Constant-volume Combustion Vessel Research Articles

A COMPREHENSIVE STUDY ON THE INFLUENCE OF RESOLVING AN INJECTOR ORIFICE AND THE INFLUENCE OF CREATING STRIPPED OFF DROPLETS ON SPRAY FORMATION USING THE VSB2 SPRAY MODEL

With respect to simulating fuel sprays applied to direct injection engines, few studies in literature have investigated resolving the injector orifice and how this may influence spray predictions for high pressure and temperature diesel engine-like conditions. In this work, we used the stochastic blob and bubble (VSB2) spray model to conduct simulations in which fuel is injected into a constant volume combustion vessel. The injector orifice is resolved into nine cells. The boundary conditions were the same as the Engine Combustion Network (ECN) noncombusting case for n-dodecane (ECN, 2020). Two simulation meshes were compared with experimental data: (1) injector orifice resolved and (2) injector orifice unresolved (grid cells in the orifice region equal orifice diameter). The resolved orifice mesh showed a liquid penetration length slightly higher and closer to experimental values. Spray predictions for an asymmetrical injection velocity was compared for both meshes. Finer structures near the leading edge of the spray (for mixture fraction and temperature fields) seen in the resolved mesh were missing in the unresolved mesh. Resolving the orifice requires a change in the core of the mesh, which also influences the results. The influence of creating new child blobs (liquid parcels are referred to as blobs in this work) stripped off from a parent blob by secondary breakup was also studied. The simulation results suggested that for high pressure and temperature diesel engine-like conditions, the influence of creating new child blobs is insignificant.

Impact of dilution on laminar burning characteristics of premixed methane-dissociated methanol-air mixtures

A constant volume combustion vessel is employed to investigate the impact of different diluents and diluent gas fractions on the methane-dissociated methanol-diluent-air premixed flames at 0.3 MPa and 343 K. The diluent gas causes the adiabatic flame temperature and thermal diffusivity to decrease, which is the main reason for the decreased laminar burning velocity, and carbon dioxide exhibits a stronger inhibitory effect than nitrogen. With increasing diluent gas fraction, the diffusive-thermal instability is enhanced while the hydrodynamic instability is inhibited, these two factors are the main reason for the insensitivity of the flame instability to the diluent gas fraction. Further, flame instability is not sensitive to the diluent gas type. R38 is the dominant chain branching reaction, while R35 and R52 are the dominant chain termination reactions in the combustion process. With the increase in the diluent gas fraction, the rates of the dominant chain termination reactions shows greater inhibition of laminar burning velocity, a larger decrease in the chain branching reaction rate, and a smaller decrease in the chain termination reaction rate will significantly reduce the free radical concentration. The maximum reaction rate reduction when diluted by carbon dioxide is greater than when diluted by nitrogen.

Characterizing the fuel-specific combustion chemistry of acetic acid and propanoic acid: Laminar flame propagation and kinetic modeling studies

In order to study the combustion chemistry of carboxyl functionality, the laminar burning velocity of acetic acid/air and propanoic acid/air mixtures was investigated in a high-pressure constant-volume cylindrical combustion vessel at 423 K, 1 atm and equivalence ratios of 0.7–1.4. Experimental results reveal that the flame propagation of propanoic acid flame is much faster than that of acetic acid flame, especially under rich conditions, and the laminar burning velocity of propanoic acid/air mixtures peaks at richer conditions than that of acetic acid. The present theoretical calculations for the isomerization and decomposition of propanoic acid radicals indicate that the primary radical products are HOCO, H and C2H5, while those in acetic acid flame are CH3 and OH based on previous studies. A kinetic model of the two acids was developed mainly based on previous and the present theoretical calculation results. It could reasonably capture the measured laminar burning velocities of acetic acid/air and propanoic acid/air mixtures in this work, as well as the previous experimental data in literature. Based on the present model, CH3- and ketene-related pathways play an important role in acetic acid flames. Under rich conditions, ketene is mostly converted to CH3 via CH2CO+HCH3+CO, and the chain-termination reaction of CH3+H(+M)=CH4(+M) is enhanced, which strongly inhibits the propagation of rich acetic acid flames. In contrast, C2H5 and ethylene chemistry play an important role in propanoic acid flames. Rich conditions promote the decomposition of C2H5, yielding ethylene and H, which can facilitate the flame propagation. This can explain the shift of the peak laminar burning velocity of propanoic acid/air mixtures towards a slightly richer condition compared with that of acetic acid/air mixtures.

Exploring fuel isomeric effects on laminar flame propagation of butylbenzenes at various pressures

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of three butylbenzene isomers (n-butylbenzene, iso-butylbenzene and tert-butylbenzene)/air mixtures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, initial pressures of 1–10 atm, and equivalence ratios (ϕ) of 0.7–1.5. The laminar burning velocities of butylbenzene/O2/He mixtures were also measured at 423 K, 10 atm and ϕ = 1.5 to provide additional experimental data under conditions that the butylbenzene/air experiments are susceptible of cellular instability. Comparison among the laminar burning velocities of butylbenzenes including both the three isomers investigated in this work and sec-butylbenzene investigated in our recent work [Combust. Flame 211 (2020) 18–31] shows remarkable fuel isomeric effects, that is, iso-butylbenzene has the slowest laminar burning velocities, followed by n-butylbenzene and tert-butylbenzene, while sec-butylbenzene has the fastest laminar burning velocities. A kinetic model for butylbenzene combustion was developed to simulate the laminar flame propagation of butylbenzenes. Sensitivity analysis was performed to reveal important reactions in laminar flame propagation of butylbenzenes, including both small species reactions and fuel-specific reactions. Kinetic effects are concluded to result in the different laminar burning velocities of four butylbenzene isomers. Small species reactions control the laminar flame propagation under lean conditions, which results in small differences of laminar burning velocities. Chain termination reactions, especially fuel-specific reactions, have important contributions to inhibit the laminar flame propagation under rich conditions. The structural features of butylbenzene isomers can significantly affect the formation of some crucial radicals such as methyl, cyclopentadienyl and benzyl radicals under rich conditions, which leads to remarkable fuel isomeric effects on their laminar burning velocities, especially at high pressures.

Insight into fuel isomeric effects on laminar flame propagation of pentanones

Laminar flame propagation was investigated for pentanone isomers/air mixtures (3-pentanone, 2-pentanone and 3-methyl-2-butanone) in a high-pressure constant-volume cylindrical combustion vessel at 393–423 K, 1–10 atm and equivalence ratios of 0.6–1.5, and in a heat flux burner at 393 K, 1 atm and equivalence ratios of 0.6–1.5. Two kinds of methods generally show good agreement, both of which indicate that the laminar burning velocity increases in the order of 3-methyl-2-butanone, 2-pentanone and 3-pentanone. A kinetic model of pentanone isomers was developed and validated against experimental data in this work and in literature. Modeling analysis was performed to provide insight into the flame chemistry of the three pentanone isomers. H-abstraction reactions are concluded to dominate fuel consumption, and further decomposition of fuel radicals eventually produces fuel-specific small radicals. The differences in radical pools are concluded to be responsible for the observed fuel isomeric effects on laminar burning velocity. Among the three pentanone isomers, 3-pentanone tends to produce ethyl and does not prefer to produce methyl and allyl in flames, thus it has the highest reactivity and fastest laminar flame propagation. On the contrary, 3-methyl-2-butanone tends to produce allyl and methyl instead of ethyl, and consequently has the lowest reactivity and slowest laminar flame propagation.

Exploration on laminar flame propagation of ammonia and syngas mixtures up to 10 atm

Reactivity enhancement is crucial for the potential applications of ammonia (NH3) as a gas turbine fuel. Doping more reactive fuels like H2, methane and syngas is a widely adopted strategy in this field, however fundamental combustion studies of NH3/reactive fuel mixtures under gas turbine-relevant high pressure conditions are still very limited. This work reports an effort to study laminar flame propagation of NH3/syngas mixtures up to 10 atm. Laminar burning velocities (LBVs) of NH3/syngas/air mixtures were measured at 298 K, various syngas contents in fuel mixtures (α) and H2 contents in syngas (β), and equivalence ratios of 0.7–1.5 in a high-pressure constant-volume cylindrical combustion vessel. A kinetic model was developed for NH3/syngas combustion based on our recent NH3 model and a recent syngas model in literature. It shows reasonable predictions on the present NH3/syngas LBVs at 1–10 atm, as well as previous data including NH3/syngas LBVs at 1 atm, NH3/syngas ignition delay times at various pressures and pure NH3 LBVs at various pressures. Modeling analysis including the sensitivity analysis and rate of production analysis provides kinetic interpretation for the effects of fuel composition (α and β), equivalence ratio and pressure on NH3/syngas LBVs. It is found that the addition of syngas shifts the chemistry from NH3 sub-mechanism to syngas sub-mechanism. A modified fictitious diluent gas method was proposed to separate the chemical and thermal effects of syngas addition, which shows that the chemical effect is more responsible for the enhanced laminar flame propagation. NH3/syngas/air flames have similar reaction networks but different preferred pathways under lean and rich conditions. Compared with pure NH3 flames, the addition of syngas also improves the importance of H + O2 (+ M) = HO2 (+ M) and consequently leads to strong pressure dependency of NH3/syngas/air flames.

Investigating the explosion of ethyl acetate in the presence of hydrogen

The explosion characteristics of ethyl acetate/hydrogen-air (EA/H2/air) mixtures have been investigated in this document using a constant volume combustion vessel. The experiments were executed using the initial pressure (1, 2, and 3 bar) and temperature (358 K) over a wide range of equivalence ratios. Hydrogen was mixed with compressed air in the volumetric proportion of 0%, 4%, 8%, and 12% and added to EA. The experimental pressure data were refined to determine the explosion characteristics. The explosion characteristics of EA/H2/air using different H2 fractions were compared. The study indicated that there was a slight difference between the peak explosion pressures of 4%, 8%, and 12% H2 fraction, however, the explosion with H2 of 12% had the maximum pressure rise rate and deflagration index when the explosion was evaluated in terms of the pressure rise rate and deflagration index. The explosion time at 8% and 12% hydrogen was relatively shorter than that of 4%.

Development of a skeletal oxidation mechanism for Fischer–Tropsch diesel surrogate based on decoupling method and particle swarm optimization

As an alternative liquid fuel, Fischer–Tropsch (FT) diesel has received significant attentions due to its characteristics of high efficiency and low emission. In this study, a surrogate fuel containing iso-hexadecane and n-dodecane with a mole ratio of 0.16:0.84 is formulated for real FT diesel by mimicking its combustion-related physicochemical properties. Mechanisms of these two components are developed based on decoupling methodology: skeletal sub-mechanisms describing iso-hexadecane and n-dodecane cracking process are constructed and combined with a reduced C0–C4 core mechanism, and then the Arrhenius parameters of certain reactions are tuned by particle swarm optimization algorithm to improve prediction accuracy. The optimized mechanisms are validated against experimental results of ignition delays, species concentrations and laminar flame speeds for iso-hexadecane and n-dodecane, respectively. Finally, by merging all the sub-mechanisms mentioned above, a skeletal oxidation model for FT diesel surrogate including 73 species and 324 reactions is obtained and employed in 3D CFD simulations to validate the ignition behavior of FT diesel sprays in a constant-volume combustion vessel; the simulation results show good agreement with experimental data.

Effects of hydrogen addition on the combustion characteristics of diesel fuel jets under ultra-high injection pressures

Many applications use hydrogen addition and high-pressure fuel injection technology to improve combustion performance. In this study, spray atomization and combustion characteristics of a diesel fuel jet, under the injection pressure of 350 MPa, injecting into a constant volume combustion vessel filled with air-hydrogen mixture at the diesel engine relevant condition are investigated by simulation method. A simplified mechanism of the n-heptane (C7H16) oxidation chemistry mechanism consisting of 26 reactions and 25 species integrated with the Kéromnès-2013 hydrogen combustion mechanism and EDC combustion model are utilized to predict the diesel fuel spray auto-ignition and combustion. The ambient gas is the mixture of air and hydrogen range in volume fraction from 0% to 10%. The ambient temperature and pressure is set to 1000 K and 3.5 MPa, respectively. The results indicate that as the hydrogen volume fraction is 2%, the minimum overall droplet SMD (Sauter Mean Diameter) is approximately 0.95 μm, which is obviously smaller than that of the case with the conventional high injection pressure. In cases that H2 v/v% larger than 4%, the maximum gaseous temperature increased significantly up to 2700 K. There are two peaks in the temperature growth rate curves as the hydrogen fraction of 8% and 10%. The high temperature at the outer edge of the spray is clearly seen due to its high value when the hydrogen fraction is larger than 4%. The hot reaction layer is the main location of CO formation. The H, OH radicals are formed at the edge of the spray where the temperature is high. The hydrogen species obviously promotes the oxidation and combustion of diesel fuel.

Experimental and kinetic modeling investigation on sec-butylbenzene combustion: Flow reactor pyrolysis and laminar flame propagation at various pressures

This work reports the investigation on pyrolysis and laminar flame propagation of sec-butylbenzene. The pyrolysis experiments were performed in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry from 780 to 1166 K at 0.04 and 1 atm. The pyrolysis products were identified and their mole fraction profiles versus the heating temperature were evaluated. The laminar burning velocities of sec-butylbenzene/air mixtures were measured at the initial temperature of 423 K and initial pressures of 1–10 atm in a high-pressure constant-volume cylindrical combustion vessel with the equivalence ratio from 0.7 to 1.5. Furthermore, a kinetic model was developed to predict the pyrolysis and laminar flame propagation of sec‑butylbenzene. Validation of the present model was performed against the new experimental data in this work. Rate of production analysis and sensitivity analysis were performed to provide insight into the chemistry in fuel decomposition and polycyclic aromatic hydrocarbons (PAHs) formation. In the flow reactor pyrolysis, the consumption of sec‑butylbenzene is mainly controlled by the unimolecular decomposition reactions and H-atom abstraction reactions at both low and atmospheric pressures. Fuel specific pathways through propenylbenzene and α-methylstyrene become the dominant formation pathways of indene and naphthalene. In the laminar flame propagation, the laminar burning velocity of sec‑butylbenzene is sensitive to the reactions of both small species and fuel-relevant intermediates under all investigated conditions. In particular, the pyrolysis reactions in the fuel sub-mechanism play inhibition effects on the laminar flame propagation of sec‑butylbenzene under rich conditions. The laminar burning velocity of sec‑butylbenzene is also compared with benzene, toluene and ethylbenzene under same initial conditions. Both the thermodynamic and kinetic effects are responsible for the difference in laminar burning velocities of these aromatic fuels.

Explosion characteristics of bio-syngas at various fuel compositions and dilutions in a confined vessel

Safety issues for production, transportation, storage, and usage of bio-syngas persist due to the variability of bio-syngas composition. Therefore, this study focuses on the effects of fuel composition variation (H2/CO/CH4) and diluents (N2/CO2) on the explosion characteristics of bio-syngas/air mixtures in a constant volume combustion vessel. The experimentally determined explosion parameters were obtained at an initial temperature of 303 K, equivalence ratios of 0.6–1.5 and atmospheric pressure with a wide range of H2/CO/CH4 compositions and dilution ratios. The results show that the concentration of H2 or CH4 in the fuel mixture has a great influence on the explosion characteristics of bio-syngas/air mixtures, while the concentration of CO in the fuel mixture plays a mild role in affecting the explosion behaviors. Moreover, it is observed that the addition of N2/CO2 to the fuel mixture can significantly reduce the potential of explosion hazards of bio-syngas/air mixtures, and CO2 dilution has stronger suppression effect on the explosion characteristics due to its larger negative influence on the thermal effect and chemical effect in comparison to that of N2 dilution. The corresponding correlations of Pmax/P0 = f(Zdilution), tc = f(Zdilution), and (dP/dt)max = f(Zdilution) were found for predicting the explosion behaviors of N2/CO2 diluted bio-syngas/air mixture at various dilution ratios.

Measurements of laminar flame speeds and flame instability analysis of E30-air premixed flames at elevated temperatures and pressures

Ethanol is regarded as one of the most promising alternative renewable fuels and as well as an oxygenate blending component in gasoline fuels, with widespread usage in many countries around the world. Laminar flame speeds can have strong influence on the stability and operability of Spark-Ignition combustion in certain operating regimes, and so the effects of different initial conditions on laminar combustion characteristics of E30 (gasoline blended with ethanol of 30% liquid volume) were analyzed in a constant-volume combustion vessel using the high-speed Schlieren method. This work presents results for equivalence ratios of 0.7–1.4, dilution ratios of 0%, 5%, 10%, and at different initial temperatures (408, 453 and 498 K) and initial pressures (1, 2 and 3 bar). It can be concluded that the laminar burning velocity has a positive correlation with initial temperature, but negative correlation with initial pressure and dilution ratio. The laminar burning velocity always reaches its maximum value at an equivalence ratio of 1.1 and does not change with varying initial conditions' the adiabatic flame temperature displays a similar variation with the initial conditions. The flame instability of E30-air mixture is enhanced as the initial pressure increases. Flame stability at lean and rich mixtures are exactly opposite at different initial temperature and dilution ratio. The laminar burning velocity was significantly promoted relative to gasoline and E10 by the addition of higher volume fractions of ethanol, highlighting one of the benefits of ethanol’s use as a blending component in gasoline fuels.

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.

Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions

Ammonia is attracting more and more attentions due to its role as both a carbon-free fuel for gas turbines and an effective H2 carrier. Only a limit number of investigations on the laminar flame propagation and laminar burning velocity of ammonia have been performed on elevated pressures, which were focused on ammonia/air mixtures and suffered strong buoyancy effect. In this work, laminar flame propagation of ammonia/O2/N2 mixtures covering wide ranges of equivalence ratios, oxygen contents and initial pressures was investigated in a high-pressure constant-volume cylindrical combustion vessel. The oxygen enrichment speeds up the spherically expanding flames and consequently reduces buoyancy effect on the laminar flame propagation of ammonia. The laminar burning velocity was observed to increase with the increasing oxygen content, but decrease with the increasing initial pressure. A kinetic model of ammonia combustion consisting 38 species and 265 reactions was constructed from previous models with updated rate constants of important reactions. The present model can reasonably reproduce the laminar burning velocity data in this work and literature, as well as the ignition delay time and speciation data in literature. Based on the model analysis, effects of oxygen enrichment, equivalence ratio and initial pressure on laminar burning velocities of ammonia were analyzed in detail. It is revealed that the enhanced flame propagation with oxygen enrichment is mainly due to the increase of adiabatic flame temperature which in turn leads to higher concentrations of key radicals like H, OH and NH2. For NH3 and its major decomposition products like NH2 and NH, reactions with oxygenated species such as OH, O, O2 and NO are generally more important in the lean flames, while the role of reactions with H, NH and NH2 becomes crucial in the rich flames. The calculated pressure dependent coefficient indicates that NH3/O2/N2 flames exhibit clear pressure dependence, while this pressure dependence is weaker than those of the hydrocarbon and biofuel flames.

Experimental and kinetic modeling investigation on laminar flame propagation of CH4/CO mixtures at various pressures: Insight into the transition from CH4-related chemistry to CO-related chemistry

In this work, laminar flame speeds of CH4/CO/air mixtures were measured at the unburnt temperature of 353 K and pressures from 1 to 10 atm in a high-pressure constant-volume cylindrical combustion vessel. Effects of pressure, equivalence ratio and CO content in CH4/CO mixtures on laminar flame speeds were investigated. A kinetic model for CH4/CO combustion was developed based on recent progress in elementary reactions and validated against previous and present experimental targets. It is found that both the thermal effect originating from different adiabatic flame temperatures and chemical effect originating from differences in the radical pool play important roles in the variation of laminar flame speed. The separate contribution of each effect varies at different pressures, equivalence ratios and CO contents. Besides, the transition from CH4-related chemistry to CO-related chemistry can be monitored by the increasing concentration of O atom in the radical pool under all the investigated conditions. Based on the modeling analysis, R18 (CH3 + CH3 (+M) = C2H6 (+M)), R19 (HCO (+M) = H + CO (+M)), R6 (CH4 (+M) = CH3 + H (+M)) and R7 (CH4 + H = CH3 + H2) have major contributions to the transition chemistry, especially under the rich conditions. Compared with the atmospheric pressure conditions, R20 (CO + O (+M) = CO2 (+M)) and R22 (H + O2 (+M) = HO2 (+M)) are enhanced at high pressures, which leads to the decrease of O atom in the radical pool at high pressures.

Experimental and kinetic modeling study of n-propanol and i-propanol combustion: Flow reactor pyrolysis and laminar flame propagation

Flow reactor pyrolysis and laminar flame propagation are investigated for n-propanol and i-propanol, which are the smallest alcohol fuels with isomeric structures. Pyrolysis products of propanol isomers at 0.04–1 atm are detected using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental observations demonstrate ethylene and propene are respective dominant hydrocarbon products in the n-propanol and i-propanol pyrolysis, while the most abundant oxygenated products are formaldehyde, acetaldehyde and ethenol in the n-propanol pyrolysis and acetone and acetaldehyde in the i-propanol pyrolysis. Higher concentrations of aromatic and oxygenated pollutants are observed in the i-propanol pyrolysis. The laminar burning velocities of propanol isomers are measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, pressures of 1–10 atm and equivalence ratios of 0.6–1.5. A general trend that n-propanol has much faster LBVs than i-propanol is noticed under all investigated conditions. A kinetic model of propanol isomers is developed and validated against the present experimental data, as well as other experimental data in literature covering wide ranges of temperatures, pressures and equivalence ratios. Rate of production analysis and sensitivity analysis are performed together to provide insight into the fuel isomeric effects on key reaction pathways and fuel reactivities. In the flow reactor pyrolysis, different dominant primary decomposition pathways of n-propanol and i-propanol lead to the differences in both molecular structures and concentration levels of pyrolysis products. In laminar flame propagation, different radical pool distributions of propanol isomers are illustrated and found to be largely influenced by fuel isomeric structures. The linear structure of n-propanol promotes the formation of more active radicals like formyl, vinyl and ethyl, while the branched structure of i-propanol facilitates the production of more stable radicals including methyl and allyl. Thus, the promoted chain-branching in n-propanol flames and enhanced chain-termination in i-propanol flames can explain the higher laminar burning velocities and reactivities of n-propanol than that of i-propanol.

Development of a Reduced Biodiesel Surrogate Fuel Model for Multi-Dimensional Computational Fluid Dynamic Simulations

This work reports the development of a reduced biodiesel surrogate fuel model for multi-dimensional Computational Fluid Dynamic simulations. The model is derived using an integrated kinetic mechanism reduction scheme and the final chemistry comprises only 83 species. The model is well validated in zero-dimensional chemical kinetic calculations under a wide range of auto-ignition and jet-stirred reactor conditions. The fidelity of the model is also further assessed in two-dimensional simulations of a constant-volume combustion vessel with respect to the experimental results of soy-methyl ester combustion. The results obtained reveal that ignition delay, lift-off lengths and soot volume fractions are reasonably well replicated by the reduced model. Furthermore, the compositions of the reduced model are varied according to the saturation/unsaturation levels of various biodiesel feed-stocks, such as palm-methyl ester and sunflower-methyl ester. In this work, it is demonstrated that the reduced model can potentially be used as a universal surrogate fuel model to predict the reactivity of biodiesel feed-stocks with low degree of saturation (≤30%) in both kinetic and fuel spray simulations.

Ignition and combustion characteristics of wall-impinged kerosene (RP-3) fuel spray with varying injection parameters

The fuel quantity and injection pressure are two essential factors to optimize the injection strategy. In this paper, we focus on the investigation for the ignition and combustion characteristics of wall-impinged kerosene (RP-3) fuel spray at different injection quantities and pressures. Experiments are conducted in a constant volume combustion vessel to simulate the Diesel engine condition, adopting a single-hole nozzle with 0.22 mm. The flame images are captured using a high-speed camera, and then the behaviors of ignition and combustion are processed and analyzed. The main emphasis is placed on the variation laws of the ignition position distance, the ignition delay time, the combustion duration, the flame area, spatially integrated natural luminosity and time integrated natural luminosity.

Numerical Study of Geometric Effects on Burning Velocity Measurement by Constant Volume Method

ABSTRACT Three axisymmetric 2D numerical model including a spherical and two cylindrical constant volume combustion vessel with different diameter to length ratio are constructed to explore the geometric effects and possibility of adopting cylindrical vessel in measurement of laminar burning velocity (LBV) by constant volume method (CVM). Two methods are adopted. First, the exact combustion properties from simulation are used to extract the LBV. Second, the estimated combustion properties based on the experimental techniques are applied. The results show that the combustion pressure and pressure rise rate are almost same in three different chambers before flame touching the wall, but in cylindrical vessel, flame surface area is changed by cylindrical confinement. The violation of spherical flame assumption applied in CVM results in the obvious overestimation of LBV. However, the discrepancy of LBV is not shown when the estimated combustion properties commonly adopted in practice are used, because the simplified properties do not capture the true burned mass fraction. Therefore, the pressure data before flame hitting the chamber wall in cylindrical vessel could perform same accuracy with that of spherical vessel in experiment. The flame impacting timing could be easily identified by the sudden drop of pressure rise rate. The only concern is that the final pressure used in calculating the burned mass fraction may be reduced by heat loss in cylinder vessel. It is suggested that before the pressure date in cylindrical vessel is utilized in determination of LBV, the final combustion pressure obtained in cylindrical vessel should be compared with the data in spherical vessel, at least compared with ideal constant volume equilibrium pressure to evaluate the deviation.

Study of laminar combustion characteristics of gasoline surrogate fuel-hydrogen-air premixed flames

This paper investigated the effects of hydrogen addition to gasoline surrogates fuel-air mixture on the premixed spherical flame laminar combustion characteristics. The experiments were carried out by high speed Schlieren photography on a constant-volume combustion vessel. Combining with nonlinear fitting technique, the variation of flame propagation speed, laminar burning velocity, Markstein length, flame thickness, thermal expansion coefficient and mass burning flux were studied at various equivalence ratios (0.8–1.4) and hydrogen mixing ratios (0%–50%). The results suggested that the nonlinear fitting method had a better agreement with the experimental data in this paper and the flame propagation was strongly effected by stretch at low equivalence ratios. The stretched propagation speed increased with the increase of hydrogen fraction at the same equivalence ratio. For a given hydrogen fraction, Markstein length decreased with the increase of equivalence ratio; flame propagation speed and laminar burning velocity first increased and then decreased with the increase of equivalence ratio while the peaks of the burning velocity shifted toward the richer side with the increase of hydrogen fraction.