Flame Lift-off Length Research Articles

Effects of an injector cooling jacket on combustion characteristics of compressed-ignition sprays with a gasoline-hydrogenated catalytic biodiesel blend

The fuel properties in Gasoline Combustion ignition modes (GCI) have been adapted by mixing high reactivity fuels for improving poor combustion stability with pure gasoline. Precise control of the injector tip temperature, as well as the fuel temperature, are considered as the essential boundary conditions to conduct the fundamental investigation on combustion process of GCI spray. Therefore, the impacts of an injector cooling jacket on the spray and combustion developments of a mixture contains 60% gasoline and 40% hydrogenated catalytic biodiesel are studied using a constant volume combustion chamber (CVCC) working under GCI mode. A dummy injector is equipped with a thermocouple to measure the effects of the cooling jacket and ambient temperature on tip temperature. The Schlieren imaging and Diffused Background Illumination Extinction Imaging techniques are employed to visualize the reacting spray development and in-flame soot formation. The results illustrate that the injector tip temperature is well-controlled with aiding of the cooling jacket, which only increases by 35 K with increasing ambient temperature by 250 K. Furthermore, it is found from the optical experiments that the liquid length, ignition delay and lift-off length are enlarged for the cooling mode compared to that of the uncooled one. The cooling jacket also brings in a larger overlapping area between liquid length and flame lift-off length. However, the in-flame soot production for the cooling jacket is increased more than twice compared to that of the uncooled one.

Coupling RSM with soot model for the study of soot formation in a momentum-dominated strained jet flames

This paper presents a comparison between the numerically determined soot volume fraction (SVF) in a momentum-dominated turbulent diffusion strained jet flames burning ethylene/hydrogen/nitrogen mixture in air with experimental results. The soot inception model used is based on the formation rate of two or three ring poly aromatic hydrocarbon with C6H6 as the soot precursor. Second-moment closure Reynolds stress method turbulent model is used in conjunction with the governing equation of mass, momentum and energy. The turbulent combustion kinetic chemistry interaction is modeled by choosing a chemical equilibrium non-premixed combustion model along with the species inlet diffusion probability density function (PDF). Soot radiation source term like P-1 radiation model is also included along with the turbulent combustion model. The soot turbulence interaction invoked uses single-variable PDF based on the temperature and mixture fraction (Z) for measurement of flame lift-off height, flame length and radiative heat loss. Equilibrium-based Lee soot oxidation model is chosen to estimate the hydroxyl (OH) and O2 species for the oxidation of soot particles. The numerical solutions for each of the experimental configurations are proposed by Mahmoud and co-workers for three different strained flames (4000 s−1, 7300 s−1, and 12,900 s−1). In each discretized finite volume cell, using a quadratic upstream interpolation for convective kinematics algorithm in spatial coordinate and time-bounded second-order accurate in temporal coordinate solution is obtained. All the above turbulence combustion interaction models are iterated with unit Courant–Friedrichs–Lewy condition. It is noted that the mixture fraction (Z) PDF used is very sensitive to the chosen turbulence combustion soot interaction models. Also, it is observed that the simulated outputs are in good agreement with the measured experimental data in terms of SVF, temperature both axially and radially. SVF increases with a decrease in the flame strain rate and shows the reverse trend in radially integrated soot volume reaction. In agreement with experiments, soot productivity shows a linear relationship in momentum-dominated flame with an increase in the strain rates for soot productivity, maximum mean SVF, exit Froude number, exit C2H4 flow rate.

Optical study on spray combustion characteristics of PODE/diesel blends in different ambient conditions

Polyoxymethylene dimethyl ethers (PODE) have been reported as a promising alternative fuel for the potential to reduce soot, CO and HC emissions of the diesel engine and improve the engine performance of gasoline compression ignition and dual-fuel engines. However, the fundamental data of spray and combustion characteristics of fuels with PODE are still deficient. In this study, the ignition delay, flame lift-off length (LOL) and soot distribution of diesel/PODE blends with PODE volume fraction of 0–100% were studied using high-speed OH chemiluminescence and planar laser-induced incandescence under different ambient temperature and oxygen concentration conditions. Results show that the ignition delay decreases and the flame LOL increases with the increase in PODE content. The increase in LOL is more distinct in lower ambient temperature conditions, whereas the decrease in ignition delay is relatively larger in higher temperature and lower oxygen concentration conditions. The total soot amount in the tested region decreases more than 50% and 95% when PODE volume fraction reaches 20% and 50%, respectively. When the PODE fraction is larger than 50%, nearly no soot can be observed in all the tested cases. The influence of PODE on soot reduction is more significant in higher ambient oxygen concentration and lower ambient temperature conditions. The soot distribution profile of the soot shows that the addition of PODE can not only reduce the soot formation but also accelerate the soot oxidation.

Multifunctional Graphene-Based Additives for Enhanced Combustion of Cracked Hydrocarbon Fuels under Supercritical Conditions

ABSTRACT As a particulate fuel additive, functionalized graphene sheets (FGS), with and without the decoration of nanoparticles, provide a means to form stable colloids with liquid hydrocarbons, act as in situ catalysts, and prevent attached nanoparticles from agglomerating or sintering during heating. Recent pyrolysis experiments and simulations have shown the synergetic effect of Pt and FGS structures accelerating fuel conversion and hydrogen formation. In this paper, the role of graphene-based additives (with and without platinum (Pt) nanoparticles) on enhancing the ignition and combustion characteristics of cracked n-dodecane hydrocarbon fuels under supercritical conditions is examined. Supercritical ignition and combustion experiments were conducted using a high pressure and high temperature windowed combustion chamber coupled to a supercritical fuel pyrolysis reactor. The combustion experiments indicated that the presence of small amounts of particulate additives (100 ppmw) to the fuel can reduce ignition times and increase subsequent combustion rates. In particular, the addition of FGS-supported Pt nanoparticles reduced ignition delay times by nearly a factor of 3 (12.4 to 4.2 ms), increased spray spreading angles by approximately 32.0% (15.4 to 20.3°), reduced the flame liftoff length by 54.0% (1.74 to 0.8 mm), and demonstrated an increase in fuel conversion for a fixed reaction time of 35.0% relative to the pure fuel baseline. These results support the notion that nanostructured fuel additives have the potential to enhance both the heat sink capacity and combustion performance of liquid hydrocarbon fuels for their use in advanced high speed propulsion systems.

Spray combustion simulation study of waste cooking oil biodiesel and diesel under direct injection diesel engine conditions

Spray combustion characteristics of waste cooking oil biodiesel (WCO) and conventional diesel fuels were simulated using a RANS (Reynolds Averaged Navier Stokes) based model. Surrogates were used to represent WCO and diesel fuels in simulations. N-tetradecane (C14H30) and n-heptane (C7H16) were used as surrogates for diesel. Furthermore for WCO, surrogate mixtures of methyl decanoate, methyl-9-decenoate and n-heptane were used. Thermochemical and reaction kinetic data (115 species and 460 reactions) were implemented in the CFD code to simulate the spray and combustion processes of the two fuels. Validation of the spray liquid length, ignition delay, flame lift-off length and soot formation data were performed against previous published experimental results. The modeled data agreed with the trends obtained in the experimental data at all injection pressures. Further investigations, which were not achieved in previous experiments, showed that prior to main ignition, a first stage ignition (cool flame) characterized by the formation formaldehyde (CH2O) species at low temperature heat release occurred. The main ignition process occurred at high temperature with the formation of OH radicals. Furthermore, it was observed that the cool flame played a greater role in stabilizing the downstream lifted flame of both fuels. Increase in injection pressure led to the cool flame location to be pushed further downstream. This led to flame stabilization further away from the injector nozzle. WCO had shorter lift-off length compared to diesel as a result of its cool flame which being closer to the injector. Soot formation followed similar trends obtained in the experiments.

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.

Numerical investigation on effects of fuel tube diameter and co-flow velocity in a methane/air non-premixed flame

In this paper, the effects of variations in the fuel tube diameter and co-flow velocity in the combustion chamber on the non-premixed laminar flame are investigated. Methane gas, as a fuel, and the dry air, as an oxidizer. The size of the combustion chamber is constant and, by changing the fuel tube diameter and co-flow velocity, changes in the numerical values of temperature, velocity, density, and concentration of the species of reactants and products in the combustion chamber are evaluated. A finite volume method (FVM) with staggered grids is used for numerical solution. Equations of continuity, momentum, energy, ideal gas state and kinetic equations with thermodynamic and thermochemical information of chemical species are solved using numerical method of SIMPLE. The convective terms are discretized using Power Law scheme (PLS).The calculations are carried out using Dryer and Glassman’s three-stage chemical kinetics. Variable under relaxation factor dependent on temperature has been used to handle the solving chemical kinetic equations. Initially, the results of calculations are compared with the experimental and numerical results of other researchers, which show an acceptable agreement.. The results show that increasing the diameter ratio reduces the length of the flame. With the large ratio of the diameters, location of the combustion’s maximum temperature is at the chamber entrance and for the small diameter ratios, its location moves to nearly outlet of the chamber. In addition, the reduction of the ratio of the diameters increases the flame lift-off. Also the results show that the optimal of diameters ratio is 0.6 in order to prevent the lift-off flame and return the flame to inlet opening of combustion chamber. Also increasing the fuel tube diameter, increases the amount of oxygen due to the return flow formation and decreases the volumes of water vapor and carbon dioxide in the centerline of the combustion chamber. The flame length attains the maximum possible value with respect to diameter ratio of 0.6 at inlet air velocity of 0.3 m/s. In addition, it is shown that increasing the air velocity increases the total flame lift-off and flame length until the air velocity reaches the value of around 0.3 m/s and by increasing the air velocity more than 0.3 m/s, the total flame lift-off and flame length decreases.

Research on the application of aviation kerosene in a direct injection rotary engine – Part 2: Spray combustion characteristics and combustion process under optimized injection strategies

Investigating the adaptability of aviation kerosene in the direct injection rotary engine is of great significance and practical value. In this paper, an optically visible constant volume combustion chamber platform with various optical diagnostic techniques was firstly set up to study the spray combustion characteristics of the fuel. Secondly, a reliable three-dimensional dynamic simulation model of direct injection rotary engine was established to investigate the combustion process of aviation kerosene under optimized injection strategies. The experimental results show that the ambient temperature of 800 K and ambient pressure of 20 bar is the ignition limit of aviation kerosene under the condition of rotary engine. With the increase of ambient conditions, the ignition delay and the flame lift-off length shorten continuously. At 850 K ambient temperature, when the ambient pressure reaches 25 bar, the lift-off length will no longer shorten and remains at about 6 mm. The simulation results indicate that under the Injection position A-Injection angle 90° (Case A3), higher combustion efficiency before top dead center can be achieved due to more appropriate distribution, concentration and atomization quality of fuel at ignition timing. The peak pressure in cylinder reaches 38.89 bar, which means a better engine dynamic performance. Meanwhile, acceptable CO and Soot emissions are obtained by the complete combustion of fuel in Case A3, however, the higher combustion temperature makes the production of pollutant NO slightly higher than other injection strategies.

Experimental study of influence of Liquefied Petroleum Gas addition in Hydrotreated Vegetable Oil fuel on ignition delay, flame lift off length and soot emission under diesel-like conditions

The fundamental behaviour on ignition and combustion characteristics of blends of Hydrotreated Vegetable Oil and Liquid Petroleum Gas was investigated in a constant high pressure, high temperature combustion chamber, using a prototype lab-scale injection system adapted from a conventional common-rail system to conduct the injection events, ensuring that fuel was liquid at any point of the injection system and avoiding the formation of fuel vapour bubbles that could alter the injected fuel behaviour.The ignition delay, flame lift-off length and the soot formation were studied by means of high-speed imaging techniques, for different operating conditions. The aim of the work is to characterize the effect of Hydrotreated Vegetable Oil-Liquid Petroleum Gas blend ratios on the previously mentioned parameters.Experimental results show that the behaviour of the fuel blends follow the expected trends of conventional diesel type fuels when varying ambient temperature, density and injection pressure. Hydrotreated Vegetable Oil, being the highest reactivity fraction, controls auto ignition of the blend. However, Liquid Petroleum Gas acts as combustion inhibitor increasing both ignition delay and lift-off length as its ratio in the blend increases. As a consequence, the differences observed in terms of flame radiation suggest that increasing Liquid Petroleum Gas fraction reduces soot formation as it promotes a higher air/mixture.

Experimental study on ignition characteristic of gasoline-biodiesel blended fuel in a constant-volume chamber

A detailed experimental study on gasoline biodiesel fuel (GB) blends was conducted to investigate its ignition and combustion characteristics under low-temperature range using an optically accessible constant volume combustion chamber (CVCC). The fuel samples were four GB blends including GB20, GB40, GB60 and GB80 corresponding to 20 %, 40 %, 60 %, and 80 % volumetric biodiesel respectively, neat gasoline, and neat biodiesel. Fuel samples were injected into the CVCC to combust using a single-hole research-grade injector. Natural soot luminous images from the combustible flame were captured by a CMOS camera to determine the ignition delay and the flame lift-off length. The ignition delay was also obtained by analyzing pressure traces from a high-frequency piezoelectric pressure transducer. The results regarding the ignition process for the pressure-based and luminosity-based ignition delays showed that both approaches presented similar tendencies. However, the pressure-based ignition delay is always a little longer than the luminosity-based ignition delay. The difference between the two definitions of ignition delay tends to decrease with the longer ignition delay or the enhanced mixing, and vice versa. As lower 60 % biodiesel fractions, the increase of biodiesel significantly reduced ignition delay and produced a lower maximum peak of heat release rate. The combustion characteristic of blend with a higher 60 % biodiesel is almost similar to pure biodiesel. In general, lift-off length lengthens with an increase in biodiesel because of its high viscosity and high surface tension. However, for the 750 K case, the lift-off length decreases due to a rapidly reduced ignition delay with the increase in biodiesel fraction (less than 60 % biodiesel). Based on experimental data, the moderate biodiesel addition (less than 20 %) can improve the ability of cold-engine starting, also solve engine misfire under low-load-condition operation due to its flammability while maintaining advantages of gasoline with great volatility and high ignition delay which significantly enhance the mixture formation process.

Radiation Model of Horizontal Jet Flame Governed by Buoyancy and Momentum

AbstractBuoyancy causes a horizontal jet flame to bend upward when it loses sufficient initial momentum. Therefore, the variation in flame trajectory must be considered to accurately predict the thermal radiant flux of a horizontal jet flame governed by buoyancy and momentum. We introduce the flame trajectory equation into the linear source equation of radiation to establish a new model. The flame length ratio is calculated by flame trajectory length, horizontal projected flame length, and vertical projected flame height in the new model. This paper also presents a formula for flame lift-off distance and flame radiant fraction. We further list the geometric parameters of the horizontal-kite shape, tile-kite shape, and flame trajectory models. Comparing the experimental data and model predictions shows that the flame trajectory model is more accurate than the horizontal-kite and tile-kite models, especially when the Froude number is low. We also study the effects of flame lift-off distance and flame length ratio on predictive accuracy. The flame trajectory model computes quickly, and a normal laptop computer needs only 1.08 s to get a picture with 24,000 data points.

Effects of inversed-delta injection rate shaping on diesel spray flame liquid length, lift-off length and soot onset

In this paper, effects of continuous injection rate ramp-down (i.e., “inversed-delta” injection rate shaping) on liquid length, lift-off length and soot onset of diesel spray flame are investigated for better understanding of its combustion characteristics. Time-sequence images of the diesel spray flame are acquired in a constant volume combustion vessel CVCC using a novel TAIZAC injector. One-dimensional modeling is performed to discuss the air entrainment and mixture distributions. During the injection period, no systematic trend is observed on the liquid and lift-off length for any injection profiles. Contrarily, the soot onset for the inversed-delta injection progressively recedes towards the nozzle while it appears constant for the conventional “rectangle” injection. After the end-of-injection EOI, the fuel vaporization and combustion recession occur at a faster rate in the rectangle injection proportional to its injection pressure, possibly due to stronger and faster entrainment waves than that of the inversed-delta injection. From 1D simulation, the liquid length and spray equivalence ratio distributions stratified “layer” seems to be continuously shifted upstream during the injection period in the inversed-delta injection. It is interesting to note that the centerline equivalence ratio value of ϕcl=2 correlate well with the soot onset observed experimentally, suggesting that the air entrainment per injected fuel could likely be promoted during injection period in the inversed-delta injection.

Understanding the diesel-like spray characteristics applying a flamelet-based combustion model and detailed large eddy simulations

This investigation analyses the structure of spray A from engine combustion network (ECN), which is representative of diesel-like sprays, by means of large eddy simulations and an unsteady flamelet progress variable combustion model. A very good agreement between modelled and experimental measurements is obtained for the inert spray that supports further analysis. A parametric variation in oxygen concentration is carried out in order to describe the structure of the flame and how it is modified when mixture reactivity is changed. The most relevant trends for the flame metrics, ignition delay and lift-off length are well-captured by the simulations corroborating the suitability of the model for this type of configuration. Results show that the morphology of the flame is strongly affected by the boundary conditions in terms of the reactive scalar spatial fields and Z–T maps. The filtered instantaneous fields provided by the simulations allow investigation of the structure of the flame at the lift-off length, whose positioning shows low fluctuations, and how it is affected by turbulence. It is evidenced that small ignition kernels appear upstream and detached from the flame that eventually merge with its base in agreement with experimental observations, leading to state that auto-ignition plays a key role as one of the flame stabilization mechanisms of the flame.

Optical Measurements of In-Flame Soot in Compression-Ignited Methyl Ester Flames

This study investigates in-flame sooting characteristics of biodiesel surrogates in compression-ignited spray flames. The aim of the study is to produce reliable experimental data on in-flame soot for validation of kinetic mechanisms and soot models. A rapeseed oil biodiesel [rapeseed oil methyl ester (RME)] was compared to neat methyl oleate (MO) and methyl decanoate (MD). In addition, neat n-heptane was chosen as a baseline fuel and the effect of blending n-heptane and MD on soot production was investigated. The study was performed in a single-cylinder research engine with optical access to a single spray for varying ambient gas temperatures (825–990 K). The in-flame soot was measured by a high-speed diffuse back-illuminated extinction imaging system, and the flame lift-off length (FLOL) and the corresponding estimated equivalence ratio at FLOL was measured from high-speed OH* chemiluminescence imaging. Results show that RME has the highest tendency to soot, closely followed by MO, independent of the FLOL equivalence ratio. Having almost identical fuel properties, this is likely due to the higher degree of unsaturation in RME compared to MO. When comparing MD to n-heptane, a much lower in-flame soot production rate is observed despite the fact that MD has a higher number of carbon–carbon bonds, concluding that the high fuel oxygen ratio in MD is effectively reducing in-flame soot production. Generally, for FLOL equivalence ratio leaner than ∼2, no in-flame soot was produced for all ambient gas temperatures. The in-flame soot production rate also showed a clear ambient gas temperature dependence for constant FLOL equivalence ratios, where the soot production rate increased with increased gas temperatures.

An Analysis of Turbulent Mixing Effects on the Soot Formation in High Pressure n-dodecane Sprays

An n-dodecane spray flame, known as Spray A, is simulated under the diesel engine conditions. The simulations are based on the well-mixed assumption where the turbulence-chemistry interactions are ignored, and employ the semi-empirical multi-step Moss-Brookes soot model coupled with the Reynolds-averaged turbulence model and a Lagrangian discrete phase spray model. A 54-species reduced n-dodecane chemical mechanism is employed in all simulations to evaluate the reaction rates. The importance of the turbulent mixing on the soot formation is analysed using three different turbulent Schmidt numbers; Sct = 0.7, 1.1 and 1.4. The non-reacting case is first validated using the mixture fraction and the velocity fields. It is found that the jet velocity and penetration length are unaffected by Sct for the inert case, however, the mixture fraction is sensitive to Sct where Sct = 1.1 leads to an excellent agreement with the measurements. Reacting simulations are compared with experimental data in terms of the ignition delay and the flame lift-off length. The results confirm the ignition delay time is marginally affected by changes in Sct. At the baseline condition, Sct = 0.7 and 1.1 result in a very similar value for the lift-off length which is in good agreement with the experiment although at higher ambient temperatures, only Sct = 0.7 agrees well with the measurements. It is found that the formation of formaldehyde and acetylene increases as the level of mixing decreases while the trend is opposite for the OH mass fraction. Consequently, with increasing Sct the soot volume fraction increases and the soot-containing region is extended. The results show that the development of the soot mass is not well captured, regardless of the value of the turbulent Schmidt number.

Effects of turbulence-chemistry interactions on auto-ignition and flame structure for n-dodecane spray combustion

The Engine Combustion Network (ECN) spray A under diesel engine conditions is investigated with a non-adiabatic 5D Flamelet Generated Manifolds (FGM) model with the consideration of detailed chemical kinetic mechanisms. The enthalpy deficit due to droplet vapourisation is considered by employing an additional controlling parameter in the FGM library. In this FGM model, β-PDF is used for the PDF integration over the control variable space. Validation results in non-reacting conditions indicate relatively good agreement between the predicted and experimental data in terms of liquid and vapour penetrations and mixture fraction spatial distribution. In reacting conditions, the effects of variance of mixture fraction and progress variable were examined. The ignition delay time and the quasi-steady flame structure are both affected by the variances. The variance of mixture fraction delays the ignition process and the variance of progress variable accelerates it. For mixture fraction, the ignition process is quicker at any stage in the case of neglecting variance. While things are more complex for progress variable, the ignition process is advanced in the case of neglecting variance at early times, but surpassed by the case of β-PDF later and until auto-ignition. When variance of mixture fraction is considered, the OH mass fraction shows a wide spatial distribution. While if not, a very thin flame is observed with a higher peak in OH, and a very large lift-off length. The variance of progress variable has little impact on the global flame structure, but makes the flame lift-off length much shorter. This study confirms the general observation, that the variance of mixture fraction is of higher importance in high temperature non-premixed combustion, however, we found that the variance of progress variable is far from negligible.

Optical investigation of mixing-controlled combustion using a novel transgenic plant oil

Fuels derived from plant oils hold promise as alternatives to petroleum-derived fuels due to their renewable production strategies, and potential to reduce net carbon emissions. Due to their high viscosity and cloud point, and low volatility however, direct use of plant oils in engines, without transesterification, poses a challenge. Recently, genetic modification has been used to alter plant lipid biosynthesis, producing novel oils that may overcome some of these challenges. In this study, 3-acetyl-1, 2-diacyl-sn-glycerol (Ac-TAG), extracted from a strain of transgenic Camelina Sativa, was studied in an optically-accessible diesel engine. Simultaneous high-speed shadowgraphy and OH* chemiluminescence imaging data were collected to obtain measurements of jet development, liquid penetration, ignition location, and flame liftoff length. A conventional diesel fuel and untransformed neat camelina oil were run at the same conditions as the Ac-TAG fuel for comparison. Even though the Ac-TAG fuel had significantly lower viscosity than the neat camelina oil, no significant differences were observed between Ac-TAG and neat camelina oil in any of the quantities studied. Relative to diesel, the biofuels had greater liquid penetration, shorter liftoff lengths and ignition delay, and consistently ignited farther upstream.

Experimental study on spray and combustion of gasoline/hydrogenated catalytic biodiesel blends in a constant volume combustion chamber aimed for GCI engines

GCI combustion mode has become an interesting topic for high thermal efficiency and low emissions, while ignition problem and knocking combustion limited its application. The blending fuel of gasoline/Hydrogenated Catalytic Biodiesel (HCB) can extend the working conditions for GCI engines because there is no mixed and ignited problem. In this paper, the spray liquid length, ignition delay and flame lift-off length of three gasoline/HCB blends with different blending ratio are measured simultaneously in a constant volume combustion chamber for providing enough data to guide the gasoline/HCB blends used in GCI combustion mode. The experimental results show that the ignition delay and lift-off length decrease with increasing the proportion of HCB in blends, while the spray liquid length shows an opposite trend. Moreover, for gasoline/HCB blends, chemical ignition delay plays a leading role in the total ignition delay and the cycle-to-cycle variation will be decreased with increasing the blending ratio of HCB. On the other hand, the spray and combustion characteristics of G70H30 and G50H50 are compared with those of diesel for further applying the blended fuel in compression ignition engines. The natural luminosity intensity of diesel is higher than that of G50H50 and G70H30 even the injection pressure of diesel is higher than that of blending fuel, which means the blending fuels have relatively lower soot emission. Meanwhile, the ignition delay and flame lift-off length of G50H50 are getting close to the diesel results. Moreover, there is a big difference in ignition delay for these three fuels under low oxygen concentration, while the difference decreases with increasing the oxygen concentration.

Auto-ignition characteristics of diesel fuel in an O2–CO2 mixture

To study diesel fuel auto-ignition in an O2–CO2 mixture, a TZ (temperature zone) model is proposed. The effect of O2 and CO2 on reaction rate is considered. The relationship between temperature and ignition delay time is obtained. Different reduced mechanisms based on steady-state assumptions are applied in three temperature zones (T ≤ 800 K, 800 K < T ≤ 1100 K, T > 1100 K). The TZ model is coupled to KIVA-3V code for simulation calculations. To support the simulations, a constant-volume combustion bomb test bench is set up to visualize diesel fuel auto-ignition in air (21%O2–79%N2), a 53%O2–47%CO2 mixture, and a 61%O2–39%CO2 mixture. Ignition delay time and the flame image in these three conditions are compared and analyzed. Then the flame temperature contour and the flame lift-off length in a 53%O2–47%CO2 mixture and a 61%O2–39%CO2 mixture are analyzed. The results show that diesel fuel auto-ignition can be achieved in the tested O2–CO2 mixture. The TZ model can predict the auto-ignition characteristics of diesel fuel in a 53%O2–47%CO2 mixture and a 61%O2–39%CO2, with errors of 12% and 10%, respectively. In these two conditions, the ignition delay time and flame lift-off length are shorter than they are in air.

Effects of oxygen concentrations on the ignition and quasi-steady processes of n-heptane spray flames using large eddy simulation

Exhaust Gas Recirculation (EGR) is a frequently used technique to reduce the production of NOx. The effect of EGR on the early flame evolution, two-stage ignition process and spray flame structures for n-heptane spray flames are investigated using large eddy simulation. The two-stage ignition process is identified based on the formation of key species and early heat release process. Results demonstrate that a longer ignition delay (ID) and flame lift-off length (LOL) under lower oxygen concentration conditions could increase the mixing time for fuel and air. However, the first-stage ignition still initiates in fuel-richer regions for the cases with higher EGR rates due to the lack of oxygen. In contrast, compared to the case with the same initial oxygen content but at a higher gas temperature of 1000 K, the first-stage ignition moves to stoichiometric mixture fraction regions at 900 K. The combustion mode analysis based on hydroxyl and formaldehyde is conducted to distinguish between the low- and high-temperature combustion regions. Most importantly, to study the stabilization mechanism, the chemical explosive mode analysis (CEMA) is conducted based on analysis on the local flow time scale and the chemical time scale. During the early stage of ignition, a balance between reaction and mixing implies that cool flame propagates from the ignition spots through the entire flow field. And during the quasi-steady state, autoignition plays a dominant role.