Temperature fields during the development of combustion in a rapid compression machine

Temperature Fields During the Development of Combustionin a Rapid Compression Machine

J. CLARKSON, J. F. GRIFFITHS*, J. P. MACNAMARA, and B. J. WHITAKERSchool Of Chemistry, The University, Leeds, LS2 9JT UK

Temperature and concentration fields have been imaged by Rayleigh scattering in one-dimension on a line andby laser induced fluorescence (LIF) of acetone in a 2-D sheet across the diameter of the cylindrical combustionchamber in a rapid compression machine. Experiments were performed in non-reactive and reactive conditions.To investigate the development of combustion, the exothermic decomposition of di-t-butyl peroxide vapordiluted by inert gas was studied. This reaction is characteristic of a conventional thermal ignition. Acetone isa major product. Inert gas mixtures, to study the temperature field in the absence of reaction, were seeded withacetone. The evidence from the experimental results supports the following interpretation. As the piston of themachine moves, it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but moreimportantly it also collects cool gas from the walls and moves this gas across the cylinder head pushing it forwardinto a plug at the center. Once the piston stops, there is a stratified component at the center, which is slightlycolder than the bulk of the gas, and for a short time afterwards there is very limited mixing by bulk transportof gas from one part to another, because the gas velocity is not very high. Diffusive transport will occur, but thetimescale is relatively slow, and the effect hardly shows before 20 to 25 ms after the end of compression. Theeffect (on the combustion of di-t-butyl peroxide) of this “temperature stratification” at the core of the cylinderis that the reaction develops more slowly in the center than elsewhere. The onset of reaction in a toroidal regionis shown unambiguously, and thermal runaway is initiated there. This is demonstrated by LIF measurementsthrough the central plane of the reaction cylinder. From the study of inert mixtures seeded with acetone, it isshown also that the colder core lies just ahead of the piston crown, but it does not reach the central plane until1 ms after the piston has stopped. Rayleigh scattering on a 1-D line in the central plane proved to beinsufficiently sensitive to show the presence of the cooler zone resulting solely from the physical compression.However, the evidence for temperature stratification becomes unequivocal from Rayleigh scattering measure-ments made in the later stages of the peroxide decomposition. Limits of sensitivity of the scattering techniquemay be inferred from this. The physical characteristics of the compression are likely to be replicated in otherrapid compression machines and are relevant to understanding the spatial development of autoignition in suchsystems, which has implications also for numerical modeling. There are rather more complicated consequences,than is the case for thermal ignition, for chain-thermal interactions which involve development through thenegative temperature-dependent regimes, as occurs in combustion of the alkanes and that of other organiccompounds. © 2001 by The Combustion Institute

INTRODUCTION

The development of the temperature field in thecombustion chamber of rapid compression ma-chines (RCM) has been a subject of interestover many years and the way in which temper-atures of gases develop during reaction after thepiston has stopped has been a matter of debate.This is particularly relevant to the quantitativevalidity of numerical modeling using compre-hensive kinetic schemes because, in such cases,the assumption of a spatially uniform tempera-ture throughout the combustion chamber is apre-requisite [1, 2]. Such modeling is used forthe kinetic understanding of the onset of engineknock [3], and experiments in rapid compres-

sion machines are important in their own rightfor validation of these numerical studies.

Until recently the only thermometric infor-mation, which could be extracted from experi-mental measurements in RCMs related to anaverage gas temperature determined from themeasured pressure [2, 4–6] Griffiths and co-workers have asserted that fast compression(such as at a piston speed of 10 m s21) is veryclose to being adiabatic, so that at the end of thecompression stroke the temperature should beuniform throughout the chamber, and they havegiven indirect evidence for this [4]. Minetti andco-workers, at Lille, have also established thateven when compression occurs at a lower rate,there is a very substantial “core gas” which is atthe adiabatic temperature [5]. In either case,during the post compression period heat loss to*Corresponding author. E-mail: [email protected]

COMBUSTION AND FLAME 125:1162–1175 (2001)0010-2180/01/$–see front matter © 2001 by The Combustion InstitutePII 0010-2180(01)00236-X Published by Elsevier Science Inc.

the combustion chamber walls must cause thecore gas volume to contract. Exothermic oxida-tion of reactive mixtures after the end of com-pression competes with these losses, but there islittle known about the way in which the temper-ature field evolves.

Recently, the Lille group have made directmeasurements of the temperature at the end ofand following compression using both singlepoint Rayleigh scattering from a laser beam andthermocouple measurements [7, 8]. They tracedthe thermal history of the core gas by bothtechniques over a considerable post-compres-sion period in a non-reactive gas, after itscompression to an adiabatic gas temperature ofabout 700 K. The Rayleigh scattered signalswere calibrated from the signal/incident beamintensity ratio at known, adiabatic, end-of-com-pression temperatures and pressures, and the(slower response) thermocouple measurementswere validated against these data. The temper-ature field associated with combustion of 2.2.4trimethyl pentane (iso-octane) was then ex-plored using the thermocouple.

Laser Rayleigh scattering has, for some time,been used as a combustion diagnostic tool butcommonly the technique has been applied toopen, low pressure flames [9]. Other than singlepoint measurements, there are relatively fewreports of its application to high-pressure sys-tems such as engines [10]. However, 2-D, lasersheet, Rayleigh scattering in a combustionbomb has been described by Kim et al. [11].Espey et al. [12] have reported planar Rayleighimaging of fuel vapor in a diesel jet and Koch etal. [13] have applied UV Rayleigh imaging to anautomobile engine.

In the present work, the potential for Rayleighscattering as a diagnostic for temperature mea-surement along a line spanning the central planeof the cylindrical combustion chamber of an RCMwas further investigated. The major interference isfrom Mie scattering and also from specular reflec-tions of the incident laser light. The study of thespatial development of the temperature fields inthe RCM by Rayleigh scattering, under bothnon-reactive and reactive conditions, was sup-ported by laser induced fluorescence (LIF) ofacetone (2-propanone) to characterize the tem-perature and concentration fields.

The first order, exothermic decomposition of

di-t-butyl peroxide (DTBP), following compres-sion to 0.6 MPa at temperatures of 530 K and570 K, was exploited as the reactive mediumbecause this combustion process develops in a“classical” thermal ignition mode. That is, reac-tion is initiated from the hottest regions of thecompressed gas, thereby causing an augmenta-tion of any temperature inhomogeneities.Moreover, acetone is a primary product ofreaction. The LIF of acetone, at 266 nm, wasstudied in a thin sheet in the vertical planeacross the central cross-section of the combus-tion chamber. Acetone was seeded at 1% byvolume in inert gases to investigate the behaviorof non-reactive mixtures.

Neither Rayleigh nor LIF measurementswere fully calibrated so we cannot report quan-titative data. Nevertheless, novel qualitative fea-tures are evident from the experimental resultsand the limits of sensitivity that might be ex-pected of Rayleigh scattering in this environ-ment are shown. We believe that the results mayhave important implications for modeling stud-ies and may enhance the understanding of spa-tial development of autoignition in rapid com-pression machines.

APPARATUS, EXPERIMENTAL METHODSAND DATA PROCESSING

A full description of the RCM and its operationcan be found elsewhere [4, 14]. A diagram of theapparatus is given in [15]. In summary, to attaina range of compressed gas temperatures, fuelvapor was pre-mixed with various proportionsof inert diluents (nitrogen, argon or carbondioxide) so that g (5Cp/Cv) could be controlled.In the present work, DTBP decomposition wasstudied in two mixtures comprising DTBP vapor(3.3 mol %) in N2 1 CO2, to achieve a com-pressed gas temperature of 530 and 570 Krespectively when the chamber wall was heatedto 315 K. The compression ratio was 10.50(6 0.15).

Pressure-time data during the compressionstroke and throughout the post-compressionperiod were measured by a pressure transducer(Kistler 601A, natural frequency 100 kHz).High-resolution records of the pressure historyand the timing mark for the laser firing se-

1163TEMPERATURES IN RAPID COMPRESSION

quence were recorded on a digital oscilloscope(Tektronix, TDS 220). The gaseous reactantswere admitted to the combustion chamber at aninitial pressure of 33 kPa, then compressed by apiston driven by compressed air. The compres-sion took 22 6 1 ms and the final, cylindricalvolume was about 30 cm3, the combustionchamber then being 4.5 cm dia. 3 1.9 cm depth.

To allow a laser beam to traverse the diame-ter of the combustion chamber, two 1.0 cmdiameter, optically flat, fused silica windowswere located on opposite sides of the chamber.The end of the chamber was fitted also with anoptically flat, fused silica window (5.0 cm diam-eter 3 3.0 cm thick) which allowed a full view ofthe chamber cross-section for imaging the scat-tered laser light or the laser induced fluores-cence. An intensified charge coupled device(ICCD) camera (Princeton Instruments, 576-G)was used to collect photons. The piston crownwas highly polished to an optically flat, mirrorfinish. This gave less interference from scatteredlight than a piston crown made from brushed,black anodized, aluminum.

Particular attention was paid to the design ofthe firing sequence to maintain a stable lasermode of operation, via its continuous firing at aconstant repetition rate while coping with thelong duty cycle of the RCM and the shot-to-shotvariation (;1 ms) resulting from the electro-mechanical operation. The solution for compat-ibility was to externally clock the laser flashlampand Q-switch at a constant 10 Hz. An “en-abling” signal was generated from the initialtrigger of the RCM, which was used to activatethe machine simultaneously with the next lasertrigger pulse. Normally two or three laser shotswould occur during the machine operation. Thefollowing laser shot would then take placesometime during the chemical reaction period.The external clock was intercepted at this stagewith a pre-set, variable delay to synchronize thisparticular laser pulse to the required imagepoint. This pulse also triggered the ICCD gatingelectronics so that the camera was exposed onlyfor the duration of the laser pulse as it passedthrough the chamber (140 ns). Once the camerahad been triggered the external clock revertedto the normal frequency of 10 Hz.

The laser system was a frequency doubledNd:YAG (Quantel 680) which had been retro-

fitted with a seed diode laser (Lightwave Elec-tronics) so as to operate on a single longitudinalmode. Single-mode operation and linewidth(;500 MHz) were monitored by means of ahigh finesse etalon placed in a reflected portionof the main beam. Typically, the laser output at532 nm was 300 mJ pulse21, and pulse-to-pulsefluctuations in the laser output were estimatedto be less than 1%.

There was always some condensation of reac-tion products within the combustion chamber.Consequently, before each experiment it wasessential to clean all surfaces scrupulously withthe windows removed. The laser beam wasrealigned after their replacement to ensure alaser beam path through the cell that was per-pendicular to the windows.

Rayleigh Scattering from a 1-D Line

The main beam was steered into the combustionchamber by three, high quality, dichroic mirrorsto remove any residual IR radiation in thebeam. A Galilean telescope arrangement wasused to condense the beam before it entered thechamber. Each single-shot pass (6–7 ns) formeda beam (;100 mm dia. at 1 s) in the horizontaldiameter of the combustion chamber close to itscentral, cylindrical plane. Neutral density filtersplaced in the beam path were used to controlthe intensity of the beam, and a series of iriseswere located along the beam path to reduce thespecularly scattered light entering the chamber.For a fixed composition, the Rayleigh scatteredsignal is an inverse function of gas temperature[7–10].

To minimize the unwanted signal from Miescattering and specular reflections off the cham-ber walls and the windows, a filtered Rayleighscattering technique was employed [16]. Thisexploits the difference in Doppler line-broaden-ing which arises from the differing speeds of(potentially Mie scattering) particles and themolecules themselves. Filtering out the peak ofthe Gaussain in the scattered laser light inten-sity and collecting only the Doppler-shifted lightfrom the extremity discriminates the molecularscattering. By operating the Nd:YAG laser on asingle longitudinal mode it is possible to tunethe output of the Nd:YAG laser to coincidewith one of the absorption frequencies in mo-

1164 CLARKSON ET AL.

lecular iodine vapor which, thus, served admi-rably as a filter. This was achieved by tuning thefrequency of the diode seed laser concomitantlywith the cavity length of the Nd:YAG oscillator.In a temperature stabilized laboratory it wasthen possible to hold the Nd:YAG laser outputon an I2 line for several hours before a modehop occurred. A heated glass cell 15 cm long 36 cm diameter containing resublimed iodinevapor and fitted with good quality windows wasplaced between the RCM end window and theICCD camera. A second smaller cell was usedon a portion of the main laser beam to check, bymeans of the easily observed laser inducedfluorescence, that the laser was correctly tunedto an I2 absorption feature.

The scattering cross-section is both tempera-ture and composition sensitive. The cross-sec-tions and their temperature sensitivity areknown for a range of substances, and from theseit would be possible to compute the overallscattering cross-section for certain mixture (in-deed this is often the case [11, 17]). In general,such calculations for reactive mixtures are un-likely to be very accurate, not only becausethere are many molecular intermediates butalso because combustion processes evolvethrough an extensive temperature range. Thislimits the scope for calibration, although thereis some compensation to be gained from thedominance of inert components (e.g., N2) whencombustion takes place in air. Because theRayleigh scattering cross-section depends onthe wavelength to the fourth power, the sensi-tivity of our experiment would have been im-proved by working with the fourth harmonic ofthe Nd:YAG laser around 266 nm, if a suitablemolecular or atomic filter had been available.For example, Golz and Andresen [18] haveemployed Fe atoms as a filter for Rayleighimaging using the output of a tunable narrowband KrF laser (248 nm).

The composition variation in the presentstudy is more simple than normally encounteredin combustion because the major componentswere nitrogen and carbon dioxide, with thereactant, DTBP, and the products ethane andacetone being present at less than 10 mol% intotal at any stage of reaction. The scatteringcross-sections of the reaction components aresomewhat greater than those of N2 and CO2.

Although iterative calculations to match com-position and temperature fields from measured,or calculated, molecular polarizabilities andtheir temperature dependences might be viable,and could yield quantitative results, there areparticular complications in the present case.That is, a falling scattering cross-section is likelyto accompany the conversion of DTBP to thesmaller product molecules, reducing the scat-tered signal under isothermal conditions, andthe peroxide decomposition is accompanied byheat release, which also reduces the overallscattering cross-section as a result of the tem-perature increase. However, this combinationhas attendant benefits for interpretation of thequalitative structure because changes of scatter-ing cross-section, originating from these twocauses, have the potential to augment eachother and thereby enhance the signals thatidentify the spatial variation in temperaturealong the laser line.

LIF of Acetone from a 2-D Sheet

For the purpose of the LIF imaging, a lasersheet (width 8 mm 3 100 mm depth) wasformed by using a cylindrical beam expandingtelescope in conjunction with a spherical lens tofocus the beam in the horizontal plane with itsvertical face to the end window. For theseexperiments the laser output at 532 nm wasfurther doubled using an external frequencydoubling crystal to obtain the fourth harmonicof the Nd:YAG fundamental. No seeding by thediode laser was necessary in these experiments.The LIF signal is sensitive to temperature be-cause the absorption cross-section falls as tem-perature increases. At 266 nm there is a 50%reduction in the fluorescence quantum yieldover the temperature range 500 to 800 K [19,20]. There are two different types of responsefrom the LIF detection in the present work.Spatial variations in temperature that arise incompressed inert gas seeded with acetone areshown as a diminished fluorescence signal inhotter regions. By contrast, regions in whichacetone is generated by reaction show an en-hancement in the fluorescence intensity. Thissignal is proportional to the concentration inisothermal conditions. However, there is a tem-perature increase associated with the formation


of acetone as a result of the exothermic decom-position of DTBP. Consequently the LIF signalis very complicated and so we do not have aquantitative calibration of acetone yield. At theconcentrations of acetone encountered in theseexperiments a sufficiently strong absorption ofthe laser beam as it traverses the combustionchamber was able to occur, such that the fluo-rescence signals show a Beer–Lambert decay ina uniform concentration field.

RESULTS

The Response of an Acetone Tracer in InertGas After Rapid Compression

The response of inert gas to rapid compressionwas studied in the mixture 0.01 (CH3)2CO 10.99 N2, for which the compressed gas temper-ature was 717 K and, from 33 kPa initially, thecompressed gas pressure was 0.84 MPa (Fig. 1,curve A). There is no expectation that acetonewould decompose to any significant extent onthe short timescale of the experiment. Thepressure trace decays in the post-compressionperiod as a result of heat loss to the chamber

walls. The points at which LIF signals wereobtained are marked in Fig. 1 (curve A).

The LIF signal after the compressed gas hasbeen allowed to cool to laboratory temperatureis shown in Fig. 2a both as an ICCD image andas the photon count integrated across the widthof the beam. There is no temperature variationacross the chamber at this stage and gas in thecylinder is obviously homogeneously mixed. Theedge of the piston is where the thin, verticallines can be seen at the edge of the image.There are corresponding step changes in inten-sity on the graph at 35k to 50k photons on theleft-hand side and at the fall below 25k photonson the right hand side. The small segments ofthe beam that extend outside the piston seal arepicked up by the camera from the recess inwhich the windows are located. The decay in thesignal from left to right, given by the integratedphoton count, is because of absorption of thebeam as it traverses the chamber.

By contrast, the signal detected at 1 mspost-compression shows a distinct enhancementof the fluorescence intensity in the central por-tion of the laser beam. Its location correspondsto the center of the chamber itself (Fig. 2b).

Fig. 1. Pressure-time records in the rapid compression machine at the conditions studied in this work. Zero on the time scalesignifies the moment at which the piston stops. The numbers marked on each curve refer to the times and conditions at whicheach of the records shown in the following were obtained. (a) Compression of N2. (b) Compression of 1.00 C4H9OOC4H9 17.00 N2 1 22.30 CO2 to 530 K. (c) Compression of 1.00 C4H9OOC4H9 1 9.77 N2 1 19.53 CO2 to 570 K.


This enhanced fluorescence signifies that gasresident in the central region at this time iscolder than that in the “toroidal” zone sur-rounding it. The extent of the enhancement in

intensity is approximately 20%, which wouldsuggest a temperature difference of approxi-mately 50 K between the center and the toroidsurrounding it [16]. On this basis the gradienton the right hand side of the temperaturedepression is approximately 50 K cm21. There isalso a left to right decay in intensity imposed onthe signal as a result of absorption of the beamintensity. By 30 ms post-compression there is noevidence of the central depression in the tem-perature (Fig. 2c), but the fluctuations of theLIF signal (which are somewhat greater thanthose in Fig. 2b at a similar magnitude for theintegrated signal) may suggest that there is aquite complex pattern of minor temperaturevariation across the central plane of the com-bustion chamber. At 30 ms there is also anincrease of the fluorescence intensity at theright hand side of the chamber, which suggeststhe development of a cool zone close to thewall, but this cannot be clearly distinguished atthe point of entry of the laser beam on the lefthand side. The signal intensity in this andsubsequent figures are representative of thenumber of scattered photons. However, theincident laser was not calibrated and (minor)variations of the intensity in successive experi-ments were not taken into account. For thisreason the ordinate is reported in arbitraryunits. Nonetheless, for comparison, the valuesdo give a meaningful indication of the relativesignal strength of the figures.

Combustion of DTBP Vapor at a CompressedGas Temperatures of 530 and 570 K and 0.6MPa

The combustion of DTBP (3.3 mol %) wasinvestigated in two mixtures comprising 1.00C4H9OOC4H9 1 7.00 N2 1 22.30 CO2 and 1.00

Fig. 2. The LIF signals from an acetone tracer (1 mol %) inN2 imaged in a sheet across the central plane of thecombustion chamber. An ICCD image and the photoncount is shown. (a) Cold, compressed gas. (b) 1.1 mspost-compression. (c) 30 ms post-compression. The ordi-nate represents the photon count integrated across thewidth of the beam, and is given as arbitrary units becausethe intensity of the incident laser beam was not calibrated.The pixel number on the abscissa is related to the positionin the field of view across the diameter of the chamber.4™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™


C4H9OOC4H9 1 9.77 N2 1 19.53 CO2. On thebasis of adiabatic compression at CR 5 10.5,these compositions ensured compressed gastemperatures of 530 K and 570 K at the end ofcompression when the cylinder is heated to 310K. These conditions correspond to those used inprevious work [14]. The pressure records areshown in Fig. 1 (lines B and C). Also marked ineach case are the points at which LIF images ofacetone (product) fluorescence in the 2-D planeand thermal imaging by Rayleigh scattering on a1-D line were obtained. Each of the opticalmeasurements were made in separate experi-ments at the prescribed conditions.

The reaction developed in a single stage inthe post-compression period at both of theselected compressed gas temperatures, but thetimescales were significantly different. The de-velopment was relatively slow at the lower tem-perature, the maximum pressure in the constantvolume chamber being reached only after about15 ms (Fig. 1, curve B). The overall change inmole numbers is relatively small (ca 6%) so thispressure change is attributable almost entirelyto the temperature change resulting from theexothermic reaction.

The compression is sufficiently rapid thatmost of the temperature increase occurs withinthe final few milliseconds of the compressionstroke and, unless excessive gas motion is arti-ficially created [21], there is hardly any heat lossassociated with the compression stage [4]. Heatloss in the post-compression period is inevitable[5, 22], but the residual gas motion is notsufficiently vigorous that the whole system couldbe approximated to a spatially uniform, “well-stirred” system [14]. Nevertheless, from theextent of the post-compression pressure rise at15 ms post-compression, on an ideal gas basisthe average increase in the gas temperature isapproximately 80 K.

The reaction occurred much more rapidlywhen the compressed gas temperature wasraised to 570 K, with some decomposition beingpossible even before the piston had stopped(Fig. 1, curve C). In this case the maximumpressure was reached ca 1 ms after the end ofcompression. The average temperature rise cal-culated from the pressure change is approxi-mately 190 K, which is close to the predictedadiabatic temperature rise for the mixture. Heat

losses over 1 ms post-compression would berelatively insignificant. This rate of developmentof the decomposition with thermal feedback isto be expected since the half-life is 3 ms at 600K, and by 650 K, t1/2 has diminished to less than0.5 ms.

The respective LIF images of acetone gener-ated as a reaction product following compres-sion to 530 K are shown in Fig. 3. These are alsodisplayed both as the primary sheet image aswell as the statistical photon count integratedacross the image width. The cold, compressed,reaction products cause a very strong Beer–Lambert absorption because of the uniformconcentration of acetone that has been gener-ated (Fig. 3a). The fluorescence signal intensityis considerably stronger than that of the traceracetone in CO2 (Fig. 2a) because the concen-tration of the product is much higher (ca. 6 mol%) and the product has cooled to laboratorytemperature. The thin line of the piston seal isjust detectable on the right hand side of theimage, but it is masked by the window reflec-tions on the beam entry side.

Although there has been no enhancement ofthe gas pressure in the chamber at 1.1 mspost-compression, there is already sufficient ac-etone formed to create a weak fluorescencesignal (Fig. 3b). There is evidence in this resultthat rather less acetone has been formed in thecenter of the chamber as a consequence of thereduced reaction rate in this cooler zone. Thepressure has begun to increase by 10 ms post-compression (Fig. 1, line B) and, commensuratewith this development of reaction, there is asomewhat stronger LIF signal from acetone(Fig. 3c). The reduced intensity at the center isclear, and (presuming that the temperature fieldis qualitatively similar to that of the non-reac-tive system) this signifies a lower temperaturesimply because hardly any acetone has beenformed. We are not able to infer any magnitudefor the temperature variations across the cham-ber because there are conflicting variations onthe intensity of the fluorescence signal owing tochanges both in the acetone concentration andthe gas temperature. The reaction is well devel-oped at 15 ms post-compression and there is amuch greater fluorescence from acetone in con-sequence (Fig. 3d). The effect of the laser beamabsorption, from left to right, is evident in the


Fig. 3. The LIF sheet images of acetone generated as a reaction product from the exothermic decomposition of DTBPcompressed to 530 K. (a) Cold products. (b) 1.1 ms post compression. (c) 10 ms post compression. (d) 15 ms post compression.(e) 25 ms post compression.


signal, but there is still a marked inhomogeneity inthe acetone concentration close to the center ofthe chamber. Even at 25 ms post-compression,although somewhat diminished in volume, there isstill a cooler core relative to that of the reactantsand products in the surrounding region (Fig. 3e).

The acetone LIF signals during reaction at acompressed gas temperature of 570 K areshown in Fig. 4. It is likely that some reactionhas taken place even in the final stages of thecompression stroke because there is a very weakLIF signal from acetone just before the end ofcompression (Fig. 4a). However, there is noindication that the cold core has penetrated to

the center plane of the chamber by that stage.By 1.1 ms after the piston has stopped, thechamber pressure is fully developed and thereactants are at their highest average tempera-ture (Fig. 1, line C). There is a substantialfluorescence signal from the acetone produced(although of lower photon count overall be-cause of the high gas temperatures) and themuch weaker signal at the center confirms theexistence of the appreciably cooler core gas inthe central plane of the combustion chamber(Fig. 4b). The photon count of the fluorescencesignal at the center integrated across the beamwidth is approximately 40% of that in the regioneither side of it. A slight increase in the imageintensity to the far left suggests that inhomoge-neities following compression may be such thatthe hottest gases reside quite close to the pe-riphery of the chamber and that a gradient hasbeen created toward the cooler core region.This would be commensurate with the forma-tion of a roll-up vortex [23,24].

Rayleigh Scattering from a 1-D Line on theHorizontal Radial Axis at the Central Plane

Rayleigh scattered signals from the horizontalradial axis are shown in Fig. 5 both as theprimary image and the statistical count of pho-tons on the central pixel line of the image,scaled to the full range of photon counts (216,;65k). The conditions are those for an inert gas(N2) and also for the decomposition of DTBP at530 and 570 K at 1.1 ms post-compression. Theedge of the piston crown and gap where thep.t.f.e. seal fit can be seen clearly in Fig. 5a as athin, dark cylindrical section. There is somescattered light detected beyond the chamberitself and these reflections lead to spurioussignals just inside the circumference of thechamber. Reflections are also visible at certainlocations inside the chamber owing to imperfec-tions of the piston crown. Some artifacts alsoappear as a result of imperfections of the iodinefilter cell. These could all be easily identifiedand distinguished from the primary signal. Alimited amount of Mie scattering was also un-avoidable. It was particularly difficult to obtain auniformly high quality of all images, the maincriteria being the cleanliness of optical align-ment of the laser beam. Nevertheless, as shown

Fig. 4. The LIF sheet images of acetone generated as areaction product from the exothermic decomposition ofDTBP compressed to 570 K. (a) 0.5 ms before the end ofcompression. (b) 1.1 ms post compression.


in Fig. 5a, a clear Rayleigh scattered line traversedthe chamber but there is no evidence to be seen ofthe low temperature zone in the central core.

Similarly, the Rayleigh scattered signal duringDTBP decomposition at a compressed gas tem-perature of 530 K (Fig. 5b) hardly reflects theevidence obtained from the acetone LIF study inthe same conditions (Fig. 3b). However, by 10 mspost-compression of DTBP at the same conditionsthe cooler central zone is clearly distinguishablein the Rayleigh scattered signal (Fig. 5c). Bycontrast to the results shown in Fig. 5b, the coolercore gas is easily identified in the Rayleighscattered signal at 1.1 ms post-compressionwhen DTBP is compressed to 570 K (Fig. 5d).

DISCUSSION

The main purpose of this work was to investi-gate the temperature field that develops duringand following rapid compression of gases in aclosed cylinder under non-reactive and reactiveconditions. The reactive system, the exothermicdecomposition of di-t-butyl peroxide, was se-lected because the thermal feedback associatedwith this first order reaction served to amplifyany temperature variations and thereby makethem more prominent. A supplementary reasonfor the choice is that acetone, as a final product,through its laser-induced fluorescence, could beused as a (non-invasive) chemical marker of

Fig. 5. Rayleigh scattering 1-D images from all three sets of reaction conditions. (a) 1.1 ms post compression in N2 (cf. Fig.2b). (b) 1.1 ms post compression during the decomposition of DTBP at 530 K (cf. Fig. 3b). (c) 10 ms post compression duringthe decomposition of DTBP at 530 K (cf. Fig. 3c). (d) 1.1 ms post compression during the decomposition of DTBP at 570 K(cf. Fig. 4b).


spatial variations. The decomposition of DTBPhas some relevance also because it was thesubject of a computational fluid dynamic analy-sis of reaction in the RCM, 7 years ago [14],some aspects of which are discussed next.

In Fig. 6 we reproduce a simulation from aKIVA II [24] calculation of the temperaturefield in the Leeds RCM. This represents theplan view of an axisymmetric section of thechamber of the rapid compression machinederived on a relatively coarse (20 3 20) grid.Results published elsewhere show the outputusing a more refined mesh, but with very similarqualitative features [14]. Although it is notintended that the result in Fig. 6 is to beinterpreted in a quantitative way, it illustrates howa cooler gas region may penetrate the core of thereaction zone as a result of gas being swept fromthe sidewall of the combustion chamber, with anattendant roll-up vortex adjacent to the pistoncrown and circumference of the chamber. This isconsistent with the pattern of development shownby Tabaczinski et al. [23] and in more recent,related work by Lee and Hochgreb [25].

Consequences of the “Cold Core” onExothermic Chemistry that Exhibits a PositiveTemperature Dependence

There is clear evidence in this study that at theend of compression a cold plug of gas is sweptacross the piston face and is able to penetratethe center of the adiabatically heated core gas.

This creates a spatially non-uniform tempera-ture field during a post-compression time inter-val that is relevant to the development of spon-taneous ignition of reactive mixtures. Thetemperature difference between the center andthe surrounding toroid may exceed 50 K. As aresult of thermal feedback, an exothermicchemical reaction that exhibits a positive tem-perature dependence enhances the spatial vari-ations in temperature, and the hottest pointsbecome the sites at which ignition would natu-rally develop. A full development of ignitiondoes not follow in the decomposition of DTBP(as studied here) because it is not stronglyexothermic (DUo

298 5 - 165 kJ mol21). Never-theless, the temperature reached in the hottestzones approaches the adiabatic temperaturechange when the reactants are compressed to asufficiently high temperature for a fast develop-ment of reaction (see Fig. 1, curve C). These arethe first experimental results to show how thenon-uniform temperature field following rapidcompression may affect the chemical develop-ment.

Consequences of the “Cold Core” onChemistry that Exhibits an Overall NegativeTemperature Dependence

As is well known, the combustion of manyorganic compounds, and alkanes in particular,exhibit a negative temperature dependence (ornegative temperature coefficient, ntc) of theoverall chemical reaction rate through temper-atures over the approximate range 750 to 850 K[6]. The underlying chemistry is reasonably wellestablished now in a quantitative way [26].When thermal feedback occurs in non-isother-mal conditions, reaction may develop to giveoscillatory cool flames and complex ignitions, asin closed vessels and stirred flow reactors [27].Single or two-stage ignitions occur in devicessuch as rapid compression machines [1, 2, 4, 5].The two-stage ignition development itself exem-plifies the occurrence of the ntc, insofar that thereaction accelerates through the initial stage butthen decelerates as the onset of the secondstage is approached. It is at this point that thereactant temperature has, on average, ap-proached about 850 K [4]. The thermokinetic

Fig. 6. The temperature field that was predicted to developby 5 ms post compression, simulated in a combustionchamber corresponding to that of the Leeds RCM, usingKIVA 11. The range of temperatures is 500 K to 570 K, withcontour lines at 10 K intervals.


interactions that cause the onset of the secondstage are discussed elsewhere [28–30].

The relevance of these features is that asystem that exhibits an ntc of rate in a certaintemperature range, is capable of responding toan inhomogeneous temperature field in a dif-ferent way from that seen in the present exper-iments. That is, reaction may be more vigorousin the colder gas, which thus serves to smoothout temperature differences, and the systemmoves to a more uniform temperature field inthe later stages as a consequence of the negativefeedback. However, if gas mixing is not verysignificant on the timescale of evolution ofignition, the composition field does not becomeuniform because different extents of reactionwill have occurred at different spatial locationsduring the course of this development. This hasimplications for the way in which the secondstage of ignition then evolves because regionsmay then exist that are chemically more activethan others. These effects are to be discussed ina later paper.

A Critique of Experimental Methods forIdentifying Spatial Structure in RapidCompression Machines and theirConsequences

Acetone LIF serves well to identify spatialvariations in temperature in the RCM when it isused as a marker in non-reactive gases up totemperatures of about 900 K. Thereafter, itsown decomposition could become troublesomeon relatively short timescales. The system can becalibrated because the temperature dependenceof the absorption cross-section is known [19,20].Although we have not pursued our own calibra-tion, the data provided by Thurber et al. [19, 20]show that, by 1 ms post-compression, the tem-perature difference between the cooler core gasand the surrounding toroid, illustrated in Fig.2b, might be as high as 50 K. This appears to besupported in the calculations by Griffiths et al.[14], as also exemplified in Fig. 6. The genera-tion of a laser sheet is easy and, with greateroptical access than in the present chamber, anextensive temperature field could be mapped.In an active free radical environment an acetonetracer not only presents a chemical perturbationbut also it is susceptible to reaction, so would

not be a viable route to studying the tempera-ture field. In the case of DTBP decomposition,the LIF of acetone can also been used toidentify the composition field represented bythe final product formation. However, unlikethe thermal calibration, there is no correspond-ing, simple quantitative relationship for theconcentration vs. LIF intensity when acetone isformed because it is being generated undernon-isothermal conditions. In both of theseapplications there is a sufficiently strong (Beer–Lambert) absorption of the excitation laserbeam for a correction for asymmetry of the LIFsignal to be required if quantitative analyses areto be sought.

Our complementary study of Rayleigh scat-tering on a one-dimensional line shows thistechnique to be rather less sensitive for thermalimaging than the acetone tracer LIF measure-ments. This limitation of Rayleigh scattering, inthese typical combustion mixtures where nitro-gen is present in high proportion, caused thecooler core gas not to be identified in theimmediately post-compression interval. Thuswe would infer that any spatial variation intemperature that is less than about 60 K isunlikely to be distinguishable from backgroundnoise in the Rayleigh scattered signal. Withoutthat awareness there is the risk of misinterpre-tation of the qualitative structure, as is exempli-fied when the results for the Rayleigh scatteringduring the onset of DTBP decomposition fol-lowing compression to 530 K (Fig. 5b) arecompared with the related LIF measurement(Fig. 3b). However, a very clear, qualitativedistinction became evident when the tempera-ture difference was amplified to approximately200 K, as shown in Fig. 5c. This is an optimalresult because the temperature difference existsat the center of the chamber. In most experi-ments, the scatter of light close to the windowsin the chamber wall precludes an unambiguousinterpretation. Nevertheless, from the resultshown in Fig. 5c, it may be inferred that tem-perature gradients at the edge of the chamberare very steep.

Rayleigh scattering studies in the chamber ofa rapid compression machine, by Desgroux et al.[7, 8], yielded single point temperature mea-surements with a higher accuracy than assessedfrom this 1-D line study, namely 6 30 K.


Although they did not investigate spatial varia-tions, even at this precision and given the scatterthat was reported for a series of measurements,it is debatable whether or not they would havebeen able to distinguish the development of a“toroidal” temperature field of the kind seen inthe present work. In any event, the compressionwas considerably slower in the apparatus thatwas used by Desgroux et al., taking 60 ms for asimilar stroke, so somewhat different spatialand temporal temperature evolution would beexpected in the post compression period. Thesubtleties of the temperature history are cer-tainly machine specific, and significant differ-ences may be realized under different operatingconditions or in chambers of different design.Lee and Hochgreb [25] have shown theoreti-cally how attention to the design of the pistoncrown and rings can suppress the formation of aroll-up vortex and thereby reduce or eliminatethe effects that are reported in the presentpaper.

Ignition delays measured in different appara-tus cannot usefully be compared directly with-out reference to the dependences on the indi-vidual equipment. Foremost is the effect of heatloss, but more subtle consequences of gas mo-tion that induce spatial temperature variations,as exposed here, may also be relevant. As hasbeen illustrated with respect to di-t-butyl perox-ide decomposition [14], the existence of temper-ature inhomogeneities also has considerablebearing on the relationship between experimen-tal measurements of ignition and numericalsimulations using detailed, zero-dimensionalthermokinetic models. However, there are mit-igating circumstances with respect to the mod-eling of alkane combustion, whereby kineticsystems that exhibit a negative temperaturedependence of reaction rate are capable ofsmoothing out the spatial variations that mayhave been created in the combustion system, asnoted in the preceding subsection and in earlierpublications [8, 31, 32].

The authors are grateful to EPSRC for supportof this project under grant GR/K97189. The au-thors wish to thank Drs M. Schreiber and J. Meyerfor permission to publish the results shown in Fig.6, which were preliminary calculations for thework described in [14].

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Received 7 November 2000; revised 5 February 2001; accepted13 February 2001


Temperature fields during the development of combustion in a rapid compression machine

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Transcript of Temperature fields during the development of combustion in a rapid compression machine