Lift-off Length Research Articles

Experimental and analytical study on capture spray liquid penetration and combustion characteristics simultaneously with Hydrogenated Catalytic Biodiesel/Diesel blended fuel

Hydrogenated Catalytic Biodiesel (HCB), with excellent volatility and cetane number while without contain oxygen and aromatic hydrocarbons, have great potential to improve engine performance and emission characteristics. In this study, the effect of HCB/Diesel blended fuels properties (by mass fraction of biodiesel in diesel) on spray combustion characteristics was investigated at inert and reacting conditions in a constant volume combustion chamber. Laser (532 nm) Mie-scattering, OH∗ chemiluminescence and high-temperature luminosity were used to capture the spray liquid length, flame lift-off length and ignition delay simultaneously with the help of two intensifier charge coupled device cameras and a high speed Complementary Metal Oxide Semiconductor camera. Encouraging results were obtained at different ambient conditions (temperatures, densities, oxygen concentration) and injection pressures. The results showed that fuels with high cetane number generate shorter ignition delay, and faster auto-ignition results in a shorter flame lift-off length. A new theory was used to interpret the effect of cetane number on lift-off length. Under reacting conditions, the combustion heat release shorten spray liquid length apparently as compared with the results under inert conditions. The liquid length was slightly affected by the injection pressure and oxygen concentrations under both reacting and inert conditions, while the flame lift-off length is significantly influenced by ambient parameters and injection pressure.

Effects of water content on evaporation and combustion characteristics of water emulsified diesel spray

Due to its potential of reducing NOx and soot emissions simultaneously while improving thermal efficiency, water emulsified diesel is considered as one of the most promising fuels for compression ignition engines. In this study, spray and combustion characteristics of neat diesel and water emulsified diesel with various water contents (10%, 20% and 30% by mass) were investigated. The influence of micro-explosion on high pressure spray characteristics of water emulsified diesel was optically observed and discussed. Experiments were conducted in a constant volume combustion chamber with a high-speed schlieren system to capture the spray and combustion processes. The results show that water content plays a significant role in affecting spray and combustion characteristics of water emulsified diesel. Under non-evaporating condition, the spray tip penetration increases with the water content but the corresponding spray angle decreases with the water content. Such an effect was found diminishing under evaporating condition. The spray volume of test fuels increases from non-evaporating to evaporating condition, and the relative volume increase of water emulsified diesel is at least 5 times higher than that of neat diesel. Both the ignition delay and flame lift-off length increase with water content. Consequently, the integrated natural flame luminosity decreases with the increase of water content. In addition, indirect evidences have proven that the occurrence of micro-explosion can enhance the breakup and evaporation processes of water emulsified diesel spray, and the use of water emulsified diesel can effectively reduce soot emission.

Dynamic adaptive combustion modeling of spray flames based on chemical explosive mode analysis

A dynamic adaptive combustion modeling framework based on chemical explosive mode analysis (CEMA) is proposed to account for different flame features such as local auto-ignition, premixed and non-premixed flamelets in diesel spray flames. The proposed modeling strategy is achieved by assigning zone-dependent combustion models on-the-fly to different flame zones segmented using a CEMA-based approach. An approximate CEMA formulation is developed to approximate the eigenvalue of the chemical explosive mode with high computational efficiency in three-dimensional (3-D) turbulent flame simulations. The utility of the CEMA-based criterion for dynamic flame segmentation is first demonstrated using CEMA-based adaptive chemistry by applying different reduced chemistry to different flame zones. The capability of the dynamic adaptive combustion modeling strategy is then demonstrated in large eddy simulations (LES) of turbulent lifted n-dodecane spray flames. Specifically, inert mixing is used for chemically inactive zones, and the well-mixed combustion model with finite rate chemistry is applied in the pre-ignition zone to capture the two-stage ignition as well as premixed reaction fronts. Adaptive mesh refinement (AMR) is further adopted near the premixed reaction fronts to capture the local flame structure and flame propagation speed. For the post-ignition zone, a recently developed tabulated flamelet model (TFM) is applied and compared with the flamelet progress variable (FPV) method. It is shown that CEMA-based adaptive chemistry induces small errors to the statistically-averaged flame structures, as CEMA is an effective and robust approach for on-the-fly flame segmentation. It is further seen that the CEMA-based adaptive modeling strategy more accurately predicts the ignition delay time and flame lift-off length compared with the low-cost flamelet models such as TFM and FPV, while the computational cost is substantially lower compared with the well-mixed combustion model using finite rate chemistry.

Experimental investigation on spray, evaporation and combustion characteristics of ethanol-diesel, water-emulsified diesel and neat diesel fuels

This paper explored the spray and combustion characteristics of ethanol-diesel (E10), water-emulsified diesel (W10) and neat diesel (D100), especially micro-explosion of E10 and W10. The experiments were conducted in a constant volume combustion chamber under cold (383 K, 0% O2), evaporating (900 K, 0% O2) and combustion (900 K, 21% O2) conditions. Results showed that the spray expansion capacities of E10 and W10 under cold condition were much weaker than that of D100 due to the larger viscosity of emulsified diesels. Under evaporating condition, the spray volume of E10, W10 and D100 increased by 59%, 34% and 21% respectively comparing with cold spray volume. The higher increasing rates of E10 and W10 were mainly due to the micro-explosion effects of ethanol and water contents. Under combustion condition, the integrated natural flame luminosity (INFL) demonstrated that the ethanol content could accelerate the oxidation of soot, while the water content could prohibit soot generation. Therefore, both ethanol- and water-emulsified diesels could inhibit the soot emission, causing lower final residual soot emission of E10 and W10 than that of D100 by 21% and 39% respectively. Moreover, the flame lift-off length (LOL) and flame spread velocity showed that the effects of micro-explosion in E10 and W10 are different. The micro-explosion in ethanol occurred earlier, which enhanced the reaction rate in upstream flame and reduced the LOL. However, the micro-explosion in W10 occurred later, which enhanced the combustion rate in downstream flame.

Two-dimensional measurements of soot in a turbulent diffusion diesel flame: the effects of injection pressure, nozzle orifice diameter, and gas density

ABSTRACTTwo-dimensional light extinction, flame luminosity, and OH* chemiluminescence images were captured at a constant ambient temperature of 823 K and two gas densities (20 and 26 kg/m3), with injection pressures of 800–2500 bar using nozzle orifices with diameters of 0.19 and 0.10 mm. Soot volume fraction and OH distribution images were obtained using the Abel inversion method, and the local equivalence ratio in the lift-off length region was predicted. The results show that the equivalence ratio along the jet’s center at the lift-off length () was found to play a critical role in soot formation. Reductions in thickened the OH zone in the upstream region of the jet, reducing the volume corresponding to the maximum soot volume fraction. The expansion of the OH zone also helped reduce the sooting zone’s width. Under high sooting conditions (e.g., >3.5), the sooting zone width in the downstream jet was independent of .

Modelling of diesel spray flames under engine-like conditions using an accelerated Eulerian Stochastic Field method

This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence–chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the triple-flame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods.

Spray and combustion characterizations of ABE/Dodecane blend in comparison to alcohol/Dodecane blends at high-pressure and high-temperature conditions

This paper presents the spray and combustion characterization of Acetone-Butanol-Ethanol mixture and alcohol fuels, blended with n-Dodecane at different volume ratio, as 20% Acetone-Butanol-Ethanol (ABE20), 20% Butanol (Bu20), 40% Butanol (Bu40), 20% Ethanol (Eth20), as well as 20% Butanol/20% Ethanol (Bu20Eth20) to evaluate the impact of the physical and chemical characteristics, especially fuel oxygen contents. The experiments were carried out in “New One Shot Engine” at high-pressure and high-temperature conditions, established as Spray A condition, defined by Engine Combustion Network (ECN): 60 bar ambient pressure, 22.8 kg/m3 ambient density and various ambient temperatures (800, 850, and 900 K). The liquid and vapor spray parameters were characterized by diffused-back illumination and Schlieren techniques in non-reactive condition (in pure nitrogen). In reactive conditions (with 15% oxygen), the lift-off length and absolute ignition delay were measured by OH∗ chemiluminescence.First, as expected because of slight difference in fuel density, mass flow rates, vapor spray penetration, and spray angle are almost similar. But, due to the highest volatility, the liquid length of ABE20 is shorter compared to pure n-Dodecane and other blends. Also its ignition delay and lift-off length are shorter than those of alcohol blends due to difference in expected cetane number and latent heat of vaporization. The increase of butanol quantity or the addition of ethanol in blends increase the liquid length due to the cooling effect from the latent heat of vaporization, while the lift-off length and ignition delay increase with oxygen contents. Finally, ABE20 seems to be a good candidate for diesel-blended fuel due to the similar behavior as n-Dodecane itself.

Effect of hybrid breakup modelling on flame lift-off length and soot predictions

An improved droplet breakup model coupled with the effect of turbulence flow within the nozzle was implemented into the general transport equation analysis code to describe the flame lift-off length and predict the soot distribution. This model was first validated by the non-evaporating and evaporating spray experimental data. The computational results demonstrate that the breakup model is capable of predicted spray penetration and liquid length with reasonable accuracy. The inclusion of turbulence enhanced the breakup model, increased the droplet breakup rate, decreased spray penetration for about 6–12% compared to the results of Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model. Then, the model was applied to investigate the influence of ambient density, temperature, oxygen concentration and injection pressure on the flame lift-off length under typical diesel combustion conditions. The predictions showed good agreement with the experimental data. The result also indicated that the turbulence inside the nozzle strengthen the rate of breakup, resulting in more smaller droplets, leading to high evaporation rate and smaller vapour penetration lengths, thus decreases the lift-off length about 8%. Finally, the model was used to explore the soot distribution. The overall trend of soot with the variations in injection pressure was well reproduced by the breakup model. It was found that the droplet with faster velocity under high injection pressure, this could lead to larger lift-off length, which will play a significant role for the fuel–air mixing process and thus cause a decrease in soot in the fuel jet. Results further indicated that the turbulence term can decrease the soot mass about 5–9% by improved the droplet breakup process.

Soot formation modelling for n-dodecane sprays using the transported PDF model

Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is modelled with the transported probability function (TPDF) method. The approach employs an acetylene-based two-equation soot model coupled with a Reynolds-averaged turbulence model and a Lagrangian discrete phase spray model. The aims are to evaluate, in the context of soot, the predictive capability of the model, the effects of turbulence–chemistry interactions (TCI), and various available chemistry mechanisms. TCI effects are evaluated by comparisons between the TPDF model and simulations using a well-mixed model neglecting turbulent fluctuations. Five test cases having variations in ambient temperature and oxygen concentration are considered.Five chemical mechanisms are first compared to experiments in terms of their ignition delay (ID) and lift-off length (LOL) under ambient O2 and temperature variations. Three relatively new mechanisms exhibit good ID performance (with both TCI approaches), while two short mechanisms also provide good LOL performance in conjunction with the TPDF approach. The two short mechanisms are considered for further comparisons. The auto-ignition process is analysed by comparing TPDF simulations with measurements from schlieren and 355 nm planar laser-induced fluorescence, detecting CH2O and polycyclic aromatic hydrocarbons, with overall good qualitative agreement, though with some differences on low-temperature reactivity.The experimental comparisons for soot consider transient soot mass and KL in the baseline condition, and steady state soot volume fraction (SVF) fields and total soot masses for all five ambient conditions. In terms of comparisons to experiment, the transient stage of the soot mass development is not well captured. An analysis of the transient KL in the baseline case shows the soot-containing region is larger in the experiment than the model, with soot extending in the experiment much closer to the jet boundary, suggesting that the model underestimates gradients around the jet head. During the steady period, however, the SVF agrees quite well. The soot models with both chemistry and TCI approaches were able to reproduce the overall soot trends with varying ambient temperature and oxygen, though the effect of the ambient temperature on the soot mass was under-predicted, in particular in its variation from 900 to 1000 K. TCI effects on soot were in overall terms relatively minor, in part due to compensating errors. Neglecting TCI showed generally higher peak soot amounts, narrower soot distributions, and more downstream soot onset and soot peak locations. These differences between the models are explained with the help of a detailed analysis of the soot phenomena.

Combustion characterization of waste cooking oil and canola oil based biodiesels under simulated engine conditions

Alternative fuels will come from a variety of feed stocks and refinement processes. Understanding the fundamentals of combustion and pollutants formation processes of these fuels will be useful for their implementation in different combustion systems. In this work, optical diagnostics were performed to waste cooking oil (WCO) and canola oil (CAO) based biodiesel sprays to assess their combustion and soot formation processes. Conventional diesel was used as a reference fuel for comparison with the biodiesels. The experiments were conducted in an optically-accessible constant-volume combustion chamber (CVCC) with simulated compression-ignition engine conditions, with different degree of exhaust gas recirculation. The liquid length and lift-off length results indicate that there was no significant interaction between the liquid phases of the fuels and their combustion regions. The flame lift-off lengths were found to be affected by both the chemical and physical properties of the fuels. It was observed that a larger difference between the lift-off length and the first-luminosity distance was correlated with lesser downstream soot formation, although the molecular structure of the fuel was found to affect the process too. Assessing the sooting and combustion characteristics of the biodiesel and diesel flames across the varied ambient O2 atmospheres revealed that the estimated soot contents of the biodiesel and diesel flames peaked at 15 and 21 vol.% O2 concentration, respectively. The peak soot contents of the WCO and CAO biodiesel flames were found be comparable, but lower than that of diesel, across the various O2 environment. The results also demonstrated that the biodiesels have higher normalized peak pressure values than diesel at all O2 conditions. Two-color pyrometry data demonstrated that the measured soot temperature and soot KL factors of the flames were similar at 15 and 21 vol.% O2, but varied with further reduction of ambient O2 concentration. Variations in the combustion duration and flame area were found to be fuel dependent.

Investigation of Biodiesel Combustion and Emissions Using Reduced Chemical Kinetics

This paper studies the differences in spray structure and emissions trends between diesel and biodiesel fuels in a compression ignition engine. A computationally efficient and predictive quasi-dimensional simulation model is combined with fuel-specific physical properties and chemical kinetic mechanisms to predict spray mixing, combustion, and emissions behavior. The results underscore the complex relationships between NOx emissions, operational parameters, and fuel chemistry and provide further evidence of a link between stoichiometry near the flame lift-off length and formation of NOx.

Numerical investigation of diesel spray flame structures under diesel engine-relevant conditions using large eddy simulation

ABSTRACTLarge eddy simulation coupled with the third-order Monotone Upstream-centered Schemes for Conservation Laws (MUSCL) differencing scheme was employed for investigating the ignition processes and flame structures of the reacting n-heptane spray over a wider range of diesel engine-relevant conditions. First, the effects of numerical schemes on the mixing and combustion processes are analyzed in detail. Comparisons of the mixture fraction profiles with experimental data from the Engine Combustion Network website show that the MUSCL gives a better prediction compared with Quasi-second-order upwind scheme. The mixing between fuel and air is much better using MUSCL scheme. As a result, ignition is initiated at fuel-leaner regions, but still richer than stoichiometric equivalence ratio. Second, the predicted ignition delay times and flame lift-off lengths are compared with experimental data under different initial conditions. The predicted results show good agreements with experimental results at different temperatures, oxygen concentrations, and densities. Finally, the effects of initial conditions on the spray flame structures are comprehensively analyzed using the temperature-equivalence ratio maps. The effects of initial parameters on the reacting spray structures and important radicals, such as OH and CO are investigated carefully. Results show that initial oxygen concentration has the greatest influence on the flame structures than ambient gas temperature and density.

Compositional effects on the ignition and combustion of low octane fuels under diesel conditions

The amount of non-paraffinic components is directly associated with fuel sensitivity (S), which is an important property for using low-octane and low-cetane fuels in direct injection engines. In this paper, we examine the effects of S and RON (research octane number) on the ignition and combustion behavior of naphtha fuel surrogates in homogeneous mixtures and diesel sprays. Two binary blends (PRF70 and PRF80) and four ternary blends (TPRF70-a, TPRF70-b, TPRF80-a, TPRF80-b), with varying amounts of iso-octane, n-heptane and toluene, are considered. Simulations are performed using a reaction mechanism with 109 species and 543 reactions. The mechanism is validated against the shock tube and rapid compression machine ignition data, and non-reacting spray data. Ignition simulations in homogeneous mixtures are performed using CHEMKIN-Pro for temperature range of 625–1250 K, equivalence range of ɸ = 0.5–2.0, and pressure of 55 bar. The study is then extended to examine the transient ignition and flame structure in liquid fuel sprays in a constant-volume combustion vessel using the CFD software CONVERGE. Results indicate that the temperature dependence of ignition characteristics in both homogeneous mixtures and sprays is strongly influenced by fuel sensitivity. In particular, it affects the NTC behavior and temperature dependence of the 1st and 2nd stage ignition processes. The ignition kernel structure in sprays is also strongly modified by fuel sensitivity, as the ignition kernel in ternary sprays involves richer mixtures, while that in binary sprays contains near stoichiometric mixtures. Consequently, the spray flame structure is also modified by fuel sensitivity. While the spray flame is characterized by partially premixed combustion involving a lean premixed zone (LPZ), a rich premixed zone (RPZ), and a nonpremixed zone (NPZ), the effect of sensitivity is to enhance the relative contribution of RPZ compared to those of NPZ and LPZ. In addition, due to enhanced ignitability, the flame in ternary sprays is located closer to the injector compared to that in binary sprays. The effect of higher RON is to increase the ignition delay time. Consequently, the ignition kernel involves relatively leaner mixtures, and the flame structure is characterized by increased contributions from LPZ compared to RPZ, and the flame liftoff length is increased. Increasing the initial reactor temperature has the opposite effect. A sensitivity analysis is performed to identify reactions that characterize the reduced and enhanced ignitability of ternary blends at low and high temperatures, respectively.

Large-eddy simulation on the influence of injection pressure in reacting Spray A

The Engine Combustion Network (ECN) Spray A target case corresponds to high-pressure liquid fuel injection in conditions relevant to diesel engines. Following the procedure by Wehrfritz et al. (2016), we utilize large-eddy simulation (LES) and flamelet generated manifold (FGM) methods to carry out an injection pressure sensitivity study for Spray A at 50, 100 and 150 MPa. Comparison with experiments is shown for both non-reacting and reacting conditions. Validation results in non-reacting conditions indicate relatively good agreement between the present LES and experimental data, with some deviation in mixture fraction radial profiles. In reacting conditions, the simulated flame lift-off length (FLOL) increases with injection pressure, deviating from the experiments by 4–14%. Respectively, the ignition delay time (IDT) decreases with increasing injection pressure and it is underpredicted in the simulations by 10–20%. Analysis of the underlying chemistry manifold implies that the observed discrepancies can be explained by the differences between experimental and computational mixing processes.

Effect of the nozzle holder on injected fuel temperature for experimental test rigs and its influence on diesel sprays

One of the test boundary conditions whose control is necessary for both experimental and numerical studies about the automotive engines research field is the temperature of the fuel inside the injector body during the injection. However, it is a difficult parameter to be directly measured in a non-intrusive way due to the injector architecture and the nature of the standard experiments that are done to characterize sprays. An experimental analysis is performed in this work employing a continuous flow test chamber, normally used in the optical diagnosis of diesel sprays, in order to compare and characterize two different designs for the nozzle holder of the test rig. The first one consisted in an aluminum holder that coats the nozzle until its tip, while the second one is made of steel and only supports the nozzle without covering it from the environment. The employed methodology was to set the test rig at a wide range of thermodynamic conditions inside the vessel such as ambient temperature and density and also the coolant temperature at the outlet of the injector casing for both cooling pieces. In this case, a dummy injector provided with a thermocouple was used to measure the tip temperature. In this way, correlations are obtained to estimate the injector body temperature out of the measurement points. Further experiments with a real single-hole diesel injector, controlling its tip temperature according to the previous results, are also discussed in this article in order to analyze the effect of this parameter on the spray evaporation process. Liquid length at inert conditions and both ignition delay and lift-off length at reactive ones were measured employing the same test vessel. All those parameters showed to be shortened with an increment of injected fuel temperature, and the lower the ambient temperature, the stronger this influence is.

A study of soot quantification in diesel flame with hydrogenated catalytic biodiesel in a constant volume combustion chamber

In this work, diffused back-illumination extinction imaging and OH* chemiluminescence imaging were employed simultaneously to measure the soot production, spray liquid length and flame lift-off length. These experiments were conducted in a high-temperature, high-pressure constant-volume combustion chamber equipped with a common-rail fuel injector with a single 180-μm-diameter orifice. Soot formation characteristics of a blend fuel (B50) with 50% hydrogenated catalytic biodiesel (HCB) and 50% 0# fossil diesel in mass were studied. Besides, the pure 0# fossil diesel (B0) under the same operating conditions was also measured as a reference. Parametric variations of injection pressure (100 MPa, 150 MPa), ambient temperature (770 K, 820 K, 870 K) and oxygen concentration (15%, 18%, 21%) have been performed for each fuel. After analysis, it was found both liquid length and flame lift-off length of B50 are shorter than that of B0. Soot onset time of B50 appears earlier and soot initial location is closer to the nozzle than that of B0. Within the measured optical access, soot area of B50 is larger than that of B0 because of shorter lift-off length, while the difference on total soot production is less noticeable because fuel properties of B0 is more favorable for soot formation.

Effects of Nozzle Diameter on Diesel Spray Flames: A numerical study using an Eulerian Stochastic Field Method

The present numerical study aims to assess the performance of an Eulerian Stochastic Field (ESF) model in simulating spray flames produced by three fuel injectors with different nozzle diameters of 100 µm, 180 µm and 363 µm. A comparison to the measurements shows that although the simulated ignition delay times are consistently overestimated, the relative differences remain below 28%. Furthermore, the change of the averaged pressure rise with respect to the variation of nozzle diameter is captured by the model. The simulated flame lift-off lengths also agree with the measurements, with a maximum relative difference of 13%. The spray flame produced by a larger nozzle diameter has a fuel-richer premixed core region despite the longer lift-of length, indicating that the higher fueling rate used with the larger nozzle diameter is a more dominating factor than the lift-off length is in influencing the air entrainment into the upstream of the spray flames. In addition, the simulated normalised flame lengths are found to decrease when the nozzle diameters increase. These predictions are in good qualitative agreement with the experimental observation. This work proves that the ESF model can serve as an important tool for the simulation of spray flames in marine diesel engines, where fuel injectors with different nozzle diameters are applied for pilot and main injections.

A computational analysis of local flow for reacting Diesel sprays by means of an Eulerian CFD model

An implementation and validation of the coupled Σ-Y ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014) is fed with the spray description, i.e. mixing formation process, provided by the Σ-Y Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain.

Effects of Fuel Unsaturation on Transient Ignition and Flame Development in Sprays

ABSTRACTFundamental studies using homogeneous mixtures in shock tube and rapid compression machine (RCM) have shown that the fuel reactivity and ignition characteristics are significantly affected by the presence and number of double bonds in the fuel molecular structure. However, this phenomenon has not been investigated in turbulent reacting sprays under realistic conditions. In this article, we numerically examine the effects of fuel unsaturation and sensitivity on the transient ignition and flame evolution in n-heptane and 1-heptene sprays in the Sandia constant volume reactor. 3D simulations of two-phase reacting flows are performed using a commercial CFD code, CONVERGE. The ignition and combustion chemistry of n-heptane and 1-heptene is modeled using a reduced CRECK mechanism, which has been previously validated against the shock tube ignition delays and reacting spray measurements. Results indicate that, at intermediate temperatures, the ignition in both homogeneous mixtures and sprays is strongly influenced by fuel unsaturation. First of all, the ignition delay is much higher for 1-heptene compared to that for n-heptane, especially at low temperatures. Secondly, ignition in n-heptane sprays occurs in fuel rich mixtures, and is characterized by a two-stage ignition process. On the other hand, ignition in 1-heptene sprays occurs in lean mixtures. Results also indicate the existence of multiple ignition locations in sprays, which is in contrast to ignition in homogeneous mixtures. The flame evolution in sprays correlates strongly with the ignition kernel evolution. Consequently, the n-heptane spray flame contains two reaction zones, namely, a rich premixed zone (RPZ) and a non-premixed reaction zone (NPZ), while the 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone in addition to NPZ and RPZ. The difference is a direct consequence of longer ignition delay in 1-heptene spray compared to that in n-heptane spray, which facilitates increased fuel-air mixing in the former. The flame lift-off location correlates with the ignition delay and ignition kernel location. Thus, at lower temperatures (1000 K and 1100 K), the lift-off length of 1-heptene flame is greater than that of n-heptane flame, which is due to longer ignition delays for 1-heptene sprays. In contrast, at higher temperatures (1200 K) the lift-off length of 1-heptene flame is smaller, which is due to the fact that the ignition kernel and, thus, lean premixed zone are located closer to the injector for 1-heptene spray compared to n-heptane spray.

Effects of injection pressure on ignition and combustion characteristics of diesel in a premixed methanol/air mixture atmosphere in a constant volume combustion chamber

The experiments on the ignition and combustion characteristics of diesel in air atmosphere (AA) and premixed methanol/air mixture atmosphere (MAA) were conducted in a constant volume combustion chamber (CVCC) equipped with a high pressure common-rail injection system. The ignition and combustion processes were recorded by a high-speed camera and a combustion pressure acquisition system. Results show that with the increase of injection pressure from 40 to 160MPa, the flame lift-off length (FLoL) becomes longer; and the ignition delay is shortened from 2.80ms to 2.03ms (AA) and 2.90ms to 2.27ms (MAA) respectively; and correspondingly the combustion duration is shortened from 5.17ms to 2.47ms (AA) and 4.83ms to 2.13ms (MAA). Compared those in AA, the ignition delay is prolonged by 0.14ms on average, while the combustion duration is shortened by 0.27ms in MAA. At high injection pressure, the moments that the maximum combustion pressure occurs and the apparent heat release rates (AHRR) starts to rise are advanced; while in MAA both of them are delayed; and the peak value of AHRR increases. With the increase of injection pressure, both spatially integrated natural luminosity (SINL) and time integrated natural luminosity (TINL) reduce significantly, but the reduction trends of TINL weaken. Due to the addition of methanol, the FLoL of diesel becomes longer, and the SINL and TINL in MAA are both lower than those in AA.