Droplet Combustion Research Articles

Certain studies on influence of nano catalysts Co3O4, SiO2 blended with CME-diesel in combustion

The growing interest for the alternative fuel has replaced the usage of diesel fuel in engines. This paper determines and evaluates the combustion characteristics of the new fuel blend comprising nanoparticles blended with cottonseed oil added diesel. The burning rates, variation of droplet size and temperature at various periods were studied. The fuel mix containing the blends of cottonseed oil in proportions of B20 (20% Cottonseed oil), B40 (40% Cottonseed oil) and B60 (60% Cottonseed oil) with Nano additive combinations of SiO2 (Silicon dioxide), Co3O4 (Cobalt oxide), Co3O4 + SiO2 with proportions of 100 ppm, 50 ppm (parts per million) and 25 ppm blended in diesel fuel by Ultrasonic dispersed techniques. The diameter of the injected fuel, time-taken to complete the combustion process, and the temperature of prepared fuel blends were tested using the self-designed droplet combustion chamber and the results were processed and analyzed. The time taken for the better blend proportions is expected to be 6 s favouring micro-explosion by eliminating agglomeration.

Effect of Polymeric Additives on Ignition, Combustion and Flame Characteristics and Soot Deposits of Crude Oil Droplets

ABSTRACT Many oil fires have resulted from the crude oil train derailments in recent years. Given the importance of crude oil shipping by rail to the energy security of the US, it is important to consider various methods that will decrease the likelihood of crude oil catching fire in case of a crude oil derailment. Present study examines the effect of polybutadiene polymer on the combustion properties and soot deposits of Bakken crude. Treating crude oil as multicomponent liquid fuels and polybutadiene as an additive, droplet combustion experiments were conducted with sub-millimeter sized spherical droplets suspended on very fine support fibers. Polybutadiene polymer additive of two different chain lengths has been investigated. Results show that both polymer chain length and origin of crude oil have a significant effect on various combustion properties like combustion rate, ignition delay, and total combustion time. Polymeric additives also change the soot deposit structure and particle size compared to the base fuel. Present research is envisioned to aid in understanding combustion behavior of crude oil as isolated droplets, as well as to generate interest in investigating more polymeric additives for liquid fuels.

Graphene oxide doped ethanol droplet combustion: Ignition delay and contribution of atomization to burning rate

Effects of graphene oxide nanomaterials addition and oxidation level on ignition delay and burning rate of ethanol droplets are experimentally investigated. Three graphene oxide samples are synthesized and characterized. Separate high-speed OH* chemiluminescence and high-speed shadowgraphy images are collected. The results suggest that increasing the loading concentration from 0 to 0.1% (by weight) generally increases the ignition delay, except for ethanol doped with the highly oxidized graphene. The results show that unlike pure ethanol droplets, atomization may occur for the doped ethanol droplets. It is demonstrated that, independent of the tested conditions, atomization occurs in the second half of the droplet lifetime. The probability density function of the atomized baby droplet diameter, initial projected velocity, and length of the projected trajectory are similar for all tested conditions and independent of the oxidation level and loading concentration of the additives. The joint probability density function calculated for the atomization-related parameters against one another suggests that the majority of the baby droplets feature a relatively short lifetime, indicating they may potentially burn inside the flame envelope. Using droplet surface regression curves versus time, the burning rate for periods in which the atomization does not occur, and for the periods that the atomization is present are estimated. The former burning rate is shown to enhance by increasing the loading concentration and reducing the oxidation level of graphene. However, supported by Fourier-Transform Infrared spectroscopy of the graphene oxide samples, it is found that a maximum increase in the latter burning rate for both loading concentrations occurs for ethanol doped with the graphene oxide that features maximum amount of infrared radiation absorption. To quantify the effect of atomization on the droplet mass loss, a conservation of mass framework is utilized, and it is shown that relatively intense atomization suppresses the droplet evaporation mass loss. Doping ethanol with graphene oxide and increasing the loading concentration from 0.01 to 0.1% enhances the overall burning rate, with a maximum enhancement of 8.4% pertaining to addition of reduced oxidized graphene oxide and for the loading concentration of 0.1%.

단파장 필터링을 통한 단일 액적 화염 내 CH₂O, CH, C₂ 화학종 분석 및 액적 연소 특성

This paper was performed to observe the flame behavior of the biodiesel, diesel, and n-heptane single fuel droplets and analyze the CH₂O, CH, and C₂ species in a single droplet flame included combustion characteristics experimentally. In order to visualize the combustion process of test fuel droplets, droplet combustion visualization and analysis were conducted using a high-speed camera and a CCD camera coupled with a short wavelength filter. A single fuel droplet was formed on the stainless wire using a syringe and ignited by a heat source provided from the power supply equipment. It was shown that the ignition delay of an n-heptane fuel droplet was shorter than those of biodiesel and diesel fuel droplets due to a higher evaporation rate in the liquid phase. A longer flame extinction and the flame life-time were also observed in the biodiesel and diesel fuel droplets than most n-heptane fuel droplets. In the case of the diesel fuel droplet, it was found that the flame life-time was slightly increased than the biodiesel fuel droplet. The C₂ species produced in the flame during combustion of all fuel droplets were distributed throughout the flame, while the CH and CH₂O chemical species tended to be located in the flame front and center area. As a result of combustion analysis, the biodiesel, diesel, and n-heptane fuels were shown the changes of the CH₂O mole fraction according to the two-stage combustion characteristics, and the mole fraction of CH species was decreased by CH + O₂ and CH + H₂O chemical reactions.

Experimental study on the evaporation and combustion characteristics of double Al/n-heptane based nanofluid fuel droplets in high temperature environment

Nanofluid fuels can be used in scramjet. However, the interactions among fuel droplets in spray and combustion process are still a gray area. The study on the evaporation and combustion of the double droplets can better demonstrate the role of nanofluid droplets. In this paper, the evaporation and combustion characteristics of the double droplets of aluminum/n-heptane based nanofluid fuels were investigated by a droplet-suspension method, whose results were compared to those of the single droplet at 973 K and 0.1 MPa. The effect of the distance of the double droplets on their evaporation behavior, ignition delay time, combustion phenomena, and micro-explosion time were studied. The influence of thermocouple wire was also analyzed. The double droplets have a synergistic effect on the evaporation behavior, ignition delay time, and micro-explosion. The ignition delay time of double droplets of the aluminum/n-heptane based nanofluid fuel droplet has debased 25.3% compared with that of the single droplet, while the maximum micro-explosion duration has increased 7.2%. With the increase of the distance of both droplets, the synergistic effect is first strengthened and then weakened.

Discussion of likely causes of possible variation of the effective ambient environment during quasi-steady droplet combustion supported by cool-flame chemistry

Droplet-combustion experiments with normal-alkane fuels, carried out onboard the International Space Station, have shown that, after radiative extinction of a droplet burning with an ordinary hot flame, often quasi-steady burning is initiated and sustained by a chemical-kinetic mechanism associated with cool flames. This second, cool-flame combustion stage occurs in a partial-burning regime in which both fuel and oxygen leak through a reaction zone that is controlled by the low-temperature chemistry of cool flames. This second stage persists for an appreciable period of time, after which it is terminated by cool-flame extinction, often at a measurable non-zero droplet diameter. This extinction diameter depends on the fuel and on the composition of the ambient atmosphere. Experiments involving large fuel droplets of different initial diameters indicate that the value of this cool-flame extinction diameter may vary with the initial droplet diameter and with the ignition energy imparted by the hot-wire igniters, even though the cool flame lies well within the region described as the inner quasi-steady zone, where flame structures are independent of initial conditions. These observations imply that the effective ambient atmosphere during the second stage likely depends on the previous combustion history. In the present contribution, evaluations are reported concerning possible influences of the ignition process, of the first-stage hot-flame combustion, and of prior cool-flame combustion on the effective atmosphere in which the second-stage combustion and extinction occur. It is concluded that analysis of influences of fuel leakage and condensation may be needed.

Single droplet combustion of aluminum nanoparticles added to a biofuel: effect of particle concentration and ambient temperature

Single droplet combustion of aluminum nanoparticles added to a biofuel: effect of particle concentration and ambient temperature

Pengaruh Panjang Rantai Karbon dan Derajat Ketidakjenuhan terhadap Karakteristik Pembakaran Droplet Asam Lemak Tunggal

This study aimed to investigate the effect of the different carbon chain lengths and the degree of unsaturation of six fatty acids as the constituent of vegetable oils. The droplet combustion was carried out at an ambient temperature and atmospheric pressure. The variation in the carbon chain length and the degree of unsaturation resulted in different physical and chemical properties of the fuel, which affect the droplet combustion characteristics. The increase of the carbon chain length results in longer ignition delay times and shorter burning durations, as well as higher droplet temperatures, burning rate constant, and specific power output. Conversely, an increase in the degree of unsaturation with the presence of double bond results in shorter ignition delay and longer burning duration, as well as higher droplet temperatures, but lower burning rate constant and specific power output. The droplet diameter evolution divides the combustion period into unsteady burning zones and quasi-linear burning zones. The flame dimension of unsaturated fatty acid is higher due to the soot formation at the top of the flame. A bluish flame related to the higher oxygen content in the molecule can be observed in saturated fatty acids. The short-chain saturated fatty acid has a large non-luminous zone because they are rich in oxygen. In contrast, the long-chain saturated fatty acid has a narrow non-luminous zone with high flame radiation.

Combustion of n-butyl acetate synthesized by a new and sustainable biological process and comparisons with an ultrapure commercial n-butyl acetate produced by conventional Fischer esterification

This paper reports a study of the combustion dynamics of n-butyl acetate (BA) using the configuration of a burning droplet. Two grades of BA were examined: one (SBA) synthesized by a new process described in the paper that uses a metabolically engineered solventogenic Clostridiumstrain through an extractive fermentation process using n-hexadecane as the extractant; and one commercially available as a high-purity (99.9%) ‘neat’ BA grade produced by conventional Fischer esterification (NBA). The initial droplet diameter was primarily 0.6 mm with some limited experiments carried out for 0.4 mm droplets to show the influence of convection. Experiments were performed in the standard atmosphere and ignition was by spark discharge. The results showed the presence of impurities in the SBA at mass concentrations totaling about 6% which included n-butanol, n-hexadecane, iso-propyl alcohol and ethyl acetate. Droplet burning rates and flame structures were not influenced by these impurities at this concentration level. In the presence of convection created by buoyancy, droplets burned faster with stretched flames and a luminosity revealing the presence of soot by incandescence at the flame tips. Reducing the initial droplet diameter to 0.4 mm eliminated the convective effect and resulted in near spherical flames. The results presented show that the new synthesis process is a sustainable alternative for BA production with burning characteristics identical to NBA in both convective and stagnant gas transport fields.

Evaporation and combustion of n-heptane droplets in supersonic combustor

The purpose of the present study is to investigate the combustion of the n-heptane droplet cloud in the supersonic combustor. The finite volume solver is developed to simulate the two-phase reacting compressible flow using single step reaction mechanism as finite rate chemistry. The focus is on the impacts of droplet size and cloud density on the performance of the scramjet. For the considered physical situation, the upper limit of the droplet size is determined to have higher combustion efficiency, and it is shown that the combustion mode is kinetic-controlled for small sizes and is evaporation-controlled for large droplet sizes. The variation of combustor’s exit total pressure and temperature is also investigated for different droplet cloud densities, demonstrating their apparent opposite behavior that must be considered to get optimum propulsion efficiency. In addition, it is illustrated that thermal choking is another criterion which should be avoided by controlling the fuel mass flow rate for intended flight conditions.

The role of the unsaturation degree on the droplet combustion characteristics of fatty acid methyl ester

Biodiesel is composed of various types of methyl ester compounds with different concentrations. The role of unsaturation degree was observed through single-molecule droplet combustion from three fatty acid methyl ester compounds. This research was conducted under normal gravity at ambient temperature and atmospheric pressure. Analysis of droplet combustion characteristics was observed along the heating, evaporation, ignition, and combustion stages. The cis configuration and allyl groups in unsaturated molecules related to different unsaturation degrees affect the droplet combustion process. Allyl and bis-allyl groups play an important role in accelerating hydrogen atom abstraction due to the weaker bond dissociation energy of the CH bond adjacent to the carbon double bond. The lower energy gap in unsaturated molecules is prone to electron excitation. Increasing the unsaturation degree resulted in shorter ignition delay and burning duration but higher droplet temperature, burning rate constant, specific power output, and flame dimensions. Soot particles formed above the flame tip on the unsaturated molecules gave a higher flame dimension. A high concentration of linoleic methyl ester in biodiesel fuel is recommended due to of high power output. However, the soot formation at a higher level of unsaturation degree should be considered.

Combustion characteristics of novel bishomocubane propellants in oxygen-enriched environments

The combustion characteristics of three novel energetic cage hydrocarbon compounds – bis(nitratomethyl)-1,3-bishomocubane (DNMBHC), diazido-dimethyl-bishomocubane (DADMBHC), and 1,3-bishomocubane dimer (BHCD), and an RP-1 surrogate fuel (RP-1-s) were investigated through droplet combustion experiments. As these novel compounds were envisioned as potential additives to RP-1 in semi-cryogenic engines, where liquid oxygen (LOX) would be used as the oxidizer, it was necessary to test these fuels in oxygen-enriched environments. Studies were conducted in two different environments – one containing 50% O2 and the other having 80% O2 with balance N2 in both cases. The influence of oxygen concentration on the variation of droplet and flame dimensions with time, time period of combustion, flame temperature, and sooting propensities were studied using videography and color-ratio pyrometry. With an increase in oxygen concentration, the flame temperatures were found to increase with a concurrent reduction in the flame stand-off ratios, resulting in faster combustion. As compared to trials in ambient air, the effect of microexplosions generated as a result of condensed phase pyrolysis reactions for the novel BHC fuels, was slightly diminished owing to the shorter combustion durations. Despite the reduced intensity of microexplosions, DNMBHC and DADMBHC droplets were found combust approximately 2.39 and 1.78 times faster than a similar-sized RP-1-s droplet with 80% ambient oxygen. Among the three novel BHC fuels, only DNMBHC fared better than RP-1-s in terms of maximum flame temperatures and had sooting propensities comparable to that of the benchmark fuel and thus was assessed as the most promising candidate.

Interactive Effects in Two-Droplets Combustion of RP-3 Kerosene under Sub-Atmospheric Pressure

To improve our understanding of the interactive effects in combustion of binary multicomponent fuel droplets at sub-atmospheric pressure, combustion experiments were conducted on two fibre-supported RP-3 kerosene droplets at pressures from 0.2 to 1.0 bar. The burning life of the interactive droplets was recorded by a high-speed camera and a mirrorless camera. The results showed that the flame propagation time from burning droplet to unburned droplet was proportional to the normalised spacing distance between droplets and the ambient pressure. Meanwhile, the maximum normalised spacing distance from which the left droplet can be ignited has been investigated under different ambient pressure. The burning rate was evaluated and found to have the same trend as the single droplet combustion, which decreased with the reduction in the pressure. For every experiment, the interactive coefficient was less than one owing to the oxygen competition, except for the experiment at L/D0 = 2.5 and P = 1.0 bar. During the interactive combustion, puffing and microexplosion were found to have a significant impact on secondary atomization, ignition and extinction.

Studies on theory and modeling of droplet and spray combustion in China: a review

Studies on theory and modeling of droplet and spray combustion in China: a review

Aluminum combustion instabilities: Dimensionless numbers controlling the instability in solid rocket motors

This article is focused on the analysis of the aluminum combustion dynamics, induced by acoustic oscillations, in solid rocket motors. This dynamics is expected to drive self-sustained pressure oscillations which can lead to structural damages of the rocket. In order to control this thermoacoustic instability, theoretical developments are presented in this paper to highlight the main parameters of the thermoacoustic source. These new theoretical developments are based on a linear model of the thermoacoustic source which has been verified with a numerical simulation in a previously published work. With some assumptions and reductions, this article clearly demonstrates that (i) the thermoacoustic source depends on 4 dimensionless numbers (Sherwood number, Stokes number, combustion Strouhal number, flow Strouhal number), (ii) if the source can be split into two contributions (flame dynamics around an evaporating droplet and droplet combustion end-dynamics), both contributions have different tendencies, (iii) the acceleration of the mean flow along the motor leads to this thermoacoustic instability. Under some assumptions (aluminum combustion model, linear or weakly nonlinear dynamics), these results make it possible to control this instability in a solid rocket motor.

Combustion characteristics of a single droplet of hydroprocessed vegetable oil blended with aluminum nanoparticles in a drop tube furnace

This study examines the burning characteristics and disruptive burning phenomena of single droplets of aluminum nanoparticles (n-Al) stably suspended in a biofuel (HVO). The biofuel used in the present work is a promising alternative fuel already tested in the aviation sector to reduce greenhouse gas and pollutant emissions. Experiments were conducted with two particle sizes (40nm and 70nm) and two particle concentrations (0.5 wt.% and 1.0 wt.%) to study its influence when added to the biofuel. The effect of size and concentration of the aluminum nanoparticles was studied at 1100 °C in a drop tube furnace. This experimental facility allows the study of combustion characteristics of falling droplets, ensuring there is no influence of the supporting fiber on the burning rate and disruptive burning phenomena occurrence. A CMOS high - speed camera coupled with a high magnification lens was used to evaluate the droplet size, burning rate, and micro-explosions. Based on this procedure, pure biofuel droplets were compared with those of biofuel blended with nanoparticles. The results suggest that the combustion characteristics of pure HVO can be enhanced with the addition of aluminum nanoparticles. Furthermore, by decreasing the particle size, a slight increase in the burning rate of nanofuels was noticed. Additionally, an increase in the particle concentration leads to a pronounced increase in the burning rate. The particle concentration also influences the delay and intensity of micro-explosions, disruptive burning phenomena detected at the end of the droplet lifetime.

Numerical modeling of radiation heat transfer from combusting droplets for a sodium fire analysis

Heat radiation is one of dominant heat transfer processes during a sodium fire event which is a concern in sodium-cooled fast reactor plants. The multi-dimensional sodium fire analysis code AQUA-SF employs the 6-flux gas radiation model to simulate radiation heat transfer in atmospheric gas and to structural walls with the consideration of the radiation effects of suspended sodium by-products as aerosols. Here, this original model does not calculate radiation heat transfer from combusting droplets of sprayed sodium. All the combustion heat transports to atmospheric gas surrounding the combusting droplet except for the heat distributed to the droplet in this model. However, some of combusting heat should transport to structural walls directly by heat radiation in actuality. Due to this modeling, atmospheric temperature and pressure are overestimated and on the other hand, temperature of structural walls is underestimated. Therefore, this study aims to model radiation heat transfer from combusting droplets numerically. In the modeling, radiation energy transport on the combusting area around a sodium droplet is formulated considering emission, absorption and scattering as interaction with surrounding gas radiation. Radiation energy from the combusting droplets is added to the source term of the radiation transport equation in the 6-flux gas radiation model. Direct radiation heat transfer from combusting droplets can be simulated by this improved model. The improved model is tested through the verification analyses of single droplet combustion and the benchmark analysis on the upward sodium spray combustion experiment. The results of the test analyses show increase in heat transfer to the walls due to the droplet radiation. As the result, the sodium fire analysis becomes more reasonably by the improved model.

Study on combustion and micro-explosion characteristics of biodiesel and ethanol mixture droplets under different oxygen concentrations and temperatures

An experimental platform for measuring droplet combustion and micro-explosion characteristics based on a tube furnace was established. Through this experimental platform, the BD60E40 droplets involving a mixture of 60 vol. % biodiesel and 40 vol. % ethanol was tested. Instantaneous gray images were obtained by means of high-speed photography during the BD60E40 droplets heating or combustion cycle. The expansion, ejection, micro-explosion, and combustion characteristics of mixed droplets under different temperatures and oxygen concentrations were studied by using the platform. The study found that the BD60E40 droplets produced high intensity primary and secondary micro-explosions when the oxygen concentration in the tube furnace was 0% and 30%. However, when the oxygen concentration was 50%, the BD60E40 droplet showed a weak micro-explosion phenomenon. The temperature was an important condition for the occurrence of micro-explosions. The BD60E40 droplet was not subject to micro-explosions during heating under 300 °C ambient temperature, but the micro-explosion occurred under 400 and 500 °C. According to the different experimental phenomena, the equation for calculating the micro-explosion intensity was established and the micro-explosion intensity of BD60E40 droplets under different oxygen concentration was calculated, and the reason for the different micro-explosion behavior of BD60E40 droplets under different oxygen concentration was established.

High-speed digital off-axis holography to study atomization and evaporation of burning droplets

Droplet formation and evaporation are key processes of spray combustion, the mechanism of which is yet not fully understood. In this study, we develop a method based on high-speed (25 kHz) digital off-axis holography (DOH) to visualize and measure burning ethanol droplets in the atomization. Ligaments and fragments from a burning drop internally blown to disintegrate are studied after hologram processing. Reconstructed images are nearly free from influences of the flame with the measured droplets showing clear borders. Size and depth location uncertainties reduce to nearly 1/10 of those of commonly used with digital in-line holography. A time-resolved observation shows the dynamic formation of secondary droplets with stages classified as vapor penetration, sheet breakup, and ligament disintegration. Sizes and velocities of fragments are extracted and their distributions are analyzed at 1.12 ms, 3.12 ms, and 13 ms after the initiation of atomization. Results show that new secondary droplets have larger sizes and smaller velocities with time. Lagrangian measurements are performed with 3D droplet tracking algorithm. Evaporation rates of 31 droplets with different positions, sizes, and velocities are obtained and the results are in good accordance with previous studies. The DOH technique is promising to investigate the combustion of individual fuel droplets in a polydisperse droplet cloud as the real spray combustion.

Study on three droplet sequential burning characteristics of coal direct liquefied diesel

Coal direct liquefied diesel (DDCL) is a new alternative fuel with high cleanliness and quality. Its hydrocarbon composition and physicochemical properties are quite different from those of petrochemical diesel. In order to study the difference in evaporative burning characteristics between DDCL and diesel during the sequential burning of multiple droplets, three fuel droplets are suspended on fibers in order by a micro-injector. The first droplet was ignited by an electrode igniter, and then the sequential burning process of three droplets was captured by a high-speed camera. An automatic processing program is written based on Matlab software to calculate the equivalent diameter of each fuel droplet. The result shows that the burning process of the DDCL droplet can be divided into three stages: initial thermal expansion, steady burning, and late micro-explosion burning. DDCL is much easier to form the burning mixture and has better flammability than diesel. The droplet spacing and fuel type have significant effects on ignition delay. The square of the normalized diameter of droplets decreases linearly with time during the steady burning period, which follows the D2 law. The reduction rates of normalized squared diameter for DDCL droplets are higher than that for diesel. The pyrolysis of macromolecular components at high temperature produces many small molecules and low boiling point components, which induce the micro-explosions in the late burning stage. The micro-explosion intensity of DDCL droplets is obviously lower than that of diesel.