POD Scale Analysis of Turbulent Premixed Flame Structure at Elevated Pressures

Turbulent combustion is usually operated at elevated pressures for practical applications to achieve high power density. There will be more wrinkles on the turbulent flame surface with pressure rising, resulting in a higher turbulent burning velocity. Wrinkling process of the turbulent flame at elevated pressure is still not well understood. One reason is that the large and small-scale wrinkles can not be distinguished because of their multi-scale nature. In this paper, the Proper Orthogonal Decomposition (POD) scale analysis is used to describe the multi-scale nature of turbulent premixed flames at elevated pressure. Turbulent premixed CH₄/air flames are conducted in a high-pressure combustion chamber under 0.1 to 0.3 MPa. Turbulence is generated and controlled by multi-grid perforated plates. Turbulent flame front is detected by OH-PLIF technique. Results show that the POD scale analysis can decompose the multi-scale of turbulent flame surface to different modes. Reconstruction of turbulent flame front shows that there is an inverse relationship between POD modes and physical length of scales. Small mode number represents large scale of flame wrinkles, while large mode number stands for small-scale wrinkles. The large and small scale of the flame wrinkles both increase with pressure rising, leading to higher flame surface density and turbulent burning velocity. This indicates that the increased turbulent flame wrinkling at elevated pressure is not only caused by flame instability, which mainly acts on the small wrinkles. Spectral analysis of the turbulent premixed flame shows that the decreased flame thickness, which enlarges the turbulence–flame interaction range, and the decreased laminar burning velocity, which extends the vortex–flame interaction time, are two dominant factors promoting the flame wrinkling at elevated pressure. Turbulent burning velocity correlation shows a linear relationship with the analytical formulation when it takes laminar burning velocity and flame thickness into account.