An Experimental Facility for Studying Flame Propagation at Elevated Pressures

Transcription

1 Paper # 7LT-21 Topic: Laminar and Trblent Flames 8 th U. S. National Combstion Meeting Organized by the Western States Section of the Combstion Institte and hosted by the University of Utah May 19-22, 213 An Experimental Facility for Stdying Flame Propagation at Elevated Pressres Garav Mittal, Bikash Parajli, Varn Anthony Davies Department of Mechanical Engineering The University of Akron, Akron, OH, An experimental facility has been designed and fabricated for stdying otward propagating spherical flames at elevated pressres and temperatres. The facility can be sed in static or dynamic mode. In the dynamic mode, the desirable conditions of elevated pressre and temperatre are attained by compression of reactive mixtre throgh a pnematic drive and hydralic damping mechanism and a spherical otward propagating flame is initiated throgh spark discharge in the compressed mixtre. A single window coincident optical layot is selected for Schlieren imaging of flame with a high speed camera. Schlieren images are digitally processed to obtain flame radis-time history. A mlti-zone model is sed to dedce flame speed from the acqired flame radis-time and pressre-time data. In the mlti-zone model, flame propagation is taken as the consective consmption of nbrned gas mixtre within the zones. The model acconts and corrects for compression effects. As part of the preliminary characterization, experimental data for flame speed of methane/air mixtres is acqired, interpreted and compared with the data from the literatre. 1. Introdction The phenomenon of flame propagation and flame strctre are significantly inflenced by mixtre pressre and preheating. Since combstion in practical engines takes place at elevated pressres and temperatres, stdy of flame propagation at simltaneosly elevated pressres and temperatres is particlarly relevant [1,2]. The importance of investigating high pressre flame propagation and associated chemical kinetics is amply recognized for the development of comprehensive chemical kinetic mechanisms [2,3-8]. As pressre increases, the controlling chemistry changes de to the increased importance of the three-body termination reactions. Investigations with methane and H 2 flames at elevated pressres have revealed that the overall reaction order shows a non-monotonic behavior with pressre, which is analogos with the three explosion limits of hydrogen/oxygen mixtres [3]. The overall reaction order initially decreases with increase in pressre de to the increased importance of the termination, H+O 2 (+M) HO 2 (+M), over branching reactions, H+O 2 OH+O etc., and then it increases again de to the branching pathways from HO 2 at higher pressres [3,9]. Conseqently, the chemical kinetics in high pressre flames is significantly inflenced by HO 2 pathways. The most poplar techniqes for laminar flame speed measrements are conterflow/stagnation flames [e.g. 1-15] and spherically otward propagating flames (OPF) [e.g. 3-5]. Flat flame and

2 bnsen brner [e.g.16,17] have also been sccessflly sed. Conterflow brners offer an excellent configration for determination of stretch-corrected flame speeds, bt typical pressres are below 7 atm de to transition to trblence with increasing pressre. OPFs (constant volme and constant pressre) in optically accessible combstion chambers have been widely sed for investigations at elevated pressres, p to 6 atm. The objective of this work is to develop an experimental facility for investigating flame propagation at elevated pressres and temperatres and sbseqently characterize and se the facility for stdying flame propagation. In the following a description of the facility is presented, followed by the model for data interpretation and preliminary characterization experiments. 2. Experimental Facility The working principle of the experimental facility is to attain the desirable conditions of elevated pressre and temperatre by compression of reactive mixtre in a careflly designed combstion chamber and then initiate a spherical OPF throgh spark discharge. The obtained images of flame evoltion can be processed to yield the data for flame speed vs stretch rate, which can be extrapolated to obtain the nstretched laminar flame speed. The cross-sectional view of the facility is shown in Fig. 1. It consists of a reactor cylinder with nearly hemispherical piston and cylinder head. The combstible mixtre in the reactor cylinder is compressed to elevated pressres and temperatres by controlled motion of the piston throgh a cstom-designed hybrid cylinder (pnematic drive and hydralic damping) arrangement. After compression, the reaction chamber assmes a nearly spherical shape. The spherical chamber is eqipped with optical window, pressre and temperatre transdcer, rptre diaphragm for safety, and electrodes for initiating an OPF by spark. Spark is initiated after compression in the constant volme spherical chamber, yielding an OPF whose radis-time history is recorded by Schlieren imaging sing a high speed camera. Fig. 1 Cross-sectional view of the experimental facility It is desirable to have a large and spherical combstion chamber for stdying OPFs. A spherical chamber avoids nde distortion of the flame, which happens in the case of cylindrical confinement 2

3 [18], and a large chamber allows constant pressre flame propagation p to a larger radis (and conseqently smaller stretch) so that the extrapolation to zero stretch can be done with increased confidence. In the present design of the facility, the combstion chamber is nearly spherical in shape, however, after carefl considerations, the diameter of the chamber is taken as 1 cm and the maximm compression stroke length of the facility is 5 cm. An increase in the diameter of the combstion chamber reqires a proportionate increase in the length of the compression stroke and exacerbates the possibility of reaction and trblence dring compression. With a combstion chamber diameter of 1 cm, the flame radis-time data p to the flame radis of 2.5 cm (half of the radis of the combstion chamber) may be sed for extrapolation [19]. It is noted that at this flame radis, pressre in the combstion chamber may rise by abot 15% and temperatre also by 5% [19]. However, the inflence of these factors on flame speed is typically less than 2% and the experimental data can be extrapolated to zero stretch with confidence when compression correction is taken into accont. In literatre, most of the experimental data from OPFs is based on flame radis less than 2.5 cm [9,2-21]. The ignition energy for the spark is provided by discharging an atomotive coil, satrated throgh a constant crrent power spply (GW Instek GPR 181HD), throgh a 1 µf capacitor. The ignition energy is easily adjsted by adjsting the crrent on the power spply. One of the electrodes is modified from a standard spark plg by removing the grond electrode and welding a stainless steel tbe and wire to it. The other electrode is adjstable and allows fine adjstment of the spark gap. The pressre in the chamber is measred sing a Kistler piezoelectric (652C) sensor and a charge amplifier. The voltage otpt of the sensor is sed to synchronize the spark generation and image acqisition throgh a time delay program in Labview. A single window coincident optical layot is selected for Schlieren imaging. This arrangement, as shown in Fig. 2, reqires only one window and minimizes the distortion of the spherical geometry of the combstion chamber. A 48 W halogen lamp sorce is focsed throgh a condenser lens on a pinhole to provide intense illmination. A 4 mm focal length achromatic doblet lens provides collimated beam throgh the 5 cm diameter window. A qartz sbstrate front srface mirror is monted on the face of the piston, which reflects the light. The reflected light is focsed by the lens and beamsplitter on a knife edge. The Schlieren image is acqired by a high speed camera (Photron Fastcam SA 3) with a 5 mm Nikon lens. Fig. 2 Set-p of the Schlieren imaging system. (1 Light sorce; 2 Condenser lens; 3 Pin hole; 4 Beam splitter; 5 Lens; 6 Qartz window; 7 Mirror; 8 Knife edge; 9 High-speed camera with lens) 3

4 Sample Schlieren images for methane/air flame with =1 at atmospheric pressre are presented in Fig. 3. These images are digitally processed to obtain flame radis-time data as shown in Fig. 4. Figre 4 also shows the experimentally measred pressre in the combstion chamber. For a flame radis of 2 cm, the pressre rise in the chamber is abot 7%. Assming isentropic compression of the nbrned gas, the change in the flame speed de to increased pressre and temperatre of the nbrned gas is negligible. However, the effect of compression-indced flow velocity may not be negligible and shold be acconted for in data interpretation. Fig. 3 Sample Schlieren images for CH 4 /air, =1, 1 atm, 296 K. Sccessive images are 1.75 ms apart Radis(cm) Pressre(bar) x x 1-3 Fig. 4 Flame radis and pressre history 3. Data Interpretation Flame radis and pressre history is sed to dedce the laminar flame speed as follows. For an OPF, the mass of the brned gas, m b, is described as dmb d rf b 4r f S dt dt 3 where r f is flame radis and b is the mass averaged density of the brned gas. A rearrangement of the terms gives - drf r b f db S dt 3 dt In this eqation, second term acconts for compression indced flow velocity in the brned gas. The o nstretched laminar flame speed, S, can be obtained by linear extrapolation of the stretched flame speed to zero stretch. 4

5 o S S LK where L is Markstein length and K is stretch rate, given by 2/ r ( dr / dt).in order to dedce, o Markstein length and S from the experimental data, it is better to work with the integrated form of the above eqation, which avoids amplification of random experimental scatter dring differentiation. An integration of the eqation gives rf ( t) dr r r ( t) b f f db o f dt S ( t t ) 2Lln (1) dt 3 dt r r Experimental data of pressre and radis can be sed to evalate the integral in the above eqation, o and S and L can be determined from a linear regression. Note that b and are time dependent. Determination of is straightforward from isentropic compression of the nbrned gas by sing T ( t) dt P( t) ln 1 T T P i i where sbscript i denotes initial conditions at the time of spark discharge and is temperatre dependent specific heat ratio of the nbrned gas mixtre. In order to determine b, a mlti-zone model is sed that tilizes the experimental time-radispressre data. In the mlti-zone model, flame propagation is taken as the consective consmption of nbrned gas mixtre within the zones. The nmber of zones eqals the nmber of time-radispressre data points that are available from the experiment. After ignition, the flame front consmes zone I first. The post combstion radis of zone I at this instant, time=t 1, is taken as the first radis point available from the experimental data and its temperatre is dedced by first calclating the isobaric eqilibrim temperatre of the brned gas based on initial temperatre and pressre, T i and P i, and then correcting for the isentropic compression of the brned gas from P i to P 1. This gives the brned gas temperatre and density of Zone I at time=t 1. Mass of the brned gas in zone I is also calclated. After the combstion of the first zone, combstion of the second zone occrs at a higher nbrned gas pressre, P 1, and temperatre, which is determined from isentropic compression of the nbrned gas. After the combstion of zone II, the oter radis of the zone II is taken as the next available time-radis-pressre data points. The temperatre of the zone II is dedced by first calclating the isobaric eqilibrim temperatre of the brned gas based on temperatre and pressre of the nbrned gas after the consmption of zone I and then correcting for the isentropic compression of the brned gas. Combstion of zone II also leads to compression of zone I, and its temperatre can be calclated from isentropic assmption. Also, since the mass of gas in zone I is already known, its radis at the present time step can also be pdated. This process is followed for the sbseqent zones. Note that in this model, every zone is considered adiabatic even thogh temperatre gradients are established in the brned gas. NASA chemical eqilibrim program from Gordon and Mcbride is sed to evalate the brned gas temperatre, moleclar weight and specific heat ratio. Figre 5 shows an example of otpt from the model. It is noted that there is a small increase in the nbrned gas temperatre and density. The brned gas density also increases de to compression, and temperatre gradient is present in the brned gas temperatre profile. Based on this model, flame speed vs stretch rate and linear interpolations are plotted in Fig. 6. For linear interpolation, only flame radis from.6 to 2 cm is taken to avoid the effect of initial spark-indced flame acceleration. Figre 6 also shows a comparison of data interpretation by f f 5

6 inclding and exclding the effect of compression indced flow velocity and variable density ratio. When these effects are exclded, d b / dt term in eqation 1 is neglected and the density ratio b / is determined based on the initial nbrned gas temperatre and pressre. An nderprediction of 4.2% in S is noted when these effects are exclded..16 Unbrned density (kg/m 3 ) Brned density (kg/m 3 ) x x 1-3 Temperatre nbrned (K) x 1-3 Brned Temperatre(K) Radis (cm) Fig. 5 Time evoltion of a) Unbrned gas density b) Unbrned gas density c) Unbrned gas temperatre. d) Temperatre profile of brned gas at flame radis of 2 cm. Calclations from the Mlti-zone model. Flame speed (cm/s) Flame speed (cm/s) Inclding compression and density effect Not inclding compression and density effect Stretch (1/s) Stretch (1/s) Fig. 6 Experimental data of stretch rate vs flame speed and linear extrapolations. CH 4 /air, =1, 1 atm, 296 K 6

7 4. Preliminary Validation Experiments Initially, the facility has been sed only in static mode and flame speed has been determined for methane/air flames at atmospheric pressre. Fig. 7 shows a comparison of the present data with the data from the literatre. An excellent agreement is noted. In the near ftre, the facility will be characterized and sed in dynamic mode where compression stroke of the facility will be sed to obtain higher pressres and temperatres before spark discharge. Flame speed (cm/s) Egolfopolos et al. Present Data Eqivalence Ratio Fig. 7 Comparison of present flame speed data with literatre [11]. CH 4 /air, 1 atm, 296 K Acknowledgement This work is spported by National Science Fondation Grant No References 1. Law, C.K.; Sng, C.J.; Wang, H.; L., T.F Development of Comprehensive Detailed and Redced Reaction Mechanisms for Combstion Modeling. AIAA Jornal 41(23) Qin, X.; J, Y. Measrements of Brning Velocities of Dimethyl Ether and Air Premixed Flames at Elevated Pressres Proc. Combst. Inst. 3 (25) Rozenchan, G.; Zh, D.L.; Law, C.K.; Tse, S.D. Otward Propagation, Brning Velocities, and Chemical Effects of Methane Flames p to 6 atm. Proc. Combst. Inst. 29 (22) Tse, S. D.; Zh, D. L.; Law, C. K. Morphology and Brning Rates of Expanding Spherical Flames in H 2 /O 2 /Inert Mixtres p to 6 atmospheres. Proc. Combst. Inst. 28 (2) Sn, H.Y.; Yang, S.I.; Jomaas, G.; Law, C.K. High-Pressre Laminar Flame Speeds and Kinetic Modeling of Carbon Monoxide/Hydrogen Combstion. Proc. Combst. Inst., 31 (27) Vries, J. de; Corbin, B.A.; Petersen, E.L. A Facility for High-Pressre Laminar Flame Speed Measrements. 5 th US Combstion Meeting of The Western States Section of the Combstion Institte at San Diego, March 25-28, 27, Paper # A2 7

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