# Observations on Jet-Flame Blowout

Authors: N. J. Moore; J. L. McCraw; K. M. Lyons
Journal: International Journal of Reacting Systems
 (2008)

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## Abstract

The mechanisms that cause jet-flame blowout, particularly in the presence of air coflow, are not completely understood. This work examines the role of fuel velocity and air coflow in the blowout phenomenon by examining the transient behavior of the reaction zoneat blowout.  The results of video imaging of a lifted methane-air diffusion flame at near blowout conditions are presented.  Two types of experiments are described.  In the first investigation, a flame is established and stabilized at a known, predetermined downstream location with a constant coflow velocity, and then the fuel velocity is subsequently increased to cause blowout.  In the other , an ignition source is used to maintain flame burning near blowout and the subsequent transient behavior to blowout upon removal of the ignition source is characterized.  Data from both types of experiments are collected at various coflow and jet velocities. Images are used to ascertain the changes in the leading edge of the reaction zone prior to flame extinction that help to develop a physically-based model to describe jet-flame blowout. The data report that a consistent predictor of blowout is the prior disappearance of the axially oriented flame branch. This is witnessed despite a turbulent flames' inherent variable behavior.  Interpretations are also made in the light of analytical mixture fraction expressions from the literature that support the notion that flame blowout occurs when the leading edge reaches the vicinity of the lean-limit contour, which coincides approximately with the conditions for loss of the axially oriented flame structure.


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## Body

## 1. Introduction

At a particular fuel velocity, a gaseous hydrocarbon jet-flame will detach from
the burner and stabilize at some axial distance downstream.  The reaction zone consists of a leading
partially-premixed flame front and a trailing diffusion flame created at the
vertically-oriented interface of the residual fuel not consumed by the leading
flame front and air. A diffusion flame
has no burning velocity so it is the premixed flame front that is generally
assumed to act as a stabilizing anchor. Many studies, like that of Muñiz and Mungal [1] and Watson                                                                                                                                                                                                                        et al. [2–4], have
investigated stable lifted flame reaction zone structures that settle at moderate
downstream positions. If the reaction
zone moves further downstream, it eventually enters a region that can no longer
support combustion due to the low fuel concentration and all reaction abruptly
ceases, a condition known as flame blowout (Kalghatgi [5], Pitts [6],
Coats [7], Chao                                                                                                                                                                                                                        et al. [8, 9].  The term                                                                                                                                                                                                                        blowout seems
more physically descriptive than the sometimes used                                                                                                                                                                                                                        blowoff since the
global reaction zone does not seem to blow off the downstream end of the jet,
but rather, to locally cease (Liñán and Williams [10]). Since the blowout phenomenon happens
typically in an abrupt and unpredictable manner, its transient characteristics
are difficult to study experimentally. Additionally, the large width of the
fuel jet, the small gradients in the scalar and velocity fields, and the
relatively low values of fuel concentration make the situation, in many ways, more
challenging to fully characterize than the situations described in the studies
of Watson                                                                                                                                                                                                                        et al. [2–4].Theories
have been developed to determine the mechanism controlling blowout. For a laminar propane jet flame, Savas and
Gollahalli [11] studied the shape of the flame front and found that near
blowout, the flame front became flat (an axially centered disk) and the chemiluminescence
weakened. The blowout conditions were
determined to be dependent on the fuel and oxidizer properties and on the burner geometry.
Chung and Lee [12] showed similar phenomena also in laminar jet flames. For turbulent flames, Broadwell                                                                                                                                                                                                                        et al. [13] proposed that at the
blowout velocity, the combustion ceases because there is not enough time for the
ignition of incoming fuel/air mixtures by entrained hot products. This work and
others (Miake-Lye and Hammer [14]) point to the primary role of large-scale
structures in facilitating hot-product transport. Similarly, Dahm and Dibble [15] applied a
blowout parameter from Broadwell                                                                                                                                                                                                                        et al.
[13] for turbulent jets in coflow and showed that an increased coflow
velocity decreased the jet blowout velocity. The blowout parameter, based on characteristic ignition time and mixing
time ratios, predicts blowout trends correctly. More recently, Han and Mungal
[16] also offered observations on flame blowout, but focused their explanations on the inability of the reaction zone to
counter-propagate against incoming reactants at blowout.  Burgess and Lawn
[17], Brown                                                                                                                                                                                                                        et al. [18], Dahm and
Mayman [19] and Montgomery                                                                                                                                                                                                                        et al.
[20] discussed related elements of flame blowout; a recent overview of this
previous research in blowout is contained in Chao                                                                                                                                                                                                                        et al. [9]. More
recently, Wu                                                                                                                                                                                                                        et al. [21] report on
lifted flames near blowout, with detailed comments on triple flames in the
pulsating region, and describe a proposed mechanism of flame pulsation and
blowout.The
current paper discusses an experimental study of the blowout phenomenon for a
lifted methane-air diffusion flame in various coflow conditions. Rather than focusing on detailed
instantaneous images of reaction zones, as has been our tact in the past, this
effort has utilized time sequences of the reaction zone at blowout. The main
focus is to investigate the transient behavior leading to global blowout. Instantaneous
measurements at blowout prove to be quite difficult with the limitations of
single-shot experimental techniques due to the abrupt onset of blowout. Two
types of experiments are described that attempt to clarify the characteristics
of flames during the blowout process, focusing on the behavior of the leading
edge reaction zone and the trailing diffusion flame at blowout. Sequences of digital images of the lifted
reaction zone are provided along with details of the flame movement for different
combinations of fuel and coflow velocities. Interpretations of the data are discussed, utilizing a relation for the
stoichiometry from Tieszen                                                                                                                                                                                                                        et al.
[22]. This allows for the assessment of past theories and the development of
a physically-based concept of flame blowout in turbulent jets, along with
proposing a new signature which indicates the imminence of flame blowout.


## 2. Experimental Setup

The experiments were performed at the Applied Energy Research Laboratory on the
campus of North Carolina State University. A vertical jet flame burner with a fuel
nozzle of 3.5 mm diameter was used to deliver 99% pure methane. The apparatus provides atop-hat velocity profile at the nozzle's exit. As shown in the schematic in Figure 1, the fuel nozzle is surrounded by
an annulus of coflowing air with a diameter of 150 mm. Care was taken to minimize the effects of
room currents on the flame apparatus by turning off laboratory ventilation
during the recording of data and limiting activity near the burner. The height of the lifted flame,                                                                                                                                                                                                                        h, is the distance from the lowest part
of the flame front to the nozzle.Figure 1
Methane is delivered
from the nozzle that is surrounded by coflowing air. The conditions allow the flame to lift to
some downstream position where there exists the lifted flame front and the
trailing diffusion flame, before proceeding to blowout. The letterh is the
distance from the fuel nozzle to the lifted flame front.For this investigation, images of chemiluminescence (Lyons and Watson [23]) from
the methane jet near blowout conditions were obtained with a Panasonic Model
PV-GS120 camera producing thirty frames per second (60 interlaced fields). The colors of the images were enhanced using
Adobe Photoshop. A rotameter measured
the fuel velocity, and the coflow velocity was measured using a TSI Veloci-calc
model 8345 anemometer. The minimum and
maximum coflow velocities used were 0 m/s and 0.65 m/s, respectively.


## 3. Results and Discussion

Images of the blowout of a methane
flame were obtained at various conditions during two different types of
experiments. In the first series of
experiments, blowout was brought about by a change in the flow conditions of a
stable lifted flame. For a constant coflow
velocity, the initial fuel velocity was set to allow the flame to stabilize at
a lifted height of approximately 14.0 cm (or 40 nozzle diameters) above the
nozzle. The same stable height was used
throughout the first experiments and was chosen because a reaction zone at that
height is stable (i.e., will not spontaneously blowout) and turbulent for each
coflow velocity tested. The fuel
velocity was then increased slightly until blowout occurred. The procedure was repeated multiple times to
determine the lowest jet velocity at which the flame would consistently
blowout. Table1 contains examples of
conditions that were digitally recorded, with the fuel velocities being the
averaged values.Table 1
Flow conditions for flame from (a) stable lifted
positions (b) to blowout.
Coflow velocity (m/s)Lifted 14.0 cm                                                                                                                                                                                                                        (a)Blowout  (b)Fuel velocity (m/s)ReynoldsnumberFuel velocity (m/s)Reynoldsnumber0.046.61011450.4109290.336.9801341.790500.431.6684835.677160.527.0585531.66848The
data of Table1 show that with increasing coflow, decreasing values of the
methane jet velocity are needed for blowout to occur.  Dahm and Dibble [15] proposed that this
reduction in fuel velocity at blowout was due to the local molecular mixing
rate. In experiments by Brown                                                                                                                                                                                                                        et al. [18], a change in the coflow velocity had a greater effect on  a flame, the further downstream the flame was stabilized, resulting in lower jet velocities for flame blowout. Additionally, the findings of Brown                                                                                                                                                                                                                        et
al. [18] confirmed the data of the current study which show that for a
given coflow velocity, the flame can on occasion extinguish at a slightly lower
fuel velocity than the experimentally-determined average blowout velocity. In
this regime (as witnessed in the current study), the coflow so dominates that
it tends to be comparable to the jet velocity and the bulk coflow velocity
carries the reaction zone downstream. As discussed elsewhere, it is proposed
that at this downstream location the flame blows out as the lean limit is reached.Images from these experiments were examined to determine the effect of coflow on the
mechanism of blowout. Figure2 shows two
sequences of images of the flame proceeding to blowout. The images in Figure 2(a) are from a flame with 0.3 m/s
coflow,  corresponding to the data on the second line of Table 1. The sequence begins after the fuel velocity
was increased from 36.9 m/s to 41.7 m/s. The image at time zero is the last one of the flame at the stable lifted
height, immediately after which the flame begins moving downstream. The flame transitions from a stable lifted flame to a quasistable
flame on the threshold of blowout.  During this transition, the length of the diffusion flame decreases as
the leading edge of the flame front drops downstream. The contrast in color for this sequence has
been increased due to the faint chemiluminescence of the actual flame. The blue flame consists of a leading-edge
flame front that anchors the trailing diffusion flame. Blowout occurs 1.50
seconds after the change in fuel velocity causes downstream movement of the
flame.(a) Enhanced images of
the flame receding downstream for 0.3 m/s coflow. The fuel velocity was increased from 36.9 to
41.7 m/s to cause blowout (line 2, Table1). The entire flame is blue in color. (b) Inverted images of the flame after being reignited at time zero,
with 34.3 m/s fuel velocity and 0.55 m/s coflow (line 4, Table 2). The distance,                                                                                                                                                                                                                        h, from the fuel nozzle to the lifted flame front is measured for
each.
(a)(b)In the second series of experiments,
the fuel velocity was set at blowout conditions, predetermined by multiple
tests, for a particular coflow velocity. With the fuel and coflow velocities held constant, the flame was reignited
at the fuel nozzle and allowed to move downstream and eventually
extinguish. Figure2(b) shows one
sequence of images from these experiments with the colors inverted and enhanced
to counteract the faintness of the flame. The larger field of view includes the nozzle but it is not visible in
the enhanced images. For this sequence,
the flame was reignited from the nozzle with 0.55 m/s coflow and 34.3 m/s fuel
velocity. After 3.53 seconds, the
chemiluminescence witnessed from the trailing diffusion flame has been
significantly reduced and the leading-edge of the flame front has moved 19.1 cm
downstream. At 25.1 cm and 3.73 seconds,
the diffusion flame is no longer visible. In the remaining 0.14 seconds until the flame entirely extinguishes, the
flame front moves 6.7 cm. Thus, the
flame moves further downstream much more rapidly in the absence of the trailing
diffusion flame compared to when it is present. The last image in the series shows that complete blowout was achieved
3.87 seconds after reignition.Data from repeated tests for each of the flow conditions revealed no trend in the
amount of time needed from reignition to blowout. However, similar characteristics of the flame
were noticed regardless of the presence or magnitude of the coflow velocity or by
which method blowout was achieved. At
downstream locations, the flame front is witnessed to decrease its recession
speed downstream as the diffusion flame diminishes. After the chemiluminescence from the trailing
diffusion flame is no longer detected, the small region of flame at the leading
edge (ablue flame ball) increases its recession speed downstream until the whole reaction is
completely extinguished. Importantly, as
Figure 2 shows and other experiments verify, blowout is                                                                                                                                                                                                                        not witnessed
while the axially oriented diffusion flame is present.As suggested by Han and Mungal
[16], the velocity of the stoichiometric contour can be estimated and used to
approximate the amount of mixing between the fuel jet and the surrounding
air. This velocity,                                                                                                                                                                                                                        US, can be calculated from(1)US=ZSU0+(1−ZS)UCF, where                                                                                                                                                                                                                        ZS is the stoichiometric
mixture fraction (0.055),                                                                                                                                                                                                                        U0 is the nozzle exit velocity, and                                                                                                                                                                                                                        UCF is the coflow velocity. To test for
blowout dependence on the stoichiometric contour velocity, the coflow and fuel
velocities were varied such that                                                                                                                                                                                                                        US remained the same, beginning with 0.50 m/s coflow and 35 m/s fuel giving                                                                                                                                                                                                                        US = 2.4 m/s, as seen in
Table 2. Despite starting at a blowout
condition and keeping                                                                                                                                                                                                                        US constant, the flame’s behavior was not consistent. At the lowest coflow velocity, the flame
stabilized and was not seen to blowout for an extended amount of time in spite
of being at a higher fuel velocity than a comparable flame in Table 1. Added to the unpredictable nature of blowout,
a difference in experimental procedures cannot be ruled out as a cause of some
discrepancy. Blowout occurred in one
second at the highest coflow velocity. These
variations in behavior predict a lack of dependence of blowout on                                                                                                                                                                                                                        US. As implied in previous studies by Han and
Mungal [16] and Watson                                                                                                                                                                                                                        et al. [3],                                                                                                                                                                                                                        US is a useful
quantity for estimating the axial velocity at the stoichiometric contour of the                                                                                                                                                                                                                        already established portions of the
flame; the leading edge of the flame has been found to favor lower speed
regions (SL to 3 SL) typically less than                                                                                                                                                                                                                        US.Table 2
Fuel velocity,U0,
and coflow velocity, UCF, for a given stoichiometric
contour velocity, US.
Times given are from reignition to blowout based on digital images.
Fuel velocityU0 (m/s)Coflow velocity UCF  (m/s)US (m/s)Time to blowout (s)36.90.42.4No
blowout after 60 s36.00.452.420.5350.52.42.4735.00.552.43.8733.50.62.42.0332.60.652.41.00


## 4. Analysis of the Scalar Field

An analysis of the scalar field for the turbulent
methane jet flame indicates a correlation between the downstream appearance of
the flame and the value of the local mixture fraction. The scalar field of methane issuing into
quiescent air is determined from the method used by Tieszen                                                                                                                                                                                                                        et al. [22]. The time-averaged mass fraction of fuel,                                                                                                                                                                                                                        Y, into air with no coflow present is
represented as(2)Y=10⋅(ρ0ρ∞)1/2(r0z)exp{−57(rz)2}. This equation, a
function of the ratio of the densities of methane                                                                                                                                                                                                                        ρo and air                                                                                                                                                                                                                        ρ∞
and the nozzle diameter                                                                                                                                                                                                                        ro,
is used to estimate the fuel concentration at a particular downstream location                                                                                                                                                                                                                        z for a given radial position                                                                                                                                                                                                                        r. It assumes self-similarity and is derived from the concentration profile
developed by Dowling and Dimotakis [24], thus a lack of dependence on the jet
velocity. The stoichiometric contour and
those indicating the 5 and 15% flammability limits of methane generated from
this approach are shown in Figure 3. Also
shown in Figure 3 are the axial locations of the flame front as the flame
progresses downstream for two different cases. Because (2) is strictly valid only when no coflow is present, both cases
have zero coflow velocity but slightly different fuel velocities, as each was
achieved by one of the two different techniques explained above. For Case 1, the flame is at a stable lifted
position before the velocity is increased from 46.6 to 50.4 m/s to induce
blowout. For Case 2, the flame was reignited
at the nozzle with the fuel velocity kept constant at 54.3 m/s. Blowout occurs at 33.8 cm for Case 1 and 38.9 cm for Case 2.Figure 3
Position of the flame
leading edge relative to the mean fuel concentration as calculated from the approach
of Tieszen                                                                                                                                                                                                                        et al. [22] for two
cases without coflow. For Case 1
(circles), the fuel velocity is increased from 46.6 m/s to 50.4 m/s (line 1,
Table 1) from the first series of experiments. For Case 2 (triangles), the fuel velocity is 54.3 m/s from the second
series. What is notable is that the
position where the axial oriented flame is lost corresponding to the approximate
position of the lean limit.For
both cases, the downstream location of the flame front when the diffusion flame
disappeared was determined from the images. As seen in Figure3, the location for Case 1 is 23.6 cm and for Case 2
is 25.1 cm. The location for both is
just within the 5% methane contour, implying that the mixture fraction at the
leading edge is found to be approaching the mean lean flammability limit
contour (and moving downstream in a direction of decreasing mixture fraction)
when the diffusion flame is witnessed to disappear and the flame to
subsequently blowout. Data from multiple
tests confirm that the disappearance of the diffusion flame occurs near the 5%
methane contour. Tests conducted with coflowing
air gave similar results; however, a more accurate representation for the
flammability limits with coflow present is necessary before the results can be
confirmed.Equations
for the time-averaged velocity profile of the flame (also from Tieszen                                                                                                                                                                                                                        et al. [22]) have been examined to verify the
validity of (2) in this study. Data from
numerous experiments utilizing particle image velocimetry to determine the
velocity of the stabilized flame were compared to the estimates provided by the
Tieszen velocity relation (Watson                                                                                                                                                                                                                        et al.
[3], Su                                                                                                                                                                                                                        et al. [25], and Muñiz
and Mungal [1]). The published PIV
measurements for each agree with the estimated velocities predicted by (2),
especially as the flame stabilizes further downstream. Each of these experimental studies used planar
laser-induced fluorescence to determine the axial and radial location of the
flame edge. Watson                                                                                                                                                                                                                        et al. [3] used the CH radical to locate the flame edge; thus
the PIV measurements were greater than one could expect from using OH, due to
the tendency of the CH zones to lie towards the centerline. To account for this, the locations of the
rich flammability limits were used to estimate the velocities for these data
sets and good general agreement was found, with better results further downstream.  The agreement of the velocity estimates
despite the presence of coflow supports the use of the similarly derived
Tieszen relation (2) for the mass fraction.These
findings support the earlier interpretations based on the digital images,
namely that the reduction and eventual disappearance of the diffusion flame
indicates the onset of blowout. In
addition, once the trailing diffusion flame is absent, the flame front is shown
to move into a downstream region in which the mixture fraction is below 5%, and
stable combustion is no longer possible. Blowout is found to be imminent for these conditions.


## 5. Conclusions

From the images generated by the two
types of experiments, general conclusions can be drawn. As shown in Figure4(a),
a stable lifted flame consists of a premixed flame front and a diffusion
flame. When the flame front moves
downstream, due to the fuel being at the blowout velocity, the diffusion flame
length begins to shorten, Figure 4(b). Once this trailing flame has disappeared, Figure 4(c), the reaction zone progresses
downstream, being unable to stabilize, and eventually extinguishes, Figure 4(d).Figure 4
Schematic of the blowout mechanism relative to the flammability limits of methane. (a) A stable lifted flame, (b) the unstable
flame has recessed downstream and the diffusion flame has shortened, (c) the diffusion flame has disappeared, (d) blowout occurs.The results of the study suggest that the trailing diffusion flame, specifically
its disappearance, plays a useful function as a flame extinction precursor
signature. The loss of the
chemiluminescence from the trailing diffusion flame functions as an indicator
of blowout being imminent. Only after
the diffusion flame disappears does blowout occur for all of the regimes
studied.It is concluded from the analysis that blowout occurs when the leading edge of the
reaction zone moves to a downstream region where most of the fuel that is
consumed is burnt locally near the leading edge, leaving little in the way of
fuel-rich gases to feed the trailing diffusion flame, only large volumes of
very fuel-lean gases.  In this sense,
blowout may be viewed for the cases studied as a lean-limit phenomenon
(Williams [26], especially Glassman comments) and this simple analysis
supports this conjecture, as well as the paper of Wu                                                                                                                                                                                                                        et al. [21], as shown in Table 3. The explanations in this paper do not explicitly address the mixing
effects (Dahm and Dibble [15]) or the velocity field considerations (Han and
Mungal [16]), nor do they contradict them, but rather offer an alternative
way based on lean limits to describe blowout and report a new visually
observable indicator that is compatible with concepts developed from the
general approach of Broadwell                                                                                                                                                                                                                        et al. [13].Table 3
Comparison of theories on blowout from past
publications.
Blowout
concepts supportedProposed
blowout mechanismBroadwell                                                                                                                                                                                                                        et al. [13]Large-scale
structures move hot product to flame leading edgeMixing
between hot products and unburned fuel allows insufficient time for
combustion to occurDahm and Dibble [15]Molecular
mixing rateReduction
in the fuel velocity with increased coflow velocity corresponds to a
consistent blowout parameterTieszen                                                                                                                                                                                                                        et al. [22]Local
flow velocity exceeds turbulent flame speedTurbulent
flame propagation towards the outer edge of the reaction zone stabilizes
flames near blowoutHan and Mungal [16]Flame
propagation against incoming reactantsFlame
base moves into a higher-velocity region due to a change in the
stoichiometric velocity of the flame surfaceWu                                                                                                                                                                                                                        et al. [21]Triple
flames/flame pulsationLessening
of the stoichiometric branch of the triple flame leads to downstream
recession and eventual blowout



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*Source: 461059-2008-02-20.xml*