Detonation Initiation by Gradient Mechanism in Propane Oxygen and Propane Air Mixtures

Transcription

1 Detonation Initiation by Gradient Mechanim in Propane Oxygen and Propane Air Mixture A.E. Rakitin, I.B. Popov, NEQLab Reearch BV, The Hague, 5AL, The Netherland A.Yu. Starikovkiy Princeton Univerity, Princeton, USA An experimental tudy of detonation initiation by high voltage nanoecond ga dicharge ha been performed in mooth detonation tube. A gradient mechanim wa ued to initiate detonation in toichiometric propane oxygen mixture with different nitrogen dilution and in propane air mixture. Initial preure from 0. to bar have been teted. Detonation wa formed within tranvere tube ize at initial preure higher than 0. bar for the propane oxygen mixture and higher than 0.8 bar for the diluted mixture with 0% of nitrogen. The dicharge energy input were J. The gradient mechanim of detonation formation imilar to the one uggeted by Zeldovich ha been hown to be the governing proce. For the mixture with air, a detonation tube with an annular dicharge chamber ha been deigned and teted. Introduction Studie of different mechanim of detonation initiation in combutible gaeou mixture are related to the neceity of development an ignition device for application in puled detonation engine (PDE). The application require a robut energy efficient method of initiation Chapman Jouguet (CJ) detonation in tube within ditance of everal caliber and time of m. The mot common approach i deflagration to detonation tranition (DDT) facilitation by the application of variou type of obtacle that turbulize the flow and increae the flame velocity, which wa initially uggeted by Shchelkin []. Alternatively, the concept of detonation initiation through a gradient mechanim wa theoretically introduced by Zeldovich []. According to thi work, an ignition delay time (or induction time) gradient formed due to a correponding temperature gradient lead to the onet of a pontaneou combution wave. The mixture firt ignite at the point with the lowet delay and then propagate by pontaneou ignition with a velocity D p determined by the hape of the gradient. Depending on the value of D p, different mode of flame propagation can be realized. When D p exceed the Chapman Jouguet velocity D CJ, a detonation wave can emerge from a contant volume exploion a the pontaneou wave decelerate to the D CJ value. Thi cae wa numerically tudied in a erie of paper reviewed in [3], where the detonation wave alway occurred from a hot pot in the region between the hock wave and the turbulent flame. Variou cenario of hot pot formation due to hock flame interaction and the role of their tructure were alo dicued in that paper. When D p fall into the interval between the peed of ound and DCJ, the pontaneou wave propagating over the induction time gradient can become coupled with the compreion wave and thu evolve into a detonation wave through hock amplification and acceleration. A imilar concept of the hock wave amplification by coherent energy releae (SWACER) wa introduced in [], where thi proce wa invetigated in more detail. Thi paper focue on the further tudy of thi mode of detonation initiation with regard to it future application in puled detonation engine, continuing the tudy in [5,6]. In the firt part, a four cell dicharge chamber i ued to enhance the efficiency of the gradient formation in term of energy input. Each of the dicharge cell i of the ame geometry a the one ued in all previou etup. Thi approach allow to etimate the required hape of the gradient along the channel and, thu, the neceary parameter of the dicharge. The econd part of the paper i dedicated to the experimental tudy of thi mechanim for the practically important propane air mixture. For optimal configuration of dicharge propagation and it interaction with the ga flow, annular dicharge channel ha been developed. DDT initiation by park dicharge The park mode of dicharge development wa realized in the whole preure range under maximum pule amplitude of 60 kv, which correponded to pule energy of 5 J. The dicharge gap in the experiment at an initial preure of 0.3 bar wa 00 mm. Thi reulted in an energy input into the ga of 0 J. Typical x t diagram baed on the preure tranducer (PT) and the infrared enor (IR) are preented in Figure. The time i counted off from the moment the nanoecond dicharge developed. The origin of the X-coordinate coincide with the grounded electrode of the dicharge gap. The negative X-value correpond to the dicharge channel and are ued to repreent the ignition proce dynamic captured with ICCD camera. Each data et in the poitive X-value region correpond to a pair of preure tranducer and infrared enor intalled at different ditance from the nozzle inlet. It i een from the data that, by the firt meauring ection, the flame front wa cloely coupled with the hock wave and propagated with a uperonic velocity

2 of over 700 m/. The temporal delay between the wave wa 0 μ, which yielded 7 mm for the gap length. Auming that the flame front and the hock wave traveled at that velocity between the nozzle inlet and the firt enor would yield their propagation with nearly zero delay after the dicharge. Such a pattern reulted in ucceful DDT between the third and the fourth enor, 30 mm from the nozzle inlet, which i tranvere tube ize. The average velocity in thi region 00 m/ wa lightly lower than the calculated CJ value. Alo, a retonation wave wa clearly een propagating backward with an average velocity of 700 m/. The DDT time did not exceed 300 μ. Figure. x t diagram of DDT, park mode. a) initial preure of 0.3 bar; b) initial preure of bar. Inert in figure a) repreent the ICCD image of ignition development in the dicharge gap. Stoichiometric propane oxygen mixture. The ignition proce inide the dicharge channel wa tudied with a LaViion Picotar HR ICCD camera. One image with a -n gate wa taken during each experiment through an interference filter with the maximum tranmittance at 3. nm and a full width at half maximum of.6 nm. The ICCD image taken 5 μ after the dicharge i hown in inert of Figure. The image intenity correpond to flame emiion inide the dicharge channel. The light line in the image deignate the channel, with it high-voltage electrode being on the left and the detonation tube being on the right. It i een that 5 μ after the dicharge the mixture wa already ignited over almot all the cell volume, which i repreented by the horizontal dahed line in Fig.,a. Thi implie imultaneou ignition of the mixture in term of the typical temporal cale of ga-dynamic procee and confirm the aumption that the propagation of both the flame wave and the hock wave occurred without a ignificant delay. After imultaneou ignition inide the cell, a DDT occurred in the detonation tube. It i een that, in the early tage (0 00 μ), a deflagration wave propagated with a velocity of approximately 700 m/, which agree well with the enor data. A detonation wave wa formed le than 300 μ after the dicharge. The park mode of DDT initiation at an initial preure of bar reulted in a ignificantly horter DDT time and run-up ditance, though under the ame energy input of 0 J from the high-voltage pule (Figure,b). The correponding x t diagram baed on preure tranducer data i preented. The average velocity became equal to the CJ value between enor and, which implied that the detonation wave wa formed before or hortly after enor. Thu, the DDT time amounted to 50 μ and the run-up ditance wa 50 mm or le. Thu it wa hown that the ignition in the ditributed park mode lead to fat DDT even in a mooth tube. DDT initiation by tranient plama At a preure of 0.3 bar, the combined tranient initiation mode wa realized with a pule amplitude of 80 kv and under a longer dicharge gap of 50 mm. The energy of the pule amounted to 5 J, wherea the energy input in the initial treamer dicharge wa le than.5 J. The DDT pattern for thi kind of initiation in term of an x t diagram i hown in Fig.,b. The initial velocity of the hock wave i equal to 600 m/, which i lightly lower than that for the park initiation at the ame initial preure. During the initial tage, the flame wave wa following the hock wave with the ame velocity, but with a notable delay of 30 0 μ. The flame wave then accelerated and caught up with the hock wave hortly after enor 3. The acceleration took more time and ditance to occur in comparion with the park mode: the average velocity between enor 3 and amounted to only 50 m/. However, the DDT did occur between thee enor, ince a retonation wave i clearly een propagating backward from ome point in that region. The DDT length and time amounted to approximately 50 mm and 00 μ, repectively.

3 Figure. a) x t diagram of DDT at an initial preure of 0.3 bar, tranient initiation mode. b) x t diagram of DDT at an initial preure of bar, tranient initiation mode. Stoichiometric propane oxygen mixture. In order to realize thi initiation mode at an initial preure of bar, the pule amplitude wa raied to 0 kv under the ame interelectrode gap of 50 mm. The pule energy wa J,. J of which wa depoited in the plama by the firt treamer dicharge. A ignificant portion of the pule energy over 3 J wa depoited during the econd dicharge. The x t diagram under thee condition i preented in Fig.,b. At the higher preure, the flame wave caught up with the hock wave ooner: the delay wa only 0 μ at enor, completely diminihing later on. The average velocity value correponded to the CJ value already between enor and. The DDT time wa lightly over 60 μ and the run-up ditance wa 50 mm, which i quite cloe to the value oberved for the park initiation under the ame condition. DDT initiation by treamer dicharge: gradient mechanim Figure 3. a) x t diagram of DDT at an initial preure of bar, treamer mode; b) Time-reolved ICCD imaging of fuel mixture ignition inide the dicharge chamber. DDT at an initial preure of bar, treamer mode. Dicharge energy W = J. Stoichiometric propane oxygen mixture. The treamer mode of dicharge development could be realized at a dicharge gap of 50 mm, an initial preure of bar, and a pule voltage of 80 kv. The pule energy and the energy input amounted to.5 and J under thee condition, repectively. The econdary re-reflected pule wa too weak to produce any ignificant effect on the preionized ga under thee condition. The treamer mode of dicharge propagation without the formation of a hot channel reulted in a different DDT pattern. The reult are preented in term of an x t diagram and in term of a time-reolved frame

4 equence captured with the UltraSpeedStar6 camera in Fig. 3. From the IR enor data, it i een that the flame front already propagated at the CJ velocity between enor and, which i quite imilar to the pattern oberved at bar under other initiation mode. However, the DDT time wa ignificantly longer and amounted to 50 μ. At the ame time, ICCD imaging howed that the ignition did not occur imultaneouly over the channel. Intead, a flame wa een originating in the region cloet to the high-voltage electrode after a delay of over 50 μ and then propagating and accelerating along the channel. The initial flame front velocity exceeded 500 m/, and accelerated to 000 m/ at the channel outlet. The correponding data point were plotted in the x t diagram in the negative X-value region. It i een that the x t trajectory of the flame wave inide the dicharge channel agree well with the one inide the detonation tube. The dahed line in the diagram i plotted to better illutrate the acceleration of the flame front inide the channel. Figure. a) Time-reolved ICCD imaging of mixture ignition inide the -channel dicharge chamber. DDT at an initial preure of 0.8 bar, propane-oxygen with 0% of nitrogen, treamer mode; b) x-t diagram of DDT at initial preure of 0.5- bar, propane-oxygen with 0% of nitrogen, treamer mode. An experimental tudy of detonation initiation by high-voltage nanoecond ga dicharge ha been performed in a mooth detonation tube with a four-cell dicharge chamber deigned to enable a gradient initiation mechanim. Stoichiometric propane-oxygen mixture diluted by nitrogen were ued at initial preure from 0. to bar. Detonation wa formed within tranvere tube ize at initial preure higher than 0. bar for the propane-oxygen mixture and higher than 0.8 bar for the diluted mixture with 0% of nitrogen (Figure ). The dicharge energy input were J. The time of detonation formation wa below 0.5 m for all condition. The value of the ignition delay time gradient, formed by non-uniform radical production during treamer dicharge and following rapid plama thermalization, ha been etimated. Role of gradient mechanim According to gradient initiation mechanim theory the condition for the gradient mechanim to lead to detonation wave formation i that pontaneou combution velocity D p i greater than ound peed V and le than Chapman-Jouguet velocity D CJ. Spontaneou combution velocity D p, or the ignition pattern inide the dicharge channel in general, i determined by the temperature gradient and by the ditribution of radical produced by the dicharge. Knowing the temperature and etimating the radical concentration at different point along the channel, it i poible to evaluate the correponding ignition delay time ditribution uing zero-dimenional chemical kinetic equation. Taking into account that D p = (d/dz) -, where z i the axial coordinate along the channel, thi will allow to etimate whether the Zeldovich gradient condition can be fulfilled. To etimate the abolute value of thee parameter, energy branching of the 00 mj put in into the ga by the dicharge wa conidered. According to model of energy exchange in nonequilibrium plama, decribed above, 0-30% of energy input i branched into direct heating of the ga by rapid thermalization mechanim. A for the radical production, it ha been hown in [7] that O atom are produced at an overall energy price of ~8 ev. A peak temperature increae wa etimated a T ~ 700 K and to the peak atomic oxygen production a 3.5%. The chemical kinetic calculation performed under thee condition uing the reaction mechanim decribed in [5] yield an ignition delay time of 33 m, which agree with the experimentally oberved reult. The reult are hown with red line in Fig. 5 together with the experimental ditribution. The condition on the gradient hold if the lope of the two curve coincide in ome region but in general Zeldovich criteria require much weaker gradient than experimentally oberved one.

5 It hould be noted, however, that the Zeldovich' criterion wa introduced uing the condition of weak excitation with rather low energy releae in chemical reaction. Under the condition of high-voltage, high-current puled dicharge the excitation i rather trong and chemical energy releae time become comparable or even le than the gadynamic time. Under thee condition we hould take into account the ignition delay time profile variation due to propagation of trong compreion wave from the reaction zone. It change ignificantly the Zeldovich condition auming contant ignition delay time gradient during all DDT proce. The baic difference between propoed criterion modification and original Zeldovich' model i conideration of compreion wave influence on the ignition delay time ditribution. It i aumed that ignition occur near the tip of the high-voltage electrode. Fat energy releae (fater than the gadynamic expanion time) will lead to temperature, preure and ound peed increae in thi ga volume. Parameter of thi high-preure, high temperature ga are: preure - P, temperature - T, ound peed - a, pecific heat ratio. Initial parameter of the ga in the vicinity of the ignition point are: preure - P, temperature - T, ound peed - a, pecific heat ratio. Thu we have a common tak of the arbitrary dicontinuity which ha a well-known olution: p p M a M a M / with the formation of a hock wave with propagation velocity M S. From thi equation one can derive M (, ). Shock wave generated by energy releae at initiation point will propagate through the ga, increaing it temperature, preure and denity: T ( ) M ( M ) (, ) T ( ) M We need to increae the temperature in the new point at ditance L from the point of initiation to the initial value in the point of initiation: Form the other hand, temperature gradient T( L) T(0) / (, ) T T( 0) T( L) / (, ) T(0) x L L Uing for L an obviou etimation L M c (, c, where E i chemical energy releae time, M S i hock E E ) wave Mach number, and c i the ound peed in the ga, one can derive a modified Zeldovich' criterion for the cae of trong initial perturbation in the reactive ytem and hort energy releae time leading to the chemical energy releae at almot contant volume condition: T / (, ) T( 0) 0 K / mm x (, ) c E For typical mixture ued in current work it i eay to etimate all parameter in equation 3. The etimation give dt/dx ~ 0 K/mm. Thi gradient i hown alo in the Figure 5 a the modified model, together with the original Zeldovich' criterion. A wa expected, the modified criterion require much horter gradient becaue of the compreion wave influence on the mixture to the right of the initiation point. Of coure the good agreement between the modified Zeldovich' criteria and the experimentally meaured gradient i rather coincidence becaue of unteady nature of the interaction. But thi correlation how that thi interaction can really modify the optimal ignition delay time ditribution and can bring it very cloe to experimentally oberved ditribution (Figure 5). Summary Nonequilibrium plama demontrate great abilitie to control ultra-lean, ultra-fat, low-temperature flame and become an extremely promiing technology for a wide range of application, including aviation GTE, piton engine, RAMjet, SCRAMjet and detonation initiation for puled detonation engine. To ue nonequilibrium plama for ignition and combution, it i neceary to undertand the mechanim of plama aited ignition and combution under variou condition and to imulate numerically dicharge and combution procee under variou condition. Two fat DDT mechanim have been oberved. When initiated by a park or a tranient dicharge, the mixture

6 ignite imultaneouly over the volume of the dicharge channel, producing a hock wave with a Mach number over and a flame wave. The delay between hock and flame wave formation i governed by the energy tranferred into tranlational degree of freedom and by radical formation. The wave then form an accelerating complex reulting in DDT. At an initial preure of bar, the DDT length and time do not exceed 50 mm and 50 μ, repectively for propane-oxygen-nitrogen mixture. Figure 5. Calculated O atom concentration ditribution over the interelectrode gap, correponding to Zeldovich model (red curve) and Modified Zeldovich criterion (green curve). Propane-oxygen with 0% of nitrogen. A gradient mechanim of deflagration-to-detonation tranition imilar to that propoed by Zeldovich ha been oberved experimentally under treamer initiation. The mixture inide the dicharge channel i excited nonuniformly by the treamer, thu forming an excited pecie concentration gradient. The gradient correpond to that of ignition delay time. The hottet pot with the hortet ignition delay i at the high-voltage electrode tip. Originating at thi point, the pontaneou combution wave tart propagating along the channel at a velocity over 500 m/ and accelerate up to the CJ velocity value at the channel output. The initiation energy i, by an order of magnitude, lower for the treamer mode when compared to the park initiation under the experimental condition, wherea the DDT time i three time longer. However, the DDT length i till within 50 mm, which correpond to tranvere tube ize, and the DDT energy i, by two order of magnitude, lower than the energy of direct planar detonation initiation. The extremely hort DDT ditance under reaonable energy input value achieved due to the gradient mechanim are the prerequiite for the deign of compact and efficient pule detonation engine. Thu plama aited ignition may be an effective tool to different application including high-peed, ultra-lean combution and control of tranient combution procee uch a deflagration-to-detonation tranition. Acknowledgement The work wa partially upported by Ruian Foundation for Baic Reearch under the project Nonequilibrium plama thermalization, AFOSR under the project Fundamental Mechanim, Predictive Modeling, and Novel Aeropace Application of Plama Aited Combution, and DOE Combution Energy Frontier Reearch Center. Reference Shchelkin, K. I., Initiation of Detonation in Gae in Rough Tube, Technical Phyic, Vol. 7, No. 5, 97, pp. 63. Zeldovich, Y. B., Librovich, V. B., Makhviladze, G. M., and Sivahinkii, G. I., On the Onet of Detonation in a Nonuniformly Heated Ga, J. Appl. Mech. Tech. Phy., Vol.,No., 970,pp Oran, E. S. and Gamezo, V. N., Origin of the deflagration to detonation tranition in ga phae combution, Combution and Flame, Vol. 8, No.,007, pp. 7. Lee, J. H. S., Initiation of Gaeou Detonation, Annual Review of Phyical Chemitry, Vol. 8, 977, pp Rakitin, A.E. and Starikovkii, A.Y., Mechanim of Deflagration to Detonation Tranition Under Initiation by High Voltage Nanoecond Dicharge, Combution and Flame, Vol.55, 008, pp , doi:0.06/j.combutflame Rakitin,A.E. and Starikovkii,A.Y., Gradient Mechanim of Detonation Initiation for PDE Application, 8th AIAA Aeropace Science Meeting and Exhibit,00, paper Alekandrov N.L., Kinduheva S.V., Nudnova M.M. and Starikovkiy A.Yu. Mechanim of ultra-fat heating in a nonequilibrium weakly-ionized air dicharge plama in high electric field. J. Phy. D: Appl. Phy. 3 (00) 550