Air turbulent swirling premixed flames in a cuboid combustor

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Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor Joonhwi Park*, Yuki Minamoto, Masayasu Shimura, Mamoru Tanahashi Department of Mechanical Engineering, Tokyo Institute of Technology, 2-12-1 Okayama, Meguro, Tokyo 152-8550, Japan

highlights

graphical abstract


Effects of mixture conditions on CH4/H2/Air turbulent swirling premixed flames are investigated.
Qualitatively different global flame features

between

the

mixture

conditions are observed.
The role of H2 in each mixture condition

remains

unchanged

even under intense turbulence.
Reaction deviate

zone from

characteristics unstrained

and

strained laminar flame profiles.
Contributions by convection and diffusion are influential on the local and global flame structures.

article info

abstract

Article history:

Effects of H2-enrichment on structures of CH4/air turbulent swirling premixed flames

Received 26 September 2019

affected by high intensity turbulence in a gas turbine model combsutor are investigated by

Received in revised form

conducting direct numerical simulations. Two stoichiometric mixture conditions, of which

21 December 2019

volume ratio of CH4:H2 ¼ 50:50 and 80:20, are simulated by considering a reduced chemistry

Accepted 25 December 2019

(25 species and 111 reactions). Results showed qualitatively different flame shapes and

Available online xxx

reaction zone characteristics between the cases. For the higher H2-ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H2-ratio case, the flame is

Keywords:

stabilized only in the inner shear layer and extinction occurs in the outer shear layer.

Direct numerical simulation

Comparison of the reaction zone characteristics with unstrained and strained laminar

Turbulent premixed flame

flames in phase space showed that H2 mass fraction for the lower H2-ratio case and re-

Hydrogen-enriched fuel

action rate profiles for both cases deviate from the corresponding laminar values. Analysis

Swirling flame

of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be

* Corresponding author. E-mail address: [email protected] (J. Park). https://doi.org/10.1016/j.ijhydene.2019.12.175 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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useful for designing practical H2-enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The demands for combustion devices to be more efficient and environmentally friendly have been increasing in recent years. One of the promising ways is using H2 as a fuel, which has great energy density and does not discharge CO2 during its combustion processes. However, there are still difficulties due to the characteristics of H2 combustion [1], such as extremely high burnt gas temperature, higher burning velocity than hydrocarbon fuels. As an alternative method, addition of H2 to natural gas (mainly consists of CH4) has brought to our attention recently. Advantages of using such a fuel in lean premixed combustion are improved flame stability with reduced emissions of pollutants, such as CO [2], NO [3] and NOx [4], lower lean blowout limit [5e7], extended flashback limit [8e10] and resistance to strain rate induced extinction [2,11,12]. Therefore, using H2-enriched natural gas as a fuel is considered to be one of the promising ways to achieve both higher thermal efficiency and reduced emissions of pollutants. Gas turbine engines are considered as one of the devices where utilization of H2-enriched natural gas is effective [13,14]. In such devices, turbulent swirling flows are widely used for enhancement of turbulent mixing and flame stabilization. Furthermore, relatively strong three-dimensionality of the flows affects the flame structures and dynamics substantially. Therefore, it is important to clarify the characteristics of CH4/H2/air turbulent swirling flames for the realization of practical H2-enriched gas turbines engines. For fundamental insight into CH4/H2 combustion, Wang et al. [15] have reported changes in the role of H2 in onedimensional CH4/H2/air laminar premixed flames under various mixture conditions. According to the results, the role of H2 in the flames shifts from an intermediate species to a reactant species when the volume ratio of H2 in the premixed mixture is increased from 20% to 30%. Furthermore, the reactions in C1 chemical pathway are promoted with the increased amount of H2 resulting from relatively higher concentrations of H, O and OH radical pools. Day et al. [16] have conducted 2-dimensional simulations of lean CH4/H2/air turbulent premixed flames, which are categorized into the laminar flamelet regime. Here, the focus was on investigating the effects of volume ratios of H2 in the mixture on the local flame structures and C1/C2 reaction pathways. They also indicated that CH4 consumption rate increase with the increase of H2 ratio. Similarly, the role of H2 changes when the volume ratio of H2 in the mixture is higher than 25%. These results indicate reaction pathway alternation with the increase of H2 volume ratios in the mixture in both laminar and relatively low level turbulent flames. However, the effects of mixture conditions on the combustion characteristics and

structures of CH4/H2/air premixed flames which are affected by strong three dimensionality of the flows and associated intense turbulent mixing, such as that in the turbulent swirling flows have not been clarified yet. There are experimental studies of H2/CH4 swirling flames focusing mainly on the flame shapes and emission characteristics under various conditions. Cheng et al. [17] have demonstrated influences of H2 volume fraction and pressure on the flame shapes and flame attachment characteristics in a laboratory scale low-swirl burner. Kim et al. [18,19] have investigated the effects of fuel composition and swirl strength on the flame shapes, CO and NOx emission characteristics of premixed swirling flames. Shanbhogue et al. [12] have reported effects of H2 enrichment and equivalence ratios on transition of flame shapes and flame stabilization mechanism. There are several numerical studies provided findings of flame structure and flow-flame interactions in weakly or moderately turbulent premixed swirling flames of CH4/air [20,21], CH4/H2/air [22] and CH4/O2/CO2 [23] by conducting large eddy simulations (LES). Huang and Yang [20] reported that mixture temperature and equivalence ratio are two most important parameter affecting structure and stability of swirlstabilized flames. Huang and Yang [21] investigated effect of swirl strength on flame length and occurrence of flame flash back. Mercier et al. [22] have studied flame shape transition between two H2 enrichment levels in both experimental and numerically. The numerical result could capture the flame shape transition observed in their experiments when considering the non-adiabaticity of combustor walls. Chakroun et al. [23] also suggested that the impact of heat loss by the combustor walls on the flamelet structures in oxy-flames. However, effects of mixture conditions on reaction zone structures and species transport characteristics in the turbulent swirling flames affected by intense turbulence, which are necessary information for turbulent combustion modeling and designing practical H2-enriched gas turbine engines, have not been fully clarified yet. Since all of the scales involved in the phenomena can not be fully resolved due to limitations of turbulence and combustion models. Therefore, in this study, direct numerical simulations (DNS) of CH4/H2/air turbulent swirling premixed flames in a cuboid combustor have been performed to clarify the effects of H2 enrichment on the flame structures in a swirl flow configuration by using a reduced chemistry consisting of 25 reactive species and 111 elemental reactions and considering temperature dependency of transport and thermal properties. This study is organized as follows. First, the computational methods and numerical conditions are addressed in Sec. DNS of CH4/H2/air swirling premixed flames. In Sec. Results and discussion, we will present the effects of H2 volume ratios in the mixture on global flame features, reaction zone

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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characteristics and balance between convection, diffusion, chemical reaction of the fuel species. Finally, conclusions are summarized in Sec. Conclusions.

DNS of CH4/H2/air swirling premixed flames Two stoichiometric CH4/H2/air turbulent swirling premixed flames in a cuboid combustor are simulated by using TTX (Tokyo Tech Combustion Simulation) code, which has been used to solve various combustion problems [24e29] by solving fully compressible governing equations. The temperature dependency of transport and thermal properties are also considered by using CHEMKIN-II packages [30,31] with vector and parallel computation modification. The governing equations are conservation of mass, momentum, energy and species mass fractions. The equations are discretized by using a fourth-order central finite difference scheme and integrated in time by using a third-order Runge-Kutta scheme. Chemical source terms are handled by the multi-timescale (MTS) method [32] and the transport and thermal properties of the species are computed by the correlated dynamic adaptive chemistry and transport (CO-DACT) [33,34]. The detailed UCSD kinetic mechanism [35], consists of 57 reactive species and 268 elementary reactions without nitrogen chemistry is reduced to 25 reactive species and 111 elementary reactions by using the path flux analysis (PFA) method [36] for H2-enriched CH4/air combustion. The reduced mechanism showed reasonable agreements to the detailed mechanism [35] in comparison of one-dimensional unstrained and strained laminar flame profiles obtained by PREMIX [37] and OPPDIF [38] and ignition delay times by SENKIN [39] for both mixture conditions. NaviereStokes characteristic boundary conditions (NSCBC) [40,41] are imposed on the inflow, outflow boundaries and combustor walls. The effects of pressure gradient and the Soret diffusion are not considered in the calculation of the diffusion velocity. The Dufour effect and radiative heat transfer are also neglected in the energy conservation equation. Fig. 1 shows the computational domain which is identical to previously reported H2/air turbulent swirling flames [28,42e44]. The dimensions of the combustor Lx  Ly  Lz is 15 mm  10 mm  10 mm and the number of uniform mesh points in each direction Nx  Ny  Nz is 1152  768  768. The inlet geometry consists of a concentric annulus of which inner and outer diameter is Din ¼ 0.6 mm and Dout ¼ 2.5 mm respectively. The combustor wall is inert, no-slip and isothermal at 700 K. Two mixture conditions of which volume ratio of CH4:H2 is 50:50 (H2-50%) and 80:20 (H2-20%) are simulated to clarify the effects of H2-enrichment on the turbulent swirling flames. The premixed mixtures are preheated to Tu ¼ 700 K at atmospheric pressure. The flame properties, laminar flame speed SL , flame thermal thickness dth ¼ ðTb  Tu Þ/jVTjmax , the maximum heat release rate uT;L and the Zel'dovich thickness dF ¼ nu =SL for both mixture conditions are listed in Table 1. Here, Tb is the burnt gas temperature and nu is the kinematic viscosity of the premixed mixture.

Fig. 1 e Schematics of the computational domain and coordinate system.

The mesh size ensures that the reaction rates of all relevant reactive species reaction rates are fully resolved in a corresponding one-dimensional (1-D) unstrained laminar premixed flame for each mixture condition. The reaction zone profiles (species mass fractions, reaction rates and heat release rate) and flame properties (SL , dth and uT;L ) showed reasonable agreements when the flames are calculated by using a half-size mesh, Dx=2 with the same reduced chemistry used in the present DNS. Temperature and species mass fractions in the burnt gas side of the corresponding 1-D unstrained laminar flame are given uniformly to the computational domain as initial conditions. The mean profiles of radial (ur ), azimuthal (uq ) and axial (ux ) velocity components of an annular swirling flow at the inlet are analytically determined as a function of radial distance r ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi y2 þ z2 . The profiles are obtained by solving steady NavierStokes equations in cylindrical coordinate system with noslip wall boundary conditions [28,43]. ur;base ¼ 0; uq;base ¼ fq

ux;base ¼

(1)





 r2 c2 þ c1 r þ ¼ fq Uq ; 3 r r

Rout

2



   R2  1 r ln  1 c3 ubx : lnR Rout

(2)

(3)

Here, Rout ¼ Dout =2; Rin ¼ Din =2 and R ¼ Rin =Rout . The integral constants c1 , c2 and c3 are determined by imposing no-slip wall conditions at r ¼ Rin and Rout and the relation between ux and ubx . c1 ¼ 

  Rout R2 þ R þ 1 ; 3ð1 þ RÞ

(4)

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Table 1 e Properties of one dimensional unstrained laminar flames. Case H2-50% H2-20%

SL [m/s]

dth [mm]

uT;L ½W =m3 

dF [mm]

nu [m2/s]

2.73 1.97

0.221 0.249

1.63  1010 1.16  1010

2.66  102 3.53  102

7.28  105 6.96  105

R3out R2 ; 3ð1 þ RÞ

c2 ¼

c3 ¼ 

(5)

2lnR : R2 ðlnR  1Þ þ 1 þ lnR

(6)

The swirl number S defined by Eq. (7) is used to determine the azimuthal velocity uq . Z

Rout

uq;base ux;base r2 dr

Rin

S¼

ðRout  Rin Þ

Z

Rout Rin

:

(7)

u2x;base rdr

By substituting Eq. (7) to Eq. (2), the external force coefficient fq is determined and the mean velocity profiles at the inlet are obtained. ðRout  Rin Þ fq ¼ S

Z

Z

Rout

Rin Rout

u2x;base rdr :

(8)

Uq ux;base r2 dr

Fig. 2 e Turbulent combustion conditions of the present DNS (red and blue squares) and previously reported H2-air cases [28,42e44] (black circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Rin

The swirl number S is set to be 1.2. The bulk mean axial velocity at the inlet ubx is set to be 190 m/s to provide nearly the same calorific input (Ein z1.16 kW) to the previous studies for stoichiometric H2/air flames [28,42]. The corresponding Reynolds number Re ¼ ubx Dout =nu is 6545 for H2-50% case and 6848 for H2-20% case. The inflow velocity perturbations u0 following the same manners to Wang et al. [45] are added to the mean velocity profiles to promote transition from laminar to turbulent flow. The perturbations consist of simple white noise in which each frequency has a randomly-given lifetime for its phase. The maximum velocity perturbation intensity u0max is

Table 2 e Numerical parameters of the present DNS based on the turbulent statistics. RelE , Ka and Da denote the € hler Reynolds number, Karlovitz number and Damko number based on integral length scale lE . RelE ¼ ðu0 =SL ÞðlE =dF Þ, Ka ¼ ðu0=SL Þ3=2 =ðlE =dF Þ1=2 and Da ¼ ðlE =dF Þ =ðu0 =SL Þ. Case H2-50% H2-20%

set to be 0.15ubx ¼ 28:5 m/s and corresponding root mean square velocity fluctuation u0rms is 10.5 m/s (z 5.5% of ubx ) at the inflow boundary. Fig. 2 shows turbulent combustion conditions of the present DNS with our previously reported H2/air cases [28,42e44] on the combustion regime diagram [46]. Several important non-dimensional numbers of the present study are summarized in Table 2. The root mean square of velocity fluctuation u0rms and the integral length scale lE are computed as u0rms

lE ¼

 1=2 2~ k ¼ 3

ðu0rms Þ3 ; ~εturb

(9)

(10)

εturb is where k~ is the turbulent kinetic energy k~ ¼ 12 u00g i u00i and ~ the turbulent kinetic energy dissipation rate computed as

~εturb ¼

lE [mm]

u0 =SL

lE /dF

RelE

Ka

Da

1.36 1.35

31.4 44.1

50.9 38.1

1598 1680

24.6 47.4

1.62 0.86

  2rn Sij Sij  Skk Skk 3 : r

(11)

Here, Sij is the turbulent strain tensor Sij ¼ 1=2ðvu00i =vxj þ vu00j =vxi Þ. f~ denotes a Favre average of a physical quantify f, f~ ¼ rf =r and f 00 does the fluctuations around f~. f is the Reynolds average of f and computed as f¼

Nt 1 X f ðx; ti Þ: Nt i¼1

(12)

Here, Nt is the number of samples in time. In the present study, 150 samples are used for the averaged statistics of the flow field. The sampling period is approximately 3:75 tL , where tL is the flow-through time tL ¼ Lx =ubx z80 msec. The diagram in Fig. 2 is constructed base on volume averaged values in the unburnt gas sides of the flames, where a Favre averaged reaction progress variable ceY is 0.1  ceY  0.4. ceY defined as cY ≡ðYs  Y us Þ =ðY bs  Y us Þ. Here, Ys is the sum of

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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species mass fractions Ys ≡YH2 þ YH2 O þ YCO þ YCO2 in Refs. [47,48]. The superscripts “u” and “b” denote the value in the unburnt and burnt gas side of the corresponding laminar conditions respectively. As Fig. 2 shows, the turbulent flames are categorized into the thin reaction zones regime. The Karlovitz numbers Ka of the present DNS results are higher than those of H2/air cases [28,42e44] due to lower SL compared with H2/air flames (10.4 m/s for 4 ¼ 1.0 and 7.28 m/s for f ¼ 0.6) at the same initial temperature and pressure (Tu ¼ 700 K and pini ¼ 0.1 MPa).

Results and discussion Effect of H2-ratios on the global flame features Figure 3 shows instantaneous snapshots of normalized heat release rate uþ T and the second invariant of velocity gradient tensor Q  . The heat release rate uT is normalized by the maximum heat release rate of the corresponding unstrained laminar flame uT;L , uþ T ¼ uT =uT;L . The second invariant of velocity gradient tensor Q is calculated as Q ¼ 1=2ðSij Sij þWij Wij Þ supposing the incompressible limit, where Sij ¼ 1=2ðvui =vxj þvuj =vxi Þ and Wij ¼ 1=2ðvui =vxj  vuj =vxi Þ. The threshold of iso-surface Q  is 1% of its maximum. For both cases, large-scale and an aggregation of fine-scale vortical structures are formed in the near field of inflow boundary due to the strong shear layer and rapid development of turbulence. However, the shapes of uþ T fields seem to be different between the cases. To figure out the difference in detail, instantaneous 2dimensional distributions of temperature T, contour lines of ~x ¼ 0 and uþ u T on the representative plane (y  0 and z ¼ 0) are shown in Fig. 4. In this study, the region of x  Lx =2 ¼ 7.5 mm is named as an “upstream region” and x > Lx =2 is named as a “downstream region” of the combustor. For both cases, the inner recirculation zones (IRZ), where heat and combustion products are recirculated by the swirling flows are formed in

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the central part of the plane (y  2:5 mm). Due to the existence of combustor walls, the outer recirculation zones (ORZ) are observed at the corner of the plane (y  2:5 and x  2:5 mm). We refer the shear layer formed between the annular jets and IRZ as inner shear layer (ISL) and the other one between the annular jets and ORZ as outer shear layer (OSL). Similar to the previous studies [28,42e44], uþ T is decreased compared to uT;L for both cases. Temperature in the ORZ is lower compared to the value in the IRZ possibly due to heat loss by contacting with isothermal walls. For H2-50% case, relatively high uþ T is observed in the near fields of swirling jets and combustor walls in the upstream region, whereas for H220% case, the region of high uþ T exists both in the upstream and downstream regions. These differences in flame length are due to increase of reactivity and diffusivity with the increase of H2-ratios. Focusing on the near field of swirling jets, the magnitudes of uþ T in the ISL and OSL are relatively similar for H2-50% case. For H2-20% case, however, higher uþ T is observed in the ISL rather than OSL. In other words, the flame is stabilized in both shear layers for H2-50%, whereas the flame extinction occurs in the OSL for H2-20% case. Thus, it is anticipated that the fresh reactants may flow through the OSL almost unreacted due to the flame extinction. As a result, the temperatures in the region from the OSL to the combustor walls become lower than H2-50% case due to entrainment cold reactants into the OSL. These different flame shapes between the cases are qualitatively similar to experimental and numerical results of pure CH4 and CH4/H2 swirling flames under the mixture conditions with lower H2-ratios in previous studies [5,12,17,22,49,50], which is attribute to reduced reactivity (or lower OH concentration) in the ORZ or extinction in the OSL. To investigate the reaction zone characteristics further, 2-dimensional distributions of fuel species mass fraction and reaction rates are examined next. Fig. 5 shows 2-dimensional distributions of fuel species mass fraction Y i . The superscript “*” denotes the normalization by the value at the inflow boundary Y ui , thus Y i ≡Yi =Y ui . For H2-50% case, CH4 is almost consumed in the upstream region,

Fig. 3 e Instantaneous snapshots of normalized heat release rate uþ T (volume rendered) and iso-surface of the second invariant of velocity gradient tensor Q  ¼ 0:01Qmax (white) for H2-50% (a) and H2-20% (b). Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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Fig. 4 e Instantaneous 2-dimensional distributions of temperature T (a, c) and normalized heat release rate uþ T (b, d) on the representative plane for H2-50% (a and b), H2-20% (c and d). The dashed-line denotes the axial position x ¼ Lx =2 ¼ 7:5 mm.

Fig. 5 e Instantaneous 2-dimensional distributions of normalized fuel species mass fractions of Y CH4 (a, c) and YH2 (b, d) on the representative plane for H2-50% (a and b) and H2-20% (c and d). The dashed-line denotes the axial position of x ¼ Lx = 2 ¼ 7:5 mm.

while the values of YH2 are approximately 0.2 in the ORZ and downstream region, indicating that leakage of H2 into the burnt gas due to high temperature equilibrium in Ref. [51] is also observed for stoichiometric CH4/H2/air mixture conditions. For H2-20% case, however, CH4 is not completely consumed in the region from the near field of OSL to the downstream. Especially, the value of Y H2 is as high as or higher than unity in the region aforementioned. Thus, these different mass fraction distributions seem to be related to extinction in

the OSL and ORZ in Fig. 4d and production and consumption of H2. To clarify the behaviors of the fuels in detail, fuel species þ reaction rate contours (uþ CH4 and uH2 ) are shown in Fig. 6. The reaction rates are normalized as uþ i ¼ ui =ui;L , where ui;L is the maximum of jui j in the corresponding unstrained laminar condition. Production and consumption reaction rates are colored by red and blue, respectively. The magnitudes of the fuel species reaction rates are decreased compared to the ui;L ,

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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þ Fig. 6 e Instantaneous 2-dimensional distributions of normalized fuel species reaction rates uþ CH4 (a and c) and uH2 (b and d) on the representative plane for H2-50% (a and b) and H2-20% (c and d). The dashed-line denotes the axial position of x ¼ Lx = 2 ¼ 7:5 mm.

which is similar to the previous studies with same swirl number condition [28,42e44]. For both cases, consumption of CH4 is observed in the reaction rate contours. For H2-20% case, the magnitudes of uþ CH4 are noticeably decreased in the near field of OSL and in the ORZ. As for uþ H2 , both consumption and production reactions are observed, while the tendencies are distinctively different. For H2-50% case, consumption of H2 is dominant in the upstream region and production reaction is observed in the reactants sides of the flame with relatively lower magnitude. For H2-20% case, consumption of H2 occurs in trailing edge of the flame with relatively high magnitude and production does in the unburnt side of the reaction zones with relatively similar magnitude of the consumption reaction suggesting that the role of H2 in each mixture condition, which is reported in [15,16] remains unchanged even in strong turbulence. In the near field of OSL and ORZ, the magnitudes of uþ H2 are decreased substantially. Therefore, the high values for H2-20% case in Fig. 5d are due to production of H2 in the unburnt side of the reaction zones and leakage of fuels to the ORZ resulted from the extinction in the OSL. In addition, comparably lower reactivity and temperature in the ORZ seem to be attributed to heat loss by cold reactants flow through the OSL as anticipated in Fig. 4d.

Effects of H2 ratios and local tangential strain rate on the reaction zone characteristics To evaluate the effects of H2-ratios and local tangential strain rates on the reaction zone characteristics observed in Sec. Effect of H2-ratios on the global flame features, the conditional average of a variable q conditioned based upon cY , which is denoted as 〈qjcY 〉 is compared with unstrained and

strained laminar flames. Here, q is fuel species mass fraction or reaction rate in this study. In the laminar flames, cY is 0 in the reactants side and 1 in the burnt gas side, while cY can be larger than unity in the turbulent flames possibly due to turbulent motions. In the process of conditional averaging, the DNS data within approximately dth from the wall are excluded due to cooling effect by the isothermal walls. In the strained laminar flame simulations, a single flame configuration (fresh reactants to hot products) as adopted in Refs. [52e54] is considered to predict the reaction zone characteristics in strong shear layers generated by the turbulent swirling flows by using OPPDIF [38]. In the single flame configuration, flame can be sustained at a much higher strain rate than the extinction strain rate in twin flames configuration [12]. The jet velocity Uj at the fuel and oxidizer side is set to be same. The thermochemical conditions at the fuel side are fresh reactants (Y ui , Tu ) and the oxidizer side are burnt gases (Ybi , Tb ) of the corresponding unstrained laminar flame. The local tangential strain rate at is defined to be the first peak of velocity gradient du=dx in the reactants side. The values of at considered in this study are 1/50 to 2 times of the mean strain rate in the preheat zones of the present DNS results which is estimated to u0rms =lz5:0  105 s-1. Here, u0rms is the velocity fluctuation computed by Eq. (9) and l is the Taylor microscale qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P g2 =ðvu00g computed as l ¼ 1=3 3i¼1 u00 =vxi Þ2 and volume averaged i i within 0.1  ceY  0.4 of the flow field. The value of l is 0.183 mm for H2-50% case and 0.166 mm for H2-20% case. Figures 7 and 8 show profiles of Y CH4 and Y H2 in phase space. As Fig. 7 shows, no significant difference between the mixture conditions is observed in the Y CH4 profiles. All of the

Y CH4 profiles decrease to nearly 0 monotonically with the

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Fig. 7 e Conditional averages of YCH4 with respect to reaction progress variable cY (dashed line) and standard deviation ± s around the mean (red area) for the case of H2-50% (a) and H2-20% (b). The corresponding unstrained (dotted line) and strained laminar flames (solid and dot-dashed lines) profiles are superimposed. The arrow indicates increase of the local tangential strain rate at . (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8 e Conditional averages of Y H2 with respect to reaction progress variable cY (dashed line) and standard deviation ± s around the mean (red area) for the case of H2-50% (a) and H2-20% (b). The corresponding unstrained (dotted line) and strained laminar flames (solid and dot-dashed lines) profiles are superimposed. The arrow indicates increase of the local tangential strain rate at . (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

increase of cY . The responses of Y CH4 to the tangential strain rate at are similar for both mixture conditions. The strained laminar flame values increase gradually with the increase of at , which suggests decreased consumption of CH4 and increased influences of relevant transports (convection and diffusion) which can modify the mixture composition in the

reaction zones. The values of 〈Y  cY 〉 are relatively higher CH4

than the unstrained and strained laminar values in the entire cY . This behavior is different from the profiles in high Karlovitz number turbulent planar CH4/air flames [55,56], where the CH4 concentration profiles lie along the corresponding laminar profiles. This deviation is rather similar to the profiles in Ref. [23], where the flame is affected by heat loss through

combustor wall. Hence, the deviation of 〈Y  cY 〉 form the CH4

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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corresponding laminar profiles may be due to the decreased consumption of CH4 as shown in Fig. 6 and/or increased influences of the relevant transports. As for the Y H2 profiles, substantial differences are observed between Fig. 8a and b. Opposite to the responses of Y CH4 to the

at , YH2 profiles decrease gradually with the increase of at for

both mixture conditions. The strained laminar profiles for H220% mixture condition decrease substantially with the increase of at , whereas the profiles are relatively similar for H2

50% mixture condition. The 〈Y  cY 〉 profile for H2-50% case H2

shows relatively similar behaviors to both unstrained and strained laminar profiles in an average sense. For H2-20% case,

〈Y H2 cY 〉 deviates from the laminar flames in the unburnt gas

side except for at ¼ 2.0 105 s-1. Thus, 〈Y H2 cY 〉 values as high as unity in the unburnt sides as observed in Fig. 5 seem to be due to decreased reaction rates (extinction) and/or convection by the swirling jets. To show the responses of Y i to at in detail, the fuel species reaction rates uþ i are evaluated by the same manners in Figs. 9 and 10. For both mixture conditions, the uþ CH4 profiles for strained laminar flames in which at  1:0  105 s-1 are relatively similar to the unstrained laminar flames. With further increase of at  2:0  105 s-1, consumption rates increase gradually. This suggests the increased contributions by convection and diffusion, which are in balance with chemical reaction. The increase of relevant transports with the increase of at in the strained laminar flames is mainly due to higher jet velocities which result in narrower reaction zone width and large species mass fraction gradient. While for the turbulent



swirling flames, the values of 〈uþ CH4 cY 〉 are decreased sub-

9

〈Y CH4 cY 〉 profiles in Fig. 7 due to increased volume fraction of the reaction zones. As for uþ H2 , the strained laminar flames show different responses to the at between the mixture conditions in Fig. 10. For H2-50% mixture condition, the maximum consumption rates of the strained flames (at  2:0  105 s-1) are decreased. In addition, consumption reaction begins to occur at relatively higher cY . With further increase of the at (at  5:0  105 s-1), the uþ H2 profiles deviate from the unstrained and strained laminar flames with the lower values of at (at  2.0 105 s-1). For H220% mixture condition, the maximum production rate increases with the increase of at within at  5:0  105 s-1. On the other hand, the peaks of consumption reaction rate appear at similar cY and the values are relatively similar except for at ¼



1:0  104 s-1. In the swirling flames, 〈uþ H2 cY 〉 profile for H2-50% case deviates from the laminar profiles. For H2-20% case,



〈uþ H2 cY 〉 deviates from the laminar profiles in the unburnt side, while it is relatively similar to the laminar profiles in the burnt gas side. This is also due to increase of reaction zones volume



fraction as considered for 〈uþ CH4 cY 〉. The comparison of DNS results with the unstrained and strained laminar flames shows that the H2 mass fraction for H2-20% case and all of the species reaction rates for both cases deviate from the unstrained and strained laminar profiles substantially unlike in high Karlovitz number turbulent planar CH4/air flames [56,57]. These results suggest that it is not sufficient to clarify the reaction zone characteristics of the turbulent swirling flames properly simply considering the effects of local tangential strain rates.

stantially and deviate from the laminar profiles unlike the

Fig. 9 e Conditional averages of uþ CH4 with respect to reaction progress variable cY (dashed line) and standard deviation ± s around the mean (red area) for the case of H2-50% (a) and H2-20% (b). The corresponding unstrained (dotted line) and strained laminar flames (solid and dot-dashed lines) profiles are superimposed. The arrow indicates increase of the local tangential strain rate at . (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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international journal of hydrogen energy xxx (xxxx) xxx

Fig. 10 e Conditional averages of uþ H2 with respect to reaction progress variable cY (dashed line) and standard deviation ± s around the mean (red area) for H2-50% case (a) and H2-20% case (b). The corresponding unstrained (dotted line) and strained laminar flames (solid and dot-dashed lines) profiles are superimposed. The arrow indicates increase of the local tangential strain rate at (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 11 e Conditionally averaged balance between convection, diffusion and chemical reaction for YCH4 (upper row) and YH2 (lower row) with respect to reaction progress variable cY (red dashed line) and standard deviation ±s around the mean (red þ þ area) for H2-50% case. C þ i , D i and R i denote contributions by convection, diffusion and chemical reaction of the species conservation equation for Yi , respectively. The corresponding unstrained laminar values are superimposed (dot-dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Effects of H2 ratios on balance between convection, diffusion and chemical reaction

instantaneous species conservation equation are evaluated [58]. An instantaneous conservation equation for species mass fraction Yi is expressed as

To investigate the balance between convection, diffusion and chemical reaction in the reaction zone, the contributions in an

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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international journal of hydrogen energy xxx (xxxx) xxx

Fig. 12 e Conditionally averaged balance between convection, diffusion and chemical reaction for YCH4 (upper row) and YH2 (lower row) with respect to reaction progress variable cY (red dashed line) and standard deviation around the mean (red area) þ þ for H2-20% case. C þ i , D i and R i denote contributions by convection, diffusion and chemical reaction of the species conservation equation for Yi , respectively. The corresponding unstrained laminar values are superimposed (dot-dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

  vYi vYi v vYi ¼ ruk r þ þ rui : rDi |{z} vxk vt vx vxk |fflfflfflfflfflffl{zfflfflfflfflfflfflk} |fflfflfflfflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflfflfflffl ffl} Ri Ci

(13)

Di

C i , D i and R i denote the contributions by convection, diffusion and chemical reaction respectively. Here, r [kg/m3] is density, uk [m/s] is the velocity in direction k, Di [m2/s] is molecular diffusivity and ui [1/s] is reaction rate. Figs. 11 and 12 show the conditionally averaged distributions of convection, diffusion and chemical reaction for YCH4 and YH2 . The superscript “þ” denotes that the values are normalized by the maximum of jrui j in the corresponding unstrained laminar flame to quantify the influences between convection, diffusion and chemical reaction. For both cases, the conditional means of contri

butions by convection 〈C þ cY 〉 and diffusion 〈D þ cY 〉 for YCH i

i

4

and YH2 are greater than the corresponding laminar values on average in the unburnt side. The peaks appear at relatively lower cY and the standard deviations of convection terms are substantially large mainly due to large velocity and its fluctuations. In the burnt side (cY > 0:6) the magni

tudes of 〈C þ cY 〉 and 〈D þ cY 〉 become negligibly small. This i

i

is attributed to reduced magnitudes of species mass fraction gradients vYi =vxk as it can be referred from Fig. 5, because the convective and diffusive transports are driven by vYi =

vxk . The contributions by chemical reaction 〈R þ cY 〉 are i

smaller than the unstrained laminar flame values, which correspond to the decreased reaction rates observed in Figs. 6, 9 and 10. The peaks appear at relatively higher cY , which is attributed to reactions with the burnt gases recirculated. These results suggest that the contributions by convection

and diffusion are more influential on the local and global flame structures of the present DNS. The influences appear strongly in the unburnt side and the profiles deviate from the laminar profiles mainly due to turbulent swirling jets. Comparison by the cases suggests that the contributions by each term for YCH4 are relatively similar as Fig. 11aec and 12a-c show. This indicates that the effects of mixture conditions do not seem to be substantial on the contributions of species conservation equation terms for YCH4 . Therefore, the

relatively similar behaviors of 〈Y  cY 〉 and higher values CH4

than the unstrained laminar values in Fig. 7 are due to the decreased contribution by reaction and increased contributions by convection and diffusion in the unburnt gas side by the swirling jets. However, the contributions by each terms for YH2 are different between the mixture conditions as observed in Fig. 11deg and 12d-g. For H2-50% case, conditionally averaged



contributions by convection 〈C þ cY 〉 and diffusion 〈D þ cY 〉 H2

H2

show relatively similar tendencies to those of CH4, because the fuel species are consumed rapidly in the upstream region.



The values of 〈C þ cY 〉 and 〈C þ cY 〉 are relatively similar in H2

CH4

an average sense. Due to larger diffusivity of H2 than CH4, the



þ

values of 〈D þ H2 cY 〉 are relatively larger than 〈D CH4 cY 〉. For H2

þ

20% case, the magnitudes of 〈C þ H2 cY 〉 and 〈D H2 cY 〉 are smaller than those of CH4. This is mainly attributed to smaller magnitudes of vYH2 =vxk than vYCH4 =vxk in the unburnt side as it can be referred from Fig. 5c and d, which yield smaller contributions by convection and diffusion of H2 than those of CH4. In



addition, it is also of interest that 〈C þ cY 〉 and 〈D þ cY 〉 for H2H2

H2

20% case contribute in opposite sign to those of the corresponding laminar conditions in the unburnt sides. Therefore,

Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175

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international journal of hydrogen energy xxx (xxxx) xxx

the deviation of 〈Y H2 cY 〉 from the laminar flame profiles for H2-20% case is attributed to convective transport by the swirling jets resulted from the decreased reaction rates in the near field of OSL and ORZ in addition to active production of H2 in the down stream region.

Conclusions DNS of CH4/H2/air turbulent swirling premixed flames in a gas turbine model combustor have been performed with a reduced chemistry (25 species and 111 reactions) to investigate the effects of H2-enrichment on CH4/air flame structures under intense turbulence. Two stoichiometric mixture conditions of which volume ratio of CH4:H2 is 50:50 (H2-50%) and 80:20 (H2-20%) are simulated. For both cases, reaction rates are decreased compared to the corresponding laminar value. However, qualitatively different global flame features and reaction zone characteristics between the cases are revealed. For H2-50% case, the flame is stabilized in both of the shear layers, whereas for H220% case, extinction occurs in the outer shear layers and H2 remain almost unreacted in the outer shear layer and recirculation zones. It is also revealed that the role of H2 in each mixture condition remains unchanged even under intense turbulence. Comparison of reaction zone characteristics in phase space showed deviation from the laminar profiles in H2 mass fraction for H2-20% case and all of the fuel species reaction rates. These suggest that considering the effect of local tangential strain rate is insufficient to clarify the reaction zone characteristics of the present DNS. In the analysis of fuel species conservation equation, convection and diffusion of H2 for H2-20% case are smaller than those of H2-50% case due to smaller H2 mass fraction gradients resulted from the decreased consumption rates in the region from the outer shear layers to the downstream. Therefore, the qualitatively different global flame features and reaction zone characteristics between the cases are due to different role of H2 in each mixture conditions and differently decreased fuel species reaction rates in the outer shear layers. For further study, effects of turbulent strain rates on the flame surface and shear layers should be investigated to elucidate the globally reduced reaction rates and extinction for H2-20% case.

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Please cite this article as: Park J et al., Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.175