The effect of main stage flow velocity on thermoacoustic instability of

stratified swirl burner Meng Han 1 National Key Laboratory of Science and Technology on Aero-Engine, Research Institute of Aero- Engine, Beihang University, Beijing,100191, P. R. China Xiao Han 2 * National Key Laboratory of Science and Technology on Aero-Engine, Research Institute of Aero- Engine, Beihang University, Beijing,100191, P. R. China Jianchen Wang 3 National Key Laboratory of Science and Technology on Aero-Engine, Research Institute of Aero- Engine, Beihang University, Beijing,100191, P. R. China Yuzhen Lin 4 National Key Laboratory of Science and Technology on Aero-Engine, Research Institute of Aero- Engine, Beihang University, Beijing,100191, P. R. China

ABSTRACT In this paper, two group experiments were conducted to study the effect of the main stage flow velocity on thermoacoustic instability and flame macrostructure. The experiments were carried out under atmosphere condition, and namely the pilot stage flame mode and the stratified swirl flame mode. The experimental results show that in the pilot stage flame mode, the thermoacoustic oscilla- tion amplitude remains constant (around 1000 Pa) with the decrease of the main stage flow velocity. But when the velocity is zero (without main stage air), the thermoacoustic instability disappeared. In the stratified swirl flame mode, the amplitude of thermoacoustic instability is the largest (around 300 Pa) when the main stage flow velocity is 6.2 m/s, and the amplitude is slightly smaller in other work- ing conditions. The time-averaged flame shape under the two flame modes is recorded and discussed. This paper highlights the effect of interactions between the pilot stage flame and main stage air or flame on thermoacoustic instability.

1. INTRODUCTION

Lean premixed prevaporized (LPP) combustors are capable to reduce NOx emissions but are often

affected by combustion instabilities generated by mutual interactions between pressure fluctuations

( 𝑝′ ) and heat release rate oscillations ( 𝑞ሶ′ ) produced by the flame[1]. Based on the Rayleigh criterion

[2], the interaction is constructive only when 𝑝′ and 𝑞ሶ′ are in phase, i.e. the phase difference is within

1 hanmeng@buaa.edu.cn

2 han_xiao@buaa.edu.cn

3 wangjianchen@buaa.edu.cn

4 linyuzhen@buaa.edu.cn

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90 degrees. Combustion instabilities are undesirable and sometimes dangerous, which should be

avoided or controlled [3].

To ensure a wide stable operating range, LPP combustors often feature staged combustion with a

central pilot flame surrounded by an annular main flame. The interaction of the pilot and main flames

is complex and often show stratification [4]. Figure 1(a) shows a typical structure for a concentrically

staged combustor, which consists of pilot and main swirlers separated by a lip structure. The resulting

flow field features three major shear layers, the inner shear layer (ISL), lip shear layer (LSL), and

outer shear layer (OSL). Swirling flames are usually stabilised inside the shear layers with relatively

low local flow velocity.

In recent decades, stratified swirl flame has received increasing concerns. Research group in Cam-

bridge has conducted a series study of undivided stratified swirl flame, including flame shape, flame

response, and nonlinear behaviour [5,6].

Kim and Hochgreb [7] observed that in an undivided stratified swirl burner, SR directly controls

the flame shape. The flame lifts off when SR equals 2 and is attached to the burner when SR equals

0.5 and 1. SR was also found to influence also the flame angle. An expanded flame was obtained with

a leaner inner stream. Chong et al. [8] examined the effect of ASR on the flame shape and found that

larger inner flow increases the flame intensity, which results in a longer flame and a larger reaction

zone. Sweeney et al. [9] conducted delicate measurements of flame structures in a stratified swirl

burner, but did not study thermoacoustic properties. However, the above three configurations do not

feature a separation lip between the inner and outer streams, which is essential to reduce NOx emis-

sions [10]. Li et al. [17] tested a single swirl burner with a non-swirled pilot, and found that the

presence of heat release in the inner shear layer leads to a destabilization of the global flame. The

flow and reaction fields of a full-scale industrial LPP burner fuelled with kerosene have been meas-

ured in Ref.12. The stratified flame structure was shown by flame surface density distribution, with

separate pilot and main flames ahead of merging downstream of the lip structure. Combustion insta-

bilities exhibiting a Helmholtz mode were observed [4], but the few operating conditions limited the

variety of observed flame shapes. A recent work of the author [13] proposed a novel BASIS burner

with dual-swirler structure. Based on this configuration, we illustrated the importance of equivalence

ratio combination of the pilot and main flame. Among the three observed flame types, the stratified

flame features thermoacoustic stability [13]. Large eddy simulation (LES) using incompressible

solver is then proved to be able to capture the time-averaged flame shape and flame dynamics in

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BASIS burner [14]. In addition, thermoacoustic instability of different premixed modes in the pilot

and main stage have been investigated [15].

In practice, the pilot and main flame often work with different equivalence ratio. More common,

with the pilot flame richer than the main flame, aiming to suppress the unsteady combustion and

reduce emissions. The interaction between the two flames is complex and have significant effects on

combustion performances. However, although flame interaction has attracted plenty of studies, but

most of them focus on transverse flame interaction in multi-injector combustors. These work is sum-

marized by a recent review paper [16]. On the other hand, flame interactions within the stratified

swirl flame is hardly been studied. Kim et at. has reported a nonlinear interaction in a stratified swirl

burner [17]. They stated the different oscillating frequency of the pilot and main flame leads to a beat

behaviour of thermoacoustic instabilities. But the burner used in this study does not feature a lip

structure. The two flame sheets merge before the exit of the burner.

In the present study, we aim to investigate the effects of main stage flow velocity on thermoacoustic

instability and flame interactions within stratified swirl flames.

2. EXPERIMENTAL SETUP AND CONDITIONS

The burner employed in this work is the Beihang Axial Swirler Independently-Stratified (BASIS)

burner, sketched in Fig. 1(a) with detailed geometrical dimensions. This burner was developed at

Beihang University aiming to represent the main aero-thermodynamic characteristics of typical low

emission combustors. The burner features a centrally staged structure with a pilot and main stage,

with swirl numbers estimated to be 0.88 and 0.56, respectively. Detailed information can be found in

Ref. [14].

Figure 2(b) shows the schematic of the test rig. The burner opens to a quartz tube of 100 mm in

diameter. Two kinds of a quartz tube with different length is chosen. A shorter one with a length of

200 mm is chosen only to measure the time-averaged flame shape, without triggering combustion

instabilities. A longer one with a length of 800 mm is used for other cases, as shown in Fig. 1. This

length is chosen to achieve a balance of the onset of combustion instabilities and the feasibility of

experiments. The burner is operated at atmospheric pressure with fully premixed mixtures of me-

thane-air. The mixing of fuel and oxidizer is achieved in premixing units located upstream of the two

swirlers. Figure 1 Detailed geometry of the BASIS combustor (a) and schematic of the test rig (b)

(not to scale). ISL stands for the inner shear layer originating from the pilot stage, while LSL and

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OSL stand for the lip shear layer and the main shear layer of the main stage, respectively. All dimen-

sions are in millimeter.

(a)

(b)

Figure 1: Detailed geometry of the BASIS combustor (a) and schematic of the test rig (b) (not to

scale). ISL stands for the inner shear layer originating from the pilot stage, while LSL and OSL

stand for the lip shear layer and the main shear layer of the main stage, respectively. All dimensions

90 120 40 50 30, 50 200 200350 fame ; rH 3 a hal = $6 ‘main fame x T Leer S bresure baal ‘261 |o100 t Tx pilot Lt stage main stage rmodture misture ‘quartz tube

are in millimeter.

In order to achieve the target stratification ratio (SR) and air split ratio (ASR), fuel and oxidizer

mass flow rates are continuously monitored before entering the premixing units. Air mass flow rates

are regulated by means of standard orifice flowmeters with an accuracy of 2 %. Mass Flow Control-

lers (Sevenstar, CS200) with an accuracy of 1 % are used for the fuel lines. In both channels, the

mixture is maintained at a constant temperature equal to 310 K, continuously monitored by a K-type

thermocouple. The time-averaged flame shapes are also measured by a digital single-lens reflex

(DSLR) camera with f/10 aperture and an exposure time of 1/2 s. A photomultiplier (PMT) of Hama-

matsu, H9306, is used to measure the global CH* chemiluminescence emission as representative of

heat release rate [18]. The above three optical instruments are equipped with CH* filters (430±5 nm).

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To monitor the pressure fluctuation ( 𝑝′ ), 5 dynamic pressure sensors (PCB, 112A22) are installed

along with the test rig as shown in Fig. 1(b). Note that S1 and S2 are flush mounted to the main

channel while S3-S5 are installed with semi-infinite tubes due to the overheated flame tube. All data

are collected by a DAQ system (National Instruments, NI9215) at a sampling frequency of 20 kHz.

A total of 100,000 data points is taken for each condition.

Two groups of operating conditions are chosen to exam to interactions between the pilot and main

stage, as listed in Tab. 1. Case group A is operated with only the pilot stage fueled with methane, as

∅ 𝑝𝑖𝑙𝑜𝑡 =0.85 and ∅ 𝑚𝑎𝑖𝑛 =0. While case group B is operated with both the pilot and main stage fueled

with methane, forming a stratified flame mode with ∅ 𝑝𝑖𝑙𝑜𝑡 =0.85 and ∅ 𝑚𝑎𝑖𝑛 =0.57. For both group A

and B, the velocity of the main stage 𝑉 𝑚𝑎𝑖𝑛 ranges from 11.0~0 m/s while keeping the 𝑉 𝑝𝑖𝑙𝑜𝑡 constant

at 6.5 m/s.

Tabl e 1 Test cases with different inlet velocity of the main s tage.

𝑚ሶ 𝑝𝑖𝑙𝑜𝑡

𝑉 𝑝𝑖𝑙𝑜𝑡

Case 𝑚ሶ 𝑡𝑜𝑡𝑎𝑙

𝑚ሶ 𝑚𝑎𝑖𝑛

𝑉 𝑚𝑎𝑖𝑛

(g/s)

(g/s)

(m/s) A-1 2.2 2.2 6.5 0 0 A-2 8.2 2.2 6.5 6 3.7 A-3 12.2 2.2 6.5 10 6.2 A-4 16.2 2.2 6.5 14 8.7 A-5 20 2.2 6.5 17.8 11 B-1 8.2 2.2 6.5 6 3.7 B-2 12.2 2.2 6.5 10 6.2 B-3 16.2 2.2 6.5 14 8.7 B-4 20 2.2 6.5 17.8 11

(g/s)

(m/s)

3. RESULTS AND DISCUSSIONS

The experimental results are shown and discussed in this section. The section has been divided into

pilot flame mode and stratified flame mode.

3.1. Pilot Flame Mode

The amplitudes and frequencies of combustion instabilities in group A are shown as Fig. 2. With

the decrease of 𝑉 𝑚𝑎𝑖𝑛 , the amplitude of pressure fluctuation remains constant at around 1000 Pa until

𝑉 𝑚𝑎𝑖𝑛 =3.7m/s. When the main stage is closed, i.e. 𝑉 𝑚𝑎𝑖𝑛 =0 m/s, the amplitude of pressure fluctuation

drops suddenly to the level of 40 Pa, featuring thermoacoustically stable. Among all the 𝑉 𝑚𝑎𝑖𝑛 above,

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the frequency is focusing at around 160 Hz, which is close to the quarter wave mode of the flame

tube, which has been predicted by the OSCILLOS.

Figure 3 shows that the time-series of pressure fluctuation ( 𝑝 ′ ) with different main stage velocity.

It could be found that the value of peak-peak jump to more than 1000 Pa, when the main stage velocity

increases from 0 m/s to 3.7 m/s. Continue to increase the main stage velocity to 6.2 m/s, the peak-

peak value of 𝑝 ′ reach to maximum (around 1500 Pa). Moreover, with the increase of main stage

velocity from 6.2 m/s to 11m/s, the peak-peak value decrease. From the time-frequency (TF) spectra

in figure 3, which is obtained by synchrosqueezing transform (SST) [15,19]. It is found that the do-

main frequency of thermoacoustic instability is around 160 Hz from the TF spectra of SST. And there

are around 320 Hz and 480 Hz, which are believed to be the harmonic frequency of 160 Hz.

g Aunplitude We 8 0 2 4 6 8 0 % Velocity of main stage (m/s) 2 4 6 8 Caley al tain eae

Figure 2: Dependence of pressure fluctuation on the bulk velocity of the main stage with only pi-

lot flame mode.

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Figure 3: Time series (top) and time-frequency spectra (bottom) of pressure fluctuation ( 𝑝 ′ ) with

different main stage velocity in pilot flame mode.

Figure 4 reports a further analysis of the pressure signals recorded by the S3 sensor for the case A1

and A5, choosing to represent the results in Fig. 2. In each subfigure, the time series are shown in the

top half while the spectrum (left) and phase space reconstruction (right) are shown below. The phase

space is reconstructed following the method proposed in [20]. Firstly, an embedding dimension order

of 3 is set for this study, trying to avoid the trajectories collapsing to show them clearly. Secondly,

the time lag τ is calculated to shift the signals apart. The auto-correlation function of the original

′ is pre-plotted and then the time where the auto-correlation value drops to zero for the first

signal 𝑃 3

time is chosen as τ. The same time lag of τ = 1.55 ms is chosen for cases A1 and A5. The differences

are clear.

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(a) (b)

Figure 4: Comparison of pressure fluctuations of S3 sensor for cases A1 (a) and A5 (b). The pres-

sure signal is shown in the top half of each subfigure, while the spectrum and the phase space tra-

jectory are shown in the bottom left and right, respectively.

aT ee

Figure 5: Time-averaged shape of pilot flame with the varying bulk velocity of the main stage

The time-averaged shapes of pilot flames are shown as Fig. 5, which are recorded by a CH * filtered

camera with long exposure time and then processed by Abel deconvolution method. It is found that

with the decrease of 𝑉 𝑚𝑎𝑖𝑛 , the flame angle and flame length reduce slightly, reaching the minimum

at 𝑉 𝑚𝑎𝑖𝑛 = 3.7 𝑚/𝑠 . However, with 𝑉 𝑚𝑎𝑖𝑛 = 0 𝑚/𝑠 , the flame becomes longer and expands more,

features clear flame sheets. The sudden transition from unstable to stable combustion is possibly due

to the mixing of the main air and the pilot flame, which induces local stretch and decreases the local

equivalence ratio, leading to unsteady flame and large heat release rate.

3.2. Stratified Flame Mode

The results of combustion instabilities in group B are shown as Fig. 5, processed with the same

methods above. Unlike group A, as 𝑉 𝑚𝑎𝑖𝑛 decreases from 11 m/s to 3.7 m/s, the amplitude increases

significantly from ~100 Pa to ~350 Pa. Further decrease of 𝑉 𝑚𝑎𝑖𝑛 leads to a lower amplitude at ~260

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Pa. This result is quite interesting, indicating that in stratified flame mode, i.e. both the pilot flame

and main flame work, the match of the bulk velocity of the two stages could be a key parameter to

control the stability of combustion system. Similarly, the peak frequency of each case focuses at

around 200 Hz, higher than 160 Hz in group A. This is caused by higher temperature distribution in

the flame tube due to the higher thermal power.

02 4 6 8 Velocity of main stage (m/s) "Weadey cf main ans

Figure 6: Dependence of pressure fluctuation on the bulk velocity of the main stage with strati-

fied flame mode.

Figure 7: Time series (top) and time-frequency spectra (bottom) of pressure fluctuation ( 𝑝 ′ ) with

different main stage velocity in stratified flame mode.

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See.

(a) (b) Figure 8: Comparison of pressure fluctuations of S3 sensor for cases B2 (a) and B4 (b). The pres- sure signal is shown in the top half of each subfigure, while the spectrum and the phase space tra-

jectory are shown in the bottom left and right, respectively.

The time-series of pressure fluctuation ( 𝑝 ′ ) with different main stage velocity are shown in figure

6. It could be found that the value of peak-peak all below 500 Pa with different main stage velocity.

In addition, the frequency of thermoacoustic instability from TF spectra all remain around 198 Hz.

; a P(t

Figure 7 shows further analysis of case B2 and B4, following the same method stated in the previous

section. It is found that case B4 shows lower amplitude at 120 Pa, but with unsteady amplitude from

the time series plot. This is also confirmed by the phase reconstruction. A limit cycle structure is

found with a wide band, showing non-linear noise from the pressure signal. However, the amplitude

of case B2 is 300 Pa, with a nearly constant amplitude. As a result, a larger limit cycle with a narrow

band. This implies a more unstable thermoacoustic instability is found for case B4.

Figure 9: Time-averaged shape of stratified flame with the varying bulk velocity of the main

stage.

The time-averaged shapes of stratified flames are shown in Fig. 7. It is found that with the decrease

of 𝑉 𝑚𝑎𝑖𝑛 , the flame angle remains nearly constant but the stratification of the two flame sheets be-

comes clearer with 𝑉 𝑚𝑎𝑖𝑛 = 6.2 𝑚/𝑠 and 𝑉 𝑚𝑎𝑖𝑛 = 3.7 𝑚/𝑠 . Considering the acceleration effect of

the pilot stage exit, the axial velocity of pilot flame ( 𝑉 𝑝,f ) is about 2 times of its bulk velocity, i.e.

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around 𝑉 𝑝,f = 12𝑚/𝑠 . It is obvious that the minimum amplitude is found with 𝑉 𝑚𝑎𝑖𝑛 = 11 m/s ,

which is most close to the 𝑉 𝑝,f . With higher 𝑉 𝑚𝑎𝑖𝑛 , more air is entering through the main stage. There-

fore, the global flame is dominated by the main stage, leading to a stronger flame interaction. The

flame is similar to a single swirl flame. While with lower 𝑉 𝑚𝑎𝑖𝑛 , the two flame is kind of decoupled.

The pilot flame and the main flame can have different flame dynamics, which intensified the flame

response to the acoustic perturbation.

3.3. Thermoacoustic Network Analysis

To further elucidate the nature of different thermoacoustic instability modes observed in figure3

and figure7. An open-source low-order thermoacoustic network solver OSCILOS [21] are used to

conduct the thermoacoustic network analysis. The network of the thermoacoustic system in this work

could be considered as the combination of a flame tube and a plenum shown by the test rig in Fig-

ure1(b). The simplified elements of BASIS burner in test rig are shown in Figure10. In simplified

elements of the thermoacoustic network include the plenum and flame tube. For the plenum duct,

both pilot and main stage are considered through the same cross-sectional area.

Figure10: Simplified elements of the thermoacoustic network

Table 2 Predicted frequencies of test cases with different inlet velocity of the main stage.

𝑉 𝑚𝑎𝑖𝑛 [m/s] 0 3.7 6.2 8.7 11

Pilot Flame Mode [Hz] 212.7 161 147 139 132

Stratified Flame Mode [Hz] \ 200.5 200.5 200.4 200.4

Table2 summarizes the predicted frequencies of the first longitude mode for both pilot and strati-

fied flame mode. In pilot flame mode, the frequency of 0 m/s in main stage have a large deviation

with experiment. However, the others of only pilot flame are consistent with experimental results in

basically. The error between prediction and experimental result is caused by main stage air, which

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changes the boundary conditions of thermoacoustic and the responds of flame. At stratified flame

mode, the frequencies with different main stage velocity are predicted by OSCILOS, which better

matches with the experimental results of stratified flame mode.

4. CONCLUSIONS

In the present paper, the flame topology and thermoacoustic instability in a stratified swirl flame

are investigated experimentally, especially on the effects of main stage flow velocity. A novel burner

is used with premixed methane as fuel. Time-averaged flame shapes and combustion instabilities are

measured and analyzed. Lower order thermoacoustic network analysis also is used to elucidate the

nature of different thermoacoustic instability modes observed in experiments.

In the pilot flame mode, only the pilot stage is fueled with premixed methane. It is found that with

the decrease of the main stage flow velocity, the amplitude of pressure fluctuation remains constant

at around 1000 Pa until the main stage is closed (main stage velocity is 0 m/s). The amplitude of

pressure fluctuation drops suddenly to the level of 40 Pa when 𝑉 𝑚𝑎𝑖𝑛 = 0 𝑚/𝑠 , featuring thermo-

acoustically stable. The flame angle and flame length reduce slightly before the main stage is closed.

And the frequencies of the system with different main stage velocity are consistent, which is around

160 Hz. The macrostructure of pilot flame has an obvious difference.

Contrary to the pilot flame mode, in the stratified flame mode, the amplitude reaches the peak at

300 Pa when 𝑉 𝑚𝑎𝑖𝑛 = 6.2 𝑚/𝑠 , while is lower at other velocities. Flame angle remains unchanged

with varying velocity. But stratification of flame become clearer due to the decreasing main stage

velocity. Moreover, the frequencies with different 𝑉 𝑚𝑎𝑖𝑛 are around 200Hz.

A low-order thermoacoustic network solver was used to explain the nature of different thermo-

acoustic instability modes. In pilot flame mode, the frequency of predicted are slightly different with

experimental result. But, the stratified flame mode have a great consistent between predicted and

experimental result.

To conclude, the combustion instabilities of the stratified swirl flame is found to be quite sensitive

to the velocity of the main stage. This indicates that the complexity of the flame interaction in strati-

fied swirl flame. Further analysis of flame dynamics and flow filed will be shown in the full-length

context.

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5. ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China

(52106128) and the National Science and Technology Major Project (2017-III-0004-0028).

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