Critical condition for the ignition of reactant mixture by radical deposition

Abstract

Motivated by recent interests in plasma assisted combustion (PAC), theoretical investigation on the ignition of pre-mixture by radical deposition is performed. Thermally sensitive intermediate kinetics are considered and the governing equations for temperature and mass fractions of fuel and radical are solved analytically. The correlation depicting the evolution of flame kernel resulted from radical and/or heat deposition is derived. Based on this correlation, we study the flame bifurcation and critical condition for the ignition caused by radical as well as heat deposition. The emphasis is placed on investigating the effects of fuel and radical Lewis numbers. For ignition by radical deposition only, it is demonstrated that the fuel Lewis number has a pronounced influence on the flame propagation and flame bifurcation. With the increase of the fuel Lewis number, new flame branches and bifurcations are observed. It is found that there are two regimes in the change of the minimum chemical ignition power with the fuel Lewis number. Unlike the fuel Lewis number, the radical Lewis number only has a quantitative influence on the flame bifurcation and critical ignition condition. For ignition by radical and thermal deposition, the flame trajectory and flame bifurcation are found to be strongly affected by additional heat deposition. The minimum chemical and thermal ignition powers are compared, and the ignition efficiency of pure chemical ignition and pure thermal ignition is shown to strongly depend on the fuel and radical Lewis numbers. Moreover, the validity of theoretical results describing the effects of fuel and radical Lewis numbers on the minimum ignition energy is confirmed qualitatively by transient numerical simulations including thermal expansion and detailed chemistry.