K.A. Watson

Scalar and velocity field measurements in a lifted CH4–air diffusion flame

Experiments have been performed to investigate the leading edge of a lifted jet diffusion flame. The first portion of this study is a simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) investigation of a lifted methane flame. The simultaneous technique is an approach for establishing the 2-D velocity field in conjunction with the flame front location indicated by laser-induced fluorescence from CH radicals within the reaction zone. The results show that the lifted flame stabilizes in a region of relatively low incoming gas velocity. Furthermore, the radial movement of large-scale vortices appears to play a crucial role in local flame extinction. The second set of experiments involves a simultaneous CH and OH PLIF investigation of the same lifted flame. The relative positions of the two radical fields have remarkable agreement. The CH profile is indicative of the fuel-rich region of the reaction zone and closely follows the inner edge of the OH profile. Furthermore, the OH zone is more than three times as thick as the CH zone, and the structures in both images support the radial motion of vortices established by the joint PIV/CH-PLIF measurements.

Simultaneous Rayleigh imaging and CH-PLIF measurements in a lifted jet diffusion flame

Abstract Simultaneous Rayleigh scattering and CH planar laser-induced fluorescence (PLIF) measurements near the stabilization region of a lifted methane–air diffusion flame are presented. The goals of this investigation are to establish flow patterns responsible for complete breaks in the CH profile that indicate local flame extinction and evaluate the stabilization mechanisms over a range of flow conditions. Considerable attention has been given to vortex–flame interactions as a primary extinction mechanism of turbulent diffusion flames. The existence of holes in the flame zone is thought to result from the radial penetration of the flame by vortices from the internal fuel jet. In this investigation, Rayleigh scattering is used as a qualitative indication of gas temperature, thereby providing valuable information about the fluid near regions of local extinction, as indicated by well-defined breaks in the CH layer. The extent of premixedness in the region upstream from the CH structure is also assessed from the Rayleigh signal level. Furthermore, the roles of premixedness in flame stabilization, the nature of the leading edge, and lift-off height oscillation are discussed.

Observations on the leading edge in lifted flame stabilization

The objective of this paper is to report some of the first experimental evidence for the “leading edge” flame as the stabilization mechanism in lifted jet diffusion flames [1–5]. CH fluorescence has been used to indicate the flame front location (i.e., region of chemical reaction) and thereby characterize features of the stabilization region [5, 6]. The “leading edge” flame phenomenon reported within refers to the outward-extending branch of CH fluorescence at the base of the streamwise CH zones. Whether the “leading edge” flame is a special case of the more general triple flame is a question which remains unanswered. It is evident from previous computational studies [7, 8] that the triple flame, when interacting with a vortex or pair of vortices, can take on characteristics of the “leading edge” flames introduced in the present study. Veynante et al. [8] illustrate the contortion of the premixed branches of the triple flame by the flowfield where the premixed branches are swept into the trailing diffusion flame. These simulated triple flame/vortex interactions are consistent with the results of this study which show a trailing diffusion flame and the leading edge reaction zone structure.The test conditions and measurement locations for this investigation are shown in Fig. 1. nDownload high-res image (204KB) nDownload full-size image n n n nFig. 1. Test conditions and measurement locations for (a) the lowest flow rate and (b) the highest flow rate. The intermediate flow rate (not shown) corresponds to a methane velocity of 21.2 m/s while the bottom of the image region is 37.4 mm from the jet exit and includes both sides of the lifted flame. n nThe axisymmetric burner consists of a 5-mm inner diameter fuel jet surrounded by a 150-mm i.d. coflow tube. Methane is delivered through the fuel jet, while low-speed air (∼0.15 m/s) passes through the coflow annulus. The stabilization regions of three lifted flames are investigated by varying the methane and air flow rates and adjusting the burner position accordingly so that the image region includes the leading edge of the reaction zone. The methane exit velocities are 15.8, 21.2, and 27.5 m/s, corresponding to jet Reynolds numbers of 4800, 6400, and 8300, respectively. Both sides of the lifted flame are imaged during the two lower flow rates (Fig. 1a) while the turbulent fluctuations and wider stabilization region resulting from the highest flow rate limit this case to one side of the flame (Fig. 1b). n nThe CH planar laser-induced fluorescence (CH-PLIF) technique has been described elsewhere [5, 6]. The setup includes a Nd:YAG-pumped dye laser which excites the Q1(7.5) transition of the B2Σ−–X2π(0,0) band of CH at λ = 390 nm. Fluorescence from the A–X(1,1), (0,0) and B-X(0,1) bands between λ = 420 and 440 nm is recorded. This approach has resulted in acceptable CH signal levels and excellent image quality (i.e., spatial resolution and contrast), which the authors find superior to the 431.5 nm laser excitation employed in earlier studies [9, 10]. The authors believe the excellent resolution resulting from the signal levels and laser sheet characteristics in this study are extremely important in uncovering the leading edge premixed branch, which is generally weaker in signal level than the trailing diffusion flame. It is likely that these stabilizing leading edge flame observations are not reported in studies with less spatial resolution or are possibly not at all detectable due to limitations in the specific CH excitation/detection scheme. n nSeveral diagnostic studies involving lifted flames present the lifted flame structure as a continuous flame surface, similar to a distorted cylindrical object, emanating from a ring-shaped structure where the flame is stabilized [4, 5, 11]. Most previous work, however, does not give experimental evidence of the mechanism of lifted flame stabilization. Figure 2 nDownload high-res image (3MB) nDownload full-size image n n n nFig. 2. CH-PLIF images illustrating the leading edge phenomenon where the CH zone extends outward at the stabilization point. (a)–(c) are from the lowest Re = 4800 flow condition (Fig. 1a); (d)–(h) are from the intermediate Re = 6400 flow condition; (i)–(l) are from the highest Re = 8300 flow condition and only include the right side of the flame (Fig. 1b). n nconsists of several instantaneous 35.1 mm × 23.4 mm CH-PLIF images which provide such evidence. The images clearly show a continuous vertical distribution of CH which represents the primary diffusion flame reported in many previous studies. In addition to the vertical trailing diffusion flame, a structure is witnessed near the flame base which curls toward the outside, or fuel-lean, portion of the reaction zone. In comparison to ideal, laminar tribrachial structures, evidence of both rich and lean branches of premixed flame is not present, only the one branch extending outward near the jet edge. However, the rich branch on the fuel side of the diffusion flame may be overlapped into the diffusion flame by the flowfield as illustrated by Veynante et al. [8]. It is believed that the branch in the CH zone is a leading edge flame, stabilized by opposing the flow in the relatively low-speed region (∼1.0 m/s) near the outside edge of the jet. n nThis branch of CH is not obviously present in all of the data; it only appears in approximately 30% of the images. The authors reason that the leading edge phenomenon may not be present around all 360 degrees of the stabilizing “ring,” but only in a portion of the flame sufficient to generate enough thermal energy to stabilize the flame globally. The reader must keep in mind that while the laser imaging techniques allow one to investigate the flame in detail locally, global behavior, most notably out-of-sheet activity, may be dominant at any given instant. With the flow inherently three-dimensional and time-dependent [11, 12], sheet imaging techniques often provide data to support a theory, but rarely provide definitive evidence. Since the measurements only investigate the flowfield in one plane, it is feasible that the leading edge structure could be present outside the measurement slice during the instances when no premixed CH branch is witnessed. In addition, recent cross-sectional images of lifted flames near the stabilization zone clearly render three-dimensional lobed structures that are consistent with this theory [11, 12]. n nThe extent of mixing and the entrainment of ambient air into the fuel is of central importance to this problem. Based on comparisons with mixture fraction images presented by Starner et al. [13], which illustrate that the portion at the base of a lifted methane flame has a flammable composition, the authors are confident that the leading edge flame lies in a flammable mixture fraction region. Furthermore, fluctuations in the axial location of the leading edge, along with its orientation relative to the trailing diffusion flame, imply axial propagation into the unburned gas region. These observations imply that the physics of flame stabilization is likely a combination of multiple mechanisms based on premixedness, strain rate considerations [6, 14], and flame propagation into nonhomogeneous flowfields with flow separation, scalar gradients, and a range of mixture fractions [13].

Simultaneous two-shot CH planar laser-induced fluorescence and particle image velocimetry measurements in lifted CH4/air diffusion flames

Joint two-shot CH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements near the stabilization region of lifted methane/air diffusion flames stabilized under different flow conditions are presented. The simultaneous technique allows for a determination of the propagation rate of the CH zone relative to the fuel flow. Simultaneous single-shot CH-PLIF and PIV techniques have been used in the past to examine lifted jet flames: however, the double-shot technique of the current study is desirable because it yields information on flame dynamics—as indicated by sequential CH-PLIF—relative to the unburned mixture. Three flow conditions were examined corresponding to three different liftoff heights. While the average velocity at the stabilization point varies between 0.83 m/s for the lowest flow condition (Red=4800) and 1.28 m/s for the highest (Red=8300), the velocity at the stabilization point during instances of zero CH movement (during the time interval of the CH pulses) is constant for all three flow conditions (1.14±0.4 m/s). Furthermore, the flame is able to stabilize itself against the incoming unburned mixture only when the gas velocity is below a certain limit, above which the flame is convected downstream with the flow.

On scalar dissipation and partially premixed flame propagation

Measurements of the scalar dissipation rate in the region immediately upstream of a lifted jet flame are presented. The scalar dissipation is determined in this isothermal region from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering. Fields of the scalar dissipation rate are presented in addition to tabulated values for three different liftoff heights ( Re d =4800, 6400, and 8300). Scalar dissipation rates do not reach levels thought to cause extinction of the leading edge based on comparison with extinction data for counterflow diffusion flames. Additionally, results are presented on the axial flame propagation velocities relative to the jet flow. The data indicate that over the three flow conditions, the flame velocity relative to the flow is approximately constant during the case of a quasi-stationary lifted flame. In light of these findings, it is suggested that concepts involving partially premixed flame propagation, rather than those of critical scalar dissipation rate, are central to modern lifted flame stabilization models.

Upstream Islands of Flame in Lifted-Jet Partially Premixed Combustion

Abstract Contemporary interest exists in understanding the roles of leading edge flow deflection, secondary jet instabilities and islands of ignited gases in permitting lifted flames to stabilize. To assess these issues, elements of the leading-edge of a lifted turbulent jet flame have been investigated using laser-imaging techniques. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. In particular, evidence of flame islands, or flame fragments, upstream of the bulk-flame leading edge are reported and discussed. This evidence is presented in the form of sequential CH-PLIF images and well as CH-PLIF/Rayleigh scattering images. Images showing thermal characteristics of the regions surrounding the edge flame are also described.

Partially Premixed Combustion in Lifted Turbulent Jets

This article reports observations of structures consistent with triple flame reaction zones in the stabilization region of turbulent nonpremixed jet flames. Previous studies of laminar mixing layers and nonpremixed jets show Hi brachial structures, however reports of triple flame structures in turbulent flowfields are sparse, The asymmetric coflow, which facilitates visualizing the luminous flame structure, is described and the observed double and triple flame structures are discussed. Flame luminosity data is presented and the relevance to CH planar laser-induced fluorescence (PLIF) studies of leading edge flame structures is discussed. Furthermore, connection is drawn to select theoretical studies of triple flames and partially premixed combustion.

Visualization of Multiple Scalar and Velocity Fields in a Lifted Jet Flame

The stabilization of lifted jet diffusion flames has long been a topic of interest to combustion researchers. The flame and flow morphology, the role of partial premixing, and the effects of large scale structures on the flame can be visualized through advanced optical imaging techniques. Many of the current explanations for flame stabilization can benefit from the flow and flame information provided by laser diagnostics. Additionally, the images acquired from laser diagnostic experiments reveal features invisible to the eye and line-of-sight techniques, thereby allowing a deeper insight into flame stabilization. This paper reports visualizations of flame and flow structures from Particle Image Velocimetry (PIV), Planar Laser-Induced Fluorescence (PLIF) and Rayleigh scattering. The techniques are surveyed and the success of visualization techniques in clarifying and furthering the understanding of lifted-jet flame stabilization is discussed.