CO2∗ ) chemiluminescence technique for equivalence ratio mapping in turbulent stratified flames

Energy 192 (2020) 116485

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Two-line (CH* /CO*2 ) chemiluminescence technique for equivalence ratio mapping in turbulent stratified flames M. Mustafa Kamal Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 May 2019 Received in revised form 9 October 2019 Accepted 3 November 2019 Available online 9 November 2019

Equivalence ratio fluctuations play an important role in exploring and controlling flame dynamic instabilities such as thermoacoustic combustion instabilities associated with lean premixed gas turbine (GT) engines. However, it is difficult to reliably measure local equivalence ratio, and so we often make estimates based on the ratios of chemiluminescent emissions. Recent studies have reported that equivalence ratio varies with chemiluminescence intensity ratio of CH
and CO
2 in the visible spectrum (431 and 410 nm, respectively). A methodology based on this relationship is adopted in the present study to construct equivalence ratio maps for a series of turbulent premixed and stratified methane/air flames under globally lean conditions (mean or global equivalence ratio, f ¼ 0:75), over a range of stratification with f spanning 0.375e1.125 for the highest level of stratification. In contrast with the previously used CH
/OH
method, the current technique allows for a single visible range camera to be used for chemiluminescence imaging. A comparison of the current estimated equivalence ratio with previously calculated local equivalence ratio from Raman-scattering measurements shows that the two-line chemiluminescence technique can be reliably used to determine equivalence ratio in the instantaneous reaction zone of turbulent flames even at higher turbulence levels and mixture stratification conditions. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Two-line chemiluminescence Turbulent combustion Stratified flames Excited species Raman measurements Gas turbine engines

1. Introduction A wide range of combustion characteristics and parameters can be determined by using diagnostic techniques that are based on the analysis of emission spectrum of a flame. Flames emit spontaneous electromagnetic radiation via several different phenomena [1]: (a) Rotation emission bands are exhibited by gas molecules at elevated temperatures; (b) Black-body spectrum is produced by solid particles such as ash, soot or char; (c) Chemiluminescence results when intermediate (or excited) species, generated during a chemical reaction, reach to their equilibrium ground-state by emitting light at their characteristic wavelength. The spectral range of the emissions varies for the different effects associated with these phenomena. Electromagnetic radiation by solid bodies and gas molecules at typical flame temperatures, occurs in the visibleinfrared (VIS-IR) and infrared (IR) ranges, respectively, whereas chemiluminescence is detected in the ultraviolet (UV) and visible (VIS) spectrum. Based on emission intensity, black-body radiation is

E-mail address: [email protected]. https://doi.org/10.1016/j.energy.2019.116485 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

considered as the major effect and chemiluminescent emission the weakest. The spectrum and magnitude of those three phenomena predominantly depend upon the combustion parameters and emission properties of a flame. In the present study, only chemiluminescent radiations are focused for flame diagnostics. The intermediate chemical species are not only formed in the flame through chemical reactions but also by thermal excitation [1e3]. Besides, these excited radicals are formed in much higher concentrations than their equilibrium ground values. In natural gas flames that do not produce large particles such as soot, the colour of the flame is dominated by exited species created in the combustion reaction. These electronically excited species are at an elevated energy state and usually formed by the reaction of intermediate combustion products such as CH, H, OH, CH2, and C2H or by the collision of these intermediate products with other bodies. When the excited radicals return to their equilibrium ground-state, they emit a photon of light at a unique characteristic wavelength or band of wavelengths. The emission of this light is known as the phenomenon of chemiluminescence. The chemiluminescence emission process involves two elementary reaction steps: (a) Two parent chemical species combine to form an excited radical R
and (b) this

2

M.M. Kamal / Energy 192 (2020) 116485

Nomenclature

K Nt

fi fo fg Ui Uo Uco Re Rei Reo r z h v

st ss

equivalence ratio for the inner flow equivalence ratio for the outer flow mean or global equivalence ratio bulk velocity in the inner flow bulk velocity in the outer flow bulk velocity in the co-flow Reynolds number Reynolds number in the inner flow Reynolds number in the outer flow radial position axial position Planck’s constant frequency wavelength chemiluminescence signal intensity mass fraction

l

I Y

Rc

constant for species total number of realizations time variance spatial variance chemiluminescence intensity ratio

Abbreviation GT gas turbine VIS-IR visible-infrared IR infrared UV ultraviolet VIS visible R radical R* excited radical HC hydrocarbon ICCD intensified charge-coupled device FSD flame surface density SR stratification ratio

excited radical then lose its additional energy by emitting a photon in the form of electromagnetic radiation, hv, to reach its ground energy state:

species A þ species B/R
þ others

(1)

R
/ R þ hv

(2)

The radiation characteristics such as wavelength, depend on the properties of the radical R and the specific chemical transition. The individual excited molecule (i.e. radical R
) is characterized by one or more spectral lines that can be combined together into characteristic band spectrum. The collective effect of the several different emitting species (or excited molecules) involved in the process, produces the chemiluminescence emission band spectrum of a particular flame. As per Eq. (2), the emitted light (hv) is in direct proportion to the concentration of the intermediate species which, in turn, depends on the rate of its formation and subsequent destruction. Besides spontaneous electromagnetic radiation, excited species are eliminated by quenching in atomic and molecular collisions with other chemical species [4]. The main emitters of chemiluminescence in hydrocarbon (HC) flames are CO
2 , CH
, OH
, and C
2 radicals. The elementary reactions that are generally considered as the main formation routes of these excited radicals, strongly depend on temperature and the involved chemical species (including both fuel and oxidiser) whether stable or intermediate [5e8]. These reactions together with the bandhead wavelengths are detailed in Table 1. OH
radical emits radiation in UV range and is mainly formed by reaction (R1) in HC flames, whereas its formation in hydrogen flames is due to reactions (R2) and (R3). Reactions R4 and R5 lead to the formation of CH
(blue

light) and C
2 (green light) emissions, respectively. CO
2 is characterized by a continuous emission represented by a broadband distributed background signal. The radiations from NO
and CN
radicals are mostly negligible except in the case of high nitrogen content flames. The peaks and range of wavelengths associated with the various excited species can be seen in the flame spectrum shown in Fig. 1. The distinct peaks associated with CH
, OH
, and C
2 provide useful information on the nature of the combustion process as they are particularly prevalent in the reaction zone. Chemiluminescence mechanism involve various flame parameters and therefore, the resultant emissions are strongly related to the combustion characteristics and flame stability. Accordingly, chemiluminescence emissions of particular excited radicals at certain wavelengths have been widely employed as a flame

Fig. 1. Chemiluminescence spectrum of a typical hydrocarbon flame. The narrow band radical emission from CH
, C
2 , and OH
are superimposed by the continuous emission from CO
2 [10].

Table 1 Formation routes (based on elementary reactions) and characteristic wavelengths of excited radicals in a combustion process [9]. Excited radicals

Formation reactions

Characteristic wavelengths (nm)

OH


R1 : CH þ O2 /CO þ OH * R2 : H þ O þ M/OH* þ M R3 : OH þ OH þ H/OH* þ H2 O R4 : C2 H þ O2 /CO2 þ CH* R5 : C2 H þ O/CO þ CH* R6 : CH2 þ C/C
2 þ H2 R7 : CO þ O þ M/CO
2 þ M

282.9, 308.9

CH
C
2

CO
2

387.1, 431.4 513, 516.5 Continuous spectrum 350-600

M.M. Kamal / Energy 192 (2020) 116485

diagnostic technique to measure important combustion parameters such as equivalence ratio, heat release rate, flame structure, and reaction zone (i.e. the flame front location) in a flame [9,11]. Chemiluminescence is a non-intrusive optical diagnostic technique and does not require the presence of tracer particles. Correlation between the chemiluminescence intensity and various flame parameters is an active topic of research. Chemiluminescence technique is commonly applied for the measurements of heat release rate fluctuations in unsteady flames in order to study instability mechanisms [12e18] and/or to determine transfer function relating instantaneous heat release rate in the flame to an external forcing factor (such as pressure variations or equivalence ratio fluctuations) [19e21]. Chemiluminescence method is also applied for the detection of the flame location by assuming the flame to exist only if the electromagnetic radiations captured in a particular spectral band surpasses a pre-determined intensity threshold. Using OH* as a flame marker or blowout precursor, Muruganandam et al. [22,23] evaluated the flame blowout conditions by identifying drop in the intensity of OH* signal below a certain threshold. Roby et al. [24] investigated CH* and OH* chemiluminescence signals for flame detection during light-off ignition in GT engines, and achieved a response time of under 2 ms. The chemiluminescence intensity emitted by the excited radicals depends on the temperature, pressure, fuel composition and properties, total mass flowrate through the flame surface, equivalence ratio, degree of premixing, turbulence intensity, flame curvature, strain rate, and possible other parameters [11,25]. These dependencies are different for every chemiluminescence species and are caused by flame’s local reaction conditions. In case of turbulent flames, these quantities may fluctuate both in space and time. High Turbulence levels cause increased strain rate potentially leading to significant structural modifications in the flame’s reaction zone. Therefore, chemiluminescence intensity of any species should be used with great caution for the evaluation of the local heat release rate in a turbulent flame. However, if the chemiluminescence signal is averaged both spatially and temporally over the entire reaction zone, the turbulence effects may diminish. Previous studies found that the chemiluminescence intensity of excited species is considerably dependent on pressure and equivalence ratio while the influence of strain rate on the intensity is found to be less pronounced [2,8,26,27]. A number of experimental and computational studies have explored the reliability and accuracy of local optical visualization to trace flame fronts and explore local flame structure [12,13,28e31]. Some research studies have compared the chemiluminescence mechanisms with high spatial resolution measurements of chemiluminescence intensity [2,3,32]. Other studies compared analytical estimates with line-of-sight measurements in both laminar and turbulent combustion, and reported positive results [26,33,34]. The relationship between local heat release rate and chemiluminescence intensity has been particularly investigated by a number of research works. Price et al. [35] reported a linear relationship between average chemiluminescence emission from C
2 radicals and volumetric flowrate of combustible mixture unaffected by turbulence. Lawn [36] studied the spatial cross-correlation of chemiluminescence signal, and concluded that chemiluminescence emission is a reliable indicator of instantaneous heat release rate in reacting flows. Balachandran et al. [37] and Ayoola et al. [38] compared spatially-resolved CH
and OH
chemiluminescence measurements, flame surface density and instantaneous heat release rate (calculated as: ½CH2 O  ½OH), and reported similar behaviour and results. The results showed that either flame surface density or OH
chemiluminescence emission data can be used to estimate instantaneous heat release rate [37,38]. Hardalupas et al.

3

[39] and Gillet et al. [40] measured trends of chemiluminescence intensity of CH
, CO
2 , and OH
radical for variations in strain rate and equivalence ratio, suggesting these excited species as reliable markers for heat release rate whereas C
2 emission is not an effective indicator. Nori and Seitzman [26,34] suggest, however, that CH
and OH
chemiluminescence signals may not be completely reliable markers for heat release rate because of the influence of pressure and equivalence ratio. Farhat et al. [41] suggests that CH
and OH
may not be effective markers of unsteady waves (such as in rarefaction zones). Computational results of Najm et al. [42] also shows that CH
, C
2 , and OH
chemiluminescence may not work as reliable local indicators of instantaneous heat release rate in flame regions with high curvature magnitudes. On the other hand, Kim et al. [28] reported a decent correlation between chemiluminescence emission signal (ranging between 350 and 390 nm) and local heat release rate fluctuations in cool flames for the case of IC engines. The various research studies by Hurle et al. [43], Clark [44], Ayoola et al. [45], Lee and Santavicca [15], and Najm et al. [46] summarize the issues related to local heat release rate measurements via. chemiluminescence in highly turbulent flames. Many studies that are mainly focused on estimating flame stoichiometry from optical signal have analysed the effect of equivalence ratio on the chemiluminescence emission. Bobusch et al. [47] have applied CH
and CO
2 chemiluminescence signals as indicator of global equivalence ratio. The relationship between equivalence ratio f and chemiluminescence intensity I is further discussed in the next section. 2. Relationship between chemiluminescence intensity and equivalence ratio Equivalence ratio f is a key operating condition for any combustion system. It has significant effects on energy conversion, heat losses, pollutant emission, and stability of a flame. Flames with equivalence ratios lower than the optimal values are close to their lean flammability limits. Fuel-lean flames result in high production of carbon monoxide and are prone to both static and dynamic instabilities. Therefore, reliable evaluation of flame stoichiometry is always essential in any combustion system particularly in the case of lean premixed combustors which are commonly used as lowNOx technology in GT engines. The intensities of CH
, C
2 , and OH
chemiluminescence signals vary with air-to-fuel ratio, and hence equivalence ratio measurements based on this phenomenon is considered both feasible and reliable under certain conditions. For this purpose, Docquier et al. [48] proposed to calculate the ratio between two chemiluminescence spectral bands (e.g., CH
=C
2 , OH
=CH
) instead of using a single chemiluminescence signal. An important advantage of such an approach is that this greatly reduces optical or geometrical parameters related interference of the optical instruments in the detectors’ field of view. Besides, this normalization ensures stable results in case of variations in local heat release rate caused by turbulent fluctuations or variations in power input. The selection of the most suitable pair of spectral signals for a specific diagnostic application is based on certain criteria. As such, the signals’ intensity ratio should be ideally a strong monotonic function of equivalence ratio and sensitive enough to other combustion parameters in order to reliably measure the equivalence ratio. Generally, the OH
=CH
intensity ratio has shown to fulfil this criteria as founded by various experimental studies [2,3,26,34,49e53]. Since OH
=CH
signal ratios are relatively insensitive to variations in flame aerodynamics, turbulence intensities [49,51] and strain rates [2,39,40,54], therefore the relationship between OH
=CH
intensity ratio and equivalence ratio

4

M.M. Kamal / Energy 192 (2020) 116485

remains effective for a wide range of flow rates, combustor designs, turbulence conditions or other operating conditions. Panoutsos et al. [2] performed computational simulation based on detailed chemical reaction mechanism to show that the ratio of chemical luminescence intenisty of OH
to CH
is independent of local strain rates. The C
2 =CH
intensity ratio has also been found to have a strong but less linear relationship with equivalence ratio [3,52,55]. In some cases C
2 =CH
ratio exhibits a non-monotonic dependence [56] and is strongly influenced by local strain rates [39]. It is worthy to note here that the chemiluminescence signals collected in a characteristic spectral band of a certain excited species (e.g., CH
at l 431 nm), also includes a ‘background’ chemiluminescence due to broadband CO
2 emissions. Some experimental studies have recorded the spectral distribution of this background signal by choosing a spectral region not influenced by the CH
, C
2 , or OH
band. Different empirical relationships between the CH
= CO
2 intensity ratio and equivalence ratio have been reported in the literature: direct relationship has been found by Refs. [39,48,53] whereas an inverse relation is observed by Ref. [57]. CO
2 = OH
ratio is mostly used for exploring the structure of non-hydrocarbon flames; however, this ratio is found to be less sensitive to equivalence ratio [26,39]. The various common trends as identified in the literature are certainly not quantitatively identical besides the qualitative disagreements that are found among some of the underlying experimental studies. The experimental curve of chemiluminescence intensity, I, and equivalence ratio, f, varies both quantitatively and qualitatively with various operating and characteristic parameters especially with the fuel composition [30,51,55]. This is partially because of the influence and interference of the continuum CO
2 chemiluminescence signal in the background [49]. Therefore, it is very important to subtract the background CO
2 intensity from the overall signal intensity in order to estimate the actual emission associated to a selected excited species. Apart from that, the Ief curve is also influenced by the optical spatial resolution of the detection equipment. These various aspects potentially influence the observed variation in chemiluminescence signal intensity captured at specific wavelengths in regard to the operating conditions [27,49]. It is, therefore, essentially necessary to estimate equivalence ratio from optical signals based on an ad-hoc calibration relating intensity ratios and equivalence ratio in a particular flame diagnostic application. Various such relationships have been observed and, subsequently, presented in the form of empirical correlations, relating chemiluminescence measurements with equivalence ratios and other parameters [27,34,39] [40,47,50,52,55,57,58]. In the current analysis, the chemiluminescence intensity ratio of CH
(431 nm) and CO
2 (407 nm) signals are chosen to measure the equivalence ratio for certain advantages. First of all, the two wavelengths are in very close proximity to each other, which eliminates the issues of nonidentical focal planes. Tautsching et al. [59] introduced three lens experimental method to detect chemiluminescence image of CH
, OH
and CO
2 at the same time; however, the images were distorted due to different optical path through focal lenses, and require correction in post-processing. Whereas working only with CH
and CO
2 signals allows for using one ICCD camera with an image doubler optical setup for detecting both chemiluminescence signals simultaneously from the same focused area on flame brush. Secondly, both CH
and CO
2 signals are within the visible (VS) range of the electromagnetic spectrum, and thus their measurements do not require any UV-lens. Finally, it is easier to subtract the CO
2 contribution in the present work. Chemiluminescence spectrum of premixed methane-air flames has been measured by many studies [10,11,60]. The narrow band radical emissions from OH
, CH
and C
2 are superimposed by the

broadband emission from CO
2 , as shown in Fig. 1. The first large peak around 310 nm represents OH
transition, while the CH
transition emits light around 431 nm as the second peak. Both peaks are superimposed by the broadband CO
2 emission. It can be seen that the CO
2 contribution in the measured signal can be of the same order or even exceeds the contribution from the radical species. Therefore, the CO
2 under each peak has to be subtracted to calculate the ‘pure’ OH
and CH
chemiluminescence intensity. The distribution of CO
2 increases at a rather large slope under OH
peak, but almost stays constant around CH
peak. Thus, detected CO
2 at the 430 nm wavelength besides CH
can be used directly in subtraction. 3. Experimental details An overview of the experimental rig, optical setup, and operating conditions is provided in this section. 3.1. Burner specifications Two-line, single camera chemiluminescence measurements were performed on a well-characterised Burner. The geometrical parameters of the burner are described in Ref. [61] while the top view and front view of the burner’s exit geometry are shown in Fig. 2. This co-annular burner was designed to generate premixed

Fig. 2. Exit geometry of the Stratified Swirl Burner. Top: top view, bottom: front crosssectional view of the exit geometry. The arrows indicate the direction of flow and swirl. fi represents the equivalence ratio of mixture exiting the inner annulus, whereas fo represent the equivalence ratio of the mixture coming out of the outer annulus [61].

M.M. Kamal / Energy 192 (2020) 116485

and stratified reacting flow conditions representative of practical combustion systems, including various degrees of stratification and high turbulence levels. The flames are stabilized on a ceramic central bluff body with minimal heat loss to the upstream structure of the burner. The equivalence ratio of the inner annulus ðfi Þ and the outer annulus ðfo Þ are independently controlled using mass flow controllers, thus allowing operator to vary the mixture stratification ratio (SR ¼ fi =fo ), for a constant global equivalence ratio (fg ). 3.2. Test conditions The operating conditions studied in the present work are shown in Table 2. The test cases are denoted by a generalised notation SwBN, where N represents the case number. The bulk velocity in the inner annulus, Ui ¼ 8.3 m/s, was set at less than half the value of Table 2 Operating conditions of the Stratified Swirl Burner in this study, including the inner fi , outer fo and mean or global equivalence ratio, fg . In all cases Ui ¼ 8.3 m/s and Uo ¼ 18.7 m/s, and Uco ¼ 0.4 m/s. The Reynolds numbers derived from the bulk velocities at the exit geometry and the hydraulic diameter of each channel are Rei ¼ 5960 for the inner flow and Reo ¼ 11,500 for the outer flow. Flame

fi

fo

fg

SR

SwB1 SwB5 SwB9

0.75 1.00 1.125

0.75 0.50 0.375

0.75

1 2 3

5

the velocity in the outer annuls, Uo ¼ 18.7 m/s, in order to generate high levels of shear-driven turbulence between the two coaxial flows. In order to provide well-characterised boundary conditions, co-flow air was supplied around the outer annulus with a bulk velocity Uco ¼ 0.4 m/s. The mixture stratification ratio (SR) defined as the ratio of the nominal equivalence ratio in the inner annulus flow to that in the outer annulus flow, was varied from unity for premixed cases to 3 for the most stratified cases. The natural luminosity photographs shown in Fig. 3 presents flame patterns of the three cases listed in Table 2. 3.3. Two-line chemiluminescence configuration The spatial distribution of CH
and CO
2 chemiluminescence in the flame is recorded simultaneously by a single intensified CCD camera (LaVision’s NanoStar, gain 55% and gate 3 ms , 1024  1024 pixel) and a Nikon Rayfact UV-105 mm f/4.5 objective lens. In order to achieve simultaneous measurements with a single camera placed perpendicular to the main flow direction, a special optical arrangement (as shown in Fig. 4) is used. The optical arrangement is composed of four fully reflective mirrors and two optical filters. The filters selectively transmit scattered light from the flame at specific wavelengths. The wavelengths correspond to a narrow spectral band, within ± 5 nm. This image double optics system is installed before the lens to create two images of the tested flame simultaneously, capturing the same field of view of the flame, as shown in Fig. 4.

Fig. 3. Natural luminosity photographs of the flames studied in the present work.

Fig. 4. Top view of the optical diagnostic setup for simultaneous imaging of the spatial distribution of CH
and CO
2 chemiluminescence in the flame.

6

M.M. Kamal / Energy 192 (2020) 116485

In the image doubler optics, fully-reflective 4e6 l mirrors (Edmund Optics Inc.) were placed at certain locations and angles through trials and errors. The flame luminescence is transmitted through two narrow band-pass filters (Comar Optics Ltd.) centered at 430 ± 5 nm and 410±5 nm to detect different chemiluminescent species CH
and CO
2 , respectively. By positioning mirrors, filters and the ICCD camera appropriately, the ICCD sensor records two flame images, one corresponding to the flame’s spatial CH* chemiluminescence (including CO*2 broad band) at 431 nm and the other

to the flame’s spatial CO*2 only chemiluminescence at 410 nm. The mirrors are arranged such that the two images are captured from the same side of the flame. A total of 1000 images were acquired at each operating point at an exposure time of t ¼ 700 ms A second configuration of the optical setup as shown in Fig. 5 was also used for the preliminary experiments to check for the effects of the optical layout on the acquired measurements. However, the comparison of the acquired images from the two different setups didn’t show any meaningful contrast. Fig. 6(a) is an example of a raw chemiluminescence image for

one of the premixed cases (fg ¼ 1:0). As expected, the I430 intensity (left side, raw signal at the CH* line) is greater than that of the I410

(right side, raw signal at the CO*2 line). The raw image shows a degree of asymmetry, which has already been minimised by rotating the burner relative to the camera until optimum radial symmetry was observed. The asymmetry can be attributed to the inlet flow conditions at the bottom of the burner, which are not perfectly uniform. To further minimise the effect of radial asymmetry, only half of the flame is considered, as shown in Fig. 6(b). 4. Data analysis This section presents a brief overview of the data analysis methodologies used to calculate derived quantities from the experimental measurements. 4.1. Calibration procedure Projection of chemiluminescence signals has been used as an

Fig. 5. The second optical diagnostic configuration for simultaneous imaging of the spatial distribution of CH
and CO
2 chemiluminescence in the flame.

Fig. 6. Mean raw images, (a) Time-averaged raw image for premixed case where f ¼ 1. Left half represents emission at 430±5 nm (i.e. CH* þ CO*2 chemiluminescence signals), right half represents emission at 410±5 nm (i.e. CO*2 chemiluminescence signal), (b) Halved line-of-sight images of premixed case where f ¼ 1. Left side represents emission at 430 nm (CH* þ CO*2 signals), right side represents emission at 410 nm (CO*2 signal).

M.M. Kamal / Energy 192 (2020) 116485

effective optical measurement of heat release rate in both laminar and turbulent flames. More importantly, chemiluminescence imaging for simultaneous detection of certain species such as CH
and CO
2 in flames have been used as an indicator of global equivalence ratio [47]. This method of equivalence ratio estimation works reasonably well for premixed flames with uniform equivalence ratio distribution. However, the line-of-sight chemiluminescence measurement may not be adequate for estimating the local equivalence ratio in stratified flames due to the spatially nonuniform mixture distribution. Irrespective of the flame type, projected chemiluminescence signals of CH
and CO
2 require proper calibration before they can be used to indicate equivalence ratio. For equivalence ratio estimation, it is assumed that the following relationships apply between species concentration and chemiluminescence intensity:

Yi ðrÞ ¼ Ki ICH* ðrÞFSDðrÞ

(1)

YO2 ðrÞ ¼ KO2 ICO* ðrÞFSDðrÞ

(2)

2

where Y is the mass fraction of certain species i and IðrÞ represents corresponding field-information of chemiluminescence signal intensity, r is the radial position from the centre of the fuel-oxidizer inlet and K is a constant. CH
and CO
2 are the markers for fuel and oxidizer, respectively. It is further assumed that the flame surface density FSD is not convoluted with the other two parameters including Y and I, and that it is independent of equivalence ratio. This assumption, however, only applies to flames in the lean regions because fuel-rich flames produce CO as well as CO2. Equations (1) and (2) can be combined to calculate the equivalence ratio as follows:

4ðrÞ ¼

Yi ðrÞ I * ðrÞ ¼ K CH YO2 ðrÞ ICO* ðrÞ

(3)

2

This shows that the field chemiluminescence signals ratio CH* = is a direct indicator of local equivalence ratio. The signal intensity of species can be expressed as: CO
2

t;s t;s I t;s CH
¼ I l  I l 1

(4)

2

(5) t;s

where t denotes time and s represents space. Accordingly, I l and 1 I t;s l represent the chemiluminescence intensity of certain pixel 2

(space) in certain image (time) at the wavelength that the optical filter allows to pass. The range of wavelength of l1 refers to 430± 5 nm (CH
), while l2 refers to 410±5 nm (CO
2 ) Now assuming Ns as the total number of pixels in each image while Nt as the total number of instantaneous realizations (images), the ratio Rc of two chemiluminescent species used for calibration is integrated and averaged in both time and space:

PNs PNt

It;s
Rc ¼ P1N P1N CH s t t;s 1 1 I CO
2

Several premixed operating conditions were used to obtain the correlation between the CH
and CO
2 chemiluminescence intensities ratio and the equivalence ratio. For stratified flames with spatially non-uniform equivalence ratio distribution, the projection of chemiluminescence signals (or in other words the line-of-sight data) needs to be transferred to field information using Threepoint Abel-deconvolution. Abel transform is a sophisticated analytical method used for deconvoluting data to allow for extraction of spatially resolved information from the temporallyaveraged line-of-sight optical measurement acquired from an axisymmetric domain with nonintrusive diagnostic techniques [62,63]. However, the validity of Abel-deconvolution is limited to measurements of only infinitely thin, perfectly parallel rays, whereas majority of optical diagnostic setups used for flame imaging consist of optical lenses that collect signals in a cone, over a non-zero solid angle. This considerably affects the measured intensity of chemiluminescence signals and thus introduce error in the deconvoluted image containing field information [63,64].

4.2. Temporal variation The temporal variation in chemiluminescence signal intensity is mainly due to the flame-turbulence interactions. The turbulent premixed and stratified flames investigated here have inherently variable structures as a result of high turbulence levels and co-flow air entrainment, causing shot-to-shot variations in CH
and CO
2 chemiluminescence images. Therefore, only time-averaged CH*

and CO*2 chemiluminescence images can be used to ensure spatial symmetry. Time averaging allows for turbulence-induced fluctuations in the chemiluminescence intensity to be averaged, providing an image of the quasi-steady local mean chemiluminescence intensity. To calculate the time variance, first the signal intensity of each pair of CH* and CO*2 chemiluminescence images are individually integrated over the respective images i.e. integrating signal inNs P I t;s tensity over all the pixels in the respective image: CH
and 1

Ns P

I t;s ¼ I t;s l2 CO
2

(6)

7

1

I t;s . CO
2

This gives an overall value of intensity for each of CH* and

CO*2 images in the pair and for all the pairs. These spatially integrated chemiluminescence intensity values are then used to generate integral chemiluminescence intensity ratio Rt for each pair. Next, for the total number of image pairs Nt , representing a times series, the mean temporal value CRt Dt of these spatially integrated intensity ratios can be calculated as:

0P 1 Ns t;s Nt Nt X X I 1 1
1 CH @P A Rt t ¼ Rt ¼ Ns t;s Nt 1 Nt 1 1 I CO


(7)

2

Finally, the time variance is calculated via the normalized standard deviations in the given time interval:

This ratio is used to draw the calibration curve with global equivalence ratio fg as discussed in Section 5.1. In the present

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  PNs  t t 2 1 R R t s ¼ Nt Rt t

study, Rx represents the ratio of two parameters integrated in only one dimension (either time or space, x ¼ t or s), as a function of the other dimension ‘x’; whereas CRx Dx is the mean of these ratios averaged in that x dimension.

The values for st indicate how the optical measurement of chemiluminescence signals is affected by the turbulence in certain period of time under each operating condition. Later in Section 5.1, Fig. 8 shows that the calculated time variances are less than 3%.

t

1

(8)

8

M.M. Kamal / Energy 192 (2020) 116485

4.3. Spatial variation Ideally, the CH
=CO
2 chemiluminescence ratio for premixed flames should have uniform distribution under the assumption of perfect mixing of fuel and air. However, the processed ratio images (Fig. 7) of premixed flames show spatial variance in equivalence ratio in the mean flame brush region. The main factors causing the spatial variance in processed images include two sources: turbulence in the flame and Abel-deconvolution in processing. As discussed previously, turbulence can lead to spatially unstable

Fig. 8. Calibration curve for chemiluminescence ratio (integrated both temporally and spatially) shows the empirical correlation of Rc and global (set) equivalence ratio, fg for the five premixed cases. The error bars show the absolute shifts due to both the time variance (blue) and spatial variance (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

emission of chemiluminescence signal. The inner structures of flame brush can generate different CH* and CO*2 chemiluminescence projection signals in different parts of flame brush under turbulence. For the second factor, there is error introduced when the projection information is transferred into field information using Abel-deconvolution [64]. Since most of the information pertaining to turbulence-induced fluctuations in chemiluminescence emissions intensity is left out by time-averaging of images, it is hard to analyse how much of error each of the two factors brings into the final result. Despite the above mentioned challenges, it is quite possible to quantify the total spatial variance in the processed images. For this purpose, the time-averaged axisymmetric images of CH* and CO*2 chemiluminescence are first deconvoluted by applying ‘Abel transform’ to obtain spatially resolved information from the lineof-sight chemiluminescence emission measurement. Then, the CH* and CO*2 chemiluminescence intensity ratio Rs is calculated for

each individual pixel from the time-averaged intensities of CH* and CO*2 signals of that very pixel. Finally, for the whole flame area, the spatially averaged ratio CRs Ds of these temporally integrated intensity ratios of individual pixels is calculated as:

0P 1 Nt t;s Ns Ns X X I 1 1
1 CH @P A CRs Ds ¼ Rs ¼ Nt t;s Ns 1 Ns 1 1 I CO


(9)

2

Similarly, to time variance calculation, the spatial variance can be expressed as:

1 s ¼ s CR Ds s

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 PNs  s s 1 R  CR Ds Ns

(10)

The calculated spatial variances are within 6e7% in the range of tested conditions. This indicates that in the stratified cases, any deviation bigger than this can be regarded as the equivalence ratio fluctuation due to the mixture stratification and turbulent mixing. 4.4. Raman spectroscopy data Fig. 7. (a) Abel deconvoluted, time-averaged mean of 1000 instantaneous chemiluminescence images for CH* and CO*2 signals, (b) Conditioned chemiluminescence intensity ratio (Rc ) for the premixed cases.

Equivalence ratio profiles derived from Raman-scattering measurements for this burner have been presented in previous investigations [61], but a short description of the estimation method

M.M. Kamal / Energy 192 (2020) 116485

involved has been reproduced here for reference. Species concentrations measurements using Raman spectroscopy allowed the reconstruction of the equivalence ratio using Eq. (11) across the flame at various axial locations represented by z. The instantaneous equivalence ratio f values are then Favre-averaged to obtain local mean equivalence ratios.

f¼

  XCO2 þ 2XCH4 þ XCO þ 0:5 XH2 O þ XH2   XCO2 þ XO2 þ 0:5 XCO þ XH2 O

(11)

The present study uses this previous data on local mean equivalence ratio fr (subscript r denotes Raman) for comparison with the mean equivalence ratio fc (subscript c denotes chemiluminescence) profiles obtained from the current chemiluminescence data. 5. Results and discussion 5.1. Empirical correlation between (I CH * =I CO* ) and f 2

Five perfectly premixed test flames (Table 3) with different global equivalence ratios ranging from fg ¼ 0:7 to fg ¼ 1:0 are used to obtain an empirical correlation between chemiluminescence intensities of CH
and CO
2 radicals and the corresponding equivalence ratio. From the spatial distribution of CH
and CO
2 chemiluminescence intensity, the spatial CH
= CO
2 ratio is computed for these five test flames. The acquired instantaneous chemiluminescence images are first corrected for the background noise. The images are expected to be distorted near the edges and, therefore, signal data close to the edges are filtered out from the images to eliminate any influence of any such distortion effects. The instantaneous realizations are then ensemble averaged. The mean (or time-averaged) images representing ICH* and ICO* are then used to calculate the distribution of 2

the spatial CH
=CO
2 ratio for the individual premixed cases. The local mean CH
=CO
2 ratios for the five premixed flames are presented in Fig. 7. The figure displays the Abel de-convoluted ensemble averages of instantaneous images for each premixed case, along with the ratio ICH* =ICO* . The ratio is obtained from the 2

emissions at 430±5 nm and 410±5 nm, by subtracting the 410 nm emissions, assuming it is associated with the CO*2 background. The intensity ratio for an instantaneous realization is given by;

Rc ¼

ICH* I430  ICO2 ;410 ¼ ICO* ICO2 ;410

(12)

2

whereas the mean (chemiluminescence) intensity ratio Rc (as calculated by Eq. (6)) for each premixed case is plotted in Fig. 8. Generally, the ratio Rc would be expected to be uniform throughout the flame under ideal conditions, yet there is considerable deviation around the edges of the flame to higher values of the ratio, suggesting that the assumption fails as the CO*2 background becomes close to zero near the reactant side of the flame. In order to Table 3 Operating conditions for the series of turbulent premixed flames including the inner fi , outer fo and mean or global equivalence ratio, fg . Premixed Flames

fi

fo

fg

SR

SwB1-a SwB1-b SwB1-c SwB1-d SwB1-e

0.70 0.75 0.80 0.90 1.00

0.70 0.75 0.80 0.90 1.00

0.70 0.75 0.80 0.90 1.00

1

9

account for this lack of spatial uniformity, each image went through a process of conditioning. Furthermore, to extract sensible values that do not divide by zero, a procedure was developed to filter the data to a signal to noise ratio above a certain threshold.  Given that mean CO*2 levels decrease near the edge of the flame brush, the near-zero values must be excluded for a sensible ratio. Therefore, based on sensitivity analysis, any data with a value less than 60% of the maximum value in the image was removed.  The median value of the spatial distribution of the intensity ratio for the five mean premixed images, along with the standard deviation (s) were then determined and a histogram plotted for each case (again, premixed only). Values in the range of the median ±1.5 $s are retained, whilst data outside of this range are considered to be the tails of the histogram and are removed.  The area of interest is selected between axial locations of 20 mm and 40 mm for all cases. The upstream location is selected to avoid data from the recirculation zone and the downstream location is selected to avoid regions affected by entrainment [61]. Fig. 7 shows the post-processed CH* to CO*2 chemiluminescence ratio maps of premixed flames. Since premixed flame of fg ¼ 0:7 is the leanest case with the smallest mean CH* to CO*2 chemiluminescence ratio Rc , it is used as the bottom limit in mapping the spatial equivalence ratio distribution. It is assumed that the current measurement cannot precisely value any location with lower CH* = CO*2 than this threshold level.

The ensemble averaged intensity ratio Rs of CH* and CO*2 signals is integrated over the total flame area (as shown in Fig. 7b) for each individual premixed case listed in the text matrix (Table 3). The resulting spatial mean values (Rc ) of the intensity ratio images are represented by the circular markers in Fig. 8 for each premixed flame. The error bars show the absolute shifts due to both the time variance (blue) and spatial variance (red) in each series of images under certain equivalence ratio, as calculated by the mean ratio (Rc ) in Eq. (6) multiplied by either temporal or spatial variance (st ; ss ). Several features are worth to note from Fig. 8. First of all, the CH
to CO
2 chemiluminescence ratio shows nearly a linear relationship with global equivalence ratio under lean burning condition between fg ¼ 0:7 and fg ¼ 1:0, which confirms the findings in Ref. [47]. Bobusch et al. [47] have shown that the CH
and CO
2 chemiluminescence ratio of premixed methane/air flame has a linear trend across a wide range of equivalence ratios and is independent of total mass flow and the strain rate as well. However, towards the lower equivalence ratios (fg ¼ 0:75 and 0:7), the data points drop off the curve. Moreover, the standard deviation increases with an increase in the global equivalence ratio. This could potentially be attributed to the increased noise contribution caused by the higher temperature gradients that are resulted from increasing flame temperatures at higher equivalence ratios. During the preliminary experiments, data points outside the reported fg -range (i.e. 0:7  1:0) were also tested. It was found that for fg < 0:65, the absolute chemiluminescence intensities of CH
and CO
2 starts to drop down, and the data becomes considerably non-linear and has apparently rather larger variance. This is due to the low signal-to-noise ratio at those conditions that linearity does not exist below fg ¼ 0:65, thus limiting the linear trend in the plot of fg vs. ðCH
=CO
2 Þ. Similarly, at higher equivalence ratios (i.e. beyond stoichiometric fg ), the CH
=CO
2 ratio increases nonlinearly with global equivalence ratio. This behaviour is of greater concern because the local flame-front equivalence ratio can cross over the

10

M.M. Kamal / Energy 192 (2020) 116485

stoichiometric condition in certain cases. These findings are contrary to those reported by Guyot et al. [10]. According to their study, for perfectly premixed flame (with globally uniform f distribution), the chemiluminescence intensity is a linear function of mass flow _ equivalence ratio f, and pressure p. Since the inclusion of rate m, extreme data points (in terms of f) were introducing non-linearity in the Rc -fg relationship, therefore such conditions were ignored and the analysis was primarily focused only on a certain range of fg (i.e. 0:7  1:0) for the sake of simplicity and accuracy. With the calibration curve, the processed CH
= CO
2 chemiluminescence ratio images can now be transferred into spatial distribution of equivalence ratio as discussed in Section 2 and then further explained in Section 4.1. Given that the graph shows a linear relationship between the mean (CH
=CO
2 ) intensity ratio for premixed cases and the global equivalence ratio, the slope of the linear curve fit is found to be 1.517.

flocal ¼ 1:517Rc þ 0:185

to the very lean burning conditions. However, only the mean flame area with enough CH* =CO*2 chemiluminescence ratio is analysed and reported in Fig. 9. In other words, the threshold value is set as fc ¼ 0:50, and the image is processed as only the calibrated pixel with fc > 0:50 is shown in the constructed map. As the flame becomes richer, the flame length gets shorter with higher value of local equivalence ratio fc , which agrees well with the actual physical condition of tested steady flames.

5.3. Equivalence ratio maps of stratified cases Based on the same calibration curve from Fig. 8, the equivalence ratio distribution in stratified flames can be generated as well. The mean equivalence ratio maps for the stratified cases are shown in Fig. 10. Like premixed cases, the equivalence ratio maps for the mildly stratified (SwB5) and highly stratified (SwB9) cases also

(13)

The fact that the curve does not intercept the y-axis at zero suggests that there is one more contribution to ICH* as f goes to zero, or that the relationship could be nonlinear as discussed above.

5.2. Equivalence ratio maps of premixed cases Once the relationship between the equivalence ratio and the chemiluminescence intensity ratio has been established, the equivalence ratio fc (subscript c denotes chemiluminescence) is mapped out by inserting the (CH
=CO
2 ) intensity ratio values at each location into Eq. (13). The equivalence ratio maps for the (time-averaged) premixed cases are shown in Fig. 9. The figure shows that the size of the constructed equivalence ratio map (i.e. the area at which chemiluminescence signal was above a certain threshold) increases as the input global equivalence ratio fg increases. The local value of the constructed equivalence ratio fc appears to decrease towards the pre-heat zone (right edge of each flame image) in almost every case. This is because of the entrainment from the co-flow air stream and the subsequent mixing that reduces the intensity of the chemiluminescence signal and hence results into lower equivalence ratio estimates at the interface. It was also noted during the experiments that the flames with lower input equivalence ratios fg have quite long flame length due

Fig. 10. Mean equivalence ratio maps of premixed (SwB1), mildly stratified (SwB5) and highly stratified (SwB9) flame cases, constructed from the simultaneous (CH* =CO*2 ) chemiluminescence signals. The spatial mean values of the equivalence ratios for the three maps are: fc ¼ 0.607±0.018 (SwB1), 0.85±0.031 (SwB5), 0.9322±0.033 (SwB9).

Fig. 9. Mean equivalence ratio fc maps constructed from the simultaneous (CH* =CO*2 ) chemiluminescence signals ratio in the premixed cases.

M.M. Kamal / Energy 192 (2020) 116485

show considerable drop in signal in the pre-heat region of the flame. In the stratified cases, where even larger temperature gradients exist due to the presence of locally rich and lean flame regions resulted from the mixture stratification, a greater contrast in signal intensity between the pre-heat and reaction zones is observed. This could potentially be corrected for if a calibration procedure is applied that considers that variations in the flame chemiluminescence intensity as a function of the local mean temperature. The possibility of any such calibration procedure could be explored with simultaneous planar Rayleigh scattering and high speed chemiluminescence measurements in the flame. 5.4. Comparison between fc ¼ and fr In order to evaluate the accuracy and reliability of the two-line chemiluminescence method for determining local mean equivalence ratios in stratified flames, the equivalence ratio (fc ) obtained from the chemiluminescence intensity ratio (CH
=CO
2 ) is compared with previously measured [61] local equivalence ratio (fr ) from Raman-scattering measurements. The comparison essentially allows for validation of the two-line chemiluminescence method. Since chemiluminescence is mainly originated in the reaction zone of a flame, the comparison is carried out in the mean flame brush constructed from instantaneous flame fronts that are located based on the temperature measurements carried out for the same flames in a previous experimental campaign [61]. The mean flame brush is marked by thermal progress variable c; where c ¼ 0 represents the reactants side of the brush, and c ¼ 1 mark the product region. The line of sight data obtained from the calibrated chemiluminescence ratio (CH
=CO
2 ) images are already filtered and Abel de-convoluted for the sake of appropriate comparisons with the line data obtained from Raman measurements. Fig. 11 shows the comparison of equivalence ratio distributions (red dots) and their

11

respective mean values (blue lines) from the Raman data with the equivalence ratio values estimated by the two-line chemiluminescence technique (black circles) at various axial locations from the base of the burner. The local equivalence ratio values for premixed data (SwB1) obtained via chemiluminescence measurements, fc , tends to decrease below cz0.7 (i.e. the pre-heat zone). The reference Raman data does not show this drop in fr in the preheat zone. This can be observed in the stratified cases (SwB5 and SwB9) as well at z ¼ 20 and 30 mm but is not the case for z ¼ 40 mm for the highly stratified case, SwB9. This shows that while the two-line chemiluminescence technique can be reliably used to determine equivalence ratio in the instantaneous reaction zone of turbulent flames even at high turbulence levels and mixture stratification conditions, this method is not reliable in the pre-heat region of the flames i.e. c < 0.7. The reason is most likely that the chemiluminescence emission is non-linearly dependent on the local temperature. This essentially results in large errors in the chemiluminescence based equivalence ratio in the preflame thermal zone where the temperature variations are relatively significant. Thus, when the flame temperature is low (i.e. the FSD is low), the two-line chemiluminescence method fails. Comparison with cases where the flame is confined and have recirculation zone, might suggest that this is only a problem when flames are tested in open environments. 6. Conclusion The single camera, two-line chemiluminescence technique was applied to the Stratified Swirl Burner under both premixed and stratified conditions. Based on the acquired chemiluminescence signals data for CH* and CO*2 radicals, a calibration curve was generated which was then used to obtain an empirical correlation between the equivalence ratio of the incoming combustible mixture and the resulting chemiluminescence emissions. Subsequently, equivalence ratio maps for both premixed and stratified flames were generated using that empirical correlation. Finally, an evaluation of the reliability and accuracy of this methodology was carried out by comparing the equivalence ratio fc profiles constructed from the chemiluminescence data with previously obtained equivalence ratio fr measurements from Raman-scattering of major combustion species. In order to make appropriate comparisons the line of sight measurements acquired from the calibrated images of chemiluminescence intensity ratio (CH* =CO*2 ) were first filtered and Abel de-convoluted.

Fig. 11. Comparison between two-line chemiluminescence data and Raman scattering data for premixed case (SwB1) mildly stratified (SwB5) and highly stratified (SwB9) cases with global equivalence ratio, fg ¼ 0.75. z represents the axial distance from base of burner (in mm). c is the ensemble averaged thermal progress variable representing the mean flame brush region; where c ¼ 0 represents the reactants side of the brush, and c ¼ 1 mark the product region.

Results show that the two-line (CH* =CO*2 ) chemiluminescence technique can be used to construct equivalence ratio map in the instantaneous reaction zone of the flame irrespective of turbulence level or mixture stratification condition in the flow field. However, these measurements are only reliable in the reaction zone, and fail where the progress variable, c, is below 0.7. This is likely to be due to a non-linear relationship between chemiluminescence emissions and local flame temperature. The analysis further suggests that the flame reaction zone is essentially anchored to an ignition temperature (or isotherm) for each case because the estimated equivalence ratio fc and, by extension, the local chemiluminescence intensities are not affected by any variations in local temperature at the instantaneous flame front. The preflame zone, on the other hand, experience significant temperature fluctuations which cause large errors in the measurements. Nevertheless, a robust relationship that takes into account local flame temperature (and thus thermal variation) in addition to chemiluminescence emission intensities could possibly allow for construction of reliable equivalence ratio maps, across the mean flame brush, from the intensity ratio of chemiluminescence signals.

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M.M. Kamal / Energy 192 (2020) 116485

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