Heat Release Calculation in a Turbulent Swirl Flame from ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2008/papers/07.2_6.pdf · chemiluminescence can be used for heat release measurements

Heat Release Calculation in a Turbulent Swirl Flame from Laser andChemiluminescence Measurements

Martin Lauer1, Thomas Sattelmayer2

1: Lehrstuhl für Thermodynamik, Technische Universität München, Garching, Germany, [email protected]: Lehrstuhl für Thermodynamik, Technische Universität München, Garching, Germany, [email protected]

Abstract This study presents a method for calculating the spatially resolved, time or phase averaged heat release of an unconfined, turbulent methane-air swirl flame from laser optical and chemiluminescence measurements. For optimal optical access the perfectly premixed flame is operated unconfined. As a consequence the air-fuel ratio of the flame is not constant due to ambient air entrainment. Therefore, the unconfined flame is a realistic model flame for technical applications with air-fuel ratio gradients. The heat release is calculated from a formulation of the first law of thermodynamics, using the density of the air-fuel mixture, the heat capacity, the fluid velocity and the fluid temperature. Since density, heat capacity and temperature are functions of the local air-fuel ratio and the reaction progress of combustion, the fluid velocity, the reaction progress of combustion and the air-fuel ratio are measured in this study. Optical methods are most suitable to obtain these values, as they are non-invasive and can provide spatially resolved data at high repetition rates.The flow velocity is measured via Particle Image Velocimetry (PIV) and the reaction progress of combustion via planar Laser Induced Fluorescence of the OH radical (OH-PLIF). The air-fuel ratio is determined from spectrally resolved chemiluminescence measurements. In former studies complete chemiluminescence spectra were measured. Because of the limitation of spectrometers to 1-dimensional spatial resolution, the air-fuel ratio of the complete flame volume could only be obtained by the sequential measurement of different planes of the flame volume. Thus, the achievable spatial resolution was only in the range of several millimeters. In this study a newly developed chemiluminescence measurement technique is presented, which overcomes this limitation. Measurements of a spectrometer are combined with bandpass filtered camera measurements to obtain the desired spectrally resolved information with high spatial resolution.

1. Introduction

The determination of the spatially resolved heat release in swirl flames is of great importance in many research fields like noise emission from flames and thermoacoustics. For example, the heat release fluctuation is used as input parameter for combustion noise models (Wäsle et al. 2005, Winkler 2007, Wäsle 2007), or for the stability analysis of burners (Auer et al. 2005, Auer et al. 2005a, Freitag et al. 2006).But the experimental determination of the heat release is problematic, since existing time resolved and spatially resolved measurement techniques in turbulent flames are too complex to be applied to the above mentioned research fields (Balachandran et al. 2005). In most cases chemiluminescence from radicals such as OH* or CH* is used to measure the heat release indirectly. It has been shown in many studies that in adiabatic, perfectly premixed, laminar or moderately turbulent flames OH* chemiluminescence can be used for heat release measurements (Ayoola 2006). But for technical flames with air-fuel ratio gradients or heat losses only qualitative relations between the chemiluminescence from OH* and the heat release of the flame have been found.In this study a new method is described, which uses the first law of thermodynamics to derive a formulation giving the heat release of a flame as the function of the following experimentally accessible flame parameters:

14th Int Symp on Applications of Laser Techniques to Fluid MechanicsLisbon, Portugal, 07-10 July, 2008

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mailto:[email protected]




• Progress variable of combustion, derived from planar laser induced fluorescence of the OH radical (OH-PLIF)

• Flow velocity, measured with particle image velocimetry (PIV)• Local air-fuel ratio, determined by evaluating the OH*/CH* chemiluminescence ratio

The link between the OH*/CH* chemiluminescence ratio and the air-fuel ratio has been shown in several studies (Haber 2000, Hardalupas et al. 2004, Ikeda et al. 2006, Nori and Seitzman 2007, Lauer and Sattelmayer 2007). But the evaluation of the OH*/CH* chemiluminescence ratio is problematic, since the OH* and CH* intensities are superimposed to the broadband emissions from CO2*. A spectrometer was needed to separate the intensities of the different chemiluminescent species. Thus, the achievable spatial resolution was limited. In this study a new procedure is presented, which combines measurements of a spectrometer with bandpass filtered camera measurements to obtain high resolution and CO2* corrected chemiluminescence intensities of the complete flame volume. The desired local air-fuel ratio can be determined from these data with high reliability and can be used for the calculation of the local heat release.The described method for determining the heat release is based on the measurement of flame parameters and an evaluation of the first law of thermodynamics. Hence the determined heat release is more reliable than heat release obtained from OH* measurements. Furthermore, the method is capable of handling flames with heat losses and air-fuel ratio gradients, which extends the field of application of OH* chemiluminescence. However, since the experimental variables cannot be acquired simultaneously, the described method is currently limited to time/phase averaged evaluations.The complete method for determining the spatially resolved heat release is tested at an unconfined swirl flame with ambient air entrainment. The flame is giving a realistic model for technical applications with air-fuel ratio gradients. Steady state flames with different air-fuel ratios as well as harmonically excited flames are investigated.In section 2 a short presentation of the theoretical background and a short review of the air-fuel ratio measurement via chemiluminescence is given. Section 3 gives an overview of the experimental setup used to check the presented method. The applied measurement techniques are presented in section 4, where the main focus is placed on the newly developed chemiluminescence measurement technique. First results are presented in section 5.

2. Theory and Background

In this section the basic approach for the calculation of the spatially resolved heat release is described. A formulation of the first law of thermodynamics is derived, from which the needed measures are identified. Since the required air-fuel ratio is derived from chemiluminescence measurements, the basics of this measurement technique, former applications and the limitations of previous publications are described also.

2.1. Basic approach from the first law of thermodynamicsThe investigated flame is described as an open, stationary system with constant pressure. For such flames the first law of thermodynamics reads:

Ql denotes the heat losses of the flame and !H0R the enthalpy of the reaction. It is defined as:

Ql =!H2 ! H0

"

products+ !H0

R +!H0 ! H1

"

reactants

!H0R = H0

products ! H0reactants


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The enthalpies of the reactants H1 and of the products H2 have to be referenced to their corresponding enthalpies at defined conditions H0(pref , Tref ) to take the chemical reaction into account. pref is 1 bar and Tref is 298.15K. Since flames at atmospheric pressure with non-preheated air-fuel mixture are under investigation in this study, the reference condition matches the condition of the reactants. This simplifies the equation above:

The desired actual heat release of the flame Qflame is the sum of the heat losses and the enthalpy of the reaction:

For a differential volume of the flame this equation reads:

The equation shows that the heat released from the combustion increases the sensible enthalpy of the fluid. The sensible enthalpy can be expressed by the fluid mass flow , the fluid temperature T and the isobaric heat capacity cp. Furthermore, the mass flow can be expressed by the fluid velocity

and the fluid density ρ. Since the velocity is a vectored quantity, the gradient of the temperature in the direction of the flow has to be considered:

The density and the heat capacity are functions of the fluid composition. They can be described by the air-fuel ratio λ and the time averaged progress variable of combustion c. The progress variable is necessary to take partial combustion into account, what can be seen in the chemical equation for partial methane combustion:

The temperature of the fluid can be estimated from the adiabatic flame temperature, which is derived from Chemkin calculations (Kee et al. 1989, Smith et al.) as a function of the local air fuel ratio. Again the time averaged progress variable is used to take partial combustion into account (Winkler 2007):

T0 is the temperature of the unburnt mixture, which is identical to the temperature of the ambient air. This leads to the final formulation:

From this formulation the desired measures for the evaluation of the heat release can be identified: The flow velocity , the progress variable of combustion c, and the air-fuel ratio λ.Note, that in this study all measured experimental variables are time or phase averaged, thus the calculation of the heat release is also limited to time or phase averaged states. Since the measured experimental variables have a discrete spatial resolution, the differential equation above is integrated numerically.

Ql =!H2 ! H0

"

products+ !H0

R

Qflame = Ql !!H0R =

!H2 ! H0

"

products

dQflame = dH2,products

dQflame

dV= (! · cp · ("v ! "T ))products

12CH4 + !

!O2 +

7921

N2

"! (1" c)

!12CH4 + !

!O2 +

7921

N2

""+ c

!H2O +

12CO2 + (!" 1)O2 +

7921

!N2

"

dQflame

dV= (!(", c) · cp(", c) · (#v ! "T (", c)))products


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2.2. ChemiluminescenceFirst publications about chemiluminescence date back to the 19th century. Since the middle of the last century it is used in combustion research (Clark 1958). The term chemiluminescence refers to radiation emitted from molecules, returning from excited state to basic state. It can be found in many chemical processes involving unstable and energetic intermediates, such as combustion processes. OH* and CH* are examples of excited molecules, often used in combustion research. They predominantly emit light in a narrow spectral band. Another molecule, which is also used often, is CO2*, which emits in a broad spectral range (Fig. 1, left hand side).

Important for the understanding and usage of chemiluminescence in combustion research are the reaction kinetics causing the formation of excited molecules. A simplified mechanism for the oxidation of methane is shown in Fig. 1 right hand side (Najm et al. 1998). OH* and CH* emerge from different side-paths of the methane oxidation and thus there is no direct link between OH* or CH* chemiluminescence and the heat release of the flame. However, Haber showed in 2000, that the OH*/CH* ratio unambiguously characterizes the air-fuel ratio of the reaction. This method has been used in a number of studies to determine the air fuel ratios of combustion processes: Haber (2000) investigated the basic effect in a methane fueled Bunsen type flame and a honeycomb flat-flame burner. Hardralupas et al. (2004) measured the local air-fuel ratio in single points of natural-gas fueled, premixed, counter flow flames. Ikeda et al. (2006) investigated the air-fuel ratio at the anchor point of premixed laminar propane/air and methane/air Bunsen type flame. Nori and Seitzman (2007) measured global chemiluminescence spectra of turbulent, premixed methane-air and syngas flames to obtain the air-fuel ratio.All the above mentioned publications used the OH*/CH* chemiluminescence ratio to measure either the global air-fuel ratio of a flame or to measure the local air-fuel ratio in single points of the flame. Wäsle et al. (2007) first measured the air-fuel ratio in a turbulent, unconfined methane-air flame with 1d spatial resolution on the burner axis based on this technique. Lauer and Sattelmayer (2007) determined the local air-fuel ratio of an identical flame with 2d spatial resolution.Since the broadband emission from CO2* interferes with the signals from OH* and CH*, a spectrometer was needed in all mentioned studies to separate the CO2* background from the desired OH* and CH* intensities. Without this separation the direct link between OH*/CH* ratio and the air-fuel ratio is lost (Lauer and Sattelmayer 2007). In the study of Lauer and Sattelmayer (2007) the achieved 2d spatial resolution was only in the range of several millimeters because complete chemiluminescence spectra were measured in every point of the flame. This limits the spatial resolution of the heat release, which can be achieved from these data. To increase the spatial resolution of the air-fuel ratio measurement, a new chemiluminescence measurement technique is developed in this study. The method is described in detail in section 4.3.Another problem dealing with chemiluminescence measurements is the line-of-sight integration. Methane-air flames are optically not dense, meaning that emitted light is not re-absorbed in the flame (Büchner 1992). As a consequence the measured intensities are integrated along the line of

280 320 360 400 440 4800.0

0.2

0.4

0.6

0.8

1.0

wavelength [nm]

OH*CH*

CO2*n

orm

aliz

ed

in

ten

sity [

-]

C H2 6 C H2 5 C H2 4 C H2 3 C H2 2 C H2

O

CH*

CH

M

O2

OH*

OO2OHCH +M3

H, OH H, M H, OH H, M

CH4 CH3 CH *2 CH2

H, OHN , CO , H O2 2 2OH

OH

OH, H, OH

H+M

OH+MHO2

CH HO3 CH O3

H M

O

O , M2H, OH, O, CH3 H, OH, M, O , H O2 2 OH

O M

O2

H O2O, O2O2

CO *2

CO2COHCOCH O2CH OH2

Fig. 1: Left hand side: A typical chemiluminescence spectrum from a premixed methane-air flame. The radical intensities from OH* and CH* are superimposed to the broadband signal from CO2*. Right hand side: The simplified reaction mechanism for methane (Najm et al. 1998). The main reaction path is marked with arrows. The chemiluminescent radicals OH* and CH* emerge from side paths of the oxidation. This means they are not linked to the heat release of the flame directly.


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sight of the sensors. A possibility for obtaining spatially resolved information from the line-of-sight integrated signals is a deconvolution algorithm (Dribinski et al. 2002). Caused by the underlying assumption of axial symmetry, this procedure is only valid for time averaged measurements in the case of turbulent flames (Lauer and Sattelmayer 2007). Since only time/phase averaged signals are under investigation in this study, a deconvolution algorithm can be applied.

3. Experimental Setup

3.1. Test RigThe experimental study is carried out with a modular, swirl stabilized burner with center body. The swirl number was held constant at S=0,55 during the experiments. The inner diameter of the nozzle

is D=40mm, the diameter of the center body is d=16mm (Fig. 2). Fuel is natural gas with a methane content of 98%. The air-fuel mixture is externally premixed to avoid any mixture fraction fluctuations in plenum and burner. The thermal power can be adjusted in the range from 10kW to 120kW, the air-fuel ratio inside the plenum can be varied between 0.8 and 2.0. At the bottom of the plenum a loudspeaker is mounted for acoustic excitation of the flow. Depending on the frequency, excitation amplitudes up to 100% of the mean flow velocity in the burner nozzle can be achieved. The test rig is operated with non-preheated air-fuel mixture.Since the flame is operated unconfined without a combustion chamber, the air-fuel ratio of the flame is not constant: As Wäsle et al. (2006) showed, a linear increase of the mass flow downstream the burner exit occurs due to turbulent mixing in the shear layer between the swirled flow and the quiescent ambient air. As a consequence the mixture becomes leaner (Wäsle et al. 2007, Lauer and Sattelmayer 2007). Only the local air-fuel ratio of the

mixture is influenced, because the premixed air-fuel mixture is of the same temperature as the ambient air. Thus it appears that in an unconfined flame the local air-fuel ratio has to be measured in order to obtain the desired local heat release of the flame.

3.2. Operation PointsIn all experiments the fuel mass flow is equivalent to 60kW thermal power. Two series of experiments are carried out. In the first, steady state flames with air-fuel ratios in the plenum of λ=1.0, λ=1.2, λ=1.4 and λ=1.6 are investigated. These experiments are used for the verification of the presented method: The integral value of the determined local heat release is compared to the expected heat release from the fuel mass flow.In the second series the air-fuel ratio is held constant at λ=1.2, and harmonically excited flames are investigated. These measurements are used to check the applicability of the presented method for dynamic processes. The excitation frequencies are 62.5Hz with 35% excitation amplitude and 125Hz with 40% excitation amplitude.All measures are acquired with 1 kHz repetition rate with phase resolved triggering for the excited measurements, resulting in 8 (at 125Hz) and 16 (at 62.5Hz), respectively, resolved phase angles.

4. Measurement Techniques

From the formulation of the first law of thermodynamics in chapter 2.1, the required experimental data for the calculation of the heat release of the flame have been identified. The progress variable


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Fig. 2: Sketch of the used test rig. The loud-speaker at the bottom of the plenum is not shown.

plenum

swirler

perfectly premixed

air-fuel mixture

is measured via planar laser induced fluorescence of the OH radical (OH-PLIF), the fluid velocity is measured by particle image velocimetry (PIV), and the air-fuel ratio is obtained from chemiluminescence measurements.These three measurement techniques are described in the following sections. The main focus is on the chemiluminescence measurement, which is described in detail in chapter 4.3. This new technique provides the desired air-fuel ratio of the flame with a high spatial resolution in the range of 0.1mm, which is a great improvement compared to former studies.Only a time/phase averaged examination is done in this study. The experimental data are taken in the flame mid-plane, which is regarded as a reference for the complete flame.

4.1. LIFThe progress variable of combustion is detected by OH-PLIF. From the sudden increase of the OH concentration, the boundary between the burnt gas and the unburnt premixed gases can be detected

(Eckbreth 1996). A Rhodamine 6G operated, frequency doubled dye laser, pumped with a Nd:YAG laser, is used for excitation of ground state OH radicals at 283nm. The laser system is operated with 1kHz repetition rate and 80µJ pulse energy. A light sheet is formed by a cylindrical lens in combination with a spherical lens. The illuminated height of the flame is 2.5D. A high speed camera with a fiber optical coupled image intensifier and an UV capable camera lens is used to detect the fluorescence signal at 308nm. The lens has a focal length of 45mm and a maximum aperture of 1:1.8.The signal is bandpass filtered for the suppression of scattered laser light. The spatial resolution of the CMOS sensor of the camera is 1024 by 1024 pixel, allowing a spatial resolution in the flame in the range of 0.1mm. With this resolution a maximum of 2048 frames can be captured. Figure 3 shows a result of a phase resolved, synchronized PIV/LIF measurement from an excited flame at 125Hz and 40 percent excitation amplitude. The left hand side and right hand side of the figure show two different phase angles with 180 deg phase shift.

4.2. PIVFor PIV measurements in the reacting flow a high speed double cavity Nd:YLF laser (527nm, 10mJ per pulse) is used. The light sheet again has a height of 2.5D. The width is about 2mm. In this study the double pulses are separated by 30µs. The detection camera is identical to the PLIF camera except for it is not intensified.The camera uses a 85mm focal length lens with a maximum aperture of 1:1.4. However, the aperture was closed to 1:5.6 during the experiments to increase the depth of focus. Additionally, a 532nm band pass filter was used for the suppression of disturbing signals. The maximum transmission of the filter amounts to 90%, the half-power bandwidth to ±10nm.The double frames were analyzed with the commercial software VidPIV from ILA. The interrogation area measured 32 by 32 pixel. A two-step adaptive cross-correlation with 12 pixel separation was used. The resulting spatial resolution of the calculated velocity field is 1.4 mm. TiO2 particles are used as tracer, due to their high temperature resistance.

4.3. Chemiluminescence measurementChemiluminescence measurements are used to obtain the desired air-fuel ratio. In former studies


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Fig. 3: An example for a phase resolved PIV/PLIF measurement in an excited flame. The phase shift between the left hand side of the figure and the right hand side is 180°. The arrows visualize the velocity field of the flame (from PIV), the color represents the time averaged reaction progress. “1” stands for totally burnt gases, “0” symbolizes unburnt air-fuel mixture.

radial coordinate [r/D]

axia

l co

ord

ina

te [

x/D

]

-1.5 -1 -0.5 0 0.5 1 1.50

0.5

1

1.5

2

2.5

3

0

0.2

0.4

0.6

0.8

1

=0

=0 +180 deg

complete chemiluminescence spectra were measured in every point of the flame. This limited the spatial resolution, which was achieved, to several millimeters. Nevertheless, the measurement of complete spectra with a spectrometer was necessary, because otherwise the required intensities of OH* and CH* could not be separated from the broadband CO2* background (Lauer and Sattelmayer 2007). In this study a new measurement procedure is presented: Measurements with a spectrometer and bandpass filtered measurements with an image intensified camera are used to combine the high 2-dimensional spatial resolution of the bandpass filtered measurements with the CO2* correction from the spectrometer measurements. Three different chemiluminescence measurements are used in this procedure:

• The chemiluminescence spectrum with 1-dimensional spatial resolution is measured on the burner axis. The used spectrometer is an Acton Research Corporation SepctraPro - 275. The attached camera is identical to the camera used for the PLIF measurements (section 4.1). The optical system of the spectrometer is a Czerny-Turner type with in-line optical path. The focal length is 275mm with an aperture ratio of 1:3.8. The used grating in this study has 150g/mm, allowing the observation of approximately 300nm of the flame spectrum with the attached camera. An UV capable camera lens with 105mm focal length and a maximum aperture of 1:4.5 is used to project the flame on the slit of the spectrometer. The slit width is set to 10µm.

• The bandpass filtered chemiluminescence of the flame is measured with the identical image intensified camera and lens used for the PLIF measurements (section 4.1). Two different bandpass filters are used:• The chemiluminescence of OH* with superimposed CO2* background is measured with an

interference filter with a maximum transmission of 16.57% at 309.65nm and a half-power bandwidth of ±5.1nm

• The chemiluminescence of CH* with superimposed CO2* background is measured with an interference filter with a maximum transmission of 48.63% at 431.39nm and a half-power bandwidth of ±5.3nm

These experimental data are line-of-sight integrated, because of the low optical density of the flame (see 2.2).

In a first step the spectrometer measurements are evaluated. The intensities of OH* and CH* are separated from the broadband CO2* background. The separation procedure is shown in Fig. 4 and described in detail in Lauer and Sattelmayer (2007).

280 320 360 400 440 480wavelength [nm]

no

rmaliz

ed

in

ten

sity [-]

1.0

0.8

0.6

0.4

0.2

0.0

line-of-sightmeasurementon burner axis

segmentation of spatial coordinateto obtain spatial resolution

wavelength coordinate

spatial coord

inate

separation of pure radical intensitiesfrom broadband CO2* background

normalized radical

chemiluminescence

intensity [-]

1.00.0

axia

l co

ord

inate

x/D

[-]

1.0

2.0

3.0

OH* CH*

Fig. 4: The separation procedure for obtaining the CO2* corrected radical chemiluminescence intensities: The spectrometer measurement on the burner axis is divided into stripes, each representing a certain axial position downstream the burner exit. Each stripe is evaluated separately for obtaining spatial resolution in axial direction. In each stripe the intensities are integrated over the remaining part of the spatial coordinate for an increased signal to noise ratio, resulting in 0-dimensional chemiluminescence spectra. The intensity of the CO2* background is assumed to be linear in the wavelength range of the radicals, allowing the separation of radical and background intensities (Haber 2000). The remaining radical intensities are integrated over the wavelength coordinate for obtaining a characteristic value for the chemiluminescence intensity of the considered radical. The integral value of each stripe results in one point of the axial resolved, CO2* corrected radical chemiluminescence intensity.


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The results are line-of-sight integrated, CO2* corrected radical intensities on the burner axis with 1-dimensional spatial resolution. Because of the line-of-sight integration these data represent the mean OH* and CH* intensities of the flame as a function of the axial coordinate.From the bandpass filtered intensities, also the intensities on the burner axis are taken as mean values of the superposition of CO2* and radical intensities as a function of the axial coordinate. From these axial intensity distributions (bandpass filtered on the one hand, CO2* corrected on the other hand) a correction function is derived, to calculate the mean CO2* corrected intensities from the mean bandpass filtered measurements.In the second step of the procedure, the bandpass filtered measurements of the complete flame are deconvoluted to obtain the local chemiluminescence intensities. Again these chemiluminescence intensity distributions in the flame mid plane are the superpositions of radical intensities and the broadband CO2* background. These 2-dimensional chemiluminescence distributions are translated to CO2* corrected radical intensities with the correction function calculated before. The complete chemiluminescence measurement procedure is shown in Fig 5. The achieved spatial resolution of this procedure ranges in the order of magnitude 0.1mm.

4.4. Calculation of the air-fuel ratio from OH*/CH* chemiluminescence ratioAfter obtaining the CO2* corrected radical intensities of the 2-dimensional chemiluminescence fields as described before, the local OH*/CH* ratio can be calculated. The chemiluminescence ratio is converted to an air-fuel ratio with an empirically determined calibration curve. For the determination of the calibration curve, the flame is shielded with a quartz cylinder to avoid ambient air entrainment, which results in flames with constant air-fuel ratio (Lauer and Sattelmayer 2007).

flame

line-of-sight integrated, band-

pass filtered measurement

line-of-sight integrated,

bandpass filtered inten-

sity on burner axis

line-of sight integrated chemiluminescence

spectrum on burner axis

line-of-sight integrated

axial radical chemilumi-

nescence intensities on

burner axis

deconvoluted, bandpass

filtered measurement

spectrometer

intensified

camera with

bandpass filter

deconvoluted CO2* corrected

radical intensity

Fig. 5: Overview of the chemiluminescence measurement technique. In the formulae * marks CO2* cor-rected radical chemiluminescence intensities, + marks the superposition of radical and CO2* intensity from bandpass filtered measurements. The bar symbolizes radial averaged intensities because of the line-of-sight integration of the measurement.


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5. Results

5.1. Steady state air-fuel ratio fieldsThe obtained air-fuel ratio fields for the steady state measurements are shown in Fig. 6. The air-fuel ratio at the burner nozzle matches the air-fuel ratio in the plenum, and the results show the theoreti-cally expected increase of the air-fuel ratio, starting about 1D downstream the burner nozzle (Wäsle et al. 2007). The result of λ=1.0 matches the result of Lauer and Sattelmayer (2007) very well. This means that the newly developed chemiluminescence measurement technique is verified with a measurement based on complete chemiluminescence spectra in every point of the flame.

The maximum observed local air-fuel ratio is increasing from 1.5 (λ=1.0) to 1.7 for the other pre-sented steady state operation points. Since the lean blow-off limit of the unconfined flame is 1.7, an incomplete conversion of the fuel mass flow has to be expected with increasing air-fuel ratio. Hence, a decreasing total heat release of the flame has to be expected with increasing air-fuel ratio.

5.2. Heat release - steady stateThe calculated spatially resolved heat release of the steady state flames is shown in Fig. 7. The re-sults show that the main part of the heat release occurs near the burner exit. The maximum local heat release is decreasing with increasing air-fuel ratio. This can be explained with the decreasing fuel content of the mixtures (Turns 2000).

!plenum=1.0

r/D [-]0.0 0.5 1.0

x/D

[-]

0.5

1.0

1.5

2.0

2.5!plenum=1.2

r/D [-]0.0 0.5 1.0

!plenum=1.4

r/D [-]0.0 0.5 1.0

!plenum=1.6

r/D [-]0.0 0.5 1.0

0.8

1.0

1.2

1.4

1.6

1.8

!(r

,x)

[-]

Fig. 6: Spatially resolved air-fuel ratios in the flame mid plane for the four investi-gated steady state flames.

!plenum=1.0

r/D [-]0.0 0.5 1.0

x/D

[-]

0.5

1.0

1.5

2.0

2.5!plenum=1.2

r/D [-]0.0 0.5 1.0

!plenum=1.4

r/D [-]0.0 0.5 1.0

!plenum=1.6

r/D [-]0.0 0.5 1.0

0.0

0.2

0.4

0.8

1.0

1.2

0.6

Q(r

,x)

[GW

/m3]

.

Fig. 7: The spatially resolved heat release of a turbulent methane-air flame with air-fuel ratio gradients. The fuel mass-flow is equivalent to 60kW thermal power. The air-fuel ratio inside the plenum is increasing from left to right.


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The integration of the spatially resolved heat release shows the expected decrease of total heat re-lease (Fig. 8). The integral value of the λ=1.0 flame is 56,5kW. This is in very good agreement with the expected value from the measured fuel mass flow. The integral value of the λ=1.6 flame near the lean blow-off limit of the burner amounts to only 27.1kW. This means more than 50% of the fuel mass flow are not burnt in the flame due to quenching of the reaction at the lean blow-off limit of the flame.

5.3. Heat release - harmonically excitedThe steady state flames were used to investigate the capability of the newly developed method for determining the spatially resolved heat release. To investigate the capability for analyzing dynamic processes of unstable flames this method is applied to harmonically excited flames also.Figure 9 shows the spatially resolved heat release of the harmonically excited flame at 125Hz. The air-fuel ratio is 1.2. The mean integral heat release is 43.0kW, which matches the value of the unexcited flame. The amplitude of the integral heat release lies in the range of the excitation amplitude of the flow velocity. The corresponding plot for 62.5Hz excitation is shown in Fig. 10.

6. Conclusion and outlook

In the presented study the spatially resolved heat release of turbulent swirl flames was obtained from laser and chemiluminescence measurements. The experimental quantities are used as input parameters for the first law of thermodynamics, which is used to calculate the heat release. This method gives more reliable results than common methods which use the chemiluminescence of a single radical to measure the heat release indirectly. The presented method is also capable of analyzing harmonically excited flames with air-fuel ratio gradients. With the new technique the phase resolved heat release of unstable flames can be determined if optical access can be provided.

1.0 1.2 1.4 1.6

air-fuel ratio [-]

inte

gra

l heat re

lease [kW

] 60

50

40

30

20

10

0

Fig. 8: The determined integral heat release of the examined steady state flames. The total heat release is decreasing with increasing air-fuel ratio.

0.0

Q(r,x) [GW/m3].0.2 0.4 0.6 0.8 1.0 1.2

!0 + 315 deg!0 + 270 deg!0 + 225 deg!0 + 180 deg!0 + 135 deg!0 + 90 deg!0 + 45 deg!0

Fig. 9: Spatially and phase resolved heat release of the λ=1.2 flame at 125Hz excitation. The excitation amplitude of the flow velocity in the burner nozzle is 40%.


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The method is validated by the comparison of the integral value of the determined heat release with the expected value from the fuel mass flow. The expected integral values are reproduced with high accuracy for flames without quenching. Also, the phase resolved heat release in harmonically excited flames show the expected values, even though these data need further analyses. For the validation of the determined local heat release, further studies must be conducted. The local fluid temperature is the most critical input parameter of the presented study. It was determined from the local time averaged progress variable of combustion and the local air-fuel ratio. The fluid temperature will be checked with alternative measurement techniques.

7. Acknowledgements

The authors gratefully acknowledge the financial support provided by the Deutsche Forschungsgemeinschaft through the Research Unit „Chemilumineszenz und Wärmefreisetzung“.

8. References

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Fig. 10: Spatially and phase resolved heat release of the λ=1.2 flame at 62.5Hz excitation. The excitation amplitude of the flow velocity in the burner nozzle is 35%.

0.0

Q(r,x) [GW/m3].0.2 0.4 0.6 0.8 1.0 1.2

!0 + 337.5 deg!0 + 315 deg!0 + 292.5 deg!0 + 270 deg!0 + 247.5 deg!0 + 225 deg!0 + 202.5 deg!0 + 180 deg

!0 + 157.5 deg!0 + 135 deg!0 + 112.5 deg!0 + 90 deg!0 + 67.5 deg!0 + 45 deg!0 + 22.5 deg!0

Büchner H (1992) Experimentelle und theoretische Untersuchung der Entstehungsmechanismen selbsterregter Druckschwingungen in technischen Vormischverbrennungssystemen. Dissertation TH Karlsruhe

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Haber LC (2000) An investigation into the origin, measurement and application of chemilumines-cent light emissions from premixed flames. M.Sc. Thesis VirginiaTech

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Wäsle J, Winkler A, Rössle E, Sattelmayer T (2006) Development of an annular porous burner for the investigation of adiabatic unconfined flames. 13th Int. Symp. On Appl. of Laser Tech. to Fluid Mechanics

Wäsle J, Winkler A, Sattelmayer T (2005) Spatial Coherence of the Heat Release Fluctuation in Turbulent Jet and Swirl Flames. Kluwer, Flow Turbulence and Combustion 74

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Kee R, Miller J, Jefferson T (1989) CHEMKIN: A General-Purpose, Problem-Independent, Trans-portable, Fortran Chemical Kinetics Code Package. Sandia National Laboratories, Sandia Report SAND80-8003

Smith G, Golden D, Frenklach M, Moriarty N, Eiteneer B, Goldenberg M, Bowman C, Hanson R, S o n g S , G a r d i n e r W J r , L i s s i a n s k i V, Q i n Z : G R I - M E C H 3 . 0 . http://www.me.berkeley.edu/gri_mech/

Turns S (2000) An Introduction to Combustion (Second Edition). McGraw Hill Boston


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Documents

Heat Release Calculation in a Turbulent Swirl Flame from ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2008/papers/07.2_6.pdf · chemiluminescence can be used for heat release measurements