Shock Wave Propagation and Flame Kernel Morphology in Laser-Induced Plasma Ignition of CH4/O2/N2 Mixture

Abstract

The application of laser ignition in the aerospace field has promising prospects. Based on the constant volume combustion chamber, the laser ignition of CH4/O2/N2 mixture with different initial pressure, different laser energy, different equivalence ratio and different oxygen content has been carried out. The development characteristics of the flame kernel and shock wave under different conditions are analyzed. In addition, the Taylor model and Jones model are also used to simulate the development process of the shock wave, and a new modified model is proposed based on the Jones model. The experimental results show that under pure oxygen conditions, the chemical reaction rate of the mixture is too fast, which makes it difficult for the flame kernel to form the ring and third-lobe structure. However, the ring structure is easier to form with the pressure and laser energy degraded; the flame kernel morphology is easier to maintain at a rich equivalence ratio, which is caused by the influence of the movement of hot air flow and a clearer boundary between the ring and the third-lobe. The decrease of the initial pressure or the increase of the laser energy leads to the increase in shock wave velocity, while the change of the equivalence ratio and oxygen content has less influence on the shock wave.

Geometry of the energy input of a shockwave generated by a nanosecond laser-induced breakdown

“…Even though the produced experimental data [18,20] show a reasonable agreement with Jones modelling, a slight over-estimation of the real radius r (t) at the first time of experiments in all tests and a slight under-estimation at the last times are noticeable. These deviations have also been reported in [23] and have been overcome by adjusting parameters a and e in equation (1). However, this adjustment is not consistent with the Jones modelling in terms of both strong shock hypothesis and acoustical limits [24] obtain results very faithful to experiments by determining these same parameters for spherical shockwaves in air at atmospheric pressure, using a modified Jones model to remove the assumption of a strong shock at t = 0, which is unrealistic in the laser breakdown domain.…”

“…However, as mentioned in [25], even if the plasma has an elongated form in the air, the shockwave becomes nearly spherical during the course of time. Consequently, depending on the observation time, the used laser energies and pressures, the radius shockwave r (t), which is used in the dimensionless term x in equation (1), has been differently defined from work to work [18,[20][21][22][23]. In [18], the shockwave shape is considered spherical, and the corresponding radius is deduced with a cross-correlation between experimental images and synthetic schlieren.…”

“…In [22], the radial component of the shockwave (r b ) is extracted using Rayleigh scattering. Lastly [23], separately investigated lengths equivalent to r a and r b .…”

On the ignition kernel formation and propagation: an experimental and modeling approach

“…The use of a laser-based ignition could provide a significant benefit from the removal of electrode interference on the ignition kernel. However, it produces a highly wrinkled early kernel and induces gas motion in the direction of the beam [18,19]. The induced gas motion is not ideal because of the potentially non-symmetric shapes and complex geometry, which makes fundamental flame studies challenging.…”