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Combustion Science and Technology
ISSN: 0010-2202 (Print) 1563-521X (Online) Journal homepage: https://www.tandfonline.com/loi/gcst20
Investigation on a Novel Type of Tubular Flame
Burner with Multi-stage Partially-Premixing
Features for Liquid-Fueled Gas Turbine
Yiran Feng, Wenyuan Qi, Mohammad Hassan Baghaei, Yuyin Zhang &
Daiqing Zhao
To cite this article: Yiran Feng, Wenyuan Qi, Mohammad Hassan Baghaei, Yuyin Zhang &
Daiqing Zhao (2019): Investigation on a Novel Type of Tubular Flame Burner with Multi-stage
Partially-Premixing Features for Liquid-Fueled Gas Turbine, Combustion Science and Technology,
DOI: 10.1080/00102202.2019.1651298
To link to this article: https://doi.org/10.1080/00102202.2019.1651298
Published online: 06 Aug 2019.
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Investigation on a Novel Type of Tubular Flame Burner with
Multi-stage Partially-Premixing Features for Liquid-Fueled Gas
Turbine
Yiran Feng
a
, Wenyuan Qi
a
, Mohammad Hassan Baghaei
a
, Yuyin Zhang
a
,
and Daiqing Zhao
b
a
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China;
b
Guangzhou Institute of
Energy Conversion, Chinese Academy of Sciences, Guangzhou, China
ABSTRACT
A new combustion system, which consists of an evaporator and
a tubular flame burner with multi-stage inlets, has been developed
to meet the growing concerns over the fuel flexibility and flame
stability of gas turbines. In the evaporator, a flash-boiling atomization
technology was adopted to enhance atomization, evaporation, and
fuel-air mixing to provide the burner with the optimized fuel-rich
mixture under various operating conditions with liquid fuels. In the
new-type of tubular flame burner, a tangential multi-stage inlet
structures were proposed to realize fast fuel/air mixing and thus
clear combustion with features of excellent stability, uniform tem-
perature profile, and non-flashback. As a practice, ethanol was
selected as the liquid fuel to examine the performances of the
evaporator and the multi-stage tubular flame burner. Results show
that ethanol spray has been fully evaporated under a wide load range
owing to the flash-boiling technology, and tubular flames can be
established stably and without flashback, and specifically in a wide
range of equivalence ratio from 0.23 to 4.6, which has been greatly
expanded compared to those of the conventional tubular flame
burners. The temperature near the burner outlet were found uni-
formly distributed in the radial direction across the burner central
area, but became very low when approaching the burner cylinder
wall, which is believed helpful for decreasing the heat loss to the
burner liner and thus preventing it from ablation. These results imply
that this new burner has great potentials in application to the
advanced gas turbine.
ARTICLE HISTORY
Received 11 March 2019
Revised 10 July 2019
Accepted 29 July 2019
KEYWORDS
Liquid fuel; flash boiling
spray; tubular flame burner;
safety combustion; partially
premixing
Introduction
Gas turbine is a critical power machinery in both commercial and military use, such as
engines of electricity generators, jet engines, and driving engines of ships and tanks. The
majority of them are operated on liquid fuels. In most of conventional gas turbines, liquid
fuels are directly injected into the burner. Poor atomization and evaporation quality of
liquid fuel usually lead to non-uniform fuel distributions and result in problems due to
insufficient time for fully evaporation of liquid droplet and mixing with air before
CONTACT Yuyin Zhang yuyinzhang@sjtu.edu.cn School of Mechanical Engineering, Shanghai Jiao Tong
University, Shanghai 200240, China
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gcst.
COMBUSTION SCIENCE AND TECHNOLOGY
https://doi.org/10.1080/00102202.2019.1651298
© 2019 Taylor & Francis Group, LLC
combustion (Lefebvre 2000; Sturgess et al. 2005). The typical issues include: (1) Carbon
deposit and coking: the carbon deposit and coking problem is the major cause to injector
clogging; (2) soot emissions: The droplet-laden fuel-air mixture usually results in incom-
plete combustion and thus high soot formation; (3) failure of blades: The crack of blades
resulted usually from high local temperature, which greatly limits the improvement of
thermal efficiency of gas turbines; (4) high NOx emissions: high local peak temperature is
the major cause for high NOx emissions.
In recent decades, a premixed type burner named Lean-Prevaporized-Premixed (LPP)
burner was proposed to solve the problems of the conventional gas turbine (Maier and Wittig
1999). In LPP combustion, the liquid fuel is usually injected into a premixed zone firstly and
mixed with the heated air stream for enhancing fuel evaporation and mixing before entering
the burner. A LPP system has advantages in reducing NOx and soot emissions, however, there
appear other problems: (1) poor fuel evaporation efficiency: Incomplete evaporation of fuel
droplets is still a difficulty in LPP and causes such problems as carbon deposit and soot
formation (Wei, Wang, Reh 2002); (2) Risk of auto-ignition: air stream in premixed zone must
be heated to adequately high in temperature so as to meet fast evaporation of liquid fuel, which
may increase danger of auto-ignition (Gokulakrishnan et al. 2008); (3) flame instability: a lean
premixed flame is intrinsically less stable compared to a diffusion flame because of the lack of
hot gas recirculation and so-called “mutual kinetic rescue”effect between neighboring
reactants (Dhanuka, Temme, Driscoll 2011; Lieuwen and McManus 2003;Molière2000).
Tubular combustion is one of the potential solutions to overcome the drawbacks of the
conventional burners or LPP burners (Ishizuka and Dunn-Rankin 2013). A tubular flame
usually forms inside a cylindrical burner into which fuel, air or their mixture are injected
through a few inlets in tangential directions. In recent years, tubular flame burner has
been widely investigated since it demonstrates a wider stable flame range from lean to rich
flammability limits in many applications (Ishizuka and Dunn-Rankin 2013). In principle,
the burned gas, of high temperature and low density, within the tubular flame burner
distributes itself around the central area of the tube, while the unburned mixture (usually
heavier due to low temperature) exist near the tube cylinder wall due to the centrifugal
force. This distribution pattern makes the flame aerodynamically stable (Ishizuka 1993),
according to Rayleigh stability criteria. In addition, the colder unburned mixtures with
higher density around the wall can form a protection zone for the burner liner from
ablation due to flame, which effects significantly lifetime of a gas turbine.
Tubular flame burners have been systematically investigated since early eighties of last
century by Ishizuka (1993), Pham, Dunn-Rankin, and Sirignano (2007), Pitz, Hu, and Wang
(2014), Kee et al. (2008), Hagiwara et al. (2000), Zhang et al. (2005,2009), Shimokuri et al.
(2014), Shi et al. (2017), The first-generation tubular flame burner adopted premixed combus-
tion, where gaseous fuels (mostly methane and propane) and fresh air were premixed at
a suitable equivalent ratio outside the burner, then the premixture was injected tangentially or
radially into the cylindrical burner, the risk of flashbackisabigissueforthistypeofburner
(Ishizuka 1993). To avoid the risk of flash back, Ishizuka, Motodamari, and Shimokuri (2007)
further developed the second-generation burner called rapidly-premixed tubular flame burner,
into which fuel and air were separately injected through the respective slits tangential to the
cylinder. The results showed that a stable tubular flame can be successfully established for
equivalent ratios range from 0.5 to 1.5 when operating on methane/air mixture. Potential
abilities of reducing NOx emission were also claimed in this type of burner although the
2Y. FENG ET AL.
flame was non-uniform, and there were luminescence strips in flame. The temperature dis-
tributions were not even, which probably resulted from the significant diffusion combustion
during the mixing. On the rapidly-premixed burner, some studies were also conducted by Shi,
Shimokuri, and Ishizuka (2013), (2015)) using gaseous fuel
As for liquid fuel, Ishizuka et al. (2009) and Pham, Dunn-Rankin, and Sirignano (2007)
have studied tubular flame by directly injecting liquid fuel into inlet slits. The particle
matter emissions were greatly reduced compared with the diffusion-flame type burners
(Ishizuka et al. 2009), however, there existed one more issue, that is, the heavy and large-
sized fuel droplets were thrown onto the cylinder wall due to the poor atomization and the
centrifugal force, which lead to combustion of fuel film and sooting flame.
In general, the tubular flame burners show some advantages in flame stability over the
conventional burners, but there exists the risk of flash back for the first generation
premixed tubular flame burner. The mixing quality of fuel and air is poor especially for
liquid fuel which may cause sooting flame and unexpected temperature distributions for
the second generation rapidly-premixed tubular flame burner. One of the most important
issues is the difficulty to apply liquid fuels to both types of tubular flame burners.
In this work, a multi-stage tubular combustion system is proposed, which consists of
aflash-boiling evaporator and a multi-stage tubular flame burner. The aim is to realize
stable, safe, and non-sooting combustion with uniform gas temperature distributions
when operating on liquid fuels. The optically-accessible evaporator was designed for
examining the effect of flash-boiling parameters on atomization and evaporation pro-
cesses. The tubular flame burner with unique multi-stage inlets and the flash-boiling
evaporator will be proved to be a promising combustion system for gas turbine with
several advantages over the conventional gas turbine burner and LPP burner.
Burner development and experiment facilities
Experiment facilities
The multi-stage tubular flame burner system is shown in Figure 1. The system consists of two
main parts: a flash-boiling spray evaporator and a multi-stage tubular flame burner. The
former was designed for fast gasification of the liquid fuel before entering the burner, the detail
of which will be given in Section 2.2. The latter was designed to realize stable, safe, and non-
sooting combustion, the detailed information of which can be found in Section 2.3.
Pressurized air from the air bomb was separated into two streams, named as the primary air
and the secondary air, and the volume flow rate of which were designated as Q
ap
and Q
as
,
respectively. The total volume flow rate of air entering the burner, Q
a
, was the sum of the
primary air (Q
ap
) and the secondary air (Q
as
), i.e., Q
a
=Q
as
+Q
ap
. The primary and the
secondary air streams were both preheated to the desired temperature using electric heaters.
The air mass flow rates were set and controlled through two flow controllers (Bronkhorst EL-
FLOW, uncertainty: ±0.5% Relative average deviation +0.1% at Full scale) located before each
heater. The fuel/air mixing processes were completed in two steps: first, the fuel was injected
into the evaporator at flash-boiling spray, evaporated quickly, and mixed with the primary air
stream to form the roughly homogenous fuel-rich mixture; second, the fuel-rich premixture
was injected tangentially into the combustion tube and re-mixed with the preheated secondary
air to form an appropriate mixture for burning.
COMBUSTION SCIENCE AND TECHNOLOGY 3
Premixed flash-boiling evaporator
A premixed flash-boiling evaporator was developed in this study to provide the burner
with homogenous, gaseous fuel-rich mixture. In order to achieve flash boiling atomiza-
tion, pressurized liquid fuels need to be preheated to a temperature higher than the boiling
point at the ambient pressure (P
a
) before injected. In this way, the liquid fuel can be fast
gasified before entering the burner, otherwise the primary air stream has to be heated up
to very high temperature for realizing complete evaporation of the fuel before entering the
burner. This heated air of high temperature will increase significantly the danger of
autoignition in the premix section.
Flash boiling sprays can be easily obtained by injecting the preheated pressurized liquid
fuel into an environment below its saturation pressure. The mechanism of flash-boiling
atomization has already been studied previously (Li, Zhang, Qi 2018; Li et al. 2017; Yang
et al. 2018). The evaporator in this work was designed with a premixing chamber, which
was a cylinder of 120 mm in length and 85 mm in diameter, into which the heated fuel
was injected and the heated air was flowed and mixed with the flash boiling spray. A six-
hole injector was mounted on the top cap center of the evaporator. A cylindrical quartz
tube in the middle section and a plane quartz window at the bottom were adopted for
visualization of the fuel spray in either the radial or the axial directions of the cylindrical
evaporator with optical diagnostic techniques. The liquid fuel was pressurized to 5–35MPa
and then heated to a certain temperature using pipe heater and the fuel temperature was
monitored by a thermocouple (TS 5 in Figure 1) and controlled using a PID control
system. At the same time, the injector was surrounded by a constant temperature oil bath
to compensate the heat loss to maintain the fuel temperature. The pressurized fuel was
provided by a nitrogen-actuated piston accumulator.
Ethanol was selected as the test fuel in this work. The ambient pressure (P
a
) was set at
0.12MPa and the fuel temperature (T
f
) was set at 473K which was apparently higher than
the boiling point of ethanol at the ambient pressure. During the experiment, the injection
Accumulator
Heater
Heater
Pressured air tank
Evaporator
Fuel injector
TS1 PS1
Mass flowm eter
PID controllers
Inputs ignal
Out put signal
Ignitor
CameraA
Camera B
High-speed camera
LEDlight
TS3 TS4
AA
A-A
Air
Heated air
Fuel/Air mixture
Control signal
Temperature sensor
Pressure sensor
TS2/PS2
Liquid fuel (ethanol)
Pipe heatingcoil
TS5
Optical diagnostic direction
Primary air(Qap)
Secondary air(Qas)
473K
Constanttemperature
Oilbath
Oil bath
PS: Pressure sensor TS: Temperature sensor
Direction of burned gas flow
Figure 1. Set-up of multi-stage tubular flame burner system.
4Y. FENG ET AL.
pressure was set at 5MPa and the injection frequency was fixed at 50Hz to generate
a homogenous fuel-rich premixture. The fuel injection duration was varied to achieve
different fuel flow rate which was related to the power output of the burner.
Mie scattering measurement was adopted to analyze the evaporation process in the
evaporator with a high-speed camera (Phantom VEO710) at the cylinder side and a plate
LED lamp at the bottom for side-view imaging, on the contrary, with the lamp at the cylinder
side and the camera at the bottom for bottom-view imaging (Figure 1). A planar laser-induced
fluorescence (PLIF) technology was also adopted to detecting the distributions of liquid and
vapor phases in the spray in the evaporation tank. Acetone was added to the fuel as the tracer
of PLIF at volume ratio of 10%. The experiment set-up of PLIF is shown in Figure 2.
Multi-stage tubular flame burner
To overcome the combustion instability of LPP burner and take the advantages of
previous tubular flame burners such as rapid mixing and safety combustion, a novel
multi-stage tubular flame burner was developed. Figure 3 shows the cross-section of the
multi-stage tubular flame burner. The burner was made of stainless steel with a length of
500 mm and a diameter of 50 mm. A cylindrical quartz tube of 150mm in length was
installed in the middle, and a quartz window was installed in the left end for observation
of the combustion processes with optical diagnostics.
High swirl flow formed when injecting streams through two pairs of slits which were
tangentially installed onto the burner as shown in Figure 3 (D-D view). The structure of
inlets is shown in Figure 4. The gaseous fuel/air (fuel rich) mixture and the secondary air
were separately introduced to the burner through the slits and rapidly remixed inside the
burner to form a rather homogeneous mixture.
The secondary air was preheated to a desired temperature by an electric heater and
controlled by a PID control system. Ignition was conducted through an electric ignitor
installed at the flange 100 mm away from the burner exit. The temperature distribution
near the outlet and in the middle of burner was measured by a Pt/Pt–13%Rh thermocouple
located at 70mm(X1) and 140mm(X2) ahead of the burner exit, as shown in Figure 3.
ND:YAG
ICCD Camera
266nm Beam
Evaporator
Fuel Injector
Reflection Mirror
Reflection Mirror
Convex Lens
Cylindrical Mirror
Figure 2. Set-up of optical imaging system of PLIF on flash boiling spray.
COMBUSTION SCIENCE AND TECHNOLOGY 5
Prevention of flash back
Safety combustion is vital to a premixed type combustion system. In this multi-stage
tubular flame burner, the width of the slits and also the equivalent ratio of the fuel-rich
premixture inside the evaporator were determined based on careful considerations to
prevent flash back. Theoretically, the flash back will not occur when the flow velocity of
the fuel-rich mixture at the inlet of the burner is higher than the premixed flame velocity,
if the quenching effect of the slits is neglected. Therefore, to reach higher level of flow
speed at the fuel slits, smaller width was preferred. However, the width of the slits cannot
be reduced infinitely, otherwise the power output of burner will be limited due to
insufficient flow rate of the premixture. On the other hand, adequate amount of air flow
is required to carry the fuel vapor formed in the evaporator to the burner, and therefore
the average equivalent ratio inside the evaporator (Ф
e
) can not be set too high, where Ф
e
was defined as:
Φe¼ð
λeC2H6O
λeO2
Þ=ðλC2H6O
λO2
Þstoic (1)
Ethanol/Air Premixture Preheated secondary Air Ignitor
Pt/Pt-13%Rh
Temperature Sensor
D-D
D
D
X1 X2
Direction of burned gas flow
Figure 3. Structure of multi-stage tubular flame burner.
AA
Length 530
A-A
Width
2
Height
150
Ethanol/Air Premixture
Preheated Secondary Air
Figure 4. Structure of inlet slits (unit: mm).
6Y. FENG ET AL.
where
stoic means stoichiometry;
λ
e-C2H6O
is the volume concentration of ethanol fuel in evaporator;
λ
e-O2
is the volume concentration of oxygen in evaporator;
As a result, 2mm-width was selected (shown in Figure 4), and the equivalent ratio in
the evaporator was set to 3.0, which has been proved to be effective for preventing flash
back. The laminar burning velocity of ethanol and contour lines of power output of
a premixed tubular flame burner were plotted against the global equivalent ratio (Ф
g
) and
the velocity of the premixture flow at the slits, where, Ф
g
was defined similarly to Equation
1 as:
Φg¼λgC2H6O
λgO2
λC2H6O
λO2
stoic
(2)
Where
λ
g-C2H6O
is the volume concentration of ethanol in tubular flame burner;
λ
g-O2
is the volume concentration of oxygen in tubular flame burner;
The laminar flame speed calculations of ethanol/air mixture at conditions of 0.1MPa
and 473K were performed using Premix Code of Converge software package. The detailed
kinetic mechanism of Marinov was used to perform the simulation, which includes 56
species and 351 reversible reactions (Marinov 2015). As seen in Figure 5, the flash-back
risk exists at a power load below 7. 9kW for the stoichiometric premixed combustion,
which significantly restrains the operation range of gas turbine for idling. However, for
rich premixed combustion such as an equivalent ratio of 3.0, the flash back will not occur
as long as the power load is not lower than 1.1kW. Since the equivalent ratio of the
premixture (Ф
e
) in the evaporator was fixed at 3.0 in the case of the multi-stage burner,
the flash back phenomenon will not happen in the combustion tube at the power load
over 1.1kW. This indicates that the range of power output can be greatly extended by this
multi-stage tubular combustion technology.
Results and discussion
Characteristics of fuel atomization and evaporation in evaporator
Two optical diagnostics were conducted to validate the flash-boiling effect on spray
atomization and evaporation in the evaporator. The first one was the Mie scattering
imaging technique. Since Mie scattering light came from scattering of liquid droplets
(Yang et al. 2013; Zeng et al. 2012; Zigan, Trost, Leipertz 2016), it is reasonable to use the
intensity of Mie scattering (represented by color in Figure 6) as the indicator for the
residual quantity of liquid phase in the spray. The experiment conditions for validation of
flash boiling effects on atomization and evaporation are presented in Table 1.
The images of ethanol spray at 0.0, 0.5, 1.0, and 1.5ms after end of injection (AEOI) are
presented in Figure 6. When T
f
= 300K and T
a
= 300K, the liquid droplets in ethanol
sprays were hard to evaporate completely before leaving the evaporator. The dense
droplets hit the wall of the quartz glass at 1. 5 ms AEOI. This phenomenon is commonly
known as the wall-wetting problem, which usually occurs in a confined room such as
COMBUSTION SCIENCE AND TECHNOLOGY 7
a combustion chamber of a gasoline engine due to poor atomization and evaporation.
When T
a
increased to 473K and T
f
remained at 300K, the wall-wetting was still observed,
though the concentrations of liquid phase reduced slightly. This slight improvement can
be explained by the effect of strengthened heat transfer between the ambient gas and the
spray droplets. Increasing T
f
to 473K, and T
a
remained at 473K, the fuel spray transmitted
from a conventional spray to a flash-boiling spray. The evaporation of ethanol spray was
greatly enhanced and fuel spray structure collapsed obviously to one plume, which
avoided wall-wetting. The collapse phenomenon of flash-boiling spray was caused by
the effect of low-pressure core (Zeng et al. 2012). With the spray collapsed to the center,
the density of the liquid droplets increased in the center area and hence the signal of Mie
scattering light becomes stronger.
In order to illustrate the difference between the evaporation rates of conventional and
flash boiling atomization, the light intensity of Mie scattering in each spray image (side-
view), which is roughly proportional to the total surface area of droplets in the spray, was
integrated and plotted against time AEOI in Figure 7. The fuel droplets in the spray under
the flash boiling conditions (T
f
= 473K, T
a
= 473K) evaporate much faster than those in
the conventional atomization. At a fuel flow rate of 230mg/s, the time for the remaining
quantity of liquid phase to be less than 1% was 1.5ms for flash boiling atomization and 2.4
and 5.4ms for the other two, respectively.
The results indicate that the fuel can be gasified efficiently by the flash boiling
evaporator and supplied to the burner in form of fully-vaporized fuel-air premixture.
The second optical diagnostic was a planar laser-induced fluorescence (PLIF) technol-
ogy for detecting the distributions of liquid and vapor phases in the spray. It is known that
the fluorescence of PLIF comes from both liquid and gaseous phases of a spray, however,
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.4 0 .6 0.8 1 1.2 1.4 1 .6 1.8 2 2.2 2.4 2 .6 2.8 3 3.2 3 .4
)s/m(yticoleVwolftilS/gninruB
Equivalant ratio Ф
g
7.9kW
5kW
1.1kW
Burning Velocity of Etha nol/Air Mixture
10kW
Figure 5. Laminar burning velocity of ethanol-air premixture and iso-power contours at various slit flow
velocities and equivalent ratios.
8Y. FENG ET AL.
it only represents the vapor phase when the liquid phase has been vaporized completely.
Whether the liquid phase exists or not can be confirmed by Mie scattering imaging. As
shown in Figure 8, the quantity of liquid phase of fuel spray was negligible at 1.5ms AEOI,
and a homogenous gaseous fuel/air mixture was found uniformly distributed in the
evaporator. It should be noted that each image in Figure 8 was the average of three single-
shot images at the same moment of injection.
Again, it has been identified by PLIF that the fuel droplets disappeared at 1.5ms AEOI
when adopting flash boiling atomization, and then the vapor distributed itself uniformly
inside the evaporator.
Characteristics of tubular flame
Formation processes of tubular flame in multi-stage burner
Images just after the ignition were captured by a high-speed camera (Phantom VEO710) to
illustrate the formation process of the tubular flame (Figure 9). The global equivalent ratio of
unburned fuel/air mixture flow was kept at Ф
g
= 1.0. After the ignitor sparked, the flame kernel
appeared around the ignitor and began to propagate upstream. The tubular flame finally
T
f
=300K T
a
=300K T
f
=300K T
a
=473K T
f
=473K T
a
=473K
0.0ms
0.5ms
1.0ms
1.5ms
Bottom-viewSide-vie w Side-view Bottom-view Side -view Bottom-view
Figure 6. Spray evaporation processes at different conditions in premix tank; color bar (Intensity of mie
scattering) (P
inj
= 5MPa, △t
inj
= 1.0ms).
Table 1. Experiment conditions for ethanol spray evaporation.
Parameter Specifications
Injection pressure (Pinj, MPa) 5.0
Ambient pressure (Pa, kPa) 103
Fuel temperature (Tf, K) 300, 373, 473
Ambient temperature (Ta, K) 300, 473
Fuel type Ethanol (99% purity)
Fuel flow rate (Mf, mg/s) 230
Equivalent ratio in evaporator (Фe) 3.0
COMBUSTION SCIENCE AND TECHNOLOGY 9
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0123456
[,gnirettacseiMfoytisnetnidezilamroN-]
Time (ms, AEOI)
Tf=300K,Ta=300K
Tf=300K,Ta=473K
Tf=473K,Ta=473K
Figure 7. Spray evaporation process in premix evaporator at various operating conditions (The
integrated intensity was normalized by the maximum).
Figure 8. Images of PLIF and mie scattering of flash boiling spray at different shot-timings AEOI.
10 Y. FENG ET AL.
stabilized at T
0
+102ms,acorrugatedflame was rotating inside the glass tube, keeping stable
in both flame diameter and flame length.
Extinction limit
In order to identify the operation range of the combustion system, the extinction limits were
measured, as shown in Figure 10. The temperature of the primary air, the secondary air, and
the ethanol fuel were preheated and kept at 473K. In most cases, Ф
e
was fixed at 3.0, except for
the cases of Ф
g
> 3.0. The global equivalent ratio Ф
g
was varied by changing Q
as
for the cases of
Ф
g
< 3.0, while adjusted by shutting down Q
as
and varying Q
ap
for the cases of Ф
g
>3.0.
The isoline of fuel injection mass was plotted in Figure 10.M
f
(mg/s) is fuel injection
mass. By fixing M
f
= 99.25mg/s and decreasing Q
a
till the flame extinction, the rich limits
were found around Ф
g
= 4.5–4.8, and by fixing the fuel flow rate and Ф
e
= 3, increasing Q
as
till the flame extinction, the lean limits were found around 0.23. It was reported by the
previous researchers that the lean extinction limit of both the premixed and the rapid-
mixed tubular burners were around Ф= 0.5, and the rich extinction limit were around
Ф= 1.68 for methane and Ф= 2.5 for propane (Ishizuka 1993; Ishizuka, Motodamari,
Shimokuri 2007). To date, there were no reports on determination of extinction limits of
liquid fuel for tubular flame burner. In fact, the combustion of liquid fuel is much more
complex than gaseous fuels since it involves the process of interaction of liquid and air,
atomization and evaporation of droplets. Considering complexities of combustion of
liquid fuel, it can be concluded that the operation range of this novel tubular flame burner
was successfully expanded in contrast to the previous tubular flame burners. In fact, the
combustion characteristics of DME in multi-stage tubular flame burner have been
numerically studied by Ren et al. (2018). According to their work, multi-stage tubular
flame burner possesses a unique distribution of species, which will promote the chemical
enthalpy supply to the flame front by recirculation reverse flow in the center, and thus the
combustion stability range has been widened.
Figure 10 shows the flame appearances from lean to rich extinction limit. Two video
cameras (SONY HDR-CX680) were used to capture the images of the flame at rate of 30fps
from different directions (Figure 1). The tubular flame was initially established at Ф
g
=1.0,
1cm
Unburned gas flow directionFlame propagation direction
Position of ignitor
T
0
+2ms T
0
+6ms T
0
+10ms T
0
+14ms T
0
+22ms
T
0
+38ms T
0
+42msT
0
+30ms
T
0
+26ms
T
0
+34ms T
0
+70ms T
0
+102ms(stable)
Figure 9. The formation process of tubular flame from ignition to stable flame (T
0
: ignition start, ms).
COMBUSTION SCIENCE AND TECHNOLOGY 11
a luminous thin zone can be observed near the cylinder wall. Increasing the air flow rate to
Ф
g
= 0.8, 0.6, 0.5, 0.4, the flame became very uniform in the central region at Ф
g
=0.4.
Furtherly increasing the air flow to Ф
g
= 0.3, the luminosity of the flame became weaker, both
the luminous zone and flame length shrank dramatically. At Ф
g
= 0.23, which was very close
to lean extinction limit, the flame shrank further, and the dark zone extended toward the end
wall, and a luminous circle was found inside the burner when viewing from the bottom.
The appearance of tubular flame at the fuel rich conditions was quite different from
that at the fuel lean conditions. Decreasing the air flow to Ф
g
=3.1,theflame diameter
became smaller compared to that at the stoichiometric conditions and the lumines-
cence became weaker. A threadlike blue tubular flame was observed at Ф
g
= 4.0, and
the flame became quiet and stable with a flame diameter even smaller than that of the
flame at Ф
g
= 3.1. Furtherly decreasing the air flow rate, the flame gradually decreased
both in length and in diameter and finally extinguished. Note that the brightness of
flame images at Ф
g
= 2.0, 2.5, 2.8, 3.1, 4.0, and 4.6 were enhanced at a small degree for
better view.
Temperature distributions at radial cross sections
In order to investigate the characteristics of temperature distributions in this new burner,
temperatures were measured at cross sections of X1 and X2 (Figure 3), which represent
Figure 10. Extinction limits of multi-stage tubular flame burner.
12 Y. FENG ET AL.
the flame temperature and exhaust gas temperature, respectively. Experiment conditions
of Ф
g
= 1.0 and 0.8 were chosen to represent stoichiometric and lean combustion. The
temperature was measured by a Pt/Pt13%Rh thermocouple with wire diameter of 0. 2 mm,
and no correction for radiation was made. At each point of a cross section, measurements
were conducted for three times.
As seen in Figure 11, an M-shaped temperature profile was found at the cross Section
X1 for both the stoichiometric and lean combustion conditions. The temperature was
relatively lower when approaching the inner cylinder wall. This feature is beneficial for
preventing the burner liner from cracking. And the temperature gradually increased to its
peak near the region of the flame front, then decreased slightly when approaching the
center region compared to that in the flame front.
A platform shaped temperature profile of the burned gas was observed at the cross section
of X2 (Figure 11), and an even temperature distribution zone was found in the wide central
region of 5mm<D< 45 mm. In the tubular flame of multi-stage burner, the gas with lower
density and higher temperature stays in the center region while the gas with large density and
lower temperature moves toward the wall side due to centrifugal force. It was reported that
temperature distribution at the exit showed obvious variation spatially for a conventional
rapidly-mixed type burner (Ishizuka et al. 2009). It is well known that turbine blade life relies
heavily on the uniformity of temperature distribution in the combustor efflux gases (Lefebvre
2000). In other words, the high peak temperature could be the major cause for the crack of
blades and burner liner. Compared to the conventional rapidly-mixed type burner, the multi-
stage tubular flame burner shows advantages in protecting the blades of the gas turbine due to
even temperature profiles at the burner outlet.
Figure 11. Profiles of flame temperature of multi-stage tubular flame burner at X1 and X2 cross
sections at different equivalent ratios.
COMBUSTION SCIENCE AND TECHNOLOGY 13
Conclusion
A novel tubular flame combustion system has been proposed for liquid-fueled gas turbine.
This system consists of a flash-boiling evaporator and a tubular flame burner with special
multi-stage structure. A homogenous fuel-rich mixture was formed inside evaporator
before entering the burner from the 5 mm-slit and the secondary air was injected through
the 30 mm-slit, which was arranged parallel to the former. This combustion system has
the following features due to its unique structure of multi-stage slits and the flash boiling
evaporator:
●The test liquid fuel (ethanol) spray can be completely vaporized into gaseous fuel in
the specially designed flash-boiling evaporator in 1.5 ms at injection pressure of
5MPa, fuel flow rate of 230 mg/s (corresponding to a thermal power of 7.9kW), when
both the liquid fuel and the primary air were preheated to 473K. This has been
proved by the optical diagnostics.
●Astabletubularflame can be obtained at global equivalent ratios (Ф
g
) from 0.23 to 4.6.
●The multi-stage tubular flame combustion system has been proved to have the ability
to effectively prevent flash back at much wider range of output power compared to
the premixed type of tubular flame burner when fixing Ф
e
= 3.0 for the fuel-rich
premixture.
●Platform-shaped temperature profiles were found at the burner outlet, with an even
distribution in the wide central region and a relatively lower temperature near the
cylinder wall. These features are beneficial to preventing the blades and the liner wall
of gas turbines from cracking.
By incorporating the flash-boiling atomization evaporator and the multi-stage inlet slits
into a tubular flame burner, two-stage fuel/air mixing can be realized, and the liquid fuel
can be burned in a more stable and safer way, because the liquid fuel can be gasified
efficiently, the risk of auto-ignition and flash-back can be reduced significantly, and the
extinction limits can be expanded several times. Finally, the new combustion system has
also shown great potential in fuel flexibility.
Funding
The research was sponsored by Intergovernmental international cooperation in science and tech-
nology innovation (NO. 2016YFE0127500) and National Natural Science Foundation of China (No.
91741130).
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