface of copper. The surface temperature was 53 °C. The novelty of the work is that the influence of free convection in gas and liquid is investigated experimentally. The analysis of experimental data has shown that in the initial period of water drop evaporation, the predominant role in the heat exchange is played by the thermal Marangoni convection.

Testing the potential role of brine reflux in the formation of sedimentary exhalative (sedex) ore deposits

Sedimentary exhalative (sedex) ore deposits are the world’s largest Zn-Pb deposits. While the geologic processes that formed these deposits are generally well understood, the fundamental hydrologic processes that drove these massive hydrothermal systems remain an area of debate. We use numerical modeling to test an emerging hypothesis, supported by recent ore genesis research and sedex basin analysis, that brine reflux flow systems produced and drove the fluids that formed sedex deposits. A previous numerical model of brine reflux, developed to study dolomitization, is adapted to a sedimentary basin with geologic features essential for sedex formation. We simulate the flow of evaporated brines through the basin and the evolution of salinity, temperature, and flow rates, and find that modeled values for these parameters for brines discharging to the seafloor exceed previously established physiochemical thresholds for ore formation (>170 g/L, >80 °C, and total discharge volumes >107 m3 per meter perpendicular to the 2D model section). Sensitivity testing of this modest-sized basin highlights the large effect that aspects of the hydrogeologic framework can have on mineralizing potential of the reflux brines. Finally, modeling alternating periods of active and inactive evaporation produces pulsed brine reflux systems capable of producing multiple deposits of different age as observed in many sedex basins. The modeling thus supports the hypothesis that seawater evaporation on the basin margin significantly inboard of sedex deposits may be responsible for their formation. Sensitivity testing suggests that numerical models with more detailed, basin-specific geologic frameworks might be useful for assessing the mineral potential of sedimentary basins.

Influence of steam condensation on vitamin C retention in green turnip undergoing low pressure superheated steam drying

AbstractIn this article, green turnip discs were dried in low pressure superheated steam for high vitamin C retention. Drying kinetics and vitamin C variation in green turnip was studied at 95 mbar in superheated steam from 75 to 90 °C, considering the influence of steam condensation on the retention of vitamin C. It was found that an amount of vitamin C was transferred in the condensate water during drying process, especially in the constant evaporation period. Due to steam condensation and assistant electrical heating, the material surface temperature was higher than the water boiling point temperature at corresponding environment pressure when superheated steam stopped condensing. The condensate occurred in material surface could enhance the loss of vitamin C in green turnip. Vacuum preheating could reduce the amount of steam condensation and material temperature rise, and hence the vitamin C retention in green turnip was higher than that in continuous superheated steam drying.Practical applicationsGreen turnip is a nutritional, economical and healthy plant, which contains various nutrients such as carotene, ascorbic acid, protein, etc. Dried green turnip has been used as a resource of food seasoning and pickled foods. Fresh green turnip having relatively high moisture contents is very sensitive to microbial spoilage. Drying is one of the most common methods used for green turnip preservation. LPSSD is an innovative technology and which has been applied for dehydration of biological materials due to high nutrients retention, better product appearance, and energy efficiency. Low pressure superheated steam drying process has an initial steam condensation period, transferring an amount of latent heat to the material. It has been demonstrated that steam condensation on sample surface have some influence on material quality. Understanding the effect of condensation on vitamin C retention in green turnip is helpful for improving the quality of dried green turnip slices.

The role of convection in gas and liquid phases at droplet evaporation

The article presents the measuring results of droplet velocity and temperature fields using non-contact optical methods: Particle Image Velocity (PIV), Planar Laser Induced Fluorescence (PLIF) and Thermal imager. The novelty of the work is that the influence of free convection in gas and liquid is investigated experimentally and theoretically and that the key criteria affecting heat and mass transfer are determined. The analysis of experimental data has shown that in the initial period of water drop evaporation, the predominant role in the heat exchange is played by the thermal Marangoni convection. However, for an aqueous salt solution, in spite of the strong influence of the surfactant, the dominant role passes to the solutal Marangoni convection (MaC). In the first seconds after the drop falling, convection and heat transfer in liquid are maximal. Under such conditions it is important to realize an accurate numerical simulation to assess the degree of wall cooling and calculate the non-stationary evaporation. When simulating heat transfer, it is incorrect to neglect free convection in gas or liquid due to their strong nonlinear influence on each other. The heat exchange in the drop is extremely conservative to convection in the liquid (the Peclet number Pe = 100 and the Nusselt number Nu = 4).

Heat transfer of aqueous salt solutions during evaporation on a structured heated wall

Evaporation and heat transfer of the layers of water and aqueous salt solutions (H2O/LiBr, H2O/CaCl2, H2O/LiCl) have been studied experimentally. Evaporation rate is greater for a structured wall than for a smooth one. When simulating droplet evaporation and heat transfer, it is necessary to take into account free convection of air and vapor. There is a change of the heat transfer coefficient for salt solutions layers on the structured surface with the presence of an extremum. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, α is lower than that of water. The maximum excess of heat transfer coefficient of micro-structured wall above the smooth wall (20–25%) corresponds to the maximum height of the liquid layer for the initial period of evaporation. With increasing time, the excess is reduced.

Non-isothermal Evaporation of Salt Solutions on a Microstructured Surface

ABSTRACTHeat transfer of a droplet and layer during evaporation of aqueous solutions of salts has been studied. The behavior of salt solutions on a smooth and microstructured surface is compared here. Evaporation rate of aqueous salt solutions is greater for a microstructured surface than for a smooth wall. The behavior of heat transfer coefficient α can be described by two time regimes: quasi-constant values of α and significant increase in heat transfer at a multiple decrease in the liquid layer height. Measurements made with application of the particle image velocimetry showed that the structured surface increases liquid speed inside the sessile drop. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, the heat transfer coefficient is lower than that for water in the entire period of evaporation on the structured surface. The maximal excess (20–30%) of α of the structured wall above the smooth surface corresponds to the maximal height of the liquid layer at the beginning of evaporation. With increasing time, the excess is reduced. A drop of heat transfer intensification with a decrease in the layer height relates to suppression of free convection (a multiple decrease in the average velocity in the drop).

Crystallization of Nanocrystals in Spherical Confinement Probed by in Situ X-ray Scattering.

We studied the formation of supraparticles from nanocrystals confined in slowly evaporating oil droplets in an oil-in-water emulsion. The nanocrystals consist of an FeO core, a CoFe2O4 shell, and oleate capping ligands, with an overall diameter of 12.5 nm. We performed in situ small- and wide-angle X-ray scattering experiments during the entire period of solvent evaporation and colloidal crystallization. We observed a slow increase in the volume fraction of nanocrystals inside the oil droplets up to 20%, at which a sudden crystallization occurs. Our computer simulations show that crystallization at such a low volume fraction is only possible if attractive interactions between colloidal nanocrystals are taken into account in the model as well. The spherical supraparticles have a diameter of about 700 nm and consist of a few crystalline face-centered cubic domains. Nanocrystal supraparticles bear importance for magnetic and optoelectronic applications, such as color tunable biolabels, color tunable phosphors in LEDs, and miniaturized lasers.

Laboratory test investigations on soil water characteristic curve and air permeability of municipal solid waste.

The air permeability coefficient has a high correlation with the water content of municipal solid waste. In this study, continuous drying methodology using a tension meter was employed to construct the soil water characteristic curve of municipal solid waste (M-SWCC). The municipal solid waste air permeability test was conducted by a newly designed apparatus. The measured M-SWCC was well reproduced by the van Genuchten (V-G) model and was used to predict the parameters of typical points in M-SWCC, including saturated water content, field capacity, residual water content and water content at the inflection point. It was found that the M-SWCC was significantly influenced by void ratio. The final evaporation and test period of M-SWCC increase with the increase in void ratio of municipal solid waste. The evolution of air permeability coefficient with water content of municipal solid waste depicted three distinct characteristic stages. It was observed that the water contents that corresponded to the two cut-off points of the three stages were residual water content and water content at the inflection point, respectively. The air permeability coefficient of municipal solid waste decreased with the increase of the water content from zero to the residual water content. The air permeability coefficient was almost invariable when the water content increased from residual water content to the water content at the inflection point. When the water content of municipal solid waste exceeded the water content at the inflection point, the air permeability coefficient sharply decreased with the increase of water content.

Flame propagation through three-phase methane-hydrate particles

A new mathematical solution for the laminar combustion of a spray containing three-phase particles is derived. The three-phase particles are represented by methane hydrate (MH) particles. These particles melt and evaporate following a spherical symmetric model that demonstrates the unique nature of that process. The methane-hydrate particles are uniformly distributed, yet the gaseous methane is not initially premixed, until they reach the particle depletion front. A one-dimensional, laminar flame then propagates into a homogeneous mixture of oxidizer, inert gases and small methane-hydrate particles. Characteristics of the laminar methane-hydrate spray combustion are examined at different methane-to-water mass ratio values within the particles. The methane-hydrate evaporation model serves as a building block for the evaluation of these characteristics. Previous constraints on the evaporation front are relaxed. Instead, the unique profiles of the evaporation process of the methane-hydrate particles are integrated over the entire evaporation period. Thus, an energy balance is employed to evaluate the flame location, velocity and temperature at which all liquid is evaporated. Using this approach it is shown that modeling the initial transient evaporation stage is crucial in MH and leads to non-linear evaporation characteristics. This in turn alters the dynamics of the supported flame temperature, location and velocity. Particle loading is shown to have significant impact on the evaporation and flame as well.

Local heat transfer to an evaporating superhydrophobic droplet

The heat flux distribution beneath a superhydrophobic evaporating droplet has been explored. High speed, high resolution infrared thermography was employed to measure the heat transfer characteristics of the evaporating droplet. Optical imaging and analytical techniques were used the capture droplet dynamics over the course of its evaporation. The droplet evaporated with a receding contact line predominantly in the constant contact angle regime. The peak local convective heat transfer was located at the triple contact line over the entire evaporation period. Peak and average heat fluxes were shown to increase as the evaporation proceeded due to the increasing contact line length density and liquid-gas interface temperature. The total thermal power across the solid-liquid interface decreased due to the decreasing solid-liquid surface area. The average heat flux to the evaporating droplet was shown to vary linearly with contact line length density.