Transverse fuel jet in turbulent cross-flow: influence of fuel composition on near field flame stabilization

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1 Paper # 070LT-0235 Topic: Laminar & Turbulent Flames 8 th US National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, Transverse fuel jet in turbulent cross-flow: influence of fuel composition on near field flame stabilization H. Kolla 1 R. W. Grout 2 A. Gruber 3 J. H. Chen 1 1 Combustion Research Facility, Sandia National Laboratories,Livermore, CA, USA 2 National Renewable Energy Laboratory, Golden, CO, USA 3 SINTEF Energy Research,Trondheim, Norway Compressible three-dimensional direct numerical simulations (DNS) of transverse fuel jet in turbulent boundary layer cross-flow of air (JICF) are performed to gain fundamental insight into the influence of fuel composition on the near field flame stabilization characteristics. A transverse jet of synthesis gas fuel, diluted by 30% nitrogen by volume, at 420 K is injected from a circular nozzle of 1 mm diameter with a bulk velocity of 250 m/s into a turbulent cross-flow of air at 750 K and free stream velocity of 55 m/s, representing nominal conditions of the premixer section of stationary gas-turbines. The aim is to develop an understanding of those mechanisms that are critical to flame stabilization in this configuration and aid the design of gas turbine burners that are fuel-flexible and intrinsically flashback-safe. The DNS incorporates detailed chemical kinetics and mixture-averaged transport properties. A chemical mechanism involving 12 species and 29 reaction steps is considered and three fuel fuel compositions with varying ratios of H 2 :CO mole fractions are investigated: 1:0 (pure H 2 ), 1:0.06 (H 2 rich) and 1:1 (H 2 lean). The laminar flame speeds for the three mixtures at nominal conditions (stoichiometric mixture, unburnt temperature of 420 K) are in the ratio 1:0.93:0.49 respectively. In spite of the large difference in the laminar propagation speeds the flame stabilizes on average at roughly the same location in all three cases: 1 mm downstream of the nozzle center in the stream wise direction and 4 mm downstream in the wall-normal direction. This location corresponds to a region of low velocity magnitudes and near stoichiometric mixtures. The dilatational influence of the flame on the turbulent flow field is likely the same in the three cases since the density ratios for the three mixtures are roughly equal to 4.5. Significant differential diffusion effects are observed with the elemental mixture fraction of hydrogen being much larger than carbon in the vicinity of the flame anchoring region. This suggests that the influence of adding CO in the fuel stream is negated somewhat by differential diffusion which continually provides an excess of H 2 to the flame compared to CO, relative to the fuel stream. 1 Introduction A transverse jet in cross-flow (JICF) is a configuration relevant in many practical combustion systems including industrial flares, boilers, furnaces, scramjets and gas turbine combustors. In systems that are required to be compact, such as gas turbine combustors or scramjets, the JICF is particularly beneficial in achieving a high level of fuel-air mixing within a confined volume and short flow residence times. However, the JICF generates a complex three-dimensional flow field involving a variety of flow structures spanning a broad range of length and time scales: shear 1

2 layer vortices, wake vortices, horse-shoe vortices and a counter-rotating vortex pair (CVP) [1]. While these flow structures are pivotal in enhancing mixing, they are challenging to study and characterize. For instance in stationary gas turbines operating in a lean premixed mode the JICF is employed in the premixer section, which is upstream of the main combustion zone, and must be designed to provide an optimal level of fuel-air mixing while maintaining intrinsic flashbacksafety. In the present study well resolved three-dimensional (3D) direct numerical simulations (DNS) are performed to understand the mechanisms that aid or disrupt flame stabilization in the near field of a transverse fuel jet in a turbulent boundary layer cross-flow of air. The aim is to fundamentally characterize mixing and flame-flow interactions in a JICF configuration to enable design of fuel-premixers with intrinsic flashback safety. The present study is part of a series [2 4] designed to numerically investigate various parameters that are likely to influence JICF near field flame stabilization. While the previous studies varied fuel nozzle geometry [2, 3] and transverse injection angle [4], here the focus is on the composition of the hydrogen-rich synthesis gas (syngas) fuel. It is well known, both from numerical [2, 3] and experimental [5 7] investigations, that a flame stabilizes in the near-field of a JICF via partially premixed flame propagation. Indeed, the flow velocity component normal to the mean flame base, measured conditionally at the instantaneous flame base location, was reported to be strongly correlated to the laminar flame speed [5]. In light of this, fuel composition can be expected to have a dominant influence since it considerably alters laminar flame properties such as propagation speed and flame thickness. This is particularly true of syngas since its two components - hydrogen (H 2 and carbon monoxide (CO) - have widely disparate chemical and transport properties. Well resolved DNS with detailed chemical kinetics and transport properties can greatly illuminate the parametric influence of fuel composition. 2 DNS configuration and numerical details Full compressible 3D DNS of reacting JICF are performed with the Sandia DNS code S3D [8]. A detailed description of the DNS configuration is presented in [3] and only the salient aspects are given here. Reacting DNS of a nitrogen diluted hydrogen-rich gaseous fuel jet injected from a circular nozzle transversely into a turbulent boundary layer cross-flow of air at atmospheric pressure is considered. The fuel stream is comprised of 30% nitrogen by volume, while the 70 % volumetric remainder is varied to study increasing amounts of CO relative to H 2. Three volumetric ratios of H 2 to CO are studied: (i) 1:0 (pure H 2 ; (ii) 1:0.06 (CO-lean), and (iii) 1:1 (CO-rich). The cross-flow is preheated to 750 K and has a free stream velocity, u cf, of 55 m/s. The jet diameter, d, is 1 mm, is at a temperature of 420 K and the jet velocity, u j, is varied to keep the momentum flux ratio, R m (ρ j u 2 j)/(ρ cf u 2 cf ), constant at The u j for the pure H 2 case is 250 m/s and the corresponding jet Reynolds number is In S3D the compressible Navier-Stokes, continuity, species and energy conservation equations are solved in their finite difference form. A skeletal mechanism for H 2 /CO oxidation kinetics [9] involving 12 species and 29 elementary reaction steps is used and the transport properties are computed using mixture-averaged formulae. For the pure H 2 case the kinetics scheme used is a subset of the H 2 /CO scheme with 9 species and 21 reaction steps. A 3D rectangular computational domain with dimensions 25mm 20mm 20mm in the x 2

3 (streamwise), y (transverse) and z (spanwise) directions, respectively, is considered. The turbulent cross flow enters the computational domain from an inflow boundary at the x = 0 plane while the y = 0 plane is an iso-thermal no slip wall boundary. The x = L x and y = L y boundaries are outflow and the z boundaries are periodic. To provide initial flow field and turbulence velocity profiles at the inflow plane, an auxiliary DNS of inert turbulent boundary layer flow with the cross flow velocity, u cf, of 55 m/s is performed separately and this is described in [2]. The computational domain is discretised into a Cartesian grid comprised of points in the x, y and z directions. The grid resolution is uniform in the x (17.8 µm) and z (18.2 µm) directions while in the y direction it varies from 10.2 to 24.3 µm with the fine region close to the wall. The solution is advanced with a time step of 4 ns and the simulations are performed on cores of the Cray XT5 (Jaguarpf) at Oak Ridge National Laboratories and Cray XE6 (Hopper) at National Energy Research Scientific Computing Center. The CPU cost is approximately 7.8 million CPU hours for a simulation for one flow through time. 3 Results and discussion 3.1 Mean flame stabilization characteristics It was reported in previous studies [3, 4] that in the near field of a JICF the intense fine scale turbulence increases micro-mixing rates and provides a sufficiently homogeneous mixture resulting in flame stabilization via partially premixed combustion. Accordingly, if the turbulence intensity, which is expected to scale with the momentum flux ratio, R m, is kept constant, one might expect the laminar flame speed, s o L, to have a dominant influence on flame stabilization. Figure 1 shows the time-averaged mixture fraction, velocity magnitude and heat release rate fields on the span wise mid-plane for the three cases. No significant differences in the mean flame characteristics for the three cases are apparent from this figure. The mean anchoring location appears on the jet leeward side and it coincides with a region of low velocity magnitude and near stoichiometric composition. It is not surprising that the location of the low-velocity bubble on the jet leeward side responsible for flame anchoring is not very different between the three cases, since R m is held constant. Furthermore, for the fuel compositions considered, the density ratios across an unstrained planar laminar flame under nominal conditions - stoichiometric mixture at 420 K - are 4.76, 4.77 and 4.82 for pure H 2, the CO-lean and the CO-rich cases, respectively. Hence, differences in the velocity field due to flame-induced density change are likely to be negligible. What is surprising, however, is the trend in flame anchoring location evident from Fig. 1. As the amount of CO increases in the fuel stream, the laminar flame speed decreases, and one might expect the flame to anchor at a progressively downstream location. But Fig. 1 suggests the contrary, that is, the flame anchoring location for the syngas cases is slightly upstream of the pure hydrogen case. For the three mixtures considered the laminar flame speeds under nominal conditions are 3.15 m/s, 2.92 m/s and 1.54 m/s respectively. This suggests that a scaling based on nominal mixture properties and flow conditions might be an oversimplification. 3

4 8th US Combustion Meeting Paper # 070LT-0235 Topic: Laminar & Turbulent Flames Figure 1: Iso-lines of mean heat release rate (black) overlaid on colour contours of mean mixture fraction (left) and velocity magnitude (right) on the span wise mid-plane for the pure H2 (top), COlean (middle) and CO-rich (bottom) cases. 3.2 Effects of differential diffusion One possible explanation for the observed trend in Fig. 1 could be differential diffusion. Smith et al. have shown that fuel mixtures comprising hydrogen and hydrocarbons could be prone to significant differential diffusion effects in both non-reacting [10] and reacting [11] flows. To quantify the extent of differential diffusion we define the elemental mixture fractions: ξh = ZH ZH,o ZC ZC,o ; ξc =, ZH,f ZH,o ZC,f ZC,o (1) where ZH and ZC are the elemental mass fractions of H and C and the subscripts f /o denote the corresponding values in fuel and oxidizer streams, respectively. The difference between the two elemental mixture fractions, ξhc ξh ξc, quantifies the extent of differential diffusion. While the interpretation of ξhc is not straightforward in the presence of chemical reactions, in nonreacting flow regions it is unambiguous; a positive value denotes an excess of hydrogen containing species compared to carbon containing species and vice-versa, relative to that present in the fuel 4

5 stream, purely due to differential molecular transport. Figure 2: Instantaneous snapshot of heat release rate iso-lines (black) overlaid on colour contours of ξ HC on the span wise mid-plane for the pure CO-rich case. The figure on the right shows an inset region close to the flame tip. Figure 2 shows an instantaneous snapshot of heat release rate and ξ HC on the span wise mid-plane for the CO rich case. While post-flame the values of ξ HC become much greater than unity, it is instructive to focus on the non-reacting portion upstream of the flame on the jet leeward side, as shown in the inset region in Fig. 2. Large portions of the leeward region where the jet fluid mixes with cross-flow fluid upstream of the flame have positive values of ξ HC, indicating an excess of hydrogen compared to carbon relative to the fuel stream. This can be readily attributed to the preferential transport of H 2 relative to CO, since these are the only species contributing to ξ H and ξ C in the absence of reactions. This effect is present even within the jet shear layer, and is consistent with the results of Smith et al. [10] who report that differential diffusion effects in non-reacting jet shear layers are considerable for jet Reynolds numbers of up to The turbulent convective transport overwhelms differential diffusion, on average, only if the jet Reynolds number is of the order of or higher. The jet Reynolds number in the present simulations is This can have significant influence on the flame characteristics. Essentially, if H 2 is continuously able to diffuse and reach the flame ahead of CO, then the flame will be oblivious to CO irrespective of how much of it is present in the fuel jet, and the flame dynamics would not deviate much from a pure H 2 jet. This might explain why the mean flame characteristics are not very different between the pure H 2 and CO-rich syngas cases. This effect could be compounded in the portions where the flame is convex to the unburnt fluid since the curvature is likely to enhance the diffusion of H 2 towards the flame relative to CO. To verify this quantitatively we compute the average values of ξ H and ξ C on a flame base, i.e. a small portion of the most upstream region of the flame, to identify the conditions in the vicinity of the flame tip. We define the flame base as the most upstream 1 mm portion of the temperature iso-surface with an iso-value of 760 K. In addition we also compute the apparent equivalence ratios of H and C: φ H,app = Z H/Z O (Z H /Z O ) st ; φ C,app = Z C/Z O (Z C /Z O ) st, (2) where Z O is the elemental mass fraction of O, and the subscript st denotes stoichiometric ratios 5

6 Figure 3: Time evolution of the flame base averaged values of apparent equivalent ratios (left) and elemental mixture fractions (right) of H and C for the CO-rich case. i.e. (Z H /Z O ) st 2*(atomic mass of H)/(atomic mass of O). Figure 3 shows the time evolution of the flame base averaged values of φ H,app, φ C,app, ξ H and ξ C for the CO rich case. At first glance φ C,app is much higher than φ H,app and very close to unity. However, this is misleading since the apparent equivalence ratios do not actually indicate the likelihood of the oxidation of a particular element. The higher value could be merely due to the fact that the amount of elemental oxygen required to fully oxidize a unit mass of carbon is three times small compared to that required to fully oxidize hydrogen. The flame base averaged values of ξ H however are consistently higher than that of ξ C, indicating that differential diffusion continually provides more hydrogen to the flame compared to carbon. This is also reflected in the time averaged values of ξ HC and heat release rate on the span wise mid-plane shown in Fig. 4. In both the CO-lean and CO-rich cases the region just upstream of the mean flame anchoring location has positive values of ξ HC, on average, indicating the predominant differential diffusion effects in the mixing of jet and cross-flow fluids. 4 Summary and conclusions Three dimensional DNS of transverse syngas fuel jet in turbulent boundary layer cross-flow of air show the influence of increasing the amount of CO relative to H 2 on the near field flame stabilization. The mean flame characteristics do not vary much, possibly owing to the momentum flux ratio, R m, being kept constant and negligible variation in the density jump across the flame under nominal conditions. The mean flame anchoring location in the CO-rich case is slightly upstream of the pure H 2 case, contrary to the trend suggested by laminar flame speed which is higher for the latter. This is likely due to differential diffusion effects which continually provide in the flame vicinity an excess of H 2 compared to CO, relative to the fuel stream, since H 2 diffuses much faster than CO, thereby negating somewhat the effect of increasing the amount of CO in the fuel stream. 6

7 Figure 4: Iso-lines of mean heat release rate (black) overlaid on colour contours of mean ξ HC on the spanwise mid-plane for the CO-lean case (left) and the CO-rich case (right). Acknowledgments This research used computational resources of the Oak Ridge Leadership Computing Facility (OLCF) at Oak Ridge National Laboratory, and National Energy Research Scientific Computing Center (NERSC). OLCF is supported by the office of Science of the US Department of Energy under contract DE-AC05-00OR NERSC is supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-05CH The work at Sandia National Laboratories was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the US Department of Energy Advanced Scientific Computing Research Office. SNL is a multiprogramme laboratory operated by Sandia Corporation, a Lockheed Martin Company for the US DOE under Contract DE-AC04-94AL The work at SINTEF is supported by the Climate Program of the Research Council of Norway and Gassnova. References [1] T. F. Fric and A. Roshko. Journal of Fluid Mechanics, 279 (1994) [2] R.W. Grout, A. Gruber, C.S. Yoo, and J.H. Chen. Proceedings of the Combustion Institute, 33 (2011) [3] R.W. Grout, A. Gruber, H. Kolla, P.-T. Bremer, J.C. Bennett, A. Gyulassy, and J.H. Chen. J. Fluid Mech., 706 (2012) [4] H. Kolla, R. W. Grout, A. Gruber, and J. H. Chen. Comb. Flame, 159 (2012) [5] E. F. Hasselbrink and M. G. Mungal. Proceedings of the Combustion Institute, 27 (1998) [6] D. Han and M. G. Mungal. Proceedings of the Combustion Institute, 29 (2002) [7] D. Han and M. G. Mungal. Combustion and Flame, 133 (2003) [8] J. H. Chen, A. Choudhary, B. de Supinski, M. DeVries, E. R. Hawkes, S. Klasky, W. K. Liao, K. L. Ma, J. Mellor- Crummey, N. Podhorski, R. Sankaran, S. Shende, and C. S. Yoo. Computational Science and Discovery, 2 (2009)

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