Investigation on the transition of a propagating planar flame

Abstract

A novel method to find the burning velocity and stretch rate of a premixed flame out of flame surface area has been developed by an image processing technique. The study is tailored for the control of precarious burning during combustion based on an automatic flame detection method. The flame propagation of premixed LPG-Air mixture is carried out in a tube of 30 mm diameter at the equivalence ratio of 1.2. A visible transition of tulip flame is observed towards the finger-shaped flame during initial propagation, which is envisaged to be triggered by the hydrodynamic instability. However, the flame is driven by stabilizing effect of diffusion and stretch. The flame structure is experimentally captured by an image processing method using Python 3.6 software to analyse the burning behaviour. The laminar burning velocity of an unstretched propagating flame has been found in congruence with the non-propagating steady flame at exit. The laminar burning velocity of steady flame is calculated by the volumetric flame formation rate. Based on the study of flame stretch rates, and flame propagation velocities at stationary burned conditions, the burning velocity of axially propagating laminar planar flame is found to be closer agreement with the higher stretched radially propagating spherical flame. The Markstein number is found to be 20.734, which divulges the presence of significant intrinsic diffusional effects in planar flame than the spherical flame during transition. The laminar and turbulent burning velocities are obtained to be accurate, respectively, at different equivalence ratios and turbulent intensities. The transition is also achieved and compared numerically by the Flamelet Generated Manifold method. The laminar burning velocity of LPG flamelet is observed to be precise at 75% propane composition and ϕ≤ 1.09. A higher laminar burning has been ascertained in lean mixtures due to the longer transition time.