Lift-off Length Research Articles

A study on tip penetration velocity and radial expansion of reacting diesel sprays with different fuels

The reacting diesel spray structure was investigated using n-dodecane, n-heptane and one binary blend of Primary Reference Fuels (80% n-heptane and 20% iso-octane in mass) based on the existing database from previous experimental results from Schlieren imaging technique in a constant pressure combustion chamber. The spray tip velocity was derived from the derivative of tip penetration versus time. The operating conditions and the injector used (single axially-oriented hole, 89mm-diameter) were chosen following the guidelines of the Engine Combustion Network. A 1D spray model was also applied here to support the analysis of experimental results. Parametric variations of injection pressure, ambient temperature and oxygen concentration have been performed for each fuel. Analysis of radial expansion and reacting tip velocity was performed in terms of an average spray radial increase (ΔR‾) and a constant (k) defining the tip penetration velocity. k values of reacting cases are always bigger than those from inert ones for both experimental and theoretical results. Based upon this parameter, quasi-steady tip penetration under the investigated conditions seems not to be affected by ambient temperature, oxygen content or fuel cetane number. Three cases with different fuels and similar ignition delay and lift-off length were further analyzed, which shows that the reactivity of the mixture has an effect on the transition timing from inert to reacting states, as well as on the initial penetration stages, but not on the quasi-steady phase. Apart from the similar tip velocity during quasi-steady phase, the full transient evolution of the tip is highly similar. The fact that this full overlap does not occur for other operating conditions indicates that early penetration stages are highly affected by the transient chemistry development, which largely depends on fuel cetane number.

Experimental investigation on the effect of n-pentanol blending on spray, ignition and combustion characteristics of waste cooking oil biodiesel

Due to their excellent physicochemical properties, biodiesel and n-pentanol are regarded as two promising alternative biofuels for automobile. However, the fundamental data of spray and combustion characteristics of n-pentanol/biodiesel blends are still scarce. The objective of this work is to investigate the effects of n-pentanol addition to waste cooking oil (WCO) biodiesel in different ratios (0%, 20%, and 40% in volume) on spray, ignition and combustion characteristics in a constant volume combustion bomb (CVCB). Ignition and combustion tests were performed at five ambient temperatures (800, 900, 1000, 1100 and 1200K) with three oxygen concentration (10%, 15% and 21%), and spray tests were conducted at the same temperature ranges but with no oxygen involved to prevent combustion happening. The result shows liquid length decreases with increase of ambient temperature, and the falling slope increases significantly with the blended n-pentanol concentration. The liquid length of B60P40 is about 6% longer than that of B100 at 800K condition, while it becomes about 10% shorter than B100 when ambient temperature rises to 1200K. Although the cetane number of n-pentanol is much less than biodiesel, an increase of n-pentanol ratio has a promoting effect on the ignition event under most conditions. Under these conditions, addition of 40% n-pentanol has cut the ignition delay of biodiesel, on average, by 30.2%. The natural flame luminosity is found to be reduced significantly with an increasing n-pentanol ratio, meaning that addition of n-pentanol could reduce the soot level under all conditions. For example, the time integrated natural luminosity (TINL) of B60P40 is only 41.2% of that of B100 under 1000K temperature conditions. The result also suggests that, for multi-component fuels, flame lift-off length (FL) is the most reliable factor that influence the soot concentration level under spray combustion processes, rather than ignition delay or soot formation time. Overall, blending n-pentanol into biodiesel has a great soot oxidation potential and could maintain suitable ignition phase at the same time.

Importance of turbulence-chemistry interactions at low temperature engine conditions

The role of turbulence-chemistry interaction in autoignition and flame stabilization is investigated for spray flames at low temperature combustion (LTC) conditions by performing high-fidelity three-dimensional computational fluid dynamics (CFD) simulations. A recently developed Tabulated Flamelet Model (TFM) is coupled with a large eddy simulation (LES) framework and validated across a range of Engine Combustion Network (ECN) ambient temperature conditions for n-dodecane fuel. High resolution grids with 0.0625 mm minimum cell size and 25 million total cell count are implemented using adaptive mesh refinement over the spray and combustion regions. Simulations with these grids and multiple LES realizations, with a 103 species n-dodecane mechanism show good agreement with experimental data for all the ambient conditions investigated. This modeling approach with the computational cost advantage of tabulated chemistry is then extended towards understanding the auto-ignition and flame stabilization at an ambient temperature of 750 K. These low temperature conditions lead to substantially higher ignition delays and flame liftoff lengths, and significantly leaner combustion compared to conventional high temperature diesel combustion. These conditions also require the simulations to span significantly larger temporal and spatial dimensions thereby increasing the computational cost. The TFM approach is able to capture autoignition and flame liftoff length at the low temperature conditions. Significant differences with respect to mixing, species formation and flame stabilization are observed under low temperature compared to conventional diesel combustion. At higher ambient temperatures, formation of formaldehyde is observed in the rich region (phi > 1) followed by the formation of OH in the stoichiometric regions. Under low temperature conditions, formaldehyde is observed to form at leaner regions followed by the onset of OH formation in significantly lean regions of the flame. Qualitative differences between species formation and transient flame development for the high and low temperature conditions are presented. The two stage ignition process is further investigated by studying the species formation in mixture fraction space by solving 1D flamelet equations for different scalar dissipation rates and homogeneous reactor assumption. Results show that scalar dissipation causes these radicals to diffuse within the mixture fraction space. This significantly enhances ignition and plays a dominant role at such low temperature conditions which cannot be captured by the homogeneous reaction assumption based model.

Early-age crack widths of thin fiber reinforced concrete overlays subjected to temperature gradients

This study aims to predict the early-age crack width of thin fiber reinforced concrete (FRC) overlays through the modified prediction equation and finite element modeling. The equation which was originally developed to predict the joint opening width of plain concrete pavements, was modified in this study to either consider the fiber aspect ratio or the residual strength ratio of FRC. The predicted crack widths were compared and validated against the measured crack widths of full-scale FRC overlays constructed in the field. Thin FRC overlays in the field had varied joint spacing from 1.12 to 3.35m and were subjected to only environmental loading, rather than traffic loading. A finite element model was generated using fracture energy to account for the fiber-bridging effect and tensile bond strength for interface bonding with the underling structures. The results from a sensitivity analysis indicated that the crack width, vertical lift-off, and debonding length all decreased as the fracture energy of the FRC increased, or as the interfacial tensile bond increased. The proposed equation was found to predict the field crack width within 0.19mm or 26% difference.

An investigation on a diesel jet’s ignition characteristics under cold-start conditions

The diesel engine is widely regarded as a critical power source for transportation vehicles, construction machinery vehicles, and military-equipment due to its advantages of significant horse power, higher fuel efficiency, greater reliability. However, diesel engines do suffer from ignition issues in extremely cold environments. Ignition characteristics during the start-up process directly impact both the start-ability and emission levels of the diesel engine. In this study, various optical diagnostics, such as Mie scattering, shadowgraphy method, and high-speed imaging, were applied to investigate the spray’s vapor and liquid phases as well as the initial flame development, while an optical constant volume chamber was used to observe the in-cylinder condition. During the experiments, the ambient temperature, injection pressure, and fuel temperature was varied in a large range to simulate different stages of the cold-start or warm-up processes. The results show that the penetration of the spray’s liquid and vapor phases, flame lift-off length, and ignition delay all observably increase with a decrease in the ambient density, ambient temperature, and fuel temperature. Furthermore, the chemical process is the dominant factor for ignition at a low ambient temperature. A low ambient temperature may lead to a misfire due to the separation between the spray’s vapor phase and the expected initial flame location. Additionally, a low ambient density could lead to misfires due to the low heating capacity of the ambient gas. The study results also show that the injection pressure does not significantly influence the ignition delay or the flame lift-off length, but a higher pressure could result in a lower ignition success rate due to over-mixing of the air-fuel. Additionally, the fuel temperature significantly influences the ignition, which advances the initial ignition time 0.1ms with every increase of 10°C under an ambient temperature of 535°C.

Simulation of Combustion Recession After End-of-Injection at Diesel Engine Conditions

Recent experimental observations show that lifted diesel flames tend to propagate back toward the injector after the end-of-injection (EOI) under conventional high-temperature conditions. The term “combustion recession” has been adopted to reflect this process dominated by “auto-ignition” reactions. This phenomenon is closely linked to the EOI entrainment wave and its impact on the transient mixture–chemistry evolution upstream of the lift-off length. A few studies have explored the physics of combustion recession with experiments and simplified modeling, but the details of the chemical kinetics and convective–diffusive transport of reactive scalars and the capability of engine computational fluid dynamics (CFD) simulations to accurately capture them are mainly unexplored. In this study, highly resolved numerical simulations have been employed to explore the mixing and combustion of a diesel spray after the EOI and the influence of modeling choices on the prediction of these phenomena. The simulations are centered on a temperature sweep around the engine combustion network (ECN) spray-A conditions, from 800 to 1000 K, where different combustion recession behaviors are observed experimentally. Reacting spray simulations are performed via openfoam, using a Reynolds-averaged Navier–Stokes (RANS) approach with a traditional Lagrangian–Eulerian coupled formulation. Two reduced chemical kinetics models for n-dodecane are used to evaluate the impact of low-temperature chemistry and mechanism formulation on predictions of combustion recession behavior. Observations from the numerical simulations are consistent with recent findings that a two-stage auto-ignition sequence drives the combustion recession process. Simulations with two different chemical mechanisms indicate that low-temperature chemistry reactions drive the likelihood of combustion recession.

A Comparison of Methyl Decanoate and Tripropylene Glycol Monomethyl Ether for Soot-Free Combustion in an Optical Direct-Injection Diesel Engine

Oxygenated fuels have beneficial effects for leaner lifted-flame combustion (LLFC), a nonsooting mode of mixing-controlled combustion associated with lift-off length equivalence ratios below approximately 2. A single-cylinder heavy-duty optical compression-ignition engine was used to compare neat methyl decanoate (MD) and T50, a 50/50 blend by volume of tripropylene glycol monomethyl ether (TPGME) and #2 ultralow sulfur emissions-certification diesel fuel (CF). High-speed, simultaneous imaging of natural luminosity (NL) and chemiluminescence (CL) were employed to investigate the ignition, combustion, and soot formation/oxidation processes at two injection pressures and three dilution levels. Additional Mie scattering measurements observed fuel-property effects on the liquid length of the injected spray. Results indicate that both MD and T50 effectively eliminated engine-out smoke emissions by decreasing soot formation and increasing soot oxidation during and after the end of fuel injection. MD further reduced soot emissions by 50–90% compared with T50, because TPGME could not completely compensate for the aromatics in the CF. Despite the low engine-out soot emissions, both fuels produced in-cylinder soot because the equivalence ratio at the lift-off length never reached the nonsooting limit. With respect to the other engine-out emissions, T50 had up to 16% higher nitrogen oxides (NOx) emissions compared with MD, but neither fuel showed the traditional soot-NOx trade-off associated with conventional mixing-controlled combustion. In addition, T50 had up to 15% and 26% lower unburned hydrocarbons (HC) and CO emissions, respectively, compared with MD.

The effect of nozzle geometry over ignition delay and flame lift-off of reacting direct-injection sprays for three different fuels

The influence of internal nozzle flow characteristics over ignition delay, and flame lift-off of reacting direct-injection sprays is studied experimentally for three fuels using two different nozzle geometries. This is a continuation of previous work by the authors, where, evaporative and non-evaporative, isothermal spray developments were studied experimentally for the same nozzle geometries and fuels. Current study reports the ignition delay through Schlieren technique, and flame lift-off length through OH* chemiluminescence visualization. The nozzle geometries consist of a conical nozzle and a cylindrical nozzle with 8.6% larger outlet diameter when compared to the conical nozzle. The three fuels considered are n-heptane, n-dodecane and a three-component surrogate to better represent the physical and chemical properties of diesel fuel. Reacting spray is found to penetrate faster than non-reacting spray due to combustion induced acceleration after ignition. Higher oxygen concentration, and ambient temperature enhance the reactivity leading to higher spray tip penetration. Injection pressure does not affect the reactivity significantly and hence, influences spray penetration through momentum—similar to a non-reacting spray. Both ignition delay and lift-off length are found to be shortest and longest for n-dodecane and n-heptane, respectively, while the surrogate fuel falls in-between the two pure component fuels. Both ignition delay and lift-off length are found to decrease with increase in oxygen concentration, ambient temperature, and density. The cylindrical nozzle, in spite of shorter lift-off length is found to have longer ignition delay, when compared to the conical nozzle. This could be due to better atomization leading to larger spread angle and evaporative cooling from the cylindrical nozzle compared to a conical nozzle. The longer ignition delay also leads to leaner equivalence ratios at the time of ignition.

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

Multiple-injection strategies are characterized by a complex and transient interplay between high- and low-temperature reactions. Tracking low-temperature reaction products such as formaldehyde (CH2O) is particularly important to understand ignition phenomena and the so-called “combustion recession” that is observed in experiments. Experimentally, it is often difficult to discriminate between formaldehyde and other species such as poly-aromatic hydrocarbons, which is why a selective excitation approach is used in this work. Simultaneous high-speed imaging of the chemically-excited hydroxyl radical (OH*) is used to improve indication of flame location and second stage ignition. During experiments in a constant-volume vessel, two 0.5-ms injections of n-dodecane, separated by 0.5-ms dwell time, are injected into a 900-K ambient. The global flame development is characterized based on high-speed diagnostics, followed by an investigation into the spatial distribution of formaldehyde at four different times after start-of-injection (aSOI). Results show significant influence of the first injection on characteristics of the second. Ignition delay and lift-off location of the second injection are prominently reduced, while flame penetration is greatly enhanced by the wake of the first injection. Formaldehyde structure is observed during both end-of-injection transients, reaching as far upstream as 6 mm from the nozzle. Even after the second injection, the flame structure still appears to be influenced by the first, with a shorter lift-off length and compressed formaldehyde structure. Based on the selective excitation procedure, it becomes clear that the interpretation of laser-induced fluorescence (LIF) images obtained by 355-nm excitation alone is prone to ambiguity.

A study on the interaction between local flow and flame structure for mixing-controlled Diesel sprays

A detailed study on the spray local flow and flame structure has been performed by means of PIV and laser-sheet LIF techniques under Diesel spray conditions. Operating conditions were based on Engine Combustion Network recommendations. A consistent comparison of inert and reacting axial velocity fields has produced quantitative information on the effect of heat release on the local flow. Local axial velocity has been shown to increase 50–60% compared to the inert case, while the combustion-induced radial expansion of the spray has been quantified in terms of a 0.9–2.1mm radius increase. As a result, the drop in entrainment rate has been quantified around 25% compared to the inert case. Streamline analysis also hints at a reduced entrainment under reacting conditions. A 1D spray model under reacting condition has been used, which confirms the modifications obtained in the main flow metrics when moving from inert to reacting conditions. When comparing the flow evolution with the flame structure, little effect of chemical activity on the spray flow upstream the lift-off length has been evidenced, in spite of the presence of formaldehyde in such regions. Only downstream of the lift-off length, as defined by OH LIF, has a strong change in flow pattern been observed as a result of combustion-induced heat release.

Application of an unsteady flamelet model in a RANS framework for spray A simulation

Abstract In the present investigation the Spray A reference configuration defined in the framework of the Engine Combustion Network (ECN) has been modeled by means of an Unsteady Flamelet Model (USFM) including detailed parametric studies to evaluate the impact of ambient temperature, oxygen concentration and density. The study focuses on the analysis of the spray ignition delay, the flame lift-off length and the internal structure of the spray and flame according to the experimental information nowadays available for validating the results provided by the model. Promising results are obtained for the nominal case and also for the parametric variations (temperature, oxygen…) in terms of liquid and vapor penetration, ignition delay (ID) and lift-off length (LOL). The model permits to predict the ID and the LOL which constitute two parameters of key importance for describing the characteristics of transient reacting sprays. Valuable insight on the details of the combustion process is obtained from the analysis of formaldehyde ( CH 2 O ) , acetylene ( C 2 H 2 ) and hydroxide (OH) species in spatial coordinates and also in the so-called ϕ - T maps. Important differences arise in the inner structure of the flame in the quasi-steady regime, which is closely linked to soot formation, when varying the ambient boundary conditions. Additionally, the auto-ignition process is investigated in order to describe in detail the spatial onset and propagation of combustion. Results confirm the impact of the ambient conditions on the regions of the spray where start of combustion takes place, so the relation between the local scalar dissipation rate and mixture fraction variance is also discussed. This investigation provides an insight of the potential of the USFM combustion model to describe the physical and chemical processes involved in transient spray combustion.

Numerical Simulations of Constant-Volume Spray Combustion of n-Heptane with Chemical Kinetics

Objectives: A reduced toluene reference fuel (TRF) mechanism of multi-component nature from the literature is utilized to simulate constant-volume spray combustion of n-heptane. The approach allows a preliminary assessment of fuel kinetic model and computational fluid dynamics (CFD) formulations in a simplified computational domain before integrating them in complex engine simulations. Methods: The operating conditions vary in ambient densities between 14.8 kg/m 3 and 30 kg/m 3 with initial oxygen concentrations ranging from 10% to 21%. The CFD models are first calibrated to replicate spray penetration lengths of the non-reacting condition. The tuned numerical models are then applied to simulate the combustion and soot formation events of reacting sprays. The soot model employed is the multi-step Moss-Brookes model with updated oxidation models. Findings: The relative errors for ignition delay and lift-off length predictions are within 35% and 22% respectively. Furthermore, simulated soot volume fraction contours agree qualitatively with the experimental soot clouds. Computed peak soot locations, however, are found to be further downstream axially as compared to the experimental results across all test cases. Application: Good agreement with experimental spatial soot distributions allows the incorporation of both fuel and soot models in engine configurations.

Effects of the turbulence model and the spray model on predictions of the n-heptane jet fuel–air mixing and the ignition characteristics with a reduced chemistry mechanism

Reynolds-averaged Navier–Stokes simulations with an improved spray model and a realistic chemistry mechanism are performed for turbulent spray flames under diesel-like conditions in a constant-volume chamber. Comprehensive numerical analyses including two turbulence models (the renormalisation group k– ε model and the standard two-equation k– ε model) with different model coefficients are made. The distribution of the fuel mixture fractions is a very important factor affecting the combustion process. In this study, we also use the entrainment gas-jet model, modifications of the the spray model coefficient and two turbulence models to investigate extensively the influence of the gas-jet theory model on the fuel–air mixture process. First, a non-reacting case is validated by comparing the liquid-phase penetration and the vapour-phase penetration and also the mixture fractions at different axis positions. Second, approriate methods are confirmed according to accurate mixture fraction distributions to validate the combustion process. Because of the large number of species and reactions, the calculation of chemically reacting flows is unaffordable, particularly for three-dimensional simulations. Hence, the dynamic adaptive chemistry method for efficient chemistry calculations is extended in this work to reduce the computational cost of the spray combustion process when a reduced chemistry mechanism is used. The results show that, in the evaporation case, the gas-jet theory model can be used to obtain a relatively accurate fuel vapour penetration length with different influential factors and that improved numerical methods can effectively reduce the mesh dependence for the spray evaporation process. It is demonstrated that the Schmidt number Sc and the turbulence models significantly influence the mixture fraction distribution. Very good agreement with available experimental data is found concerning the ignition delay time and the flame lift-off length for different oxygen concentrations owing to the accurate fuel mixture fraction.

Study on LTC for light duty engines – Part 2 – Spray enhancements

This paper presents the 2nd part of a study on low temperature diffusive combustion in a small HSDI (High Speed Direct Injection) engine with the objective of avoiding simultaneously NOX and soot formation, aiming to extend the range of operation conditions where this combustion strategy has been achieved and. Also, it was intended to reduce CO and HC emissions, considered very high in the 1st part of the tests. New hardware configuration was chosen in order to improve air entrainment in the lift-off length such as increasing injection pressure, reducing fuel injection rate and nozzle hole diameter. Also, a parametric study on the behavior of in-cylinder air thermodynamic characteristics towards the diffusion-flame low temperature combustion was also carried out in order to evaluate the important aspects of each one and observe how the LTC range has been extended.The strategy is based mainly in a significant reduction of the equivalence ratio at the lift-off length cross section and the combustion temperature, maintaining the conventional mixing controlled diesel jet structure. In this 2nd part of the study, injection pressure was increased and new nozzles with reduced holes were used to enhance air entrainment and the injection rate was reduced to extend the duration of diffusion-flame combustion phase.The new proposal allowed extending Diffusion-flame LTC range, being possible to achieve this combustion modality with higher air temperature which lead to considerable reduction in HC, CO and combustion noise while NOx and Soot remained insignificant.

Modelling of spray and combustion processes by using the Eulerian multiphase approach and detailed chemical kinetics

This research deals with computational modelling of non-reactive and reactive turbulent spray processes. The spray process is modelled using the Euler Eulerian multiphase approach together with a size-of-classes model where the discrete phase is considered as continuum and divided into sub-classes. The combustion process is modelled by taking into account chemical kinetics and solving homogeneous gas phase reactions. The combustion model is implemented into a commercial computational fluid dynamics code, and used in combination with previously validated spray sub-models. Several non-reactive cases are modelled by comparing the fuel spatial and temporal development to the available experimental data. The modelled results show excellent agreement for fuel penetration and mixture distributions. Furthermore, the developed method is validated by modelling reactive spray processes within constant volume vessel, and by comparing results to the Engine combustion network experimental data. The vessel conditions correspond well to diesel-like conditions in terms of gas residuals, pressure and temperature. Finally, the given results show a good agreement for the lift-off length and the ignition delay trends compared to the experimental data, but a slight discrepancy in the combustion process occurrence is observed.

Implementation of two color method to investigate late cycle soot oxidation process in a CI engine under low load conditions

Soot emissions from diesel engines are an important concern in meeting emissions regulations. Soot emissions are the result of two competing processes: soot formation and soot oxidation. Mechanisms of soot formation are discussed extensively in the literature. Equivalence ratio at lift-off length along with residence time and gas temperature play an important role for soot formation in a diffusion flame. Mixing capability and bulk gas temperature are the most important parameters that influence the in-cylinder soot oxidation process. Normally, research studies of soot formation-oxidation processes have been developed under controlled and not completely representative conditions of engine operation in the field. Therefore, the main objective of this work was to develop a simplified methodology to evaluate in-cylinder soot oxidation under ‘real’ engine conditions. In particular the impact of mixing process and bulk gas temperature on late cycle soot oxidation was evaluated. The experimental measurements were made in a production light-duty diesel engine varying those parameters that have been demonstrated in the literature as the most relevant in soot formation – oxidation processes; injection pressure, ambient density and intake air temperature. To measure soot, two color method was applied by means of an optoelectronic pyrometer. To evaluate the mixing capability a specific “tracer” Apparent Combustion Time (ACT−1) based on the experimental heat release and injection parameters was defined. The relationship between both parameters was used to explain the soot oxidation process.

Ignition and combustion characteristics of n-pentanol–diesel blends in a constant volume chamber

Pentanol is considered as one of the most promising alternative biofuels due to its excellent physicochemical properties. The objective of this work was to compare the ignition and combustion characteristics of different n-pentanol/diesel blends in an optical constant volume combustion chamber. The tested fuels included 20% (D80P20) and 40% (D60P40) of n-pentanol blended with diesel in volume, and pure diesel (D100). Broadband chemiluminescence technique was used to measure the timing and location of spray ignition. A high-speed CCD camera with two ND8 dimmer lenses was used to capture the incandescence radiated from the soot particles during combustion. A wide range of experimental conditions was investigated. The ambient temperature ranged from 800K to 1200K and the oxygen concentration ranging from 10% to 21%, covering both the conventional and low temperature combustion regimes. The results show that pure diesel has shorter ignition delay and distance comparing to pentanol blends. A larger blending proportion of pentanol D60P40 advances the ignition phase more than the D80P20 in low oxygen concentration conditions. Due to the fuel-borne oxygen and the dilution effect, the natural flame luminosity is reduced significantly with the increase of pentanol ratio in most conditions except under the intermediate temperature region of 1000K. In that condition, the shorter ignition delay and flame lift-off length of pentanol blends cause a slightly increase in the natural flame luminosity. The natural flame luminosity images showed that the oxygen-contained structure of pentanol could accelerate soot oxidation under all conditions. This indicates that pentanol blends could decrease final soot emissions in internal combustion engines.

Simultaneous capture of liquid length of spray and flame lift-off length for second-generation biodiesel/diesel blended fuel in a constant volume combustion chamber

Simultaneous imaging of OH∗ chemiluminescence and Mie scattering of fuel spray have been performed under reacting conditions in a constant volume combustion chamber aimed for a second-generation biodiesel/diesel blended fuel. Mie scattering of 532nm laser light technique was used for the liquid fuel measurement. In order to remove the interference signal from soot radial, filtered Mie scattered light from the fuel droplet was recorded with a charge coupled device (CCD) camera. The spray liquid length and the flame lift-off length for this high cetane-number fuel were investigated under different ambient gas conditions (temperature, density, oxygen concentration) and injection pressure. Experiments showed that the spray liquid length under the reacting condition was shorter than that under non-reacting condition. The flame lift-off length was significantly influenced by the ambient gas temperature, density, oxygen concentration and injection pressure, while the spray liquid length was slightly affected by the injection pressure and oxygen concentration. The overlap between liquid fuel and combustion flame easily happened at high temperature, high oxygen concentration, and low injection pressure conditions.

Lagrangian conditional moment closure model with flame group interaction for lifted turbulent spray jet flames

A new Lagrangian conditional moment closure (CMC) model is developed for multiple Lagrangian groups of sequentially evaporating fuel in turbulent spray combustion. Flame group interaction is taken into account as premixed combustion by the eddy breakup (EBU) model in terms of the probability of finding flame groups in the burned and the unburned state. Evaporation source terms are included in the two phase conditional transport equations, although they turn out to have negligible influence on the mean temperature field during combustion. The Lagrangian CMC model is implemented in OpenFOAM [1] and validated for test cases in the Engine Combustion Network (ECN) [2,3]. Similar ignition delays and lift-off lengths are predicted by the incompletely stirred reactor (ISR) and the Eulerian CMC models due to relatively uniform conditional flame structure in the domain. The improved Lagrangian CMC model shows no abrupt reaction or oscillatory behaviour with an appropriate model constant K and gives results in better agreement with measurements lying between the predictions by ISR and Lagrangian CMC without flame group interaction.

The effect of oxygenated fuel properties on diesel spray combustion and soot formation

Diesel engines have advantages due to their potential for high fuel conversion efficiency. However, it is necessary to reduce pollutant emissions, particularly NOx and soot to meet emission mandates. Many studies have shown that oxygenated fuel additives can effectively reduce soot emissions. In this research, to investigate the effect of oxygenated fuel properties on combustion and soot emission, CFD simulations were carried out using newly developed chemical mechanisms for various oxygenated fuels such as tri-propylene glycol methyl ether, methyl decanoate, and dimethyl ether. The results show that the formation of soot precursors decreases with increased oxygen content and decreased number of C–C bonds in the fuel. The fuel molecular structure strongly affects soot formation in addition to fuel-air mixing. Comparisons of fuel blends (oxygenated fuels with n-hexadecane) including methyl decanoate, which have the same fuel oxygen ratio, show that the effects of the ignition delay and lift-off length control soot formation for fuels with similar fuel oxygen ratio and number of C–C bonds.