observations of the emergence of detonation from a turbulent flame brush

2809

Proceedings of the Combustion Institute, Volume 29, 2002/pp. 2809–2815

OBSERVATIONS OF THE EMERGENCE OF DETONATIONFROM A TURBULENT FLAME BRUSH

GERAINT THOMAS and RICHARD BAMBREYDepartment of Physics

University of Wales AberystwythCeredigion SY23 3BZ, UK

Experimental schlieren images and pressure records are presented from tests where a detonation wasobserved to emerge from a turbulent flame brush. This paper describes how an initial flame front formedfollowing autoignition behind a reflected shock wave, at a point away from the reflecting end wall. Thetest gas was stoichiometric ethylene/oxygen diluted with 75% argon. Typical reflected shock pressures andtemperatures were 110 kPa and 947 K, respectively. This paper then describes in detail the sequence offlame propagation, compression wave formation, and eventual onset of detonation events that were ob-served. Estimates of flame front velocity and changes in the velocity of unburned ahead of the flame gasare estimated from the schlieren images and found to be of the order of several hundreds of meters persecond, respectively. Just prior to ignition, the flame accelerates rapidly forming a steep compression frontat which point a detonation is seen to form. This paper discusses factors that could contribute to weak ornon-ideal ignition away from the wall and considers how chemical and gas dynamic processes could con-tribute to the final transition to detonation event. Possible factors considered are spatial variations in extentof pre-exothermic multistep chemical reactions and propagation of a flame front through such a region.

Introduction

In a recent paper, Thomas et al. [1] described howdetonation could result following the passage of firstan incident and then a reflected shock through aninitially spheroidal and essentially laminar flamebubble. Increasing the incident and reflected shockvelocities led to an ever increasing intensity of com-bustion at a now highly convoluted flame front. Be-yond a critical incident shock velocity, detonationwas observed to emerge from the highly turbulentflame brush. Detailed numerical simulations byKhokhlov et al. [2], using a similar interaction con-figuration, showed clearly that the main cause of am-plification of the rates of chemical reaction was thegeneration of acoustic instabilities by Kelvin-Helm-holtz and Richtmeyer-Meshkhov instabilities, ampli-fying the initial baroclinic vorticity [1–3].

Unfortunately, despite the ability to generate anonset of detonation during certain tests if criticalconditions were exceeded, the complexity of themultiple internal shock process and the intensity ofthe resulting shock enhanced combustion emissionmeant that the final onset of detonation could notbe observed in any detail. A summary of other testsattempting initiation of detonation was given morerecently by Thomas and Bambrey [4].

One experimental approach that gave reproduci-ble onset of detonation events involved autoignitionaway from the end wall of a shock tube operated inreflected shock mode. In this paper, we report in

detail the result of one such experiment, in whichpremature autoignition of the reflected shock gasgave rise to a turbulent like reaction front. After afurther delay, a detonation wave was observed toemerge from the accelerating reaction front, in amanner conducive to detailed study. This observa-tion of detonation emergence is similar to those ob-tained in several previous studies of reflected shockignition events [5,6]. The present work also consid-ers further why and how the events could arise andevolve as they do. The paper therefore describes theexperimental observations in detail and discussespossible mechanisms to account for the observed ig-nition, local acceleration of the reaction front, andsubsequent onset of detonation.

Experimental Details

The experiments were undertaken using a stan-dard shock tube arrangement, described in detail byThomas et al. [1] and Thomas and Bambrey [4]. Thetest section was 76 � 38 mm in cross-section andincluded a 230 mm long window section for opticalaccess. In addition to schlieren photography, diag-nostics included pressure and light emission sensors.The main differences between the present experi-ments and those reported earlier were, first, no pre-existing spark ignited flame and, second, the mixturetested was stoichiometric ethylene/oxygen dilutedwith 75% argon.

2810 DETONATIONS AND SHOCK INDUCED COMBUSTION

Fig. 1. Spark schlieren images with strong or ideal ig-nition at the rear reflecting wall. Initial pressure 5.3 kPa,incident shock Mach number 2.64, reflected shock pres-sure 190 kPa, and temperature 1273 K. Time after shockreflection: frame 1, 150 ls; frame 2, 175 ls; and frame 3,225 ls.

Fig. 2. Spark schlieren images with mild or non-idealignition behind a reflected shock in C2H4 � 3O2 � 12Ar.Initial pressure 5.3 kPa, incident shock Mach number 2.31,velocity 738 ms�1, reflected shock pressure 123 kPa, andtemperature 1083 K. First frame, 1.2 ms after incidentshock reflection from rear wall (immediate right of eachimage). Frame interval 50 ls.

Experimental Results

Schlieren

In reflect shock studies of autoignition, it is usuallyassumed that ignition occurs at the rear reflecting

wall. The usual procedure is to note the time atwhich the shock reflects from the end wall and thesubsequent time at which reaction is first observed,usually determined by monitoring pressure or com-bustion emission.

When observed using a conventional schlieren sys-tem, the anticipated sequence of events is as shownin Fig. 1. Here the arrival and reflection of the in-cident shock wave are shown and, after a short delay,the onset of reaction that in this case accelerates rap-idly to detonation. The mixture was C2H4 � 3O2 �12Ar and the incident shock Mach number was 2.64giving a reflected shock temperature and pressure of1273 K and 190 kPa. This form of behavior was de-scribed as strong ignition by Vermeer and Oppen-heim [5] among others.

Ignition in the reflected gas at points away fromthe end wall is also observed, as in the present study.For reasons that will be clarified in a later section,we term these non-ideal ignitions. An example ofsuch an event is shown in Fig. 2. Here the reflectedshock, which moved to the left as viewed, has alreadyexited the test section window. The reflecting endwall is located to the immediate right of the images.In this test, ignition was first observed after a delayof 1200 ls, relative to the time the incident shockwave reflected from the end wall. Initially a smallignition kernel is seen to form, which grows steadilyover a period of some 440 ls. This sequence ofevents is not unique, and a number of similar imageswere obtained for reflected shock temperatures inthe range 900–1100 K.

However, for certain test conditions, flame frontpropagation rates were observed to increase dra-matically at later times with compression frontsformed ahead of the reaction front followed shortlythereafter by the emergence of detonation. An ex-ample of such an onset of detonation event is shownin Fig. 3. In this sequence of images, the first framewas taken 1788 ls after reflection of the incidentshock at the end wall. The subsequent framing in-terval was 15 ls. In this test, the initial pressure was5.3 kPa, the incident shock Mach number was 2.2,and the theoretical reflected shock temperatures andpressures were 110 kPa and 974 K, respectively.

By the first frame of Fig. 3, the autoignition kernelhas already grown to the point where the reactionfront has touched both side window of the test sec-tion (38 mm apart) and upper and lower walls(76 mm apart). The area of contact with the glasswindow is identified by the disappearance of a tur-bulent-like structure at the center of the projectedreaction volume. Also seen on this and subsequentimages is a small spherical flame kernel, some15 mm in diameter, close to the upper wall and tothe right of the main reaction volume.

After an additional 15 ls, a length of the reactionfront away from the upper wall has propagated quiterapidly and a localized compression front has started

OBSERVATIONS OF THE EMERGENCE OF DETONATION 2811

Fig. 3. Spark schlieren images showing the emergenceof detonation in reflected shock region, gas mixture C2H4

� 3O2 � 12Ar. Initial pressure 5.3 kPa, incident shockMach number 2.2, velocity 700 ms�1, reflected shock pres-sure 110 kPa, and temperature 974 K. First frame, 1.8 msafter shock reflection from end wall. Frame interval 15 ls.Pressure gauge location along botom wall is indicated onthe final frame.

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Fig. 4. Pressure time history, obtained 40 mm from theend wall, during test shown in Fig. 3.

to form, inclined at 45� to the vertical, between thespherical kernel and the main reaction volume. Thisis seen to have advanced after an additional 15 ls,overlaying the spherical kernel. As the images are atwo-dimensional projection of three-dimensionalevents, it is surmised that the accelerating reactionfront and the flame kernel do not lie in the samevertical planes and are displaced away from eachother across the width of the test section. In addition

to the accelerating reaction front seen in the upperhalf of the test section window, a second compres-sion front also forms in the lower half.

Detonation was seen in the next frame, obtainedafter an additional 15 ls, and appears to to haveemerged from a point along a line, normal to theviewing direction, defined by the intersection of theupper compression front and the spherical kernel.As an outline of the smooth spherical kernel is stillclearly superimposed within the detonation bubblevolume, the latter is not believed to have been di-rectly involved in the final onset of detonation.

Pressure

A pressure record obtained during the test inwhich the images in Fig. 3 were recorded is pre-sented in Fig. 4. The records shows both the inci-dent and the initial reflected shock. The main pres-sure increases due to combustion and transition todetonation can also be seen. There is also a secondpressure major pulse whose exact origin is uncertainbut which can be shown to have propagated from asecond explosive event upstream of the window sec-tion. Also indicated on Fig. 4 is an earlier gradualpressure increase, commencing some 200–300 ls af-ter the passage of the reflected shock. It should benoted that the gauge was placed 40 mm from theright-hand edge of the schlieren images and on thebottom as indicated in Fig. 3.

Discussion

The reaction front images presented in the pre-ceding section show an autoignition event that leadsto an onset of detonation. Similar images and asso-ciated pressure histories have been obtained previ-ously by other researchers and have been variouslydescribed as spotty or weak ignition. They differfrom strong ignition in that autoignition is followed


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Fig. 5. (�) Measured autoignition delay times behind areflect shock in C2H4 � 3O2 � 12Ar plotted against in-verse temperature, after Brown and Thomas [7]. Solid lineis correlation due to Hidaka et al. [10]. (�) Actual delaytime based on the residence time in the reflected shock ofthe first gas element observed to react in Fig. 3.

Fig. 6. Schematic distance-time diagram showing resi-dence times of a gas particle located away from the endwall in the incident shock t� and reflected shock t�. Timesare referenced to the instant that the incident shock is re-flected from the closed end wall, t0.

by a slower deflagration phase, there is no rapid orinstantaneous appearance of detonation, and thepressure transients are less violent.

Unfortunately, although the occurrence of suchfeatures been described in several previous reflectedshock studies, less attention has been paid to under-standing why the ignition kernels are observedwhere they are, away from the back wall. To drawattention to the uncertainties in our understandingof what causes weak or spotty ignition, we term idealignition that where ignition occurs first at the endwall and non-ideal when ignition it is remote fromthe wall. One contributory factor is the difference inparticle residence histories throughout the reflected

shock volume, along the tube axis, a matter discussedin more detail in a later section.

Autoignition behind Reflected Shock Waves

Autoignition delays for C2H4 � 3O2 � 12Ar ob-tained using the present shock tube apparatus havealready been reported by Brown and Thomas [7].Their measured delays plotted against reciprocaltemperature are presented in Fig. 5, supplementedby more recent unpublished data from this labora-tory. Also plotted as a solid line is a correlation dueto Hidaka et al. [8].

Plotted in Fig. 5 is a single point (solid symbol)corresponding to the actual autoignition delay timeof the gas at the point where reaction was first de-tected on the schlieren image in Fig. 3. Unlike theother measurements, this specific delay time wascomputed from the actual residence time of the gaselement in the reflected shock gas, not the time sincereflection. The basis for this calculation is illustratedin Fig. 6, which defines the residence times of anygas element away for the reflecting end wall in firstthe incident and then the reflected shock wave. Agas particle is overtaken by the incident shock a timet� before the shock reaches the end wall. The gasparticle is eventually engulfed by the reflected shockat t�. From this schematic, it is obvious that signifi-cant spatial gradients in extent of reaction must existalong the shock tube axis, due to the different resi-dence times at each plane away from the reflectingend wall. Thus, even for an ideal shock tube, it isincorrect to treat the reflected shock gas volume asa spatially uniform reactor at the ideal reflectedshock temperature and pressure.

Non-ideal Ignition

As illustrated above, when an ideal shock tube isoperated in the reflected shock mode, gas ignitionmust occur first at a point close to the end wall. How-ever, several investigators have reported instanceswhere the ignition point moves, as in Figs. 2 and 3,away from the reflecting end wall [4–6,9], a phenom-enon which we term non-ideal ignition.

One consequence of such non-ideal ignitions isthat estimates of autoignition delay times will be afunction of the measurement location. Experimen-tally, ambiguity in the determination of the true au-toignition can arise if multiple observation locationsare used. Petersen et al. [9], for example, were awareof this and compensated by extrapolating their au-toignition delay times to events occurring at the endreflecting wall. Allied with this is the spatial variationin the extent of preautoignition reactions.

The result of these complicating phenomena arewell illustrated by the present observations. In Fig.7a, the pressure record shown in Fig. 4, obtained ata point 40 mm from the end wall, is reproduced with


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Fig. 7. Records from test shown in Fig. 4 at increasetime resolution. (a) Pressure gauge output, 40 mm fromthe end wall, and vertical markers corresponding to imagesin Fig. 3. (b) Photodiode outputs from gauges at the rearwall (solid) and midpoint of window section (dashed).

greater time resolutiuon. Fig. 7b shows correspond-ing outputs from photodiodes located at the closedend wall and at the midpoint of the window section.If only the end wall emission were noted, the delaywould be greater than actually obtained in practice.Even the use of a delay time from the photodiodegauge located closest to the the ignition plane givesan incorrect absolute value for the autoignition delaytime, as this does not take account of the true resi-dence time a gas element at reflected shock condi-tions. There is clear evidence of early reaction onthe pressure records, indicated by a gradual pressureincrease prior to the major pressure excursion. Theimages in Fig. 3, whose relative timing are indicatedby fiducial markers on Fig. 7a, show clearly that themain pressure peak at a gauge close at the end wallis due to a flame acceleration event and not pureautoignition.

The reason usually advanced for the occurrenceof spotty ignition kernels is the sensitivity of ignitiondelay time s to variations in temperature, T. Previous

investigators have considered that stochastic varia-tions in the temperature field and sensitivity inds/dT will give a number of random ignition loca-tions behind the reflected shock. In practice, this isunrealistic because gas in a turbulent field will fluc-tuate about a mean, not achieve a steady-state tem-perature within a distribution of temperatures. Theidea of a constant temperature finite duration igni-tion hot spot is therefore not a credible one. How-ever, a decrease in overall ignition delay time s dueto fluctuations in temperature over a characteristictime scale shorter than s is still possible. Even so, ifstatistical variations contribute, an additional factormust still be included to account for ignition movingaway from the reflecting end wall; otherwise, igni-tion will fluctuate about the ideal delay time at theend wall.

Other influences could be variations in chemicalreaction rates at intermediate temperatures [10,11]and shock-boundary layer interactions, as discussedby Gamezo et al. [12], both of which could enhancethe influence of turbulence on finite rate homoge-neous gas-phase chemistry as outlined above.

Emergence of Detonation

As a result of the non-ideal ignitions describedabove, distinct reaction fronts are seen to develop inthe middle of the test section. Unfortunately, due tothe two-dimensional projection of a three-dimen-sional event, it is not possible to determine exactlyin which plane parallel to the test section windowsignition occurs. The reaction fronts initially evolvegradually, with the reaction front velocity betweenthe first two frames of Fig. 3 of the order of 350 �150 ms�1.

Also evident in the images in Fig. 3 is a small flamebubble. This plays no part in the transition processbut is useful in providing some indication of the bulkvelocity in the unburned mixtures. This indicates amean unburned gas velocity increasing from ap-proximately 0 to 200 � 10 ms�1 over the time lead-ing to the final onset of detonation.

In the second frame of Fig. 3, a compression frontis seen to form in the upper half of the tube windowahead of the reaction front and to the left of theflame bubble. By the third frame, a second com-pression front is forming in the lower half of thetube. From motion between frames, it is possible toestimate the velocity of the compression front, whichis of the order of 870 � 60 ms�1. The sound speedin the initial shocked reflected gas was 552 ms�1 but,as can be seen in Figs. 4 and 7, the gas pressure hasincreased to between 2 and 2.5 bar by this point intime. Unfortunately, the pressure records shown inFigs. 4 and 7 were obtained at the lower wall asviewed in Fig. 3, and the pressure histories relatingto compression front formation and onset of deto-nation in the upper half of the tube are not availableexplicitly.


Nevertheless, using these as a guide, and assumingadiabatic compression, the sound speed would to becloser to 620 ms�1. Given the local gas velocitiesestimated above, the compression front is propagat-ing into the reflected shock gas at close to sonic ve-locity. Also significant is the change in unburned gastemperature due to the adiabatic compression dueto work done by the partial expansion of burnedgases behind the reaction front. Considering frames3 and 4 in Fig. 3 and the corresponding pressuresgiven from Fig. 7a, the gas temperature would beincreased to 1195 K at 2.2 bar and 1240 K at 2.5 bar.The ideal steady-state autoignition delay at thesetemperatures would now be of the order of 80 ls, asignificant reduction from that at the initial reflectedgas temperature of 974 K.

The exact moment of onset of detonation cannotbe observed exactly in Fig. 3, but transition appearsto have occurred between the compression front andthe reaction front behind it. The flame bubble didnot appear to play a role as it has not been fullyengulfed by the detonation and is partially visible inframe 4 of Fig. 3. The velocity of the detonation,using an initiation point estimated by ray tracing, wasapproximately 1890 ms�1 compared to a theoreticaldetonation velocity at the reflected gas conditions of1687 ms�1

A key factor in the observed events is a clear ac-celeration in the propagation rate of a section of theconvoluted reaction front, leading to the formationof a compression front and hence onset via a local-ized hot spot transition. It is also interesting to spec-ulate whether the spatial variations in extent of re-action discussed above in respect of non-idealignition could have contributed to the localizedflame front acceleration, as the flame propagatedinto a region with a gradient in extent of pre-exo-thermic multistep chemical reaction. Now, unlikethe Zeldovitch or mechanism where a reaction-phase wave propagates into a negative gradient inextent of reaction, the present ignition locationwould give rise to a flame propagating into an evolv-ing reaction field with a positive gradient in extentof reaction with respect to the direction of wavefront propagation.

Conclusions

The present tests, based on reflected shock tech-niques, have provided a useful means of undertakingdetailed observation of the onset of detonation.However, even though many of the problems asso-ciated with studies of transition following prolongedflame acceleration have been eliminated, the finalonset events are still highly stochastic at the timeresolution required to capture onset. Nevertheless,

the test have provided valuable quantitative data onthe local evolution of reaction fronts and the sur-rounding gas dynamic flow field over the times lead-ing up to onset of detonation.

Two key features contribute to the evolution offinal onset of detonation: first, an as yet unexplainedearly ignition away from the end wall for a reflectshock autoignition test, possibly linked to changes inchemistry at intermediate temperatures, 750–1100 K [10,11], modulated by gas dynamic pro-cesses; and second, a rapid local acceleration of aturbulent-like reaction front that leads to localizedcompression as the reaction products expand. It isinteresting to note that in almost all cases greaterflame acceleration, which led to the onset of deto-nation, was observed in the gas volume between thereaction front and the end wall. This suggests a pos-sible chemiacoustic interaction or, perhaps, an influ-ence of chemical origin in the gas with significantresidence time in the reflected shock.

Acknowledgments

Financial support from the U.K. Engineering and Physi-cal Research Council GR/L89242 is gratefully acknowl-edged. We are also grateful to Dr. Steven Box for additionalethylene autoignition delay data included in the paper.

REFERENCES

1. Thomas, G. O., Bambrey, R. J., and Brown, C. J., Com-bust. Theory Modelling 5:573–594 (2001).

2. Khokhlov, A. M., Oran, E. S., and Thomas, G. O.,Combust. Flame 117: 323–339 (1999).

3. Bately, G. A., McIntosh, Ac., and Brindley, J., Proc. R.Soc. London Ser. A 452:199–221 (1996).

4. Thomas, G. O., and Bambrey, R. J., Shock Waves, inpress, (2002).

5. Vermeer, D. J., Meyer, J. W., and Oppenheim, A. K.,Combust. Flame 18:327–336 (1972).

6. Fieweger, K., Blumenthal, R., and Adomeit, G., Com-bust. Flame 109:599–619 (1997).

7. Brown, C. J., and Thomas, G. O., Combust. Flame117:861–870 (1999).

8. Hidaka, Y., Kataoka, T., and Suga, M., Bull. Chem. Soc.Jpn. 47(9):2166 (1974).

9. Peterson, E. L., Davidson, D. F., and Hanson, R. K.,J. Propul. Power 15:82–91 (1999).

10. Cadman, P., Thomas, G. O., and Bulter, P., Phys.Chem. Chem. Phys. 2:5411–5419 (2000).

11. Goy, C. J., Moran, A. J., and Thomas, G. O., Auto-Ignition Characteristics of Gaseous Fuels at Represen-tative Gas Turbine Conditions, ASME paper 2001-GT-0051, 2001.

12. Gamezo, V., Khokhlov, A. and Oran, E. S., Combust.Flame 126:1810–1826 (2001).


Elaine Oran, NRC, USA. Do you know of any quanti-tative measurements of heat losses to walls, and tempera-ture gradients at walls, in shock-tube experiments?

Author’s Reply. The potential effects of heat and mo-mentum transfer on shock flow near walls are well known.There is an extensive body of published work on this topicfor hypersonic flows over surfaces, but there is little quan-titative data and supporting detailed model validation avail-able for simple shock tubes. Some work on heat transferfrom detonations in shock tubes was reported by Edwardset al. [1], while the most recent heat transfer measurementsof relevance to shock flows were reported by Buttsworth

[2]. An illustration of the effect that heat transfer couldhave on chemical reaction was reported by Thomas et al.[3], where heat transfer to a cold wall was observed to delayslightly the onset of reaction in a 1 mm thick layer close tothe wall.

REFERENCES

1. Edward, D. H., Brown, D. R., Hooper, G., and Jones,A. T., J. Phys D. Appl. Phys. 3:365 (1973).

2. Buttsworth, D. R., Shock Waves 12:87 (2002).3. Thomas, G. O., Ward, S., and Williams, Rh. L., and

Bambrey, R. J., Shock Waves 12:11 (2002).

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observations of the emergence of detonation from a turbulent flame brush

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