Papers review for Flame Stability

Papers review for Flame Stability
By using tangential swirl burner(TSB)
Prepared by : jameel tawfiq jameel
Supervisor: Dr. Qahtn Al-Fatlawee
Supervisor: Dr. Mohammed AL- faham

My notes or lines
1. How to get the combustible stability of the
flame.
2. As well as reducing polluting emissions to the
environment.
3. Work on designing modern combustion
burner techniques to avoid combustion
problems.

Experimental Burner Rig Setup
A tangential generic swirl burner of a thermal capacity up
to 150 kW, figure 3-1, has been used to investigate different
flame flashback mechanisms. The system has two circular
tangential inlets of 67 mm diameter each. The inlet area
can be varied by using different inserts (25-25), (50-50) and
(70-70) respectively in order to achieve variable swirl
numbers. The system can also be used with three different
nozzle configurations varying the exhaust diameter from 62
to 76 mm. Air was provided by a centrifugal fan via flexible
hoses and two banks of rotameters to control the air flow
rate. Another bank was used for natural gas injection

The burner baseplate is designed mainly to allow central
fuel injection in order to start burner operation. During
stable operation, both air and gas are injected tangentially.
Premixed gas injectors extend across the inlet ducts to allow
adequate mixing. central fuel injectors are extended from
the baseplate through the centre of the burner plenum
opening just before the nozzle.
The approach considers the following assumptions:
1. Isothermal conditions
2. Constant static pressure across the exit
3. Constant density throughout the burner
4. Uniform axial velocity in the exhaust

Introduction
This article reviews the scientific issues
associated with operating steady flowing
combustors on syngas, such as gas turbines,
boilers, and furnaces. A number of
important practical problems must be dealt
with in developing a combustion system
capable of combusting syngas, particularly if the
system must also emit low levels of
•CO and NOx emissions.

•Some of these issues are well understood
•and utilize well-developed engineering knowledge,
such as required line or
orifice sizes needed to pass a given fuel flow rate in
order to supply the combustor
with a given heating rate. There are a number of other
issues, however,
substantially less well understood, which involve
complex interactions between
fundamental combustion phenomenon and fluid
mechanics. In particular, burners
involve complex, poorly understood interactions
between swirling flow dynamics, propagation.

In related fashion, there are a number of operability issues
related to
operating the combustor in a safe, efficient, and reliable
manner. This includes
having the combustor reliably hold the flame so that it
neither flashes back nor
blows out, and burns the fuel in a ‘‘quiet,’’ steady fashion.
We briefly review four
of the most critical of these operability issues below, all of
which are strongly influenced
by fuel properties:

This article addresses:
 Blowoff ,
 Autoignition
 Flashback
 Combustion instability

tangential swirl burner(TSB)
(TSB) system consists from :
• Tangential swirl burner system .
• Central air / fuel injector pipe .
• Stand to fixed the system.
• Cylinder gas (LPG).
• Compressor air
• Thermal camera .
• Sensor to avoid flashback.

Blowoff
Blowoff means the flame is being detached from its
stable position and physically blown out. The most
common reason that can cause flame blowoff is
very lean mixtures or robust combustion
instabilities. Recently swirl combustors have been
Extensively
employed with lean premixed combustion to
reduce NOx emissions, which are required to
operate close to their blow off limits, hence the
strong possibility of being subjected to this issue.

This problem becomes significantly important, especially
for aircraft engines. Blow off is a complicated
phenomenon that involves interaction between chemical
reactions, flame propagation and flow dynamics. Blow off
phenomena have been extensively investigated, and a
variety of experimental techniques and numerical
simulations have been employed to achieve better
understanding to prevent its occurrence
It is thought that blow off is governed by the equilibrium
between chemical kinetics and fluid mechanics. Thus this
relation can be described by using a Damköhler number
based on a well-stirred reactor which represents the
relation between residence time and chemical kinetic
time

Da=τu/τc
Where
τu : The period that reactants remain in the reaction zone
or flow time scale [s]
τc : The required time for chemical reaction [s]
Thus blow off will occur if the residence time is shorter
than the chemical time. The residence time is denoted by
the ratio between the characteristic length (recirculation
zone), and the characteristic velocity scale, while chemical
time scale represents the ratio between thermal
diffusivity, and laminar flame speed SL, thus blow off can
be correlated with the Damköhler number as follows:

Where
Da : Damköhler number [-]
d : characteristic length (recirculation zone length) [m]
Ub : average bulk burner exit axial velocity [m/s]
α : thermal diffusivity [m2/s]
SL : Laminar flame speed [m/s]

However, determining the characteristic velocity, the
characteristic length and hence the residence time is not
simple. The characteristic velocity is a function of
approaching flow velocity Ub which may change with
Reynolds number due to changes in burning gas
temperature while the characteristic length is a function of
the recirculation zone shape. Additionally, calculations of
thermal diffusivity for a fuel of different blends is
sophisticated. Hence the evaluation of chemical time scale
becomes more complex. This investigation of the chemical
kinetic is not sufficient to have a comprehensive
understanding about blow off; fluid mechanics must be
accounted too.

Nonetheless, blow off trends of a wide range of fuel
compositions can be determined by using the Damköhler
number
The relation between flame strain and blow off can be used
to predict the mechanism of blow off. Whereas flame strain
occurs under certain levels of flow velocity gradients and it
can withstand limited levels of strain before extinction, this
level is denoted by an extinction strain rate Kext which is a
function of the fuel composition, chemical time scale and
stoichiometry, thus Damköhler number can be written as
follows;

Da=Kext/K
Kext : extinction strain rate [1/s]
K : flame stretch
Many mechanisms can be used to enhance flame
resistance to blow off. Swirl burners and bluff-body
techniques have been employed in many applications to
achieve flame stability. They can work as premixed flame
holders, enabling mixing between combustible mixture
with hot combustion gases and promote the continuous
ignition of reactants

The flame holding represents a crucial factor in flame
stability. Consequently, many studies have demonstrated
different characteristics of flame holding and its effect on
blowoff limits by using bluff-bodies. Lieuwen, et al. studied
the effect of bluff-body geometries. The team pointed out
that vortex shedding results from the bluff-body playing an
important role in the ratio of gas expansion across the
flame and hence the level of flame extinction, and as a
consequence the blowoff mechanism. Roy Chowdhury and
Cetegen correlated the turbulence level with strain rate;
they found that strain rate increased with increasing
turbulence, consequently leading to localised extinction
zones along the flame surface .

Chaparro and Cetegen [122] also indicated that blowoff
equivalence ratios are affected directly by upstream flow
extinction.
Due to recent trends from using lean premixed combustion
and alternative fuels, introducing hydrogen or high
hydrogen blends as alternative fuels have increased
significantly. Blowoff and flame extinction limits are directly
proportional to the percentage of the hydrogen content of
a certain fuel. Many studies have proven that the
equivalence ratio at which blowoff occurs can be decreased
by increasing hydrogen percentage Working at this
conditions enables burners to operate at lower flame
temperatures consequently low NOx emissions

Autoignition
Under certain conditions inside the premixer such as high
pressure or high temperature, it is possible that the mixture
ignites without an external energy source this occurs
especially in premixed flows an increase in temperature
may be caused by early incidence of chemical reactions of
the mixture inside the premixer at high pressure and/or
high temperature. This results in considerable amount of
heat generation leading to spontaneous ignition. In general,
autoignition is defined as the homogeneous ignition of the
combustible mixture upstream of the combustion chamber

Autoignition is a serious issue that can stain the
performance of gas turbines and can cause considerable
damage to the system. Thus it is crucial to predicting the
ignition time of each mixture to achieve more secure
operation conditions. Autoignition time delay is defined as
the time interval between the injection of the mixture into
the combustion chamber and the onset of the flame. This
time can be represented using Wolfer equation
Where
ti : the ignition time delay [ms]
P : the pressure [bar]
Tm : the initial mixture temperature
[K]

Nevertheless, in addition to the effect of pressure and
temperature, fuel composition plays an important role in
determining the autoignition time delay. Natural gas or
methane is the favourable fuel for stationary gas turbines
and has reasonably weak ignition behaviour at low
temperatures. However, this behaviour can be affected
drastically when adding higher hydrocarbon blends.
Recently, hydrogen has proved to be a clean energy carrier,
and it can be added to natural gas or methane to enhance
some operational requirement, as a consequence of its
delay time, wide flammability limits, high energy density
and high laminar flame speed.

However, the percentage of hydrogen addition significantly
impact the autoignition mechanisms. The higher the
hydrogen concentration, the more the hydrogen chemistry
dominates the mixture ignition, whereas the non-linear
relationship between logarithmic of ignition delay and
equivalence ratio becomes increasingly evident with
increasing hydrogen fraction
In swirl combustors, strong aerodynamic effects are directly
linked to predicting autoignition times. However, weak
aerodynamic system designs make them more likely to
suffer flashback. Thus, ignition delay times of the fuel used
and the local conditions should be taken into consideration
when designing the premixer to avoid autoignition

Flashback
Flashback is the upstream flame propagation from the
combustion zone to the premixing zone inside the
combustor. It occurs when the flame speed exceeds the
approaching flow velocity [39]. Flashback is a main
operability issue and an inherent instability problem of lean
premixed combustion systems which can cause the burner
to overheat with considerable damages to the system
hardware and increase in pollutant emissions [68, 140,
141] as can be seen from figure 2.5.

Fuel composition represents an important factor affecting
flashback mechanisms. This effect is characteristic of
turbulent flame speed ST variations. Thus introducing
hydrogen or hydrogen fuels to premixed combustion
systems that use swirl combustors have brought many
concerns regarding flashback. More reactive gases than a
natural gas such as hydrogen can present flame speeds
about four times greater than those of natural gas.
Swirl lean premixed flames are more likely to be subjected
to flashback. Swirling flows are complex and can interact
with the flame in these systems causing flame flashback.

Figure 2-5. System hardware damages due to flashback.

According to previous literature, there are four types of
flashback mechanisms
Boundary Layer Flashback
This type of flashback was firstly described by. They investigated
laminar non-swirling flow flashback, pointing out that the reason of
flame flashback in the wall boundary layer is due to the reduction of
the approaching flow velocity near the wall in such way that it
became lower than the flame speed. Thus, they proposed the so-
called “Critical gradient” model. This model defines the flashback
limits according to the equilibrium between the wall-parallel
boundary layer velocity u(y), where (y) represents the wall - normal
coordinate and the flame speed Sf (y) at a distance dq from the wall
which represent the quenching distance, as can be seen in figure 2.6.

Recently Lin, et al. [149] proposed a methodology for
correlating the flashback propensity with turbulent flame
speeds (ST) for H2 rich gases based on measurement of
critical velocity of the flame and flow as follows
gc : the critical velocity gradient of the flame [1/s]
Le : Lewis number which represents the ratio between
thermal diffusivity to mass
diffusivity [-]
δlo : (un-stretched) laminar flame thickness [m]
The critical velocity gradient of the flow is represented by
the following equation;
gf=0.03955×Ub74⁄×υ-3/4⁄×d1/4

gf : the flow velocity gradient [1/s]
Ub : the bulk velocity at the combustor inlet [m/s]
υ : the kinematic velocity of the combustible mixture [m2/s]
d : the diameter of combustor inlet [m]
Thus, according to these relations, the flame stability
depends on the balancing between (gf) and (gc), whereas
flashback takes place when (gc) exceeds (gf).
Turbulent Core Flashback
Turbulent core flashback takes place when the turbulent
flame velocity (ST) exceeds the local flow velocity in the
core flow .

Then the flame can propagate upstream in all the zones of
the burner that have lower velocities than the turbulent
flame speed. Thus one of the fundamental design
parameters of new burners is how to ensure that flow fields
do not have strong local velocity deficits with axial flow
velocities that must be higher than the turbulent flame
speed (ST). Turbulent flame velocities depend on chemical
kinetics, turbulent structures, length scales and velocity
fluctuations.
Although, there is vast literature on different techniques and
simulations to predict the turbulent flame velocity of various
combustion systems, there is still a crucial need to have a more
practical and robust methodology for investigating turbulent flame
velocity especially for H2-fuels. Consequently, designing combustion
systems to have a higher axial flow velocity, hence avoid turbulent
core flashback, is still based on practical experience

Flashback due to Combustion Instabilities and Coherent
Structures
Combustion or thermo-acoustic instabilities inside the
combustion chamber can cause upstream flame
propagation both in the turbulent core and boundary layer,
consequently flame flashback. These types of instabilities
arise when unsteady or periodic heat release couples with
one or more acoustic modes of the combustor. It is possible
for these oscillations to interfere with the combustion
system causing a considerable amount of vibrations which
lead to failure or collapse of the combustion system.
one of these mechanisms is high pulsation levels, which
cause a modulation of the flow field in the burner

Another cause of instabilities is equivalence ratio
perturbations which have a direct effect on a chemical
reaction, flame speed and indirectly on the flame area
which in turns control the amount of heat release
response.
Swirling flows and their coherent structures, especially the
precessing vortex core (PVC), also represent another source
of unsteadiness or fluctuation.

Flashback due to Combustion Induced Vortex
Breakdown (CIVB)
Combustion induced vortex breakdown has been identified as one
source of sudden flame transition and flashback in swirl combustors.
This sudden transition can cause severe system failure due to the risk
of overheating of upstream burner positioned components. This type
of flashback is substantially different from the previous three
flashback types. It occurs primarily in swirl stabilised combustors
without a centre body at stable operation due to small changes in the
flow field and the interaction of the turbulence and chemistry at the
tip of the central recirculation zone [65, 160, 161]. It can occur even if
the flow velocity is higher than the turbulent flame speed [162]. The
earliest study that denotes this phenomenon was done by Kröner, et
al. [66]. The group investigated the flashback limits for swirl burners
with cylindrical premixing tubes without using centrebodies

They found that the quenching effect of chemical reactants plays the
dominatn role in flashback limits. Another study [65] found that flame
propagation is governed by mutual feedback between chemical
reaction and flow. In other words, flashback has high sensitivity to the
momentum distribution in the vortex core, whereas the chemical
reaction leads to variation in pressure boundary conditions at the inlet
of the combustion chamber, consequently, promoting the propagation
of the vortex breakdown. The study has proved the possibility of
significant flashback resistance mechanisms when introducing axial
jets, however, altering swirl levels. The effect of the axial jet has also
been investigated by [163] who found that the increase in axial jet
diameter leads to thicker vortex cores. Hence the vortex breakdown
will be shifted downstream and consequently stabilise the burner
operation window.

They also correlated the phenomenon of stagnation of the
recirculation zone with the effect of changing the
equivalence ratio of the axial position. Based on their
investigations the upstream flame envelope and the
stagnation point play the dominatn role in the onset of the
CIVB. If the recirculation zone can pass the stagnation point
in the upstream direction, the volume expansion upstream
of the recirculation zone generates positive azimuthal
vorticity consequently positive axial velocity. Thus stable
flame prevents the occurrence of the CIVB. However, upon
further upstream flame propagation (a further increase of
equivalence ratio) baroclinic torque will increase, thus the
high possibility of CIVB occurrence increases as well.

Kröner, et al. [66] proposed a new correlation to define
flashback limits. This correlation stated that flame
quenching occurs at a critical value of the quench parameter
(Cquench) which represents the ratio between the
characteristic chemical time scale (τc) and flow or mixing
time (τu)

Papers review for Flame Stability

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