Fuel Droplets Research Articles

SIMULATION OF LIQUID FUEL SPRAY FORMATION AND DISTRIBUTION IN A REACTING TURBULENT FLOW

The paper presentsthe computational experiments of the liquid fuels spray and its droplets distribution in a turbulent reacting flow. Primary and secondary atomization of two types of liquid fuel droplets (isooctane and dodecane) in the presence of combustion was described by the equations of continuity, momentum, internal energy, concentration of components of reacting substances and a two-parameter model for calculating turbulent flow. The study results of the spray, dispersion and combustion of droplets of hydrocarbon liquid fuels in a model combustion chamber when changing the injector injection angle were obtained. The injection angle values varied from 2 to 10 degrees. Based on the computational experiment temperature profiles and concentration characteristics of combustion products and gas in the combustion chamber at various times were obtained. Numerical calculations of the droplets’ Sauter mean diameter distributions have the same dispersion pattern for dodecane. This suggests that the accuracy and adequacy of developed complex model of the formation and distribution of spray in a reacting flow has been confirmed by its strong correlation and good agreement of the modeling results with experimental data from other researchers. This kind of modeling methods and the obtained computational experiments results from them are widely used not only in traditional thermal power engineering, but also in the study of technological processes in the new generation engines chambers, combustion of alternative types of fuels and optimization of their combustion.

Surface deformation of moving droplets of slurry fuels

Experimental research findings are reported on the characteristics of surface transformation of droplets of promising fuel slurries in the air, as they move at subsonic velocities typical of combustion chambers of power plants. The main components of the fuels were water, coal processing waste, and coal. Typical shapes of droplets and the duration of their deformation cycles were identified. Droplets containing more than 70% of the solid phase remained practically undeformed. The lowest relative velocities of droplets leading to their fragmentation were determined. The key characteristics of secondary droplets (their number, sizes, velocities, and surface area of liquid) were calculated on the basis of the experimental findings. These characteristics were compared with those of initial droplets. Disruption conditions in the chosen range of the gas jet pressure (P ≤ 6 bars) can only be provided for fuel slurry droplets containing less than 60% of a coal component. The effect of a group of factors on deformation characteristics was identified. These include air jet and droplet velocities, droplet sizes, temperature, concentration, and type of components and additives. Approximation equations were derived for the mathematical description of the experimental data. Using certain criteria, the conditions necessary and sufficient for the disruption of water–fuel slurries on impact with an air jet were estimated.

Secondary crushing of droplets of water-oil emulsions

When fuel oil is supplied to an accurate chamber, its spraying process takes place due to injectors and sprinkler devices. This process is called primary droplet crushing. It is often not enough to burn fuel efficiently. Since the size of fuel droplets in the combustion chamber often reaches several millimeters, which increases underburning and leads to uneven burnout. In such cases, it is advisable to use secondary crushing of water-oil fuel droplets. Secondary crushing of droplets reduces the average size of droplets in the fuel spray torch by several times. At the first stage of secondary crushing of droplets, their collisions with each other in the jet are realized, after which the formed droplets are subjected to collisions with the walls of thermal equipment. At the next stage, pyrolysis of such droplets occurs at the periphery of the jet, which leads to the formation of solid particles and subsequent collisions of droplets of water-oil fuels with them. At the final stage, the formed secondary fragments are subjected to intense heating in the combustion chamber, which allows for micro-explosive grinding. The present study is aimed at studying the characteristics of secondary crushing of water-oil fuel droplets with the addition of specialized additives. The results of the conducted studies have shown that the use of an additive based on a special combination of positively and negatively charged ions reduces the size of fuel droplets by 25%. It has been found that when using such an additive, the ratio of the free surface areas of droplets increases several times with a combination of all secondary grinding modes.

Combustion Behavior of Fuel Droplets with Metallic, Non-Metallic and Organo-Metallic Boron Additives

ABSTRACT There is considerable interest in the utilization of fuels derived from boron materials, with their high calorific value, in various applications ranging from propellants to pyrotechnics. Conversely, their impact on the combustion behavior of conventional hydrocarbon fuels remains largely unclear. In this study, ignition, combustion, micro-explosion and simultaneous atomization behaviors of gasoline-based alternative fuels containing metallic (28–35 µm MgB2), nonmetallic (1 µm amorphous boron, 10 µm AlB12 with 86–88% and 95–97% purity) and organo-metallic boron derivatives (triethyl borate (TEB) containing C2H5 groups and trimethyl borate (TMB) containing CH3 groups) were investigated. The experiments were carried out at droplet scale and recorded using a high-speed camera and a thermal camera. The findings revealed a systematic reduction in ignition delay for each gasoline fuel enriched with boron derivatives. 2.5%AlB12/G and pure TMB droplets were the fastest extincting fuel droplets (0.9678 s and 1.245 s, respectively). The highest maximum flame temperature was recorded as 626 K and 610 K for pure TMB and 80%TEB/G, respectively. Droplet diameter regression analyses showed that the diameters of fuel droplets containing predominantly metallic and nonmetallic boron derivatives decreased in accordance with the d 2 -law. This study demonstrated that cost-effective and easily producible amorphous boron and organometallic boron derivatives are promising energy carriers for hydrocarbon fuels.

Experimental Study of Evaporation Characteristics of Acoustically Levitated Fuel Droplets at High Temperatures

Examining fuel droplet evaporation is crucial for enhancing fuel engine efficiency, conserving energy, and reducing emissions. This study utilizes experimental methods involving ultrasonic standing wave levitation and high-speed cameras to investigate the impact of temperatures and droplet properties, including initial diameter and composition, on the evaporation process. The evaporation behaviors of fuel droplets, like hexadecane and diesel, are documented across a temperature spectrum spanning 150 °C to 550 °C, with an initial droplet equivalent diameter ranging from 0.10 to 0.30 mm. The evaporation rate positively correlates with temperature and may vary by 15% to 71% between hexadecane and diesel droplets.

Combustion Characteristics of r-GO/g-C3N4/LaFeO3 Nanohybrids Loaded Fuel Droplets

ABSTRACT Graphene oxide (GO), reduced graphene oxide (r-GO) and graphitic carbon nitride (g-C3N4) are two-dimensional carbon-based nanosheets that show promise in reducing emissions with their superior catalytic activity in capturing species such as NOx and CO2 thanks to their oxygen-based functional groups and active edges on their surfaces. These active surfaces also provide a scheme for the substitution of materials with high calorific value or high catalytic activity for combustion. This study focuses on the fabrication of functional nanohybrid structures customized for combustion with LaFeO3 metal oxide nanoparticles substituted on these nanosheets and their effect on the combustion behavior of gasoline. The fabrication of r-GO/g-C3N4/LaFeO3 nanohybrid structures was carried out by a two-step hydrothermal method. The structural characterizations of the samples were confirmed by SEM and XRD analyses and their chemical states were confirmed by Raman and XPS techniques. Combustion experiments were carried out by droplet scale combustion of gasoline-based nanofuel droplets containing dilute (0.2 wt.%) and high (0.7 wt.%) concentrations of GO, r-GO, g-C3N4, g-C3N4/LaFeO3 and r-GO/g-C3N4/LaFeO3 nanoparticles. The experimental process was recorded with a high-speed camera and a thermal camera. The nanofuel droplets containing 0.2 wt.% g-C3N4/LaFeO3 nanohybrid structures had the highest maximum flame temperature of 519 K, and the nanofuel droplets containing 0.7 wt.% r-GO/g-C3N4/LaFeO3 particles had the highest maximum aggregate temperature of 1177 K. The ignition delay time decreased for all droplets with 0.2 wt.% and 0.7 wt.% particle loadings. At 0.2 wt.% concentration, g-C3N4 doped fuel droplets exhibited the lowest extinction time, while at 0.7 wt.% concentration, the lowest extinction time was measured for r-GO/g-C3N4/LaFeO3 doped fuel droplets. Fuel droplets containing g-C3N4 particles had the highest burning rate and were the fastest extinguishing fuel droplets in the electric field. In this study, it has been demonstrated that the combustion rate and energy value of hydrocarbon fuels can be increased and soot formation can be reduced at the same time with the new generation of graphene-based functional materials to be created, and thus, many combustion problems can be solved simultaneously with these functional particles.

The Modeling of Fuel Auto-Ignition Delay and Its Verification Using Diesel Engines Fueled with Oils with Standard or Increased Cetane Numbers

This article contains the results of mathematical modeling of the self-ignition delay (τc sum) of a single droplet for various fuels, and the results of measurement verification (τc) of this modeling in diesel engines. The result of modeling the τc sum (as a function of the diameter and ambient temperature of the fuel droplet) revealed two physical and two chemical stages that had different values of the weighting factor (WFi) in relation to the total delay of self-ignition. It was also found that the WFi values of individual phases of the self-ignition delay differed for different fuels (conventional and alternative), and in the total value of τc sum. The measured value of the self-ignition delay (τc) was determined in tests using two diesel engines (older—up to EURO II and newer generation—from EURO IV). The percentage difference in the Δτc sum value obtained from modeling two fuels with different cetane number values was compared with the percentage difference in the Δτc value for the same fuels obtained during the engine measurements. Based on this analysis, it was found that the applied calculation model of the self-ignition delay for a single fuel droplet can be used for a comparative analysis of the suitability of different fuels in the real conditions of the cylinder of a diesel engine. This publication relates to the field of mechanical engineering.

Time-Varying Oscillatory Response of Burning Gel Fuel Droplets.

Gel fuel droplets exhibit disruptive burning due to the rupture of their gellant shell, which causes the release of unreacted fuel vapors from the droplet interior to the flame in the form of jets. In addition to pure vaporization, this jetting allows convective transport for fuel vapors, which accelerates gas-phase mixing and is known to improve droplet burn rates. Using high-magnification and high-speed imaging, this study found that the viscoelastic gellant shell at the droplet surface evolves during the droplet's lifetime, which causes the droplet to burst at different frequencies, thereby triggering a time-varying oscillatory jetting. In particular, the continuous wavelet spectra of the droplet diameter fluctuations show that the droplet bursting exhibits a nonmonotonic (hump-shaped) trend, where the bursting frequency first increases and then decreases to a point where the droplet stops oscillating. The changes in the shell structure are captured by tracking the temporal variation of the area of rupture sites, spatial movement of their centroid, and the degree of overlap between the rupture areas of successive cycles. During the initial period (immediately following its formation) when the shell is newly formed, it is weak and flexible, which causes it to burst at increasingly high frequencies. This is because the area at and around the rupture site becomes progressively weaker with each rupture in an already weak shell. This is shown by a high degree of overlap between the areas of successive ruptures. On the other hand, the shell flexibility during the initial period is demonstrated by a reversal in the motion of rupture site centroids. However, at later stages when the droplet has undergone multiple ruptures, the depletion of the fuel vapor causes accumulation of gellant on the shell, thus causing the shell to become strong and rigid. This thick, strong, and rigid shell suppresses droplet oscillations. Overall, this study provides a mechanistic understanding of how the gellant shell evolves during the combustion of a gel fuel droplet and causes the droplet to burst at different frequencies. This understanding can be used to devise gel fuel compositions that result in gellant shells with tailored properties, and therefore, control the jetting frequencies to tune droplet burn rates.

Characteristics of child droplets during micro-explosion and puffing of suspension fuel droplets

The paper presents the findings of experimental research whereby it was possible to determine the number and size of child droplets formed during puffing and micro-explosion of droplets of suspension fuels. The following main suspension components were used: water, rapeseed oil, coal, wood, rubber. For the first time, the impact of a group of the following factors on the child-droplet behavior has been comprehensively investigated: initial droplet size, heating temperature, component concentration, as well as the size and concentration of solid particles. The conditions for the consistent occurrence of two modes of droplet fragmentation were identified: micro-explosion and puffing. Recommendations were formulated for the development of secondary atomization technologies for liquid and composite fuel droplets by means of micro-explosion.

Conditions for and Characteristics of the Dispersion of Gel Fuel Droplets during Ignition

We formulated and experimentally proved a hypothesis on the causes of dispersion (puffing and microexplosion) of binary fuel droplets, including those in the composition of gel fuels. This hypothesis is based on the concepts of wetting thermodynamics and the theory of the two-component surface energy of substances and materials. An effective and reliable criterion was established that allowed the assessment of the possibility of the onset of puffing and microexplosion during the high-temperature heating of binary liquids. Microexplosions were found to occur only when isothermal conditions were necessarily reached at the liquid–liquid interface during the mixing of mutually insoluble components, provided that one component had to be polar, and the second had to be dispersive. In addition, it was necessary to provide external heating conditions under which the value of the surface free energy of the liquid–liquid interface formation tended to zero.

Plasma Actuation for the Turbulent Mixing of Fuel Droplets and Oxidant Air in an Aerospace Combustor

In order to explore plasma-assisted turbulent mixing in aerospace engines, the dielectric barrier discharge plasma actuation for the turbulent mixing of fuel droplets and oxidant air in a ramjet combustor was studied using computational fluid dynamics. A two-way coupling of turbulent air and discrete droplets was realized by Eulerian–Lagrangian simulation, and the dielectric barrier discharge plasma action on flow was modeled by body force. The results show that the plasma actuation can rearrange the recirculation zone behind the evaporative V-groove flameholder, and the main mechanism of actuation is to increase the local momentum of the fluid; the actuation dimension, actuation intensity, and actuation position of the dielectric barrier discharge plasma have strong effects on the turbulent mixing of fuel droplets and oxidant air; and a relatively optimal turbulent mixing can be achieved by adjusting the actuation parameters.

Характеристики прогрева и испарения капель реактивного топлива и его суррогатов

The paper presents the results of experimental study of heating and evaporation of TS-1 aviation kerosene and its surrogates (n-decane and SU4) in a high-temperature gas flow (gas temperature range is 450–750 K, gas flow velocity range is 0–6 m/s). The conditions for temperature and gas flow rate are established, under which it is possible to provide good agreement on the time of complete evaporation, the dynamics of changes in the sizes (radii) and average temperatures of droplets of traditional aviation fuels using the example of kerosene TS-1 and its surrogates (SU4 and n-decane). When processing videograms, we used our own image processing algorithms developed in the MATLAB software package.

Effect of the relative arrangement of atomizers on composite liquid fuel spraying characteristics

The paper presents the experimental research findings for the characteristics of promising composite liquid fuel spraying with two atomizers, when varying the scheme of their relative arrangement. The nozzle arrangement was shown to have a significant effect on the spraying characteristics (droplet sizes and velocities), the volume and rate of the combustion chamber filling with aerosol. Following the experimental research, the relative efficiency indicators of spraying water and composite fuels were calculated. The research findings for the spraying of water and composite liquid fuels based on coal and coal processing waste were compared. It was established that in the interaction of sprayed liquid jets, it is possible to control the average size and, thus, number of secondary fragments by intensifying their transformation during coalescence and disruption. A mathematical processing of the experimental results was performed using a set of dimensionless criteria introduced. Schemes were shown that make it possible to provide minimum sizes of slurry droplets and maximum relative efficiency factor of spraying. • One- and two-atomizer spraying characteristics of fuels were compared. • Droplet number, sizes, velocities and trajectories were determined. • Fuel droplet sizes can be changed by the liquid jet interaction with air. • Maximum efficiency factor of fuel spraying was determined. • Spraying schemes for more complete filling of the chamber with fuel aerosol were proposed.

State of the Art of Fuel Droplet Evaporation

The process of fuel droplet evaporating is one of the most important factors that directly affect the efficiency of the combustion process. Therefore, the current study reviews previous studies that focused on the process of evaporation of a drop of fuel. The review is divided into points. The first part is concerned with modeling of the evaporation process under different initial condition and temperature of the droplet. The second part present the experimental studies concerned with measuring the evaporation time of the droplet, as well as the shape of the droplet during the evaporation process. Most of the studies related to this subject can be divided into three categories: The first category is the studies that are concerned with the process of heating the droplet and studying the evaporation time for different types of fuels or by adding nanomaterials to the fuel and studying their effect on the evaporation process. The second category is the studies concerned with the mechanics of droplet evaporation and the study of droplet shape. The third category is the studies that focus on studying the effect of initial conditions, such as temperature and pressure, as well as the concentration of gasses surrounding the drop and their types. There are other studies concerned with projecting the electric field onto the drop during the evaporation process and studying its effect.

Jetting Dynamics of Burning Gel Fuel Droplets.

Jetting in burning gel fuel droplets is an important process which, in addition to pure vaporization, enables the convective transport of unreacted fuel vapors from the droplet interior to the flame envelope. This aids in accelerating the fuel efflux and enhancing the mixing of the gas phase, which improves the droplet burn rates. In this study, Schlieren imaging was used to characterize different jetting dynamics that govern the combustion behavior of organic-gellant-laden ethanol gel fuel droplets. To initiate jetting, the gellant shell of the burning gel fuel droplet was subjected to either oscillatory bursting or isolated bursting, or both. However, irrespective of the jetting mode, the jets interacted with the flame envelope in one of three possible ways. Based on the velocity and the degree to which a jet disrupts the flame envelope, it is classified as either a flame distortion, a fire ball outside the flame or a pin hole jet (localized flame extinction), where the pin hole jets have the highest velocity (1000-1550 mm/s), while the flame distortion events have the lowest velocity (500-870 mm/s). Subsequently, the relative number of the three types of jetting events during the droplet lifetime was analyzed as a function of the type of organic gellant. It was demonstrated that the combustion behavior of gel fuels (hydroxypropyl methylcellulose: HPMC at 3 wt.%) that tend to form thin-weak-flexible shells is dominated by low-velocity flame distortion events, while the gel fuels (methylcellulose: MC at 9 wt.%) that facilitate the formation of thick-strong-rigid shells are governed by high-velocity fire ball and pin hole jets. Overall, this study provides critical insights into the jetting behavior and its characterization, which can help us to tune the droplet gasification and the gas phase mixing to achieve an effective combustion control strategy for gel fuels.

Evaporation Characteristics and Morphological Evolutions of Fuel Droplets After Hitting Different Wettability Surfaces

Evaporation Characteristics and Morphological Evolutions of Fuel Droplets After Hitting Different Wettability Surfaces

A molecular dynamics study of evaporation mode transition of hydrocarbon fuels under supercritical conditions

The mode transition of evaporation for single- and multi-component hydrocarbon fuels is investigated at the molecular level. This study scrutinizes first the subcritical and supercritical evaporation of n-hexadecane droplets and liquid films by molecular dynamics (MD) simulations. The mode regime map of n-hexadecane droplets is obtained. Then the mode transition of evaporation of a three-component droplet and a six-component droplet is studied. A critical dimensionless number τ0.9P of 0.5 based on the average displacement increment (ADI) of fuel atoms is used to identify the evaporation mode transition of fuels with any type and number of components. It is found that in the diffusion mode of evaporation, the entropy becomes the dominant factor in the evaporation of fuels, and the disorder of the fuel molecules increases significantly compared with that in the classic evaporation mode. Compared with the case of the quiescent droplet, with increasing relative velocity between the droplet and the ambient gas, the mode transition becomes easier, although this is a non-linear process. Fuel droplets and liquid films with different initial sizes are investigated to understand the size effect. In addition, for the same ambient temperature and pressure, the smaller the normalized specific heat transfer surface area of the fuel is, the easier the mode transition of evaporation is. A correlation was proposed to compare the possibility of mode transition of evaporation for single- and multi-component fuels.

Effect of Internal Decay Power and Surface Cooling on Nuclear Fuel Droplet Solidification During MFCI in a SFR—A Numerical Study

Melting of the nuclear core is one of the severe accident scenarios in a Sodium-cooled Fast Reactor (SFR). During such an event, molten corium may come into contact with the coolant sodium. This interaction of the molten fuel and the coolant is commonly termed molten fuel–coolant interaction (MFCI) in the nuclear industry. In this study, a numerical analysis is carried out to study the solidification of a molten fuel droplet in the liquid sodium pool. In the first part of the study, the effect of constant internal heat generation on the solidification of the droplet is evaluated with convective heat dissipation prescribed at the droplet surface. The internal heat generation (decay power) and the heat transfer coefficient are varied as parameters, and the time required for complete solidification of the molten droplet is obtained. Based on the results, the freezing of the droplet is categorized into three regimes: conduction limited, transition, and internal heat generation dominated regimes. It is observed that the solidification process of nuclear fuel droplets generated during MFCI is not influenced by internal heat generation and lies in a conduction-limited regime for decay power level prevailing in a medium-sized SFR. Hence, in the next part of the study, the numerical analysis is carried out by incorporating the time-dependent decay power and the temperature-dependent heat transfer coefficient in the computational model by developing user-defined subroutines depicting a realistic scenario of an accident. The results of the analysis show that because of the high subcooling of sodium, film boiling is ruled out; nucleate boiling with a maximum heat transfer rate occurs briefly. The heat transfer coefficient then declines as the interface temperature between the droplet and the sodium decreases rapidly until the natural convective regime is reached. A parametric study on the droplet diameter is also carried out by varying the diameter from 0.5 to 10 mm, spanning the typical particle size spectrum expected during MFCI.

Ignition of water-coal fuel droplets during radiative-convective- conductive heating in relation to boilers operating on the technology of circulating fluidized bed

• Inert material significantly accelerates the ignition of a drop of coal-water fuel. • The effect of an inert material is most pronounced at moderate temperatures (Tg = 873 K). • At high oxidizer temperatures, the inert material does not affect the ignition process. • Inert material leads to dispersion of the near-surface layer of the fuel. One of the most promising options for reducing anthropogenic heating of coal-fired power plants on the environment is the use of not homogeneous coal as the main fuel, but coal-water suspensions. But the combustion of coal-water fuels is associated with the solution of the problem of large values of the ignition delay times for such fuels. Therefore, an important task of great practical importance is the selection (or development) of technologies for combustion of coal-water suspensions in the furnaces of steam and coal-water boilers. The article presents the results of experimental studies of the coal-water fuel drops ignition processes. They were carried out in order to establish the temporal characteristics and conditions of ignition under conditions of a flare mode and combustion in a circulating fluidized bed of an inert material. According to the results of experiments, it was found that at moderate temperatures of the environment (T g ≤ 873 K), particles of inert material (sand) accelerate the ignition of coal-water fuel drops. With high-temperature heating, the ignition delay times for these two fuel combustion technologies are identical in relation to the combustion of coal-water fuels prepared from typical (for energy purposes) semi-anthracite and high-volatile A bituminous coal. Colloquium. The energy crisis in autumn 2021 showed the problem of the rapid transition of the world community from traditional thermal energy to the energy of non-traditional renewable energy generators (wind, sun, biomass). It became obvious that in the next few decades, developed countries will be forced to use coal as their main energy source. Therefore, at present, an urgent task is to reduce the concentration of anthropogenic oxides in the flue gases of coal-fired power plants. One of the most promising technological solutions to this problem is the combustion of coal in the composition of coal-water fuel (CWF). But the technologies of combustion of coal-water fuels in the furnaces of steam and hot water boilers have not yet been developed, mainly due to the problem of large values of the ignition delay times for droplets of such fuels. Therefore, furnace devices of the traditional design of the chamber type with flare combustion of CWF have not yet been developed. One of the promising technologies for the combustion of coal-water fuel can be the technology of a circulating fluidized bed of inert material. But the results of studies of the coal-water fuels drops ignition processes under conditions corresponding to combustion devices operating on the technology of a circulating fluidized bed have not yet been published.

Mathematical modeling of heat transfer in a droplet of coal-water fuel leading to its fragmentation

• Fragmentation model has been developed for heated droplets of fuel suspensions. • The main mechanisms of heat transfer are taken into account. • The heating conditions effect on the disintegration inertia has been determined. • Model allows for varying solid and liquid components concentrations in wide ranges. • Model is used to describe the conditions in the combustion chambers of power plants. Based on the results of the experimental studies performed, physical and mathematical models were developed to describe the processes of heat transfer in droplets of potential coal-water fuels (CWF) before their fragmentation caused by intense heating. The model is based on the solution of a system of heat transfer equations taking into account specific features of the CWF droplets fragmentation identified in the course of experiments. CWF droplet fragmentation delay times were recorded in the course of heating using the disintegration criterion identified in the experiments performed. In the case of CWF, complete evaporation of water from fuel composition droplet serves as the criterion. A series of experiments were carried out to test the model and to clarify the mechanism for implementing the processes investigated. The effect of a group of the following factors on the fragmentation delay times was determined: the temperature (773–2273 K), the initial droplet sizes (0.05–1.55 mm) and the component composition of the fuel (solid particles concentration varied between 40 and 60 wt%). The conditions for the intensification of the processes of fragmentation and ignition of potential fuel suspensions were singled out.