Experimental study of compact swirl flames with lean premixed CH4/H2/air mixtures at stable and near blow-off conditions

doi:10.1016/j.expthermflusci.2020.110294

Experimental Thermal and Fluid Science

Volume 122, 1 April 2021, 110294

https://doi.org/10.1016/j.expthermflusci.2020.110294 Get rights and content

Highlights

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Flame-flow dynamics of compact lean premixed swirl flames are investigated.
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Hydrogen stabilized flames are studied with up to 80% H₂ fraction.
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The weakened root burning intensity is identified to be the onset of blow-off.
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The flame root is unstable due to the straining caused by shear deformation.
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The inner shear layer vortices are anticipated to promote the blow-off.

Abstract

An experimental study on the compact lean premixed flames stabilized on a bluff-body and swirl burner is performed. The flames are featured with a strengthened Inner Recirculation Zone (IRZ). The objective is to identify the critical causes of blow-off near the lean limit. Meanwhile, the effects of hydrogen addition on the flame stabilization are also examined. Premixed CH₄/H₂/air mixtures are adopted with varied hydrogen fractions up to 80%. Flames far from and close to the blow-off conditions are both obtained for comparison. Simultaneous PIV/OH-PLIF measurements are performed, and flow-flame dynamics are analyzed. Results show that the flow straining near the flame root is dominant to induce the root extinction and initiate the blow-off. The strain is observed to mainly result from the shear deformation. The weakened root burning intensity is demonstrated with the CH* chemiluminescence. With hydrogen addition, the flame root is strengthened due to the enhanced resistance to the strain. The increased burning velocity and temperature with hydrogen are also beneficial for the flame stabilization. The inner shear layer vortices are anticipated to promote the blow-off by convecting cold unburned mixture into the IRZ.

Introduction

Lean premixed combustion is promising to reduce the NO_x emission, whereas its application is hindered by the blow-off event in the vicinity of lean limits [1], [2], [3], [4], [5]. In this study, the near blow-off dynamics of relatively compact lean premixed flames are examined. The compact flames are generally attained with a strengthened Inner Recirculation Zone (IRZ). Such flames are short relative to the flame holder as addressed by Dawson et al. [6], and the flame stabilization is more susceptible to the IRZ flame.

Blow-off of the compact and short flames was generally attributed to the cooling of the IRZ flame. Dawson and Kariuki et al. [6], [7], [8] suggested that large amounts of cold backflow in the downstream region are responsible for the cooling. The numerical results of Hodzic et al. [9] were also in favor of this idea. Chaudhuri et al. [10] and Chowdhury et al. [11] proposed that the cooling can be induced by the local extinction in shear layers, which results in leakage of the cold and unburned mixture into the IRZ. The extinction was also observed around the flame root and was proposed to cause the flame detachment and blow-off [12], [13], [14], [15]. A comprehensive understanding of these proposed mechanisms is essential. In our previous study, the weakened flame root was identified to be the onset of IRZ cooling and blow-off, after comparing the burning intensity at conditions far from and near the lean limit [16]. However, the related flow-flame dynamics were not investigated. A further demonstration of the flow straining, vortices and their influence on the flame stabilization is performed in the present study.

To attain more stable flames, one solution is to introduce reactive fuels into the mixtures such as hydrogen (H₂). Previous studies have revealed that with H₂ addition, the blow-off limits shifted to lower equivalence ratios [1], [17], [18], [19], [20]. This effect was attributed to the increased laminar burning velocity [21], higher adiabatic flame temperature [22], and enhanced resistance to the strain-induced extinction with H₂ addition [1], [18], [23]. The high diffusivity of hydrogen was also observed to stabilize the flame recently by Vance et al. [24] and Jiménez et al. [25]. In this study, stable flames are also conducted with H₂ addition. The purpose is to investigate how the hydrogen will affect the flame stabilization, especially by examining their influence on the flow-flame coupling. Lean premixed CH₄/H₂/air mixtures with hydrogen fractions up to 80% are adopted in the present study. The hydrogen fraction in previous studies was mostly less than 50% [17], [18], [24], [25], [26], [27]. As indicated by Day et al. [28], the next-generation combustion systems may have to operate with alternate fuels that contain even 90% H₂. The study of highly hydrogen-enriched flames is expected to be significant.

The flame structure is measured by Planar Laser-Induced Fluorescence (PLIF) of hydroxyl (OH). Flamelet is extracted with the OH-PLIF results. The flow field is captured using the Particle Image Velocimetry (PIV). Importantly, simultaneous measurements of the OH-PLIF and PIV are conducted to investigate the flame and flow dynamics at stable and near blow-off conditions. The experimental methods will be introduced in Section 2 and the results will be analyzed in Section 3.

Section snippets

Burner and mixture properties

The burner schematic is shown in Fig. 1, which has been introduced before in detail [16]. To reinforce the IRZ, the combination of a bluff-body and a swirler was adopted similar to [4], [29], [30], [31]. The burner outlet diameter is 35 mm. The bluff-body is similar to that of Dawson et al. [6] with a roof diameter of 25 mm, and the blockage ratio is about 51%. The swirler is imparted by eight 45° vanes with a swirl number of about 0.7 [16], [29]. The swirler is installed 50 mm upstream from

Mean flow fields

The flame brushes (〈c〉 = 0.1, 0.5 and 0.9), mean velocity vector fields, and contour fields of the mean axial velocity (v) are superimposed and shown in Fig. 6. The mean non-reacting flow field is also displayed, where a smaller IRZ can be observed compared to the reacting flow fields. A dash-dot line at y = 25 mm above the bluff-body is displayed in Fig. 6. This line is also situated at the end of the non-reacting IRZ. Blow-off is drawing near from A1 to A3 with decreasing ϕ. It is seen that

Conclusions

Compact lean premixed CH₄/H₂/air flames stabilized on a bluff-body and swirl burner were investigated at stable and near blow-off conditions. The effects of hydrogen addition on the flame stabilization were studied with up to 80% H₂ fraction. The burning intensity was obtained with the CH* chemiluminescence. Simultaneous PIV and OH-PLIF measurements were performed to investigate the flame and flow dynamics. The effects of flow straining and shear layer vortices on the lean flame stabilization

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study is supported by National Natural Science Foundation of China (Nos. 91841302 and 51776164).

References (46)

S. Taamallah et al.
Fuel flexibility, stability and emissions in premixed hydrogen-rich gas turbine combustion: Technology, fundamentals, and numerical simulations
Appl. Energy
(2015)
Y. Huang et al.
Dynamics and stability of lean-premixed swirl-stabilized combustion
Prog. Energy Combust. Sci.
(2009)
C. Karagiannaki et al.
A comparison of the characteristics of disk stabilized lean propane flames operated under premixed or stratified inlet mixture conditions
Exp. Therm Fluid Sci.
(2014)
R. Ciardiello et al.
Experimental assessment of the lean blow-off in a fully premixed annular combustor
Exp. Therm Fluid Sci.
(2020)
J. Runyon et al.
Lean methane flame stability in a premixed generic swirl burner: Isothermal flow and atmospheric combustion characterization
Exp. Therm Fluid Sci.
(2018)
J.R. Dawson et al.
Visualization of blow-off events in bluff-body stabilized turbulent premixed flames
Proc. Combust. Inst.
(2011)
J. Kariuki et al.
Measurements in turbulent premixed bluff body flames close to blow-off
Combust. Flame
(2012)
J. Kariuki et al.
Heat release imaging in turbulent premixed methane–air flames close to blow-off
Proc. Combust. Inst.
(2015)
E. Hodzic et al.
Large eddy simulation of bluff body flames close to blow-off using an Eulerian stochastic field method
Combust. Flame
(2017)
S. Chaudhuri et al.
Blowoff dynamics of bluff body stabilized turbulent premixed flames
Combust. Flame
(2010)

M. Stöhr et al.

Dynamics of lean blowout of a swirl-stabilized flame in a gas turbine model combustor

Proc. Combust. Inst.

(2011)

W. Zhang et al.

Measurements on flame structure of bluff body and swirl stabilized premixed flames close to blow-off

Exp. Therm Fluid Sci.

(2019)

M. Emadi et al.

Flame structure changes resulting from hydrogen-enrichment and pressurization for low-swirl premixed methane–air flames

Int. J. Hydrogen Energy

(2012)

R. Schefer

Hydrogen enrichment for improved lean flame stability

Int. J. Hydrogen Energy

(2003)

P. Strakey et al.

Investigation of the effects of hydrogen addition on lean extinction in a swirl stabilized combustor

Proc. Combust. Inst.

(2007)

V. Di Sarli et al.

Laminar burning velocity of hydrogen–methane/air premixed flames

Int. J. Hydrogen Energy

(2007)

S.K. Mishra et al.

Adiabatic flame temperature of hydrogen in combination with gaseous fuels

Int. J. Hydrogen Energy

(1989)

G.S. Jackson et al.

Influence of H2 on the response of lean premixed CH4 flames to high strained flows

Combust. Flame

(2003)

F.H. Vance et al.

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

Proc. Combust. Inst.

(2019)

C. Jiménez et al.

Stabilization of ultra-lean hydrogen enriched inverted flames behind a bluff–body and the phenomenon of anomalous blow–off

Combust. Flame

(2018)

M. Li et al.

Experimental study of hydrogen addition effects on a swirl-stabilized methane-air flame

Energies

(2017)

H. Kim et al.

Flame characteristics of hydrogen-enriched methane–air premixed swirling flames

Int. J. Hydrogen Energy

(2009)

M. Day et al.

A combined computational and experimental characterization of lean premixed turbulent low swirl laboratory flames

Combust. Flame

(2012)

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Experimental study of compact swirl flames with lean premixed CH4/H2/air mixtures at stable and near blow-off conditions

Highlights

Abstract

Introduction

Section snippets

Burner and mixture properties

Mean flow fields

Conclusions

Declaration of Competing Interest

Acknowledgments

Appl. Energy

Prog. Energy Combust. Sci.

Exp. Therm Fluid Sci.

Exp. Therm Fluid Sci.

Exp. Therm Fluid Sci.

Proc. Combust. Inst.

Combust. Flame

Proc. Combust. Inst.

Combust. Flame

Combust. Flame

Proc. Combust. Inst.

Exp. Therm Fluid Sci.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Proc. Combust. Inst.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Combust. Flame

Proc. Combust. Inst.

Combust. Flame

Energies

Int. J. Hydrogen Energy

Combust. Flame

Experimental study of compact swirl flames with lean premixed CH₄/H₂/air mixtures at stable and near blow-off conditions