Experimental and numerical analysis for high intensity swirl based ultra-low emission flameless combustor operating with liquid fuels

Abstract

Flameless combustion offers many advantages over conventional combustion, particularly uniform temperature distribution and lower emissions. In this paper, a new strategy is proposed and adopted to scale up a burner operating in flameless combustion mode from a heat release density of 5.4–21MW/m3 (thermal input 21.5–84.7kW) with kerosene fuel. A swirl flow based configuration was adopted for air injection and pressure swirl type nozzle with an SMD 35–37μm was used to inject the fuel. Initially, flameless combustion was stabilized for a thermal input of 21.5kW (Q̇‴=5.37MW/m3). Attempts were made to scale this combustor to higher intensities i.e. 10.2, 16.3 and 21.1MW/m3. However, an increase in fuel flow rate led to incomplete combustion and accumulation of unburned fuel in the combustor. Two major difficulties were identified as possible reasons for unsustainable flameless combustion at the higher intensities. (i) A constant spray cone angle and SMD increases the droplet number density. (ii) Reactants dilution ratio (Rdil) decreased with increased thermal input. To solve these issues, a modified combustor configuration, aided by numerical computations was adopted, providing a chamfer near the outlet to increase the Rdil. Detailed experimental investigations showed that flameless combustion mode was achieved at high intensities with an evenly distributed reaction zone and temperature in the combustor at all heat intensities. The emissions of CO, NOx and HC for all heat intensities (Ф=1–0.6) varied between 11–41, 6–19 and 0–9ppm, respectively. These emissions are well within the range of emissions from other flameless combustion systems reported in the literature. The acoustic emission levels were also observed to be reduced by 8–9dB at all conditions.