1 American Institute of Aeronautics and Astronautics

Colorless Distributed Combustion (CDC): Effect of Flowfield Configuration

V. K. Arghode1 and A. K. Gupta2 Dept. of Mechanical Engineering, University of Maryland

College Park, MD 20742, USA

K. H. Yu3 Dept. of Aerospace Engineering, University of Maryland

College Park, MD 20742, USA

Colorless Distributed Combustion (CDC) is being investigated as it shows great potential for lower NOx emissions, noise reduction and uniform thermal field for gas turbine combustors. CDC is characterized by distributed reaction zone which leads to uniform thermal field and provide significant improvement in pattern factor, reduced NOx emission and lower sound levels. Basic requirement for CDC is mixing between the combustion air and product gases to form hot and diluted oxidant prior to its mixing with the fuel. This leads to spontaneous ignition of the fuel and distributed reactions. This requirement can be met with different configuration of fuel and air injections with carefully characterized flowfield distribution within the combustion zone. In the present investigation four sample configurations have been examined. They include a diffusion flame configuration and three other configurations that provide potential for CDC mode combustor operation. For all four modes same fuel and air injection diameters are used to examine the effect of flow field configuration on combustion characteristics. The results are compared for flowfield and fuel/air mixing using numerical simulations and global flame signatures, exhaust emissions, acoustic signatures, and thermal field using experiments. Both numerical simulations and experiments are performed at heat load of 25kW, using methane as the fuel at atmospheric pressure using normal temperature air and fuel. Lower NOx emissions, better thermal field uniformity, and lower acoustic levels have been observed when the flame approached CDC modes as compared to the baseline case of a diffusion flame. The reaction zone is observed to be distributed over the combustor volume in the CDC mode.

Keywords: Distributed combustion, Colorless combustion, Recirculation, Flowfield configurations, Spontaneous ignition, Emissions.

I. Introduction Due to stringent emission norms and concern on cleaner environment, many combustion techniques have been

investigated in the past to achieve low pollutant emissions (NOx, CO) for gas turbine combustors. Some examples include rich burn quick quench lean burn (RQL), catalytic combustion and ultra lean premixed combustion [1]. Recently, Colorless Distributed Combustion (CDC), which is based on the principle of avoiding stabilization of a concentrated flame front and achieving distributed reactions, has emerged as a promising option to reduce NOx and CO emissions in addition to improved pattern factor, stable combustion and noise reduction. The name colorless is due to negligible emissions from the flame in the visible range as compared to conventional flames. In previous studies on furnace flames, avoidance of concentrated flame front has been achieved by recirculation and mixing of large amount of combustion products with the fuel and air stream prior to spontaneous ignition and distributed reaction. Preheating of air stream has been employed to achieve energy savings as well as to aid in spontaneous

1Graduate Student, Member AIAA 2Distinguished University Professor, Fellow AIAA, email: akgupta@umd.edu 3Associate Professor, Associate Fellow AIAA

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-253

Copyright 2009 by the authors. All rights reserved. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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ignition of the fuel. However, it has been recognized that preheating of air is not necessarily required to achieve distributed reactions. The concept has been successfully demonstrated to achieve low NOx and CO emissions, stable combustion and noise reduction using range of gaseous, liquid as well as solid fuels [2-6]. Much work has been reported under normal pressure furnace flame conditions with low thermal loadings. In this case the work is known by different names such as High Temperature Air Combustion (HiTAC) [2,3], Flameless Oxidation (FLOX) [4] and MILD combustion [5,6]. However, recently the development of the concept for gas turbine application that requires high thermal loadings and elevated pressure combustion conditions has gained interest [7,8].

The basic concept of HiTAC [2] (as well as FLOX or MILD combustion) and its comparison with conventional furnace combustion is shown in Figure 1. In conventional furnace flame, fuel and air are injected close together so that direct mixing between fuel and air takes place and a concentrated flame is stabilized in the region where stoichiometric mixture of fuel and air is present (F*A). This results in high flame temperature region (>1800K) which results in high NOx levels, especially the thermal NOx as known from the Zeldovich mechanism [9,10]. The flame is stabilized by the entrainment of product gases (BA) which provides the high temperature ignition energy source. However in HiTAC, preheated air (H) and fuel (F) injection locations are separate, and both fuel and air streams first entrains the product gases (B) avoiding direct mixing between the fuel and air streams. Mixing between air and entrained product gases forms the hot and low oxygen concentration (diluted) mixture (BH), weak reaction between fuel and entrained product gases can take place (F*B) followed by main reaction zone between fuel-product gas mixture and the hot and diluted oxidizer (F*B)*(BH). To avoid a concentrated flame front it is important that the fuel is uniformly distributed and then spontaneously ignites to provide distributed (volumetric) reaction zone. For spontaneous ignition of fuel the temperature and oxygen concentration of air and product gas mixture (BH) should be such as to appropriately minimize the ignition delay time and support spontaneous ignition. Due to distributed reactions the hot spots and high temperature stoichiometric regions are minimized in the combustion region resulting in lower NOx levels (thermal NOx).

The common key feature in most of the previous studies is air injection at high velocities to avoid the stabilization of flame, appropriate separation of air and fuel jets and recirculation of large amount of product gases to aid spontaneous ignition and distributed reactions. Similar concept of separate injection of fuel and air at high velocity with desirable recirculation and mixing of product gases can be applied for combustors operating at higher heat release intensities (5-50MW/m3-atm [11]) for gas turbine application. These requirements can be met with different configurations of fuel and air injection with carefully characterized flowfield. The objective of the present work is to investigate the effect of three different flowfield configurations that provide the potential for CDC mode of combustor operation.

II. Combustor Design Strategy The recirculation of product gases can be achieved by entrainment into the injected air jets. To understand the

variation of recirculation ratio (recirculation ratio is defined as the ratio of mass of entrained product gases to the mass of injected air) with the injected air jet and product gas characteristics, available correlation in the literature [12] was used. The correlation used was for a variable density, nonreacting air jet, injected in a quiescent medium and is given in equation 1. It may be noted that the entrainment rate in a reacting jet is lower than a nonreacting jet [13]. However considering the recirculation ratio achieved prior to spontaneous ignition, the correlations for variable density, nonreacting jet can be used. The temperature of recirculated gases will be different for different equivalence ratio; the recirculation ratio variation with recirculated gas temperature is shown in Figure 2(a). The air injection temperature and jet diameter is 300K and 0.1875 respectively. It can be observed that recirculation ratio decreases, at a given distance from air injection for higher temperature of recirculated gas (recirculation ratio ~ 1/Trec1/2). From Figure 2(a) it can be observed that for variation of recirculation gas temperature from 14002000K the recirculation ratio decreases from 2.5 to 2 at distance of 4.5 from air injection. The effect of air injection diameter on recirculation ratio is shown in Figure 2(b). The air injection temperature is 300K and the recirculated gas temperature is 1700K. It can be observed that the recirculation ratio varies significantly with air injection diameter (recirculation ratio ~ 1/djet) and recirculation ratio and at distance of 4.5 from the air injection port the recirculation ratio increases from 0.38 to 3.84 with decrease in air injection diameter from 0.4375 to 0.125.

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densitygas edrecirculatdensityjetinjected

diameterjetd)(dd

fluxmassjetinitialmfluxmassedrecirculatm

,1dx32.0

m

mratio rec.

rec

jet

jet

21recjetjet

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jet

rec

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jet

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(1)

At higher recirculation ratios the oxidizer (mixture of recirculated product gases and injected air) temperature will increase and the oxygen concentration will decrease. To obtain the values of oxidizer temperature and oxygen concentration, chemical kinetic simulations for adiabatic well stirred reactor (residence time = 100ms) with methane fuel and air at 300K and equivalence ratio of 0.8 is performed using the GRI 3.0 reaction mechanism [14]. The product gas from the first reactor is mixed with air in the second reactor to form the oxidizer. The ratio of mass flow rates in first reactor and the mass flow rate of air in the second reactor will govern the recirculation ratio. The results are presented in Figure 3(a). It can be observed that with recirculation ratio in the range of 2-3 the temperature of oxidizer is about 1530-1655K and oxygen concentration of 9.4-8vol.%. For spontaneous ignition of fuel and oxidizer mixture the ignition delay times should be well below the residence time in the combustor. The ignition delay time decreases exponentially with increase in temperature and increases linearly with increase in oxygen concentration. The correlation for ignition delay time for methane fuel is given in equation 2 [15]. Chemical kinetic calculations have been performed to calculate of ignition delay for closed homogeneous reactor using GRI 3.0 mechanism [14]. They are presented here along with the results obtained from those using equation 2 (see Figure 3(b)). It can be observed from Figure 3(b) that the ignition delay is about 1 ms for recirculation ratio of 2-3.

K,cm/moles,ondssec,unitsK1400T),T/23000exp(]O[1025.6tig

K1300T,)T/12000exp(T]O[109.2

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3

12

16

31.0

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(2)

In addition, the effect of positioning of the air injection ports, exhaust diameter, equivalence ratio, heat load, air injection temperature and partition wall on recirculation ratio and mixing between the product gases and the injected air was investigated numerically [16]. Based on the calculations, the air injection diameter and positioning and the dimensions of combustor can be appropriately chosen. The details of the combustor dimensions are presented in the following section.

III. Geometry and Configurations Figure 4 shows a schematic of the combustor. The combustor has a square cross-section with aspect ratio

(length/width) of 1.5. Fuel and air injection holes are on the diagonals of both top (see Figure 4(e)) and bottom side (see Figure 4(f)) of the chamber in the sets of four and the combustion product gas exit is from the top side (see Figure 4(e)). It can be noted that the positioning of air and fuel injection ports on both top and bottom side is same. Different combinations of fuel and air injection can be used to examine the effect of flowfield configuration. In the present study four different configurations have been investigated. In the first configuration, air is injected below the fuel injection location as shown in Figure 4(a). Due to jet expansion along the length of the pipe the air flow is more uniform at the inlet of the combustor. This is the diffusion flame configuration (herein referred as Diff.) and the flame is stabilized due to a weak recirculation region present in the combustor. This configuration is also used for start-up of the combustor and then later transitioned to other configurations (which cannot be started directly). Second configuration is the opposed flow configuration (herein referred as Opp.) where air is injected from the top side and fuel is injected from the bottom side of the combustor, see Figure 4(b). This configuration enables a large separation between the air and fuel injection streams. The injected air first entrains the product gases and forms a high temperature and diluted oxidizer as it reaches the bottom side where fuel is injected. Better mixing between air and product gases before meeting the fuel stream is expected in this configuration. In the third configuration both air and fuel are injected from the top side (herein referred as Top), the air and fuel will entrain the product gases, see Figure 4(c). The positioning of fuel injection stream in between the air and recirculating stream enables better

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mixing between the product gases and the fuel jet and aids in entrainment of fuel jet into the injected air jet. After enough entrainment of product gases and mixing between the fuel and hot and diluted oxidizer spontaneous ignition is expected take place some distance downstream of injection. In the fourth configuration both air and fuel is injected from the bottom side (herein referred as Bot., see Figure 4(d)). Both air and fuel streams will entrain the product gases and spontaneous ignition is expected to occur downstream. In this configuration also fuel stream is placed between the injected air and the recirculated gases to aid in better mixing and entrainment of fuel stream in the air jet. In this configuration the direction of recirculated product gases is from the top side to the bottom side along the centreline of the combustor, which is opposite to the case of configuration Opp. and Top.

The position of fuel injection and air inlet for the Diff. configuration is shown in Figure 4(f). For Diff. configuration the air is injected below of the fuel injection through a pipe to allow for the jet expansion and achieve uniform low velocity flow at the inlet of the combustor. The positioning of the air injection ports at the pipe inlet is shown in Figure 4(g).

IV. Numerical The jet profile and mixing between fuel and air under non-reacting condition is examined for the three (Opp.,

Top and Bot.) configurations. The mass flow rates of air and fuel (methane) corresponds to heat load of 25kW and equivalence ratio of 0.8 (methane mass fraction of 0.045 for perfect mixing). The corresponding fuel injection velocity is 97m/s and air injection velocity is 128m/s. The temperature for both air and fuel jet is 300K and operating pressure is 1atm. The momentum ratio for the two jets (air/fuel) is 28 hence the air jet is the dominant jet for the present case. The flowfield and species distribution is solved using a steady state, implicit, finite volume based method. SIMPLE algorithm is used for pressure velocity coupling. Full hexahedral grid is used to minimize the grid size and appropriate refinement of grid is performed in the regions with higher gradients. Geometrical symmetry is used to reduce the computational time and only one-eighth of the geometry with grid size of about 0.5 million cells was modeled. Realizable k-e model with standard wall functions is used to model turbulence. Realizable k-e model has been shown to provide more accurate prediction of profile and spreading of non-reacting round jets [17]. Convergence is obtained when the residuals for all the variables are less than 1e-04. The centerline jet velocity profile obtained from the numerical solution is compared with the correlation given in equation 3 [18]. For all simulations commercial software FLUENT code is used.

velocityjetinitialVdiameter,jetDvelocitycenterlinejetVjet, theof centerlinethealongdistanceX

DX575.6

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i

x

1

i

x

==

==

=

(3)

V. Experimental The CDC test combustion facility has optical quartz windows from three sides and the fourth side has ports for

diagnostic probes and igniter. The combustor was operated at a heat load of 25kW (heat release intensity 5MW/m3-atm) using methane fuel. Both methane and air was injected at ambient temperature of 300K and the combustor operated at atmospheric pressure. The fuel injection velocity was 97m/s and the air injection velocity varied from 114m/s to 146m/s and this combination provided equivalence ratio variation to change from 0.9 to 0.7, respectively.

The combustor was allowed to run for about 20minutes in each configuration before taking the experimental data. The exhaust gas sample was collected using an iso-kinetic, water-quenched sampling probe. The NO concentration was measured using the chemiluminescence method, CO concentration was measured using the non-dispersive infrared method and O2 concentration (used to correct the NO and CO emissions at standard 15% oxygen concentration) was measured using the galvanic cell method. The emission readings were observed to stabilize within 3 minutes for change in experimental condition (here change in equivalence ratio for the same configuration). The experiments were repeated three times for each configuration and the uncertainty was estimated to be about 0.5 ppm for NO and 20% for CO emissions. Digital camera with constant f-stop setting of 3.2 and shutter speed of 1/13s was used for global flame imaging for all the configurations. For sound level measurements, piezoelectric microphone and signal processing unit, employing the fast-Fourier transform (FFT) to convert the pressure fluctuations into the sound pressure level spectrum and calculate the A-weighted sound pressure level (to entail human response), was used. The frequency range of 22-21kHz for the sound spectrum and the time constant of 10s was used. The microphone was positioned at a distance of 3 (from the centre) along the length towards the top side, 1.5 (from the centre) along the width and 1.5 normal to the fourth (steel plate having ports for diagnostic probes) side of the combustor. The time gap of 3 minutes between two experimental data sets (here change in equivalence

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ratio for the same configuration) was employed. The experiments were repeated 3 times for each configurations and the uncertainty in sound pressure level was estimated to be about 0.5dBA. Temperature was measured using R-type thermocouple (50 microns). 10000 samples with sampling frequency of 1kHz were used to calculate the mean temperature. The experiments were repeated 3 times for each configuration and the uncertainty in temperature measurement was estimated to be about 20K. The uncertainty in temperature measurement due to radiation losses is further estimated to be about 50K [19].

VI. Results A. Jet behavior and fuel/air mixing

The contours of velocity magnitude in the plane having air and fuel injection holes for configurations Opp., Top and Bot. are shown in Figure 5. It can be observed that the air jet is the dominant jet for all the three configurations and the velocity decays to about 15% of the initial jet velocity at a distance of 6 (combustor length is 9) from the air injection for all three configurations. The centerline velocity profile for the three configurations and its comparison with the correlation (equation (3)) is shown in Figure 6(a). It can be observed that the velocity profile is very similar for all the three configurations and a fair matching with the correlation is obtained. This implies that the confinement and different configurations does not have significant effect on the air jet.

The fuel jet decays quickly for all three configurations and is entrained into the air jet for both Top and Bot. configurations (See Figure5). The effect of configuration is significant on the fuel jet as quantified using the decay length. The decay length for fuel jet is obtained where the centerline CH4 mass fraction of the fuel jet is 0.075 and the decay length is reported at the distance from the fuel injection side along the length of the combustor. The fuel jet decays earliest for configuration Opp. (decay length=1.25) followed by configuration Top (decay length=1.4) and Bot. (decay length=1.75). This can also be observed from Figure 5. For configuration Opp. the jet decays quickly due to presence of cross flow from the air jet which is turning towards the exhaust. For both configuration Top and Bot., due to presence of strong air jet in the vicinity the fuel jet decay is delayed.

The relatively early decay of fuel jet for configuration Top as compared to configuration Bot. results in faster mixing between the air and fuel jet and can be seen from Figure 6(b). Along the centerline of air jet the CH4 mass fraction reaches the value of 0.025 (0.045 for perfect mixing) at a distance of 1.44 for configuration Top as compared to 1.65 for the Bot configuration. It can be noted that the delay in mixing between fuel and air jet (obtained by separation as described in the introduction) is desirable to achieve reactions closer to CDC mode. It is recognized that the flowfield and mixing will be different in reacting conditions; however, comparative study of non-reacting condition provides important insights about the effect of flow field configurations on the jet profile and fuel air mixing.

B. Global flame behavior Global pictures of combustion zone operating in the four configurations with equivalence ratio varied from 0.7-

0.9 are shown in Figure 7. The combustor has optical access from three sides (quartz window size of 46.5 for the combustor side of 69). A black painted metal sheet is placed in the background and pictures are taken in dark to minimize the effect of light reflection from the background. For configuration Diff., which is closer to a traditional diffusion flame mode, flame fronts can be observed and reaction zone is seen to be not distributed uniformly in the combustor. Both bluish (near fuel injection) and yellowish regions of the flame could be observed, see Figure 7(a), (e), (i). As the fuel is injected at an angle of 45 towards the four corner of the combustor, reaction zone is mostly confined towards the corners of the combustor and low light intensity region is present in the central region. It can be observed that as the equivalence ratio is increased from 0.7 to 0.9 the low light intensity central region increases. This can be attributed to higher penetration of fuel jet due to decrease in air flow velocity with increase in equivalence ratio (note that air flow was varied to change the equivalence ratio). As the fuel injection velocity is large (97m/s) the flame is not attached to the fuel injection ports and is stabilized due to the weak recirculation region present in combustor. In Opp. configuration (see Figure 7(b), (f), (j)) very low visible emission is observed and the reaction zone is observed to be more uniformly distributed. With increase in equivalence ratio the visible emission as well as the glow of combustor wall is also observed to increase; this can be attributed to higher (adiabatic) temperature of product gases associated with increase in equivalence ratio. It can also be observed that the glow of combustor wall is higher at the top side as compared to the bottom side; this is because near the top side of the combustor mostly product gases are present. With air injection from the top side the air stream entrains the product gases and relatively low temperature gas mixture (air and product gas) is present near the bottom side of the combustor. From the bottom side fuel is injected which reacts with the oxidizer to form relatively high temperature product gases which turns towards the central region of combustor (away from the walls) and

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moves towards the top side of the combustor. Global pictures for configuration Top are presented in Figure 7(c), (g), (k). At lower equivalence ratio (phi=0.7) the lower portion of the combustor is observed to have very low light emissions, this may be due to absence of reaction zone in the lower portion of the combustor. At this condition the reaction zone is observed to be shifted towards one side of the combustor (right side), this may be reason for higher glow of the combustor wall on the right side of the combustor. This configuration was also observed to be unstable. Early mixing between the fuel and air jet for this configuration, as seen from the numerical simulations (See Figure 6(b)), may be the reason for unstable combustion in this configuration. As the equivalence ratio is increased the reaction zone is observed to increase covering the lower portion of the combustor. Relatively higher glow from the combustor wall with increase in equivalence ratio was also observed due to higher temperature of the product gases. In this configuration, though the reaction zone was not covering the complete combustion volume, distinct flame fronts were not observed as seen in the case of configuration Diff. For Bot. configuration (see, Figure 7(d), (h), (l)) the reaction zone is observed to be uniformly distributed in the combustor, however, higher visible emissions as compared to configuration Opp. were observed. Higher glow from the combustor walls is observed with increase in the equivalence ratio, this can be related to higher (adiabatic) flame temperatures with increase in equivalence ratios. In this configuration, since both air and fuel is injected from the bottom side of the combustor the reaction proceeds mostly along the length of the combustor towards the top side, which results in relatively high temperature product gases present on the top side of the combustor (also seen from the higher glow from the combustor wall on the top side of the combustor).

C. NO and CO emissions The NO emission levels corrected to 15% O2 in the exit gases for the four configurations are shown in Figure

8(a). NO is produced through two major reaction mechanisms for hydrocarbon flames, namely thermal (Zeldovich) and prompt (Fenimore) mechanism [9,10]. In the thermal mechanism NO is formed in the high temperature zone and is the major contributor to NO formed in the diffusion flames. Prompt NO is formed in the reaction zone by rapid reaction of hydrocarbon radicals (CH) with molecular nitrogen [9]. At higher temperature zones (T>1800K) thermal mechanism is the major contributor to the total NO formation, however prompt mechanism can be important where the contribution from thermal NO is very small (rich flames, low temperature regions). The distributed reactions mostly suppress the formation of NO through thermal mechanism due to avoidance of high temperature regions. In all the four configurations very low NO levels (single digit NO levels) are observed. It may be noted that the configuration Diff. is a lifted diffusion flame mode, instead of a conventional attached diffusion flame mode, because the velocity of fuel injection is very high (~97 m/s) [18]. Hence fuel dilution with the product gases takes place before the ignition (due to weak recirculation of product gases present), hence reducing the high temperature reaction zones. Lower NO levels are observed especially for configuration Opp and Bot.. Except for configuration Opp, the corrected NO level is observed to vary insignificantly with increase in equivalence ratio. For a particular configuration the residence time (tR) can be approximated by, m/)V(~tR & , V is the combustor volume, is mean combustor gas density and m& is mass flow rate through the combustor. The NO level increase with increase in residence time of the gases in the combustor. With increase in equivalence ratio from 0.7 to 0.9 the mass flow rate through the combustor decreases and the mean gas density decreases due to increase in the adiabatic flame temperature which results in similar residence time with increase in equivalence ratio and may not have significant effect on the NO levels. It may be noted that for diffusion flame the reaction zone is stabilized at the stoichiometric region (or near stoichiometric region for fuel diluted with product gases) irrespective of the additional flow of the air, hence with same amount of dilution air (corrected concentration), no heat losses and same residence time the NO levels will be nearly constant even with increase in the equivalence ratio. Hence for the case having more hot spot zones the corrected concentration of NO will not vary significantly with increase in equivalence ratio (configuration Diff., Top and Bot). For the adiabatic, same residence time, well stirred reactor case, NO formation increases significantly with higher equivalence ratio (higher temperature) due to absence of hot spot regions. Hence the increase in NO is much higher than the reduction in NO due to addition of dilution air (corrected concentration case). This implies that the corrected NO levels will increase with increase in equivalence ratio for the case which is closer to well stirred reactor (configuration Opp.). It may be noted that the configurations Opp., Top and Bot. will have significantly higher residence time (of hot gases) than the configuration Diff. due to presence of strong recirculation region inside the combustor. Hence even with presence of more hot-spot zones in configuration Diff the NO levels are almost similar to other configurations. Furthermore, as the configuration Bot. is a forward flow configuration (air is injected from the bottom side and exit is on the top side) as opposed to configurations Top and Opp. which are reverse flow configuration (both air and exhaust on the top side), the residence time for configuration Bot. can be lower than the other two configurations resulting in lower NO levels.

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Figure 8(b) shows the CO concentration levels corrected to 15% O2 in the exit gases. It can be observed that the lifted diffusion flame (configuration Diff.) produced significantly higher CO levels, and configuration Opp. and Bot. produced the lowest CO levels. This can be attributed to non-uniform mixing in configuration Diff. (from regions of variable stoichiometry near the flame front) and more uniform reaction zone in configuration Opp and Bot. Configuration Top produced intermediate levels of CO this may be because of unstable combustion as discussed in the previous section which leads to non-uniform mixing and variable stoichiometry to produce higher CO level. It can also be observed that the CO emission increases significantly with increase in equivalence ratio. This is due to lack of availability of oxygen as well as dissociation of CO2 at high temperatures (higher equivalence ratio) [1].

D. Noise level The sound pressure levels (A-weighted) for the four configurations are shown in Figure 9. For the reacting case

(see Figure 9(a)) it can be observed that configuration Opp. produced lowest noise levels (about 3dBA reduction) as compared to the other configurations. This may be because for configuration Opp. the injected air jets entrain large amount or product gases before meeting the fuel (due to large separation between air and fuel jets). This results in higher temperature of oxidizer (mixture of air and product gases) resulting in smooth and spontaneous ignition of fuel and steady combustion which reduces the combustion noise. The combustion noise is generated due to unsteady heat release rate which results in local expansion (and contraction) of gas volume resembling the monopole combustion source [20]. Lower noise levels are observed with increase in equivalence ratios for all the configurations. This can be because at higher equivalence ratio (and constant heat load) the air injection velocity is lower to reduce the jet noise and the oxidizer temperature is higher (due to higher temperature of product gases) resulting in stable ignition and combustion of fuel resulting in lower noise levels. The stable ignition and steady combustion will lead to reduction in the combustion noise as discussed above. Highest noise level is observed for configuration Top at equivalence ratio of 0.7. This may be due to unstable combustion as observed in the global images of this configuration. The noise level for non-reacting case is presented in Figure 9(b). The noise levels are about 5-10dBA lower than the reacting flow case, and the sound levels do not vary significantly with change in equivalence ratio (Top, Opp., Diff.) or increases slightly at lower equivalence ratio (Bot.). The slight increase in sound level for configuration Bot. can be due to higher jet velocity (146m/s) at equivalence ratio of 0.7 as compared to 114m/s at equivalence ratio of 0.9.

E. Thermal field Temperature is measured in the centre plane of the combustor, see schematic of combustor in Figure 4.

Direction x is defined along the length of the combustor with x=0 is at the bottom side and direction y is along the width of the combustor with y=0 is at the centreline of the combustor. Temperature is measured at x=1.5, 3, 4.5, 6 and 7.5 and y=0, 0.5, 1, 1.5, 2 and 2.5 to obtain the thermal field in the centre plane of the combustor. The results are presented in Figure 10 for the four configurations at equivalence ratio of 0.8. For configuration Diff. the temperature near the centreline (y=0, 0.5 and 1) is lower as compared to the temperature near the wall of the combustor (see Figure 10(a)). This is because the fuel is injected at 45 angle towards the corners of the combustor, and as observed from the global pictures for the configuration Diff. (see Figure 7(e)) the reaction zone is mostly confined towards the corners of the combustor. Therefore, near the wall of the combustor temperature is higher due to presence of flame in the vicinity. Due to this reason the temperature field in the centre plane is not uniform for the configuration Diff.. Thermal field for configuration Opp. is shown in Figure 10(b). It can be seen that thermal field is more uniform as compared to the configuration Diff. Temperature near the walls (y=2, 2.5) is lower as compared to temperature near the centreline. This is due to entrainment and mixing of product gases with the injected air (resulting in relatively lower temperature mixture) near the walls of the combustor. However, near the centreline the temperature is higher due to presence of the product gases. Figure 10(c) shows the thermal filed for configuration Top. Large variations along the y-direction can be observed and temperature is lower near the wall as compared to the centreline temperature. The large variation in thermal field can also be understood by closely examining the global picture of the flame, see Figure 7(g). The thermal filed is for the centre plane on the left side of the flame photograph. Due to non-symmetrical nature of the reaction zone (reaction zone shifting towards the right side of the combustor) the temperature drops significantly as one move away from the centreline towards the walls of the combustor. It can also be observed that the temperature is lower in the lower portion of the combustor (x=1.5, 3). Relatively more uniform thermal field is observed for the configuration Bot. (see Figure 10(c)). From Figure 7(h) it can also be seen that the reaction zone is uniformly distributed in the combustor. Slightly higher temperature is present near the centre of the combustor; this may be due to the presence of high temperature product

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gases present in the centre region of the combustor; however the overall thermal field is more uniform as compared to other configurations.

VII. Conclusions Investigation and comparison of four different flowfield configurations is performed with respect to flowfield

and fuel air mixing (numerical simulations) and global flame signatures, NO and CO emissions, noise level and thermal filed uniformity (experiments) at constant heat load of 25kW and equivalence ratio variation of 0.7-0.9. The flow field configurations were observed to have minimal effect on the dominant air jet, however it had significant effect on the fuel jet characteristics as well as the fuel/air mixing. Distributed reaction zone, better thermal filed uniformity and lower NO (7ppm, phi=0.7) and CO (20ppm, phi=0.7) emissions were observed for configurations Opp. and Bot.. As configuration Bot. is a forward flow configuration, this may be advantageous for practical use in the combustors as well as may have lower residence time of product gases in the combustor resulting in lower NO emissions. Lower visible emissions and lower noise levels (3dBA reduction) were observed for Opp configuration. This is postulated due to the fact that this configuration has large separation between air and fuel stream, hence air stream first entrains large amount of product gases before meeting the fuel stream to allow spontaneous ignition and stable combustion. Configuration Top was observed to be unstable and noisier and produced more NO and CO emissions as compared to other configurations and one of the reasons may be due to early mixing between the fuel and air. This configuration also showed large variation in the thermal filed due to non symmetrical reaction zone. Configurations Opp. and Bot. shows promise for further investigation and to achieve combustion characteristics closer to the colorless distributed combustion (CDC) regime.

Acknowledgments This research was supported by ONR, Program Manager, Dr. Gabriel D. Roy. The support is gratefully

acknowledged. References

[1] Lefebvre, A. H., Gas Turbine Combustion, Taylor and Francis, Second Edition, 1998. [2] Tsuji, H., Gupta, A. K., Hasegawa, T., Katsuki, M., Kishimoto, K., Morita M., High Temperature Air Combustion: from energy conservation to pollution reduction, CRC Press, (3rd printing), 2003. [3] Gupta A. K., Thermal Characteristics of Gaseous Fuel Flames using High Temperature Air, J. Engineering for Gas Turbines and Power, Vol. 126, 2004, pp. 9-19. [4] Wunning, J. A., Wunning, J. G., Flameless Oxidation to Reduce Thermal NO-Formation, Progress in Energy and Combustion Science, Vol. 23, 1997, pp. 81-94,. [5] Cavaliere A., Joannon M., Mild Combustion, Progress in Energy and Combustion Science, Vol. 30, 2004, pp. 329-366. [6] Ozdemir I. B, Peters N., Characteristics of the Reaction Zone in a Combustor Operating at Mild Combustion, Experiments in Fluids, Vol. 30, 2001, pp. 683-695. [7] Flamme, M., New Combustion System for Gas Turbines (NGT), Applied Thermal Engineering, Vol. 24, 2004, pp. 1551-1559. [8] Levy, Y., Sherbaum, V., Arfi, P., Basic Thermodynamics of FLOXCOM, the Low-NOx Gas Turbines Adiabatic Combustor, Applied Thermal Engineering, Vol. 24, 2004, pp. 1593-1605. [9] Miller J. A., Bowman C. T., Mechanisms and Modeling of Nitrogen Chemistry in Combustion. Progress in Energy and Combustion Science, Vol. 15, 1989, pp. 287338. [10] Correa, S. M., A review of NOx Formation Under Gas-Turbine Combustion Conditions, Combustion Science and Technology, Vol. 87, 1992, pp. 329-362. [11] Vincent, E. T., The Theory and Design of gas Turbines and jet Engines, McGraw-Hill Book Co., First Edition, 1950. [12] Ricou, F. P., Spalding D. B., Measurements of Entrainment by Axisymmetrical Turbulent Jets, Journal of Fluid Mechanics, Vol. 11, n1, 1961, pp. 21-32. [13] Han, D., Mungal M. G., Direct Measurement of Entrainment in Reacting/Nonreacting Turbulent Jets, Combustion and flame, Vol. 124, 2001, pp. 370-386. [14] Smith G. P., Golden D. M., Frenklach M., Moriarty N. W., Eiteneer B., Goldenberg M., Bowman C. T., Hanson R. K., Song S., Gardiner W. C., Lissianski V. V., Qin Z., http://www.me.berkeley.edu/gri_mech/. [15] Li S. C., Williams F. A., Reaction Mechanisms for Methane Ignition, Journal of Engineering for Gas Turbines and Power, vol. 124, 2002, pp. 471-480. [16] Arghode, V. K., Gupta, A. K., Numerical Simulations of Gas Recirculation for CDC Combustor, 7th High Temperature Air Combustion and Gasification International Symposium, Phuket, Thailand. 13-16 January 2008. [17] Shih, T, Liou W. W., Shabbir, A., Yang, Z., Zhu, J., A New k-e Eddy Viscosity Model For High Reynolds Number Turbulent Flows, Computers and Fluids, Vol. 24, 1995, pp. 227-238. [18] Turns, S. R., An Introduction to Combustion, McGraw-Hill, 2000, Second Edition.

9 American Institute of Aeronautics and Astronautics

[19] Bradley, D., Matthews, K. J., Measurement of high gas temperatures with fine wire thermocouples, Journal of Mechanical Engineering Science, Vol. 10, n4, 1968, pp. 299-305. [20] Strahle, W. C., Combustion Noise, Progress in Energy and Combustion Science, Vol. 4, 1978, pp. 157-176.

Figure 1. Comparison of ordinary combustion and high temperature air combustion for furnaces [1].

10 American Institute of Aeronautics and Astronautics

0

1

2

3

4

5

2 3 4 5 6 7 x (in)

Re

circ

ula

tion

ra

tioTrec (K) 1400 Trec (K) 1600Trec (K) 1800 Trec (K) 2000

0

1

2

3

4

5

2 3 4 5 6 7 x (in)

Re

circ

ula

tion

ra

tio

d_air (in) 0.125 d_air (in) 0.1875d_air (in) 0.3125 d_air (in) 0.4375

(a) Recirculation ratio variation with recirculated gas temperature

(b) Recirculation ratio variation with air injection diameter

Figure 2. Variation of recirculation ratio with (a) recirculated gas temperature and (b) air injection diameter for a free jet.

0

500

1000

1500

2000

2500

0 1 2 3 4 5Rec ratio

Tem

pera

ture

(K

)

0%

5%

10%

15%

20%

25%

O2

(vol%

)

Toxidizer (K) O2oxidizer (vol%)

0.1

1

10

100

0 1 2 3 4 5Rec ratio

Ign

itio

n de

lay

(ms)

Chemkin (OH max) Correlation

(a) Temperature and oxygen concentration (b) Ignition delay time

Figure 3. Effect of recirculation ratio on (a) temperature and oxygen concentration and (b) ignition delay time (phi = 0.8).

11 American Institute of Aeronautics and Astronautics

(a) Diff. (b) Opp. (c) Top (d) Bot.

(e) Top side (f) Bottom side (g) Pipe inlet

Dimensions: D=2.5, H=6, L=9, W=6,

a=2, ad=0.5, f=1.5, fd=0.25, d_air=0.1875,

d_fuel=0.0625, d_exit=1,

=45

Figure 4. Schematic of the flowfield configurations.

(a) Opp. (b) Top (c) Bot. Figure 5. Contours of velocity magnitude for the three configurations.

Air Fuel

Air Fuel Fuel

Air

Air (d_air)

ad

W

Fuel (d_fuel) a

Exit (d_exit)

f W

fd

Fuel (d_fuel) a

L W2

Fuel D H

Exit Fuel Air

H

Exit

W2

D

Air

H

Exit

W2

Fuel D Fuel H

Exit

L W2

D

Air

Air

L L

f

W

Air (d_air) Air (d_air)

Center plane

Pipe inlet

Bottom side

Top side

W

Velocity (m/s)

12 American Institute of Aeronautics and Astronautics

0

0.4

0.8

1.2

0 10 20 30 40(X/D)

(Vx/

Vi)

Opp Top Bot Correlation

0

0.01

0.02

0.03

0.04

0.05

0 5 10X (in)

CH4

(ma

ss fra

ctio

n)

Opp Top Bot

(a) Centreline velocity decay for the three configurations

(b) Methane concentration variation

Figure 6. Comparison of centreline velocity and the CH4 mass fraction

13 American Institute of Aeronautics and Astronautics

(a) Diff., phi=0.7

(b) Opp., phi=0.7

(c) Top, phi=0.7

(d) Bot., phi=0.7

(e) Diff., phi=0.8

(f) Opp., phi=0.8

(g) Top, phi=0.8

(h) Bot., phi=0.8

(i) Diff., phi=0.9

(j) Opp., phi=0.9

(k) Top, phi=0.9

(l) Bot., phi=0.9

Figure 7. Global flame photographs for the four configurations.

14 American Institute of Aeronautics and Astronautics

0

3

6

9

12

15

0.6 0.7 0.8 0.9 1Equivalence ratio

NO

@15

%O

2 (pp

m)

Diff Opp Top Bot

(a) NO emissions

0

300

600

900

1200

1500

0.6 0.7 0.8 0.9 1Equivalence ratio

CO@

15%

O2

(ppm

)

Diff Opp Top Bot

(b) CO emissions Figure 8. (a) NO and (b) CO emissions for the four configurations.

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70

75

80

85

90

0.6 0.7 0.8 0.9 1Equivalence ratio

SPL

(dBA)

Diff Opp Top Bot

(a) Reacting flow

70

75

80

85

90

0.6 0.7 0.8 0.9 1Equivalence ratio

SPL

(dBA)

Diff. Opp. Top Bot.

(b) Non-reacting flow Figure 9. Sound pressure levels for the four configurations in (a) reacting flow condition and (b) non-reacting

flow condition.

16 American Institute of Aeronautics and Astronautics

0

500

1000

1500

2000

0 1.5 3 4.5 6 7.5 9X(in)

Tme

an

(K)

Y (in) 2.5 Y (in) 2 Y (in) 1.5Y (in) 1 Y (in) 0.5 Y (in) 0

0

500

1000

1500

2000

0 1.5 3 4.5 6 7.5 9X(in)

Tme

an

(K)

Y (in) 2.5 Y (in) 2 Y (in) 1.5Y (in) 1 Y (in) 0.5 Y (in) 0

(a) Diff., phi=0.8 (b) Opp., phi=0.8

0

500

1000

1500

2000

0 1.5 3 4.5 6 7.5 9X(in)

Tme

an

(K)

Y (in) 2.5 Y (in) 2 Y (in) 1.5Y (in) 1 Y (in) 0.5 Y (in) 0

0

500

1000

1500

2000

0 1.5 3 4.5 6 7.5 9X(in)

Tme

an

(K)

Y (in) 2.5 Y (in) 2 Y (in) 1.5Y (in) 1 Y (in) 0.5 Y (in) 0

(c) Top, phi=0.8 (d) Bot., phi=0.8 Figure 10. Temperature field in the center plane (x is along the length and y is along the width of the

combustor, centerline is y=0).