The effect of chamber cross section variation on a highly diluted and preheated combustion chamber characteristic

Abstract

Increased temperature homogeneity, reduced pollutant emissions, and combustion stability are key features expected from industrial furnaces employing flameless combustion. This study systematically examines the effects of furnace geometry, specifically convergent versus divergent axial cross-sectional profiles, and wall temperature on flameless combustion characteristics within a laboratory-scale fixed-volume chamber. Utilizing two-dimensional axisymmetric simulations with detailed chemical kinetics coupled to the (EDC) turbulence–chemistry interaction model, the investigation explores variations in height-to-average diameter ratios ($H/D_{\mathrm{ave}}$), through comprehensive parametric analysis supported by model validation and grid independence tests. Findings indicate that divergent furnace geometries promote enhanced internal gas recirculation and flame stability at lower $H/D_{\mathrm{ave}}$, whereas convergent geometries yield superior performance at higher ratios. The stable reaction zone volume is strongly influenced by wall temperature and geometry. Reaction zone volume depends on wall temperature, furnace geometry, and $H/D_{\mathrm{ave}}$. In divergent geometry at 1200 K with $H/D_{\mathrm{ave}}=2.6$, the reaction zone occupies over 80 % of the furnace volume. Increasing the $H/D_{\mathrm{ave}}$ to 5 at the same wall temperature reduces the reaction zone volume to less than 10 %. As wall temperature increases, the contribution of radiant heat transfer becomes more significant. Specifically, divergent geometries with low $H/D_{\mathrm{ave}}$ ratios and convergent geometries with high $H/D_{\mathrm{ave}}$ ratios exhibit greater radiative heat transfer contributions. Carbon monoxide emissions are influenced by the ignition delay and the rate of CO₂ decomposition. At wall temperatures around 1200 K, carbon monoxide levels are more affected by ignition delay, whereas at temperatures exceeding 1400 K, CO emissions are predominantly governed by the rate of CO₂ breakdown.