Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames

Transcription

1 archivum combustionis Vol. 26 (2006) No.3-4 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames Department of Heat Technology and Refrigeration, Technical University of Lodz, Poland ul. Stefanowskiego 1/15, Fax: (48) (42) , The aim of this study was to examine the influence of walls on laminar burning velocity for flames propagating in propane-air and ethylene-air mixtures near the quenching limit. Experiments were carried out in a narrow wedge-shaped channel and recorded by a camera. Results of measurements of laminar burning velocity under quenching conditions were compared with laminar burning velocity obtained for adiabatic and unstretched flames. It was found that the measured laminar burning velocity is lower than adiabatic laminar burning velocity for lean mixtures. For flames propagating in rich mixtures its value can equal the adiabatic laminar burning velocity. This phenomenon can be explained by the Lewis number effect ( Le < 1). Additionally, quenching distance for flames propagating in ethyleneair mixtures was determined. These results were used to determine the critical Peclet number. Values of the number for ethylene-air flames are between Pe = Introduction Laminar burning velocity u L is one of the most important parameters, which represents in a synthetic way processes occurring in the laminar flame front. Andrews and Bradley [1] have analyzed different measurement techniques used to determine laminar burning velocity, against a background of their own measurements in which they have observed initial phase of a flame propagating in a spherical combustion chamber. Their method took into consideration the flame thickness, which was essential for the flame propagation in mixtures near flammability limits. They have compared laminar burning velocity for methane-air flames obtained by different authors and different methods. They found significant scattering of this parameter caused by different non-adiabatic conditions of the experiments and by neglecting flame thickness in the computations. It is worth to say that at the time when their paper was published flame stretch and preferential diffusion have not been taken into consideration jet and their laminar burning velocity was higher than currently assumed. Yamaoka and Tsuji [2] used an experimental method to determine the laminar burning velocity under conditions close to adiabatic, but still their values were subject to some flame stretch. The most popular method used for determination of laminar burning velocity was proposed by Law [3]. He used the symmetrical counterflow flame configuration. Upon ignition, two stable flat flames were established in the stagnation flowfield. The experiment basically

2 2 involves determining the axial velocity profile along the centerline of the flow by LDV. The minimum point of the velocity profile is identified as a reference upstream flame speed which is corresponding to the imposed stretch rate K. Laminar burning velocity for unstretched flame ( K = 0 ) can be determined by linear extrapolation of the experimental data to zero stretch rate. There are some doubts if the linear extrapolation to conditions K = 0 is proper. It is considered that the error due to linear extrapolation can be as high as 5% 15% [4] in comparison with the nonlinear extrapolation. For that reason Vagelopoulos and Egolfopoulos [4] developed a new method for direct measurement of laminar burning velocity. In contrast to the Law method [3], the flame is stabilized at some distance between the nozzle and the stagnation plane. This method allows measuring the laminar burning velocity in the near-zero strain rate zone. Results obtained by this technique are lower than that proposed by Law. It is known that any flame can be quenched in a channel if the distance between the walls is small enough. The minimum plate separation for which flame propagation can be achieved is named quenching distance D g. First detailed measurements of D g can be found in the work of Potter [5]. Quenching distance depends on kind of fuel, mixture concentration and directions of flame propagation (upward or downward) [6]. Quenching distances for methane-air flames and for propane-air flames were determined by Jarosinski [6] and Jarosinski and Podfilipski [7], respectively. Values of quenching distances for these flames are reproduced in Fig. 1 and Fig. 2. Fig. 1. Quenching distance as a function of equivalence ratios for upward and downward flames propagating in methane-air mixtures [6].

3 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames 3 Fig. 2. Quenching distance as function of equivalence ratio for propane-air mixtures. Experimental points: o - quenching distance for downward propagating flames, - quenching distance for upward propagating flames [7]. For lean methane mixture and rich propane air mixture quenching distance depends on direction of flame propagation (upward or downward). Flame stretch and preferential diffusion are the physical factors responsible for the differences ( Le < 1 ). A critical Peclet number Pe c was adopted to describe the quenching flame phenomena in narrow channels. This number is defined as follows where: a thermal diffusivity and u L laminar burning velocity. DguL Pec = (1) a It was established for propane-air mixtures that Pe = 42 [8]. However, the critical Peclet number decreases for rich mixtures, where Le < 1. c

4 4 Jarosinski [6] compared flame thickness δ g and a quenching distance D g and found that for D g methane air mixture is approximately constant over the entire range of mixture compositions and equal to 2. δ g Daou and Matalon [9] using numerical methods have analyzed flame behavior in narrow channels with and without heat losses. Their calculations show that flames with heat losses are thicker in comparison with those propagating under adiabatic conditions. They found that quenching distance is approximately 15 times larger than flame thickness, calculated using a L =. In their calculations thermal diffusivity was taken for the mean temperature u L ( T T )/ 2. b u The aim of this work was to examine the influence of walls on laminar burning velocity for flames propagating in propane-air and ethylene-air mixtures near quenching limit and to determine quenching distances for flames propagating in ethylene-air mixtures. 2. EXPERIMENTAL DETAILS Quenching distance and laminar burning velocity under quenching conditions were measured in a vertical 45mm 12mm tube, about 1065mm long. A wedge-shaped aluminum bar was placed into the tube with rectangular cross-section to obtain the narrow channel. Distance between the slit walls decreased along the channel length from 12mm to 1mm. Each end of the tube was equipped with a valve (Fig. 3). Valves allowed to fill the tube with the mixture and to open the end during ignition. A change of the direction of flame propagation was realized by inversion of the channel. Fig. 3. Geometry of the quenching channel.

5 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames 5 The tube was filled by displacement. About 10 vessel volumes were passed through the channel before ignition. One end of the tube was always open during the experiments. Ignition of the mixture was located near the open end of the channel. The photographic records of the combustion process were made by a conventional Panasonic S-VHS video camera. 3. RESULTS The laminar burning velocity under quenching conditions was determined during downward propagation of the flame. Position of the flame as a function of time was taken from successive frames. This dependence was nonlinear: flame propagation velocity gradually decreased. Laminar burning velocity was determined by an equation of tangent to the extinction point. The applied method is shown in Fig. 4. Fig. 4. Flame position as a function of time for the ethylene-air mixture with equivalence ratio. Tangeht at the extinction point. Φ = 0.65 It was observed that flames propagating in lean propane-air mixtures are flat contrary to those propagating in rich mixtures. Values of laminar burning velocity for propane-air mixture determined in this way are compared with those obtained under adiabatic conditions (papers [2, 4, 10] in Fig. 5). Velocities taken for this comparison have the following properties: line 2 with some stretch; lines 3 and 4 without stretch. As it can be seen, the maximum value of laminar burning velocity determined in a quenching channel is shifted from Φ = 1. 1 to Φ = 1. 2.

6 6 Fig. 5. Laminar burning velocity as a function of equivalence ratio for propane-air mixture. Comparison of laminar burning velocity under quenching conditions determined in the present work (1) with adiabatic laminar burning velocities (2), (3) and (4). Laminar burning velocities: 2 Yamaoka and Tsui [2], 3 Egolfopoulos al. [10], 4 Vagelopoulos and Egolfopoulos [4]. Quenching distances measured for ethylene-air flames are shown in Fig. 6. Fig. 6. Quenching distance as a function of equivalence ratio for ethylene-air mixtures.

7 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames 7 As it can be seen quenching distances do not depend on direction of flame propagation in the range of mixture concentration between 0. 5 < Φ < For the flame moving upward the range of flammable mixture concentration is wider than for downward flames. Behavior of rich propane-air and ethylene-air flames is similar. The large difference between rich flammability limits for upward and downward propagation flame is caused by preferential diffusion. For this mixture concentrations Le < 1. Experimental results for downward propagating ethylene-air flames were used to determine the critical Peclet number. Values of Peclet number for ethylene-air flames are shown in Fig. 7. We can see that all points for the whole equivalence ratio range are located between Pe = For rich mixture the values of Peclet numbers somehow decrease. Laminar burning velocities obtained by Vagelopoulos and Egolfopoulos [4] were used to calculate these values. Fig. 7. Critical Peclet number as a function of equivalence ratio for ethylene-air flames under quenching conditions. As it was mentioned earlier Daou and Matalon [9] using numerical methods have found the relationship shown below: Dg ~ 15 (2) L For the experimentally obtained results this relationship becomes: D g ~ ( ) 15 (3) L

8 8 As we can see agreement of the data is satisfactory. Results of measurements of laminar burning velocity under quenching conditions for ethylene-air flames are shown in Fig. 8. These values are compared with laminar burning velocity obtained for adiabatic and unstretched flames [10]. Fig. 8. Laminar burning velocity as a function of equivalence ratio for ethylene-air mixture. Comparison of laminar burning velocity under quenching conditions (1) with adiabatic laminar burning velocity (2). 1 laminar burning velocity under quenching conditions determined in the present work, 2 adiabatic laminar burning velocity from Egolfopoulos et al. [10]. There is a difference between adiabatic laminar burning velocity and laminar burning velocity obtained under quenching conditions for flames propagating in lean propane-air mixtures and also lean ethylene-air mixtures. This difference is about 10cm/s for propane-air flames and about 6cm/s for ethylene-air flames. However for rich propane-air flames values of this parameter are equal to the data published by Yamaoka and Tsui [2], Vagelopoulos and Egolfopoulos [4] and Egolfopoulos et al. [10] (see Fig. 5). For the flames propagating in rich mixtures of ethylene-air, the results agree with data found by Egolfopoulos et al. [10] for the range between < Φ < for adiabatic conditions. For Φ > 1. 8 laminar burning velocities near quenching conditions are lower than adiabatic. The difference is about 4cm/s.

9 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames 9 It was expected that heat loss from flame to the walls would cause reduction of flame temperature and what would reduce intensity of chemical reaction and in consequence of laminar burning velocity. As it is seen in Figures 5 and 8, laminar burning velocity under quenching conditions is lower than the adiabatic one for the flames propagating in lean mixtures. For flames propagating in rich mixtures its value equals the adiabatic laminar burning velocity for the equivalence ratios between 1.45 a 1.65 for both propane-air and ethylene-air flames. On the other hand for mixtures richer than Φ = it is lower than the adiabatic laminar burning velocity for flames propagating in ethylene-air mixture. It means that in addition to the cooling effects of the walls there exists some additional mechanism influencing limit flame during its propagation in the narrow channel in ethylene-air and propane-air mixtures. Increase of laminar burning velocity above expected limit value can be explained by a Lewis number (Le) dependence, which for rich propane-air and ethylene-air mixtures is less than unity. If a deficient reactant is more diffusive ( Le < 1 ) then the diffusivity of the mixture then a reaction zone is additionally supplied with it, and the flame temperature and laminar burning velocity become higher. For rich propane-air and ethylene-air mixtures the deficient reactant is oxygen and also it is the more diffusive one. It was found that flames propagating in rich propane-air mixtures under quenching conditions are convex. It is necessary to remember that for Lewis number different from unity, flame convexity effect plays a significant role in the flame behavior. It was stated in [12], that for the mixture corresponding to Le > 1 increased flame stretch reduces reaction rate, while for mixture with Le < 1, its influence is opposite. Law [3] showed for rich propane-air flames, that propagation velocity of flames quenched by stretch is much higher than that unstretched one. (Fig. 10). Similar results can be found in numerical investigation of Kurdyumov and Fernandez-Tarrazo [13]. They have considered a flame propagating in narrow duct with adiabatic and cold isothermal walls. It was found that, when flames with Le < 1 propagated in circular ducts with isothermal walls, their velocities were higher than those with adiabatic walls of the same circular duct radius. This effect is due to the higher flame curvature near a cold wall. Fig. 9. Maximum flame temperature as a function of flame stretch for various Lewis numbers [11].

10 10 Fig. 10. Comparison of flame speed at quenching stretch to adiabatic laminar burning velocity [3]. 4. CONCLUSIONS 1. The critical, quenching Peclet number for flames propagating in a broad range of ethylene-air mixture compositions is located between Pe = As it was expected laminar burning velocities for flames propagating in lean propane-air and ethylene-air mixtures under near quench conditions are lower than under adiabatic condition. 3. Laminar burning velocities for flames under near quench conditions in rich propane-air and ethylene-air mixtures, for which Le < 1, for < Φ < (propane-air) and < Φ < (ethylene-air) equal to laminar burning velocity under adiabatic condition. Lack of a drop of the u L value for these flames is caused by an influence of preferential diffusion. 4. Laminar burning velocity for flames under near quench conditions in ethylene-air mixtures for Φ > is lower than this one under adiabatic condition for the same mixture compositions. It is because the flame curvature decreases in this case so the preferential diffusion however, heat transfer from the flame to the walls is still the main factor affecting the flame propagation velocity.

11 Laminar Burning Velocity under Quenching Conditions for Propane-Air and Ethylene-Air Flames 11 References [1] Andrews G. E., Bradley D.: The Burning Velocity of Methane-Air Mixtures, Combustion and Flame, 19, 1972, pp [2] Yamaoka I. and Tsuji H.: Determination of Burning Velocity Using Counterflow Flames, Twentieth Symposium (International) on Combustion, The Combustion Institute, 1984, pp [3] Law C. K., Zhu D. L. and Yu G.: Propagation and Extinction of Stretched Premixed Flames, Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp [4] Vagelopoulos C. M., Egolfopoulos F. N.: Direct Experimental Determination of Laminar Flame Speeds, Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, pp [5] Potter Jr. A. E.: Flame Quenching, Progress in Combustion and Fuel Technology, vol. 1, ed. J. Durcarme, M. Gerstain and A. H. Lefebvre, New York, Pergamon Press, 1960, pp [6] Jarosinski J.: Flame Quenching by a Cold Wall, Combustion and Flame, 50, 1983, pp [7] Jarosiński J., Podfilipski J.: Properties of Propane Flames, Eighteenth International Colloquium on the Dynamics of Explosions and Reactive Systems, Seattle, 2001, pp [8] Jarosinski J., Podfilipski J. and Fodemski T.: Properties of Flames Propagating in Propane Air Mixtures Near Flammability and Quenching Limits, Combust. Sci. and Tech., 174, 2002, pp [9] Daou J. and Matalon M.: Influence of Conductive Heat-Losses on the Propagation of Premixed Flames in Channels, Combustion and Flame, 128, 2002, pp [10] Egolfopoulos, F. N., Zhu, D. L., and Law, C. K.: Experimental and Numerical Determination of Laminar Flame Speeds: Mixtures of C2-Hydrocarbons with Oxygen and Nitrogen, Proc. Combust. Inst. 23, 1990, [11] Law C. K.: Dynamics of Stretched Flames, Twenty Second Symposium (International) on Combustion, 1988, pp [12] Law C. K., Sung C. J.: Structure, Aerodynamics, and Geometry of Premixed Flamelets, Prog. Energy Combust. Sci., 26, 2000, pp [13] Kurdyumov V. N. and Fernandez-Tarrazo E.: Lewis Number Effect on the Propagation of Premixed Laminar Flames in Narrow Open Ducts, Combustion and Flame, 128, 2002, pp