Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure
Introduction
Flame stabilization is a phenomenon to be controlled in numerous industrial applications (furnace, boiler, rocket engine, turbine reactor, etc.). It requires the suitable mixing of fuel, oxidizer, and released heat. In industrial applications the flame is usually anchored at the nozzle to avoid flame instability (leading to pressure variation), extinction (leading to unburned hydrocarbon emission, etc.) and blowout for safety reasons. Partially premixed combustion is involved in more and more industrial applications to reduce NOx emission and fuel consumption. This mode of combustion exhibits particular properties and needs to be better understood. From a fundamental point of view, the turbulent lifted flame is an interesting configuration with which to investigate the properties of partially premixed combustion, as this phenomenon results from a balance of the effects of combustion and turbulence involving particular interactions between aerodynamics and mass and heat transfers. Upstream of the flame base the jet fluid mixes with the ambient fluid, leading to partial premixing within the shear layer where the flame stabilizes. In this region the flame edge is submitted to high shear and mixture fraction gradients, leading to a complex phenomenon.
In this context, numerous theories have been proposed in the past, as reviewed by Pitts [1]; they fall into two categories. One approach is based on turbulent flame propagation [2], [3], [4], [5] and the other on extinction, considering either non-premixed flamelets [6], [7] or the role played by large-scale structures [8], [9]. More recently partially premixed propagation has been introduced in the description of turbulent lifted flame discussed in Peters' review [10].
The stabilization of turbulent flames on jets is controlled by specific combustion properties at the flame base induced by the local mixture fraction gradients and their superimposition on local turbulence upstream of the reaction zone. In laminar flows, the flame base develops into a triple flame structure (fuel-lean and fuel-rich premixed branches and a trailing diffusion flame) across a mixture fraction gradient, and flame stabilization is controlled by the combined effects of mixture fraction gradient and aerodynamical flow properties, which influence the heat release and mass fluxes at the flame base [11], [12]. In turbulent flows, these interactions and their consequences on flame behavior become very complex, and the triple flame becomes distorted, with branches overlapping into the trailing diffusion flame [13]. Numerous studies are still required to better understand the interaction between the flame edge and the turbulent flow, and to provide validation tools for numerical modeling. Recent experimental studies show that in turbulent flows the mixture at the flame base is within the flammable limits [14], [15], [16] and the velocity field in the stabilization region is slow enough to allow propagation of a leading-edge flame [17], [18]. This was confirmed by the experimental observation of a leading-edge flame in turbulent jet flames [19]. Nevertheless the properties of the flame edge in turbulent fields are not well characterized and further investigations are required.
The aim of the present study is to provide a large database of flame base locations (liftoff height and radius) for turbulence-free jets over their stability domain for different fuels (methane, propane, ethylene, and air-diluted ethylene) and different nozzle diameters (from 2 to 5 mm i.d.). The axial and radial locations of the flame base are measured by OH planar laser-induced fluorescence (PLIF). Blowout velocities are measured and their values are analyzed to determine whether the process can be described by a simplified model based on the large-scale structures of the jet. The locations of the flame base near the blowout condition are compared with those derived from the usual assumptions involved in the model proposed to describe the blowout condition [4], [8]. Axial and radial locations of the flame base are measured and statistically analyzed over the stability diagrams of the flames investigated. The large database of flame base locations (probability density function (pdf), average, standard mean deviation) is examined and exhibits a correlation between flame base behavior and the region of the jet where flame stabilizes. These results show the role played by the large-scale structures of the jet in the fluctuations of the flame base. This role is confirmed by comparing the amplitude of the flame base fluctuations with the local size of the large scales deduced from PIV measurements.
Despite the large database of flame base locations, physical phenomena governing the stabilization of turbulent lifted flames are not yet fully understood and there are still several directions for future investigation.
Section snippets
Apparatus
The burner consists of a stainless-steel tube with an inside diameter, D, of 2, 3, 4, or 5 mm. It is now well established that the basic jet velocity profile is a determining parameter of the instability that controls the development of the turbulence within the jet (Fig. 1). The basic velocity profile depends on the experimental conditions (nozzle contraction, Reynolds number, roughness, jet exit configuration). To obtain a flat velocity profile and a thin basic mixing layer from the
“Blowout condition”
For all fuels and nozzle diameters, when possible, lifted flames are investigated over a wide range of initial velocities, U0, from velocities below the “liftoff velocity” (flame in the hysteresis region) to velocities leading to flame blowout. Some experimental limitations have been encountered for the highest velocities when the flame dimensions become too large for the dimensions of the experiment. The experimental determination of “blowout velocity” is delicate, because at this stage the
Conclusions and summary
A parametric investigation of turbulent lifted flames has been carried out for three fuels and various nozzle diameters. Blowout velocities are determined and experimental data are compared with an approach proposed by Broadwell et al. [8] based on the role of inviscid large-scale structures of turbulent jets. From this approach it is possible to propose an expression of blowout velocity without a detailed description of the blowout phenomenon. Measurements of flame base location near blowout
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