Droplet Flame Research Articles

Experimental study of fuel distribution and combustion characteristics under subatmospheric pressure in an afterburner

The fuel distribution and ignition characteristics of an afterburner with typical components under subatmospheric inlet conditions are important references for improving the performance of the afterburner at high flight altitudes. In this work, lean ignition limits, outlet temperature distributions, fuel droplet size distributions and flame propagation processes in the afterburner were obtained experimentally. The results show that with increasing fuel injection angle, the fuel atomization characteristics improve, but the flame intensity fluctuation increases, and the ignition performance deteriorates; thus, a proper ratio of liquid-phase fuel to vapor-phase fuel is one of the key elements for successful ignition. On the one hand, the increased stabilizer blockage ratio results in a localized flow velocity increase near the trailing edge of the stabilizer, which effectively promotes fuel atomization in the shear layer, resulting in a fuel droplet size reduction of 25–30 μm. On the other hand, the extent of the recirculation zone increases, and a large low-pressure and low-turbulence recirculation zone is not conducive to fuel atomization in the flow direction. During the ignition process in an afterburner with all the typical components, the flame kernel first propagates along the flow direction for a certain distance and then propagates and anchors toward the shear layer. At subatmospheric pressures, the decrease in the intensity of the combustion reaction causes a reduction in the heat released by combustion, and the fuel droplet size increases by 10–25 μm, which requires more heat for evaporation. Therefore, the flame propagates an increased distance along the flow as the pressure decreases. Furthermore, the difficulty of ignition increases, and the ignition equivalence ratio increases by 0.1–0.15.

Hypergolic fuel impacting a gelled oxidizer wall: Droplet dynamics, heat release, ignition, and flame analysis

The combination of high-concentration solutions of hydrogen peroxide, known as High Test Peroxide (HTP), with the green fuel composed of tetramethylethylenediamine, dimethylaminoethanol, and methanol (TMEDA/DMEA/MeOH, 1:1:1 vol%), catalyzed by 1 wt% of copper chloride dihydrate, has significant potential for space hypergolic propulsion applications in terms of performance and safe operation. Besides the well-known and mature propulsion systems adopting liquid and solid propellants, gel propellants also present interesting characteristics that can be better explored to enable the creation of alternative propulsive systems. The present work employed HTP at 98 wt% with 6 wt% of fumed silica as a gelling agent to create gelled HTP (GHTP). Droplets of the green fuel impinged on a layer of GHTP placed over a glass slide, acting as a solid oxidizer, mimicked an element of a new hybrid gel/liquid hypergolic propulsion system. Data from infrared and visible light cameras enabled a detailed analysis of the dynamics of a reactive droplet impinging on a gelled oxidizer wall, as well as the heat release, ignition, and flame spread of the hypergolic GHTP/green fuel pair under open-air conditions. The heat release was observed to be distributed across concentric annular areas for both the fuel surrogate and the fuel with catalyst, before and after ignition, demonstrating a strong correlation with non-reactive droplet fluid dynamics, which was corroborated by spread diameter analysis processed from visible image data. Vapor and Ignition Delay Times (VDT and IDT) were found to be as low as 4 and 17 ms, respectively, for the highest droplet impact velocity and catalyst concentration. The reaction rate in the gas phase, considering the flame spread area, showed a dependency of both impact velocity and catalyst concentration, with the latter exhibiting a more pronounced and clear effect. The surface temperature ranges where first vaporization and ignition occurred were 60 °C ∼ 70 °C and 120 ∼ 170 °C, respectively, which is close to the boiling point of methanol and the auto-ignition temperature of TMEDA. This finding, along with the chemical mechanisms in the gas phase related to the presence of a catalyst in the fuel, may be important for hypergolic ignition. The relatively low ignition temperatures and the short ignition times represent additional advantages of the present hypergolic combination for propulsion applications.

A laminar spray group combustion experiment (LSGCE) with in situ image diagnostic methods and image analysis algorithms.

Spray combustion is important for different engines. The understanding of spray combustion should be further promoted especially in the non-dilute region, and there is lack of well-defined spray experiments. In this study, an experimental platform was developed. Using this platform, a cylindrical quasi-laminar spray can be formed and ignited by a thin and straight hot wire, making it a simple configuration. Two image diagnostic methods were also developed to capture in situ microscopic droplet images and macroscopic droplet-flame images synchronously. Different image analysis algorithms were developed to obtain droplet statistics (diameter, velocity, and number density) and flame information (size, location, and flame propagation speed) from the raw images. The design, diagnostic methods, and image analysis methods are detailedly presented. This experimental platform can cover a wide range of operating conditions, with Gig in a range of 0.01-0.06 and temperature in a range from room temperature to 1400K. In addition, this platform is small in size and is capable of further implanting into a ground-based microgravity facilitaty. The whole experimental system can be applied in spray ignition and combustion studies and can provide legitimate data for further model development.

Enhancing Droplet Combustion Dynamics in Trimethyl Borate-Based Blends: Exploring Energetic Phenomena

ABSTRACT Trimethyl borate (TMB) is an excellent alternative to alter the combustion of conventional fuels due to the combination of boron, stable methyl groups, and oxygen, which can improve the combustion behavior in many ways compared to alcohols and etheric hydrocarbon structures. In this research, the combustion and energetic phenomena trends of trimethyl borate blends was investigated on a droplet scale. The camera systems were used at the combustion characteristics look at how the size of the droplets, the structure of the flame, and the flame temperature changed over time. The additions of 20%, 40%, 60%, and 80% trimethyl borate fuel to gasoline were tested for their ability to burn. As the amount of TMB increased, high variations in droplet deformation and high breakups from the hemispheric geometry occurred. At this point, changes were observed in droplet shape change independent of mixing ratio. TMB droplet had the highest flame temperature of 600 K and the lowest extinction time of ~ 1270 ms. As the gasoline content of the droplets increased, the droplet flame temperature tended to decrease. Also, the shortest ignition delay time was observed for pure TMB and fuel droplets containing 40%, 60% and 80% TMB (~0.714 ms).

Exploring electric field forces and polarization influence on combustion in Fe-additive fuel droplets

The application of electrical techniques such as electric field, polarization, and ionization in the field of combustion makes many positive developments possible. The electric field effect offers significant advantages in controlling flame stability and reducing soot formation during the combustion of fuels. Also, the conditioning effect can improve particle oxidation during combustion for hydrocarbon fuels containing metal particle inclusions with high calorific value. This study focuses on investigating the combustion and atomization behavior of Fe additive diesel fuel droplets at electric field strengths of E = 5 V/m, E = 6.7 V/m, and E = 10 V/m and in positive (↑) and negative (↓) electric field directions. The experiments were carried out by igniting a fuel droplet suspended on a ceramic wire with an arc at the center of the electric field between two conductive plates in a combustion chamber with optical apertures. The combustion processes were recorded with an optical system consisting of a high-speed camera and a thermal camera. Combustion and atomization behavior were characterized by sequential flame and droplet atomization images, respectively. The droplet shape change and flame behavior results were comparatively evaluated according to the d2-law. Ignition delay and extinction times were determined by processing threshold technique and maximum flame temperatures were determined by processing thermal camera images. The results showed that increasing the electric field intensity in the negative electric field direction resulted in improved soot oxidation, and Fe particles burnt efficiently. Ignition delay times showed a decreasing trend with an electric field effect and an increasing trend with the addition of Fe particles. The highest maximum flame temperature and lowest extinction time were recorded as 785 K and ∼ 578.34 ms for Diesel/Fe/100(↓) fuel droplet, respectively and Fe particles did not exhibit microexplosion phenomena at E = 10 V/m. In this study, it has been shown that an effective oxidation can be achieved for Fe particles by increasing the mobility of ionized species in the flame zone under the effect of electric field and a homogeneous thermal field can be created in the flame zone even in fuels with particle addition at increasing electric field intensities.

Detailed numerical simulation and experiments of a steadily burning micron-sized aluminum droplet in hot steam-dominated flows

Detailed numerical simulations are conducted in comparison with experimental results to study the flame structure and burning rate of a steadily burning aluminum droplet in hot steam-dominated environments. The droplet surface temperature, flame temperature, and flame stabilization position are measured along with the droplet burning rate estimated from the droplet size evolution. A numerical model accounting for detailed transport properties and chemical kinetics is presented and applied to unveil the flame structure, species and temperature distributions, and heat/mass transfer between the droplet and the surrounding gas. The numerical results of the temperature, velocity, and species distribution profiles demonstrate that the aluminum vapor flame is of classical diffusion flame structure, where near the droplet, there is a non-negligible amount of AlOAl apart from the main product Al2O3(l). This supports the deposition and formation of an alumina cap on the surface proposed in the literature. The simulation correctly captured the flame temperature and flame stabilization distance for a range of droplet sizes. Net heat flux analysis shows that conduction heat from the flame front accounts for less than 30% of the heat needed in aluminum evaporation, which warrants further quantification on other heat sources. The experimental and numerical results enrich the knowledge of the heat/mass transfer and chemical reactions near the droplet, which helps deepen the understanding of aluminum droplet burning.

Insights into the flame transitions and flame stabilization mechanisms in a freely falling burning droplet encountering a co-flow

The present study investigates the flame dynamics of a contactless burning fuel droplet under free fall subjected to a co-flow. The dynamic external relative flow established due to co-flow and droplet acceleration results in a series of droplet flame transitions. Different flame structures were observed, including a wake flame, reversed wake flame and enveloped flame. Following ignition, the droplet is allowed to fall through the central tube of a co-flow arrangement, and, at its exit, the droplet flame encounters the co-flow. The wake flame, which was established based on the droplet's instantaneous velocity of descent, encounters the abrupt relative velocity jump due to the co-flow. This causes the droplet flame to go through various transitions as it approaches equilibrium with the surrounding flow. Once it equilibrates, the droplet flame evolves in response to the instantaneous relative flow velocity. The droplet flame evolves by altering both its shape and the stabilization mechanism. Two stabilization mechanisms were identified for the droplet wake flame: edge-flame stabilization and bluff-body stabilization. The stabilization mechanism for different flame structures and the transition events have been theoretically analysed, and the relation between flame shape evolution and flow velocity has been determined based on the flow-field characteristics at the corresponding Re (Reynolds number) range. Furthermore, these correlations are employed in a mathematical formulation based on the spring–mass analogy, which predicts the droplet flame evolution after encountering the co-flow, including all the transition events.

Investigation of evaporation and combustion characteristics of diesel and fatty acid methyl esters emulsified fuel droplets

To address the problems of energy shortages and environmental pollution, biodiesel and its emulsified fuel have gained wide attention. In this work, the fatty acid methyl esters (FAME) emulsified water-in-oil fuel (BW) is thoroughly investigated using the droplet suspension experiment with a focus on the main characteristics of puffing/micro-explosion, droplet ignition delay and flame regime, etc. The role of water content at different ambient temperatures is examined in comparison to a counterpart blend of diesel/water (DW) fuel. Results show that the increase in water content and temperature has a significant promotion effect on the strength and number of puffing/micro-explosion process, however, the higher viscosity of BW leads to the lower values. And owing to the heat absorption by water evaporation, only the micro-explosion with high intensity induced by water nucleation has a favorable effect on the evaporation rate. There exists a critical concentration of water addition, after which the enhancement of evaporation is expected. Moreover, it is indicated that the diesel with water additives has a better atomization effect, and the size of child droplets generated by DW droplets breakup is smaller, which is in the range of 0.08–0.1 mm compared to the 0.09–0.13 mm for BW droplets. It has been shown that the increase in water content hinders the rise of droplet flame temperature, extends the ignition delay time, and shortens the maximum premixed-flame length. Compared to diesel droplets, the larger combustion rate of BW droplets is correlated with their shorter flame length and the higher boiling point of FAME.

New Insight into the Effects of Gaseous CO2 on Spherically Symmetric Droplet Flames

This study investigated the effect of CO2 on the burning behavior and radiative properties of a single ethanol droplet flame in microgravity. Measurements of the droplet burning rate, the flame size and temperature, and the radiative emissions were performed, under microgravity conditions for ethanol droplets burning in N2 and CO2 environments, using the 1.5 s drop tower facilities at the Korea Maritime and Ocean University (KMOU). The non-monotonic sooting behaviors (caused by the elevated O2 concentrations) were found to have a significant influence on radiative heat losses in N2 environments, resulting in non-linear droplet burning behaviors with O2 concentrations. Due to the unique nature of CO2 in microgravity, which absorbs radiative energy from the flame and raises the temperatures of the surrounding gases, the CO2 environments suppressed the radiative heat losses from the flame, regardless of the non-monotonic sooting behavior observed at the higher O2 concentrations. These experimental findings highlight the complicated physics of CO2 gas radiation in microgravity, which has not been quantitatively explored.

Comparison of the Flame Dynamics of a Liquid-Fueled Swirl-Stabilized Combustor for Different Degrees of Fuel-Air Premixing

Abstract This study investigates the flame dynamics of lean premixed kerosene combustion for two different degrees of fuel–air premixing using a swirl stabilized burner with an axially movable twin fluid fuel injection nozzle. Thermal power, equivalence ratio, and atomizing air mass flow are varied systematically for both nozzle positions investigated. Measurements of the droplet size distribution at the nozzle exit are provided for all operation points. NOx emission measurements and OH*-chemiluminescence flame images show that stationary combustion characteristics significantly change with the nozzle position. Flame Transfer Functions (FTFs) are presented and interpreted for all operation points. The FTFs for the two configurations differ most in the low frequency range where the influence of the droplet dynamics is expected to be highest. For both configurations, a change in thermal power does not affect droplet size, flame shape, NOx emissions, and FTF. The observed trends in response to changes in equivalence ratio and atomizing air mass flow are opposite for both configurations. NOx emissions and flame shape are independent of the atomization air mass flow in the highly premixed configuration but not in the partially premixed configuration. In contrast to this, the FTF is affected by changes of the atomization air mass flow in both configurations, but again the trends are opposite. The observed trends for the highly premixed configuration are modeled and reproduced by a change in the phase relation between the equivalence ratio fluctuations and other instability driving mechanisms.

Experimental study on microwave-induced puffing, micro-explosion, and combustion characteristics of ammonium dinitramide-based liquid propellant droplets

Microwave ignition technology has the advantages of high ignition energy, stable ignition, and spatial multi-point ignition. These advantages make this technology promising for future application in green single-component propellants. In this paper, the ignition characteristics of ammonium dinitramide (ADN)-based liquid propellant droplets under the influence of microwaves at room temperature are investigated using experimental methods. The effects of microwave power on puffing, micro-explosion, and combustion behavior of ADN-based liquid propellant droplets were studied. The droplet and flame diameters were statistically related to time, and the microwave-assisted droplet ignition mechanism was analyzed. A new rectangular waveguide resonant cavity was designed in which the droplet is placed at the maximum electric field strength of the device. The droplet morphology and flame profile inside the resonant cavity were photographed with a high-speed camera. The experimental results showed that the microwave positively influenced the puffing, micro-explosion, and combustion behavior of droplets. When the microwave power was increased from 200 to 280 W, the total droplet evaporation time and ignition delay time were reduced by 56.5% and 35.2%, respectively. The positive effects of microwaves on combustion have been summarized as the thermal effect of microwaves on polar molecules and the promotion of fuel oxidation reactions by microwave-induced plasma. The plasma was found to control the development of the initial flame propagation front and to influence the temperature during the combustion reaction process. In this paper, we propose the mode of droplet combustion under microwave induction as a plasma discharge and several stages of the droplet combustion process. This research provides novel insight into the study of the microwave ignition mechanism of liquid fuels.

Investigation of combustion characteristics on triethyl borate, trimethyl borate, diesel, and gasoline droplets

In this study, the evolution of fuel droplet diameter, flame structure, and maximum flame temperature over time was observed using a high-speed camera and a thermal camera at the same time. Traditional fuels (diesel and gasoline) and new generation fuels (triethyl borate and trimethyl borate) fuels were investigated within the droplet experiments. It has been shown that the flame structure of diesel and gasoline fuel droplets has a large non-luminous region that is seen during the combustion process, unlike triethyl borate and trimethyl borate fuels. The curves of the time-dependent variation of the dimensionless square of the droplet diameter (D/D0)2 of the fuel droplets considered in the study generally exhibited curve properties conforming to the D2-law. In the flame formed by the trimethyl borate droplet, the highest flame temperature was observed by the thermal camera, while the shortest burning time was detected. According to the experimental conditions in the study, the shortest ignition delay was measured for trimethyl borate, while the longest ignition delay time was observed for diesel fuel droplet. Also, it was determined that the lowest burn rate constant was detected for the gasoline droplet, while the highest burn rate constant was seen for diesel fuel.

Combustion characteristics of trimethyl borate, diesel, and trimethyl borate-diesel blend droplets

In this study, droplet tests were carried out for pure and blended forms of diesel fuel, which is the traditional fuel, and trimethyl borate fuel, which is new generation fuels. In this context, the evolution of fuel droplet diameter, flame structure, and flame temperature over time was observed using a high-speed camera and a thermal camera simultaneously. Measurements were made for the addition of 20%, 40%, 60%, and 80% trimethyl borate fuel to diesel fuel in fuel blends. The curves of the time-dependent change of the dimensionless square of the droplet diameter (D/D0)2 of the fuel droplets considered in the study, in general, have been determined to comply with the D2-law. Also, as the amount of diesel rose, there were high changes in droplet distortion and high deviations from the hemispherical geometry, and also elongation in the dominant shape droplet shape changed times. While the highest flame temperature was monitored by the thermal camera in the flame formed by the trimethyl borate droplet, as the amount of diesel fuel in the mixture increased, the maximum flame temperature decreased, but the burning time of the droplet was prolonged. On the other hand, the shortest ignition delay was measured for trimethyl borate, while the longest ignition delay time was detected for diesel fuel droplet. In general, it has been observed that the addition of trimethyl borate reduces the ignition delay, shortens the extinction time, and increases the temperature during combustion compared to diesel.

Forced ignition and oscillating flame propagation in fine ethanol sprays

The present work investigates forced ignition and oscillating propagation of spray flame in a mixture of fine ethanol droplets and air. The Eulerian-Eulerian method with two-way coupling is used and a detailed chemical mechanism is considered. Different droplet diameters and liquid fuel equivalence ratios (ER) are studied. The evaporation completion front (ECF) is defined to study the interactions between the evaporation zone and flame front. The gas composition at the flame front is quantified through an effective ER. The results show that the kernel trajectory is considerably affected by droplet size and liquid ER. Generally, the flame ER reaches the maximum when the ECF starts to move from the spherical center. It gradually decreases and reaches a constant value when the flame freely propagates. Quasi-stationary spherical flame is observed when the liquid ER is low, whilst kernel extinction/re-ignition appears when liquid ER is high. These flame behaviors are essentially affected by the heat conduction and species diffusion timescales between the droplet evaporation zone and flame front. The dependence of the minimum ignition energy (MIE) on liquid ER is U-shaped and there is an optimal liquid equivalence ratio range (ERo) with the smallest MIE. Long and short ignition failure modes are observed, respectively for small and large liquid ERs. When liquid ER is less than ERo (long failure mode), larger energy is required to initiate the kernel due to very lean composition near the spark. For liquid ER larger than ERo (short mode), ignition failure is caused by strong evaporative heat loss and rich gas composition due to heavy droplet loading. Flame speed oscillation is seen when the initial droplet size is above 10 μm. The oscillation frequency linearly increases with the liquid ER. Meanwhile, larger droplets lead to smaller oscillation frequencies.

Micro-explosion enhanced combustion of Jatropha oil/2, 5-dimethylfuran (DMF) blended fuel droplets

The present study investigated the micro-explosive combustion characteristics of Jatropha oil-2, 5-dimethylfuran (DMF) blended fuel droplets using fiber-support and high-speed backlit imaging technique. The flame temperature and soot optical thickness (KL) are measured by two-color pyrometry method. The results show that the combustion of Jatropha oil-DMF blended droplets containing four stages: ignition delay, first micro-explosive combustion, d2 law combustion and second micro-explosive combustion stage. The micro-explosion in second and fourth stage is caused by superheating of DMF and pyrolysis, respectively. The flames of DMF, diesel and Jatropha oil droplets belong to typical diffusion combustion. However, the micro-explosive flame of Jatropha oil-DMF blended droplets belongs to premixed combustion. When the DMF concentration reaches 50%vol, the ignition delay is equivalent to that of diesel. The average burning rate of Jatropha oil-DMF blended droplets increases nonlinearly with air velocity. When the DMF concentration is 75%vol, it reaches the peak, which is 66.4% higher than that of diesel. Furthermore, the effect of micro-explosion on droplet flame deformation is summarized into four modes: slight fluttering flame, local flame separation, large deformation flame, large deformation flame with local flame separation. When there is air flow in the horizontal direction, a “soot shell” forms around the droplets. This is due to the upward buoyancy interacts with the horizontal airflow force to form a rotating airflow force, which concentrates the soot particles around the droplets.

Insights into the dynamics of wake flame in a freely falling droplet

The combustion of a freely falling dodecane droplet has been studied experimentally in a droptower-like facility under ambient conditions. A unique ignition mechanism is used by igniting the droplet in pendant mode and releasing it to fall freely. This unveils a different type of droplet wake flame behavior which is explored in this study. Initially, the droplet flame transitions from fully enveloped to a wake flame configuration due to forward extinction. The wake flame has similar characteristics as a laminar lifted triple-flame. As the droplet accelerates, the flame stand-off increases continuously. The change in wake flame topology and intensity occurs in two different regimes corresponding to different droplet diameters. A new non-dimensional parameter has been derived to account for the local balance between buoyancy and momentum diffusion that alters the fuel availability. To explain the flame topological evolutions and transitions for different droplet diameters and Reynolds numbers, a theoretical formulation has been proposed based on the momentum diffusion from surrounding due to relative motion. Further, at very high Reynolds number, flame stretching or shedding regime occurs, causing momentary spikes in flame intensity due to the interaction with asymmetric vortex shedding induced by the Bernard–Von Karman instability. Interestingly, the flame shedding height follows the buoyant flickering scaling, even for the momentum-dominant droplet wake flame. Additionally, the circulation build-up mechanisms are shown to be responsible for the flame shedding events for droplet wake flame at high Reynolds number.

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.

Pengaruh Campuran Bahan Bakar Biodiesel WCO - Diesel terhadap Karakteristik Api Hasil Pembakaran Spray Difusi pada Concentric Jet Burner

Biofuels from waste cooking oil (WCO) represent a sizable opportunity not only in terms of energy production but also as a way for sustainable development despite their low yield, higher viscosity, lengthy production time and cost. Alternatively, biodiesel can be blended at an appropriate blending ratio with convention diesel oils. The biodiesel and its blends is proved to give better emission characteristics than conventional diesel oils. This study aims to experimentally investigate the effect of the fuel blend on the combustion characteristics of WCO biodiesel. The characteristic are the droplet size, flame height, flame width and temperature distribution. In this study, the blended fuel are B0 (Solar), B10, B20, B30, B40 and B100 (WCO biodiesel). Measurement and visualization of the combustion flame for each variation of the fuel mixture was were tested at different pressures, namely 4 bar and 5 bar. The experimental results show that the droplet size increases with increasing WCO concentration in the fuel; on the other hand, visualization and calculations show that the height and width of the flame of the fuel mixture decreases Observation on the temperature distribution shows that the WCO biodiesel mixture has the potential to increase the fire temperature at certain points

Two-phase free jet model of an atmospheric entrained flow gasifier

We present a steady-state two-phase fluid dynamic model based on a classical free jet approach for the simulation of the main reaction zone in an atmospheric entrained flow gasifier. Radial Gaußian profiles of the mixing ratio and velocity for single-phase free jets taken from literature are adapted to the two-phase free jet. Exchange of momentum and mass between the jet and the surrounding is described by a parameter derived from atomization experiments under ambient conditions. With the free jet equations, a pattern of gas phase velocity and temperature is calculated. Droplets are introduced at the nozzle, accelerated, heated up and evaporated. Initial droplet size fractions are measured under ambient conditions. Sub-process models for fuel decomposition, oxidation and the water gas shift reaction are included in the model. The interaction of the sub-process models and the free jet equations are considered via balance equations for momentum, mass and enthalpy, solved in each control volume. The two-phase free jet model (2Ph-FJM) calculates the local composition, velocity and temperature of the gas phase as well as velocity, temperature and evaporation of the fuel droplet fractions. Simulation results are approved for a set of experimental data (i.e. droplet velocity, droplet size distribution and flame structure via OH*-chemiluminescence imaging) from the bench-scale atmospheric entrained flow gasifier REGA.

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.