Experimental Thermal and Fluid Science 28 (2004) 709–714

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Experimental Thermal and Fluid Science 28 (2004) 709–714

www.elsevier.com/locate/etfs

Recirculation phenomena in a natural gas swirl combustor

Aldo Coghe *, Giulio Solero, Gianfranco Scribano

Dipartimento di Energetica, Politecnico di Milano, Fac. di Ingegneria Milano Bovisa, via La Masa 34, 20156 Milano, Italy

Abstract

This paper presents the experimental results obtained in a natural gas swirl combustor (input thermal power¼ 17 kW) through

different techniques (laser Doppler Anemometry for flow field characterisation, temperature measurements by thin thermocouples,

emission spectroscopy of the flame front and pollutant emissions analysis at the exhaust). The main aim of the performed research

was to investigate the recirculation phenomena induced by the swirl motion imparted to the air stream (swirl number S ¼ 0:82)inside the combustor: in fact, different recirculating regions (central and corner) have been observed and, by integration of the

velocity profile measured by LDV, the corresponding flow rate has been estimated. Particularly, it has been found that flame

confinement in the presence of intense swirl generates a wide central recirculation zone and a large corner vortex. The hot reverse

stream propagating on the burner axis prevents penetration of the fuel jet, induces a rapid mixing and burning and provides flame

stabilisation. The corner recirculation of hot burned gases favours entrainment in the outflowing reactants mixture contributing to

their progressive pre-heating and leaning, thus influencing combustion process development and pollutants formation (especially

thermal NOx).

� 2004 Elsevier Inc. All rights reserved.

Keywords: Swirl burners; Combustion diagnostics

1. Introduction

The multidisciplinary nature of the Combustion Sci-

ence requires different investigation approaches. Among

them, the fluid dynamic analysis is without doubt one of

the most important tools to ascertain the stability and

efficiency of a combustion device. One of the method-

ologies recently developed to minimize the environ-mental impact of combustion processes is founded upon

the improvement and the optimization of the mixing

process between the reactants (fuel and air): in fact, it is

well known [1,2] that, independently from the combus-

tion technology used, the main features of the combus-

tion process (under the point of view of efficiency,

stability and pollutant emissions) are strictly linked to

the efficiency of the mixing process between the reac-tants, because the chemical kinetic time scales are usu-

ally shorter than the turbulent time scales. Therefore,

the fluid dynamic analysis of a combustion process can

*Corresponding author. Tel.: +39-2-2399-8537; fax: +39-2-2399-

8566.

E-mail address: aldo.coghe@polimi.it (A. Coghe).

0894-1777/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.expthermflusci.2003.12.007

provide useful informations about the main character-

istics of the phenomenon.

This paper deals with the experimental characterisa-

tion of a natural gas swirl combustor, analysing by

different techniques both the flame front behaviour and

the recirculating regions induced by the swirl inside the

combustion chamber. Swirl effect is often used in com-

bustion devices in order to improve flame stability andenhance mixing process [3]. The present analysis has

been carried out in order to quantify the degree of

mixing between hot products and cold reactants which is

generated by the swirling air motion, both inside and

outside the flame region, suggesting improvement to the

burner design, by promoting or hindering the develop-

ment of these phenomena.

2. Experimental set-up

Fig. 1 reports a schematic view of the investigated

burner. As it can be seen, the burner is equipped with an

axial + tangential swirl generator and with a fuel injector

coaxial to the air stream: it is a configuration quite

similar to those used for typical industrial appliances(diffusive atmospheric pressure burners). A cylindrical

Fig. 1. Schematic view of the investigated burner.

710 A. Coghe et al. / Experimental Thermal and Fluid Science 28 (2004) 709–714

quartz combustion chamber (internal diameter¼ 192

mm) has been used for flame confinement, making

possible flame visualization and measurements by laser

anemometry. A natural draught hood provides the

exhaust and sampling of the burned gases.

Table 1 reports the main operating conditions used

for the experimental measurements reported in thispaper. The air stream was supplied by the laboratory air

compressed line and was divided into two separately

metered streams (axial and tangential air flow) to allow

continuous control and regulation of the swirl strength

at the burner exit. The axial air entered through four

radial inlets and passed through a honeycomb flow

straightener to produce a uniform axial stream. The

tangential air was introduced downstream in the cylin-drical pipe through four tangential inlets to impart

angular momentum. Natural gas was supplied through a

central pipe with an axial injection nozzle, 8 mm diam-

eter, derived from a real burner injector design. The

inner pipe had an outer diameter of 15 mm and was

located concentrically in the outer pipe of inner radius

Rb ¼ 18 mm. The tangential and axial air and the fuel

flowrates were metered and stabilised by calibratedthermal mass flowmeters and controllers. The burner is

fed by natural gas provided by the network distributor

in the Bovisa laboratory area (90% methane, 4% ethane,

1.5% propane, 4% N2 and a small percentage of higher

hydrocarbons).

Momentum ratio (an important parameter connected

to the interaction between the two coaxial streams [4]) is

defined as

Table 1

Main operating conditions of the investigated burner

Air flow rate (g/s) 8.8

Air mean velocity efflux vair (m/s) 8.7

Reynolds number of air jet 20,700

Natural gas flow rate (g/s) 0.35

Natural gas mean velocity efflux vf (m/s) 10.7

Reynolds number of natural gas jet 5600

Input thermal power (kW) 17

Air swirl number S 0.82

Fuel/air momentum ratio MR 0.91

Fuel/air equivalence ratio U 0.69

MR ¼ qf � v2fqair � v2air

ð1Þ

Swirl number of the air stream is [5]:

S ¼ Gh

Gx � Rb

ð2Þ

where Gh ¼ axial flux of angular momentum, Gx ¼ axial

flux of axial momentum, and Rb ¼ burner radius.

The value 0.82 of the swirl number has been esti-mated using the above mentioned definition by direct

measurement through LDV and subsequent integration

of mean axial and tangential radial velocity profiles at

the efflux, in isothermal conditions [6].

Fig. 2 shows a typical image of the investigated flame,

with the indication of two measurement traverses h=Rb

(Rb¼ radius of the air efflux¼ 18 mm). It can be ob-

served the typical calyx-shaped flame, due to swirl effect[7] and low equivalence ratio.

Owing to the high complexity of the flow inside the

combustion chamber (due to the high swirl intensity

imparted to the air stream), the experimental charac-

Fig. 2. Typical image of the analysed swirling flame.

-10

-5

0

5

10

15

20

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

r/R

V (m/s)

0

200

400

600

800

1000

1200

1400

T (°C)

V (m/s)T (°C)

-10

-5

0

5

10

15

20

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

r/R

V (m/s)

0

200

400

600

800

1000

1200

1400

T (°C)

V (m/s)T (°C)

10

15

20

1000

1200

1400V (m/s)T (°C)

(b)

(a)

A. Coghe et al. / Experimental Thermal and Fluid Science 28 (2004) 709–714 711

terisation has been carried out through different tech-

niques.

Measurement by LDV of the reactive flow field has

been performed at increasing distances h=Rb from the

efflux, in order to analyse the formation of the recircu-lating regions and their possible influence upon flame

development. Velocity fields were measured using a two-

component fiber optics Laser Doppler Velocimeter

equipped with an Argon ion laser and a Bragg cell with

40 MHz frequency shift for directional ambiguity reso-

lution. The optical system was operated in the back-

scatter mode and the signal processors were two Burst

Spectrum Analysers (BSA––Dantec). MicrometricAl2O3 particles were used as scattering centres: at least

10,000 instantaneous velocity data were acquired for

statistical analysis, with estimated statistical errors of

less than 2% in the mean values and 5% in the r.m.s.

fluctuations.

Mean temperature was measured using a Pt/Pt-13%

Rh bare wire thermocouple with 0.3 mm diameter bead.

The amplified signals were sampled at a 500 Hz sam-pling frequency and the mean value was based on 5000

instantaneous data. A correction was made for the

radiation error, following [8] and using the measured

velocity values for the evaluation of convective heat

transfer coefficient.

Emission spectroscopy in the visible range from the

flame front has been carried out to investigate the

presence of CH* radicals (k ¼ 431 nm), which can beconsidered as flame front tracers [9].

Finally, burned gases have been sampled for analysis

of the pollutant emissions (NOx and CO), through a

system based, respectively, upon chemiluminescence

and infrared analysis.

V (m/s)

-10

-5

0

5

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

r/R0

200

400

600

800T (°C)

(c)

Fig. 3. Mean axial velocity and temperature profiles at different dis-

tances h=Rb from the efflux. (a) h=Rb ¼ 0:66, (b) h=Rb ¼ 2, and

(c) h=Rb ¼ 4.

3. Experimental results

Fig. 3a–c reports the results relative to the radial

semi-profiles of mean axial velocity and temperature

(already corrected for radiative losses) at increasing

distances h=Rb from the efflux. The radial coordinate has

been normalised to the radius R of the cylindrical

combustion chamber.

From the analysis of the profiles, different regions(progressively interacting) can be distinguished:

• At h=Rb ¼ 0:66, it was observed a central relatively

‘‘cold’’ region characterised by a velocity peak (2.5

m/s) which can be attributed to the natural gas jet, al-

ready decelerated by the formation of the central

toroidal recirculation region, induced by swirl. The

air stream can be clearly identified by the velocitypeak at 17 m/s. The two parallel streams are sepa-

rated by the incipient formation of the central recir-

culation region induced by swirl effect. The central

recirculating hot products oppose to the fuel jet

(decelerating it, as previously noticed) and mix to de-

velop combustion and stabilise the flame close to the

burner exit. Fuel/air momentum ratio is the control-

ling parameter of this process, together with the swirl

motion. A recirculation region, characterised by a

quasi uniform temperature value (about 900 �C, pro-gressively cooling towards the quartz walls), is pres-ent at the region between the outflowing reactants

and the chamber walls. This reverse flow is induced

712 A. Coghe et al. / Experimental Thermal and Fluid Science 28 (2004) 709–714

by a corner recirculation due to the air stream radial

expansion and the wall confinement. The high tem-

peratures measured in this zone indicate the presence

of a large fraction of already burned gases which are

entrained by the reactant flow. At this short distancefrom the burner exit, the external recirculating flow is

hotter than the inner reverse stream and produces

a fast pre-heating of the incoming air jet.

• At h=Rb ¼ 2, the central fuel jet completely disap-

pears in the central recirculation region, indicating

complete mixing with the coaxial air stream and

counter-propagating burned gases. The consequent

development of combustion reactions is clearly visibleby the temperature increase (up to 1200 �C) in the

correspondence of the burner axis (see also the flame

visualization in Fig. 2). This traverse seems as crucial

for the onset of combustion reactions. Anyway, mea-

sured combustion temperature is quite low due both

to temporal and spatial averaging produced by the

thermocouple and thermal radiative transfer through

the transparent combustion chamber. Reactant airstream at lower temperature is still noticeable (at

0:22 < r=R < 0:5) both from velocity and tempera-

ture profiles, but the interesting fact is the more

intense strength of the corner recirculation phenome-

non with respect to previous measurement position,

reaching velocity values of )6 m/s and temperatures

higher than 1000 �C.• The profiles result more uniform at h=Rb ¼ 4, where

it can be noticed the weakening of the recirculating

regions, both central and peripheral: combustion

reactions should be almost completed, as indicated

by the uniform temperature profile.

The previous results are confirmed observing Fig. 4,

which compares the value of peak temperature mea-

sured in the central region of the burner with the emis-

0

20

40

60

80

100

120

0 1 2 3 4 5 6h/R

CH* intensity [%]

Fig. 4. Temperature peak values in the core region of the burne

sion intensity derived from CH* radicals, measured by a

line of sight technique, as a function of h=Rb coordinate.

The profiles deriving from the two different tech-

niques match well, both for initial increase and maxi-

mum at h=Rb ¼ 2� 3, confirming that this positionrepresents the full development of combustion reactions,

which are completed at h=Rb ¼ 5.

Moreover, a quantitative analysis of the recirculating

phenomena was obtained by integration of the mean

velocity radial profiles in the different regions (previ-

ously described). Thus, it was possible to estimate the

flow rate corresponding to every distinct stream: reac-

tants (ascending flow) or products (reverse flow). Fig. 5reports the behaviour of the recirculated flow rates as a

function of h=Rb: the values have been normalised toM0,

that is the injected mass flow rate through the burner.

Mre represents the flow rate corresponding to the posi-

tive velocity part of the radial profile, Mir is the central

recirculating flow rate andMcr is the result of integration

of the peripheral recirculation zone.

There is a progressive mixing of reactants with re-circulating hot products and thus the ascending (posi-

tive) mass flow rate initially increases. It can be observed

that the contribution of the central recirculation region

is very low, while the corner recirculating flow rate (and,

consequently, the positive flow) is maximum (about 1.5

times the total injected flow rate) in the correspondence

of h=Rb � 2, that is just the crucial traverse for com-

bustion reactions development. Therefore, it is evidentthe possibility of a strict interaction between the flame

and the surrounding recirculating flow, which can con-

tribute to local pre-heating and leaning of the reactant

mixture with respect to feeding conditions (so that the

value of the equivalence ratio U ¼ 0:69 fed to the burneris purely indicative).

Fig. 6 represents the NOx emissions measured at the

exhaust (for this result, also swirl number of the air

7 8 9 10 11 12b

0

200

400

600

800

1000

1200

1400

T (°C)

CH* intensityT peak central region (°C)

r and CH* emission intensity vs. distance from the efflux.

0.5

0.75

1

1.25

1.5

0.4 0.5 0.6 0.7 0.8 0.9Equivalence ratio

EINO

x [g

/kg]

S=0.7S=0.82

Fig. 6. NOx emissions in the exhaust gases as a function of equivalence

ratio U, for two different swirl numbers of the air stream.

Fig. 7. Qualitative schematization of the flow field inside the com-

bustion chamber.

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5h/Rb

Mre/M0 Mir/M0 Mcr/M0

M/M0

Fig. 5. Recirculating flow rates evaluated by the measured axial

velocity profiles as a function of h=Rb.

A. Coghe et al. / Experimental Thermal and Fluid Science 28 (2004) 709–714 713

stream and equivalence ratio were varied in order to

analyse the influence of these parameters upon NOx

formation). Nitric oxides emissions resulted quite low,

but high if compared to temperature levels measured

inside the combustion chamber (maximum at about

1200 �C, a value at which the formation of thermal NOx

would have to be very low). This result can be explained

by considering that the analysed flame is characterised

by intense local temperature fluctuations, which can

affect NOx formation. In fact, high turbulence intensitylevels have been registered in critical points of the

velocity measurements. In any case, it is interesting to

notice that NOx emission is almost insensible to the

value of equivalence ratio (in the lean regime), but is

strongly influenced by swirl intensity: a moderate in-

crease of swirl number from 0.7 to 0.82 can reduce NOx

formation up to 30%. This suggests the great influence

of fluid dynamic recirculating phenomena induced bythe swirl upon the main features of fuel/air mixing and

combustion processes, almost independently from the

feeding equivalence ratio conditions. Particularly, in-

crease of swirl strength promotes the generation of the

recirculating phenomena: the central one, which induces

rapid mixing and provides flame anchorage close to the

burner efflux and, consequently, the stabilisation of the

combustion process; while the corner recirculation re-

gion, characterised by hot products descending andbeing entrained by the fresh mixture, more strictly

interacts with the flame development and its radial

spreading (local pre-heating and progressive leaning of

the reactant mixture), favouring the reduction of

NOx formation, through internal burned gases recircu-

lation.

Fig. 7 finally resumes qualitatively the macro-scale

flow field derived from the measurements inside thecombustion chamber, with evidence of interaction

between the calyx-shaped flame, stabilised by the

central recirculation region, and the corner recircu-

lating stream, mainly formed by already burned

gases.

4. Conclusions

The recirculation phenomena inside a swirl natural

gas combustor have been analysed by means of different

experimental techniques. The results obtained put into

evidence that the onset of different recirculation regions,

connected to swirl strength, can strictly influence the

main combustion features. In fact, while the toroidal

central recirculation region is important for reactantsmixing and flame stabilisation, the corner recirculating

zone (close to the combustion chamber walls) induces

entrainment of a large amount of hot burned gases into

the outflowing reactant mixture.

The LDV measurements allowed a quantitative esti-

mation of the recirculating flows generated by the swirl

motion of the inlet air and a detailed analysis of their

influence on the flame stabilisation and combustiondevelopment. The observed large amount of internal

exhaust gas recirculation suggests the possibility of

714 A. Coghe et al. / Experimental Thermal and Fluid Science 28 (2004) 709–714

using this mechanism, depending on swirl intensity

and chamber diameter, to control flame dilution and

abatement of thermal NOx. A tentative explanation was

also provided of the observed decrease of NOx emissions

with increase of swirl intensity. In fact, swirl increase canhelp the reduction of temperature gradient during the

development of combustion reactions, also improving

reactants pre-heating and leaning the mixture with re-

spect to feeding conditions, positively influencing NOx

emissions at the exhaust, which resulted quite indepen-

dent (almost in the lean regime) from the equivalence

ratio fed to the burner.

Acknowledgements

This work has been supported by the MIUR (Min-

istry of University and Research) in the framework of a

FIRB Project. The authors are grateful for the assis-

tance provided by Mr. Stefano Dal Mas during the

conduction of the experimental activity.

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