Importance of the inlet air velocity on the establishment of flameless combustion in a laboratory combustor

Abstract

This article examines the importance of the inlet air velocity (V_air) on the establishment of flameless combustion in a 10 kW laboratory scale combustor. Variations in V_air were accomplished by changing the air nozzle diameter while maintaining constant all remaining input parameters. Initially, laser-Doppler anemometry was employed to evaluate the combustor flow aerodynamics under non-reacting conditions. Subsequently, flue-gas composition data and hydroxyl radical chemiluminescence (OH^*) imaging were obtained as a function of V_air. For two of these combustor operating conditions, spatial distributions of temperature, recorded with fine wire thermocouples, and of O₂, CO₂, unburned hydrocarbons, CO and NO_x concentrations, measured with the aid of a sampling probe, were also obtained. The OH^* images showed that as V_air increases at a constant excess air coefficient (λ) of 1.3, the main reaction zone, typical of flameless combustion condition, remains approximately in the same region of the combustor, because of the flow aerodynamics similarity, but the OH^* intensities decrease, which indicates higher entrainment ratios of the fuel and burned gases by the central air jet. For λ greater than 1.7, however, flameless oxidation could not be established regardless of the air jet momentum. This suggests that the establishment of the flameless combustion condition in future gas turbines through the dilution of the reactants with a substantial amount of flue gases in configurations where the combustion air is provided by a central high-momentum air jet that is surrounded by a number of low-momentum fuel jets may be problematic.

Introduction

Under flameless oxidation conditions combustion takes place in a distributed reaction zone rather than in a thin flame front, with relatively uniform and low temperatures and temperature fluctuations, in comparison to conventional flames. The fuel is oxidized in an environment that contains a significant amount of flue gases and a low concentration of oxygen, as a result of internal or external exhaust gas recirculation. The radiative heat fluxes are relatively high and uniform, there is no visible flame, the level of noise is low, soot formation is negligible and NO_x and CO emissions are very low.

Flameless combustion is a technology relatively well established for industrial applications where heat is extracted from within the furnace while processing material, and is among the most promising technologies that can meet the stringent demands of reduced pollution and increased efficiency in future gas turbines. Gas turbine would operate lean adiabatic combustion (high oxygen content) while furnaces feature higher equivalence ratio (low oxygen content) [1]. This may pose difficulties in establishing the flameless combustion regime in gas turbines. In this case, there is a need to recirculate, by aerodynamic means, a large amount of hot combustion products, with a relatively high oxygen content, which can make difficult to establish the proper reactants dilution for the onset of the flameless combustion condition. For instance, Li et al. [2] examine this combustion mode in a gas turbine combustor operating at atmospheric conditions. They found that the flameless combustion mode occurred only for a limited range of conditions at fuel lean conditions, high preheat temperature and high air flow rates.

Very recently [3], we examined the operational, combustion and emission characteristics of a small-scale combustor as a function of the excess air coefficient (λ), which implied also changes in the inlet air velocity (V_air). We observed that for λ below 1.5 it was possible to establish the flameless combustion regime, while for higher λ the fuel oxidation occurred in a conventional lean combustion mode. Due to the burner configuration, it was not possible to analyze independently the importance of λ and V_air on the combustion regime. In the present article we examine separately the effect of V_air on the establishment of flameless combustion.

Review articles on flameless combustion, also called flameless oxidation, moderate or intense low oxygen dilution (MILD) combustion, high temperature air combustion or colorless distributed combustion, include those of Wünning and Wünning [4], Cavaliere and de Joannon [5] and Tsuji et al. [6]. The effect of the initial air–fuel jet momentum on the establishment of flameless combustion has been studied by a number of investigators [7], [8], [9], [10], [11], [12], [13]. Szegö et al. [10] found that a certain fuel jet momentum threshold was needed to achieve flameless combustion conditions in a recuperative furnace. This momentum ensured the penetration of the fuel jets to a region classified as the oxidation zone. Also in a recuperative furnace, Mi et al. [11] reported an investigation on the importance of the initial air–fuel injection momentum rate and the air–fuel premixing on flameless combustion. The authors concluded, numerically, that there is a critical momentum rate of the inlet fuel–air mixture below which the flameless combustion cannot occur. Also, they found, both experimentally and numerically, that, above this critical rate, both the momentum rate and the inlet fuel–air mixedness affect only marginally the stability of and emissions from the flameless combustion. In combustors other than recuperative furnaces, Mancini et al. [7] and Derudi et al. [8] also evidenced the threshold below which flameless combustion cannot occur not only for gaseous hydrocarbon fuels but also for highly reactive fuels, as hydrogen-containing fuel mixtures.