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Characterization of PF flames under different swirl conditions basedon visualization systems

A. González-Cencerrado ⇑, A. Gil, B. Peña

Centre of Research for Energy Resources and Consumption (CIRCE), C/Mariano Esquillor Gómez, 15, 50018 Zaragoza, Spain

h i g h l i g h t s

Influence of swirl number on different combustion flames has been studied in detail. Flame luminous information is captured by means of a CCD based visualization system.

Relevant variations concerning dynamical and physical flame features are presented.

Spatial–temporal characterization of flames highlights the effect of swirl increase.

A new approach to frequency analysis of semi-industrial PF flames is performed.

a r t i c l e i n f o

Article history:

Received 30 September 2012

Received in revised form 5 December 2012

Accepted 23 May 2013

Available online 11 June 2013

Keywords:

SwirlImage processing

CCD camera

Pulverized coal

Gas flame

a b s t r a c t

The complex phenomena underlying reacting flows are at present an active research topic for improving

efficiency and control of industrial combustion processes. The present study assesses the use of a reliable

and low cost visualization system based on CCD (charged couple device) technology for the identification

of different flame states as a function of swirl number. The experiments were performed in a 500 kWthpulverized coal (PF) swirl burner. The visualization system was previously used for similar analysis of

an atmospheric gas swirl burner of 50 kWth. Both flames were analyzed under low and high swirl condi-

tions in order to identify possible common features concerning swirl flames. The analysis procedure pro-

posed in this work extracts different statistical and spectral parameters from the intensity of radiation

stored by each CCD element. The representation of flame features as two-dimensional distributions

has led to the identification of typical structures in swirl flows and has allowed us to detect changes

in flame structure as swirl number increases. Besides, the quantification of aforementioned parameters

has led to relevant findings concerning frequency analysis and flame stability as a function of swirl. Spe-

cifically, results show that the characteristic frequency in terms of flame flicker is sensible to changes in

swirl conditions.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Pulverized fuel (PF) flames from swirl burners involve many

influential variables and exhibit large spatial and temporal varia-tions in flow dynamics and heat release. In spite of the vast amount

of related literature, the complexity of reacting flows under PF

flames demands a deeper understanding based on reliable experi-

mental data at industrial scale. Among the potential techniques

currently under research, flame visualization-based systems pro-

vide a non-intrusive alternative for the analysis of flame structure.

In this context, advanced technologies such as Particle Image

Velocimetry (PIV) or Laser Doppler Anemometry (LDA) have been

widely used in recent years [1–4]. These laser-based diagnostic

methods provide very accurate information from the flame in

terms of its internal structure, flow oscillations or associated insta-

bilities. Nevertheless, their applicability is mainly restricted to very

particular environments: free of soot, with no presence of multi-component fuels or multiple phases and under atmospheric pres-

sure [5]. In addition to this, large physical scales and the conse-

quent reduced optical access to the flame, make very difficult

their practical implementation on industrial combustion systems.

There are numerous experimental analyses of gaseous swirl

flames that usually include reactive conditions and that lead to

the spatial characterization of flames. Table 1 includes a summary

of these experiences indicating remarkable details and measure-

ments performed in each case. However, literature concerning

experimental analysis of pulverized fuel flames including reactive

conditions and leading to spatial characterization is limited to a

few studies [6,7].

0016-2361/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.05.077

⇑ Corresponding author. Tel.: +34 976 762 562; fax: +34 976 762 616.

E-mail address: [email protected] (A. González-Cencerrado).

Fuel 113 (2013) 798–809

Contents lists available at SciVerse ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l

http://dx.doi.org/10.1016/j.fuel.2013.05.077mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2013.05.077http://www.sciencedirect.com/science/journal/00162361http://www.elsevier.com/locate/fuelhttp://www.elsevier.com/locate/fuelhttp://www.sciencedirect.com/science/journal/00162361http://dx.doi.org/10.1016/j.fuel.2013.05.077mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2013.05.077http://crossmark.crossref.org/dialog/?doi=10.1016/j.fuel.2013.05.077&domain=pdf

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CCD based visualization systems offer a cost-effective alterna-

tive for monitoring and characterization of combustion flames gi-

ven its robustness and simplicity of design [8,9]. Different studies

performed through this techniques have provided valuable infor-

mation concerning flame temperature and emissivity [10–12], sootconcentration [13,14] or quantitative measurements of radical

emissions [15]. Vision-based techniques have been also used for

the analysis and assessment of biomass co-firing at industrial

scales [9,16] finding interesting results that suggest different im-

pacts on flame characteristics.

The present experimental work deals with the challenge to ob-

tain a better understanding of strongly radiating turbulent and

reacting flows from the images acquired with a high speed CCD

camera. Specifically, the effect of changes in swirl number is inves-

tigated in two different flames. On the one hand, the visualization

system is used in a laboratory scale atmospheric gas flame, being

the results physically interpretable from comparison with PIV

measures. On the other hand, the procedure is applied to a semi-industrial scale burner of pulverized fuel, involving a participating

media and a more complicated flow dynamics.

Both facilities and the visualization system are described in Sec-

tion 2. A flame visualization system based on a high-speed CCD

camera was used to capture the image sequences containing the

spatial–temporal luminous information from the flame. After a

suitable digital image processing, different flame features are ob-

tained in terms of statistical and spectral parameters. Results are

Nomenclature

CCD charged couple deviceD nozzle diameter (m) f characteristic frequency (Hz)HSB high swirl burnerIRZ internal recirculation zone

LDA laser doppler anemometryLPP lean premixed prevaporizedLSB low swirl burnerL characteristic length (m)PF pulverized fuelPLIF planar laser induced fluorescencePVC precessing vortex coreQ volumetric flow rate (m3/s)

Re Reynolds number: Re ¼ 4 _m=plDS-PIV stereo particle image velocimetryS g ,s geometric secondary swirl: S g ,s = 2/3 tan aSt Strouhal number: St = ( f L)/V St ⁄ alternative Strouhal number, frequency parameter:

St ¼ ð f D3e Þ=Q V fluid velocity (m/s)a vane anglel dynamic viscosity at ambient temperature (kg/m s)/ air–fuel ratio

Table 1

Summary of experimental analysis performed in swirl flames using advanced technologies.

Ref. Fuel React. Technology Burner Conditions Measurements

[1] Propane Yes S-PIV atmosph. LPP Low/high S Velocities, frequencies, flow structures, vorticity

[2] — No LDA,acoustic vortex burner Re = 15,000; S = 1.01 Velocities, flow structures

[3] — N o L DA , fl ow v isua lizat ion Front w all (sca le 1/10) D if f. c onfig urat ions I nte ra ct ion swirling jets, flow v isua liza tion,

velocities

[4] N .g . ye s PIV, L DA , p hot og ra phy L ea n non-premix ed swirl-

stabilized

S = 0.82;20 kW Two injection topologies, velocities,

temperatures

[38] Methane Yes S-PIV, PLIF, Rayleigh

scattering

Stratified premix.

turbulent LSB

27–40 kW; / = 0.62 Velocities, temperatures, fuel distribution

[39] M ethane Ye s PIV, O H-PL IF ima ging Premix . t urbule nt LS B R e = 77–221; / = 0.7 Velocities, OH concentration, flame fronts,

structures

[40] Methane Yes S-PIV, OH-PLIF Gas turbine model

combustor

10.3 kW; / = 0.75; S = 0.55;

Re = 15,000

Velocity fields,flame structures

[41] N.g. Yes CH-chemiluminiscence,

photography, acoustic

Premix. swirler injector a = 45, diff. operationpressures

Combust. instabilities,oscillations, CH

concentrations

[42] Gas

(unspecif.)

Yes PIV, OH-PLIF, Rayleigh

thermometry

LSB 27–40 kW;Re = 20,000–

30,000; S = 0.5; / = 0.62

Velocities, vorticity, temperature

[6] — No Hot-film sensor,

visualization

Coa l burne r S = 0. 344–0. 410 Flow visua liza tion, frac ta l dime nsion,turbulence

[37] Pr opane Yes LDA,high speed Pr emix. atmosph. camera,

acoustic, chemilm.

S = 0–1.5 Frequency analysis

[43] — No LDA, high-speed filming,

acoustic

Changeable blade swirler Re = 16,000; S = 1 Velocities, structures, frequencies

[44] Methane Yes LDA,OH-PLIF, Raman

scatt., chemilm.

Lean premix. 25–30 kW; / = 0.7–0.83 Velocities, OH concentration, temperature,

instabilities

[45] N. g. Ye s O H-PL IF, PIV, Ra ma n spe ct . Le an premix ed D if fe re nt pressures Veloc ity field s, O H c once nt rat ion

[46] Methane Yes LDA, OH/CH-PLIF, Raman

scatt.

Gas turbine model

combustor

7.6–34.9 kW Flow fields

[47] Methane Yes LDA, CH-PLIF, Raman scatt. Gas turbine combustor Re = 7500–60,000;

S = 0.55–0.9

Flow field, species and temperature

measurements, visualization of reaction zones

[48] Gas

(unspecif.)

Yes Photography, emission

spect.

Lean direct injection a = 30–65 Species distribution,structure

[49] Gas

(unspecif.)

No PIV, hot-wire anem., high-

speed photo

Lean swirl Re = 5700–61,000; S = 0.6–

3.7; confined/unconf.

Structure, PVC, IRZ dependency with geometric

and flow features

[7] Coal Yes PIV, LES Swirl coal burner S = 0.91 Velocities,species concentration,temperature,

PIV–LES comparison

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presented either spatially or as quantitative data depending on the

analysis procedure.

Section 3 briefly defines the processing procedure and the cor-

responding outcomes which are later interpreted as flame param-

eters. Section 4 presents the results concerning the two flames and

discussion of their main findings. The conclusions and perspectives

are finally summarized in Section 5.

2. Experimental setup

Two different experimental facilities were used for the study. In

a first stage, a laboratory premixed gas swirl burner (50 kWth) was

used. The purpose was to characterize the flame and establish the

basis for the subsequent analysis of a 500 kWth PF flame, where the

visualization of the whole flame was not possible. This enables

swirl flame comparison at very different scales, providing useful

information to complement the current analysis as a tool for the

identification of swirled flame features.

2.1. Test rigs

The laboratory scale facility comprising the gas burner belongs

to the Thermal Engineering, Energy and Atmosphere (ITEA) group

from Carlos III University, Spain. The atmospheric burner was de-

signed to study swirl flames by means of visual analysis. Accord-

ingly, its geometry allows good optical access to the whole flame.

Fig. 1 shows the geometry and dimensions of the burner. The main

body corresponds to a cylindrical plenum and two coaxial central

Fig. 1. Atmospheric gas swirl burner (courtesy of M. Legrand). (a) Real image and (b) scheme of the top and front view of the burner geometry [1]. Dimensions are in mm.

Fig. 2. Experimental test facility: 500 kWth PF swirl burner.

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pipes ending with a nozzle of 26 mm diameter. Two symmetrical

opposite pipes with tangential outlets are used to generate the

swirl motion. The radial distance of these two pipes can be ad-

justed to control the swirl level. An air-propane flow is premixed

and then fed into the burner. Further details about burner geome-

try and design can be found in Refs. [1,17].

Pulverized coal combustion experiments were performed in a

500 kWth test facility (Fig. 2). It consists of a downward oriented

swirl burner dedicated to pulverized fuels (mainly biomass andcoal) [18]. The premixed burner has two coaxial injectors for pri-

mary and secondary air, a refractory quarl to promote flame stabil-

ity an a cylindrical combustion chamber of 3 m long and 1 m of

internal diameter. Different ports along the furnace are available

for probe installation. Additionally the facility is fully instrumented

and monitored through and advanced control and data acquisition

system. Coal is introduced with primary air by the inner duct of the

burner and is swirled by a volute. With regard to secondary air, a

movable-vane swirler mechanism provides the rotational motion

to the air flow. The swirl number is controlled with high precission

by adjusting the vanes angle (a). The geometric swirl number def-inition is given by this expression S g ,s = 2/3 tan a. Secondary air ispreheated up to 250 C with an additional natural gas burner in or-

der to enhance combustion efficiency. A multifuel feeding systemprovides the on-line regulation of the real mass flow of fuels. Pol-

lutant emissions are measured at stack using a complete analyzer

system with gas sampling, sample conditioning, analyzer and sys-

tem control units. The set of standard analyzers are composed of

NDIR (Non-dispersive Infrared) absorption photometers and elec-

trochemical sensor for O2. Further details can be found on Refs.

[19,20].

2.2. Visualization system

The flame monitoring system, based on a CCD (charged coupled

device) camera was designed for the visualization of combustion

flames in both, atmospheric and highly confined enclosures [21].The harsh environment during tests in the PF facility and the small

dimensions of the inspection ports determined the final design of

the probe. On the one hand, the system relies on a 17 mm remote

head, located at the end of the probe, which accommodates the ac-

tive sensor and is fitted to an optical assembly. On the other hand,

a protective system keeps low work temperatures and good lens

conditions. It consists of a cylindrical water-cooled jacket made

of stainless steel with a purgue of air and dimensions: 680 mm

long and 35 mm outer diameter. This probe is inserted in one of

the inspection ports near the burner throat. This arrangement pro-

vides a suitable flame view area of 315 mm (h) 230 mm (v),

approximately.

As regards camera specifications its main characteristics are

listed in Table 2. It achieves a high-speed frame rate of 120 frames/s (fps) which allows a suitable frequency analysis [22].

A full characterization stage was reported in a previous work

(Ref. [20]) but it is briefly presented here. This reduces the distor-

tion in the acquired flame images caused by thermal fluctuation or

electronic noise from the system which could lead to misinterpre-

tations. Following parameters were defined and quantified: non-

uniform responsivity, instrumental background, noise ratio and

saturation equivalent exposure [21]. They depend on the CCD sen-

sor physical structure and the conversion process into an electricalsignal. Several tests in absence of light were performed in order to

create a master dark frame which was built from the time average

of gray values for each individual pixel. By subtracting this frame to

each image, the systematic error is removed. This is the first step of

the processing procedure. The global performance parameter for

non-uniformity was quantified as the standard deviation of the

dark frame, being around 2.53%, which is an acceptable figure.

Thermal fluctuation and electrical noise under dark conditions

were also estimated for every pixel as the standard deviation com-

posing the time signal (around 1%). Finally, size and distortion of

the visualized area were analyzed through a set of tests performed

on a graph paper uniformly illuminated. The final resulting images

were scaled using this experimental fitting.

2.3. Fuel and experimental program

Tests were first conducted in the 50 kWth atmospheric swirl

burner. Several air-propane mixtures were premixed and then

burned under high (HSB) and low (LSB) swirl conditions. Swirl

number for the LSB configuration was set around 0.48 while for

HSB it was near 0.6. It was controlled by changing inner/outer flow

ratio. Reynolds conditions (Re), referred to the total mass flow rate,

were similar for both situations (Re = 14,000). CCD camera was lo-

cated perpendicular to the flame vertical axis to permit a complete

flame view (see Fig. 4a).

Table 2

Camera specifications.

Parameter

Active area (mm) 6.4 (h) 4.8 (v)

Active pixels 659 (h) 494 (v)

Signal/noise ratio >50 dB (0 dB gain)

Electronic shutter 1/10,000 s

Operating temperature 5 C to +45 C

Dimensions (H W L) Head ø17 46 mm (ø D)CCU 44 29 66 mm

Spectral range 400–1000 nm

Max. Spectral response 500 nm

Focal length 7 mm

Table 3

Fuel characterization.

Coal

Moisture (as received,%wb) 2.30

Proximate analysis (%d.b)

Ash 14.60

Volatile matter 26.00

Fixed carbon 57.10

Ultimate analysis (%d.b)Carbon 69.60

Hydrogen 4.00

Nitrogen 2.05

Sulfur 0.50

HHV (MJ/kg), (d.b) 27.80

Mean particle diameter (mm) 0.045

Table 4

Summary of main parameters of combustion tests.

Furnace operating conditions

Thermal Power input (kWth) 500

Coal feed rate (kg/h) 68.4

Excess air (%) 14.6 ± 1.5

Primary air to fuel ratio 2.3

Primary air

Flow rate (kg/h) 157.4 ± 0.08

Temperature (C) 15

Geometric swirl number 1.54

Reynolds number 19,144

Secondary air

Flow rate (kg/h) 503.2 ± 12.30

Temperature (C) 254

Geome tric swirl numbe r ( nominal) 0 .67

Reynolds number 25,800

Mean furnace temperature (C) 980 ± 15

Swirl numbers (S g ,s) 0.28; 0.40; 0.67; 0.79; 1.20


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With regard to experiments performed in the 500 kWth pulver-

ized fuel swirl burner, a wider range of swirl numbers was ana-

lyzed, as it is indicated in Table 4. The fuel used in the tests wasa South African coal, with low sulfur content and pulverized to a

mean particle size of 0.045 mm. Ultimate and proximate analyses

are shown in Table 3. According to particle size distribution analy-

sis, 100% of the pulverized coal is under 0.1 mm and 70% under

0.055 mm.

Experimental research at laboratory large scale mainly consist

on combustion tests varying secondary swirl number, while main-

taining the other relevant parameters nearly constant (Table 4).

Once steady conditions have been achieved in the combustion

chamber, after a suitable preheating time, secondary swirl number

is progressively modified by adjusting the secondary vane angle

using a vane actuator. Each swirl position is fixed during a suitable

time period (not less than 15 min) and an average of 3–4 videos are

then recorded for each swirl condition. Furnace gas temperaturewas around 980 ± 15 C. A slight temperature increase could not

be avoided due to the slow warming of refractory walls.

Although most of the videos were recorded at stable conditions,

some flame images were also registered at transition stages, when

swirl conditions were changing (while secondary vanes were in

motion). Transition periods take few minutes until vanes reachthe correct angle. During this interval, the flame is continuously re-

corded in order to analyze it under variable conditions. Frame rate

(120 fps), electronic shutter (1/10,000), iris aperture and time

duration (42 s) were the same for all the videos.

3. Analysis procedure

Luminous radiation from the flame reaches the CCD sensor and

is converted into an electric charge. The charge from each cell is

transformed into a pixel gray value (ranging from 0 to 255) in

the corresponding image. According to the camera acquisition

speed, 120 images per second (fps) are recorded and transmitted

to the frame grabber installed in the PC, where they are finally

stored. A collection of thousands of images containing the lumi-nous flame information is the basis for the subsequent analysis.

It is made over each video starting with the correction of non-uni-

formity patterns (Section 2.2) followed by the calculation of the

different flame parameters explained below. The video represents

a three-dimensional matrix of gray values, where two dimensions

correspond to space and the third one to time. Fig. 3 outlines the

two different analyses that can be done as a function of data

selection.

Spatial analysis: the time signal of each individual pixel is ana-

lyzed through statistical and spectral methods. The correspond-

ing results are represented in form of two dimensional spatial

distributions which leads to a qualitative characterization of

the flame area.

Fig. 3. Outline of the processing procedure leading to different flame characterization.

Table 5

Flame parameters definition.

Signal

characterization

Statistical/spectral

parameter

Mathematical

expression

Flame brightness Mean value ( x) x ¼ 1N PN

i¼1 xiFluctuation

amplitude

Standard deviation (r) r ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1N

PN i¼1 xi xð Þ

2q

Oscillation

frequency

Flicker (F)F ¼

PN 1k¼0

X f kð Þj j f k

PN 1

k¼0 X f kð Þj j


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Temporal analysis: gray values contained in each image are

averaged. Therefore just one temporal flame luminous signal

is obtained for each video. From that signal, flame features are

calculated leading to a quantitative estimation of the flame

state.

Statistical and spectral flame parameters linked to geometrical,

luminous and oscillating properties are calculated from the flame

videos using specific processing algorithms [20]. Table 5 shows

the mathematical expression for each parameter considered in

the analysis. All the equations are referred to a base set of gray val-

ues ( x1 . . . xi . . . xN ) which depends on the type of analysis. These

parameters comprise different statistical moments: mean value

and standard deviation, which can be interpreted as the flame

brightness and the amplitude of the flame luminous fluctuations,

respectively. Both are expressed in gray units (g.v.) that range from

Fig. 4. Spatial distribution of flame features under low (LSB) and high (HSB) swirl conditions in a 50 kW th propane swirl burner. (a) Snapshots of real images, (b) Mean value

(flame brightness, g.v.), (c) Standard deviation (luminous fluctuation amplitude, g.v.) and (d) Flicker (Hz).


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0 to 255 according to the camera specifications. Flame brightness is

usually related to the flame size, the volatile content of the fuel and

the radiation level of chamber walls, when they are present [23]. At

this point, it is important to take into account the existing differ-

ences of luminous emission between the gas and the PF flame.

They are later commented in more detail but the main difference

is related to the contribution of particles in the PF flame. Therefore,

the information provided by this parameter cannot be interpreted

in the same way concerning the two flames.

Apart from statistical parameters, the Fast Fourier Transform

(FFT) is used to calculate the Power Spectral Density ( f k) and an

averaged frequency, called flicker (F ). This parameter corresponds

to the mean frequency at which the flame fluctuates. Those fluctu-

ations are caused by the variability of heat release, turbulent mix-

ing and the resulting eddies existing in a combustion flame [24].

Therefore, it can be used as an estimator of different flame condi-

tion and combustion efficiency [25].

4. Results and discussion

In this section main results regarding both flames are presented

and discussed. It must be noticed that the sources of flame radia-

tion strongly depends on fuel composition and combustion condi-

tions. Consequently, these facts must be taken into account for a

correct interpretation of the flame parameters.

Specifically, spontaneous emission of electromagnetic radiation

is derived mainly from solid material (soot, ash, particles), from

some gas molecules (basically CO2 and H2O), due to the high tem-

perature reached in flames, and finally, from several excited spe-

cies coming from specific chemical reactions [26]. In the PF flame

the contribution of particles represents most of the radiation

reaching the sensor element, while for the gas flame luminous

emission is derived from the other sources. Besides, the confine-

ment of the PF flame influences its structure and behavior [27]

apart from the interference of thermal radiation from furnace walls

and participating media [5].

Therefore, it is important to bear in mind these obvious differ-

ences between the two flames which makes that the results cannot

Fig. 5. Averaged gray values (g.v) corresponding to the central axis for both flames,

showing the relative displacement as swirl number is increased.

Fig. 6. Comparison of flame front location obtained from different analyses performed in the same burner. (a) PIV measurements (turbulent kinetic energy ( m2/s2)) for LSB

and HSB conditions, white lines represent the approximate averaged flame front location [1] and (b) Standard deviation distributions (g.v.) from spatial analysis for LSB andHSB conditions. Countours of high standard deviation values define similar flame fronts.


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be considered fully comparable. Nevertheless, luminous informa-

tion from both flames allows a suitable first-step characterization

in terms of dynamical structure and oscillatory behavior under dif-

ferent swirl conditions.

4.1. Results from atmospheric gas flame

A thorough flame characterization had been performed bymeans of Stereo Particle Image Velocimetry (S-PIV) in a previous

work [1]. Low and High Swirl Burner configurations (LSB and

HSB respectively) under isothermal and reactive conditions were

covered in the study. Reactive flow results have been compared

to current analysis in order to improve the interpretation of the

results.

Here, LSB and HSB configurations were tested under reactive

conditions leading to two different flow topologies. Fig. 4 shows

the actual image and the spatial distribution of flame parameters

for both swirl conditions applied to the propane flame. Brightness

distribution (mean gray value) clearly reveals the great difference

in flame shape between high and low swirl conditions (Fig. 4b).

Since the gray level is, in a rough sense, directly related to temper-

ature [12], concentric distribution of brightness levels representsan approximate map of flame temperatures. Accordingly, the core

of this structure represents the more intense combustion zone of

the flame, with recirculation of active species due to the radial ad-

verse pressure gradient generated by swirling flow [28]. Therefore

this area may correspond to the internal recirculation zone (IRZ)

identified in experimental and numerical studies [29,30].

Another typical effect of swirl flames that can be appreciated in

the tests is the attachment of the flame to the burner nozzle when

swirl number is increased [28,31]. Fig. 5 shows graphically this ef-fect. The analysis is applied to Fig. 4b where mean values distribu-

tion of both flames (HSB and LSB) is illustrated. A’A and B’B

sections represent the mean gray values obtained from these

images versus axial dimension, and they correspond to the central

axis of the flame. The relative displacement of the flame front be-

tween both swirl conditions can be easily observed and quantified

in approximately 7 mm.

Flow structure produced by swirl burners is characterized by

the presence of strong toroidal recirculation zones which are

mainly responsible of flame stabilization [28,32]. Those zones of

reversal flow have been extensively documented and they are usu-

ally represented by spatial distribution of streamlines which are

calculated from measured time-mean velocity distribution (see,

for example Fig.1.2 in Ref. [27]). These lines set constant stream

conditions and show two circular flow structures which produce

the formation of the internal recirculation bubble. Recirculation

patterns created in these areas affect the hot products and species

from chemical reactions which are responsible of luminous emis-

sion. Standard deviation of this luminous fluctuation along the

flame area is the basis for the current analysis. When it is analyzed,

the resulting spatial distributions (Fig. 4c) show specific circular

zones with constant values indicating the presence of particular

structures similar to those established by theoretical streamlines

which have been also defined for this particular burner [17]. These

results are obtained under different swirl conditions being evident

even with low swirl number. This fact suggests that standard devi-

ation values obtained from spatial analysis are related to the flow

patterns and associated dynamics and highlight the importance of

this parameter as a potential tool for the analysis of flamestructure.

As regards flicker distribution, frequency along the two flames

is very different. Under HSB conditions, the flame shows lower

oscillation frequencies than flame under LSB conditions. In both

cases frequency increases in the radial direction, being highest in

the flame visible boundary.

In addition, higher swirl causes an increase of both the width

and length of the flame reverse flow zone, a well-known effect

[28]. In the current analysis, the area of higher brightness also in-

creases in size for higher swirl numbers, as can be shown in Fig. 4b.

In addition, the location of the two toroidal zones changes with in-

creased swirl, standing closer to the nozzle. This effect also indi-

cates the attachment of the flame under HSB conditions.

Finally, comparison between S-PIV measurements under reac-tive conditions and current results has shown some similarities.

Legrand et al. [1] identified the approximate flame front location

by computation from seeding density gradients (white lines in

Fig. 6a). In the current analysis similar flame front boundaries have

been determined, located by means of standard deviation spatial

distributions. Black lines in Fig. 6b) represent the contour of high

standard deviation values which approximately match with the

flame fronts found by PIV measurements.

4.2. Pulverized coal flame results

4.2.1. Pollutant emissions

CO, SO2 and NO x emissions trends as a function of swirl number

were analyzed and compared with luminous radiation parameters.Fig. 7 shows averaged values during the corresponding time periodFig. 7. Averaged values of (a) CO, (b) NOx, and (c) SO2 emissions as a function of swirl number corresponding to pulverized coal combustion tests.


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as function of swirl. CO emissions were low, showing good com-

bustion performance for all swirl conditions. A slight reduction in

CO concentration with swirl number can be appreciated in

Fig. 7a), although standard deviation greatly increases at the high-

est swirl number (1.2), which may be attributed to an increase of

flame fluctuation amplitude.

NOx emissions trends with swirl strongly reflect differences in

mixing patterns. Increase and decrease of NO x with swirl have been

experimentally reported [31]. The NOx reduction at low-

to-medium swirl flames is attributed to flame stabilization because

the enhancement of the fuel–air mixing process. Under fuel rich

conditions in the near burner region fuel-nitrogen is mostly devol-

atilized and reacts favoring N2 formation over NOx. Our results

(Fig. 7b) show a NOx decrease starting with the nominal swirlnumber with a previous increase for lower values indicating an

improvement in the mixing process [31]. This initial trend might

indicate that mixing of secondary air with volatile products takes

place prior to complete combustion which produces that oxygen

react with the fuel nitrogen to form NO. This could result in a slight

increment of NOx before the mixing improvement.

Finally, SO2 concentration slightly increases with swirl number

(Fig. 7c). As expected, an inverse dependence of SO2 and CO emis-

sions with swirl number can be appreciated in Fig. 7a and c, as SO2emissions are directly related to fuel sulfur content and combus-

tion completion.

4.2.2. Results from digital analysis

Results from atmospheric swirl flame measurements and emis-sion trends enables more accurate description of PF flame features

without ignoring the existing differences of luminous flame emis-

sion between both. It allows the connection between digital results

and the dynamical processes present in swirl flames.

The five different swirl numbers tested have been arranged

according to high and low swirl conditions. The nominal swirl

number (0.67) divides both groups (LSB and HSB), and is included

in the LSB group. Qualitative results of flame features are presented

below according to this arrangement (Figs. 8 and 9).

Brightness contours (Fig. 8a) show an increase of flame width

and luminous level with the swirl rise. As a consequence of swirl

increase the spreading angle of the flame becomes wider. This ef-

fect, that can be observed in current spatial results, is already

known [28] and it has been also experimentally proved under sim-

ilar conditions [33]. A brighter inner zone is noticeable for thenominal swirl condition. This might indicate the formation of recir-

culation flows leading to the internal recirculation zone (IRZ).

Recirculation zones are only formed beyond a critical swirl number

>0.6 [28], which just matchs with the current nominal value

(0.67). This explains why this area does not appear in the flames

with lower swirl conditions. On the contrary, brightness distribu-

tions for HSB conditions (Fig. 9a) highlight the presence of this area

also showing the flame widening.

If the area of higher brightness is compared for all the flames

(LSB and HSB), one more feature concerning swirl flames can be

appreciated. According to experimental works [33], the volatile re-

lease and particle heating in the IRZ tends to be greater for inter-

mediate swirl values. In this case, flame image corresponding to

nominal swirl conditions (0.67) shows in fact a larger and more

uniform area of maximum brightness (Fig. 8a). Bearing in mind

Fig. 8. Spatial distribution of flame features under relative low swirl conditions (LSB). All figures correspond to the pulverized coal flame operating under nominal conditions.


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that volatile release increases flame brightness [16], the current

observation agrees with published results.

With regard to standard deviation distributions for LSB condi-

tions, Fig. 8b shows how the area of higher values enlarges for

higher swirl numbers. According to the results obtained for the

atmospheric gas swirl flame, this feature is linked to flow patterns

and recirculation zones. Therefore, it can be deduced that increas-

ing swirl number produces an increase of recirculation patterns

Fig. 9. Spatial distribution of flame features under high swirl conditions (HSB). All figures correspond to the pulverized coal flame operating under nominal conditions.

Fig. 10. Time evolution of some flame features under different swirl conditions during a coal combustion test. Each value is extracted from the averaged luminous flame

signal of the corresponding video.


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also in the pulverized fuel flame. This result is verified also for HSB

conditions (Fig. 9b). The recirculation zone becomes much more

intense showing an elongation in the left side. Considering the

rounded shape of this new structure, it might correspond to the

swirled secondary air inlet. Intensification of recirculation patterns

consequently involves the improvement of mixing process. This ef-

fect agrees with the obtained NOx trend previously discussed.

Distribution of flame flicker shows clear differences betweenLSB and HSB conditions. For low swirl numbers (0.28 and 0.40),

distribution of averaged frequencies is similar, with a wide zone

of low frequency in the upper central part and a high frequency

band in the right (Fig. 8c). Nevertheless, for higher swirl numbers

the lower frequency zone apparently changes moving towards

the central left side (Fig. 9c). In consequence, higher frequencies

are moved and reallocated to the right and upper part of the flame.

The changes in oscillating frequencies reveals how the inner

dynamical behavior of the flame is altered by changing the swirl

number of secondary air flow.

Finally, some interesting conclusions can be deduced from

quantitative results. The evolution of flame parameters, obtained

from the averaged luminous signal, is measured during the tests.

In the course of transition stages, secondary vane angle is progres-

sively modified according to the procedure detailed in Section 2.3.

In these periods flame stability is expected to be altered. In order to

evaluate the usefulness of flame features in detecting those varia-

tions several videos were recorded during these periods. Fig. 10

shows standard deviation and flicker values along the different

swirl periods during a combustion test. Each value is derived from

the corresponding video. The evolution of both features clearly

show opposite trends. Additionally, they are notoriously altered

in the transition periods which proves its usefulness detecting

flame variations.

Apparently an increase in swirl number implies wider fluctua-

tion of flame luminous radiation and a magnification of low fre-

quencies over the higher. The improvement in mixing patterns as

a result of the swirl rise explains the increment of the fluctuation

amplitude. Otherwise, according to specific publications concern-ing flicker measurements in flames [23], flicker decrease indicates

that changes in flame geometry prevail over changes of heat re-

lease rates.

Another finding derived from temporal analysis is related to the

frequency variation of the luminous flame signal as function of

swirl condition. Frequency analysis concerning both, isothermal

and reacting swirl flow applications provides a suitable tool for

detection of PVC dynamics. Through the calculation of a widely

used dimensionless number, Strouhal number (St), swirling flows

are analyzed in terms of their precession frequencies. Theoretical

calculations (usually based on LES) are obtained and contrasted

to experimental measurements in order to validate the numerical

simulations. These are commonly performed by acoustic methods

which provide the characteristic frequency of a certain flow but

they are usually made under non-reactive conditions since the dif-

ficulties to measure on real combustion flames.

Here,a frequency parameter defined in previous publications[27,28,32] was calculated using flicker as a characteristic fre-

quency. This value is basically another form of Strouhal number

and it is directly derived from it. When Strouhal is applied to a bur-

ner, the velocity of the fluid is derived from the isothermal burner

flow rate and based on the burner exhaust area. The characteristic

length is the corresponding burner exhaust diameter (De). There-

fore, the difference between this parameter and the basis definition

of Strouhal is the term p/4. Many authors have used this alterna-tive form of Strouhal number [34–36].It is calculated using the fol-

lowing expression.

St ¼ f D3e

Q ð1Þ

where f is the characteristic frequency (flicker value in this case,Hz), De is the exit diameter of the burner (m) and Q is the total

air flow rate (m3/s). Fig. 11 shows the resulting trend by calculating

its value for the different frequencies obtained under each swirl

condition, where a parabolic evolution of St ⁄ versus swirl intensity

is obtained. A total of 16 experimental data support the curve where

each point has been defined from different experiments performed

under similar swirl conditions. Several studies concerning swirling

flows show similar trends even using different frequency data,

which are usually obtained by means of acoustic techniques or

numerical methods [29,37]. This effect has not been studied before

on coal flames at semi-industrial scales due to the complexity of

measurements in these environments. In this sense, our analysis

represents a promising option to further progress in the under-

standing of pulverized coal flames.

5. Conclusions

An extensive experimental study of different swirl flames has

been performed by means of a CCD based visualization system.

The analysis was focused on a pulverized fuel flame of 500 kWthunder several swirl conditions. Additionally, an atmospheric gas

swirl flame of 50 kWth was also analyzed performing a similar

analysis in order to complement the results and obtain additional

information regarding different swirl flames. In this case, PIV mea-

surements allowed to prove the usefulness of visualization system

to identify the flame dynamical differences when swirl changes.

The spatial characterization of both flames through statistical

and spectral parameters has led to the identification of typicalstructures of swirl flames such as recirculation areas and flow pat-

terns. Several differences concerning flame shape or oscillating fre-

quency between low and high swirl conditions were detected. A

clear increase of flame width and spreading angle was noticed

for the coal flame with the swirl rise. Luminous flame intensity

in the inner area of the flame clearly increases from nominal value

and higher swirl numbers. This observation may be related to the

improvement of the mixing patterns as a result of the swirl rise.

The expected increase in recirculation patterns was also identified

through the remarkable variation in standard deviation values

which show a clear increment with the intensification of swirl.

Finally, several parameters from temporal analysis of the lumi-

nous flame radiation signal were proved to be potentially useful for

the identification of changes in the luminous flame rate and there-fore to diagnose the actual state of the flame in terms of stability.Fig. 11. Frequency parameter (St

⁄

) versus swirl number obtained from coalcombustion in a PF swirl burner.


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A modified Strouhal number has been defined by means of

flame flicker, similarly as in the characterization of swirling flows.

From the analysis, an inverse dependence of swirl number with

this parameter has been found, which agrees with related litera-

ture predictions.

Pollutant emissions obtained in current experimental combus-

tion tests have shown the expected trends, in accordance with an

improved mixing process as swirl number is increased. Particu-larly, NOx measurements exhibit close connections with swirl con-

ditions and, in consequence, with other swirl-related parameters,

as flicker. Further research is needed to finally establish a more

accurate trend.

In summary, in-depth flame characterization under very differ-

ent swirl conditions allowed obtaining valuable information about

main variations of flame structure and oscillatory behavior. Up to

know, spatial details and frequency information could be only ob-

tained using more advanced and complex technologies and almost

entirely focused in gas flames. In this sense, the actual system

means an innovative tool for flame analysis in a wide scale range

including PF flames under reactive conditions.

Acknowledgements

This work has been supported by the project ENE2010-16011

(Spanish Ministry of Science and Education, R&D Energy Program).

The authors gratefully acknowledge J. Nogueira and M. Legrand

(University Carlos III) for their support during the tests with the

gas burner and for the opportunity of using their facilities. Also

we whish to thank to C. Bartolomé and I. Ramos, researchers of

the Thermal Division of CIRCE, for their help during tests.

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