The qualitative mixture concentration distributions and the flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures were investigated in a high-temperature high-pressure constant volume combustion vessel. The techniques of laser induced exciplex fluorescence (LIEF) and flame natural luminosity imaging have been used to visualize the liquid and vapor phases of the spray and the flame development process respectively. Results reveal that, as the wall temperature increases, the vapor-rich field extends to the region close to the wall and the area of high vapor fluorescence intensity increases. However, the wall temperature only has a little influence on the liquid phase of the spray. The flame luminosity intensity increases and the ignition delay decreases as the wall temperature increases. Cases at higher wall temperature show more soot emissions, higher soot formation and oxidation rate. In the condition of Pa = 4 MPa, the fuel has evaporated completely before impinging on the wall. The flame area and height at different wall temperatures are nearly the same. The ignition position is observed near the impingement point. However, in the condition of Pa = 2 MPa, there are some liquid phase of the fuel impinging on the wall. The flame area and height increase as the wall temperature increases. The ignition position is always observed in the wall jet region. The distance between ignition position and the axis of the spray decreases as the wall temperature increases.

In this paper, polycyclic aromatic hydrocarbons (PAHs) and soot formation in rich partially premixed flames and nonpremixed flames were studied using a blend of n-heptane and toluene. The flames were diluted with Ar, N-2, and CO2 to control the flame temperature. Laser-induced fluorescence, laser-induced incandescence, and two-color pyrometry were used to study the effects of flame temperature on the PAHs and soot evolution. Results show that temperature distributions are similar for different gas dilutions at low flow rates. However, the high-temperature area increases dramatically in size at high flow rates and it depends on diluents in the order Ar > N-2 > CO2 with regard to flame temperature. With an increase in the flow rate of Ar, the higher growth rate of flame temperature and bigger region of high-temperature region can lead to a higher growth rate from small to large PAI-Is and higher soot volume fraction. However, for CO2 dilution, the increased flow rate results in the increase of formation of large PAHs, but soot formation is reduced due to the fact that lower flame temperature suppresses the soot formation. Therefore, it can be concluded that the evolution of PAHs and soot strongly depends on flame temperature.

•Spray characteristics of gasoline/PODE and diesel/PODE are studied in CVC.•PODE has longer spray tip penetration compared to gasoline and diesel (Ta = 300 K).•PODE shows smaller spray cone angle than diesel under evaporating condition.•Lower equivalence ratio along both directions can be obtained by fueling PODE.

Partially premixed combustion (PPC) and reactivity controlled compression ignition (RCCI) are two new combustion modes in compression-ignition (CI) engines. However, the detailed in-cylinder ignition and flame development process in these two CI modes were not clearly understood. In the present study, firstly, the fuel stratification, ignition and flame development in PPC and RCCI were comparatively studied on a light-duty optical engine using multiple optical diagnostic techniques. The overall fuel reactivity (PRF number) and concentration (fuel-air equivalence ratio) were kept at 70 and 0.77 for both modes, respectively. Iso-octane and n-heptane were separately used in the port-injection (PI) and direct-injection (DI) for RCCI, while PRF70 fuel was introduced through direct-injection (DI) for PPC. The DI timing for both modes was fixed at –25°CA ATDC. Secondly, the combustion characteristics of PPC and RCCI with more premixed charge were explored by increasing the PI mass fraction for RCCI and using the split DI strategy for PPC. In the first part, results show that RCCI has shorter ignition delay than PPC due to the fuel reactivity stratification. The natural flame luminosity, formaldehyde and OH PLIF images prove that the flame front propagation in the early stage of PPC can be seen, while there is no distinct flame front propagation in RCCI. In the second part, the higher premixed ratio results in more auto-ignition sites and faster combustion rate for PPC. However, the higher premixed ratio reduces the combustion rate in RCCI mode and the flame front propagation can be clearly seen, the flame speed of which is similar to that in spark ignition engines but lower than that in PPC. It can be concluded that the ratio of flame front propagation and auto-ignition in RCCI and PPC can be modulated by the control over the fuel stratification degree through different fuel-injection strategies.

With the development of new combustion mode,the spray wall-impingement exists widely in combustion process of modern internal combustion engine.Studying the mechanism of spray wall-impingement is beneficial to improve the thermal efficiency and reduce the pollutant emissions.In this paper,the wall-impingement research status of the droplet and spray has been shown from experiments and numerical simulations,respectively.The disadvantages of the existing research are pointed out,and a reference for further research on spray wall-impingement is provided.

Spray wall-impingement has large effects on the pollutant emissions and thermal efficiency of engines. Different wall temperatures can affect the gas–liquid phase transition of the spray, and the temperature gradient from the ambient to the wall will cause a different mixture process in the region nearby the wall. However, there are few studies on spray wall-impingement with different wall temperatures, particularly on the liquid–gas phase transition of the spray. Therefore, in this paper, the effects of different wall temperatures on spray-impingement have been investigated in a high-temperature, high-pressure constant volume combustion vessel. Cooling equipment was used to adjust the temperature difference between the wall and the ambient gas. n-Dodecane was chosen as the diesel surrogate to study the spray process. The spray wall-impingement was tested by changing the injection pressures (Pi) and wall temperatures (Tw). The ambient temperature (Ta) and ambient pressure (Pa) were kept constant at 773 K and 4 MPa, and the distance (L) between wall and injector was set to 35 mm to mimic the radius of the combustion chamber in heavy-duty diesel engines. A laser-induced exciplex fluorescence (LIEF) technique was used to probe the vapor and liquid phases of the injected fuel. Results show that the liquid phases of the spray do not reach the wall except in the condition of low wall temperature and high injection pressure. The liquid penetration develops and then becomes constant after 1 ms from the start of injection. With the increase of injection pressures (600–1600 bar), the liquid concentration of the spray decreases; however the liquid penetration decreases insignificantly. The wall temperature and the injection pressure have little influence on the liquid interpenetration process. For the vapor phase of the spray, the high concentration regions (equivalence ratio (φ) > 1) mainly distribute in the area of 10 mm away from the impact point on the wall. With the decrease of wall temperatures, the high-concentration regions are enlarged at the near wall regions. However, at the injection pressure of 1600 bar, the influence of wall temperatures on the equivalence ratio is small. The decreasing wall temperature deteriorates the mixing process of the fuel and ambient gas, but the effect is weakened with the increase of injection pressure.

The near-wall spray characteristics of a diesel jet after impingement under the conditions of different impinging distance,injection pressure,ambient pressure and temperature in a high-temperature high-pressure constant volume combustion vessel were investigated.The development process of the free spray and impinging spray was measured by using high-speed photography.And also,the vapor concentration and flow field during the fuel injection development were analyzed with the CFD simulation.The results show that,the injection pressure has a little influence on the spray cone angle at the ambient temperature of 500-900 K and the ambient pressure of 1-4 MPa.It also has a little effect on the spray penetration when the ambient temperature is up to 700 K or more.In particular,the spray penetration has a trend of slight reduction with the increase of the injection pressure when the impinging distance reduces to 30 mm and 20 mm.The ambient temperature is found to have a greater influence on the free spray penetration in comparison with the impinging spray.In addition,as the impinging distance reduces,the spray penetration decreases and the spray cone angle increases as well,but the height of impinging spray firstly increases and then decreases.Among the four kinds of boundary parameters mentioned above,the impinging distance has the most influences on the development characteristics of wall-impinging diesel spray.

The four alcoholic isomers of butanol were added into T20 diesel surrogate (80% n-heptane and 20% toluene in volume) in co-flow partially premixed flames at volumetric fractions of 20% and 40% in order to investigate the effect of butanol addition on polycyclic aromatic hydrocarbons (PAHs) and soot formation. For excluding the influence of toluene, the flame of T16 (84% n-heptane and 16% toluene in volume) and T12 (88% n-heptane and 12% toluene in volume) were also investigated to compare 20% and 40% butanol blend-flames in the same content of toluene. Laser-induced fluorescence and laser-induced incandescence were used to probe the distributions and concentrations of PAHs and soot volume fraction. A detailed n-heptane-toluene-butanols-PAH kinetic model was constructed in order to clarify the chemical effects of the different butanol-blended fuels on PAH formation. The results show that the reduced toluene content (due to butanol addition) is the dominant factor for PAH and soot reduction, compared with the base fuel of T20. The different PAH formation for the tested butanol-blended fuels is attributed to the different reaction pathways of butanol isomers. The branched-carbon-chain butanols (tertiary butanol and isobutanol) tend to produce more propargyl radicals than those of straight-carbon-chain butanols (normal butanol and secondary butanol), thus the PAH formation is higher for the branched butanols. The soot volume fractions with tertiary butanol and isobutanol addition are even higher than that without an additive in the fuel, while the addition of normal butanol and second butanol can reduce soot volume fractions further caused by its oxygenated molecular structure compared with T16 and T12. The soot formation tendency can actually be sequenced by tertiary butanol>isobutanol>secondary butanol>normal butanol at both 20% and 40% blending ratios. The concentration of four-ring PAHs is proportional to soot volume fraction, which means that four-ring PAHs can be used as the indicator of soot formation.

Effects of oxygenated fuels on soot reduction strongly depend on the base fuel. Interesting candidates from oxygenated fuels in this respect include both n-butanol and 2,5-dimethylfuran (DMF), because they have already been used in diesel engines recently. However, information is rather limited on n-butanol and DMF added into a diesel fuel surrogate in fundamental flames to investigate the mechanism of soot reduction. In the current work, both n-butanol and DMF was successively added into diesel surrogate (80% n-heptane and 20% toluene in volume, named as 120) in co-flow partially premixed flames. The effects of different oxygenated structures on polycyclic aromatic hydrocarbons (PAHs) and soot were investigated at the same oxygen weight fractions of 4% and the same volume fractions of 20%. The diagnostics on PAHs, soot volume fractions and soot sizes were conducted by using both laser-induced fluorescence (LIF) and two-color laser-induced incandescence (2C-LII). A combined detailed kinetic model (n-heptane/tolueneibutanols/DMF/PAHs) has been obtained in order to clarify the chemical effects of the different oxygenated fuels on PAHs formation. Results show that the reduced toluene content due to the addition of oxygenated fuels is the dominant factor for the reduction of soot, as compared with the base fuel of 120. The oxygenated structure of n-butanol has a higher ability to reduce PAHs and soot as compared with the addition of DMF. This is due to the fact that the consumption of DMF leads to much formation of C5H5 which enhances the formation of PAHs and subsequent soot. However, the formation of PAHs can be inhibited remarkably as blending n-butanol because only small hydrocarbons like C2H2 and C3H3 etc. are formed. The formation rate of A4 is more similar to that of soot in comparison with the smaller ring aromatics. For the size of soot particles, the distribution range is shrunk from 19-70 nm for 120 to 20-40 nm for the addition of oxygenated fuels. As compared to the effects of oxygenated structures, DMF20 presents a little wider distribution on soot sizes than that of B16.8. Some larger soot particles are detected in DMF20 flame but cannot be found in B20 flame. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

In order to distinguish small aromatics preferably, a Nd : YAG Laser was used to supply an excitation laser, which was adjusted to 0.085 J x cm(-2) at 266 nm. Benzene, toluene, naphthalene, phenanthrene, anthracene, pyrene and chrysene were used as the representative of different rings aromatics. The fluorescence emission spectra were researched for each aromatic hydrocarbon and mixtures by Laser induced fluorescence (LIF). Results showed that the rings number determined the fluorescence emission spectra, and the structure with same rings number did not affect the emission fluorescence spectrum ranges. This was due to the fact that the absorption efficiency difference at 266 nm resulted in that the fluorescence intensities of each aromatic hydrocarbon with same rings number were different and the fluorescence intensities difference were more apparently with aromatic ring number increasing. When the absorption efficiency was similar at 266 nm and the concentrations of each aromatic hydrocarbon were same, the fluorescence intensities were increased with aromatic ring number increasing. With aromatic ring number increasing, the fluorescence spectrum and emission peak wavelength were all red-shifted from ultraviolet to visible and the fluorescence spectrum range was also wider as the absorption efficiency was similar. The fluorescence emission spectra from one to four rings could be discriminated in the following wavelengths, 275 to 320 nm, 320 to 375 nm, 375 to 425 nm, 425 to 556 nm, respectively. It can be used for distinguish the type of the polycyclic aromatic hydrocarbons (PAHs) as it exists in single type. As PAHs are usually exist in a variety of different rings number at the same time, the results for each aromatic hydrocarbon may not apply to the aromatic hydrocarbon mixtures. For the aromatic hydrocarbon mixtures, results showed that the one- or two-ring PAHs in mixtures could not be detected by fluorescence as three- or four-ring PAHs existed in mixture. This was caused by radiation energy transfer mechanism, in which the ultraviolet light was lost in mixtures but the fluorescence intensities were increased with the one- or two-ring PAHs adding. When the mixture only contained three- and four-ring PAHs, the fluorescence emission spectrum showed the both characteristics of three- and four-ring PAHs fluorescence. When three- and four-ring PAHs existed in mixtures at the same time, the fluorescence emission spectra were related to each concentration, so the rings number could be discriminated to a certain extent.

Partially premixed laminar flames were formed using our purpose-built burner. The soot reduction mechanism of blends of diesel and oxygenated fuel was explored. The mixture of toluene and n-heptane(volume ratio, 20:80)(T20) was used as a diesel surrogate. Methanol, ethanol, n-butanol, methyl butyrate, and2,5-dimethylfuran(DMF) were blended with T20, whilst retaining a 4% oxygen content. Laser-induced fluorescence(LIF) was used to obtain spatial fluorescence spectra of polycyclic aromatic hydrocarbons(PAHs)in partially premixed co-flow flames. Laser-induced incandescence(LII) was used to measure soot concentration(volume fraction). The formation and growth of PAHs in flames varied with the fuel blend. Four-ring aromatics(A4) exhibited similar formation and oxidation to soot, so A4 was suitable for estimating soot formation and oxidation. With oxygenated additives, the content of toluene is reduced in T20 fuel, which is the major reason for the reduction of PAH fluorescence spectral intensity and soot concentration. The contribution of different oxygenated additives to PAH formation also affected soot reduction. The PAH-LIF spectral intensity and soot concentration of n-butanol/T20 blends were lower than those of fuels containing methanol, ethanol, methyl butyrate, and DMF. Therefore, n- butanol more effectively reduced PAHs and soot emission during the

Aiming at the measurement requirement of liquid fuel layer combustion,a boundary controllable,flame steady,partially premixed burner was designed in this paper. The mixture of n-heptane and toluene was chosen as the test fuel. The flame structures,temperature measurement,laser diagnostics results were all close to those of similar partially premixed flames,which showed that the designed burner,the co-flow partially-premixed flame and the diagnostic methods were reliable. The results of laser diagnostics revealed the formation and oxidation processes of soot and PAHs under different flame heights,and the measured data provided the good support to verify and develop the soot formation and oxidation mechanism under fuel rich conditions.

Toluene reference fuel (TRF) is a kind of surrogate fuel for diesel. Clarifying the combustion mechanism of TRF is a key to understand the combustion of diesel. Alcohols are added into diesel to reduce the soot emission from engines. The formation mechanism of soot precursors, i.e., polyaromatic hydrocarbons (PAHs), in the premixed laminar flames of TRF, methanol-TRF blend, ethanol-TRF blend, and n-butanol-TRF blend were investigated experimentally and theoretically. A partly reduced n-butanol combustion mechanism, including the sub-mechanism of methanol and ethanol, and a partly reduced soot formation mechanism were merged into a partly reduced mechanism of TRF to simulate the combustion of the TRF-alcohol blends. Additions of methanol, ethanol, and n-butanol can reduce the mole fractions of the PAHs in TRF flames at an equivalence ratio of 2.0. The mole fractions of the PAHs in the flames are in the order of n-butanol-TRF < ethanol-TRF < methanol-TRF (<TRF) at a fixed oxygen content of 4.0%. The simulation results show that benzene is overwhelmingly produced from the toluene in TRF. Additions of the alcohols into TRF have negligible effect on the branching ratios of toluene consumption, but may increase the mole fractions of OH and O, which accelerate the oxidation of PAHs formed in the flames, at the fixed oxygen content of 4.0%. The decrease of PAHs can be attributed to the effect of dilution by the additions of the alcohols. (C) 2015 Elsevier Ltd. All rights reserved.

High-speed imaging and thermodynamic analyses were applied to an optically accessible diesel engine,at engine speed of 1,200,r/min and fuel-injection quantity of 20,mg per cycle,to investigate the effects of blends of 3 types of oxygenated fuels with diesel on natural luminosity of combustion. 20%of methyl butyrate,n-butanol and 2,,5-dimethylfuran(DMF)were blended with diesel by volume,respectively,referred to as MB,20,B,20 and DMF,20. Results show that D,100 has the largest gap between the ignition delay derived from heat release and natural luminosity. Obvious blue flame from chemiluminescence of combustion process is observed for all three blends. The ability for soot reduction of these three oxygenated fuels is in the order of DMF,20,MB,20 and B,20. Longer ignition delay plays the most important role in soot reduction in the combustion process of DMF,20,while for MB,20 and B,20,oxygen content prevails. Blending of oxygenated fuels reduces the area and temperature,and cuts down the soot formed in combustion process.

This paper describes the application of laser-induced incandescence as a quantitative measurement tech-nique for volume fraction and size of soot particles in the flame. The soot volume fraction distribution was quantified by the two color LII method,while the soot particle size distribution was measured by the time-resolved LII method. LII models with two different sub-models for heat-conduction were compared under the gas pressure of 0.1-3.0,MPa. LII measurement was carried out on a co-flow partial-premixed burner flame with three different fuels. Mixing fuel of n-heptane and toluene was used as a basic case,and oxygenated bio-fuels of ethanol and n-butanol were added, respectively,as comparative cases. The results show that the addition of oxygenated bio-fuel is an effective way to reduce soot emission in the flame,and that n-butanol has a better soot-reduction effect than ethanol.