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ADVANCES IN UNDERSTANDINGOF FLAMEACCELERATION
FOR THE IMPROVING OF THE COMBUSTION EFFICIENCY
C. GERLACH, A. EDER, M. JORDAN, N. ARDEY, F. MAYINGER
Lehrstuhl A f�ur Thermodynamik, Technische Universit�at M�unchen85747 Garching, GermanyPhone: +49 89/289 16229, Fax +49 89/289 16218E-mail: [email protected]
1. Abstract
The propagation of gaseous explosions is governed by the interaction ofchemical kinetics with the molecular and turbulent heat and mass trans-port. Combustion processes like de agration and detonation depend onthe di�erent valence of physical e�ects under certain conditions. Geome-try and the expansion ow of the ame itself a�ect the turbulence andtherefore the transport of fuel into the reaction zone. The present paperdiscusses the di�erent hydrogen combustion processes and reports on theexperimental investigations of transport phenomena during ame propa-gation with highly blocking obstacles. Several facilities have been operatedwith sophisticated optical measurement techniques like high speed schlierenvideographie, laser induced predissociation uorescence and laser dopplervelocimetry to obtain detailed information about the combustion process.It will be shown that the turbulent quenching of ames leads to an amountof free radicals resulting in sensitive clouds of those radicals with corre-sponding high chemical reaction rates, which has a strong in uence on thee�ciency of the combustion processes.
2. Introduction
Combustion processes can be devided into two di�erent kinds, de agrationand detonation. The de agration is due to the velocity (subsonic speed)of the ame front more controllable compared to a fast de agration or de-tonation (supersonic speed). A detonation leads to highest pressure peaks,which is desired in many technical applications. The transition from the
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de agration to the detonation (DDT) is di�cult to predict due to the de-pendency on several parameters like geometry, mixture and the thermody-namical quantities. Therefore DDT processes are of great interest for theimproving of the combustion e�ciency as well as for the safety design ofindustrial systems and buildings.
Several projects for the observation of hydrogen combustion e�ects havebeen carried out at the Lehrstuhl A f�ur Thermodynamik ([1], [2], [3]). Atthe moment several facilities are in operation for the observation of turbu-lent de agrative combustion as well as DDT processes like shock focusing,critical ame acceleration and hot-jet ignition.
3. De agration, Detonation and the Transition from De agra-tion to Detonation
3.1. DEFLAGRATION
A laminar ame front propagates transversal to the ame surface with acharacteristic laminar ame velocity sl depending on the reaction rate, massand thermal di�usivity, temperature and pressure of the unburned gas.
A relative small Lewis number of Le = 0:3 � 0:5 (ratio of thermaldi�usivity a to mass di�usivity D) expresses the more important massdi�usion term for a H2/Air mixture. The unburned gas is ignited by theheat release of the combustion process. Due to the viscosity and the heatloss by the surrounding walls the surface of the ame front tends to curve.So the e�ective ame velocity increases due to a ratio of the ame surfaceAl to the cross-section of a geometry A0.
Pressure waves are originated from the ame itself with ongoing propa-gation. The ame front starts to wrinkle and to create a cellular structure.A higher pressure gradient across the ame front leads to a more cellularstructure [7]. Figure 1 shows this e�ect for several H2 concentrations in anexplosion tube.
At higher velocities of the expansion ow the in uence of turbulenceincreases. Macro scale vortices (approximately 10% of the tube dimension)extend the surface of the ame front, which leads to a higher e�ective amevelocity. Micro vortices in the dimension of the reaction zone (Kolmogorovmicro scale �k) increase the mixture of burned and unburned gas and alsoaccelerate the combustion process. Nevertheless the ow consists of a con-tinous spectrum of vortices from micro to macro scale, a classi�cation inlength scales eases the characterization of the combustion process.
If the chemical reaction time exceeds the lifetime of the micro vor-tices, which is expressed in the Karlovitz number Ka > 1 (ratio of che-mical reaction time �c to the lifetime �k of the Kolmogorov micro vortices),the in uence of quenching e�ects extends. The high turbulence leads to a
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Figure 1. High speed schlieren photographs of H2/Air ames at several H2 concentra-tions
quenching of the ame due to the mixing of cold unburned gas with thereaction zone. Therefore the temperature decreases and the ignition tem-perature can not be reached. This leaves free radicals (OH, O, H etc.) thatcan build highly sensitive areas with the surrounding unburned gas.
3.2. DETONATION
The Detonation di�ers essentially from the de agration process. The amefront is strongly coupled to the leading shock front. Due to the increa-sing temperature of the gas mixture in the shock system it ignites after acharacteristic induction time (depending on the mixture) behind the shockfront. The reaction zone has to release as much energy as needed for theshock system to maintain the ignition conditions for a stable detonation.
The detonation also maintains due to transversal components of theshock front which are re ected at the surrounding walls. They superimposewith other re ected shock components. The resulting regular pattern of thesuperimposing shock system can be visualized with smoked foils and givesa good impression of the course of the detonation. The cell size �d of thispattern only depends on the gas mixture. The minimum tube diameter tokeep up a detonation were determined in di�erent scales of explosion tubes
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([13], [14], [16]).Figure 2 shows the typical propagation of a detonation in the Preheatable
Detonation Tube (PhD) at the Lehrstuhl A f�ur Thermodynamik. It has alength of 6 m and a inner diameter of 66 mm. The photo is taken by meansof schlieren techniques through a window section of 60 � 50 mm.
Figure 2. Schlieren image of a detonation propagating in the PhD Tube (20 H2 vol.-%in air)
3.3. DEFLAGRATION TO DETONATION TRANSITION (DDT)
Under some circumstances a de agration with its precursor pressure wavesystem can turn into a detonation. The in uence of turbulence makes itvery di�cult to predict a possible DDT process precisely. Three di�erentDDT mechanisms are to be explained:
� shock focusing� exceeding critical ame speed� local explosions resulting from reignition of partially quenched volumes
(hot-jet ignition)
As mentioned above the superposition of shock fronts allows to reachthe conditions for the initiation of a detonation process. Special geometrieslike ba�es or cups provoke a focusing of a shock system, and lead to anignition of the burnable gas.
A de agration produces a strong precursor pressure wave system. Withthe ongoing propagation in combination with the energy release of the amefront the pressure wave can increase to a shock front. A supersonic amewith an uncoupled shock system is referred to as fast de agration. If the
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ame behind the shock system is strongly accelerated by turbulence it cancouple to the shock system and develop into a detonation. The transitionfrom de agration to detonation is visualized in �gure 3. The ame accel-erates over a run-up length of 3 m with annular obstacles (BR = 60%,obstacle path length 2 m, distance between the obstacles d = 185 mm).
Figure 3. Selected frames of the transition from de agration to detonation by ameacceleration in the PhD tube (15.9, 16.0 and 16.1 vol.-% H2)
At highly blocking obstacles (BR�90%) the observed combustion pro-cesses can not only be described by increasing of the turbulence. The e�ec-tive turbulent burning velocity behind a jet producing ori�ce di�ers fromthe turbulent velocity induced by the expansion ow. Several experimentshave been carried out to obtain more information about the Hot-Jet igni-tion mechanism, which will be described in the following.
4. Experiments at the L.View Facility
The L.View facility in Pisa consists of a rectangular test section (677 �677 � 3200 mm) which is divided into two chambers. The �rst chamberhas a length of 1050 mm and is seperated from the second chamber by awall with a central round ori�ce with a diameter of 100 mm, resulting in ablockage ratio of BR = 98:3%.
The second chamber has a weak rupture disk to the ambient with thedimensions of 300�300 mm. Two axial fans inside the dividing wall ensure ahomogenous H2/Air mixture of the same equivalence ratio in both chambersbefore ignition. To visualize the ame propagation a video camera with aframe rate of 25 Hz is used. The combustion is recorded simultaneouslythrough the front windows and, re ected by a 45� mirror, through the topwindows. To enhance the contrast, aerosols of a NaCl solution are added,which are stimulated to emit light at the high combustion temperature. The ow velocity is measured inertialess and non-intrusively for the horizontalcomponent parallel to the main ow and the vertical component at threedi�erent positions (]1� ]3 in �gure 4) with a two component LDV system.
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Figure 4. Selected frames of a 9 vol.-% H2 in air ame at the L.View facility with anori�ce (BR = 98:3%)
In �gure 4 selected frames of a H2/air ame (9 vol.-%) are shown. Themixture was ignited at the lower right corner.
About 300 ms after ignition the ame reaches the ori�ce. Due to thehigh penetration velocity of the gas through the ori�ce, a sudden ignition inthe second chamber would be expected. However, during 240 ms betweenpicture 3 to 5 a ame jet is observed in the second chamber, but no exten-ding ame propagation. Then, 600 ms after the ignition of the �rst chamber,the gas in the second chamber ignites with a very high reaction rate.
The relatively low repition rate of the video camera allows no su�cientresolution of the process. With a methane/air mixture (lower laminar bur-ning velocity sl) it can be observed that the ignition process in the secondchamber happens at distinct points behind the ori�ce. High speed video
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records will be taken at the L.View facility to resolve these e�ects on hy-drogen, too.
5. Experiments at the PuFlaG Facility
Corresponding to the investigations at the L.View facility measurementshave been made at the PuFlaG (Pulsed Flame Generator) facility. Due tothe smaller scale of the experimental setup it is possible to apply highlysophisticated optical measurement techniques. The tube with an inner di-ameter of 80 mm is equipped with four quartz windows (190�60 mm). Dif-ferent obstacles with a central ori�ce of varying diameters causes a blockageratio between 95% and 99.7%.
Figure 5. Selected frames of a ame (self uorescence); left: 12 vol.-% H2 in air; right:12.2 vol.-% H2 in air ame at the PuFlaG facility (BR = 99:7%)
The total gas volume is about 25 liters in front of the obstacle andabout 36 liters behind the obstacle. Four high-speed piezo-capacitive pres-sure transducers are used to record the pressure history during the com-bustion process.
Due to the character of schlieren records, visualizing only the densitygradient, it is impossible to distinguish between exhaust gas and the ameitself. Therefore self uorescence of the OH-radical, which is a characteristicspecies in the chemical reaction of the hydrogen/air combustion processes,
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has been used to obtain information about the ame itself. The very weakemission of the radical had to be ampli�ed by a two-stage intensi�er infront of the video camera.
In �gure 5 selected frames of a 12 vol.-% H2 ame compared to a 12.2vol.-% H2 ame are shown. Both ames have approximately the same ve-locity in front of the ori�ce (approximately 3.3 m/s) but as the richer amereignites after a certain time (2 ms after the ame reaches the ori�ce), theleaner mixture is quenched completely. The ignition of the second chamberoccurs approximately 20 ori�ce diameters behind the obstacle. After an ig-nition in the second chamber a very high combustion rate can be observed.
Figure 6. Propagation velocities during the combustion of various H2/air mixturesthrough an ori�ce (BR = 97%)
The resulting high velocities as shown in �gure 6 have been obtained bymeasuring the propagation of the ame from frame to frame in the schlierenrecords. The velocity strongly increases to a maximum behind the ori�ce.With the expansion of the ame to the wall the axial propagation decreases.DDT processes are not object of the investigations at this facility and havenot been observed.
6. Experiments at the MuSCET Facility
Experiments in the MuSCET facility, a horizontal square cross section ex-plosion tube (268�268 mm) with a length of 3.5 m, were performed toimprove the understanding of the in uence of ow obstacles on ame pro-pagation.
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Figure 7. LIPF and high speed schlieren records behind the ori�ce (BR = 85%, 16Vol.-% H2 in Air)
At obstacles with a blockage ratio above 50% a strong contraction of theexpansion ow in front of the obstacle could be observed. The obstacle leadsto a acceleration of the ow in combination with an increasing turbulencein the shear layer of the jet after the ori�ce. In this case the turbulence issu�cient for the quenching of the ame.
Figure 7 shows the accumulation of highly reactive OH-radicals in a zone
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below the ori�ce. For the visualization of the OH-radicals Laser InducedPredissociation F luorescence (LIPF) is used. The OH-radicals are excitedwithin a thin lightsheet (thickness � 1 mm) by a wavelength of 248 nm witha tunable KrF excimer laser. The excitiation is realized by the transitionA2�+; V = 3 � X2�; V = 0 (P1(8)). The uorescence of the OH-radical(A2�+; V = 3 �! X2�; V = 2) at 295 - 304 nm is recorded by an UVintensi�ed CCD camera applied with a special re ectance �lter to avoidadditional uorescence and rayleigh scattering signals. The pulse of thelaser durates for about 17 � 10�9 s, the lifetime of the excited state is inthe range of 10�10 � 10�5 s [19]. Therefore the ow situation seems to be'frozen' at the time of record.
The mixture of the OH-radicals with the surrounding unburned gasbuilds a high sensitive cloud that simultaneously ignite forming a localexplosion as seen in the Schlieren records. The LIPF pictures were takenas a series of single shots triggered at di�erent time to give an impressionof the propagating combustion process.
Several local explosions can amplify themself transitting to a globaldetonation in the tube if the sensitive cloud volume is su�ciently large.Initial conditions for the spherical detonation from a local ignition werementioned by Desbordes [17]. The process to the fully developed detonationis referred to as 'Hot-Jet Ignition'.
7. Conclusion
The main goal of this experimental work is to deliver su�cient data fora numerical simulation of all kinds of combustion processes. At this verymoment di�erent codes for de agration and detonation processes have tobe used due to the di�ering nature of de agration and detonation. So farqualitative information as well as conservative limits for the DDT had beencollected, but more quantitative data is needed to correctly predict a DDT.It has been shown that the DDT process consists of several physical phe-nomena, which are di�cult to describe. These e�ects have a very strongin uence on the combustion process and the transition point between de- agration and detonation so that they have to be taken into considerationfor the further development of integrated de agration/detonation codes.For the validation of quenching e�ects at larger scales a new facility calledHot-Jet Explosion Tube (transition from a tube with a diameter of 200 mmand a length of 4 m via an ori�ce to a tube with a diameter of 500 mmand a length of 3 m) has been set into operation at the Lehrstuhl A f�urThermodynamik.
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Acknowledgements
It is gratefully acknowledged, that the work presented in this paper hasbeen supported by the German Ministry of Education, Science, Researchand Technology BMBF and the European Comission.
Nomenclature
Le Lewis number, Le = a=Da thermal di�usivityD mass di�usivitysl laminar burning velocityAl Surface of the ameA0 Cross-section of the tube� densityp pressure�k Kolmogorov micro scaleKa Karlovitz number, Ka = �c=�k�c Chemical reaction time�k Lifetime of the Kolmogorov micro vortices�d Detonation cell widthBR blockage ratio, ratio between blocked and unblocked area
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