Numerical and theoretical investigation of ethanol/air flame instability

Abstract

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.

Keywords:

• ethanol
• detailed numerical simulation
• hydrodynamic instability
• thermal-diffusion instability
• high pressure

Disclosure statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been corrected with a minor change. This change does not impact the academic content of the article.

Additional information

Funding

This work was supported by National Natural Science Foundation of China No. 51876139 and 51506146. All simulations are performed on TianHe-HPC1 system at National Supercomputer Center in Tianjin, China and the HPC system at State Key Laboratory of Engines, Tianjin University.