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Flame kernel evolution and shock wave propagation in ethanol/air mixtures with laser ignition in a constant-volume chamber was investigated by means of high-speed Schlieren photography. At initial pressure of 0.1 MPa and temperature of 333 K, ignition was performed using a pulsed laser at the second harmonic wavelength with six different laser energies from 75 to 200 mJ, for the equivalence ratios ranging from 0.8 to 1.4. A cross-shaped plasma spot is captured ∼3.0 µs after the laser triggering, whereas the shape of the flame kernel is initially circular and then transforms to an ellipsoid structure. Following the contraction of the plasma, the flame kernel growth rate is initially very different in the four directions and then approaches a steady flame speed. The contraction of the plasma zone is accompanied by a rapid shock wave propagation where, the wave propagation speed is found to be negligibly influenced by the variations in either laser energy or mixture equivalence ratio. Under the test conditions in this study for ethanol/air mixtures, the initial shock wave speed is ∼480 m/s which decays to ∼380 m/s after 20 µs. The shock wave front propagates with a time dependency of about t0.6 when t

The cellular instabilities of premixed C1–C3 alkane–air mixtures were investigated at an initial temperature of 400 K and different initial pressures (0.25–1.5 MPa). Flame propagation images were recorded using high-speed Schlieren photography techniques. Cellular instabilities were analyzed using both qualitative and quantitative methods. The results showed that with an increase in the initial pressure, the flame instabilities of these three alkane–air mixtures increased as a result of the enhanced hydrodynamic instability. For a given initial pressure and with an increasing equivalence ratio, the thermal-diffusive instability was the main influencing factor on the flame instability of methane when Φ ≤ 1.0, whereas it was the result of hydrodynamic and thermal-diffusive instability when Φ > 1.0. The flame instability of ethane and propane was caused by the enhanced hydrodynamic and thermal-diffusive instability when Φ ≤ 1.0, whereas it was determined by diffusion-thermal instability when Φ > 1.0. Under the conditions with the strongest cellular instability, the cellular instability of these three alkane–air mixtures tended to increase with an increasing number of carbon atoms in the fuel.

Atomization of fuel sprays is a key factor in the control of combustion quality in direct-injection engines. In the present work, the near-field spray patterns of 2-methylfuran (MF), ethanol (ETH) and isooctane (ISO) under non-flash boiling and flash boiling conditions were investigated using an ultra-highspeed imaging technique. Fuel was injected from a single-hole solenoid injector into an optically accessible constant volume chamber at the injection pressure of 40 MPa. Various conditions were tested, ranging from non-flash boiling conditions (ambient) to flare-flash boiling conditions with fuel temperatures of 20 °C and 80 °C and different back pressures. High-speed imaging was performed using a long-distance microscope coupled with an ultra-highspeed camera (1 million fps). Results showed that under flash boiling conditions, near-nozzle spray patterns changed significantly and clear radial expansion was observed due to bubble formation and explosion. Among the three fuels, MF showed the most intense flash boiling behavior due to it having the highest vapor pressure. The effects of different non-dimensional numbers were also considered and it was found that saturation ratio and cavitation number were the two main governing factors for the near-nozzle spray behaviors. During the end of the injection process, the low effective pressure led to poorly atomized spray with a compact liquid column and large ligaments; this could result in poor air/fuel mixing and thus higher HC and particle emissions. Significant improvements were observed at Rs = 0.2 where flash boiling greatly promoted the spray atomization, even with low fuel velocity.

Cyclopentanone, a product of biomass pyrolysis of agricultural waste, has certain advantages as a biofuel candidate but so far little is known about its combustion characteristics. In this paper, the laminar flame characteristics of cyclopentanone, including stretched flame propagation speed, unstretched flame propagation speed, and laminar burning velocity, were measured and compared with gasoline and ethanol, using the outwardly propagating spherical flame method and the high-speed Schlieren photography technique. The experiments were conducted in a constant-volume vessel using various fuel-air equivalence ratios (ϕ = 0.8–1.6) at elevated initial temperatures (T 0 = 423, 448 and 473 K) and a fixed initial pressure (P 0 = 0.1 MPa). Linear and non-linear extrapolations were used to characterise the relationship between the stretch rate and the stretched flame propagation speed when Markstein length was near to or away from zero respectively. Empirical functions were obtained to calculate the laminar burning velocities of cyclopentanone for various fuel-air equivalence ratios and initial temperatures. The results show that Markstein length of cyclopentanone decreases when equivalence ratio is increased, and the turning point of equivalence ratio at which it changes from positive to negative is slightly below 1.4. The maximum laminar burning velocity of cyclopentanone appears at the equivalence ratio of approximately 1.2, regardless of the initial temperature. The laminar burning velocity of cyclopentanone has a smaller difference to that of ethanol and gasoline when equivalence ratio is leaner than stoichiometric, but when equivalence ratio increases from 1.0 to 1.4, it becomes increasingly lower than that of ethanol and higher than that of gasoline. The maximum laminar burning velocity of cyclopentanone is 0.82 m/s; for gasoline it is 0.72 m/s and for ethanol it is 0.86 m/s, at an initial temperature of 423 K and pressure of 0.1 MPa.

2-Methyl tetrahydrofuran (2-MTHF) is one of the promising second-generation biofuel candidates, raising increasing interest because of the recent breakthrough in producing 2-MTHF from biomass. As a potential gasoline extender and renewable oxygenate, its high energy density and avoidance of competing with food make it attractive compared with ethanol. In the current work, the laminar burning characteristics of 2-MTHF-air mixtures are studied with varying equivalent ratios (0.88–1.43) and initial temperatures (333 K, 363 K and 393 K) at ambient pressure in a constant-volume vessel. The results are compared with ethanol and isooctane, a representative component in gasoline. The stretched flame speed, un-stretched flame speed, Markstein length, flame thickness, Markstein number, laminar burning velocity and burning flux are calculated and analyzed. The result shows that for most tests, the ranking of un-stretched flame propagation speed is ethanol, 2-MTHF and isooctane. The peaks of un-stretched flame speeds against varying equivalent ratios appear at Φ = 1.1–1.2 for three tested fuels. The Markstein length indicates that generally isooctane has the most diffusion-thermal stable flame than 2-MTHF and ethanol when Φ

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.

The effect of N2 and CO2 dilution on MTHF/air premixed flame propagation was investigated at initial temperature and pressure of 393 K and 0.1 MPa, respectively. The flame speed measurements were conducted on spherically expanding premixed flame in a constant volume vessel using high speed Schlieren technique. The level of dilution was varied from 5 to 15% at equivalence ratio (ϕ) ranging from 0.8 to 1.5. CO2 has a larger impact on flame speed reduction than N2, where the peak flame speed at ϕ = 1.1 dropped from 0.58 m/s to 0.44 m/s and then to 0.24 m/s as the CO2 dilution increased from 0% to 15%. Markstein length was found to increase with dilution in the premixed flames which could be attributed to the decrease in the probability of hydrodynamic instability owing to increasing flame thickness especially for rich conditions. Numerical simulations using two detailed kinetic mechanisms showed good agreement with the experimental measurements. Comparison between the kinetic effects and physical dilution effects of CO2 on flame speed showed that the latter dominated overall, with the relative contribution of kinetic effects being highest for Φ = 1.1. Reverse CO + OH ⇔ CO2 + H was identified as the key reaction directly affected by CO2 dilution leading to CO2 dissociation and H consumption. The competition for H radical with primary chain branching/propagating reactions hindered the MTHF/air combustion in CO2 diluted flames.

Using high-speed schlieren and shadow photography, the instabilities of hydrogen-air outwardly propagating spherical laminar premixed flame were studied in constant volume combustion bomb, at different fuel equivalence ratio (0.3 ~ 3.0), temperature (300K ~ 400K), pressure (1 ~ 3bar). The results show that: the flame instabilities decrease with increasing fuel-air equivalence ratio, increase with increasing pressure and have no significant change with temperature.

The study of pedestrian simulation is one of the remarkable topics in the current traffic and simulation field. The meanings and contents of the study were stated in the beginning. A brief history of the study was discussed. In the next, the mechanism and applicable conditions of five typical microscopic pedestrian simulation models, including benefit cost cellular model, cellular automata model, magnetic force model, social force model and queuing network model, were analyzed besides a comparison. Furthermore, five pieces of typical pedestrian simulation software were proposed. Finally, the problems and the future tendencies of pedestrian simulation were discussed.