Combustion and Flame 158 (2011) 2420–2427

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

A numerical assessment of the novel concept of crevice containmentin a rapid compression machine

Gaurav Mittal a,⇑, Mandhapati P. Raju b, Anil Bhari a

a Department of Mechanical Engineering, The University of Akron, Akron, OH 44325, USAb Optimal Inc., Plymouth, MI, USA

a r t i c l e i n f o

Article history:Received 13 August 2010Received in revised form 27 February 2011Accepted 20 April 2011Available online 9 May 2011

Keywords:Rapid compression machineTwo-stage ignitionCrevice containment

0010-2180/$ - see front matter � 2011 The Combustdoi:10.1016/j.combustflame.2011.04.013

⇑ Corresponding author.E-mail address: gaurav@uakron.edu (G. Mittal).

a b s t r a c t

Rapid compression machines (RCMs) typically incorporate creviced pistons to suppress the formation ofthe roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effectsinside the RCM due to the crevice zone being at lower temperature than the main reaction chamber.In this work, such undesirable effects of a creviced piston are highlighted through computational fluiddynamics simulations of n-heptane ignition in RCM. Specifically, the results show that in an RCM witha creviced piston, additional flow of mass takes place from the main combustion chamber to the crevicezone during the first-stage of the two-stage ignition. This phenomenon is not captured by the zero-dimensional modeling approaches that are currently adopted. Consequently, a novel approach of ‘crevicecontainment’ is introduced and computationally evaluated in this paper. In order to avoid the undesirableeffects of creviced piston, the crevice zone is separated from the main reaction chamber at the end ofcompression. The results with ‘crevice containment’ show significant improvement in the fidelity ofzero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in thefirst-stage of ignition. Although the implementation of ‘crevice containment’ requires a modification inRCM design, in practice there are significant advantages to be gained through a reduction in the rateof pressure drop in the RCM combustion chamber and a quantitative improvement in the data obtainedfrom the species sampling experiments.

� 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Rapid compression machines (RCMs) are valuable tools forinvestigating chemical kinetics at low-to-intermediate tempera-tures and elevated pressures. Until few years ago, interpretationof RCM data was plagued with uncertainties arising from the influ-ence of temperature non-uniformity due to the presence of pistonmotion induced roll-up vortex, and facility dependent heat losscharacteristics [1,2]. The CFD evidence of the roll-up vortex inthe context of RCMs and its consequences on ignition were firstdiscussed by Griffiths and coworkers [3,4]. Lee and Hochgreb [5]made a remarkable contribution and developed upon the idea ofincorporating a crevice on the periphery of piston, which was ini-tially proposed by Park and Keck [6], and computationally showedthe efficacy of the creviced piston in suppressing the vortex. Thephenomenon of the roll-up vortex and its suppression was subse-quently investigated by many groups [7–14] and the importance ofa creviced piston in RCMs for obtaining quality experimental datais now well recognized.

ion Institute. Published by Elsevier

Another important aspect is to correctly model RCM experi-ments to properly account for the facility dependent heat losscharacteristics, so that the experimental data can be used for vali-dating detailed chemical kinetic mechanisms. Such modeling isusually conducted with a zero-dimensional (Zero-D) code, suchas SENKIN [15], while accounting for the heat loss effects[16–18]. However, not all approaches that are used to accountfor heat loss effects are appropriate. It was highlighted in Ref.[19] that the assumptions of adiabatic core and Newtonian heatloss results in severe discrepancy in the modeled temperature,whereas the ‘adiabatic volume expansion’ provides a much betterway of dealing with the heat loss. As such, the use of the crevicedpiston goes a considerable way to the attainment of the homoge-neous temperature field within the main body of the reactionchamber, and hence enables a closer approximation of the Zero-D modeling to the reality [20]. In our previous investigation withhydrogen ignition in an RCM [21], it was shown that SENKIN sim-ulations in conjunction with the Zero-D modeling approach withan ‘adiabatic volume expansion’ perform very well in adequatelypredicting the ignition delay, as compared with the resultsobtained from computational fluid dynamics (CFD) simulations.Recognizing that the ignition characteristics of hydrocarbon fuels,

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G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427 2421

especially those exhibiting two-stage ignition phenomena, are sig-nificantly different from hydrogen ignition, similar CFD studieswere conducted for n-heptane [20]. Results showed that theZero-D model performed very well in adequately predicting thefirst-stage ignition delays, although quantitative discrepancy forthe prediction of the total ignition delays and pressure rise in thefirst-stage ignition was noted even when the roll-up vortex wassuppressed and a well-defined homogeneous core was retainedwithin the RCM [20]. It was highlighted [20] that although a crev-iced piston offers the advantage of suppression of the vortex, mul-ti-dimensional effects are not completely avoided during theconditions of two-stage ignition and the approach of Zero-D mod-eling could introduce a discrepancy of up to 30%. It is desirable toattempt to minimize this discrepancy and improve the fidelity ofthe Zero-D simulations.

The extent of agreement between the CFD and Zero-D couldpossibly be improved by incorporating multi-zone modeling ofRCM; for instance by including separate zones for the core region,boundary layer and the crevice, and allowing for the exchange ofmass and energy amongst the zones [e.g. Ref. [5]]. In this work,however, an alternative approach is proposed which would requiresome modifications to the design of the RCMs so that the multi-dimensional effects are minimized and, consequently, the fidelityof the Zero-D simulations is improved. Through comparisons withthe results of CFD calculations, we first address the quantitativeshortcomings of Zero-D modeling that arise from validation ofexperimental results obtained in RCMs that incorporate a pistoncrevice to eliminate the effects of the roll-up vortex. Subsequently,a novel concept to overcoming the problems is introduced andcomputationally evaluated through the modeling of reaction inn-heptane/oxidizer mixtures with a skeletal kinetic model in anRCM. As confirmed previously using the same skeletal kinetic mod-el [20], n-heptane combustion is well known to exhibit two-stageignition and NTC behavior in its ignition delay. The principle ofthe concept is to separate the crevice zone from the main reactionzone by a seal which is engaged only when the piston reaches theend of its travel. We call this ‘crevice containment’. In the follow-ing, we sequentially describe the specifications of the presentnumerical simulations, the approach of the ‘crevice containment’and its advantages.

2. Numerical specifications

The specifications of the rapid compression machine simulatedhere have been detailed in [13]. Briefly, this RCM system consists ofa driver cylinder, a reactor cylinder and a hydraulic motion controlchamber. The machine is pneumatically driven and hydraulicallystopped. Moreover, the bore of the reactor cylinder is 50.8 mmand it incorporates an optimized creviced piston head design topromote homogeneous reaction zone.

Table 1 shows the compositions and physical conditions forwhich reactive simulations are conducted. The compositions in Ta-ble 1 are similar to those chosen in Ref. [20] and have higher levelof inert gas dilution than air. Diluted mixtures are taken to obtainlong ignition delays up to 50 ms so that the potential multi-dimen-sional effects are enhanced. In the simulations, the initial mixture

Table 1Summary of simulated conditions.

Composition (mole %)n-heptane/O2/N2/Ar

Compressedpressure, PC

(bar)

Compressedtemperatures, TC

(K)

Equivalenceratio

0.562/10.307/58.209/30.922

�10 710–880 0.6

0.562/6.184/62.969/30.285

�15 710–890 1

temperature is kept fixed at 297 K, and the compressed tempera-ture at the top dead center (TDC) is varied through a variation ofthe compression ratio from 8.84 to 16.46. The physical volume ofthe crevice zone is 4.77 cm3 and that of the main reaction chamberat TDC varies between 27.50 and 38.40 cm3. The details of thenumerical procedure and computational grid are identical to thosereported in Ref. [20] and are mentioned briefly in the following.

2.1. Computational fluid dynamics simulations

CFD simulations are conducted for an axisymmetric configura-tion using the FLUENT package [22], as reported previously [20].Computations are performed from the beginning of the compres-sion stroke with a compression time of 30 ms. Initially the gas mix-ture at rest is specified with a uniform temperature of 297 K andknown pressure. A fixed temperature of 297 K and no-slip condi-tions are specified at the cylinder wall boundary and the pistonsurface. FLUENT simulations use the segregated implicit solverwith the Pressure-Implicit Split-Operator (PISO) algorithm for thepressure–velocity coupling, the Pressure Staggering Option (PRE-STO) scheme for pressure, and the second-order upwind discretiza-tion for density, momentum, and species.

The kinetic mechanism of n-heptane and the associated ther-modynamic parameters are taken from the model of Liu et al.[23] in CHEMKIN [24] format. This skeletal mechanism of Liuet al. [23] consists of 43 species and 185 reactions and has a con-cise representation of low-temperature kinetics, including forma-tion of alkyl peroxy radicals, subsequent isomerization, andaddition of second O2. Furthermore, this skeletal mechanism wasvalidated [23] against ignition delay measurements from RCMand shock tube at pressures up to 44 bar, species profiles for n-hep-tane oxidation in a plug flow reactor at 3 atm, and species distribu-tion profiles in n-heptane-air counterflow diffusion flames.

In the simulations, the piston starts from rest and its motion isgiven in a manner similar to the piston motion in an engine, byspecifying dimensions for the crank radius and the connectingrod length. At the TDC, the piston comes to rest and remains therefor the subsequent time steps. The specified piston trajectory issomewhat different from that realized in our RCM. However, wehave noted that the resulting fluid dynamics and temperaturefields are negligibly influenced by the choice of the trajectory[21]. The CFD simulations are conducted with and without ‘crevicecontainment’. Furthermore, in the calculations in which ‘crevicecontainment’ is simulated, the effect of the seal, which would berequired in practice to separate the main reaction zone from thecrevice zone at TDC, is simulated by changing the entrance of thecrevice to ‘wall’ when the piston reached the TDC. Throughoutthe calculations, a time step of 41.667 ls is taken, which corre-sponds to 0.25 crank angle degree of a compression stroke of30 ms (180 crank angle). The choice of this time step was partiallygoverned by the need for having a consistent computational gridduring the entire compression stroke. It was found that the timesteps larger than this value result in the failure of deletion/mergingof the cells adjacent to the moving piston, and eventually lead tonegative cell volumes. At every time-step, convergence is ascer-tained by monitoring residues for the governing equations of con-tinuity, momentum, energy and species. Independence withrespect to the size of the time-step and grid distribution has al-ready been reported [20] by comparing the results with a grid dis-tribution finer by a factor of 4 and time-step smaller by an order ofmagnitude, and therefore will not be presented again.

2.2. Zero-dimensional simulations

In addition to the CFD simulations, Zero-D simulations are alsoconducted using SENKIN [15]. As discussed in [13,20], the Zero-D

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Fig. 1. Comparison of CFD vs Zero-D simulations. (a) non-reactive conditions,circles (Zero-D), line (CFD) (b) reactive conditions, dashed line (Zero-D), solid line(CFD). Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/10.307/58.209/30.922 by mole percentage. Conditions at TDC: PC = 10.38 bar, TC = 761 K.

2422 G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427

calculations include the compression stroke and take the effect ofheat loss during compression and post-compression period into ac-count through an approach based on volume expansion, whichuses the empirically determined heat loss parameters obtainedfrom the non-reactive pressure history. In order to simulate anRCM experiment of a given reactive mixture with Zero-D, a corre-sponding non-reactive experiment is first carried out with a non-reactive mixture of the same values of the mixture heat capacity,initial pressure, and initial temperature as for the reactive mixture.The non-reactive pressure history is used as the reference to backcalculate the time-dependent ‘effective volume’ of the core region[13,20]. As discussed in [20,21], the volume expansion based ap-proach assumes that there is no mixing between the cold boundarylayer and the hot core region, and the only way the effect of near-wall heat loss penetrates the core region is through the expansionof the core region caused by the cooling of the boundary layer.Therefore, even though the geometric volume of reaction chamberremains unchanged after the piston reaches the TDC, the core re-gion experiences an expansion that can be modeled as adiabaticvolume expansion and the ‘effective volume’ of the core regioncan be derived from the non-reactive pressure trace. For Zero-Dmodeling of the RCM experiment, conditions specified in the SEN-KIN calculations are those of a closed adiabatic system with thetime-varying ‘effective volume’. Numerical calculations are per-formed using the Sandia SENKIN code [15] in conjunction withCHEMKIN [24].

In this conceptual study, for every reactive CFD simulation, thecorresponding non-reactive CFD simulation is conducted by sup-pressing the fuel oxidation chemistry during the calculations andthe obtained pressure trace is used as the reference for derivingthe ‘effective volume’ for the Zero-D simulation. The reactive CFDsolutions are then compared with those obtained from the reactiveZero-D simulations using SENKIN. As such, CFD calculations repre-sent ‘computational experiments’, which will be simulated throughthe Zero-D modeling.

3. Results

3.1. Results without ‘crevice containment’

In the following, we present and discuss a representative case tocompare the CFD simulated results for ignition delay with theZero-D modeling without ‘crevice containment’. This discussionwill highlight the problems associated with the use of crevice inRCM and the results presented here will also serve as a baselinefor assessing the improvement when ‘crevice containment’ isintroduced in due course.

Figure 1a illustrates a comparison of Zero-D and CFD simulatedresults for non-reactive conditions. Unlike actual experiments, forsimplicity the non-reactive simulation is conducted by suppressingthe fuel oxidation chemistry during calculations without changingthe mixture composition. The mixture, n-heptane/O2/N2/Ar = 0.562/10.307/58.209/30.922 by mole percentage, is com-pressed to 10.38 bar and 761 K. For the CFD simulation, the maxi-mum instantaneous temperature is plotted. Furthermore, in orderto obtain the pressure and temperature traces from the Zero-Dsimulation, the non-reactive pressure trace from the CFD calcula-tions is first used to back calculate the ‘effective volume’ as de-scribed previously [13,20]. The effective volume is then used forthe Zero-D simulation. It is noted that based on this approach, bothpressure and the maximum instantaneous temperature are exactlyreproduced by the Zero-D solution, as shown in Fig. 1a.

Figure 1b presents a comparison of the CFD and Zero-D simu-lated pressure traces for the reactive conditions of Fig. 1a. Bothpressure traces reveal the phenomenon of two-stage ignition. Fol-

lowing the convention, the first-stage ignition delay (s1) and totalignition delay (s) are defined as the time duration from the end ofcompression to the inflection point in the pressure history duringthe first-stage ignition and hot-ignition, respectively. In Fig. 1b,the CFD simulated total ignition delay of 22.87 ms is approxi-mately 32% greater than the Zero-D simulated ignition delay of17.26 ms. In contrast, the predicted first-stage ignition delay forboth simulations is practically the same. This example shows thatalthough the pressure and temperature traces are adequatelyreproduced by the Zero-D with ‘adiabatic volume expansion’ fornon-reactive conditions, the same approach introduces errors inthe prediction of the total ignition delay for reactive conditionswith two-stage ignition. In Fig. 1b, pressure traces also reveal thatthe Zero-D-simulated pressure rise during the first-stage ignition ishigher than the CFD-simulated pressure rise. Solid circles, markeda–b in Fig. 1b, are the time instants at which the temperature fieldsare presented in Fig. 2. Specifically, Fig. 2a and b show the temper-ature fields before the onset and after the end of the first-stageignition. It is noted that the effect of the roll-up vortex is com-pletely absent and temperature evolution proceeds homoge-neously except for the boundary layer and the cool crevice zone.

The discrepancy between the Zero-D and CFD results, as seen inFig. 1b is due to other multi-dimensional effects which becomeimportant when there is heat release due to reaction. Important in-sights into the multi-dimensional effects are obtained by compar-ing the mass fraction inside the crevice with/without chemicalreaction. A comparison for the case under discussion is shown inFig. 3a. Result shows that for the non-reactive situation, the massfraction inside the crevice increases during compression and con-tinues to increase gradually during the post-compression perioddue to the flow of mass into the crevice. The overall effect of thismass flow is to reduce the temperature and pressure in the main

cylinder wall

axis

Piston Motion

(a) Time = 0 ms (TDC) (b) Time = 16.66 ms

761 715 668 622 576 529 483 436 390 343 297

898 838 778 718 658 598 538 477 417 357 297

crevice

Fig. 2. Temperature fields (K) at different times for reactive simulations. Conditions: Same as in Fig. 1b.

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(b)

Fig. 3. Comparison of reactive (solid line) and non-reactive (dashed line) CFDsimulations. Conditions: same as in Fig. 1. (a) Mass fraction inside the crevice (b)pressure difference between the main reaction chamber volume and the crevice.

G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427 2423

combustion zone and also avoid the formation of the vortex. Assuch, the effect of this mass flow for the non-reactive situation isautomatically accounted for in the ‘adiabatic volume expansion’approach. However, for the reactive situation, Fig. 3a shows thatthe mass fraction inside the crevice is increased as a result offirst-stage ignition in the main combustion chamber. During thefirst-stage ignition, significant chemical heat release takes placein the main combustion chamber which is maintained at high tem-perature, and practically no heat release occurs in the cooler cre-vice. As a consequence, additional mass flows into the crevicefrom the main reaction chamber and the pressure and temperaturerise in the main chamber are lower than what it would have been

in the absence of the crevice. The Zero-D approach of ‘adiabaticvolume expansion’ can successfully take into account the effectof heat loss to the wall and predict the temperature and pressureof the main reaction chamber for non-reactive mixtures and forreactive mixtures until there is negligible heat release, which isthe situation before the onset of the first-stage ignition. Therefore,in Fig. 1b, the Zero-D model result for first-stage delay closelymatches with the CFD predicted ignition delay. However, theZero-D model fails to account for the effect of non-uniform heat re-lease during the first-stage ignition and the consequent compres-sion of additional mass in the crevice. Therefore, it predictshigher pressure and temperature rise due to the first-stage ignitionand consequently shorter ignition delay than the prediction of theCFD analysis, as seen in Fig. 1b. Figure 3b shows a trace of the pres-sure difference between the main reaction chamber volume andthe crevice for reactive and non-reactive situation. Notable is thepressure difference during the first-stage ignition which may beinterpreted as the driver for the additional mass flow.

Figure 4 presents a comparison of the CFD and Zero-D simulatedresults over the entire NTC regime. Specifically, Fig. 4a comparesthe results of first-stage ignition delay (s1) and overall ignition de-lay (s) at compressed pressure near 10 bar, and Fig. 4b comparesthe magnitude of pressure rise (DP) due to first-stage ignition. Itis seen from Fig. 4a that the first-stage ignition delays are accu-rately reproduced by the Zero-D simulations. In addition, theZero-D simulated total ignition delays follow the same trend asthe CFD results and are consistently lower. As expected, Fig. 4bshows that DP from CFD simulations are much lower than fromZero-D and the discrepancy increases with decrease in tempera-ture. It happens because the intensity of the first-stage ignition,as perceived by the pressure rise during the first-stage ignition, in-creases with decrease in temperature. This, in turn, magnifies theeffect of non-uniform heat release and the discrepancy in DP, asnoted in Fig. 4b.

Figure 5 shows similar results at a compressed pressure of�15 bar for a different mixture with the molar composition of n-heptane/O2/N2/Ar = 0.562/6.184/62.969/30.285. While the overallbehavior in Figs. 4 and 5 is similar, the discrepancy in the total igni-tion delay and DP is slightly lower at the higher PC of �15 bar. Thisis consistent with heat loss effects being reduced at higher pres-sures due to the lower thermal diffusivity of the reacting mixture[20] and the discrepancy at increased pressures is expected to beeven lower.

Based on the above discussion, it is evident that although use ofa creviced piston avoids the formation of vortex in the reactionchamber, it also creates complications by allowing additional massflow into it when there is significant heat release before the finalevent of hot-ignition. Consequently, two sources of non-uniform

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Igni

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Fig. 4. (a) Comparison of CFD (circles) and Zero-D (triangles + lines) simulatedresults of first-stage ignition delays (s1) and total ignition delays (s) (b) DP for CFD(filled symbols) vs Zero-D (open symbols). Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/10.307/58.209/30.922. Pressure at TDC: PC � 10 bar.

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Fig. 5. (a) Comparison of CFD (circles) and Zero-D (triangles + lines) simulatedresults of first-stage ignition delays (s1) and total ignition delays (s) (b) DP for CFD(filled symbols) vs Zero-D (open symbols). Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/6.184/62.969/30.285. Pressure at TDC: PC � 15 bar.

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2424 G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427

heat release are created which confer a multi-dimensional effect,namely the cooler boundary layer and the crevice zone. Such mul-ti-dimensional effects cannot be easily modeled with a Zero-Dmodel, and use of CFD for validating detailed kinetic mechanismsis impractical because of enormous computational needs. The exis-tence of some boundary layer, and its multi-dimensional effect,cannot be avoided, but the consequence of the crevice on Zero-Dmodeling can be countered by resorting to the approach of ‘crevicecontainment’, which is discussed in the following section.

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Pre

Time (ms)

Fig. 6. Comparison of CFD (dashed line) vs Zero-D (solid line) simulations with‘crevice containment’. Conditions: Same as in Fig. 1b.

3.2. Results with ‘crevice containment’

In ‘crevice containment’, our idea is to separate the crevice zonefrom the main reaction zone by means of a seal which engages onlywhen the piston reaches TDC. During the compression stroke, cre-vice is connected to the main chamber and therefore avoids theformation of the roll-up vortex. During the post-compression per-iod, crevice and the main reaction chamber are separated by a sealto avoid the flow of additional mass into the crevice when chemicalheat release takes place in the main chamber. Again, CFD and Zero-D simulations are conducted for ‘crevice containment’. As men-tioned previously, in the CFD calculations, the effect of the seal issimulated by changing the entrance of the crevice to ‘wall’ whenpiston reached the TDC. Results for the same conditions as inFig. 1b are shown in Fig. 6. In comparison to Fig. 1b, a significantimprovement is noted in the prediction of the Zero-D model interms of the overall ignition delay and first-stage pressure rise.

Figure 7a compares the results of the CFD vs Zero-D for the first-stage ignition delay (s1) and overall ignition delay (s) with ‘crevicecontainment’. It is evident from Fig. 7a that ‘crevice containment’significantly reduces the discrepancy between the CFD and Zero-D results for the total ignition delays. Figure 7b presents a compar-

ison of the first-stage pressure rise from CFD vs Zero-D. Again, bycomparing with Fig. 4b, it is evident that a considerable improve-ment is made possible. Figure 7c compares percentage errors inthe predicted ignition delay and pressure rise with/without crevicecontainment. The discrepancy between the Zero-D vs CFD calcula-tions without crevice containment is attributed to both the coolerboundary layer and the crevice volume; whereas with crevice con-tainment it is attributed only to the cooler boundary layer. FromFig. 7c, it is inferred that the influence of the cooler boundary layeris much smaller than that of the crevice. Similar results for a differ-ent molar composition of n-heptane/O2/N2/Ar = 0.562/6.184/62.969/30.285 and compressed pressure of �15 bar are shown inFig. 8.

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Fig. 7. (a) Comparison of CFD (circles) and Zero-D (triangles + lines) simulatedresults of first-stage ignition delays (s1) and total ignition delays (s) with theapproach of ‘crevice containment’ (b) DP for CFD (filled symbols) vs Zero-D (opensymbols). (c) % error in ignition delay prediction |100 � [sCFD � szero-D]/sCFD| and DPprediction |100 � [DPCFD � DPzero-D]/DPCFD| with and without crevice containment.Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/10.307/58.209/30.922.Pressure at TDC: PC � 10 bar.

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Fig. 8. (a) Comparison of CFD (circles) and Zero-D (triangles + lines) simulatedresults of first-stage ignition delays (s1) and total ignition delays (s) with theapproach of ‘crevice containment’ (b) DP for CFD (filled symbols) vs Zero-D (opensymbols). (c)% error in ignition delay prediction |100 � [sCFD � szero-D]/sCFD| and DPprediction |100 � [DPCFD � DPzero-D]/DPCFD| with and without crevice containment.Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/6.184/62.969/30.285.Pressure at TDC: PC � 15 bar.

G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427 2425

The influence of the crevice containment on the temperaturefield is depicted in Fig. 9. It is noted that a small vortex is formeddue to the roll-up of the gases after the closure of the crevice. How-ever, this perturbation is very small and the crevice successfullysuppresses the bulk of the vortex.

3.3. Specific benefits of including ‘crevice containment’

Crevice containment offers the following three distinct advanta-ges. First, it prevents post-compression mass flow into the creviceand considerably improves the accuracy of the Zero-D modeling.Second, it reduces the rate of post-compression pressure drop. Acomparison of the pressure traces for the non-reactive simulationswith/without ‘crevice containment’, as shown in Fig. 10, illustratesthis clearly. The reduced rate of pressure drop for ‘crevice contain-ment’ results in mitigation of heat loss induced increase in ignitiondelays. Figure 11 shows the CFD simulated ignition delays with/

without ‘crevice containment’ to demonstrate this aspect. Last,‘crevice containment’ can significantly improve quantitative mea-surements from species sampling. Species sampling in RCM, espe-cially for heavy hydrocarbons with low vapor pressure, can beconducted by rupturing a diaphragm at predetermined time anddumping the reacting mixture into a large container [25,26]. Incurrent RCMs with crevices, but no containment, this procedure re-sults in mixing of gases in the main combustion chamber with theunreacted reactants in the crevice. This mixing can be completelyavoided if the crevice is effectively sealed, because only the reac-tants in the main reaction chamber will be dumped for sampling.

4. Concluding remarks

In this work, a novel approach of ‘crevice containment’ is intro-duced and evaluated through CFD simulations of n-heptane igni-

cylinder wall

axis

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crevice

Piston Motion

Fig. 9. Temperature fields (K) at different times for non-reactive simulations depicting the perturbation due to crevice containment. Conditions: same as in Fig. 1b.

0

2

4

6

8

10

12

-30 -20 -10 0 10 20 30 40

Pres

sure

(ba

r)

Time (ms)

without crevice

containment

with containment

Fig. 10. Comparison of CFD simulated non-reactive pressure traces with/without‘crevice containment’. Mixture molar composition: n-heptane/O2/N2/Ar = 0.3/5.5/31.06/16.5. Conditions at TDC: PC = 10.38 bar, TC = 761 K.

2

10

18

26

34

42

50

58

725 775 825 875

Igni

tion

Del

ay (

ms)

TC (K)

τ

τ1

Fig. 11. Comparison of CFD simulated results with (circles + line) and without(triangles) ‘crevice containment’. Mixture molar composition: n-heptane/O2/N2/Ar = 0.562/10.307/58.209/30.922. Pressure at TDC: PC � 10 bar.

2426 G. Mittal et al. / Combustion and Flame 158 (2011) 2420–2427

tion in an RCM. It is shown that apart from the upside of suppres-sion of the vortex, incorporation of a crevice in an RCM has a down-side also; namely enhancement of multi-dimensional effects inRCM, especially during the conditions of two-stage ignition. Thesemulti-dimensional effects compromise the accuracy of zero-dimensional modeling. The downside, however, can potentiallybe overcome with ‘crevice containment’, making the zero-dimen-sional modeling more accurate. Moreover, additional benefits arise

from crevice containment, namely a reduction in post-compressionpressure drop and a significant potential for the quantitativeimprovement in the experimental data obtained from species sam-pling. Reduced post-compression pressure drop also leads toappreciable reduction in ignition delays.

Although the concept of ‘crevice containment’ is straightfor-ward, there could be some challenges in its practical implementa-tion due to the requirement of engaging a seal at the end of thecompression stroke. One possibility is to have a combustion cham-ber that slightly steps inward at the TDC following the same profileas the piston taper. The containment could then be achievedthrough the mating surfaces at the TDC. Although the proposedconcept remains only an aspiration at present, we believe thatthe approach is implementable. We anticipate that this novel con-cept will represent yet another important step in the improve-ments in the design of RCMs and their utility and acceptabilityamongst combustion scientists.

Acknowledgment

This work was supported by The University of Akron, College ofEngineering.

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