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Spray ignition represents a critical process in numerous propulsion and energy conversion devices. The combustion of single droplet is low cost utilization and an effective analysis method for fuel characterization based on its properties. Nowadays, many investigations had been done in single droplet combustion like free falling droplet, suspended droplet in normal gravity and microgravity. A review is presented of recent developments of the combustion of single droplets with discussion and future work in the field. This paper reviews the finding in the literature up to now in this evolving field specifically the experiment detail and the observation area. Moreover, it highlights the prospect experimental that can extend to explore for further research. [ABSTRACT FROM AUTHOR]

• Evaporation of nanofluid droplets was studied by coarse-grained MDS. • Stronger solid–liquid interactions and higher initial volume fractions increase the deviation of the d2- law. • Several typically morphologies of dryout nanofluid droplet were observed. • The underlying mechanism of crust formation can be characterized by Pe· φ 0. The evaporation of nanofluid droplet is an important process in nanofluid combustion, spray drying, and so on. We investigate the influence of nanoparticles on the evaporation behavior of spherical droplets by coarse-grained molecular dynamics simulations. The results show that the solid–liquid interaction and the initial volume fraction of the nanoparticles can greatly affect the crust formation. The underlying mechanisms can be characterized by multiplying the Peclet number(Pe) and initial particle concentration(φ 0). Greater Pe· φ 0 can result in morphologies with crust due to the quick movement of nanoparticles to droplet surface. Finally, a phase diagram is constructed to predict the morphology of the dryout nanofluid droplets. Our results extend the fundamental understanding of the mechanisms that control the drying of droplet suspensions. [ABSTRACT FROM AUTHOR]

For a liquid fuel, its vaporization characteristics are important. The determination of vaporization rates for alternative fuels such as biodiesel and its diesel blends is reported in this work. For this purpose, suspended droplet technique in conjunction with optical measurement of the droplet diameter is used. The physical processes occurring during the vaporization are studied at temperatures lower than the boiling point of the respective fuels to ascertain all the regimes of vaporization. Further, the vaporization characteristics are related to the composition of the fuel, which governs its thermal and physical properties. This study becomes important due to absence of such a comprehensive data in literature, and attempts to create a useful data-base for vaporization rates and related features of biodiesel fuels produced from sources such as jatropha, karanja, palm and coconut, and their diesel blends. The overall evaporation rates of neat and fuel blends are reported systematically. For neat fuels, the vaporization rates are seen to correlate directly with two parameters namely, heat transport and mass transport parameters, constituted using the gas-phase thermal and physical properties, as well as the liquid-phase density. Also, the vaporization rate is seen to inversely vary with a parameter called long chain saturation factor. All these parameters depend on the composition of the fuel. [ABSTRACT FROM AUTHOR]

The evaporation characteristics of karanja biodiesel and its blends with diesel measured in a suspended droplet facility are reported in this article. The results have been compared with the evaporation rates of pure single component hydrocarbon fuels, such asn-decane,n-heptane, and neat fossil diesel. Unlike pure hydrocarbons, the evaporation rate for karanja biodiesel is seen to be linear. The evaporation rate decreases as the biodiesel content in the blend increases and its variation tends to be nonlinear particularly at higher temperatures. The karanja and its blends with diesel have a longer heating phase and droplet lifetime compared to diesel. [ABSTRACT FROM PUBLISHER]

Highlights: [•] An experimental study of micro-explosion by means two different approaches has been carried out. [•] Temperature of micro-explosion and fall in temperature after micro-explosion are evaluated. [•] Separation of the continuous and dispersed compounds has been observed. [•] The influence of the thermocouple and the heat transfer are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents a two-phase numerical model to simulate transient vaporization of a spherical two-component liquid fuel droplet. The model considers variation of thermo-physical properties in both liquid- and vapor-phases, as functions of temperature and species concentrations. Multi-component diffusion and surface tension effects are also considered. The model has been validated using the experimental data available in literature. The validated model is used to study the vaporization characteristics of both suspended and moving methanol–ethanol blended droplets in an atmospheric pressure environment. Relative strengths of forced convection and Marangoni convection are studied and compared for both suspended and moving droplets. Results in terms of streamlines, isotherms and isopleths at different time instants are reported and discussed. For low relative velocities, solutal Marangoni effects are seen to be important. [Copyright &y& Elsevier]

Abstract: A numerical model for the complete thermo-fluid-dynamic and phase-change transport processes of two-component hydrocarbon liquid droplets consisting of n-heptane, n-decane and mixture of the two in various compositions is presented and validated against experimental data. The Navier–Stokes equations are solved numerically together with the VOF methodology for tracking the droplet interface, using an adaptive local grid refinement technique. The energy and concentration equations inside the liquid and the gaseous phases for both liquid species and their vapor components are additionally solved, coupled together with a model predicting the local vaporization rate at the cells forming the interface between the liquid and the surrounding gas. The model is validated against experimental data available for droplets suspended on a small diameter pipe in a hot air environment under convective flow conditions; these refer to droplet’s surface temperature and size regression with time. An extended investigation of the flow field is presented along with the temperature and concentration fields. The equilibrium position of droplets is estimated together with the deformation process of the droplet. Finally, extensive parametric studies are presented revealing the nature of multi-component droplet evaporation on the details of the flow, the temperature and concentration fields. [Copyright &y& Elsevier]

Three-phase liquid-phase microextraction (LLLME) was developed for high performance liquid chromatography and capillary electrophoresis. The new branch of this mode, entitled suspended droplet liquid phase microextraction (SD-LPME), involves the use of a suspended droplet of an aqueous solvent. In this technique, the droplet is floated freely in the surface-center of an immiscible organic solvent, which has been laid on the surface of the aqueous sample while being agitated by a stirring bar. The performance of this technique is demonstrated in the determination of one of benzodiazepines, lorazepam, which is extracted into a single drop of aqueous solution. SD-LPME has provided good enrichment (645-fold), but relatively poor reproducibility: RSD%: 5.04, n = 5 (primarily due to repeated manual manipulation), simplicity, relatively fast extraction and back-extraction times (30.0 and 60.0 seconds, respectively). This method has allowed a direct transfer of the extracted analyte into a high performance liquid chromatography-diod array detector (HPLC-DAD). [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a novel suspended droplet microextraction method was developed for the detection of trace of organic compounds in water samples. The process was executed in a rotating extraction vial without the use of a stir bar. A single drop of octan-1-ol placed on top of the water sample was used as the solvent. The droplet remained on top of the water sample as a thin layer with an expanding surface area during the extraction stage, while during the sampling stage, the droplet was collected and sampled by inserting a needle. The volume of the microdroplet used was 3μL or less, to ensure high organic compound sensitivity. The microextraction experimental setup was simple, utilizing centrifugal forces and possesses the advantages of low cross-contaminant/interference and applicability to water samples apt to emulsification. Nitrobenzene was selected as a model organic compound, and samples were analyzed using gas chromatography (GC) or UV–vis spectrometry. Analysis of the microextraction method results showed a relative standard deviation (RSD) less than 3.82%. [Copyright &y& Elsevier]

The experimental temperature transition and freezing times of sucrose solution droplets were acquired to validate a numerical model. In addition, experiments were undertaken to determine the effect of sucrose on the external mass transfer from droplets. Results showed that the addition of a solute significantly affected the mass transfer from freezing droplets. For the accurate modelling of any spray crystallisation process of aqueous solutions, the solute effects must be quantified. Comparisons of numerical model predictions against experimental data revealed that discrepancies increased with decreasing air temperature. This is likely to be due to solidification becoming increasedly non-equilibrium at high freezing rates. Therefore, to increase the accuracy range of the numerical modelling approach, non-equilibrium effects will need to be incorporated in future models. [Copyright &y& Elsevier]

A comprehensive numerical model is developed to study the evaporation of a suspended droplet in forced convective high-pressure environments. This model includes real gas effects, liquid-phase internal circulation, variable thermophysical properties, high-pressure effects, solubility of inert species into the liquid phase, and gas- and liquid-phase transients. Numerical predictions for the suspended droplet within a zero-gravity environment are in very good agreement with the microgravity experimental data. Numerical results show that at higher ambient pressure the droplet swells initially due to the heat-up of the cold droplet, and its subsequent regression rate is far from following the d² law during the early stages of droplet evaporation. The numerical results also show that the droplet lifetime decreases with increasing ambient pressure or ambient temperature. The center temperature of the droplet at lower ambient pressure follows the surface temperature faster than at higher ambient pressure. The mass fraction of nitrogen dissolved at the droplet surface (liquid-phase side) increases with time at very high ambient pressure, such as 8 MPa, while it keeps almost the same level for its entire lifetime at the lower ambient pressure, such as 0.1 MPa. The results also indicate that the solubility of nitrogen cannot be neglected at higher ambient pressure; however, the solubility of nitrogen can be neglected at low ambient pressures. [ABSTRACT FROM AUTHOR]

The objective of this study was to develop a simple experimental and numerical method to study the temperature transition of freezing droplets. One experimental approach and several numerical methods were explored. For the experimental method, a droplet was suspended in a cold air stream from the junction of a thermocouple. The droplet’s temperature transition was able to be accurately measured and the freezing of the droplet observed. The numerical models developed were able to predict the temperature transition and the freezing time of the droplet. Of the numerical methods, a simple heat balance model was determined to be an accurate means of predicting the freezing time of the droplet. [Copyright &y& Elsevier]

In this study, a set of GTL–diesel fuel blends (G20, G50, G80, and G100, where the number represents the percentage of GTL fuel in the fuel blend) are prepared. Subsequently, using the suspended droplet method in a controlled heating chamber, the evaporation behaviour of these GTL fuel blends is visualised using a high-speed camera connected to a long-distance microscope. It is found that, among the tested fuel blends, puffing is not observable for G100, whereas micro-explosions are absent for G20. In comparison, the remaining fuel blends experience both these phenomena. In addition, the highest enlargement factor is observed for G20, followed by G50 and G80, whereas G50 has the highest micro-explosion intensity, followed by G80 and G100. Finally, the numbers and sizes of the child droplets are determined by adjusting the detection threshold, and it is found that G50 has the highest number of child droplets, followed by G80, G100, and G20. The results indicate that the presence of 50% GTL fuel in a GTL–diesel fuel blend (by volume) can lead to the best droplet micro explosions compared to the rest of the tested fuel blends. • The GTL fuel can reach the superheated state and promote micro-explosions at 500 °C. • 50% GTL fuel in a GTL–diesel fuel blend yield to the best micro explosions. • Entrapment of lighter evaporated components by heavier components yield to puffing. • Micro-explosion result from release of the accumulated energy. • Higher surface tension fuel blend result in better micro-explosion. [ABSTRACT FROM AUTHOR]

• We produced HFO emulsions using ultrasonically induced cavitation and measured their droplet size distribution with Cryo-SEM. • We created a new methodology for the analysis of suspended droplet experiments when thermally-induced secondary atomization occurs. • The new methodology adopts a Dynamic Mode Decomposition of the normalized distance of the interface from the centroid of the droplet. • It is possible to reconstruct the evolution of the droplet from the first 2 modes available with reasonable accuracy. • The first two modes and their dynamics can be related to evaporation and breakup. Clean and efficient processing of heavy fuels is a major challenge for several combustion driven prime movers like internal combustion engines, used in marine or power generation sectors. Emulsification was recognized in the past as practical technology for heavy fuels combustion since it engenders an enabling phenomenon called micro-explosion that proceeds during the spray process. Micro-explosions allow finer secondary break-up, leading to improved mixing, and subsequent cleaner and fuller burning. However, the translation of this technology to real applications is still not fully exploited due to lack of basic understanding and characterization of the evaporation process which includes both micro-explosions and puffing. Ultrasonically induced cavitation is a promising technology for the production of water-in-oil emulsions at industrial scale. Fundamental research performed in the field of liquid fuels gasification and combustion mostly regards ideal or simple mixtures and not all the considerations made in these cases apply for real fuels. In this work, we investigated the evaporation characteristics of ultrasonically produced heavy fuel oil (HFO) emulsions with a set of newly developed methodologies. We characterized the emulsions by using a state-of-the-art microscopy technique, the Cryogenic Scanning Electron Microscopy, Cryo- SEM and obtained accurate droplet size distribution up to nano-scale. We tested the fuel emulsion in a suspended droplet experiment and reconstructed the interface from the obtained images. The normalized squared diameter profile is not representative of the complex physics involved in heavy fuel evaporation; therefore, it was substituted with the normalized distance of the interface from the centroid of the droplet. By using this procedure, it is possible to highlight both evaporation and stochastic events like puffing and ejections. A dimensionality reduction algorithm, the dynamic mode decomposition (DMD), was then performed on the evolving interface to highlight the main modes describing the emulsion system and the dynamics. The overall objective was to develop a strategy for optimizing emulsions for improved combustion performance. From the experimental data, it was observed that a water concentration of 5% by mass decreases the vaporization time of the mixture and that the presence of water favors puffing and ejections with different intensity depending on the percentage of water enhancing the volatilization of the fuel. [ABSTRACT FROM AUTHOR]

An investigation was performed to determine the heating characteristics of sludge pyrolysis oil (SPO), heavy fuel oil (HFO), and their blends. The ignition temperature, burnout temperature, flammability index (C), and combustion characteristics index (S) of various fuels were evaluated using a thermogravimetric analysis (TGA). Furthermore, the heating behavior of the SPO, HFO droplets, and their blends was also investigated using a suspended droplet experimental system. The TGA results showed that the combustion process had three stages. In the volatile decomposition and burning stage, the fuel blends with a higher SPO content exhibited higher flammability and combustion characteristics indexes due to the release and oxidation of the lightweight components. However, there were less differences in the flammability and combustion characteristics indexes for the high weight components that burned out in the final stage. Distinct stages could be observed in the suspended droplet experiments. Different vaporization/combustion rates for different fuels were also evaluated, and the results showed that adding SPO to HFO significantly improved combustion performance. A constant combustion rate was obtained with the addition of SPO to HFO. A higher SPO content in the fuel blend increased the frequency and magnitude of the micro-explosion and reduced residual products. • Combustion behaviors of sludge pyrolysis oil and heavy fuel oil were studied. • Sludge pyrolysis oil improves the fluidity and enhances combustion performance. • Sludge pyrolysis oil enhances combustion performance for volatile components. • The droplet vaporization/combustion rates of blended fuels were evaluated. • Sludge pyrolysis oil increased the frequency and magnitude of the micro-explosion. [ABSTRACT FROM AUTHOR]

• Fluid flow and heat transfer in a laser-heated suspended droplet is numerically investigated. • Marangoni flow is induced by the localized heating of a focused laser. • Increasing the laser power intensifies the Marangoni flow inside the droplet. • Increasing the laser beam size decreases the non-uniformity of the temperature distribution. • Marangoni flow and heat transfer highly depends on surface tension temperature coefficient. In this study, the Marangoni flow and heat transfer inside a laser-heated water droplet suspended in the immiscible oil phase is numerically investigated. It is shown that the Marangoni flow is induced by non-uniform temperature distribution across the droplet interface caused by the localized heating source. Effects of the laser power, laser beam size and surface tension temperature coefficient are investigated. Simulation results show that the temperature rising rate and average temperature rising linearly increase with laser power due to more heat generated by photothermal effect and the Marangoni flow is intensified due to larger interface temperature gradient. With increasing laser beam diameter, the temperature distribution becomes more uniform due to more uniform laser energy distribution. The Marangoni flow is reverse when the surface tension temperature coefficient changes from the positive to the negative and vice versa, resulting in the transformation of the Marangoni flow and temperature distribution. [ABSTRACT FROM AUTHOR]