Spray Flame Characteristics Research Articles

Multiple structures and transition mechanisms of laminar fuel-rich ethanol/air counterflowing spray flames

Structures of laminar non-premixed ethanol/air spray flames in the axisymmetric counterflow configuration are studied under fuel-rich conditions by means of numerical simulations. The monodisperse ethanol spray is carried by air and directed against an air stream. Both streams enter at 300 K, and the system is at atmospheric pressure. Up to three different structures of these flames for identical boundary and initial conditions are identified, and regime diagrams are presented that show their conditions of existence in terms of the gas strain rate on the spray side of the configuration, [Formula: see text], starting from 55/s at an initial spray velocity of 0.44 m/s. The equivalence ratio on the spray side, [Formula: see text], is varied between 1.1 and 1.6, and initial droplet radii, [Formula: see text], from 10 to 50 [Formula: see text]m are considered. The most stable spray flame structure is characterized by two chemical reaction zones. For some conditions, single chemical reaction zones on either side of the counterflow configuration are found. Conditions under which these different flame structures exist are analyzed. Previous studies identified only two different structures for non-identical boundary conditions, and in this study, three different structures are presented for the first time. Moreover, the transition mechanisms of one structure to another are analyzed. The competition between the energy-consuming spray evaporation and the exothermic chemical reaction rates as well as the location of the spray determines the existence of the different flame structures. This transition of the different flame structures may explain spray flame characteristics such as flame pulsation or flame instabilities.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

Optical diagnosis study of fuel volatility on combustion characteristics of spray flame and wall-impinging flame

Finding suitable alternative fuels and reducing carbon emissions are important development directions for future engines. Many carbon-neutral fuels, such as alcohol fuels are widely used, and they are more volatile than traditional diesel fuel. Therefore, exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics and the development of efficient and clean combustion technology. In this paper, the flame structure, flame development characteristics and soot evolution of three fuels, including n-hexane, 74%v n-heptane mixed 26%v isooctane (PRF26) and 53%v n-dodecane mixed 47%v isooctane (D53) were tested, that the cetane number of three fuels is essentially same. High speed self-luminous imaging and RGB two-color method were used to investigated the effect of fuel volatility on combustion. Results can be seen as following. Firstly, during the early stages of n-hexane combustion, a blue premixed flame can be found, and there is a pale-yellow flame in the later stage. As the boiling point increases, the area of the blue flame in the first stage gradually decreases, the area of the yellow flame in the later stage gradually increases, and the spatially integrated natural luminosity (SINL) of flame gradually increases. In addition, as the boiling point increases, the initial spontaneous combustion ignition point is away from the nozzle, the ignition delay time reduces and flame area becomes larger. Furthermore, the flame of high boiling fuels result in higher soot emissions. Secondly, in the event of flame wall-impingement, the near-wall flame of the three fuels has higher flame SINL and more soot formation in comparison to the free spray flame. The flame development of the three fuels is similar. However, there are some differences in the flame brightness distribution. The bright flame areas in n-hexane and PRF26 are concentrated near the wall, while the bright flame area in the D53 flame is concentrated on the wall and the flame head area rolled up with a larger distribution area. The flame near the wall is impacted by the wall temperature that the lower wall temperature leads to prolonged fuel evaporation and mixing time, which increases the ignition delay time. In conclusion, fuel volatility has less influence on flame structure and development, but a significant impact on ignition delay time and emissions. Regardless of whether wall impingement occurs, higher volatile fuels can extend the ignition delay time, increase the flame area and decrease soot emissions from combustion.

Investigation of the mechanism of global and terminal flame uplifted in subsonic horizontal spray jet flame under sub-atmospheric pressure: From macroscopic to microscopic perspective

This work explored how subsonic horizontal spray flames behave under sub-atmospheric pressure and the mechanisms behind their combustion. For examine a horizontal spray flame with rated air volume flow and equivalence ratio, large eddy simulation (LES) and probability density function (PDF) were employed, based on the discrete phase (DPM) random trajectory model. The simulation results are in good agreement with the previous experiments. Through the micro-analysis of the burning time, burning path, and rate of the burn path of droplets under different atmospheric pressures, the combustion characteristics of the flame have been revealed; as well, a component change analysis was conducted to investigate the combustion mechanisms of the flame. Density distributions in buoyancy plumes can reveal the morphologic mechanisms in the corresponding flame regions. A detailed explanation of the varied mechanisms in horizontal spray flames in low-pressure environments is provided, in conjunction with the results of previous experiments on macroscopic flame morphology. There is also a proposed chain of influence across certain characteristics of horizontal spray flames under low pressure. This improves our understanding of mechanisms governing flame behavior in sub-atmospheric pressure environments by allowing us to theoretically design and regulate flames.

Reaction zone characteristics of iso-pentanol swirl spray flames using OH-PLIF and 2C-LII

This paper focuses on the reaction zone and soot emission characteristics of swirl spray flames of iso-pentanol and blends. OH planar laser induced fluorescence (PLIF) measurements were used to study the lift-off and local extinction features. Soot emission was investigated using the planar time-resolved two-colour laser induced incandescence (2C-LII) diagnostic technique. Fuels used in this study were n-heptane, iso-pentanol, ethanol and blends of n-heptane/iso-pentanol and n-heptane/ethanol. Different operating conditions were tested and categorized into three flame types including stable, near blow-off, and far away from blow-off. A clear double flame sheet structure appeared in most flames, referred to as the inner and outer regions in this study. Also, all flames other than pure iso-pentanol displayed an open-up spray in a cone-like “V” shape. Results of stable and near blow-off conditions showed that the addition of iso-pentanol to an n-heptane flame caused the flame to become more attached to the bluff-body. In stable flames, the addition of iso-pentanol to n-heptane increased the occurrence of local extinctions. Whereas the addition of ethanol to n-heptane decreased occurrences of local extinction. Across near blow-off conditions, the impact on lift-off height caused by the addition of ethanol to n-heptane was less than that of the addition of iso-pentanol to n-heptane. In the far from blow-off conditions, less local extinction in pure iso-pentanol was found compared to the n-heptane/iso-pentanol mixture. Soot volume fraction of the iso-pentanol flame was less than the n-heptane flame, and the addition of iso-pentanol to the n-heptane flame reduced the soot volume fractions to a lower level than in the pure n-heptane flame.

Numerical analysis of heat transfer characteristics of spray flames impinging on a wall under CI engine-like conditions

Design of Compression Ignition (CI) engines with improved thermal efficiencies needs better understanding of the heat transfer mechanism from spray flame to the combustion chamber wall. In this regard, heat transfer occurring during the interaction between impinging spray flame and wall, under CI engine-like conditions, is investigated in this study using 3-Dimensional numerical simulations based on an Eulerian–Lagrangian framework. Simulations are performed for different fuel spray injection velocities (which are representative of different fuel injection pressures in CI engines), to examine their influence on the heat transfer between impinging spray flame and wall. To couple the convective and radiative heat transfer at the wall surface with the conduction heat transfer occurring within the finite thickness wall, Conjugate Heat Transfer (CHT) is incorporated in the simulations. A Non-Adiabatic Flamelet/Progress Variable (NA-FPV) approach is employed as the combustion model of n-dodecane, which is considered to be the fuel for liquid spray. Dynamics of the liquid film formed on the wall surface by impinging fuel droplets are captured using a particle-based approach. Contribution of radiative heat flux is taken into consideration using the Discrete Ordinates (DO) method. Results indicate that the total heat flux (sum of convective and radiative heat fluxes) at the wall surface increases with the fuel injection velocity. It is observed that the total wall heat flux is largest in the stagnation zone where the spray flame impinges directly on the wall surface, while the radiative heat flux at the wall surface becomes larger as the distance from this stagnation zone increases. Additionally, it is found that the influence of fuel injection velocity on the radiative heat flow rate at the wall surface is rather small. This radiative heat flow rate when expressed as a percentage of the total wall heat flow rate, ranges from ≈ 18% to 30% (depending on the 3 cases investigated), indicating that its contribution cannot be neglected for the CI engine-like conditions under which the present simulations are performed. Furthermore, to characterize the heat transfer occurring during spray flame-wall interaction process, correlations between the Nusselt number Nu (corresponding to the wall heat loss) and Reynolds number Re (of the flow field) of the form Nu∝Ren, are analysed and compared with that of a previous experimental study to assess their applicability. It is found that, depending on how the Nusselt number Nu is defined (either using the total wall heat flux or the convective heat flux), the value of the correlation index n changes. When Nu is calculated based on the total wall heat flux (which includes the contribution from the radiative heat flux), the value of n is found to be 0.49 which is close to the correlation index value of n=0.4 reported in the recent experiments performed at Toyota Central R&D Labs., Inc.

Numerical Prediction of Turbulent Spray Flame Characteristics Using the Filtered Eulerian Stochastic Field Approach Coupled to Tabulated Chemistry

The Eulerian stochastic fields (ESF) method, which is based on the transport equation of the joint subgrid scalar probability density function, is applied to Large Eddy Simulation of a turbulent dilute spray flame. The approach is coupled with a tabulated chemistry approach to represent the subgrid turbulence–chemistry interaction. Following a two-way coupled Eulerian–Lagrangian procedure, the spray is treated as a multitude of computational parcels described in a Lagrangian manner, each representing a heap of real spray droplets. The present contribution has two objectives: First, the predictive capabilities of the modeling framework are evaluated by comparing simulation results using 8, 16, and 32 stochastic fields with available experimental data. At the same time, the results are compared to previous studies, where the artificially thickened flame (ATF) model was applied to the investigated configuration. The results suggest that the ESF method can reproduce the experimental measurements reasonably well. Comparisons with the ATF approach indicate that the ESF results better describe the flame entrainment into the cold spray core of the flame. Secondly, the dynamics of the subgrid scalar contributions are investigated and the reconstructed probability density distributions are compared to common presumed shapes qualitatively and quantitatively in the context of spray combustion. It is demonstrated that the ESF method can be a valuable tool to evaluate approaches relying on a pre-integration of the thermochemical lookup-table.

Spray Flame Characteristics of Bio-Derived Fuels in a Simulated Gas Turbine Burner

Abstract Fuel injection plays an important role in liquid-fueled gas turbine combustion. The strong interdependence of liquid breakup and atomization, turbulent dispersion of these droplets, droplet evaporation, and fuel–air mixing make the spray modeling an extremely challenging task. The physical processes are even more difficult to predict for alternative fuels with different thermophysical properties. In this study, spray flames of unheated and preheated vegetable oil (VO) produced by an air-blast (AB) atomizer in a swirl stabilized combustor are investigated experimentally. Phase Doppler particle analyzer (PDPA) is used to measure the instantaneous diameter and axial velocity of droplets at different axial and radial locations in both flames. Experiments are conducted at an equivalence ratio of 0.79 and atomizing air to liquid ratio by a mass of 2.5 to obtain stable VO flames. Radial profiles of mean axial velocity and Sauter mean diameter (SMD) are presented to show the effect of fuel preheating. Joint probability density functions (joint PDF) are presented to show the correlation between droplet diameter and axial velocity. Results are analyzed to show that both sprays exhibit self-similar droplet diameter distributions at different axial and radial locations when normalized properly. Thus, the vast amount of PDPA data in the spray can be reduced to simple distribution functions. A method to reconstruct the joint PDF from experimentally determined distribution functions is presented. We envision that the joint PDF approach outlined in this study could be implemented in high-fidelity computational fluid dynamic models to improve spray predictions in future studies.

Effects of carbon dioxide addition on diesel spray flame characteristics in oxygen-carbon dioxide atmospheres

In order to study the diesel flame combustion characteristics in the different OCD atmospheres, a DCER model is proposed, which considers the air entrainment rates and the influences of OCD atmospheres. Firstly, FLOL and X+ calculation formulas are obtained. Furthermore, the chemical model of primary reference fuel (n-heptane) and the geometric structure of CVCC are established. Secondly, to verify the model, the experiment is carried out in a visualized CVCC and processes of flame combustion are captured by a high-speed camera. Furthermore, results of flame structure, temperature maps, FLOL and FNL are discussed at 3 MPa and 850 K in air, 50%O2/50%CO2(50 CDC), 43 CDC and 35 CDC, respectively. Through comparison between the DCER model simulation and the experiment, the conclusions are as follows: combustioncharacteristicsare quite different in several atmospheres. Comparing the temperature maps, there is an obvious distinction in the upper part of the flame upstream region in 50 CDC and 43 CDC. The DCER model can predict the diesel FLOL in 50 CDC, 43 CDC and 35 CDC with errors of 5.35%, 6.47% and 3.65%, respectively. The changes of OCD have a great influence on FLOL and FNL. With CDC decreasing, FLOL and FNL decline stably and the maximum of FNL appears earlier.

Combustion and soot characteristics of hydrotreated vegetable oil compression-ignited spray flames

Quantitative measurements of combustion and in-flame soot in hydrotreated vegetable oil (HVO), biodiesel (rapeseed methyl ester [RME]) and diesel fuel spray flames are presented in this study. The measurements were performed in a compression ignition chamber with optical access having engine-like thermodynamic conditions. The simultaneously employed optical techniques were high-speed diffuse back-illuminated light extinction imaging for measuring the optical depth (KL) of in-flame soot, and OH* chemiluminescence for imaging high temperature reaction zones. The results show that diesel produces a considerably higher amount of in-flame soot compared to RME and HVO, which is likely due to the aromatics present in diesel, while RME and HVO produce similar levels. RME was found to produce slightly less soot, which is likely due to the fuel-bound oxygen, since HVO and RME have a similar number of carbon-carbon bonds. The soot production during the premixed and mixing-controlled period were found to have similar trends. The time lag between start of ignition and soot formation was found to be constant for varying ambient temperatures for individual fuels, where soot was detected earliest after ignition for diesel, followed by HVO and RME, matching the measured sooting tendencies. The stabilised flame lift-off length of HVO was found to be shorter than that of diesel, while RME had the shortest of them all.

Experimental study on the combustion characteristics of high-pressure octanal spray flames

In view of the stringent emission regulations, oxygenated fuels are growing in popularity owing to their potential to reduce emissions in diesel vehicles and off-road applications. It is important to explore the combustion characteristics of oxygenates that can match the energy content and density of diesel whilst reducing the formation of pollutants. Furthermore, having similar thermo-physical properties to diesel could yield them as drop-in fuel candidates. In this work, the combustion characteristics of a long carbon chain oxygenate, octanal (C8H16O), and diesel, were investigated using the two-colour pyrometry method to study the temperature and soot distributions in the spray flames. This investigation was done by injecting the fuels at high pressures in an optically accessible constant volume chamber under a diesel engine-like environment, at high ambient pressures and temperatures. The soot lift-off length for octanal was longer than for diesel, which reduced the amount of time available for soot to grow. For the same injected mass of diesel and octanal, the combustion of octanal was faster due to the presence of fuel-bound oxygen. The two-colour pyrometry results revealed that the spatial distribution of soot for octanal was slightly different from that of diesel. The chemical structure of octanal, consisting of a long chain alkane with an aldehyde functional group at the end and without the presence of any aromatic structures, hindered the soot formation significantly throughout the combustion process when compared to diesel.

Experimental investigation on horizontal jet spray flame characteristics with a constant air-fuel ratio in different sub-atmospheric pressures

It is hoped that horizontal liquid fuel spray burners can be better applied to small heating and heat transfer equipment in high-altitude areas. The morphology and temperature characteristics of diesel horizontal spray flame were investigated in a low-pressure chamber (0.05–0.10 MPa). The results show that when the air-fuel ratio is constant, the horizontal projection length of the and uplifted feature of the flame are proportional to the atmospheric pressure, which is completely contrary to the change rule when only the volume flow of air is kept constant. When the air-fuel ratio is constant, the flame trajectory at different atmospheric pressures can be normalized into two equations with 0.08 MPa as the dividing line. The dimensionless flame fluctuation length β can reveal the stability of the horizontal spray flame. An increase of β indicates that the diameter of the flame holder and swirling intensity need to be moderately increased at high altitudes. That is also confirmed from the perspective of the air entrainment coefficient. In addition, an accurate flame length prediction model is established. Dimensionless temperature analysis reveals that the air-fuel ratio needs to be closer to the stoichiometric ratio when compared with normal pressure.

Numerical analyses of laminar flames propagating in droplet mists using detailed and tabulated chemistry

Numerical analyses of laminar flames propagating in mono-dispersed quiescent droplet mists are addressed. Attention has been given to the evolution of combustion reactions interacting with different mists and the assessment of chemistry simplifications. In the first part, investigations are exclusively performed in a detailed description of the chemistry. Focus is concentrated on simplifications typically assumed in simulations of turbulent spray combustion. Namely, evaporative cooling and differential diffusion. These phenomena are systematically studied together with the characteristics of the observed flammability limits. In the second part, simulations conducted with the flamelet generated manifold (FGM) method are compared with detailed chemistry results. Flame propagation speed and profiles of selected quantities are used. Results demonstrate to what extent common simplifications applied to turbulent spray combustion, as well as FGM tables constructed with single-phase flamelets, are suitable for representing spray flame characteristics.

Large Eddy Simulation of Swirled Spray Flame Using Detailed and Tabulated Chemical Descriptions

Accurate characterization of swirled flames is a key point in the development of more efficient and safer aeronautical engines. The task is even more challenging for spray injection systems. On the one side, spray interacts with both turbulence and flame, eventually affecting the flame dynamics. On the other side, the structure of turbulent spray flame is highly complex due to equivalence ratio inhomogeneities caused by evaporation and mixing processes. The first objective of this work is to numerically characterize the structure and dynamics of a swirled spray flame. The target configuration is the experimental benchmark named MERCATO, representative of an actual turbojet injection system. Due to the complex nature of the flame, a detailed description of chemical kinetics is necessary and is here obtained by using a 24-species chemical scheme, which has been developed for numerical simulations of spray flames. The first Large Eddy Simulation (LES) of a swirled spray flame using such a detailed chemical description is performed here and results are analyzed to study the complex interactions between the spray, the turbulent flow and the flame. It is observed that this coupling has an effect on the flame structure and that flame dynamics are governed by the interactions between spray, precessing vortex core and flame front. Even if such a detailed kinetic description leads to an accurate characterization of the flame, it is still highly expensive in terms of CPU time. Tabulated techniques have been expressly developed to account for detailed chemistry at a reduced computational cost in purely gaseous configurations. The second objective is then to verify the capability of the FPI tabulated chemistry method to correctly reproduce the spray flame characteristics by performing LES. To do this, results with the FPI method are compared to the experimental database and to the results obtained with the 24-species description in terms of mean and fluctuating axial gas velocity and liquid phase characteristics (droplet diameter and liquid velocity). Moreover, the flame characterization obtained with the FPI approach is compared to the results of the 24-species scheme focusing on the flame structure, on major and minor species concentrations as well as on pollutant emissions. The potential and the limits of the tabulated approach for spray flame are finally assessed.

Transported Joint Probability Density Function Simulation of Turbulent Spray Flames Combined with a Spray Flamelet Model Using a Transported Scalar Dissipation Rate

ABSTRACTNumerical simulations of a turbulent ethanol/air spray flame are performed using a combined formulation of a joint gas mixture fraction-enthalpy probability density function (PDF) and a spray flamelet model, where the scalar dissipation rate of the mixture fraction is described with a transport equation. Spray evaporation is included in the flamelet model through the pre-calculation of counterflowing laminar spray flames with a detailed reaction mechanism of 38 species and 337 elementary reactions for ethanol/air combustion. A transport equation for the mean scalar dissipation rate of the mixture fraction is derived, taking into account the spray evaporation. The computed results are compared with the experimental data provided by A. Masri, University of Sydney, Australia. All major spray flame characteristics, including the Sauter mean diameter, the droplet velocity, and its turbulent fluctuations as well as gas temperature, generally show good agreement with experiments. Different models for the scalar dissipation rate of the mixture fraction are not significantly different for the flame under consideration except for inside the chemical reaction zone, where the new model shows some improvement compared to the experiment.

Secondary atomization and spray flame characteristics of carbonated W/O emulsified fuel

The secondary atomization behavior and spray flame characteristics of carbonated W/O emulsified fuel were studied. The dissolved CO2 in the carbonated emulsified fuel was able to reduce the bubble nucleation energy needed for secondary atomization. First, the spray characteristics of carbonated fuels were studied in a N2 environment at 923K, using high-magnification shadow imaging of the spray droplets. This allowed the spray droplets to be clearly visualized during secondary atomization. The dissolved CO2 was shown to enhance secondary atomization by decreasing the bubble nucleation energy, resulting in the production of finer spray droplets. The spray flame characteristics of n-dodecane, emulsified fuel, and carbonated emulsified fuel were then studied in a laminar counterflow burner. Direct photography of the flame using a color high-speed video camera and magnified shadow imaging using a monochrome high-speed video camera were used. Compared the ignition point of luminous flames in n-dodecane, an ignition delay was found in the emulsified fuel, which was not found in the carbonated emulsified fuel. Compact luminous flames were also formed in the carbonated emulsified fuel. In the magnified flame images, bright spots of higher luminosity than the surrounding area were observed in both the emulsified fuels. These proved to be a useful marker for secondary atomization. In the carbonated emulsified fuel, the bright spots appeared earlier, indicating that the dissolved CO2 reduced the droplet temperature at which secondary atomization occurred. This effect countered the ignition delay caused by water addition in the emulsified fuel. A higher frequency of bright spot occurrence was also found in the carbonated emulsified fuel. It was demonstrated that carbonation of emulsified fuel improved the spray flame characteristics through the enhancement of secondary atomization.

Fuel unsaturation effects on NOx and PAH formation in spray flames

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the detailed CRECK mechanism (http://creckmodeling.chem.polimi.it/index.php/current-version-kinetic-mechanisms/low-and-high-temperature-complete-mechanism). The reduced mechanism and spray models are validated against the shock tube ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network (ECN). 3-D simulations are performed using the CONVERGE software to examine the structure and emission characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to β scission reactions resulting from the presence of double bond in 1-heptene.

Spray and combustion of waste cooking oil biodiesel in a compression-ignition engine

An experimental study was carried out to compare the spray and combustion characteristics of waste cooking oil biodiesel with those of conventional diesel in a common-rail direct-injection compression-ignition engine. The indicated mean effective pressure of approximately 0.7 MPa was tested under an engine speed of 800 r/min. The fuels were injected at an injection timing of −5 crank angle degree after top dead center with injection pressures of 80 and 160 MPa. The experiments were performed to investigate the combustion and emission characteristics in a metal engine, while spray and combustion flame characteristics in an optical engine. The morphological and compositional features of particulate matter were also analyzed by using transmission electron microscopy and thermogravimetric analysis. The direct imaging of the spray indicated a longer injection delay for waste cooking oil biodiesel. However, it exhibited longer liquid penetration length and narrower spray angle than diesel as the spray developed. Waste cooking oil biodiesel combustion showed a longer ignition delay with a slightly lower peak of in-cylinder pressure and heat release rate than diesel due to the deteriorated atomization and evaporation of fuel spray. In terms of emission, waste cooking oil biodiesel showed benefits in reduction of carbon monoxide, unburned hydrocarbon and particulate matter emissions. The particulate matter from waste cooking oil biodiesel was composed of smaller primary particles and more volatile organic fraction than diesel soot. The combustion flame imaging verified the retarded combustion phase of waste cooking oil biodiesel. Waste cooking oil biodiesel combustion showed lower flame luminosity accompanied by shorter visible flame duration, at both injection pressures. The flame spatial fluctuation and flame nonhomogeneity were also lower for waste cooking oil biodiesel.

F232 CO_2溶解エマルジョン燃料の二次微粒化挙動と噴霧火炎特性(OS-03:革新的技術のための燃焼研究(5))

Secondary atomization behavior and spray flame characteristics of carbonated W/O emulsified fuel were studied. An emulsified fuel or carbonated emulsified fuel was injected into a flame stabilized in a laminar counterflow field. Direct photography of the flame using a color high-speed video camera and magnified shadow imaging of spray droplets using a monochrome high-speed video camera were used. As a result, tip position of spray flame became shorter and spray flame was compact when carbonated emulsified fuel was used. This was because dissolved CO_2 enhanced secondary atomization, leading to improvement of fuel vapor ignition at upstream region. Secondary atomization behavior of carbonated emulsified fuel droplet was also observed at upstream region.

Combustion Properties of Spray Flames of Canola Methyl Ester and Diesel Blends at Lean Inlet Equivalence Ratios

The objective of this study was to document the combustion characteristics of spray flames of Canola Methyl Ester (CME) and No 2 diesel (petroleum fuel) blends. Three blends with 25%, 50% and 75% volume concentration of CME were studied. The fuel was atomized and burned in a heated stream of air at supply equivalence ratios of 0.62 and 0.75. Measurements of global CO and NO emissions, in-flame temperature and in-flame concentrations of combustion products were made. The near-injector homogeneous gas-phase reaction zone increased in size with the addition of CME. The global NO emission decreased significantly with the increase in CME content in the fuel blend at the equivalence ratio of 0.62; however, the global NO emission index of all the tested flames was comparable at the equivalence ratio of 0.75. The in-flame NO concentration profiles and flame temperature profiles followed similar trends, suggesting that the thermal mechanism of NO formation was dominant in these flames.