A series of two dimensional (2D) detailed numerical simulations of premixed cylindrical expanding ethanol/air flames in a constant volume encloser under elevated pressures and equivalence ratios are performed to investigate cellular flame instability. The results demonstrate that at pressure of 10 atm and temperature of 358 K, ethanol/air flame cellular instability increases non-monotonically with equivalence ratio (ϕ) from 0.8 to 1.6 and has the most intense instability at ϕ = 1.2. The trend is similar in both theory and simulation, while the latter overpredicts the critical flame radius compared with the theory. At the equivalence ratio of 1.2, the flame instability increases monotonically with pressure from 2 to 20 atm. The 2D simulation results are theoretically analysed by Peclet number, logarithmic growth rate of disturbance, flame thickness and critical flame radius for the onset of flame instability. It is found that hydrodynamic (DL) instability is insensitive to ϕ. In contrast, thermal-diffusion (TD) instability is overwhelming and changes dramatically with increasing ϕ mainly due to molecular diffusion. Therefore, the destabilising effect of logarithmic growth rate on the ethanol/air flame surface with increasing ϕ is actually due to the weakening stable effect of TD instability. The almost constant critical Peclet number and drastically reduced flame thickness lead to a great decrease in the critical flame radius with pressure rising both in theory and 2D simulations. It is concluded that the onset of instability advances with higher pressure and the acceptable quantitative comparison in terms of critical flame radius was observed between simulation and theory.

Whereas discharge characteristics of a gliding arc have previously been investigated in a low-speed subsonic flow without considering the effect of shock waves and extremely high turbulence, we here studied an important physical feature of the gliding arc in a supersonic flow that generates complex shock waves and extremely high turbulence. The gliding arc in a 1.2 Mach supersonic jet air flow is investigated by high-speed imaging, schlieren imaging, electrical measurements, and the large eddy simulation. The experimental results show that high-repetition transitions between the spark-type discharges and the glow-type discharge with a frequency of 1.4–2.1 MHz can be observed in the supersonic flow after the gliding arc is generated between the narrowest gap of knife-shaped electrodes, whereas the similar phenomena are invisible in a low-speed subsonic flow. It can be recognized that the high-repetition transitions occur in the region with complex structures of shock waves and expansion waves. The large velocity/density gradient generated by the complex wave structures and the small-scale vortices produced by the strong turbulence is located near the position where the high-repetition transitions occur. The high-repetition transitions between the spark-type discharges and the glow-type discharges are found to be mainly caused by the combined effects of the strong turbulent eddy transportation in the supersonic flow and the drastic change in the flow properties across the shock wave, shedding light on the mechanism of the high-repetition discharge transitions of the gliding arc in a supersonic flow.

As a kind of non-Newtonian fluid with special rheological features, the study of the breakup of power-law liquid jets has drawn more interest due to its extensive engineering applications. This paper investigated the effect of gas media confinement and asymmetry on the instability of power-law plane jets by linear instability analysis. The gas asymmetric conditions mainly result from unequal gas media thickness and aerodynamic forces on both sides of a liquid jet. The results show a limited gas space will strengthen the interaction between gas and liquid and destabilize the power-law liquid jet. Power-law fluid is easier to disintegrate into droplets in asymmetric gas medium than that in the symmetric case. The aerodynamic asymmetry destabilizes para-sinuous mode, whereas stabilizes para-varicose mode. For a large Weber number, the aerodynamic asymmetry plays a more significant role on jet instability compared with boundary asymmetry. The para-sinuous mode is always responsible for the jet breakup in the asymmetric gas media. With a larger gas density or higher liquid velocity, the aerodynamic asymmetry could dramatically promote liquid disintegration. Finally, the influence of two asymmetry distributions on the unstable range was analyzed and the critical curves were obtained to distinguish unstable regimes and stable regimes.