No abstract

Ignition delay times of n-nonane and n-undecane in 4% oxygen/argon have been measured behind reflected shock waves in a heated shock tube at temperatures of 1168−1600 K, pressures of 2, 10, and 20 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Ignition delay times are determined by using CH* emission and pressure signals monitored at the sidewall. Results show that ignition delay times of two fuels decrease as the temperature or pressure increases, and a decrease in equivalence ratio results in a shorter ignition delay time. For fuel-lean and stoichiometric mixtures, n-nonane has ∼25%−35% longer ignition delay times than n-undecane. For fuel-rich mixtures, ignition delay times of two fuels are very close. Correlations for ignition delay times of two fuels as a function of temperature, pressure, and equivalence ratio are formulated through regression analysis. The experimental data are in good agreement with shock tube data available, and the trends of experimental data were captured well by the predictions from the LLNL and JetSurF mechanisms under conditions studied. Comparison of ignition delay times for nine n-alkanes from propane to n-undecane reveals that the n-alkanes have the similar ignition delay behavior and their ignition delay times are close to each other. Reaction path analyses and sensitivity analyses are performed to investigate the consumption of fuels and identify the important reactions in the ignition process. To our knowledge, we provide the first ignition delay time data for n-undecane at elevated pressures, and our measurements for n-nonane are at a broader range of conditions than previous studies. Current results contribute toward understanding the ignition characteristics of n-nonane and n-undecane, and they provide validation targets for corresponding kinetic mechanisms.

In this paper, the propagation characteristics of a ring Airy vortex beam (RAVB) in slant atmospheric turbulence are studied. The effects of some key parameters, such as the truncation factor, the zenith angle, and the propagation distance, on the propagation characteristics of RAVB are analyzed. The results show that the beam quality becomes worse with an increase in the propagation distance, zenith angle, and truncation factor. When the propagation distance exceeds 3 km in turbulence, the value of the aperture-averaged scintillation index will reach the peak value. On the other hand, the mean signal-to-noise ratio of this beam will be affected by the propagation distance. The beam quality can be improved by increasing the topological charges. When the truncation factor is 0.1, RAVB performs best when the distance is long.

For a tightly focusing imaging system, the aberration of the lens will result in a distorted focusing spot and undermine the system performance. In this paper, the expression of the tightly focused light field of Laguerre-Gaussian vector vortex beam (LGVVB) is deduced. Then the intensity distribution, focal shift and spot size that influenced by spherical aberration are calculated in detail. Since the vector vortex beams have multiple degrees of freedom to control the form of the intensity distribution on the focal plane, the effects of aberration on the focal spot are analyzed when some key parameters are changed. The results show the Pancharatnam topological charge is the most influential parameter to change the light distribution on the focal plane. According to the change rule, a scheme is proposed to minimize the effect of aberration by jointly adjusting the parameters of the LGVVB.