Droplet Burning Rate Research Articles

Numerical investigation of a burning fuel droplet pair with different spacings and sizes

The combustion of double fuel droplets with different initial centre spacings and different droplet radii in convective environment are studied numerically by using the VOF (volume of fluid) model. The influences of these two factors on the droplet burning rates in the process of spray combustion are studied. The combustion processes of the two droplets with different initial centre spacings (shown as d2-t curves) and with different droplet radii (shown as (d/d0)2-t curves) are constructed respectively. Then functional analysis that reflects the global characteristic of combustion curves is proposed to process the simulation results. By analysing the functional values of combustion curves of the double droplets with different droplet centre spacings and radii, some results are obtained as follows: (1) The burning rates of the front droplets are always greater than those of the back droplets in convective environment, regardless the different droplet spacings or droplet radii. (2) The functional values of the combustion curves of the front droplets decrease nonlinearly as the initial centre spacing increases with a rapid decline first and a slow decline later, while the functional values of the combustion curves of the back droplets decrease linearly as the initial centre spacing increases. (3) The functional values of droplets combustion curves increase linearly as the droplet radius increases. In this case, the different initial centre spacings do not obviously affect the trends of the droplet burning rates versus the droplet radius.

Numerical Simulation of Single Droplet Combustion Characteristics in High Temperature Convection Environment

In order to better study the combustion characteristics of small droplets during spray combustion, a mathematical model of single fuel droplet evaporation and combustion was established based on the numerical simulation method of VOF(Volume of Fluid). Numerical simulation was carried out using FLUENT. The combustion process and flame propagation characteristics of the small droplet in the convection environment under different pressures of 0.1 MPa, 0.2 MPa and 0.3 MPa, were studied respectively, and the influence of different ambient pressures on combustion was analyzed. The results show that due to the influence of air convection, the ignition position was located in the downstream area of the droplet, forming a wake flame. Then the flame gradually overcame the convection effect and developed upstream, and finally transformed into the enveloped flame. With the further development of combustion, the combination of fuel vapor diffusion and combustion promoted the development of the flame front in the direction of airflow. After reaching the balance each other, the position and temperature of the flame front would be relatively stable. The increase of ambient pressure promoted the combustion process, which increased the droplet burning rate and flame temperature. At the initial stage of ignition, the temperature rise rate of the average droplet temperature at 0.3 MPa was 1.18 K/ms, which was 32.6% higher than that 0.89 K/ms at 0.1 MPa.

Systematic Measurement of Hydrocarbon Fuel Droplet Burning Rate Constants and Ignition Delays

In this experimental study, a new data reduction approach was developed for measuring droplet burning rate constants that automatically finds the most linear part of the -squared curve for the longest period of time. The approach, here called , was developed in response to the observation that burning rate constants can vary by up to 10%, depending on the reduction approach used, especially for fuel types such as nonvolatile simple hydrocarbons and kerosenes. To evaluate this new method, burning rate constants were measured for 10 fuels, including two alcohols, four simple hydrocarbons and four kerosene blends. The measured rates ranged from 0.74 to , typically grouped by similarities in thermophysical properties of the fuel types. Notably, and its simpler equivalent yielded burning rates with small statistical uncertainty ( average) across all fuels and were associated with the highest coefficient of determination for all measurement approaches, indicating that these two methods are viable options for improved consistency in measuring burning rate constants. In addition, a systematic method is presented for measuring ignition delays using a short-duration plasma source for ignition. The ignition delays ranged from 65 to 110 ms for most fuels, correlating well with the fuel boiling temperature. Combined, this work provides baseline data and data reduction tools suitable for future droplet combustion studies.

Pressure Effect in Droplet Combustion of Blended Fuel on Ethanol and Kemiri Sunan (Reutealis Trisperma (Blanco) Airy Shaw) Biodiesel

Bio oil from seeds of Kemiri Sunan (reutealis trisperma (Blanco) airy shaw) plant is particularly attractive to be studied since its high potential as alternative of biodiesel. This paper describes the results of a fundamental study of the combustion characteristics of a single droplet of blended fuel of Kemiri Sunan (reutealis trisperma (Blanco) airy shaw) biodiesel and ethanol in various levels of ambient pressure. The ambient pressures of the burning chamber were varied at 1, 3, and 5 bars. The fuels were prepared by mixing the biodiesel with the ethanol at concentrations of 0%, 10%, 20%, and 30% vol/vol. The single droplets of blended fuels were suspended using micro-syringe on a tip of the thermocouple. It was ignited and combusted using an electrical heater. The ignition and combustion processes of the single droplets were recorded using a high-speed camera. It was found that the ignition delay of the droplet decrease with the increase of ambient pressure and the concentration of ethanol in blended fuels. Also, the burning rate of blended droplets increased with increasing biodiesel concentration and pressure. The maximum droplet temperature slightly increased during the combustion with the increasing ethanol concentration, but not with increasing ambient pressure.

Droplet combustion and sooting characteristics of UCO biodiesel, heating oil and their mixtures under realistic conditions

The droplet combustion characteristics of conventional heating oil, UCO biodiesel and their mixtures (10 and 20% biodiesel by volume) were examined in a free-falling droplet combustion facility designed to approach the typical temperature and oxygen conditions around individual droplets in real boiler flames. Moreover, and given the importance of avoiding effects due to gravity or convection in this type of experiments, the quite spherical soot shells recorded (infrequent event in free-falling droplet combustion studies) reveal a configuration close to 1-D, even though tests were performed at normal gravity. The droplet combustion characteristics were evaluated at four different oxygen levels (0,3,5 and 10% O2), and comprise droplet size evolution, burning rates and sizing of the occurring soot shells and envelope flames. Results show indistinguishable behaviors between heating oil and their mixtures, whereas biodiesel displays longer initial heating transients and higher quasi-steady burning rates. Even though the UCO biodiesel studied is highly unsaturated, no signs of polymerization were observed for any of the tests. In addition to these combustion characteristics, an aspirating soot probe was used to quantitatively evaluate the propensity to soot of each fuel at different oxygen levels. The amount of collected soot was found to significantly decrease with biodiesel addition, even though similar results were obtained for both mixtures. The practically identical droplet combustion characteristics between heating oil and its mixtures with biodiesel, in addition to the lower propensity to soot of the latter, support the feasibility of using such blends as drop-in replacement for conventional heating oil. The experimental results obtained are thought to constitute a valuable database for the development and validation of modeling tools for the prediction of droplet evaporation and combustion of practical fuels in realistic conditions.

Combustion Characteristics of Horizontal Diesel Spray Burner in Low‐Pressure Chamber Based on Multivariate Correlation Analysis

At present, the burning problem of low power burners widely used in emergency and stove heating on the plateau has gradually attracted people’s attention. The fuel injection pressure, inlet air flow, air distribution regulation coefficient, and environmental pressure were considered respectively, and then the four factors and six levels of a uniform experimental investigation were conducted in a low‐pressure chamber with adjustable atmospheric pressure. The flame morphology, flame temperature, and NO emission were investigated, and their correlation with each independent variable was analyzed. In addition, burning of droplet swarm was obtained with the simulate numerical calculation by the ANSYS FLUENT. The results showed that the low‐pressure environment had a negative correlation with the maximum width and position of flame, the characteristics of flame uplifted and NO emission. Air volume determines the length of the flame, and atmospheric pressure and fuel injection pressure determine the maximum width of the flame. The shape of the horizontal jet flame in plateau environment is more dependent on the regulation of air volume. In order to reduce NO emission in plateau environment, it may be necessary to maintain a large air supply volume and reduce fuel injection pressure appropriately. At the microscopic level of droplet combustion, change mechanisms of macro flame shape in different sub‐atmosphere were analyzed. The mechanism of flame morphological change in the plateau environment is the increase in droplet burning time and path. The adjustment of flame under sub‐atmospheric pressure through the change of operating parameters is essential to improve the burning rate of droplets.

Combustion characteristics and liquid-phase visualisation of single isolated diesel droplet with surface contaminated by soot particles

The combustion generated soot contamination effect on a single diesel droplet ignition and burning was investigated experimentally for the first time. Diesel droplet flame was used to contaminate the droplet to be investigated prior to ignition. Distinct differences in lifetime and stability of the burning of the neat and contaminated droplet samples were observed in their heating, boiling and disruptive phases. For a soot-contaminated droplet surface, the evaporation rate became weaker as a result of slower mass transfer thus contracted the flame formation. Contrary to the burning rate enhancement of droplet with stable and uniform suspension of particles observed by other researchers, the slightest contamination of soot particles in a fuel droplet surface can significantly reduce the burning rate. Denser agglomeration of soot can form a shell on the droplet surface which blocks the flow of gas escaping through the surface thus distort the droplet even further. At late combustion stage, bubbles are observed to rapture on the surface of the soot-contaminated droplet. Strong ejections of volatile liquid and vapour that would explode shortly after parting from the droplet are observed. It seems that the explosion and burning of ejected mixture have little interactions with the enveloped flame surrounding the primary droplet. Enhanced visualisation of droplet liquid-phase has clearly indicated the cause of declining trend in the burning rate and flame stand-off ratio of soot-contaminated diesel droplet. These insights are of significance for understanding the effect of fuel droplet contamination by combustion generated soot particles.

Rheological Properties and Ignition and Combustion Characteristics of Biochar–Algae–Water Slurry Fuels

This study examined the rheological properties, ignition, and combustion characteristics of biochar–algae–water (BAW) slurry fuels. A pine sawdust biochar with a median particle size (D50) of approximately 12 μm and algae Chlorella vulgaris in dry powder form with D50 of approximately 19 μm were used. The BAW slurries with a constant solid loading of 40 wt % and algae/biochar ratio varying from 0 to 0.2 by weight were prepared. The apparent viscosity was measured using a Haake VT550 cone-and-plate viscometer. The stability of the slurries was characterized using a “drop rod” method. Ignition and combustion characteristics of the slurries were studied using a suspended single-droplet technique. A single droplet of a slurry fuel with a diameter ranging from 0.5 mm to 1.5 mm was suspended on a silicon carbide fibre and burned in air at 1023 K in an electrically heated tube furnace. The ignition and combustion processes of the droplet were recorded using a CCD camera at 200 fps. The ignition delay time, burnout time, and burning rate were determined. The BAW slurries showed shear-thinning flow behavior. The slurries had higher viscosity and greater stability at higher algae proportion in the solid. The ignition and combustion process of BAW slurries followed the sequence of water evaporation, devolatilization, ignition, and combustion of the solid residue. The combustion of the residual solid was diffusion controlled under the experimental conditions and the burning rates of the BAW slurry droplets ranged from 0.15 to 0.25 mm2 s−1.

Evaporation and combustion characteristics of hydrocarbon fuel droplet in sub- and super-critical environments

An experimental study on evaporation and combustion phenomena of single suspended hydrocarbon droplets was conducted in sub- and super-critical pressure environments under normal gravity. Droplet temperature and photographs were obtained by utilizing embedded thermocouple and high-speed video camera respectively. Experimental results show equilibrium vaporization stage exists during droplet combustion in sub-critical pressure environments, which accords with quasi-steady assumption. However, the equilibrium vaporization stage disappears during droplet combustion in super-critical pressure environments, quasi-steady assumption is not applicable. Droplet combustion time decreases rapidly with the increase of ambient pressure at sub-critical conditions and phase equilibrium controls the droplet burning rate. In super-critical conditions, the interface between droplet and ambient gas becomes ambiguous, droplet combustion time does not decrease and approaches a stable value; phase change disappears and diffusion coefficient starts to affect the droplet burning rate. The trend change of combustion time variation at critical pressure indicates that the critical pressure point is an important basis for judging whether the droplet enters the super-critical combustion or not. The ratio of droplet evaporation time to droplet combustion time has little change in sub-critical pressure environments, but it decreases rapidly in super-critical pressure environments, this indicates droplet evaporation process completes earlier in super-critical pressure environments.

Correlation of the droplet burning rate in rotatory natural convection

Thermal and fluid flow analyses of hydrocarbon liquid droplet evaporation and combustion in natural convection and in rotation are carried out numerically and experimentally. The regression of the liquid droplet square diameter and the evolution of the liquid droplet temperature in rotatory natural convection are determined numerically by using the implicit finite difference method and Thomas algorithm. Comparison between numerical and experimental results shows qualitative and quantitative satisfactory agreement. Correlation expressing the burning rate in the function of Dgheim, Prandtl, and Grashof numbers is proposed. This correlation takes into consideration the burning of the heptane liquid droplet in rotatory natural convection.

A Burke–Schumann analysis of diffusion-flame structures supported by a burning droplet

A Burke–Schumann description of three different regimes of combustion of a fuel droplet in an oxidising atmosphere, namely the premixed-flame regime, the partial-burning regime and the diffusion-flame regime, is presented by treating the fuel and oxygen leakage fractions through the flame as known parameters. The analysis shows that the burning-rate constant, the flame-standoff ratio, and the flame temperature in these regimes can be obtained from the classical droplet-burning results by suitable definitions of an effective ambient oxygen mass fraction and an effective fuel concentration in the droplet interior. The results show that increasing oxygen leakage alone through the flame lowers both the droplet burning rate and the flame temperature, whereas leakage of fuel alone leaves the burning rate unaffected while reducing the flame temperature and moving the flame closer to the droplet surface. Solutions for the partial-burning regime are shown to exist only for a limited range of fuel and oxygen leakage fractions.

Experimental investigation on droplet burning characteristics of diesel-benzyl azides blend

In this work, the droplet burning characteristics of neat benzyl azides, diesel-benzyl azides blends (10%, 30%, 50% of benzyl azides (wt.)) and diesel fuel were investigated under atmospheric pressure. The ambient temperature around the flat-flame burner is about 1123K. The initial droplet temperature is about 300K. The effects of ambient oxygen concentration (14%, 21% and 30%) and initial droplet diameter (∼257μm and ∼456μm) on the burning characteristics were also studied. Experiments were carried out using the droplet free falling technique. A CCD camera with a micro lens was used to collect droplet photos, through which the droplet burning rate and flame size were deduced. A strobe light which could freeze the droplet visually was also used to collect ignition delay and combustion duration. The flat-flame burner manipulated by varying the composition of the premixed gas consisting of propane, oxygen, and air was used to provide combustion environment. The results show that the addition of benzyl azides not only increases the burning rate but also causes micro-explosion before the end of the flame because of the occurrence of liquid-phase reaction. With the increase of benzyl azides mass fraction, the ignition delay decreases, the combustion duration first increases and then decreases. Furthermore, due to the faster evaporation rate of benzyl azides, the droplet flame sizes increase with the benzyl azides mass fraction increasing during burning. However, the diesel-benzyl azides blend has stronger sooting propensity than diesel. Meanwhile, the ambient oxygen concentration and droplet initial diameter have important effects on the burning characteristics of diesel-benzyl azides droplet.

Nanoaluminum/Nitrocellulose microparticle additive for burn enhancement of liquid fuels

Addition of metal and metal oxide nanoparticles to hydrocarbon fuels has shown the ability to increase the volumetric energy density, decrease ignition delay, increase heat of combustion, and catalyze fuel decomposition in recent research. However, energetic metal nanoparticles are prone to aggregation, which occurs at an increased rate near the regressing surface of a burning liquid droplet where local concentrations increase and can form a transport-inhibiting shell, ultimately decreasing the droplet burning rate. Alternatively, gas ejections from the droplet can disrupt shell formation and transport nanoparticles from the droplet to the flame zone. This study quantifies up to a 12.1% decrease in the burning rate constant of Kerosene droplets when 6.1wt% nanoaluminum (nAl) particles are added (the maximum stable loading) with a hydrocarbon-based surfactant in a free-falling single droplet combustion experiment. Addition of nitrocellulose (NC) particles to the nanofuel diminishes or fully counteracts the burning rate decreases and provides a means of tuning the burning rate constant higher than that of pure Kerosene (maximum 13.8% increase over control with 2.3wt% nAl and 0.6wt% NC added). To reach stable nanofuels at higher particle loadings up to 15.0wt% solid additives, nAl and NC were electrosprayed into composite mesoparticles (MP) before suspending with surfactant in Kerosene. These MP-based nanofuels boast increased dispersibility and additive loadings and thus higher achievable burning rates (maximum 26.5% increase over control) than physically mixed analogs. A mechanism is proposed in which droplet disruptions influenced by NC addition include cyclical inflations, during which the liquid gasification rate increases, e.g. by expanding the outer surface area of the droplet.

Combustion characteristics of butanol isomers in multiphase droplet configurations

This study reports results of experiments on the isolated droplet burning characteristics of butanol isomers (n-, iso-, sec-, and tert-) under standard atmosphere conditions in an environment that promotes spherical combustion. The data are compared with predictions from a detailed numerical model (DNM) that incorporates complex combustion chemistry, radiative heat transfer, temperature dependent variable fluid properties, and unsteady gas and liquid transport. Computational predictions are generated using the high temperature kinetic models of Sarathy et al. (2012) and Merchant et al. (2013).The experiments were performed in a free-fall facility to reduce the effects of buoyancy and produce spherical droplet flames. Motion of single droplets with diameters ranged from 0.52mm to 0.56mm was eliminated by tethering them to two small-diameter SiC filaments (∼14µm diameter). In all the experiments, minimal sooting was observed, offering the opportunity for direct comparison of the experimental measurements with DNM predictions that neglect soot kinetics.The experimental data showed that the burning rates of iso- and sec-butanol are very close to that of n-butanol, differing only in flame structure. The flame stand-off ratios (FSR) for n-butanol flames are smaller than those for the isomers, while tert-butanol flames exhibited the largest FSR. DNM predictions based upon the kinetic model of Sarathy et al. over-predict the droplet burning rates and FSRs of all the isomers except n-butanol. Predictions using a kinetic model based upon the work of Merchant et al. agree much better with the experimental data, though relatively higher discrepancies are evident for tert-butanol simulation results. Further analyses of the predictions using the two kinetic models and their differences are discussed. It is found that the disparity in transport coefficients for isomer specific species for Sarathy et al. model fosters deviation in computational predictions against these newly acquired droplet combustion data presented in this study.

Droplet burning rate enhancement of ethanol with the addition of graphite nanoparticles: Influence of radiation absorption

A droplet stream flame was used to measure the burning rate of ethanol droplets with the addition of graphite nanoparticles. Two particle sizes, 50 nm and 100 nm, were used for this study. Results indicate that as particle concentration is increased, the burning rate of the resulting nanofluid droplet also increases. The maximum enhancement of 62% was observed with the addition of 3 wt.% 50 nm graphite nanoparticles. To understand the burning-rate-enhancement phenomenon, a model was developed to estimate the radiation absorptivity by the hybrid droplet from the stream flame. The computational models determine the ratio of radiation retention by the entire depth of the fluid (volumetric absorptivity) using optical properties of both the particles and the fluid along with the penetration of radiation within the nanofluid using the well-known Monte Carlo algorithm that incorporates the aforementioned calculated optical properties of the nanofluid. Results indicate that radiation absorption by the hybrid droplet does play a role in the enhancement of burning rate. More importantly, the absorption is not uniform within the hybrid droplet. It is localized in the region near the droplet surface, promoting boiling at the droplet surface. This mechanism is believed to be responsible for the observed increased burning rate.

Partial-Burning Regime for Quasi-Steady Droplet Combustion Supported by Cool Flames

A simplified model for droplet combustion in the partial-burning regime is applied to the cool-flame regime observed in droplet-burning experiments performed in the International Space Station with normal-alkanes fuels resulting in expressions for the quasi-steady droplet burning rate and for the flame standoff ratio. The simplified predictions are found to produce reasonable agreement with the experimentally measured values of burning-rate constants but not with their apparent dependencies on pressure or on the initial droplet diameter. Good agreement is found, however, with newly measured and numerically calculated flame standoff ratios in this droplet combustion supported by cool flames.

Combustion of n-Butanol, Gasoline, and n-Butanol/Gasoline Mixture Droplets

Fuels derived from biofeedstocks are receiving attention for their potential as additives to conventional petroleum-based transportation fuels. Normal butanol, in particular, can enhance performance compared to ethanol because of its higher energy density. To better understand the combustion dynamics of n-butanol in the context of gasoline, experiments are reported here to examine the isolated droplet combustion characteristics of an 87 octane (ethanol-free) gasoline and a mixture of gasoline (0.9, v/v) and n-butanol (0.l, v/v, B10), along with n-butanol. The experiments are performed in an ambience that minimizes convection and promotes spherical droplet flames. The initial droplet diameters range from 0.52 to 0.63 mm, and the experiments are carried out in room-temperature air at normal atmospheric pressure. Measurements of the evolution of the droplet diameter show that butanol and B10 droplets have burning rates that are almost identical to gasoline, even though other features of the burning process, such as soot formation and the relative position of the droplet and flame, are quite different. With butanol mixed with gasoline, the mixture flames are comparatively closer to the droplet than for gasoline droplets. A scale analysis is developed that expresses the droplet burning rate in terms of temperature-dependent properties. The results support the experimentally observed similarity of burning rates for butanol, gasoline, and their mixtures, even though soot formation is neglected.

Cool-flame extinction during n-alkane droplet combustion in microgravity

Recent droplet-combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets, following radiative extinction of the visible flame, can continue to burn quasi-steadily in a low-temperature regime, characterized by negative-temperature-coefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial size burning in oxygen/nitrogen, oxygen/nitrogen/carbon dioxide, and oxygen/nitrogen/helium environments at pressures from 0.5 to 1.0atm, with oxygen concentrations from 14% to 25% by volume. These large n-alkane droplets exhibited radiative extinction of the hot flame, followed by quasi-steady low-temperature burning, which terminated with diffusive extinction accompanied by the formation of a vapor cloud, while small droplets did not exhibit radiative extinction but instead burned to completion or disruptively extinguished. Results for droplet burning rates in both the hot-flame and cool-flame regimes, as well as droplet extinction diameters at the end of each stage, are presented. The cool-flame extinction diameters for all three n-alkanes are shown to follow a similar trend as functions of the oxygen concentration, predicted here from a simplified theoretical model that is based on the reaction-rate parameters for the oxygen molecule addition to the alkyl radical and for ketohydroperoxide decomposition.

Combustion characteristics of alkane two-droplet array Part 2. Two droplet array of n-Docosane

Combustion of two n-Docosane droplets in air at room temperature is studied experimentally. The droplets are disposed vertically one above another and burning history of upper droplet is registered. A new accurate method for measuring an equivalent diameter of burning droplet is developed. As a result, burning rate constants of Docosane and Octadecane droplets are measured more accurately. The dependence of upper droplet burning rate on interdroplet spacing is analyzed.

Hydrocarbon Droplet Turbulent Combustion in an Elevated Pressure Environment

New experimental data on the effect of turbulence on the combustion of n-heptane and n-decane droplet at elevated pressure are reported. Turbulent flow field in the central volume of a spherical vessel was generated by a set of eight axial fans. Flow characterization demonstrated that turbulence is isotropic and homogeneous within the 40 mm central volume of the vessel. The combustion of n-heptane and n-decane droplet was examined by varying turbulence intensity via the fans rotational speed and ambient pressure and temperature inside the vessel/chamber. Results showed that droplet burning rate becomes sensitive to turbulence only at elevated ambient pressure. Droplet flame extinction was found to depend on the heat loss from the droplet envelop flame, which increases with turbulence intensity.