air premixed flames using high-repetition-rate OH PLIF

Combustion and Flame 193 (2018) 145–156

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Experimental study of CO2 diluted, piloted, turbulent CH4 /air premixed flames using high-repetition-rate OH PLIF Dong Han∗, Aman Satija, Jay P. Gore, Robert P. Lucht School of Mechanical Engineering, Purdue University, 500 Allison Road, West Lafayette, IN 47907, USA

a r t i c l e

i n f o

Article history: Received 30 October 2017 Revised 9 March 2018 Accepted 12 March 2018

Keywords: Turbulent premixed flames Carbon dioxide addition High-speed diagnostics OH PLIF Flamelet structure Turbulent burning velocity

a b s t r a c t High-speed planar laser-induced fluorescence (PLIF) was applied to turbulent premixed flames with CO2 addition. Instantaneous PLIF images capturing emissions from the OH radical via the A2  -X2  (1,0) band were collected. Three flames with varying levels of CO2 addition were established at the same adiabatic flame temperature, Reynolds number, and Lewis number, thereby minimizing the effects of differences in flame temperature and transport. The chemical effects of CO2 addition were investigated through critical combustion parameters revealing turbulent flamelet structure and burning velocity. The flamelet structure analysis suggests that the development of the mean flame brush thickness follows the turbulent diffusion law with a secondary effect introduced by CO2 addition. The flame surface density of these flames is affected by CO2 addition mainly through modification of mean progress variable distributions. The flame length is extended by CO2 addition with an enhancement of unburned pocket formation in the downstream portion of the flame. The apparent size and consumption speed of the fine scale unburned reactant pockets are similar among flames with varying CO2 addition. The global consumption speed of flames with CO2 addition is reduced predominantly by a reduction in the laminar flame speed. The global combustion intensity shows a constant value within uncertainty limits for flames with CO2 addition. The local combustion intensity before x/D = 1.5 is observed to be lower for the flames with CO2 addition due to the attenuation of pocket formation in the flame brush development region. Its effect on global combustion intensity is counterbalanced by the contribution of pocket formation process in the downstream portion of the flame. © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Turbulent premixed combustion plays an important role in the development of high efficiency and low NOx emission stationary gas turbines. Improved understanding of turbulent lean premixed flames has been gained from time and space resolved as well as time-averaged measurements of radiation emissions [1], temperature [2], species concentrations [3], and velocity [4]. A recent paper by Driscoll [5] reviewed the advancement in understanding of turbulent flamelet structure and burning velocity. A complete database of turbulent premixed flames with different nozzle exit velocities was generated by Chen et al. [2]. However, further efforts on experimental and numerical studies of atmospheric and high pressure lean premixed flames are still needed for the development of low NOx emission technologies in industrial gas turbines.

∗

Corresponding author. E-mail address: [email protected] (D. Han).

Exhaust gas recirculation (EGR) is a promising technique to further reduce NOx emission due to its effects on the NOx formation process in premixed combustion [6]. The stringent regulation on NOx emissions requires an advanced understanding of EGR effects on turbulent premixed combustion process. Carbon dioxide is a major component of EGR in hydrocarbon combustion systems. Its effects on modification of laminar flame structure, and flame speed have been studied previously [7–9]. Investigation of the effects of CO2 addition for turbulent premixed flames has been the subject of several recent studies [10–13]. In this study, we performed multi-kHz OH planar laser induced fluorescence (OH PLIF) measurements on turbulent premixed flames with varying levels of CO2 addition. The flames were operated at the same adiabatic flame temperature, Reynolds number, and Lewis number to minimize thermal and transport effects caused by CO2 addition. Modifications observed on flame structure and burning velocity are expected to be the result of the chemical effects of CO2 addition [13]. The CO2 addition changes laminar flame speed, laminar flame thickness and time scale, and consequently modifies some portions of flame topology depending on these parameters. Temperature and major species measurements

https://doi.org/10.1016/j.combustflame.2018.03.012 0010-2180/© 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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D. Han et al. / Combustion and Flame 193 (2018) 145–156

Nomenclature A AL AT Apo c D I0 l LT Re SL0 ST, GC ST, LC ST, LCP T t u U V

x α δT  τ

flame surface area area of unwrinkled flame surface (area of c¯ = 0.5 surface) area of wrinkled flame surface area of pocket surface progress variable Burner diameter stretch factor integral length scale length of flame front perimeter within the laser sheet equivalent radius of a pocket unstretched laminar flame speed turbulent burning velocity (global consumption speed) turbulent burning velocity (local consumption speed) turbulent burning velocity (local consumption speed of unburned pocket) temperature time instance RMS velocity fluctuation mean axial velocity volume side length of an interrogation box fitting constant turbulent flame brush thickness flame surface density flame development time, τ = x/U

Subscription r radial direction or radial location x axial direction or axial location u unburned reactant b burned product po pocket up unburned reactant pocket for these flames were performed in our previous study using coherent anti-Stokes Raman Scattering [13]. The OH PLIF measurements in this paper focus on experimental evidence on turbulent flamelet structure represented by flame brush and flame surface density, and turbulent burning velocity represented by global and local consumption speed. The chemical effects of CO2 addition on those combustion parameters are first investigated in this study, which is of importance in the development of valid computational models such as large eddy simulation (LES) with minimum adjustable parameters [5]. Mean turbulent flame brush thickness δ T and flame surface density (FSD)  are two critical parameters for quantifying turbulent flamelet structure. Mean turbulent flame brush thickness δ T indicates the spatial region over which reaction layers are located. The recent progress in the understanding of mean flame brush development in turbulent premixed flames was reviewed by Lipatnikov and Chomiak [14]. They pointed out that measured spatial distributions of progress variable presented in a dimensionless form using δ T collapse into a universal curve for a wide range of different flames. This universal curve which has been parameterized using different functions shows a good agreement with various experimental measurements [12,15,16]. The development of δ T within a turbulent premixed flame was investigated by researchers [5,17]. Karlovitz et al. [17] claimed that the increase of δ T is mainly

controlled by the turbulent diffusion law. The Taylor theory of turbulent diffusion predicted δ T well for some experimental data [18–20], even though it does not account for the non-isotropic nature of turbulence or heat release in flames. Flame surface density  describes the distribution of flamelets within the flame brush. Experimental measurements of the flame surface density using direct or indirect methods have been published for model validation as well as for advancing the fundamental understanding of turbulent premixed combustion [21–24]. Lee et al. [21] performed PLIF measurements for turbulent premixed flames established on a nozzle type burner. They claimed that the maximum FSD increased with an increase in the ratio of normalized turbulent fluctuation velocity and laminar flame speed u /SL0 or decrease of integral length scale l. A similar effect of integral length scale on flame surface density was also reported by Deschamps et al. [22] using laser-induced Rayleigh scattering (LIRS). Filatyev et al. [23] reported that the maximum FSD decreases as mean flame brush thickness increases due to turbulent diffusion. The flame surface density approach has also been employed as a simple algebraic scheme or in terms of a modeled balance equation in turbulent premixed combustion simulation including Reynolds averaged Navier–Stokes (RANS) [25] and LES [26]. Turbulent burning velocity is another critical quantity for turbulent premixed flames. The capability of adequately predicting turbulent burning velocity provides an assessment of computational simulations. Turbulent burning velocity has been reported in literature mainly in terms of local consumption speed ST, LC and global consumption speed ST, GC [5]. The global consumption speed at different levels of turbulence was reviewed by Lipatnikov and Chomiak [14]. Kobayashi et al. [27] reported that the increase in normalized global consumption speed ST, GC /SL0 with an increase in normalized turbulent fluctuation velocity u /SL0 is rapid at higher pressures for small turbulence fluctuation velocity u . They attributed this phenomenon to hydrodynamic instability which enlarges the total flame area in high-pressure environments. Aldredge et al. [28] found that a decreasing sensitivity of ST, GC to increases in turbulence fluctuation velocity u occurs when u is approximately 2.5 times the laminar flame speed SL0 . Abdel-Gayed et al. [29] observed a decrease in global combustion intensity ST, GC /SL0 with increasing u /SL0 due to quenching phenomenon with various fuels. The local consumption speed ST, LC was typically evaluated using flame surface density and flame brush thickness from laserimaging measurements [5]. Numerical calculations from Hemchandra and Lieuwen [30] suggested that the local consumption speed of an attached flame is not only a function of the local flow and flame conditions but also of upstream conditions. The local consumption speed ST, LC and global consumption speed ST, GC both imply how fast fuel burns in a flame, but they should not be compared due to lack of correlation between their definitions. The aforementioned quantities characterizing turbulent flamelet structure and burning velocity are often experimentally investigated by PLIF of molecules and radicals [21,23,31]. Hydroxyl radical (OH) is generally abundant in flames from reaction zone to products, which determines that OH PLIF is typically employed for flamelet structure analysis with a thin flame front assumption between reactants and products. The time resolution of conventional PLIF measurement is on the order of few Hz, limited by experimental instruments, in most cases by camera and laser power [32]. The recent development of solid-state lasers, high-speed cameras, and high-speed intensifiers makes multi-kHz PLIF imaging possible [33,34]. Johchi et al. [35] reported unburned reactant pocket consumption speed in a turbulent premixed jet flame using 10 kHz PLIF. Trunk et al. [36] estimated local displacement speed for freely propagating flames using 10 kHz simultaneous OH PLIF and particle imaging velocimetry (PIV).

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2. Description of key combustion parameters A brief description of and mathematical definitions for key combustion parameters adopted for this paper are presented in this section. These parameters provide insights into turbulent premixed flamelet structure and burning velocity. The measurement of these parameters is also necessary to assess the capability of using computational tools including DNS or LES to predict these quantities with a minimal number of adjustable parameters. The progress variable c was introduced to model premixed flames [37]. It is defined as a normalized temperature or normalized mass fraction as

c=

T − Tu Y − Yu or c = Tb − Tu Yb − Yu

(1)

which implies a single-step global reaction from reactants to products, where temperature rises from Tu to Tb . In the PLIF measurements, the instantaneous flamelet front is assumed to be infinitely thin, and no intermediate values of temperature are resolved. Mean turbulent flame brush thickness δ T is a characteristic measure of the transition zone between the unburned and burned states in a premixed flame. Several evaluation methods have been proposed in the literature [15,38,39]. In this paper, δ T was determined using the maximum gradient method of mean progress variable [15]. The following equations were used to evaluate the mean flame brush thickness along the burner centerline and those along the radial direction at a specific axial location x∗ .

   dc¯(x, r = 0)   δT,x = max−1   dx

   dc¯(x = x∗ , r )   δT,r (x∗ ) = max−1   dr

(2)

(3)

Flame surface density  quantifies the surface area of the flamelet per unit volume in turbulent premixed flames [40]. It is defined as

 = lim

AT

 x → 0 x 3

or x =

AT

x 3

(4)

Eq. (4) is used to calculate the flame surface density within a cubic interrogation box of size x. In PLIF measurements, information is available only in the plane of a laser sheet. It was assumed that the flame surface area per unit volume equals the flame perimeter per unit area in the laser sheet as,

x =

LT

x 2

(5)

The accuracy of this 2D approximation employed in planar experimental measurements was evaluated by Bell et al. [24] using DNS. They pointed out that a multiplication factor of 1.35 should be used to multiply  from 2D measurements to yield correct 3D values. In this paper, the flame surface density values are shown without correction. The global consumption speed ST, GC is a measure of the total fuel consumption rate, typically in Bunsen type burners. It is defined as

ST,GC =

m˙ u

ρu Ac¯

(6)

where Ac¯ is defined as the area of the c¯ contour of the flame. The choice in the selection of c¯ value is somewhat of an arbitrary decision. For examples, Filatyev et al. [23] used c¯ = 0.5 for ST, GC evaluation, while Kobayashi et al. [11] employed c¯ = 0.1 in their work. Measurements and simulations can be compared in a meaningful fashion as long as the same c¯ contour is selected.

Fig. 1. Schematic of instantaneous flame fronts with unburned reactant pockets on the product side and burned product pockets on the reactant side. The wrinkled flame front area AT , and unwrinkled flame front area AL , and flame front area of burned or unburned pockets Apo are labeled.

The local consumption speed provides an evaluation of fuel consumption rate at a region of interest (ROI) or within an interrogation region. It is defined as



ST,LC = SL0 I0

∞ −∞

 dη

(7)

where η is normal to the flame brush or unwrinkled flame surface AL . If we consider the control volume shown in Fig. 1, we have



AT + A po =

∞ −∞



A po AT + = AL AL

 dV =

∞

−∞



∞ −∞

 AL d η

 dη

(8)

(9)

For instances where unburned or burned pockets are seldom A A observed, AT >> Apo and the flame surface density  can be calL

L

culated by measuring the wrinkled flame front area in the laser diagnostic images using below equations.

AT ≈ AL



∞ −∞

 dη

(10)

AT AL

(11)

ST,LC = SL0 I0

An alternative way to compute local consumption speed is to use the correlation between ST, LC ,  max , and δ T shown in Eq. (12). The correlation was derived based on the empirical expressions shown in Eqs. (13) [41] and (14) [42].

ST,LC = SL0 I0 max δT

(12)

 = 4max c¯(1 − c¯ )

(13)



c¯ = 1 + exp

 −4 η − η −1 ( c¯=0.5 ) δT

(14)

The stretch factor I0 is defined as the ratio between the local consumption speed of a laminar flamelet and the unstrectched laminar flame speed. The stretch factor I0 was assumed to be unity for the flames that we investigated. The pocket consumption speed is a measure of the consumption rate of unburned reactant pockets formed through wrinkling and interaction of flame fronts or local extinction events. It is defined as

ST,PC = lim

1 Vup t

t→0 Aup

(15)

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D. Han et al. / Combustion and Flame 193 (2018) 145–156

Fig. 2. Cross-sectional schematic diagram of the PARAT burner with dimensioned zoom-in views of important aspects [13].

where Vup is the volume change of an unburned reactant pocket within a time intervalt and Aup is the averaged surface area of the pocket on two images. For this paper, the small-scale unburned pockets were tracked in sequential images, and their statistics (sizes and consumption rates) were evaluated from the OH PLIF images. It is important to note that with a 2D measurement, the measured or apparent consumption speed is a combination of true consumption speed and out-of-plane motion. Also, some observed pockets might be a portion of 3D contiguous flame surface intersecting with the laser sheet. However, the data acquired using the 2D measurement can still be utilized in comparison with cross-sectional data from 3D simulation results. 3. Experimental methods 3.1. Burner and flame operating conditions The piloted axisymmetric reactor assisted turbulent (PARAT) burner was used to establish turbulent premixed flames with varying levels of CO2 addition. The turbulent premixed flames were stabilized on an 18-mm diameter nozzle exit with an annular pilot flame. A cross-sectional schematic diagram of the PARAT is shown in Fig. 2. The details of burner dimensions, turbulence generation mechanism, and burner translation system are documented in [13]. The CH4 , CO2, and H2 flow rates were controlled by digital mass flow controllers with an accuracy of about 2%. The airflow rate was determined from the upstream pressure gauge reading of an orifice plate which was calibrated using a dry test turbine meter. The heat release rates of the pilot H2 flame and CH4 flame were calculated based on their flow rates and lower heating values. The nominal jet exit Reynolds numbers are based on the mixed cold gas properties [43], the exit velocity of the central jet burner, and the burner diameter. The laminar flame speeds were calculated using CHEMKIN with the GRI 3.0 mechanism. The dilution effect of pilot flame products on the laminar flame speed was expected to be less than 2%, following the expression by Metghalchi and Keck [44]. The uncertainty of laminar flame speed was estimated to be about 5% by accounting the uncertainties in flow rates. The laminar flame thermal thicknesses (characteristic flame thickness) and quenching

Table 1 Operating conditions for the OH PLIF measurements. Velocity and turbulence boundary conditions are measured at the burner exit. Flame # Reynolds number (±50) Adiabatic temperature (±50 K) Equivalence ratio (±0.02) CO2 % by total mass (±0.1) CH4 mass flow rate (±2 mg/s) Air mass flow rate (±20 mg/s) CO2 mass flow rate (±4 mg/s) Pilot H2 mass flow rate (±0.03 mg/s) Pilot H2 heat release percent of total (%) Lewis number Laminar flame speed (±2 cm/s) Laminar flame thermal thickness (μm) Quenching strain rate (1/s) RMS velocity fluctuation (m/s) Integral length scale at burner exit (mm) Turbulent Reynolds number

1

0.80 0.0 111 2440 0.00

0.99 34 70 4860

2 10,0 0 0 2030 0.84 5.0 110 2300 124 2.7 6 0.98 30 80 3850 1.7 1 80

3

0.89 10.0 109 2150 246

0.97 25 90 2760

strain rate for the flames were estimated, following Lipatnikov [15], within the framework of the activation energy asymptotic theory. The turbulence boundary conditions and statistics were reported based on particle imaging velocimetry (PIV) done in previous studies [13]. The turbulent Reynolds number was calculated based on integral length scale and velocity fluctuation. Three turbulent premixed flames with CO2 addition varying between 0% and 10% were established for OH PLIF measurements at conditions identical to those in our previous study [13], as shown in Table 1. The flames were operated at identical jet Reynolds numbers (10,0 0 0), and the equivalence ratios were chosen to yield the same adiabatic flame temperature (2030 K) to minimize the thermal effects on flame propagation and structure. The unburned mixture Lewis numbers differed by a maximum of 2%, thereby minimizing transport effects on the flame structure. The discussion of CO2 effects on turbulent premixed flames is mostly based on these three flames.

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3.2. Measurement system and data processing A high-data-rate laser-imaging system [45] was employed for the OH PLIF measurements. The laser system consists of a frequency-doubled Edgewave Nd: YVO4 solid-state laser and a Sirah Credo dye laser. The Nd: YVO4 laser was operated at 532 nm with a pulse duration about 6 ns (FWHM), and the dye laser was pumped by the laser beam emitted from the Nd: YVO4 laser. The laser beam at a wavelength about 566.6 nm emitted out of the dye laser was frequency doubled to 283.3 nm to excite the Q1 (7) (1, 0) transition in the A2  -X2  electronic system of OH radical. The laser pulse energy of the OH excitation beam was about 0.1 mJ/pulse, corresponding to an average power of 0.9 W. The imaging system consisted of a Vision Research Phantom v7.3 high-speed camera, a LaVision high-speed IRO intensifier, and a UV-grade lens with an f-number of 4.5. The camera system was positioned perpendicular to the direction of the laser propagation and the plane of the laser sheet. A Semrock interference filter (FF01-320/40-25) with a transmission of 74% at 310 nm was used to selectively transmit the OH fluorescence and block the scattered laser radiation at 283 nm. The high-speed camera was operated at 90 0 0 frames per second (fps) with a window size of 640 × 320 pixel2 . The imaging region was set to 24 × 18 mm2 , with each pixel corresponding to 38 × 38 μm2 . The signal to background ratio for the OH PLIF measurements was about 12, which was determined by obtaining a ratio of the averaged fluorescence intensity and averaged non-fluorescence intensity on all pixels. The background signal was obtained using off-resonant excitation. The raw PLIF images were, first, corrected with background subtraction. After that, the images were binarized with a threshold value which is constant for each panel but may vary between different panels. The threshold values were slightly higher than zero to minimize the intermittence caused by pixel to pixel nonuniformity of the imaging system. The binarized pixel values represent the instantaneous reaction progress variable c with 0 for unburned reactants and 1 for burned products. The profiles of mean progress variable c¯ were calculated based on ensemble averages of 10 0 0 instantaneous OH PLIF images. The c¯ values represent the probability of finding the burned products at a given location within the flame brush. The OH-PLIF measurements also allow for the estimation of the flame surface density  using the method explained in Filatyev et al. [23]. The 2D approximation discussed in Section 2 was applied without the 3D correction factor. A schematic diagram of the mean flame surface density calculation procedure is shown in Fig. 3. An edge detection method embedded in the Matlab program was employed to mark instantaneous “flame fronts” after binarization. The “flame front” highlighted using this method has a uniform finite thickness, which was determined by the Matlab program instead of combustion physics. The values associated with pixels within the flame front layer were set to unity, while all other pixels were assigned a value of zero. At each region of interest, an interrogation box of size 0.38 × 0.38 mm2 (10 × 10 pixel2 ) was chosen to calculate the local flame surface density  . The perimeter of the “flame front” in the box was determined using the average area of the unity-value pixels within the box, divided by the average thickness of the “flame front” at that location. The selection of box size was found to have a negligible effect on the calculation of ¯ calculation [23]. The mean values of  for the regions of interest were calculated based on all local  values for each interrogation box within the region of interest for all 10 0 0 images. Obtaining the the measurement uncertainty of flame sturcture parameters and turbulent burning velocities introduced the uncertainties in flame condition is challenging. Because these quantities were computed or estimated using imaging processing method without a direct correlation to operating conditions such as flow

Fig. 3. A schematic of the procedure for mean flame surface density calculation.

rate or gas composition. In this work, the measurement uncertianty was evaluated only based on the uncertainty during the data processing. The measurement uncertainty for progress variable was introduced mainly by the variation of threshold values in binarization. The variation in single digit threshold value from user selection introduced about 2% fluctuation in c¯ locations. The variation of the threshold values also introduces about 2% fluctuations in the mean flame brush thickness, flame surface density, and local consumption speed. The uncertainty caused by finite pixel resolution is negligible. The uncertainties for these parameters are omitted in the plot for clarity using large symbols. The uncertainty of the global consumption speed calculated using angle method is introduced mostly from the roundish shape of the flame surface, the uncertainty of the inflow bulk velocity at the burner exit (∼2%), and the uncertainty in the mean progress variable profiles (∼2%). The area method was employed at the same time for ST, GC values to evaluate the uncertainty from roundish shape of the flame surface [11,13]. Both methods lead to results that are the same to within 5%. The overall uncertainty in the global consumption speed was calculated to be around 9% using the arithmetic summation of the three uncertainty values. It is important to note that the uncertainty values evaluated using aforementioned methods represent mostly the uncertainty in the data processing with limited connection to the operating conditions. 4. Results and discussion 4.1. Experimental observations Figure 4 shows instantaneous raw OH PLIF images without any correction for the turbulent premixed flames with CO2 addition listed in Table 1. Each image consists 9 panels. These panels are temporally independent but spatially correlated. Each panel has a 15% overlap with an adjacent panel along the burner axial centerline. The areas with finite intensities in the images indicate

150

D. Han et al. / Combustion and Flame 193 (2018) 145–156

Fig. 4. Instantaneous OH images of flames with (a) 0%, (b) 5%, and (c) 10% CO2 addition. Each image consists of 9 panels which are spatially correlated but not temporally correlated. Each panel has a 15% overlap in the axial direction with adjacent panels.

combustion products with finite concentrations of the OH radical, while the areas with zero intensity show unburned reactants with no detectable OH radical. The color scheme of each pixel indicates the OH fluorescence intensity received by the camera. Some of the intensity variation in the images might be attributed to non-uniformity of the laser sheet and the imaging system. The maximum OH fluorescence intensity was found to decrease with CO2 addition presumably due to a decrease in the OH concentration in the CO2 diluted flames, although some of the intensity decreases may result from the increased collisional quenching rate caused by the increased CO2 concentration [46]. It was observed that the flames with CO2 addition have longer flame length with many intermittent unburned reactant regions and pockets inside the flame brush. In the upstream regions, the flame without CO2 addition have larger wrinkled structures. As the level of CO2 addition increases, these wrinkled structures are observed in the further downstream regions. In these regions, the unburned and burned pockets are observed as the intersection of laser sheet with contiguous flame surface and isolated pockets. In the furthest downstream panels, an increasing number of intermittent regions and pockets are presented with increasing of CO2 addition. These intermittent regions and pockets are expected mostly as isolated detached fuel pockets. The increasing number of pockets or intermittent regions is probably due to the higher instability for the

flames with higher CO2 addition. Figure 5 shows typical time sequences of the flame structure observed at representative locations for flame 1. Similar phenomena were also observed in flames 2 and 3. In the regions before x/D = 2 of the flames, the continuous shape of the flame front is observed as shown in Fig. 5(a) and (b). The continuous flame front is wrinkled by turbulent flow eddies of various scales. Mass and heat transfer, enhanced by turbulence, generates both in-plane and out-of-plane motion of reactants and products. The out-of-plane motion of combustion products as observed in Fig. 5(a) of the 3D flame either propagates or transported into the image plane. The formation of the burned pocket is controlled by turbulent fluctuations. These fluctuations enlarge the in-plane flame surface area, which consequently enhances the local consumption speed. The in-plane motion of combustion products connects two wrinkled flame fronts forming an unburned pocket as shown in Fig. 5(b). The burnout of reactant pockets subsequently reduces the consumption speed by decreasing the flame surface area [15]. In the flame tip region as shown in Fig. 5(c), the continuous shape of the main jet flame has been replaced by many fine scale unburned pockets, which was also observed by other researchers in turbulent premixed jet flames [47]. The large unburned reactant pocket observed in Fig. 5(c) breaks down into two small pockets due to the interaction of two flame fronts. The breakdown process of large unburned pockets increases

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151

Fig. 5. Example of sequential OH PLIF images from flame 1 with highlights showing (a) burned product pocket formation process, (b) unburned reactant pocket formation process, and (c) unburned reactant pocket breakdown process. The symbols t0 , t1 , and t2 are arbitrary time instances during the measurements. The image sequences (a) and (b) show every other image in the time sequence.

the flame surface area, which accelerates the reactant consumption speed. 4.2. Turbulent flame brush The mean progress variable c¯ profiles of flames with 0%, 5%, and 10% CO2 addition are plotted as a function of distance along the burner centerline in Fig. 6. These profiles are compared with those from previous temperature measurements using CARS [13]. These c¯ values from OH PLIF measurements were fitted using the complementary error function [14]



c¯(ξ ) = 0.5erfc −ξ

√

π



where the dimensionless distance ξ is defined as ξ =

(16) x−xc¯=0.5

δT.x

. The

values of fit parameters xc¯=0.5 and δ T, x shown in Table 2 are close to those reported in the past CARS measurements for these flames [13]. The discrepancies between mean progress variable profiles based on CARS measurements and Eq. (16) for flames with CO2 addition are not observed in PLIF measurements. It is most likely caused by the spatial averaging effect of CARS measurements in CO2 diluted flames. The spatial averaging effect was enhanced by the unburned pockets, which could result in an underestimation of local temperature. The mean progress variables plotted along the radial direction at representative axial locations from x/D = 0.25 to 1.5 are shown in

Fig. 6. Mean progress variable profiles along the burner centerline for flames with 0%, 5%, and 10% CO2 addition computed based on ensemble averages of 10 0 0 instantaneous OH PLIF images.

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D. Han et al. / Combustion and Flame 193 (2018) 145–156 Table 2 Parameters for dimensionless distance calculation. Flame brush location

0% CO2 x/Dc = 0.5 2.9 r/Dc = 0.5 0.52 0.49 0.48 0.42 0.39 0.36

r/D = 0 x/D = 0.25 (r > 0) x/D = 0.5 (r > 0) x/D = 0.75 (r > 0) x/D = 1.0 (r > 0) x/D = 1.25 (r > 0) x/D = 1.5 (r > 0)

5% CO2

δ T, x /D 1.4

δ T, r /D 0.08 0.12 0.17 0.24 0.29 0.32

x/Dc = 0.5 3.5 r/Dc = 0.5 0.54 0.52 0.48 0.44 0.42 0.41

10% CO2

δ T, x /D 2.2

δ T, r /D 0.05 0.11 0.17 0.21 0.25 0.29

x/Dc = 0.5 3.9 r/Dc = 0.5 0.53 0.50 0.50 0.45 0.44 0.41

δ T, x /D 2.5

δ T, r /D 0.05 0.09 0.15 0.19 0.23 0.28

Fig. 7. Mean progress variable profiles along the radial direction at representative axial locations for flames with 0%, 5%, and 10% CO2 addition computed based on ensemble averages of 10 0 0 instantaneous OH PLIF images.

Fig. 7. Beyond x/D = 1.5, measurements were inside the flame brush and thus were not applicable for the radial flame brush thickness calculation using Eq. (16). At x/D = 0.25, a rapid increase in mean progress variable is observed for all three flames because the of the heat release from hydrogen pilot flames. The flames were lifted and anchored by the pilot flames. The radial c¯ profiles measured further downstream of a flame show a gradual increase of mean flame brush thickness along the axial direction. This increase is predominately due to the development of the flame front towards the burner centerline, with a minimal spatial variation of c¯ = 1 surfaces. The values of mean flame brush thickness along the radial direction listed in Table 2 were acquired by fitting Eq. (16), where r−r the dimensionless distance ξ is defined as ξ = δc¯=0.5 . The develT.r

opment of the mean flame brush thickness along the radial direction is plotted in Fig. 8 as a function of axial distance. The data were fit using a function based on Taylor turbulent diffusion theory [5,14] as



1/2   1 / 2 l τ u δT,r = α 2u l τ 1−  1 − exp − uτ l

(17)

The measurements display reasonable agreement with Eq. (17) with different α values of 1.5, 1.4, and 1.3 for flames 1, 2, and 3, respectively. This result suggests that the development of mean flame brush thickness of turbulent premixed flames is controlled by the turbulent diffusion law with same differences introduced by CO2 addition. The effect of CO2 addition on mean flame brush thickness is possibly due to an alteration caused by

Fig. 8. Mean flame brush thickness at representative axial locations for flames with 0%, 5%, and 10% CO2 addition. The experimental data are fitted by functions based on Taylor turbulent diffusion theory shown as solid lines.

heat release rate, which was not considered in the Taylor turbulent diffusion theory. The change in heat release rate and laminar flame speed may also cause other changes in turbulence. The increase of

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Fig. 9. Mean flame surface density values at representative regions for turbulent premixed flames with CO2 addition of 0%, 5%, and 10%. Each region covers a radial range from r/D = −0.67 to + 0.67 and an axial range as shown in each panel. The mean values are computed from 10 0 0 images.

flame length and centerline flame brush thickness may suggest a spatial delay of flame propagation. Such delay may be reflected in RMS turbulent velocity, integral length scale, and mean dissipation rate. However, the effect of volumetric expansion on the flow across the flame front on the mean velocity and RMS fluctuation values may be small due to the random orientation of flame front. The temperature increase may increase the viscosity and integral length scale, which may lead to a decrease in the dissipation rate. Future investigation is needed to quantify to the effect of laminar flame speed on turbulence. 4.3. Flame surface density The mean flame surface densities at representative axial regions as a function of mean progress variable are shown in Fig. 9. Each region covers a radial range from r/D = –0.67 to + 0.67 and an axial range as shown in each panel. Upstream of x/D = 1.87, all three flames were plotted, in c¯ space, between 0 to 1. However, as the measurement moves further downstream, the c¯ space reduces on the reactant side corresponding to the flame brush development of each flame, consequently leading to fewer scattered data plotted. Figure 9 shows that the mean flame surface density curves are quasi-symmetric in the mean progress variable space. A significantly higher maximum value of mean flame surface density ¯ max is observed in the top left panel due to an interference with the H2 pilot flame. The effect of H2 pilot flame decreases rapidly as observed in regions between x/D = 0.5 and 1. All other panels ¯ max about 0.4 mm−1 with a less than show a nearly constant  10% reduction between x/D = 1 and 3.6. The CO2 addition rate has a negligible effect on the mean flame surface density in the regions before x/D = 3.6. One possible reason for this observation is that the turbulent fluctuation velocity is maintained the same among the flames, which in an important parameter for flame wrinkling structure. The mean values of flame surface density at representative axial regions as a function of centerline progress variable and axial locations are shown in Fig. 10. Each region covers a radial range

Fig. 10. Mean flame surface density at representative regions for turbulent premixed flames with CO2 addition of 0%, 5%, and 10%. The mean values are plotted as a function of the progress variable along the centerline (top) and the axial location (bottom) of each region. Each region covers a radial range from r/D = −0.67 to + 0.67 and an axial range of x/D = 0.25. The mean values are computed from 10 0 0 images.

from r/D = –0.67 to 0.67 and axial range of x/D = 0.25. The mean flame surface density increases with progress variable first and decreases after reaching maximum value near c¯ = 0.5. The CO2 addition shows minimal effect on the mean flame surface density as a function of progress variable. The mean flame surface density as a function of axial location also increases first and decreases after reaching a maximum value. The mean flame surface density profiles are stretched to the downstream side with increasing of CO2 addition. The flame surface density is lower in regions with x/D < 3 but higher in regions with x/D > 4. In the furthest locations downstream, significant higher values are observed for the flame 3 with 10% CO2 addition. This is because the fluorescence signal

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Fig. 11. PDFs of fine-scale unburned reactant pocket equivalent radius in turbulent premixed flames with 0%, 5%, and 10% CO2 addition.

Fig. 12. The PDF of local consumption speed calculated with unburned reactant pocket in turbulent premixed flames with 0%, 5%, and 10% CO2 addition.

for this flame at these regions are low and comparable to background and camera noise. The intermittent regions caused by nonuniformity of the camera detector were mistaken as unburned or burned pocket. Such error introduced in data processing increased the mean flame surface density. This type of error also caused some non-zero values are also observed beyond x/D = 6. 4.4. Consumption of fine-scale unburned pockets Fine-scale unburned pockets beyond x/D = 3.6 were identified to investigate the effect of CO2 addition on unburned reactant pocket formation and consumption. The unburned pockets of interest were surrounded by burned regions and did not break down into smaller pockets within the time sequence analyzed. The mechanism of the fine scale formation is shown in Fig. 5. The crosssectional areas of the detected unburned pockets were recorded, and their changes within measurement temporal resolution were calculated. The criteria were applied to 10 0 0 sequential OH PLIF images, and approximately 40 0, 50 0, and 60 0 unburned pockets were detected for flames 1, 2 and 3, respectively. Equivalent radii Re of the unburned pockets were calculated by treating these pockets as circles with an equivalent area. The probability density functions (PDF) of the radii of the unburned pockets for the flames are shown in Fig. 11. The PDFs for all three flames show minimal difference for different levels of CO2 addition. The expected radius of the small-scale unburned pockets for all three flames is approximately 0.5 mm, which is close to the integral length scale of these flames. The local consumption speed of the unburned reactants can be estimated from the evolution of each pocket in a sequence of consecutive images. In this study, the local consumption speed of unburned pockets was evaluated based on Eq. (15) as

ST,LCP =

Aup 2π Re t

(18)

The terms in [18] are defined in the nomenclature and the operator  represents the change in the quantities following it. This estimation method generates errors for non-spherical geometries as discussed in [35] and due to out-of-plane motion of pockets driven by local flow field. Quantifying these errors requires knowledge of the 3D geometry for each pocket and for the local flow field, which is beyond the scope of our 2D measurements. The PDFs of local consumption speed of unburned pockets are shown in Fig. 12. The

Fig. 13. Correlations between reactant pocket size and consumption speed from flame 1.

most probable measured consumption speed is about 1.3 m/s for all three flames. This speed is much higher than their unstretched laminar flame speeds. The difference in the unstretched laminar flame speed introduced by CO2 addition does not appear to have a significant effect on the fine-scale unburned pocket consumption speed. Figure 13 shows the correlation plot between the unburned reactant pocket size and the measured consumption speed for flame 1. Similar correlations were observed for flames 2 and 3. This plot shows a weak dependence of the consumption speed on the pocket size. A similar weak correlation was also observed by Johchi et al. [35]. The slight increase in consumption rate for smaller unburned pockets is possibly caused by an increase in the surface to volume ratio as well as heat transfer to unburned reactants through conduction or radiation from products [35,48].

D. Han et al. / Combustion and Flame 193 (2018) 145–156

Fig. 14. The PLIF measurements based global consumption speed ST, GC and ST, GC normalized by SL0 the laminar flame speed with 0%, 5%, and 10% CO2 addition. The PLIF measurements are compared with past CARS measurements [13].

155

Fig. 15. The mean and RMS fluctuation values of ST, LC /SL0 for turbulent premixed flame with 0%, 5%, and 10% CO2 addition estimated using two methods: 1) ST,LC /SL0 = I0 AT /AL without pockets, and 2) ST,LC /SL0 = I0 max δT with pockets. S

4.5. Turbulent burning velocity The global consumption speed ST, GC was evaluated through the mean progress variable contours of c¯ = 0.2 using Eq. (6) for comparison with our previous measurements [13]. The ST, GC value decreases with the increase of c¯ value selected for calculation (e.g. ST,GC = 1.8 using c¯ = 0.2 while ST,GC = 1.4 using c¯ = 0.5 for the flame with 0% CO2 addition). The values of ST, GC and u SL0

ST,GC SL0

are plot-

ted as a function of as shown in Fig. 14. Figure 14 shows that the global consumption speed decreases with CO2 addition while S

the ratio with respect to unstretched laminar flame speed ST,GC is L0 almost constant within the uncertainty limits. The effects of CO2 addition on global consumption speed show agreement with our previous study based on CARS temperature measurements [13]. The local consumption speed ST, LC was first evaluated using Eq. (11), which does not account for the flame surface area of unburned and burned pockets. This method was not employed for the regions downstream of x/D = 2.5 because the flame surface area of pockets was much more significant. The interrogation region chosen covers a radial range from r/D = –0.67 to 0.67 and an axial range of x/D = 0.25. Within each interrogation region, the value of AT was calculated using an in-house length computing code following the image processing procedure as shown in Fig. 3. The value of AL was computed using the length of c¯ = 0.5 contour based on 10 0 0 images within the interrogation region. The mean and RMS fluctuation values of ST, LC were calculated based on 10 0 0 OH PLIF images. The ratio of local consumption speed and laminar flame speed

ST,LC SL0

for representative regions are plotted in Fig. 15. S

The mean and RMS fluctuation values of ST,LC increase gradually as L0 x/D increases until x/D = 1.5 and then remain constant after that for all three flames. The level of CO2 addition has a minimal effect S

on the value of ST,LC . L0 An alternative estimation of local consumption speed was performed using Eq. (12). This method was applied to the flame brush development area at axial locations x/D ≤ 1.5. The values of δ T at the representative axial locations were adopted from the previous section. The values of  max at those locations were interpolated from spatially averaged values calculated using the aforementioned method. The mean and RMS fluctuation values of ST,LC SL0

ST,LC SL0

are shown in Fig. 15. The mean values of increases along the axial direction before x/D = 1.5, which shows an agreement with

our first method. However, the mean value of ST,LC calculated usL0 ing Eq. (12) are higher than those using Eq. (11). The discrepancy between these two methods increases along the axial direction, which suggests an increased probability of finding unburned or burned pockets with increasing x/D. The discrepancy between these two methods also decreases with CO2 addition. This indicates a reduced contribution of the flame surface area from the pocket formation for the flames with CO2 addition before x/D = 1.5. It is important to note that in this region, the unburned and burned pockets are mostly likely observed at the intersection of laser sheet with contiguous flame surface. The increase in the number of unburnt pockets observed in the flame without CO2 addition is due to the larger wrinkled structures in this region as shown in Fig. 4. S

The RMS values of ST,LC calculated without pockets show signifiL0 cantly higher values than those with pockets. This is because that the flame surface density assessed without including pockets will have a broader distribution due to the wide size distribution of the subtracted pockets. 5. Summary and conclusions In this study, we performed 9 kHz OH PLIF measurements in turbulent premixed flames with and without CO2 addition. The OH PLIF measurements were applied to investigate the effects of CO2 addition on flamelet structure and burning velocity. The flames studied were operated at the same adiabatic flame temperature, Reynolds number, and Lewis number to minimize thermal and transport effects caused by CO2 addition. The flamelet structure of all three flames was characterized by the flame brush thickness and the flame surface density at representative locations. The unburned pocket formation and consumption speed were investigated using the statistical methods and assuming the circular shape of the unburned pockets. Turbulent burning velocity including global and local consumption speeds were reported. The OH PLIF measurements suggest that the CO2 addition extends the flame length by spatially delaying the progress of the reaction as well as creating widely distributed unburned reactant pockets. The wide distribution of unburned reactant pockets is expected as a result of reduced laminar flame speed with an increase of CO2 addition. The mean reaction progress variable c¯ for turbulent premixed flames with and without CO2 addition collapses to a universal curve with a dimensionless coordinate associated with mean flame brush thickness δ T . The development of the mean flame brush thickness δ T, r follows the turbulent diffusion law, with

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differences introduced by CO2 addition. The flame surface density  is essentially the same in the regions before x/D = 3.6 for the flames with and without CO2 addition. The CO2 addition modifies the flame surface density  significantly in the regions beyond x/D = 3.6 of the flame with a higher possibility of finding unburned reactant pockets in a wider spatial distribution. The consumption speed ST, LCP of the fine scale unburned reactant pocket is much higher than upstretched laminar flame speed SL0 . The difference introduced by CO2 addition on laminar flame speed SL0 hardly alters this consumption rate. The local combustion intensity ST, LC /SL0 is observed to be lower in flames with CO2 addition due to the attenuation of pocket formation in the flame brush development region upstream of x/D = 1.5. The local combustion intensity downstream in the flame is expected to increase with increasing number of pockets. As a result of these counteracting phenomena the global combustion intensity ST, GC /SL0 shows a remarkably constant (within uncertainty limits) value independent of the level of CO2 addition. This is an important practical result for combustor designers interested in utilizing variable exhaust gas recirculation (EGR). Acknowledgment Funding for this work was provided by the U.S. Department of Energy, National Energy Technology Laboratory (NETL), and University Turbine Systems Research (UTSR) Program under Grant no. DE-FE0011822 and by the U.S. Department of Energy, Division of Chemical Sciences, Geosciences, and Biosciences under Grant no. DE-FG02-03ER15391. References [1] J. Ji, Y.R. Sivathanu, J.P. Gore, Thermal radiation properties of turbulent lean premixed methane air flames, Proc. Combust. Inst. 28 (20 0 0) 391–398. [2] Y.-C. Chen, N. Peters, G.A. Schneemann, N. Wruck, U. Renz, M.S. Mansour, The detailed flame structure of highly stretched turbulent premixed methane–air flames, Combust. Flame 107 (1996) 223 IN222. [3] A. Bohlin, E. Nordström, H. Carlsson, X.-S. Bai, P.-E. Bengtsson, Pure rotational CARS measurements of temperature and relative O2 -concentration in a low swirl turbulent premixed flame, Proc. Combust. Inst. 34 (2013) 3629–3636. [4] M.J. Dunn, A.R. Masri, R.W. Bilger, A new piloted premixed jet burner to study strong finite-rate chemistry effects, Combust. Flame 151 (2007) 46–60. [5] J.F. Driscoll, Turbulent premixed combustion: Flamelet structure and its effect on turbulent burning velocities, Prog. Energy Combust. Sci. 34 (2008) 91–134. [6] Y. Tanaka, M. Nose, M. Nakao, K. Saitoh, E. Ito, K. Tsukagoshi, Development of low NOx combustion system with EGR for 1700 C-class gas turbine, Mitsubishi Heavy Ind.: Techn. Rev. 50 (2013) 1. [7] T. Le Cong, P. Dagaut, Oxidation of H2 /CO2 mixtures and effect of hydrogen initial concentration on the combustion of CH4 and CH4 /CO2 mixtures: experiments and modeling, Proc. Combust. Inst. 32 (2009) 427–435. [8] D.L. Zhu, F.N. Egolfopoulos, C.K. Law, Experimental and numerical determination of laminar flame speeds of methane/(Ar, N2 , CO2 )–air mixtures as function of stoichiometry, pressure, and flame temperature, Symp. (Int.) Combust. 22 (1989) 1537–1545. [9] H. Guo, G. Smallwood, F. Liu, Y. Ju, O. Gulder, The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counterflow CH4 /air premixed flames, Proc. Combust. Inst. 30 (2005) 303–311. [10] K.B. Fackler, M.F. Karalus, I.V. Novosselov, J.C. Kramlich, P.C. Malte, Experimental and numerical study of NOx formation from the lean premixed combustion of CH4 mixed with CO2 and N2 , J. Eng. Gas Turbines Power 133 (2011) 121502–121507. [11] H. Kobayashi, H. Hagiwara, H. Kaneko, Y. Ogami, Effects of CO2 dilution on turbulent premixed flames at high pressure and high temperature, Proc. Combust. Inst. 31 (2007) 1451–1458. [12] C. Cohé, C. Chauveau, I. Gökalp, D.F. Kurtulus¸ , CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames, Proc. Combust. Inst. 32 (2009) 1803–1810. [13] D. Han, A. Satija, J. Kim, Y. Weng, J.P. Gore, R.P. Lucht, Dual-pump vibrational CARS measurements of temperature and species concentrations in turbulent premixed flames with CO2 addition, Combust. Flame 181 (2017) 239–250. [14] A.N. Lipatnikov, J. Chomiak, Turbulent flame speed and thickness: phenomenology, evaluation, and application in multi-dimensional simulations, Prog. Energy Combust. Sci. 28 (2002) 1–74. [15] A. Lipatnikov, Fundamentals of premixed turbulent combustion, CRC Press, 2012. [16] I.G. Shepherd, Flame surface density and burning rate in premixed turbulent flames, Symp. (Int.) Combust. 26 (1996) 373–379.

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