Fuel effects on NO formation in diesel-like jets in a vessel

Combustion and Flame 206 (2019) 201–210

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Fuel effects on NO formation in diesel-like jets in a vessel Tamara Ottenwälder a,∗, Thomas Raffius b, Christian Schulz b, Gerd Grünefeld b, Hans-Jürgen Koß b, Stefan Pischinger a a b

Institute for Combustion Engines, RWTH Aachen University, Forckenbeckstr. 4, Aachen 52074, Germany Institute of Technical Thermodynamics, RWTH Aachen University, Schinkelstraβ e 8, 52062 Aachen, Germany

a r t i c l e

i n f o

Article history: Received 16 November 2018 Revised 28 January 2019 Accepted 30 April 2019

Keywords: Di-n-butylether n-octanol Diesel NO Laser-induced fluorescence (LIF)

a b s t r a c t We demonstrated recently that NO concentration measurements are feasible even in the core of largely non-sooting diesel-like jets by combined laser-induced fluorescence (LIF) and spontaneous Raman scattering (SRS). However, the question arises whether previous findings hold also for other diesel-like jets. The current study focuses on fuel effects. In the previous NO measurements, n-heptane was used. It is replaced by pure di-n-butylether (DNBE) and a tailor-made blend of 50% DNBE and 50% n-octanol. These fuels are promising biofuel candidates and lead to an interesting variation of mixing during combustion (MDC). The determination of NO concentrations turns out to be generally feasible with the blend on the jet centerline in the quasi-steady phase of the injection event. The corresponding uncertainty is about ± 28 %. By contrast, some of the NO-LIF measurements in sooting DNBE jets are discarded, primarily due to increased light attenuation. For the remaining NO concentrations with DNBE the corresponding uncertainty is about ± 40 %. For the blend, results indicate that NO formation is very similar to the one in the n-heptane jets. Thus, the net effect of changed volatility and oxygenation is seemingly weak. By contrast, quasi-steady centerline NO concentrations are apparently significantly affected by MDC for pure DNBE. Relatively high NO concentrations are observed in this case, although products of highly fuel-rich fluid parcels are also present there. This study indicates the importance of MDC in such jets. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction This study aims at providing a deeper understanding of nitric oxide (NO) formation in diesel-like jets with alternative fuels. Such combustion processes are currently being developed to reduce CO2 and pollutant emissions as well as fuel consumption of diesel engines, e.g., [1–3]. However, there is a lack of multi-species (including NO) and temperature measurements in such jets with sufficient temporal and spatial resolution. For instance, to our knowledge, fuel effects on NO formation in diesel-like jets have previously not been investigated by laser-induced fluorescence (LIF) measurements under engine-relevant conditions. This is conducted in the present work. For instance, in PPCI LTC (partially premixed compression ignition low-temperature combustion) a number of different quantities were measured [4]. However, NO was not probed in the jets by LIF

∗

Corresponding author. E-mail addresses: [email protected], [email protected] (T. Ottenwälder), thomas.raffi[email protected] (T. Raffius), [email protected] (C. Schulz), [email protected] (G. Grünefeld), [email protected] (H.-J. Koß), [email protected] (S. Pischinger).

because of diagnostic challenges. Due to lacking NO-LIF data, NO formation was discussed based on OH data in that prior study. This may be improved by NO-LIF measurements in the future. Thus, it is one aim of the present study to investigate the feasibility of such measurements in similar jets. In this environment, NO-LIF generally suffers from several diagnostic difficulties. Firstly, light attenuation can be dramatic in highpressure combustion, in particular, in the ultraviolet (UV) range, which is generally relevant for NO detection by LIF [5]. Secondly, the quantification of NO-LIF generally requires the knowledge of detailed temperature and major-species data, which are usually not available [6]. Thus, NO concentrations were previously determined only a few times in conventional diesel combustion [5,7]. However, we demonstrated more recently that NO, CO, O2 , temperature, and attenuation measurements are feasible by LIF and spontaneous Raman scattering (SRS), respectively, in diesel-like jets in a high-pressure combustion vessel [8–10]. Compared to conventional diesel-type combustion, light attenuation turned out to be somewhat reduced in these generally low-sooting jets. LIF and SRS diagnostics were also modified in our prior works to achieve measurements even in the central region of the jets. In particular, spectrally highly resolved ( ∼ 0.4 nm) detection of LIF and SRS emissions was employed, using an imaging spectrograph. Thereby,

https://doi.org/10.1016/j.combustflame.2019.04.053 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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the effect of interfering LIF emissions of O2 and combustion intermediates/products on measurement uncertainty was greatly reduced. This and the reduced attenuation are major reasons why the uncertainty of the measured centerline NO concentration was found to be about ± 30 % in our work, whereas it was about ± 50 % in prior measurements in diesel engines [5,7]. The latter were conducted early in the cycle in [5] and late in the cycle in [7]. Late-cycle measurements with lower uncertainty were also presented in [5], but our prior and current measurements are rather comparable with the early-cycle results of [5], since we probe the jets basically during injection relatively close to the premixed burn. Previous qualitative planar LIF measurements in rather conventional jets demonstrated that NO primarily only formed in a shelllike structure on the periphery of the jets during injection [6,11– 13]. By contrast, in our prior 1-d LIF imaging in diesel-like jets with enhanced premixing [8,14,15], NO was found throughout the jet cross-section during injection. This is qualitatively consistent with theoretical expectations [4]. While conventional initial centerline mixtures are too fuel rich for NO formation, enhanced premixing leads to intermediate stoichiometry in the corresponding region of the “modern” jets. Initial centerline NO formation was investigated in more detail by our improved LIF measurements [8]. Surprisingly, the measured NO concentrations were found to be essentially consistent with corresponding 0-d constant-pressure simulations. This may be explained by a weakness of effects of turbulent mixing during combustion (MDC). This assumption is essentially consistent with O2 , CO, and temperature measurements by SRS and LIF in these jets [10,16]. Mixing and air entrainment downstream of the lift-off length (LOL) are denoted MDC in this article for brevity. According to [17,18], MDC can be affected by the heat-release rate. The latter was found to be particularly low for pure DNBE (di-n-butylether) in prior engine experiments, due to its high CN (cetane number) [2,19]. Thus, increased MDC was expected for DNBE in [10]. This appeared to be consistent with new experimental findings in that study. Many previous studies focused on premixing in diesel (-like) jets, i.e., mixing upstream of the LOL, e.g., [4]. By contrast, the effects of mixing downstream of the LOL, i.e., MDC, are not as well understood [17]. In this study, fuel effects on NO formation are particularly investigated. For this purpose, fuels are selected which lead to a variation of MDC. N-Heptane was used in [8]. In the present work, pure DNBE and a blend that consists of 50% (by volume) DNBE and 50% n-octanol are used under the same conditions. Compared to n-heptane, volatility and oxygen content of the blend are different. However, the apparent structure of the flames was previously found to be similar for these two fuels [10]. The question is whether this holds also for NO formation. DNBE and n-octanol are also used because they are promising biofuel candidates [2,3,20– 22]. The current blend was particularly recommended in [22]. 2. Experimental 2.1. NO-LIF measurements In this work, rotational–vibrational transitions of NO are excited via the A2  + ← X2  (0, 2) band system at ∼ 248 nm [23]. A band head, which is in the tuning range of the KrF∗ laser, is employed, so that several rotational transitions are excited simultaneously. This detection scheme is described in more detail in [7,15,23–26]. The experimental set-up is essentially described in [8]. Briefly, the focused laser beam (at 247.94 nm) has a pulse energy of ∼ 16 mJ in the probe volume. A 25 mm long section of the laser beam is imaged onto the entrance slit of an imaging spectrograph,

which is equipped with a grating that has 3600 grooves per mm. The resulting spectral resolution is ∼ 0.4 nm. In the exit plane of the spectrograph, an image-intensified charge-coupled device (ICCD) camera is installed. The spatial resolution of the laser measurements in the direction of observation, y, is limited by the 3 mm width of the laser beam, and the one along the laser beam (x-axis) is also 3 mm. The resolution in the z-direction is 1.2 mm. The distance between the nozzle hole exit of the investigated jets and the probe volume on the centerline is denoted nozzle distance (ND). Measurements are conducted at ND = 40 mm and 50 mm. 2.2. Diesel-like jet, high-pressure vessel, and additional diagnostics A 3-hole (diameter: 109 μm) diesel-like nozzle is attached to a piezo diesel injector. Fuel-rail pressures, prail , of 700 bar, 10 0 0 bar, and 1500 bar are used. The energizing time (ET) of the injector is 10 0 0 μs. The optically-accessible constant-volume high-pressure vessel, is described elsewhere [14,15]. It provides dried undiluted air at stationary ambient pressure and temperature, Ta and pa , of 50 bar and 800 K, respectively. A set-up for OH∗ imaging is implemented by a CMOS (complementary metal oxide semiconductor) camera with intensifier and filter combination for ∼ 308 nm detection [10]. The imageacquisition frequency is 10 kHz. The OH∗ -luminescence is used to detect the LOL, OH∗ penetration length (OHPL), and ignition delay (ID) [10]. High-speed (10 kHz) jet visualization is also conducted by shadowgraphy, using a collimated light beam and a CMOS camera, to detect the gaseous penetration length (GPL) [14,15]. A high power LED (light-emitting diode) and a CMOS camera are used to detect the liquid penetration length (LPL) in a separate set of experiments [27]. 3. Results and discussion 3.1. Overall jet characterization According to [10], the transient behavior of the entire spraycombustion event is briefly described in the following. Figure 1 illustrates both injection and high-temperature combustion as a function of tasoi (time after start of injection). The two panels of Fig. 1 correspond to the DNBE/n-octanol blend (a) and pure DNBE (b). Each panel shows results for three different prail -values as explained in the legend. The fuel injection event is characterized by GPL and LPL. The LPL curves show that a QSP (quasi-steady phase) is established for each of the fuels and injection pressures. High-temperature combustion (HTC) is characterized in terms of ID and LOL (solid, colored curves) in Fig. 1 [14,28–31]. It is averaged over 20 injections. The curves indicate that a QSP is established in the HTC events for all fuels and prail values. It is denoted QSPHTC in the following. The question arises whether the NO-LIF measurements are conducted in the QSPHTC . Figure 1 shows that this depends on both fuel and prail . For instance, for each fuel the classification of the LIF measurements depends on prail close to the end of injection (EOI), as indicated by the correspondingly colored dots which are explained in the legend. Some of those dots have two colors (black and green) indicating that these measurements are performed in the transient late period for 700 bar only. As noted in [10], current DNBE/n-octanol jets are generally similar in terms of the global quantities to the corresponding nheptane jets. The only significant difference is the quasi-steady LPL, which is by ∼ 45 % longer for the blend. The prior investigation also demonstrated that soot formation is very low for these two fuels, whereas it is relatively high for DNBE due to its low LOL. The low LOL of DNBE is attributed to its high CN.

T. Ottenwälder, T. Raffius and C. Schulz et al. / Combustion and Flame 206 (2019) 201–210

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Fig. 1. Overall jet characterization on the centerline for the blend (a), and DNBE (b). Operating conditions: Ta = 800 K, pa = 50 bar, and ET = 10 0 0 μs. Injection pressure is color coded. “NO-LIF” indicates time and location of laser measurements. Legend also explains their classification in terms of the period of the combustion event in which they are conducted. EOI: end of injection; ID: ignition delay; OHPL: OH penetration length; LOL: lift-off length; LPL: liquid penetration length; GPL: gaseous penetration length; ∗ : partial or complete semi-quantification failure.

Table 1 Properties of investigated fuels at 20◦ and 1 bar [10] and references therein. CN of the blend: 58. CN: Cetane number.

Boiling temperature Heating value Density CN Oxygen content Vapor pressure Surface tension Dynamic viscosity Enthalpy of vaporization Stoichiometric air/fuel ratio

Unit

DNBE

n-octanol

◦

142.4 38.4 769 100 12.3 6.4 22.2 0.64 345.5 12.7

195 38.4 824 39.1 12.3 0.125 27.5 7.3 341 12.7

C MJ/kg kg/m3 – % mbar mN/m mPas kJ/kg kg/kg

While the presented LPL data are adopted from [27], the data for the other global jet quantities shown in Fig. 1 are not. ID, LOL, and OHPL data were also presented in [27], but they were not recorded during NO-LIF measurements, in contrast to the ones shown in Fig. 1. Thus, the current data are slightly more relevant. However, by comparing the current and previous data, it can be stated that the jets appear to be essentially reproducible. This is important because many results of the prior study are currently used. 3.2. NO-LIF raw data To demonstrate the quality of the raw data, typical ICCD frames are depicted in Fig. 2. The panels correspond to the different fuels. The length of the 1-d probe volume (along x-axis) is slightly larger than the widths of the jets. Each frame is averaged over 20 laser shots, i.e., 20 injection events, for clarity. These data are acquired in the QSPHTC (tasoi = 2.7 ms) and at ND = 50 mm. Thus, they show dispersion spectra from the quasi-steady jet core, i.e., around x = 0. In both panels of Fig. 2, the most striking signal arises from NO-LIF around 236 nm, indicating a very good signal-to-noise ratio (SNR ∼ 10 in Fig. 2) of the NO measurements. This may

Fig. 2. Spatially resolved mean dispersion spectra for blend (a) and DNBE (b) at ND = 50 mm, tasoi = 2.7 ms, and prail = 10 0 0 bar. Other operating conditions as in Fig. 1.

be surprising at least in the case of the sooting DNBE jet in Fig. 2(b)). Obviously, NO-LIF detection is not dramatically affected by light attenuation and interfering broadband LIF from polycyclic aromatic hydrocarbons (PAH). The latter is reduced by anti-Stokes detection with increased spectral resolution ([8] and references therein). Accordingly, only relatively weak broadband LIF is observed between the NO- and O2 -LIF bands in Fig. 2. This is also demonstrated by “raw” dispersion spectra obtained from Fig. 2 by averaging the pixel intensities over areas which correspond to a spectral resolution of ∼ 0.1 nm and a spatial resolution of 3 mm (around the centerline). They are shown in Fig. 3. There are both ensemble-averaged and single-shot spectra. In particular, the former show the smooth, structureless wavelength dependence of the broadband LIF. (This is less obvious in the single-shot spectra due to increased shot noise.) The interfering “background” of the NOLIF band is therefore determined by interpolating the intensities

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ND = 40 mm. The other conditions are equal to the ones of the other spectra in Fig. 3. These conditions are explained in the following section. Note that the raw PAH-LIF signals in the “background” channels (BG1 and BG2) are similar at both ND values. However, light attenuation is significantly stronger at ND = 40 mm compared to ND = 50 mm, as demonstrated in the following section. Thus, PAH-LIF is significantly higher after attenuation correction at the shorter ND value. Despite higher PAH-LIF, the NO-LIF band at ∼ 236 nm can still be clearly observed in the inset of Fig. 3. PAH-LIF is not the dominating source of uncertainty in the concentration determination based on NO-LIF, as discussed in the next section. Note also that there appears to be an emission line at ∼ 238.5 nm in the inset spectrum of Fig. 3. It was also observed in corresponding attenuation measurements by N2 -SRS [32]. As suggested in that prior work and the references given therein, it may be caused by SRS from carbon-carbon double bonds (C=C) in intermediates. It may, therefore, indicate fuel breakdown. 3.3. Determination of NO concentrations

Fig. 3. Typical emission spectra measured in quasi-steady jet core at ND = 50 mm, tasoi = 2.7 ms, and prail = 10 0 0 bar for blend (black) and DNBE (red). Other operating conditions as in Fig. 1. Inset shows “worst case” DNBE spectrum for ND = 40 mm, tasoi = 2.7 ms, and prail = 10 0 0 bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

averaged over the spectral channels BG1 and BG2. The raw NO-LIF intensity is obtained by averaging the spectra in the grey area around ∼ 236 nm to reduce shot noise. The same strategy was applied to n-heptane jets in [8]. Figure 3 confirms that it is not necessary to modify it for the sooting DNBE jets. Note also that there is a higher level of broadband LIF for DNBE compared to the blend in Figs. 2 and 3. This seems to reflect the different soot formation of these fuels. Accordingly, the broadband LIF is denoted PAH-LIF in the following. Since PAHs are soot precursors, significant PAH-LIF was generally found only in sooting regions of diesel (-like) flames in [4]. Thus, increased PAH-LIF is currently also interpreted in this way. Figure 2(b) shows that significant PAH-LIF is detected throughout the DNBE jet indicating highly fuel-rich combustion. However, NOLIF is also measured across the jet cross section in Fig. 2(b) indicating rather near-stoichiometric combustion. Thus, the products of highly fuel rich and near stoichiometric combustion are seemingly well mixed in the DNBE jet. This may be explained by increased MDC observed previously in the DNBE jets [10]. Accordingly, Fig. 2(b) illustrates that O2 -LIF is also observed throughout the jet cross-section. Note also that the “simultaneous” detection of O2 -, NO-, and PAH-LIF throughout the DNBE jets is achieved in every single shot, however, with poorer signal-to-noise ratios (SNR). An example is given in Fig. 3. Overall, the DNBE jet appears to be approximately homogeneous. By contrast, the central region and periphery of the jet are resolved for the blend in Fig. 2(a). This is indicated by pronounced dips in the NO- and O2 -LIF profiles around the centerline. Accordingly, MDC appears to be rather low for the blend, as suggested previously. As stated in [10], centerline attenuation-corrected PAH-LIF (around 263 nm) was significantly higher at ND = 40 mm compared to ND = 50 mm for the quasi-steady DNBE jets. Essentially, this is also observed in the NO-LIF spectra in the anti-Stokes range. A “worst-case” example is depicted in the inset of Fig. 3. It is an ensemble-averaged raw centerline spectrum measured at

According to [8] and references therein, absolute NO concentrations, [NO], are inferred from LIF-signal intensities, I, by the following equation:

[NO] = I · C · A · F (T ) · Q

(1)

where C, A, F(T), and Q result from absolute calibration, light attenuation, temperature dependence, and quenching, respectively. Generally, the attenuation correction is particularly important, because its measurement uncertainty dominates the resulting error of [NO]. In particular, the LIF-quenching correction is generally less important. Another reason is that calibration in a free jet (N2 jet seeded with varying amounts of NO) is conducted at the same ambient pressure as the diesel-like jet measurements, so that the C uncertainty is ∼ 3 % [8]. Similarly, the effect of temperature uncertainty is rather small as also discussed in that prior work. The temperature dependence of NO-LIF, F(T), primarily arises from the population of the probed ground-state levels (Boltzmann fraction). Accordingly, since excited states are probed, F(T) increases by ∼ 78 % from 10 0 0 K to 260 0 K for fixed mole fractions. Information on minor factors that contribute to F(T) is given in [26]. Total, absolute measurement uncertainties presented in the following section are determined according to error propagation taking all of the factors in Eq. (1) into account. This includes shot noise, interfering emissions, drifting laser power, and the standard sampling error. More information is given in the discussions of the individual factors and elsewhere [8–10,15,16,32]. Since centerline. QSPHTC data are particularly important in this article, corresponding measurement uncertainties are primarily discussed in the following. Firstly, attenuation correction is considered. Since both laser beam attenuation and LIF trapping occur, the A factor includes both effects. For this purpose, A is defined as the inverse of the total light transmission determined by N2 SRS measurements, using the same light paths and approximately equal wavelengths [9,32]. This is achieved by employing the same laser, which is only slightly ( < 1 nm) tuned, and the N2 anti-Stokes emission, which nearly coincides spectrally with the NO-LIF emission band used. A-values are generally higher around the centerline compared to the periphery, presumably, due to longer light paths and more fuel-rich combustion in the central region. For brevity, this is not shown for the current oxygenates, but it was demonstrated for n-heptane jets in [32]. In Fig. 4, relevant centerline A data averaged over the QSPHTC are given as a function of prail . Measurement uncertainty and

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Fig. 4. Centerline QSPHTC light attenuation data for emission at ∼ 235 nm. Full and reduced data sets are explained in the text. Attenuation correction factor (a), its uncertainty (b), and its shot-to-shot variability (c). Operating conditions are Ta = 800 K, pa = 50 bar, and ET = 1000 μs.

shot-to-shot fluctuations are depicted in panels (b) and (c), respectively. They are determined as described in [32]. A fluctuations are inferred from ensembles of single-shot N2 -SRS measurements. Figure 4(a) shows that at least some of the NO-LIF measurements are considerably affected by light attenuation. For instance, if A equals 10, this implies that the detected signal has to be multiplied by 10 for attenuation correction. However, this is not a severe problem if the A uncertainty is sufficiently low. Thus, A uncertainties given in Fig. 4(b) are considered in the following paragraph. There are two types of curves in Fig. 4, which correspond to a full and a reduced data set. The latter arises from NO concentration determination failure for some of the used combinations of ND, prail , and tasoi . If A-uncertainties are larger than 60% in the full data set, NO concentration determination becomes unreliable and is therefore not conducted. Primarily NO data for DNBE are affected.

205

At ND = 40 mm, for prail of 70 0, 10 0 0, and 150 0 bar, this holds for tasoi -values of 1.4/1.9/2.7 ms, 1.4/1.9 ms, and 1.4 ms, respectively. At ND = 50 mm, only the NO measurement at prail = 700 bar and tasoi = 1.9 ms is “discarded”. In Fig. 1, the NO-LIF measurements which are “discarded” at least for one of the prail -values are marked by asterisks. This indicates that early-cycle measurements at ND = 40 mm are particularly affected. Indeed, the NO data are “discarded” for all of the prail -values only for DNBE at ND = 40 mm and tasoi = 1.4 ms, as noted above. Recall also that all of the QSPHTC measurements (all tasoi values) are “discarded” only with DNBE at ND = 40 mm and prail = 700 bar. Accordingly, there are no A-data points for this combination of ND and prail in the reduced DNBE data set in Fig. 4 (a)/(b)/(c). However, the corresponding A-data of the full data set are indeed consistently particularly high in all of the panels of that figure. Figure 4 also shows that the reduced A-data set for DNBE is roughly comparable to the one of the blend. These data are essentially also similar to the corresponding ones for n-heptane in [8]. Note also that [NO] determination failure is an issue for the blend only at ND = 40 mm, prail = 70 0/10 0 0 bar with tasoi = 1.4/1.9 and 1.4 ms, respectively. The origin of the observed A-data variations is briefly discussed in the following. Note that the fuel and prail dependencies observed in the different panels of Fig. 4 are generally similar. One reason for this is the fact that both A-values and A-fluctuations contribute to A-uncertainties. Particularly high A-data in the full DNBE data set at ND = 40 mm shown in Fig. 4(a)/(b)/(c) can be primarily explained by increased soot formation [10]. Interestingly, the corresponding data for ND = 50 mm are significantly lower in particular in Fig. 4(a). This strong axial gradient appears to be also consistent with that prior study. It seems to confirm that soot and its precursors contribute significantly to absorption only for DNBE [32]. Presumably, also the finding that A-data decrease generally with increasing prail in all panels of Fig. 4 can be at least partly explained in this way. The effect of prail on soot formation is well known and reflected by the generally varying quasi-steady LOL values in Fig. 1. In Fig. 4, there is one striking exception to that rule. A-data are relatively high for the blend at ND = 40 mm and prail = 1500 bar in all panels. Note that these particular measurements are conducted relatively close to the LOL as depicted in Fig. 1(a). Apparently, this leads to increased A-data. Strong indications that this is caused by intermediate species which occur at second-stage ignition were reported in [32]. Similarly, early-cycle measurements are particularly affected by absorption for all fuels, presumably, due to intermediates. In Fig. 4(b)/(c), A-data for ND = 50 mm and prail = 700 bar in the full DNBE data set are “surprisingly” high, although the corresponding A-value is not significantly increased in panel (a). The high A-uncertainty is primarily caused by interfering PAH-LIF, indicating poorer mixture preparation. Presumably, this is also the reason for increased fluctuations in A. According to [8], F(T) is determined based on temperature measurements by N2 -SRS which are described in detail in prior works as follows. While the basic methodology was presented in [16], its application to the current diesel-like jets was reported in [10]. Note that these temperature mean values and fluctuations are based on single-shot measurements to avoid averaging errors. However, in the present state of the diagnostic, these N2 -SRS and NO-LIF measurements cannot be conducted simultaneously. This is not a severe problem because sufficiently reproducible QSPHTC centerline jet conditions are primarily considered, which exhibit low temperature variabilities at high temperature. The latter is important because the F(T) curve is relatively flat in the high-temperature range [26].

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Fig. 6. -dependence of the adiabatic flame temperature and Q for DNBE. Black horizontal lines show mean temperature and Q-values. Error margin indicated for Q.

Fig. 5. Centerline QSPHTC temperature (panels (a) and (b)) and F(T) uncertainty data (c). Operating conditions as in Fig. 4.

Currently relevant centerline temperature data averaged over the QSPHTC are presented as a function of prail in Fig. 5. Note that mean temperatures in panel (a) are generally higher than 20 0 0 K. There is only one exception which is attributed to the proximity of the concerning laser measurement (for the blend at ND = 40 mm and prail = 1500 bar) to the LOL. In Fig. 5(b), temperature fluctuations are generally lower than about 10 %. Only the measurement close to the LOL exhibits somewhat higher variability. Importantly, Fig. 5(c) demonstrates that the resulting uncertainty of F(T) is always relatively low ( < 5 %), as explained in the prior paragraph. The LIF quenching correction is conducted as follows. Basic information is given in [33]. Due to the similarity of the results for n-heptane and the blend, Q factors for the blend are determined according to [8]. Briefly, they are based on prior temperature and species measurements, 0-d constant-pressure homogeneous reactor calculations, and quenching cross-sections in [34]. The calculations yield species concentrations which were not directly measured, assuming adiabatic equilibrium conditions. This was found to be particularly precise for most centerline data in the QSPHTC . The resulting uncertainty in [NO] was estimated to be about ± 5 % there. However, for some of the centerline QSPHTC data, which were measured particularly close to the LOL or in which relatively poor mixture preparation was found, the calculations are less reli-

able and the corresponding uncertainty contribution was reported to be up to ± 15 %. These uncertainties are therefore also estimated for the blend. However, the determination of Q and its uncertainty is modified for DNBE jets, because -values (equivalence ratio) are less precisely known due to poorer mixture preparation and enhanced MDC. For the centerline QSPHTC data, a larger range of possible mean -values is estimated as follows. Corresponding mean temperatures were reported to be in the range from ∼ 2300 K to ∼ 2660 K for all prail and ND values [10]. These temperature values are close to the maximum possible temperatures of DNBE/air combustion, i.e., the adiabatic-equilibrium temperature for  = 1, which is 2661 K [35]. This indicates that approximately complete, near-stoichiometric combustion is established in the central region of the quasi-steady jets. Thus, a range of mean -values is estimated as illustrated in the upper subfigure of Fig. 6. This plot shows the -dependence of the adiabatic-equilibrium temperature around  = 1. The measured mean temperature (2490 K) is indicated by a black horizontal line. It intersects the temperature curve at certain -values which are marked by vertical lines. This leads to an estimated mean Q-value as depicted in the lower subfigure of Fig. 6. In this plot, the -dependence of calculated normalized equilibrium Q-values is shown. The mean Q-value, which approximately corresponds to the range of mean Q-values discussed above, is marked by a black horizontal line. Its uncertainty is estimated to be about ± 10 %, as also illustrated in Fig. 6. Accordingly, -values are presumably in the range from ∼ 0.6 to ∼ 1.4 as also indicated by dashed vertical lines in Fig. 6 (lower plot). Although essentially near-stoichiometric combustion is therefore investigated, there are likely small sooting regions, at least at ND = 40 mm [10]. Thus, there may be a small fraction of intermediate species, which are neglected in the previous discussion. Unfortunately, quenching cross-sections are not known for these species ([8] and references therein). They are currently neglected because they are essentially unimportant for the comparison of NO formation for the considered fuels. As demonstrated below, [NO] is generally higher for DNBE than for the other fuels (centerline QSPHTC conditions). This main finding cannot be affected by possible additional quenching by these intermediates, because this would only

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lead to even higher [NO] for DNBE. If the concerning quenching cross-sections are particularly low, this is also unimportant because these intermediates are minority species. In contrast to centerline [NO] uncertainties discussed so far, corresponding data are not given for the jet periphery. They cannot be precisely determined, primarily, because of relatively poor flame repeatability, using ensembles of “only” 20 shots [9]. Limited mean flame repeatability also contributes to centerline [NO] uncertainties in the transient periods of the injection event [8]. However, this does not mean that these uncertainties are always particularly high, as demonstrated in the next section. For instance, total relative late-cycle uncertainties for DNBE are similar to corresponding QSPHTC data, because A uncertainties are lower after EOI due to leaner mixtures. Flame repeatability is generally high for centerline QSPHTC measurements [8]. Thus, it is not an important factor for the measurements which are primarily presented in the following. 3.4. Centerline [NO] data The reduced data set defined in the prior section yields centerline results depicted in Fig. 7. To investigate the temporal evolution of the jets, [NO] data are given as a function of tasoi . Ensemble averaged (20 shots) results and dimensionless shot-to-shot fluctuations are presented in Fig. 7(a)/(b)/(e)/(f) and (c)/(d)/(g)/(h), respectively. The upper four panels correspond to the blend while the lower ones show DNBE data. The left/right columns of panels correspond to ND = 40 mm and ND = 50 mm, respectively. prail is color coded according to legend. Total, absolute measurement uncertainties are indicated by bars in Fig. 7(a)/(b)/(e)/(f). Corresponding relative uncertainties averaged over the QSPHTC and all prail values are ± 28 % and ± 40 % for the blend and DNBE, respectively. Thus, for the blend it is very similar to n-heptane results [8]. Note also that there are no error bars shown in Fig. 7(c)/(d)/(g)/(h), because they cannot be precisely determined. The depicted relative [NO]-variability is denoted “actual fluctuations” since apparent single-shot measurement uncertainties are “subtracted” [8]. This means that measurement uncertainties caused by photon statistical noise (shot noise) are taken into account according to error propagation, in order to determine real [NO] fluctuations approximately. Shot noise is inferred from the signal intensities in terms of numbers of photoelectrons generated in the image intensifier. However, real NO fluctuations are expected to be higher than the measured ones due to the limited spatial resolution of the diagnostics [36]. Despite missing uncertainties, given “actual fluctuations” yield interesting information presented below. Importantly, [NO] data are generally indeed essentially constant in the QSPHTC in all panels of Fig. 7. There is only one striking exception. The data point is marked by + in Fig. 7(b). Its low [NO]-value can be, presumably, explained by a short residence time of the concerning fluid parcel in high-temperature combustion, which is indicated by a relatively long ID (prail = 700 bar) and the proximity of that laser measurement to the OHPL curve shown in Fig. 1(b). Interestingly, there is generally little effect of prail observed in all panels of Fig. 7. Two obvious exceptions in the QSPHTC are described in the following. The first one is the data point marked by + discussed in the prior paragraph. Secondly, [NO]-variability is clearly higher for prail = 1500 bar compared to the lower prail values in Fig. 7(c). This is most likely an effect of the particularly short ( ∼ 8 mm) axial distance between quasi-steady LOL and probe-volume location for the blend at ND = 40 mm and prail = 1500 bar shown in Fig. 1(a). However, all of the concerning centerline measurements are conducted within the flame, as

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demonstrated by single-shot spectra (not shown for brevity). They always exhibit significant PAH-LIF, which is not detected in pure air or the early cool flame. Actually, all measurements are likely performed in high-temperature combustion, because the shot-to-shot standard deviation of the LOL is only about 1 mm [10]. Apart from the two exceptions discussed in the preceding two paragraphs, there is also little effect of measurement ND observed in the QSPHTC in all panels of Fig. 7 (compare the two columns of panels). This may be surprising for DNBE, i.e. in Fig. 7(e)/(f). Recall that strong axial gradients were previously found in other species (CO, PAH, OH, and O2 ) measurements in that jet region [10]. Apparently, increased MDC reported in that prior study has little effect on mean [NO]. The most striking fuel effect observed in the QSPHTC in Fig. 7 is that [NO]-values are generally higher for DNBE than for the blend, compare panels (a) and (b) to (e) and (f), respectively. (Overlapping error bars are found only for some of the concerning data points.) This finding may be surprising because one might expect that NO formation was reduced in a more sooting jet [37]. However, the current observation may be explained by increased MDC for DNBE. Consistently, the QSPHTC [NO]-values for DNBE are generally also higher than the corresponding ones for n-heptane presented in [8]. In fact, the entire [NO]-data for the blend shown in Fig. 7(a)– (d) are generally similar to the previous results for n-heptane. The most striking difference is that [NO]-values were found to be generally higher after EOI (tasoi = 3.7 ms) compared to the QSPHTC for n-heptane, while this is currently only observed for the higher prail -values and ND = 50 mm for the blend, see Fig. 7(b). However, this is a less important difference, since the present study focuses on the QSPHTC . The fast temporal evolution of the unsteady jets shortly after EOI is not well resolved because only a few NO-LIF measurements are conducted in the transient, late period as illustrated in Fig. 1. As suggested in [8], particularly high [NO]-values after EOI may be explained by more fuel-lean mixtures, which are expectedly found in that period [4]. However, since these mixtures exist only for a short time, they are currently not always probed. Note also that all QSPHTC [NO]-values presented in Fig. 7 are non-vanishing. Indeed, NO-LIF is generally observed in the corresponding single-shot spectra (examples shown in Fig. 3). This is also consistent with [8,14]. Recall that this is an interesting finding because quasi-steady jet-core NO-LIF signals were previously found to be vanishing in conventional diesel jets [6,12]. This was attributed to particularly fuel-rich centerline mixtures caused by poor premixing and weak MDC. Accordingly, the current finding may be primarily explained by enhanced premixing for the blend and n-heptane. By contrast, MDC most likely plays an important role in sooting DNBE jets [10]. To investigate MDC effects, the QSPHTC [NO] data of Fig. 7 are plotted as a function of the distance between (quasi-steady) LOL and probe volume, z, in Fig. 8. Each data point is averaged over the available QSPHTC data for one combination of fuel type, prail , and ND. n-Heptane data are adopted from [8]. Error bars show measurement uncertainties. In Fig. 8, z is considered because one might think that the availability in the jet center of NO transported by MDC from the side of the flame where the reaction occurs (in the classical model) and NO is formed increased almost linearly with increasing z. This appears to be indeed the case at least for the data of the blend and n-heptane in Fig. 8. However, most of the DNBE data points are higher than one would expect for a simple linear relationship. It is plausible that this is caused by particularly strong MDC for DNBE which was suggested in [10]. The classical model of diesel jets may also not be suitable, because a more homogeneous flame structure was reported for PPCI LTC jets in [4,38] which are rather similar to the current ones.

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Fig. 7. Centerline [NO] of reduced data set for the blend (a–d) and pure DNBE (e-h). Mean values (a,b,e,f) and shot-to-shot variabilities (c,d,g,h). prail is color coded. Other operating conditions as in Fig. 1. Left column: ND = 40 mm; right column: ND = 50 mm.

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Excellence Initiative by the German federal and state governments to promote science and research at German universities. The work was also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy Exzellenzcluster 2186 “The Fuel Science Center” ID: 390919832. The authors also thank technician R. Herwig for his assistance. References

Fig. 8. Comparison of centerline (QSPHTC ) [NO] data. Fuels are color coded according to legend. Other operating conditions as in Fig. 1. z: distance between LOL and probe volume.

Figure 8 also confirms that the results for the blend and nheptane are essentially very similar. This seems to be consistent with the conclusion drawn in [10] that the net effect of changed volatility and oxygenation is weak for these fuels. 4. Summary and conclusions Recently, initial NO formation was investigated by LIF and SRS measurements in largely non-sooting diesel-like n-heptane jets. Surprisingly, quasi-steady centerline [NO]-values were found to be essentially consistent with simple 0-d homogeneous-reactor simulations. This was apparently consistent with weak MDC observed in these jets. Moreover, a weak effect of prail was previously reported. The question arises whether this holds also for other diesel-like jets in particular, with different fuels and varying MDC. Thus, corresponding NO-LIF measurements are currently conducted in DNBE and DNBE/n-octanol jets, because they were recently characterized by SRS and increased MDC was found for pure DNBE. These fuels are also promising biofuel candidates. First, diagnostics are discussed. Primarily, [NO] measurements are more challenging in DNBE jets due to increased soot formation. Consequently, some of these measurements are discarded and a relatively high uncertainty ( ∼ ± 40 %) is determined for the reduced set of quasi-steady centerline data. By contrast, NO-LIF measurements are generally feasible with the blend. The corresponding uncertainty is about ± 28 %. Second, fuel effects on [NO] are considered. Overall, results for the blend are very similar to the ones previously found for nheptane, which are summarized in the first paragraph of this section. This seems to be consistent with prior SRS measurements which indicated that these jets are indeed very similar. Thus, the prior conclusion that the net effect of changed volatility and oxygenation was seemingly weak is also confirmed. By contrast, some of the results for pure DNBE are strikingly different. In particular, higher quasi-steady centerline [NO]-values are generally found compared to the other fuels. This may be surprising because of the increased sootformation of DNBE. However, this is basically explained by enhanced MDC, which apparently leads to a rather “well mixed” flame structure. With regard to the conducted prail sweep it can be concluded that there is no noticeable effect for DNBE, which is rather similar to the corresponding findings for the other two fuels. Acknowledgments This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the

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