Combustion regime transition of H[formula omitted] flames during steady and transient operation of a sequential combustor

Abstract

The combustion regime transition in a sequential burner (SB) supplied with H2 (48 kW) is experimentally studied during steady and transient operation. The test rig is a simplified model of an industrial sequential combustor featuring two-staged combustion chambers separated by a mixing section in which dilution air and fuel are injected. The temperature, velocity and composition of the hot vitiated gas flowing through the SB are defined by the products from the first stage (30 kW natural gas-air flame at equivalence ratio 0.7), and by the mass flow of dilution air m˙DA. To study the combustion regime transition during steady operation of the combustor, m˙DA is fixed at several values between 22 g/s and 7 g/s. For transient operation investigations, m˙DA is suddenly changed between 20 and 7 g/s, which triggers a fast transition of the combustion mode. High-speed hydroxyl radicals OH* chemiluminescence is used to characterize the combustion process, and optical emission spectroscopy (OES) and tunable diode laser absorption spectroscopy (TDLAS) are respectively used to extract mean and time-resolved temperatures of the vitiated gas in the SB. In particular, we investigate the transition from a propagation-driven turbulent flame anchored at the inlet of the sequential combustion chamber, to a flame stabilized by autoignition inside the mixing section of the burner when the dilution air mass flow is suddenly reduced. Zero-dimensional (0D) simulations are used to analyze the underlying combustion regime transition. A 0D reactor network is developed and calibrated with the experimental data. This simplified low-order model predicts well the flame location for both steady and transient operation. Moreover, the good agreement between the numerical results and the experimental data demonstrates that time-resolved TDLAS successfully enables measurement of small temperature variations in the vitiated flow associated with non-perfect mixing of the different streams in the SB.