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Diesel fuel jet lift-off stabilization in the presence of laser-induced plasma ignition


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Proceedings of the Combustion Institute 32 (2009) 2793–2800

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Diesel fuel jet lift-o? stabilization in the presence of laser-induced plasma ignition
Lyle M. Pickett a,*, Sanghoon Kook a, Helena Persson b, ¨ Oivind Andersson b
a b

Sandia National Laboratories, P.O. Box 969, MS 9053, Livermore, CA 94551, USA Lund University, Department of Energy Sciences, Box 118, S-221 00 Lund, Sweden

Abstract The mechanisms a?ecting lift-o? stabilization at diesel conditions were investigated by laser-igniting a diesel fuel jet upstream of its natural lift-o? position. Single-nozzle fuel sprays penetrating into an optically accessible constant-volume chamber were ignited using laser-induced plasma formation both prior to natural autoignition or after a quasi-steady lift-o? length was established. Fuel sprays ignited readily, with reaction kernels growing in connected regions. After laser-ignition, the lift-o? persists upstream of the natural lift-o? position for a substantial period of time indicating that upstream ignition has a strong in?uence on lift-o? stabilization. While not discounting the role of ?ame propagation downstream of the ignition event, these results show that upstream ignition sites can start a chain of events that e?ectively controls lift-o?. Lift-o? eventually returns to its natural position, but only after injection times that are too long for practical engines. The time of return to the natural position depends upon the relative distance of the laser-ignition site to the natural lift-o? length. A theory for fuel jet lift-o? stabilization based on ?ame propagation into pure fuel-ambient reactant streams fails to predict the long upstream stabilization away from the natural lift-o? length because turbulent velocities are higher in upstream regions of the fuel jet. Likewise, upstream lift-o? stabilization by autoignition of pure reactants (no mixing with combustion products) fails because of cooler temperatures and shorter residence times. A potential mechanism explaining the transient lift-o? response to laser-ignition is o?ered based on turbulent mixing with high-temperature combustion products found at the jet edges. Published by Elsevier Inc. on behalf of The Combustion Institute.
Keywords: Diesel combustion; Lifted ?ames; Plasma ignition; Soot

1. Introduction After the initial autoignition phase, the reaction zone of a diesel fuel jet stabilizes at a location

*

Corresponding author. Fax: +1 925 294 1004. E-mail address: lmpicke@ sandia.gov (L.M. Pickett).

downstream of the fuel injector [1,2]. The ?ame remains lifted during the ‘‘di?usion-burn” phase of heat release, allowing fuel and ambient gases to premix upstream of the reaction zone and strongly a?ecting combustion and soot formation processes downstream [3,4]. Unburned hydrocarbons are also formed between the injector and the lift-o? length shortly after the end of fuel injection [5]. Because of the signi?cance of lift-o? to

1540-7489/$ - see front matter Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2008.06.082

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these combustion and emission formation processes, a sound understanding of diesel ?ame lift-o?, and its response to engine operating parameters, is critical for control and optimization of diesel combustion. Motivated by this need, recent experimental and modeling e?orts have endeavored to provide insight into lift-o? stabilization at diesel conditions. A review of several computer modeling approaches seeking to capture experimental lifto? length trends may be found in Ref. [6]. Brie?y, these methods range from detailed chemistry modeling in well-stirred reactors (no subgrid combustion model) [7,8], to representative interactive ?amelet (RIF) models [6,9,10], or hybrid models including both premixed ?ame propagation and nonpremixed ?ames [2,11,12]. Well-stirred reactor models exhibit lifted ?ames [7,8]. RIF models using a single ?amelet predict an attached ?ame after autoignition [6,10]. However, if multiple ?amelets (MRIF) are released during the course of injection [2,6], or multiple steady ?amelets are computed to identify the scalar dissipation rate of ?ame extinction [9], then lifted ?ames are predicted. In general, the above models (single RIF excluded) mimic experimental lift-o? length trends with changes in ambient temperature, ambient pressure, and fuel injection pressure, indicating some capability for modeling lift-o? length at diesel conditions despite the fact that the combustion models are based on di?erent assumptions. In addition, simple scaling arguments for lift-o? based on either ?ame propagation or autoignition roughly agree [1]. Matching of experimental lifto? trends therefore does not prove that the underlying assumptions for the models are correct and further insight is needed. For atmospheric-condition ?ames, it is generally accepted that the mode of non-premixed ?ame lift-o? stabilization is by ?ame propagation [13]. However, recent experimental results show that lift-o? stabilization mechanisms may be substantially di?erent for the high-temperature, highpressure conditions typical of diesel combustion. In particular, there is strong evidence that selfignition, rather than solely ?ame propagation, serves a role. These evidences include a cool ?ame (?rst-stage ignition) upstream of ?ame lift-o? [1,2,5,14], shorter lift-o? lengths for fuels with shorter ignition delays [1,2], and igniting kernels that are upstream of, and detached from, the high-temperature reaction zone downstream [1,2]. Nevertheless, these examples of ignition do not exclude the possibility of ?ame propagation in other fuel jet regions. A second important factor that may cause differences between diesel sprays and typical steady nonpremixed ?ames is the transient nature of diesel injection and combustion. Studies have shown that diesel fuel jet regions that autoignite during

the premixed burn essentially become the location of a rapidly-stabilized lift-o? that changes little during the rest of injection (provided ambient and injector conditions are constant) [1,2,15]. This is true for injection durations considered long for an engine (e.g. 6 ms), showing that a ‘‘quasisteady” lift-o? length is easily de?ned. However, combustion products formed during the premixed burn may continue to a?ect lift-o? during the rest of injection. The lift-o? stabilizing in?uence of combustion products formed during the premixed burn was demonstrated, for example, in a recent modeling study where ramping rate-of-injection shapes (increasing velocity after lifto? stabilization) did not increase lift-o? [8]. The presence of premixed-burn combustion products can also explain the shorter lift-o? for higher cetane number fuels, as has been observed experimentally [1,2], because transient jet penetration is less for shorter ignition delay periods after the start of injection (ASI). In this study, we intentionally decouple the self-ignition/premixed-burn region and the quasisteady lift-o? by igniting the fuel jet in a region upstream of the naturally occurring lift-o? length. A high-energy laser is focused to form a plasma at speci?c spatial locations and times ASI, thereby igniting the fuel spray. By monitoring the response of the reacting jet, we can gain insights into the fundamental mechanisms a?ecting lifto? stabilization at diesel conditions, thereby providing guidance for further combustion model development at these conditions. 2. Experimental details Experiments were conducted in a large constant-volume combustion vessel under simulated diesel engine conditions. The facility is the same as that used for many of the earlier lift-o? length studies [1,3]. A detailed description of the facility is given in Ref. [16]. The vessel has a cubical combustion chamber (108 mm on a side) with optical access provided by side-port windows. Flow velocities induced by the vessel mixing fan are small (<0.5 m/s) relative to the fuel jet, creating an essentially quiescent environment within the vessel [16]. A single-hole, common-rail fuel injector is mounted in a metal side-port such that the diesel spray is directed into the center of the chamber. The injector and ambient experimental conditions for this study are listed in Table 1. Extended injection durations were used (10.8 ms) to allow su?cient time to monitor the response of laser-ignition. The fuel pressure decreased by only 2% because of a large fuel-line volume and low ?ow rate (small ?ow nozzle area). As a result, the rate-of-injection retained a nearly top-hat shape throughout injection. In addition, the large chamber volume allowed the reacting

L.M. Pickett et al. / Proceedings of the Combustion Institute 32 (2009) 2793–2800 Table 1 Ambient and fuel injector conditions Ambient temperature, Ta Ambient density, qa Ambient oxygen vol.% Ori?ce pressure drop, DP Ori?ce diameter, d Bosch nozzle shape Injection duration Fuel Fuel cetane number 750–900 K 14.8 kg/m3 21% 150 MPa 0.090 mm KS1.5/0.86 10.8 ms #2 Diesel 46 Table 2 Laser and imaging parameters Laser wavelength Laser energy Laser pulse duration Lens focal length Estimated spot dia/len. Laser position 1 [mm] Laser position 2 [mm] Camera framing period Camera resolution Camera lens (visible) Camera ?lters

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532 nm 60 mJ 8 ns 125 mm 0.02 mm/1.1 mm x = 20, r = 1.5 x = 35, r = 3.9 19.5 ls 256 ? 112 50 mm f/1.2 <500 nm SPF

jet to impinge and spread along the back wall without interacting with the incoming jet. Laser-induced plasma ignition was accomplished by focusing the second-harmonic of a Nd:YAG laser into the fuel spray as indicated in Fig. 1. The optical parameters and laser-spot positions are given in Table 2. The intent of the laser-spot positions was to probe upstream of the natural lift-o?, forcing ignition near combustible regions at the jet edge. Using the settings of Table 2, we found that ignition was successful for every injection, provided the laser pulse was delivered after the jet head had reached the plasma site. High-temperature chemiluminescence was recorded during each injection to visualize ignition and lift-o? length transients using a highspeed CMOS camera and settings given in Table 2. A 500-nm short-pass ?lter was used to collect strong chemiluminescence (e.g. CH*) while rejecting the laser wavelength and the strongest emission from soot incandescence, if soot forms downstream of the lift-o? distance. The imaging setup is not sensitive to much-weaker cool-?ame chemiluminescence. In addition to imaging, luminosity from the reacting jet was recorded with a pair of photodiodes to identify ignition timing, as well as overall soot incandescence level.

3. Results and discussion Sample chemiluminescence images and the resulting transient lift-o? distance are shown ?rst. Next, e?ects of laser beam location relative to the natural lift-o? length are investigated by varying ambient temperature and/or laser position. Finally, possible mechanisms for lift-o? stabilization are discussed. 3.1. Chemiluminescence imaging A chemiluminescence image sequence demonstrating the transient lift-o? of a laser-ignited fuel jet is shown in Fig. 2. The sequence corresponds to a condition where natural autoignition begins at approximately 1.3 ms ASI and lift-o? stabilization occurs shortly thereafter. At 3.9 ms ASI the laser is then ?red at Position 1 (20 mm), forming a plasma upstream of the natural lift-o? length. Selected chemiluminescence images are shown to highlight di?erent lift-o? transients. A light border line is also provided to indicate the low-level (100/4096 counts) detection limit of a chemiluminescence region. The minimum axial distance from the injector to this border line is de?ned as the lift-o? distance and it is plotted as a dotted line in Fig. 3. Times corresponding to the chemiluminescence images of Fig. 2 are indicated by circle symbols and labels. Figures 2 and 3 show that the premixed burn (PB) occurs at approximately 50–60 mm from the injector and the ?ame tends to stabilize at 40–50 mm shortly thereafter. Average (solid) and individual (dashed) curves show that lift-o? distance decreases only slightly with increasing ASI without laser ignition. The slight decrease in natural lift-o? can be explained by compression of surrounding ambient gases through diesel heat release [17]. Even though the estimated ambient temperature increase is small (maximum is less than 2%), these changes a?ect lift-o? length because of a strong temperature dependency [1,3]. A correction to lift-o? for temperature, density, and injection pressure variation [1] is shown as a gray solid line in Fig. 3, indicating that a

Injector

532 nm 1 cm, 60 mJ

f = 125 mm

Fig. 1. Schematic of combustion vessel and laser ignition setup.

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Fig. 2. Chemiluminescence image sequence with laser ignition at 3.9 ms ASI. Ambient temperature 850 K. Other conditions given in Table 1. Scale in mm.

Fig. 3. Lift-o? distance with and without (natural) laser ignition. Dotted line and circle symbols: images of Fig. 2. Dashed line: individual natural injection. Solid line: mean of 10 natural injections. Gray solid line: natural lift-o? corrected for ambient and injector changes during injection.

near-steady lift-o? distance is expected if ambient and fuel injector conditions were to remain constant. At 3.2 ms ASI a chemiluminescence kernel spontaneously forms upstream of the main ?ame body. The kernel grows in size and eventually merges with the reacting jet downstream. This type of self-ignition event (labeled A) is frequently observed during natural injection conditions and was documented previously at similar operating conditions in Ref. [1] and at other conditions in Ref. [2]. This particular self-ignition event occurs long after the premixed burn and it is typical for the quasi-steady period of combustion. Indeed, chemiluminescence images show six clear instances of upstream self-ignition kernels after 3.2 ms ASI for the individual natural injection

given in Fig. 3. These events correspond to rapid decreases in calculated lift-o? distance. At the time of laser-induced plasma ignition (3.9 ms ASI), a chemiluminescence kernel is formed. The kernel remains small for about 0.1 ms before the kernel intensity and size begins to increase signi?cantly. The kernel grows in a connected reaction zone before it merges with the downstream ?ame body (the original lift-o? at 45 mm) after only 0.25 ms after the time of plasma formation—an average speed of 100 m/s, which is similar to jet convective speed estimates. The original lifted ?ame appears largely unaffected by the upstream kernel’s growth as there is little change in its axial position during this time. Although the downstream portion of the laserignition region moves quickly downstream, the upstream reaction zone does not. The calculated lift-o? distance remains close to the original plasma location by the time that the ?ame merges downstream. Afterwards, the laser-a?ected lift-o? gradually increases to the natural lift-o? distance, but over a period of 5 ms. This time is signi?cantly longer than practical injection durations in an engine and the eventual merge back to the natural lift-o? distance could be discovered only by resorting to a long injection duration (10.8 ms) in the experiment. An immediate observation from the results above is that ignition events have a stabilizing e?ect on ?ame lift-o? that persists for a long time relative to jet convective times. In many respects, the laser-ignition event (LI) resembles the selfignition event (A). In both cases the ignition kernel appears to grow as a propagating ?ame until it merges with downstream reaction zones, and this combustion stabilizes the ?ame upstream for a signi?cant period of time thereafter. Although not excluding ?ame propagation or ignition from

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in?uencing lift-o? stabilization after the upstream ignition event, these results show that an upstream ignition site can initiate a chain of events that e?ectively controls lift-o? distance. 3.2. Laser beam location and timing relative to natural lift-o? The e?ect of the laser focal spot location and timing on lift-o? distance is demonstrated in Fig. 4 at the same ambient and injector conditions as Fig. 2. Two laser positions (20 and 35 mm) are shown with laser timing (3.9 ms) after the natural lift-o? length was established. Positioning the laser beam closer (35 mm) to the natural lift-o? length results in a shorter time period for the lift-o? to relax back to the natural distance, but the relaxation time remains signi?cant (about 2.5 ms). Rather than allowing the fuel jet to autoignite and establish a quasi-steady lift-o? prior to laser perturbation, another approach is to force ignition prior to autoignition. As indicated in the Introduction, this can change the location of the premixed burn, potentially altering the lift-o? distance afterwards. Laser timings of 0.2–0.4 ms were used to produce ignition prior to the natural ignition delay (1.3 ms) for the 850-K condition. By this time the spray has already penetrated to the 20-mm laser position. Figure 4 shows that forcing ignition prior to the premixed burn tends to stabilize ?ame lift-o? upstream for a substantial period of time (9 ms) before it returns to the natural distance. Compared to laser excitation at 3.9 ms (20 mm), this return time is much longer. We comment about potential reasons for this di?erence in the ?nal section. One consequence of lift-o? stabilization closer to the injector is that the reacting fuel–air mixture

becomes more fuel rich and there is a signi?cant increase in soot formation, as shown for natural lift-o? variation in Ref. [4]. A simple way to demonstrate the higher soot formation when using laser ignition is to show the natural luminosity from the fuel spray collected by a photodiode, as given in Fig. 5. Luminosity is plotted on log scale to show both chemiluminescence with low intensity, as well as soot incandescence with orders of magnitude higher intensity. Injections without laser ignition or later laser timings exhibit low luminosity after autoignition, a characteristic of luminosity by chemiluminescence [1]. This implies that the natural level of fuel–air mixing upstream of the lift-o? is su?cient to produce little or no soot formation. Laser ignition signi?cantly increases luminosity levels by the formation of soot. Laser ignition at 3.9 ms is followed by strong soot luminosity, and then a gradual decline back to chemiluminescence-dominated levels. The 3.9-ms timing shown is the same injection as Fig. 2, where bright soot luminosity regions are visible in these images after laser-ignition. Soot luminosity levels decline to chemiluminescence levels, however, as the lift-o? distance increases to the natural lift-o? distance and the reacting fuel–air mixture becomes more fuel-lean. Combustion and soot formation occurs rapidly ASI for the 0.3-ms laser timing. Peak luminosity levels actually exceed that of the 3.9-ms timing by nearly a factor of two, indicating higher sootformation levels. The time of high soot luminosity is also longer than that of the 3.9-ms timing. This is again consistent with the substantial period of time required for the lift-o? to relax to the natural lift-o? distance as observed in Fig. 4. Another way to change the relative position of the laser-induced plasma to the lift-o? length is to leave the laser position ?xed while varying the natural lift-o? length. This can be done by changing

Fig. 4. Lift-o? distance with laser ignition at di?erent locations and timings. 35-mm position is an individual injection. Ambient temperature 850 K.

Fig. 5. Luminosity from the fuel jet with and without laser ignition. 3.9-ms timing, with circle symbol for the image labeled ‘‘SOOT”, is same injection as Fig. 2.

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the ambient temperature, ambient density or fuel injection pressure [1,3], for example. We have varied all of these parameters but will show only ambient temperature results here (Fig. 6). The laser timing and position are 3.9 ms ASI, and 20 mm, respectively, similar to many results above. For ambient temperatures of 800 K and above, Fig. 6 shows that the laser timing occurs after the premixed-burn and quasi-steady lift-o? stabilization. For the 750-K condition, natural ignition occurs after 6 ms ASI, visible chemiluminescence takes place in fuel-lean mixtures after the vaporized jet impinges upon vessel walls, and combustion e?ciency is poor. Therefore, the laser timing at 3.9 ms ASI ignites the spray and produces reaction for a condition that would not ignite, or have a visible lift-o? otherwise. The lift-o? distance tends to return to the natural distance more quickly for higher ambient temperatures. As the distance from the laser-ignition site to the natural lift-o? length is less for higher ambient temperatures, this result is consistent with results from Fig. 4 where the laser position, rather than the natural lift-o? length, was changed. Although the time to return to the natural lift-o? distance decreases, the rate at which the lift-o? distance increases is lower for high ambient temperatures. This might be expected because the governing forces that cause the lift-o? to return to the natural lift-o? distance would likely be higher when this distance is large. Despite the faster rate of return at low ambient temperature, the laserignited lift-o? does not return to the natural lifto? distance prior to the end of injection for the 800-K and 750-K conditions. The extended separation distance from laser-ignition site to natural lift-o? length is apparently too great even though the injection duration is quite long.

3.3. Potential mechanisms for lift-o? stabilization after laser ignition The results above show that the laser-induced plasma timing and position have signi?cant e?ects on the lift-o? distance and soot formation and that these e?ects persist for long time periods. This persistent upstream lift-o? stabilization cannot be explained by lift-o? stabilization concepts limited to mixing and combustion of only pure fuel and ambient gases (no mixing with combustion products). For example, lift-o? stabilization based on ?ame propagation into pure fuel-ambient reactant streams would require turbulent ?ame speeds that match much higher jet velocities in the near-injector region—?ame speeds that it could not reach prior to laser ignition. Likewise, upstream lift-o? stabilization by self-ignition of pure reactants fails because of cooler vaporizedjet temperatures and shorter residence times near the injector. A general idea o?ered in the Introduction is that high-temperature combustion products formed during the premixed burn may continue to a?ect lift-o?. In this section we explore this idea further to develop a working hypothesis for the mode of lift-o? stabilization after laser-ignition. To begin, we identi?ed the location of high-temperature combustion products in the fuel jet by performing shadowgraph imaging of the reacting spray. The experimental setup is not described here because the diagnostic is fairly routine [18] and results are used only for discussion purposes. A sample shadowgraph image is shown in Fig. 7 for the identical conditions as the chemiluminescence images in Fig. 2 (850 K) but without laser ignition. The small-scale structure outside

Fig. 6. Mean lift-o? distance with and without laser ignition as a function of ambient temperature. All other conditions are given in Table 1. Laser position and timing is 20 mm, 3.9 ms ASI.

Fig. 7. (Top) Shadowgraph images of reacting jet at 850 K ambient temperature. (Bottom) Time-averaged boundary of steady reacting jet at 850 K (solid) and 900 K (dashed), along with average chemiluminescence at 850 K.

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of the jet represents natural temperature (refractive-index) gradients along the walls and within the vessel, while the ‘‘smeared” structure shows the vapor boundary or hot-combustion-product (low density) border of the jet. The reacting section (lift-o?) can be identi?ed as the location with a strong radial expansion at 40–50 mm, the same distance as the natural lift-o? in Fig. 2. A border line is generated on the image to highlight the vapor and high-temperature product boundary. The time-averaged (3–6 ms ASI) position of this jet boundary is shown at the ?gure bottom (solid), together with the shadowgraph jet boundary at 900 K (dashed) and the time-averaged chemiluminescence at 850 K over the same period. There are several signi?cant observations from Fig. 7. First, the radius of the high-temperature product boundary is larger than that of the chemiluminescence region. This is expected because combustion products exist at the jet edge where there is low mixture fraction (less than stoichiometric), while the chemiluminescence declines because of lack of reaction and lower temperature. Second, the jet radius of reactants entering the lift-o? location is much smaller than the combustion product radius after the lift-o? length. Third, experimental conditions with a shorter lift-o? length (e.g. 900 K) tend to produce a combustion product boundary that has a larger jet radius than that of a non-reacted portion of the fuel jet (upstream of lift-o? length at 850 K). From these observations we propose that laser ignition upstream (20 mm) of the natural lift-o? length (45 mm for 850 K) can temporarily produce a combustion product region at the jet outer edge, similar to a shorter lift-o? (900-K) condition. This high-temperature reservoir of gases, labeled R in Fig. 7, is de?ned as the volume of the laser-ignited jet minus the volume of the natural jet (the original 850-K border). This reservoir, consisting of combustion products and entrained fresh ambient gases (oxygen), exists outside the non-reacting jet spreading angle. A conceptual picture of how this high-temperature reservoir a?ects combustion and lift-o? stabilization is shown schematically in Fig. 8 for four di?erent zones of the jet. Zone 1 represents an ignition site, either natural (event A, in Fig. 2) or laser-generated (LI), occurring in the midst of a non-reacted fuel/ambient mixture. This ignition event leads to a type of reaction propagation in the surrounding fuel ambient mixture, as depicted in Zone 2. The current study does not identify if this reaction propagation is by connected conduction layers, as in a ?ame, or by mixing and subsequent volumetric ignition. However, the heat release expands the jet and feeds the high-temperature product reservoir (Zone 3) at the jet edge. High-temperature, low-density gases are known to reduce the local strain and turbulence level of shear ?ows, which would tend to stabilize the

e eratur ir o Temp High-uct Reserv Prod

1 Fuel + Ambient

Reaction 2 Propagation

4 Fuel-rich Products

Ambient

3

High-T Produ emperatu re c t Re s ervoir

Fig. 8. Schematic of lift-o? stabilization sequence after an upstream ignition event (natural or laser).

reaction zone. In addition, turbulent mixing and entrainment of this hot reservoir can ignite more of the fresh fuel–air charge that penetrates inside the reservoir (Zone 2). This additional combustion then serves to replenish the high-temperature reservoir, as shown by the arrows connecting Zone 2 and Zone 3, producing a feedback e?ect for lift-o? stabilization. It is interesting that combustion product re-entrainment (e.g. Zone 3) has been suggested as a potential lift-o? stabilization mechanism itself [19]. Lift-o? gradually moves downstream to the natural lift-o? length, unlike the modeling results of Ref. [8], because the high-temperature product reservoir is eventually depleted. Lift-o? tends to advance back to the natural lift-o? length more rapidly at low ambient temperature because the incoming fuel–air mixture temperature and combustion product reservoir temperature are lower. As a result, a greater mass of entrained high-temperature products are needed to produce ignitable/combustible mixtures. This acts to more quickly deplete the product reservoir mass and increase the lift-o? distance. However, low ambient temperatures also have a large product reservoir, simply because of the long distance from the natural lift-o? length to the ignition location (e.g. a long Zone 3). As a result of large mass in the high-temperature reservoir, the ultimate return to the natural lift-o? length requires more time at lower ambient temperature, or may not even occur by the end of injection, as shown in Fig. 6. The larger mass of Zone 3 can also explain the results of Fig. 4, where it was shown that early laser ignition, e?ectively forcing the premixed burn location and timing, tends to stabilize the lift-o? length upstream for a longer period of time than when auto-ignition and a quasi-steady lift-o? length are already established. A possible explanation for this result is that the mass of the hightemperature product reservoir formed upstream is larger when igniting the leading edge of the

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jet. It is well known that the penetrating jet head moves slower and has a larger spreading angle than a steady jet because of momentum exchange at the head of the jet. This would logically create a higher mass of combustion products in the nearinjector region when this mixture ignites. Although this study dealt with laser ignition upstream of the natural lift-o? length, the general idea depicted in Fig. 8 may be important for lifto? stabilization with natural self-ignition. Selfignition events upstream of (or near) the lift-o? length (like event A in Fig. 2) can serve to continuously re?ll high-temperature reservoirs at the jet edges. The reservoirs never fully deplete because self-ignition events occur with enough frequency to stabilize the ?ame. These self-ignition events are driven by the same physics that cause the initial autoignition and premixed burn at a given axial location, which ?lls the initial high-temperature product reservoir and serves to stabilize the ?ame thereafter. A dependency of ?ame lift-o? upon cetane number also follows from this idea. While the ideas for lift-o? stabilization center on ignition as the initial event, we cannot dismiss ?ame propagation as playing a vital role for overall stabilization. For example, the ignition kernels tend to grow in a ?ame-like pattern (Fig. 2). Flame propagation may therefore be necessary to ?ll combustion product reservoirs at the jet edges, which in turn stabilize the ?ame. This idea is consistent with the combustion model developed in Ref. [2]. Finally, from a practical perspective we note that there are many potential sources for ignition generation in the near-nozzle region in working engines. These include high-temperature residual gases, interaction with neighboring sprays, nonuniform charge gas temperature distribution, or pre-injections, for example. Results from this study show that if ignition sites do occur upstream, there is little chance that the lift-o? will return to a ‘‘natural” lift-o? length because of the lift-o? persistence upstream and typical short injection durations for engines. 4. Conclusions The mechanisms a?ecting lift-o? stabilization at diesel conditions were investigated by laserigniting a diesel fuel jet upstream of its natural lift-o? position. Laser ignition is found to have a strongly stabilizing in?uence as lift-o? persists upstream near the ignition site for a substantial period of time. Lift-o? eventually returns to its natural position, but only after injection times

that are too long for practical engines. The time of return to the natural position depends upon the relative distance of the laser-ignition site to the natural lift-o? length. A potential mechanism explaining the transient lift-o? response to laserignition is o?ered based on turbulent mixing with high-temperature combustion products found at the jet edges.

Acknowledgment Support for this research was provided by the U.S. Department of Energy, O?ce of Vehicle Technologies. Sandia NNSA #DE-AC0494AL85000.

References
[1] L.M. Pickett, D.L. Siebers, C.A. Idicheria, SAE Paper 2005-01-3843. [2] C. Pauls, G. Grunefeld, S. Vogel, N. Peters, SAE ¨ Paper 2007-01-0020. [3] D.L. Siebers, B. Higgins, SAE Paper 2001-010530. [4] L.M. Pickett, D.L. Siebers, Combust. Flame 138 (2004) 114 135. [5] M.P.B. Musculus, T. Lachaux, L.M. Pickett, C.A. Idicheria, SAE Paper 2007-01-0907. [6] R. Venugopal, J. Abraham, SAE Paper 2007-010134. [7] P.K. Senecal, E. Pomraning, K.J. Richards et al., SAE Paper 2003-01-1043. [8] H. Juneja, Y. Ra, R.D. Reitz, SAE Paper 2004-010530. [9] R. Venugopal, J. Abraham, Combust. Sci. Tech. 179 (2007) 2599–2618. [10] I. Magnusson, M. Balthasar, T. Hellstrom, THIE¨ SEL Conference, Valencia, Spain, 2006. [11] F.A. Tap, D. Veynante, Proc. Combust. Inst. 30 (2005) 919–926. [12] J.W. Campbell, G. Hardy, A.D Gosman, SAE Paper 2008-01-0968. [13] N. Peters, Turbulent Combustion, Cambridge University Press, 2000. [14] C.A. Idicheria, L.M. Pickett, SAE Paper 2006-013434. [15] C.A. Idicheria, L.M. Pickett, Proc. Combust. Inst. 31 (2007) 2931–2938. [16] Engine Combustion Network Experimental Data Archive, available at <http://www.ca.sandia.gov/ ECN/>. [17] B.S. Higgins, D.L. Siebers, SAE Paper 2001-010918. [18] G.S. Settles, Schlieren and Shadowgraph Techniques, Springer-Verlag, 2001. [19] J.E. Broadwell, W.J.A. Dahm, M.G. Mungal, Proc. Combust. Inst. 20 (1984) 303–311.


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