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An experimental study of hydrogen autoignition in a turbulent co-flow of heated air
Proceedings of the
Proceedings of the Combustion Institute 30 (2005) 883–891
Combustion Institute www.elsevier.com/locate/proci
An experimental study of hydrogen autoignition in a turbulent co-flow of heated air C.N. Markides, E. Mastorakos* Hopkinson Laboratory, Engineering Department, University of Cambridge, UK
Abstract The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Autoignition; Turbulence; Hydrogen ignition
1. Introduction Understanding the effects of turbulence on autoignition in inhomogeneous mixing layers is not only a topic of fundamental importance, but also crucial for the development of the new generation of low NOx diesel and homogeneous charge compression injection engines and lean premixed prevaporized (LPP) gas turbines, whose further
*
Corresponding author. Fax: +44 1223 332662. E-mail address: [email protected] (E. Mastorakos).
development is hindered by our capabilities to predict the interaction between turbulence and the slow chemistry leading to autoignition. Experimental work with autoignition has mostly concentrated on uniform mixtures to produce information relevant to kinetic modelling. Autoignition in non-uniform mixtures has been dominated by analyses, experiments, and computational modelling of laminar counterflow layers between cold fuel and hot air [1–6], which have provided an excellent understanding of the development of ignition kernels in the presence of strain rate. Recently, these experiments have been extended to turbulent counterflows of hydrogen [7,8]. Both
1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.024
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these experiments and successful probability density function (PDF) modelling [9] have produced the very interesting result that, depending on the temperature of the air stream, autoignition may not occur at all (at least during the residence time available, as determined by the bulk strain rate). It was also shown that increased turbulence in the air stream resulted in a higher critical temperature necessary for autoignition, suggesting a delaying effect on the pre-ignition reactions. These findings are in subtle contrast to direct numerical simulations (DNS) that have shown that turbulence may accelerate autoignition. The DNS results [10–16] have revealed that, locally, autoignition occurs at a Ômost reactiveÕ mixture fraction (nMR) and at regions with low values of the scalar dissipation rate (v). Since these simulations did not last very long relative to the turbulence turnover time and also had to resolve the fuel–air interface, the autoignition time was found to strongly depend on the initial condition (i.e., the initial value of v), with the turbulence affecting autoignition time only insofar as it affected the emergence of the lowest value of the conditional v|nMR. Therefore, autoignition could be promoted by fast mixing due to the earlier emergence of well-mixed nMR spots. There has been no evidence from DNS so far that autoignition can be precluded completely due to high bulk strain rates. The possibility of autoignition not occurring in situations of sustained, high mean values of v, has been theoretically demonstrated in [17,18], where the critical value of the mean of v depended additionally on the fluctuations of v, and consequently the turbulence. There seems to be a difference between flows of parabolic nature (e.g., mixing layers, such as those studied by DNS) where v decays and autoignition always occurs eventually, and counterflows, where v does not decay as the reactants flow radially outwards [19]. This discrepancy may explain why in the latter flow, critical conditions exist where autoignition does not develop at all [7–9]. The present paper presents an experimental effort to observe autoignition in turbulent non-premixed flows of a parabolic nature, in an attempt to improve our understanding of the above subtle points concerning turbulent autoignition. From a more practical point of view, measurements of autoignition times in the presence of turbulence and data for modelling must be collected. In addition, of key interest is the uncovering of a connection between the ensemble-mean behaviour (e.g., the average ignition timing measured from many cycles in a diesel engine), and the possibility of relatively rare events causing dangerous autoignition (e.g., in a gas turbine premix duct). The statistics of autoignition have not been previously explored experimentally, but could be the outcome of calculations such as those by PDF or large Eddy simulation methods that are more appropriate for
ÔextremeÕ events, such as ignition and extinction. Therefore, the present data can serve as an additional test for advanced turbulent reacting flow models. The objectives of this paper are: (i) to describe a new experiment for determining the effects of turbulence on non-premixed autoignition and (ii) to quantify the autoignition location in axisymmetric flows of hydrogen in hot co-flowing air, as a function of air temperature and velocity. In the following section, a brief description of the experimental methods is given, followed by the results and their discussion. The paper closes with a summary of the main conclusions. 2. Experimental methods 2.1. Apparatus Figure 1 shows the experimental arrangement. Air was electrically preheated and flowed into a circular quartz tube, after passing through a perforated plate (3.0 mm holes and 44 % blockage) to promote turbulence. The heaters were designed and manufactured in-house, to avoid performance and reliability issues encountered with commercially available models and were heavily insulated. Air temperature measurements at the exit of the heaters were used to control the supplied voltages so as to provide a steady temperature at the en-
Fig. 1. Apparatus schematic (not to scale).
C.N. Markides, E. Mastorakos / Proceedings of the Combustion Institute 30 (2005) 883–891
trance of the quartz tube. The main test section consisted of a 0.50 m long and 25.00 mm inner diameter (R = 12.50 mm) vacuum insulated quartz tube to reduce heat losses. This ÔjacketedÕ tube provided full optical access and required no additional insulation. Diluted fuel mixtures were injected from a centrally located nozzle and autoignition, visualized by hydroxyl radical (OH) chemiluminescence, was observed at some distance downstream. Hydrogen (H2) and nitrogen (N2) were supplied from compressed cylinders (99.999 % purity), and flowed into the test section axially and continuously, through a 2.25 mm internal diameter (d) stainless steel, thin-walled (0.32 mm) injector at ambient pressure. The fuel nozzle was located 63 mm downstream of the perforated plate to allow the turbulence to develop. A 500 mm long and 0.25 mm diameter mineral insulated K-Type (Cr/Al) thermocouple was placed through the injector tube, all the way to the injection location, and allowed a real-time measurement of the fuel injection temperature (Tfuel). A certain amount of preheating of the fuel was unavoidable due to heat transfer from the hot co-flowing air. Thus, Tfuel was not an independent parameter, but depended on the air temperature (Tair), and the mixture flow rate and composition. To minimize this effect, the injector was encased in a 2.0 mm thick-walled ceramic tube for most of its length, which kept the fuel at a lower temperature. Tair was measured 25 mm upstream of the injector with a 0.25 mm diameter ceramic sheathed barewire R-Type (Pt/Pt and 13%Rh) thermocouple, protruding 9 mm into the flow from the wall. Air velocities (Uair) of up to 35 m/s, with Tair up to 1015 K, have been achieved. Fuel velocities (Ufuel) ranged from 20 to 120 m/s, with Tfuel between 650 and 930 K. An additional 1.0 mm diameter, mineral insulated N-Type (Nicrosil/Nisil) thermocouple was placed at the exit of the tube to monitor heat losses along the length of the tube and to detect combustion. An air temperature drop of 3 K in the first 100 mm was typical. Air and hydrogen flow rates were measured by digital mass flow controllers (Bronkhorst Hi-Tec El-Flow), while calibrated rotameters were used for the nitrogen flow rates, with indeterminate (random) experimental errors of ±1–2% and ±6%, respectively. The random error associated with Tair and Tfuel was ±4 K, accounting for both temperature drifts during a run (±1 K) and measurement error, including hardware performed cold junction compensation. The raw measurements of Tair were corrected for both radiation and conduction losses, with a method similar to that of [6], but modified for the present geometry and conditions. The correction was approximately 5 K due to radiation and a further 5 K due to conduction. Tfuel was only corrected for radiation. Due to these corrections, the reported values of
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Tair and Tfuel have determinate (systematic) uncertainties of ±6 and ±1–2 K. Measurements of temperature across the section and of mean velocity under hot conditions were made with 76 lm diameter ceramic sheathed bare-wire K-Type thermocouples and a miniature high temperature Pitot probe. These measurements showed that the mean temperature was approximately uniform (to within 5%) for radii of about r/R = 0.15–0.75, but decreased sharply towards the walls to about 85–90% of the centreline temperature. The mean velocity was also uniform across the tube for r/ R = 0.15–0.65 at operating temperatures. Hot– wire measurements in cold flow conditions exhibiting Reynolds number similarity with the actual hot flows were used to measure the initial mean and fluctuating velocities. In cold conditions, the turbulence intensity was about 12–13% and uniform across the flow for r/R = 0.2–0.8 while the integral lengthscale (Lturb ) was about 4– 5 mm for r/R = 0.20.7 and increased nearer the jet and the wall of the tube. Furthermore, the mean velocity was uniform within the extended region r/R = 0.150.8, because of the absence of the thermal boundary layer. Therefore, we may conclude that the air flow was essentially uniform across the tube cross-section in terms of velocity, turbulence, and temperature, in the immediate region away from the nozzle and walls, and consequently that specifying Uair and Tair is sufficient to accurately define the initial conditions of the co-flow at the injection plane. Similar results were established at all axial locations where data are presented in this paper. Power density spectra of the velocity showed a well-developed turbulence and a 5/3 roll-off at the expected inertial sub-range frequencies. Turbulence Reynolds numbers based on Lturb and the velocity fluctuations spanned the range 90– 160, similar to what was achieved in DNS. The large-eddy turbulent time scale (sturb) based on Lturb was in the range 1–2 ms. 2.2. Operation and data analysis Due to the large thermal inertia of the insulation, approximately one hour of operation with the required flow rate of air at elevated temperature was allowed for the experiment to reach thermal equilibrium. Once the heater exit thermocouples indicated steady conditions, the fuel was switched on, and final adjustments to the flow rates were made before the system reached its final state. The basic measured variables were Tair and Tfuel, and the volumetric flow rates of the air, hydrogen, and dilution nitrogen. The dilution of the fuel stream is described by the mass fraction of hydrogen at injection ðY H2 Þ. Images of OH chemiluminescence were taken with a LaVision Nanostar ICCD camera fitted
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with a Nikon Nikkor UV lens, with and without an additional OH filter band-centred at 307±10 nm. Up to 2000 images per operating condition (set of Tair, Uair, Tfuel, and Ufuel) were taken to observe enough autoignition events. The exposure time was between 50 and 150 ls and high gain settings were used so as to capture the weak chemiluminescence signals. Pixel intensity in the Ôraw snapshotsÕ was the result of transverse, lineof-sight integration of OH chemiluminescence. To remove background noise, and increase signal-to-noise ratio, individual images were processed with adaptive Wiener filtering, median smoothing, and fourth order low-pass filtering in the frequency spectrum. These Ôprocessed snapshotsÕ were normalized, and the image background noise was removed by setting the pixel intensity to zero below a certain threshold, and to unity above it. Hence, the presence of OH at a pixel location resulted in a normalized unity signal, while the absence of OH in zero. Regions of presence of OH chemiluminescence were deemed an Ôautoignition spot,Õ with axial location denoted by Lign. All Ônormalized snapshotsÕ corresponding to a particular operation point were superimposed to compile the two-dimensional joint PDF of autoignition spot location. Alternatively, the autoignition spot location could have been quantified based on the average OH chemiluminescence, obtained by averaging all instantaneous processed snapshots, before normalization. In practice the two approaches produce similar results, yet their interpretation is slightly different. In this paper, we have used only the PDFs and report solely on the Ôaxial ON/OFF presenceÕ of OH, without dealing with the absolute chemiluminescence intensity or attempting to radially quantify the phenomenon. The mean (ÆLignæ) and standard deviation (LRMS) of Lign were calculated directly from the PDFs by double numerical integration. A further definition of autoignition length was based on the minimum axial location (Lmin) of an OH spot observed during a data run. This location was defined, where the PDF reached a rise height from the background of 3 % of the peak value. The minimum autoignition length so derived agreed very well (within ±5 %) with direct long-exposure photographs. For certain experiments, based on the results of [4], the fuel was doped with small amounts of methane ðY CH4 Þ so as to make the autoignition event visible. It was observed that, for these conditions, the presence of traces of methane did not significantly affect the phenomenon. In these cases, a CCD Digital Video Camera Recorder, set to a maximum exposure of 250 ls and a slowest shutter speed of 1/3 s, filmed for approximately 40 s and generated about 125 still images for each data point. For these images, the minimum autoignition length (Lmin) was determined. The estimated random error, due to the instruments
and the processing of the images in the determination of the instantaneous Lign, was of the order of ±4%, but Lmin was more sharply determined to within ±1%. All lengths have an absolute systematic uncertainty of ±5% due to scaling. Finally, various high-speed images were taken (Kodak Ektapro-HS4540 Motion Analyzer) at 13.5 kHz (frame intervals of 74.1 ls), but the spectral sensitivity was insufficient to detect hydrogen autoignition. This motivated detailed measurements with acetylene (not shown here), as this fuel gave adequate illumination. These measurements confirmed the main findings of the present paper concerning autoignition and demonstrated the same qualitative features, such as those of the emergence of the kernel and the subsequent flame propagation, which is something we have observed for all fuels tested. Hence, we treat the qualitative behaviour of acetylene as typical autoignition behaviour for other fuels as well, and the high-speed acetylene image sequences taken, as sufficient to show the characteristic evolution of turbulent autoignition kernels. The experimental investigation of hydrogen autoignition was organized as follows. To simulate a mixing pattern that is relatively straightforward to treat theoretically, equal jet and co-flow velocities (Uair = Ufuel) were used, to investigate the effect of background turbulence, Tair, dilution, and methane addition on autoignition. The effect of the jet velocity was also investigated by a series of experiments where Ufuel > Uair. In such cases, the turbulence across the test section was affected by the mean shear introduced by the jet. 3. Results and discussion 3.1. Operating envelope—bulk behaviour Visual observations over a wide range of operating conditions showed the following qualitatively distinct flow regimes. For low Tair and high Uair and Ufuel or lower Y H2 , no autoignition was observed within the length of the tube under investigation, although for certain conditions very faint and infrequent autoignition spots could be heard from further downstream. As Tair decreased further, local explosive autoignition completely disappeared, at first replaced by a situation of low heat release in the tube which raised the temperature at the exit by a few to tens of degrees before no effect could be detected at all. We call this the ÔNo IgnitionÕ regime. For a certain range of higher Tair, lower Uair and Ufuel, or higher Y H2 , a statistically stable situation was observed, where instantaneous autoignition occurred in the form of random spots, at about 10–50 d from the injector. Figures 2A–C demonstrates typical examples. These spots resulted in neither flashback nor acted as flame
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Fig. 3. Single acetylene autoignition kernel sequence at 13.5 kHz. Flow from below.
Fig. 2. Examples of instantaneous raw OH chemiluminescence snapshots. (A–C): Uair = 26 m/s, Tair = 1000 K, Ufuel = 120 m/s, and Y H2 ¼ 0:13; (D) Same but Tair = 1020 K—propagation towards the injector.
anchoring points, but were short-lived ignition kernels that died out (quenched). Autoignition in these conditions produced an intense noise, with decreasing pitch and loudness as the temperature decreased or velocity increased. This is the ÔRandom SpotsÕ regime. At low Uair and Ufuel, and high Tair, as soon as the fuel was switched on, autoignition and subsequent flashback occurred. The first appearance of an ignition kernel was randomly located in space, and flashback occurred with what seemed like a triple flame [20] and resulted in a normal jet diffusion flame. Figure 2D shows such an event in progress. Generally, the temperature needed to cause a flashback increased monotonically with both coflow air and fuel jet velocities. This is the ÔFlashbackÕ regime. A further ÔLifted FlameÕ regime has been observed at even higher velocities and temperatures, in which a stable lifted hydrogen flame is achieved. This observation is consistent with the experiments of [21], where lifted hydrogen jet flames in a co-flow of hot combustion products were examined. Unlike the ÔNo IgnitionÕ and ÔRandom SpotsÕ transition, which contains intermediate states that make this boundary unclear, such as those of low heat release and very infrequent spots, the boundary between the ÔRandom SpotsÕ and ÔFlashbackÕ regimes is quite sharp, tested, and found to stretch 1–2 K with relatively high consistency. Although more data are necessary to accurately map out the regime boundaries, the main features outlined here have been observed under a wide range of velocities, and with other fuels including acetylene and various hydrogen/methane mixtures. In the rest of this paper, we consider the ÔRandom SpotsÕ regime, since to the authorsÕ knowledge, it has not been previously characterized. 3.2. Visualization of autoignition kernels To get a better understanding of the spatial and temporal manifestation of the autoignition kernels in the ÔRandom SpotsÕ regime, high-speed films of acetylene autoignition were taken at various conditions. Figure 3 illustrates a typical se-
quence. The films exhibited an evolution from a dark background to a small spot, which became a spherical shell flame front propagating in all directions. Following this, quenching of the spot was observed, with the image returning to a completely dark background. The life-span of the kernels was of the order of 100–200 ls, in which time they were advected by the flow a distance of the order of a few millimeters. Both the spot and these flamelets contribute to the OH images such as those observed in Fig. 2. Therefore, each autoignition eventÕs location was measured to within 2– 5 mm in the axial direction, depending on whether the ICCD camera would capture parts of the resulting flame or the autoignition event itself, and by how much this kernel had moved during exposure. The frequency of the appearance of the kernels was very dependent on the conditions. It increased monotonically with increased Tair and decreased Uair, but even close to flashback conditions when the frequency was maximum, successive kernel realizations appeared to be independent events. Although the lengthscale of the first appearance of a kernel could not be determined, because the light intensity at this early stage was too small, it was evident that the kernels at birth had dimensions a very small fraction of their eventual fully grown size (a few mm). The generation of double flame propagation following autoignition in a non-premixed medium and the subsequent decay of the flamelets is completely consistent with the DNS data [10]. The random appearance, quenching, and reappearance of autoignition spots in a statistically steady situation are possibly a new experimental finding. It should be noted that the quenching observed in Fig. 3 involves flame disappearance rather than pre-ignition kernels that failed to ignite as observed by DNS [15], as only fully fledged combustion could be visualized in these images. 3.3. Autoignition lengths Calculation of ÆLignæ and LRMS was done by integrating over the PDFs compiled by processing many images such as those in Fig. 2. Figure 4 shows typical PDFs for high and low Tair cases, and comparison with average OH chemiluminescence images. It is clear that, for low Tair autoignition occurs farther from the injector, but also that the PDFs rise sharply at the injector-side. This allows an unambiguous definition of the min-
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Fig. 4. Two-dimensional PDFs of autoignition location and corresponding average OH chemiluminescence. Axial intensity profiles are also shown. (A) Uair = Ufuel = 26 m/s, Tair = 1010 K, and Y H2 ¼ 0:13; (B) Same but Tair = 1000 K.
imum autoignition length Lmin, which could have been similarly defined in terms of the average OH. The PDFs show a smooth tail at long distances from the injector, which is consistent with the fuzzy boundary between the ÔRandom SpotsÕ and ÔNo IgnitionÕ regimes. Figure 5 shows the effects of Tair, Uair, Ufuel, Y H2 , and Y CH4 on the autoignition lengths. Autoignition shifted downstream with decreasing Tair, increasing Uair, and increasing Ufuel for all conditions tested. Comparisons between Figs. 5A and B or C and D show that Lmin was about 60–70% of the mean, ÆLignæ (although this is partly a reflection of the ÔcontaminationÕ of the PDF from flamelets evolving from the ignition spots). When the two velocities were equal, Figs. 5A and B, the measurements only covered a small range of variables. Within this range, it was confirmed that relatively small quantities of methane in hydrogen ðY CH4 ¼ 0:05; 0:07; and 0:10Þ and the degree of dilution ðY H2 ¼ 0:13 and 0:26Þ do not seem to have as significant an effect on autoignition as the velocities and temperatures, similar to the results of [4]. More extensive experiments with unequal jet and co-flow velocities, Figs. 5C and D, show that the trends are non-linear, and that the axial location increases sharply as Tair decreases, which explains why the boundary between the ÔNo IgnitionÕ and the ÔRandom SpotsÕ regimes is not well defined. In contrast, for every one of the curves shown, flashback would have occurred for a temperature higher than the hottest point plotted by about 1–2 K, which shows that the boundary between the ÔRandom SpotsÕ and the ÔFlashbackÕ regime is sharper. In addition to the observed difference between Lmin and ÆLignæ, the randomness of the autoignition location can be quantified with LRMS. LRMS was large (up to 40% of the mean), which is an over-estimation because the PDFs include contributions from the short-lived propagating flamelets, as previously discussed. However, the wide
Fig. 5. Mean and minimum autoignition lengths as a function of Tair. (A) ÆLignæand (B) Lmin for Uair = Ufuel and various Y H2 and Y CH4 ; (C) ÆLignæand (iv) Lmin for varying Uair, Ufuel but fixed Y H2 ¼ 0:13 and Y CH4 ¼ 0. In (A and B), markers are empty for Y CH4 ¼ 0 and filled for Y CH4 6¼ 0. In (C and D), markers are for Ufuel: d (40 m/s), + (70 m/s), · (100 m/s), and * (120 m/s). In all four subplots, lines are for Uair: solid (20 m/s), dashed (26 m/s), dashed dot (28 m/s), and dotted (32 m/s).
spread of autoignition locations was also captured and confirmed in many high-speed films that do not have this problem (not shown). Hence, there seems to be a real effect of the turbulent flow in the spatial distribution of the ignition kernel, although the unavoidable fluctuations of Tair, which were not measured here, must also make a contribution that should be elucidated. 3.4. Autoignition delay times Mean and minimum (Æsignæ and smin) residence times until the point of autoignition could be defined simply as ÆLignæ/Uair and Lmin/Uair. However, this does not account for the velocity of the mixture in the jet, which can be a factor of 2–5 times higher than Uair. In addition, many of our data show autoignition at close distances to the injector nozzle. Hence, the jet momentum cannot be ignored. An improved attempt at a suitable definiˆ as tion can be made based on a mean velocity U ˆ and smin = Lmin/U ˆ . For smin: in Æsignæ = ÆLignæ/U smin ¼
Lmin ¼ ^ U
Z 0
Lmin
dx ; U ðxÞ
ð1Þ
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with U(x) being the mean centreline velocity in the jet. For jets in turbulent co-flows with comparable velocities, the normalized axial centreline velocity air can be described by UUðxÞU ¼ F ðdxeff Þ, where deff ¼ fuel U air qfuel 12 dð qair Þ . The functional relation F ðdxeff Þwas obtained from [22]. It was checked in a limited number of cases, with Pitot tube and hot wire measurements, and good agreement was found. The densities of both streams were calculated from their known composition, and the measured Tair and Tfuel. Figure 6 shows the results for the residence times from injection to the minimum autoignition length (smin). Similar trends were found for Æsignæ. The residence time was hence of the same order as the turbulent timescale. The effect of increasing Tair was a decrease in smin, as expected. On the other hand, we note that, for a given Tair, the data for different Ufuel but for the same Uair collapse approximately on a single band, while the bands of curves for different Uair do not. Thus, it can be concluded that the residence time until the point of autoignition was mostly affected by Uair, and consequently the background turbulence intensity, and not Ufuel or the bulk straining of the flow, at least within the experimental envelope that was investigated. This suggests that the phenomena are not simply kinetically controlled, but that the turbulence and mixing play a role in determining the location of autoignition. The fact that for the same Tair, a higher Uair results in delayed autoignition, reveals
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a delaying effect of the air stream on autoignition, consistent with [8]. Significant effects of air flow velocity and turbulence were also observed for a more complex chemistry and geometry in the experiments of [23], where a liquid fuel spray was perpendicularly injected into a heated air stream, with the air velocity being lower than the fuel injection velocity. Here, it was found that as the turbulence was strengthened, the autoignition delay increased in the region above 1300 K. It is possible that the higher Uair result in higher v, which in turn delay autoignition. Measurements of n and tests with different turbulence promoting grids and injector diameters must be performed to understand these observations better (e.g., changing the perforated hole size while keeping its blockage the same should not affect the turbulence intensity but will directly affect the integral length scale, while changing the injector diameter will not affect the background turbulence, but will affect the mixing patterns prior to autoignition). Measurements of n will also assist the interpretation of the ÔNo IgnitionÕ regime, as it is not clear at present if this lack of ignition is because nMR disappears due to excessive mixing, or because the chemistry becomes too slow relative to the turbulence thus revealing a critical condition. Figure 6 can also be considered as an Arrhenius plot and used to define effective activation temperatures (Tact). The slopes range from about 25,000 to 40,000 K in the Æsignæ plots, and 60,000– 110,000 K for smin. These values are higher than those found in uniform hydrogen–air mixtures (e.g., experimental data at 1 bar [24] show a Tact of about 30,000 K at the same temperature range). Furthermore, the Arrhenius plots show deviations from straight lines, and they may also depend on the velocity. The data do not span a wide enough range to draw solid conclusions, but it seems that a single Tact cannot be defined. Turbulent mixing can cause non-linearity in Arrhenius plots even with one-step chemistry [17], and hence the present data suggest that the pre-ignition reactions have been influenced by the turbulence. However, this conclusion must be re-examined with other fuels that show a less complex kinetic behaviour than hydrogen, where thermal feedback may not be a prerequisite for autoignition [2].
4. Conclusion
Fig. 6. Residence times defined by Eq. (1) as a function of Tair for Uair = Ufuel (A) and Uair„Ufuel (B). Symbols as in Fig. 5.
It has been shown that it was possible to achieve a variety of non-premixed autoignition phenomena, with a jet of (diluted) hydrogen or hydrogen/methane mixture injected into a turbulent co-flow of hotter air. Four ÔregimesÕ were identified: ÔNo Ignition,Õ ÔRandom Spots,Õ ÔFlashback,Õ and ÔLifted FlameÕ. The boundary between ÔRandom SpotsÕ and ÔFlashbackÕ was very sharp (i.e., spanned 1–2 K 2air temperature for a given
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set of air and injection velocities), while between ÔNo IgnitionÕ and ÔRandom SpotsÕ it could only be defined within a range of about 5 K in air temperature. In the statistically stable ÔRandom SpotsÕ regime, independent kernels appeared and were advected with the flow before they disappeared. Flashback was found to be promoted by a decrease in air and/or injection velocities. Optical measurements of autoignition lengths and subsequent analyses showed that, inside the tested range, autoignition was relatively insensitive to changes in hydrogen dilution and the presence of traces of methane. Both minimum and mean autoignition lengths increased by a decrease in air temperature or an increase in either air or injection velocities. In an attempt to quantify the autoignition delay time, a residence time until autoignition location was defined in terms of the decay of the centreline velocity of the jet within the co-flow. This residence time approximately collapsed the data within each band of similar air velocities, more so for the case where the jet and co-flow velocities were equal. The autoignition delay at the same air temperature increased with increasing air velocity. This suggests that the present phenomena are not only chemically controlled and that turbulent mixing delays autoignition.
References [1] A. Lin˜an, Acta Astronautica 1 (1974) 1007–1039. [2] T.G. Kreutz, C.K. Law, Combust. Flame 104 (1–2) (1996) 157–175. [3] T.G. Kreutz, C.K. Law, Combust. Flame 114 (3–4) (1998) 436–456. [4] C.G. Fotache, T.G. Kreutz, C.K. Law, Combust. Flame 110 (4) (1997) 429–440.
[5] C.G. Fotache, C.J. Sung, C.J. Sun, C.K. Law, Combust. Flame 112 (3) (1998) 457–471. [6] C.G. Fotache, T.G. Kreutz, D.L. Zhu, C.K. Law, Combust. Sci. Technol. 109 (1995) 373–393. [7] J.D. Blouch, C.J. Sung, C.G. Fotache, C.K. Law, Proc. Combust. Inst. 27 (1998) 1221–1228. [8] J.D. Blouch, C.K. Law, Combust. Flame 132 (3) (2003) 512–522. [9] J.D. Blouch, J.-Y. Chen, C.K. Law, Combust. Flame 135 (3) (2003) 209–225. [10] E. Mastorakos, T.B. Baritaud, T.J. Poinsot, Combust. Flame 109 (1-2) (1997) 198–223. [11] H.G. Im, J.H. Chen, C.K. Law, Proc. Combust. Inst. 27 (1998) 1047–1056. [12] R. Hilbert, D. Thevenin, Combust. Flame 128 (1–2) (2002) 22–37. [13] S. Sreedhara, K.N. Lakshmisha, Proc. Combust. Inst. 28 (2000) 25–34. [14] S. Sreedhara, K.N. Lakshmisha, Proc. Combust. Inst. 29 (2002) 2051–2059. [15] T. Echekki, J.H. Chen, Combust. Flame 134 (3) (2003) 169–191. [16] P. Domingo, L. Vervisch, Proc. Combust. Inst. 26 (1996) 233–240. [17] E. Mastorakos, A. Pirez da Cruz, T.B. Baritaud, T.J. Poinsot, Combust. Sci. Technol. 125 (1997) 243–282. [18] E. Mastorakos, R.W. Bilger, Phys. Fluids 10 (1998) 1246–1248. [19] E. Mastorakos, A.M.K.P. Taylor, J.H. Whitelaw, Combust. Flame 91 (1) (1992) 55–64. [20] H.G. Im, J.H. Chen, Combust. Flame 119 (4) (1999) 436–454. [21] R. Cabra, T. Myhrvold, J.-Y. Chen, R.W. Dibble, A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29 (2003) 1881–1888. [22] Y. Sakai, T. Watanabe, S. Kamohara, T. Kushida, I. Nakamura, Int. J. Heat Fluid Flow 22 (2001) 227–236. [23] Y. Mizutani, T. Takada, in: ICDERS, 17th, 1999. [24] S.A. Bozhenkov, S.M. Starikovskaia, A.Yu Starikovskii, Combust. Flame 133 (1–2) (2003) 133–146.
Comments Antonio Cavaliere, University of Naples, Italy. Has the noise in the movie shown during the presentation been analyzed in frequency? Is this frequency correlated to the extinction/ignition process and if so, in which way? Reply. Analysis of the pressure signal from these experiments has been done, but was not presented in this paper. The ignition and extinction processes are both evident in the pressure traces. The frequency of occurrence of auto-ignition (fign) correlates very closely with the auto-ignition length (Lign), so that decreased Lign results in significant increase in fign. These results are being prepared for publication. d
Peter Jansohn, Paul Scherrer Institute, Switzerland. Does the conclusion ‘‘ignition delay times increase with higher velocity i.e., turbulent velocity fluctuations’’ hold
true for both types (co-flow with same velocity, jet flow with (strongly) different velocities for air vs. fuel) of experimental conditions? Reply. Yes, in both cases and keeping everything else the same, the ignition delay time increases monotonically with the co-flow air velocity for all conditions tested. d
Jacqueline H. Chen, Sandia National Laboratories, USA. How is the ignition delay time affected by the temperature of the air co-flow? In particular, how near are you to the cross-over temperatures, Tc on average and are the ignition kernels affected more by the local scalar dissipation rate due to the proximity of the kernel temperature to this critical temperature Tc where branching and recombination rates are the same?
C.N. Markides, E. Mastorakos / Proceedings of the Combustion Institute 30 (2005) 883–891 Reply. The effect of the air co-flow temperature (Tair) on the ignition delay time (tign) is presented in Fig. 6 of the paper. Tc for hydrogen/air chemistry is approximately 925 K at 1 atm [1]. The range of Tair in our experiments is between 945 and 1010 K, so it is not appreciably higher. The initial temperature of the most reactive mixture fraction is expected to be slightly lower than Tair because the fuel stream is colder by about 50– 300 K. The effect of the scalar dissipation rate in the locality of the auto-ignition kernels has not been examined here. We cannot be sure whether at the higher velocities, when tign is larger, this is indeed caused by higher dissipation rates at the ignition locations, but relevant measurements are in progress.
Reference [1] B. Pellett, Chinitz, Review of Air Vitiation Effects on Scramjet Ignition and Flameholding Combustion Processes, 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (2002). d
Peter A.M. Kalt, University of Adelaide, Australia. The temperature of the fuel jet is not controlled in
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the same way as the ambient air. Changes in jet velocity will accordingly change the mass flux of colder fuel, and the local temperature. Furthermore, auto-ignition may be occurring in regions of steep local temperature gradients. The correlation between jet velocity and auto-ignition height could be strongly coupled to these issues of dilution with ‘‘cold’’ fuel. For example, the lift-off height in the Cabra/Dibble vitiated coflow burner ([20] in paper) is sensitive to fluctuations in temperature as small as 5–10 K. Please comment. Reply. Evidence from analysis and DNS of autoignition in non-premixed flows suggests that ignition occurs at the ‘‘most reactive’’ mixture fraction ([10] in paper), which is determined by the chemistry and the two stream temperatures. Because the most reactive mixture fraction is very lean for the present conditions (detailed chemistry calculations show that it is less than 0.05), the initial, ‘‘frozen,’’ temperature of the fluid particles that are likely to auto-ignite is very little affected by the fuel temperature. We chose to present the data in terms of air temperature for this reason. The sensitivity to air temperature is very strong, as Fig. 6 of the paper shows.
Proceedings of the Combustion Institute 30 (2005) 883–891
Combustion Institute www.elsevier.com/locate/proci
An experimental study of hydrogen autoignition in a turbulent co-flow of heated air C.N. Markides, E. Mastorakos* Hopkinson Laboratory, Engineering Department, University of Cambridge, UK
Abstract The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Autoignition; Turbulence; Hydrogen ignition
1. Introduction Understanding the effects of turbulence on autoignition in inhomogeneous mixing layers is not only a topic of fundamental importance, but also crucial for the development of the new generation of low NOx diesel and homogeneous charge compression injection engines and lean premixed prevaporized (LPP) gas turbines, whose further
*
Corresponding author. Fax: +44 1223 332662. E-mail address: [email protected] (E. Mastorakos).
development is hindered by our capabilities to predict the interaction between turbulence and the slow chemistry leading to autoignition. Experimental work with autoignition has mostly concentrated on uniform mixtures to produce information relevant to kinetic modelling. Autoignition in non-uniform mixtures has been dominated by analyses, experiments, and computational modelling of laminar counterflow layers between cold fuel and hot air [1–6], which have provided an excellent understanding of the development of ignition kernels in the presence of strain rate. Recently, these experiments have been extended to turbulent counterflows of hydrogen [7,8]. Both
1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.024
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these experiments and successful probability density function (PDF) modelling [9] have produced the very interesting result that, depending on the temperature of the air stream, autoignition may not occur at all (at least during the residence time available, as determined by the bulk strain rate). It was also shown that increased turbulence in the air stream resulted in a higher critical temperature necessary for autoignition, suggesting a delaying effect on the pre-ignition reactions. These findings are in subtle contrast to direct numerical simulations (DNS) that have shown that turbulence may accelerate autoignition. The DNS results [10–16] have revealed that, locally, autoignition occurs at a Ômost reactiveÕ mixture fraction (nMR) and at regions with low values of the scalar dissipation rate (v). Since these simulations did not last very long relative to the turbulence turnover time and also had to resolve the fuel–air interface, the autoignition time was found to strongly depend on the initial condition (i.e., the initial value of v), with the turbulence affecting autoignition time only insofar as it affected the emergence of the lowest value of the conditional v|nMR. Therefore, autoignition could be promoted by fast mixing due to the earlier emergence of well-mixed nMR spots. There has been no evidence from DNS so far that autoignition can be precluded completely due to high bulk strain rates. The possibility of autoignition not occurring in situations of sustained, high mean values of v, has been theoretically demonstrated in [17,18], where the critical value of the mean of v depended additionally on the fluctuations of v, and consequently the turbulence. There seems to be a difference between flows of parabolic nature (e.g., mixing layers, such as those studied by DNS) where v decays and autoignition always occurs eventually, and counterflows, where v does not decay as the reactants flow radially outwards [19]. This discrepancy may explain why in the latter flow, critical conditions exist where autoignition does not develop at all [7–9]. The present paper presents an experimental effort to observe autoignition in turbulent non-premixed flows of a parabolic nature, in an attempt to improve our understanding of the above subtle points concerning turbulent autoignition. From a more practical point of view, measurements of autoignition times in the presence of turbulence and data for modelling must be collected. In addition, of key interest is the uncovering of a connection between the ensemble-mean behaviour (e.g., the average ignition timing measured from many cycles in a diesel engine), and the possibility of relatively rare events causing dangerous autoignition (e.g., in a gas turbine premix duct). The statistics of autoignition have not been previously explored experimentally, but could be the outcome of calculations such as those by PDF or large Eddy simulation methods that are more appropriate for
ÔextremeÕ events, such as ignition and extinction. Therefore, the present data can serve as an additional test for advanced turbulent reacting flow models. The objectives of this paper are: (i) to describe a new experiment for determining the effects of turbulence on non-premixed autoignition and (ii) to quantify the autoignition location in axisymmetric flows of hydrogen in hot co-flowing air, as a function of air temperature and velocity. In the following section, a brief description of the experimental methods is given, followed by the results and their discussion. The paper closes with a summary of the main conclusions. 2. Experimental methods 2.1. Apparatus Figure 1 shows the experimental arrangement. Air was electrically preheated and flowed into a circular quartz tube, after passing through a perforated plate (3.0 mm holes and 44 % blockage) to promote turbulence. The heaters were designed and manufactured in-house, to avoid performance and reliability issues encountered with commercially available models and were heavily insulated. Air temperature measurements at the exit of the heaters were used to control the supplied voltages so as to provide a steady temperature at the en-
Fig. 1. Apparatus schematic (not to scale).
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trance of the quartz tube. The main test section consisted of a 0.50 m long and 25.00 mm inner diameter (R = 12.50 mm) vacuum insulated quartz tube to reduce heat losses. This ÔjacketedÕ tube provided full optical access and required no additional insulation. Diluted fuel mixtures were injected from a centrally located nozzle and autoignition, visualized by hydroxyl radical (OH) chemiluminescence, was observed at some distance downstream. Hydrogen (H2) and nitrogen (N2) were supplied from compressed cylinders (99.999 % purity), and flowed into the test section axially and continuously, through a 2.25 mm internal diameter (d) stainless steel, thin-walled (0.32 mm) injector at ambient pressure. The fuel nozzle was located 63 mm downstream of the perforated plate to allow the turbulence to develop. A 500 mm long and 0.25 mm diameter mineral insulated K-Type (Cr/Al) thermocouple was placed through the injector tube, all the way to the injection location, and allowed a real-time measurement of the fuel injection temperature (Tfuel). A certain amount of preheating of the fuel was unavoidable due to heat transfer from the hot co-flowing air. Thus, Tfuel was not an independent parameter, but depended on the air temperature (Tair), and the mixture flow rate and composition. To minimize this effect, the injector was encased in a 2.0 mm thick-walled ceramic tube for most of its length, which kept the fuel at a lower temperature. Tair was measured 25 mm upstream of the injector with a 0.25 mm diameter ceramic sheathed barewire R-Type (Pt/Pt and 13%Rh) thermocouple, protruding 9 mm into the flow from the wall. Air velocities (Uair) of up to 35 m/s, with Tair up to 1015 K, have been achieved. Fuel velocities (Ufuel) ranged from 20 to 120 m/s, with Tfuel between 650 and 930 K. An additional 1.0 mm diameter, mineral insulated N-Type (Nicrosil/Nisil) thermocouple was placed at the exit of the tube to monitor heat losses along the length of the tube and to detect combustion. An air temperature drop of 3 K in the first 100 mm was typical. Air and hydrogen flow rates were measured by digital mass flow controllers (Bronkhorst Hi-Tec El-Flow), while calibrated rotameters were used for the nitrogen flow rates, with indeterminate (random) experimental errors of ±1–2% and ±6%, respectively. The random error associated with Tair and Tfuel was ±4 K, accounting for both temperature drifts during a run (±1 K) and measurement error, including hardware performed cold junction compensation. The raw measurements of Tair were corrected for both radiation and conduction losses, with a method similar to that of [6], but modified for the present geometry and conditions. The correction was approximately 5 K due to radiation and a further 5 K due to conduction. Tfuel was only corrected for radiation. Due to these corrections, the reported values of
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Tair and Tfuel have determinate (systematic) uncertainties of ±6 and ±1–2 K. Measurements of temperature across the section and of mean velocity under hot conditions were made with 76 lm diameter ceramic sheathed bare-wire K-Type thermocouples and a miniature high temperature Pitot probe. These measurements showed that the mean temperature was approximately uniform (to within 5%) for radii of about r/R = 0.15–0.75, but decreased sharply towards the walls to about 85–90% of the centreline temperature. The mean velocity was also uniform across the tube for r/ R = 0.15–0.65 at operating temperatures. Hot– wire measurements in cold flow conditions exhibiting Reynolds number similarity with the actual hot flows were used to measure the initial mean and fluctuating velocities. In cold conditions, the turbulence intensity was about 12–13% and uniform across the flow for r/R = 0.2–0.8 while the integral lengthscale (Lturb ) was about 4– 5 mm for r/R = 0.20.7 and increased nearer the jet and the wall of the tube. Furthermore, the mean velocity was uniform within the extended region r/R = 0.150.8, because of the absence of the thermal boundary layer. Therefore, we may conclude that the air flow was essentially uniform across the tube cross-section in terms of velocity, turbulence, and temperature, in the immediate region away from the nozzle and walls, and consequently that specifying Uair and Tair is sufficient to accurately define the initial conditions of the co-flow at the injection plane. Similar results were established at all axial locations where data are presented in this paper. Power density spectra of the velocity showed a well-developed turbulence and a 5/3 roll-off at the expected inertial sub-range frequencies. Turbulence Reynolds numbers based on Lturb and the velocity fluctuations spanned the range 90– 160, similar to what was achieved in DNS. The large-eddy turbulent time scale (sturb) based on Lturb was in the range 1–2 ms. 2.2. Operation and data analysis Due to the large thermal inertia of the insulation, approximately one hour of operation with the required flow rate of air at elevated temperature was allowed for the experiment to reach thermal equilibrium. Once the heater exit thermocouples indicated steady conditions, the fuel was switched on, and final adjustments to the flow rates were made before the system reached its final state. The basic measured variables were Tair and Tfuel, and the volumetric flow rates of the air, hydrogen, and dilution nitrogen. The dilution of the fuel stream is described by the mass fraction of hydrogen at injection ðY H2 Þ. Images of OH chemiluminescence were taken with a LaVision Nanostar ICCD camera fitted
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with a Nikon Nikkor UV lens, with and without an additional OH filter band-centred at 307±10 nm. Up to 2000 images per operating condition (set of Tair, Uair, Tfuel, and Ufuel) were taken to observe enough autoignition events. The exposure time was between 50 and 150 ls and high gain settings were used so as to capture the weak chemiluminescence signals. Pixel intensity in the Ôraw snapshotsÕ was the result of transverse, lineof-sight integration of OH chemiluminescence. To remove background noise, and increase signal-to-noise ratio, individual images were processed with adaptive Wiener filtering, median smoothing, and fourth order low-pass filtering in the frequency spectrum. These Ôprocessed snapshotsÕ were normalized, and the image background noise was removed by setting the pixel intensity to zero below a certain threshold, and to unity above it. Hence, the presence of OH at a pixel location resulted in a normalized unity signal, while the absence of OH in zero. Regions of presence of OH chemiluminescence were deemed an Ôautoignition spot,Õ with axial location denoted by Lign. All Ônormalized snapshotsÕ corresponding to a particular operation point were superimposed to compile the two-dimensional joint PDF of autoignition spot location. Alternatively, the autoignition spot location could have been quantified based on the average OH chemiluminescence, obtained by averaging all instantaneous processed snapshots, before normalization. In practice the two approaches produce similar results, yet their interpretation is slightly different. In this paper, we have used only the PDFs and report solely on the Ôaxial ON/OFF presenceÕ of OH, without dealing with the absolute chemiluminescence intensity or attempting to radially quantify the phenomenon. The mean (ÆLignæ) and standard deviation (LRMS) of Lign were calculated directly from the PDFs by double numerical integration. A further definition of autoignition length was based on the minimum axial location (Lmin) of an OH spot observed during a data run. This location was defined, where the PDF reached a rise height from the background of 3 % of the peak value. The minimum autoignition length so derived agreed very well (within ±5 %) with direct long-exposure photographs. For certain experiments, based on the results of [4], the fuel was doped with small amounts of methane ðY CH4 Þ so as to make the autoignition event visible. It was observed that, for these conditions, the presence of traces of methane did not significantly affect the phenomenon. In these cases, a CCD Digital Video Camera Recorder, set to a maximum exposure of 250 ls and a slowest shutter speed of 1/3 s, filmed for approximately 40 s and generated about 125 still images for each data point. For these images, the minimum autoignition length (Lmin) was determined. The estimated random error, due to the instruments
and the processing of the images in the determination of the instantaneous Lign, was of the order of ±4%, but Lmin was more sharply determined to within ±1%. All lengths have an absolute systematic uncertainty of ±5% due to scaling. Finally, various high-speed images were taken (Kodak Ektapro-HS4540 Motion Analyzer) at 13.5 kHz (frame intervals of 74.1 ls), but the spectral sensitivity was insufficient to detect hydrogen autoignition. This motivated detailed measurements with acetylene (not shown here), as this fuel gave adequate illumination. These measurements confirmed the main findings of the present paper concerning autoignition and demonstrated the same qualitative features, such as those of the emergence of the kernel and the subsequent flame propagation, which is something we have observed for all fuels tested. Hence, we treat the qualitative behaviour of acetylene as typical autoignition behaviour for other fuels as well, and the high-speed acetylene image sequences taken, as sufficient to show the characteristic evolution of turbulent autoignition kernels. The experimental investigation of hydrogen autoignition was organized as follows. To simulate a mixing pattern that is relatively straightforward to treat theoretically, equal jet and co-flow velocities (Uair = Ufuel) were used, to investigate the effect of background turbulence, Tair, dilution, and methane addition on autoignition. The effect of the jet velocity was also investigated by a series of experiments where Ufuel > Uair. In such cases, the turbulence across the test section was affected by the mean shear introduced by the jet. 3. Results and discussion 3.1. Operating envelope—bulk behaviour Visual observations over a wide range of operating conditions showed the following qualitatively distinct flow regimes. For low Tair and high Uair and Ufuel or lower Y H2 , no autoignition was observed within the length of the tube under investigation, although for certain conditions very faint and infrequent autoignition spots could be heard from further downstream. As Tair decreased further, local explosive autoignition completely disappeared, at first replaced by a situation of low heat release in the tube which raised the temperature at the exit by a few to tens of degrees before no effect could be detected at all. We call this the ÔNo IgnitionÕ regime. For a certain range of higher Tair, lower Uair and Ufuel, or higher Y H2 , a statistically stable situation was observed, where instantaneous autoignition occurred in the form of random spots, at about 10–50 d from the injector. Figures 2A–C demonstrates typical examples. These spots resulted in neither flashback nor acted as flame
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Fig. 3. Single acetylene autoignition kernel sequence at 13.5 kHz. Flow from below.
Fig. 2. Examples of instantaneous raw OH chemiluminescence snapshots. (A–C): Uair = 26 m/s, Tair = 1000 K, Ufuel = 120 m/s, and Y H2 ¼ 0:13; (D) Same but Tair = 1020 K—propagation towards the injector.
anchoring points, but were short-lived ignition kernels that died out (quenched). Autoignition in these conditions produced an intense noise, with decreasing pitch and loudness as the temperature decreased or velocity increased. This is the ÔRandom SpotsÕ regime. At low Uair and Ufuel, and high Tair, as soon as the fuel was switched on, autoignition and subsequent flashback occurred. The first appearance of an ignition kernel was randomly located in space, and flashback occurred with what seemed like a triple flame [20] and resulted in a normal jet diffusion flame. Figure 2D shows such an event in progress. Generally, the temperature needed to cause a flashback increased monotonically with both coflow air and fuel jet velocities. This is the ÔFlashbackÕ regime. A further ÔLifted FlameÕ regime has been observed at even higher velocities and temperatures, in which a stable lifted hydrogen flame is achieved. This observation is consistent with the experiments of [21], where lifted hydrogen jet flames in a co-flow of hot combustion products were examined. Unlike the ÔNo IgnitionÕ and ÔRandom SpotsÕ transition, which contains intermediate states that make this boundary unclear, such as those of low heat release and very infrequent spots, the boundary between the ÔRandom SpotsÕ and ÔFlashbackÕ regimes is quite sharp, tested, and found to stretch 1–2 K with relatively high consistency. Although more data are necessary to accurately map out the regime boundaries, the main features outlined here have been observed under a wide range of velocities, and with other fuels including acetylene and various hydrogen/methane mixtures. In the rest of this paper, we consider the ÔRandom SpotsÕ regime, since to the authorsÕ knowledge, it has not been previously characterized. 3.2. Visualization of autoignition kernels To get a better understanding of the spatial and temporal manifestation of the autoignition kernels in the ÔRandom SpotsÕ regime, high-speed films of acetylene autoignition were taken at various conditions. Figure 3 illustrates a typical se-
quence. The films exhibited an evolution from a dark background to a small spot, which became a spherical shell flame front propagating in all directions. Following this, quenching of the spot was observed, with the image returning to a completely dark background. The life-span of the kernels was of the order of 100–200 ls, in which time they were advected by the flow a distance of the order of a few millimeters. Both the spot and these flamelets contribute to the OH images such as those observed in Fig. 2. Therefore, each autoignition eventÕs location was measured to within 2– 5 mm in the axial direction, depending on whether the ICCD camera would capture parts of the resulting flame or the autoignition event itself, and by how much this kernel had moved during exposure. The frequency of the appearance of the kernels was very dependent on the conditions. It increased monotonically with increased Tair and decreased Uair, but even close to flashback conditions when the frequency was maximum, successive kernel realizations appeared to be independent events. Although the lengthscale of the first appearance of a kernel could not be determined, because the light intensity at this early stage was too small, it was evident that the kernels at birth had dimensions a very small fraction of their eventual fully grown size (a few mm). The generation of double flame propagation following autoignition in a non-premixed medium and the subsequent decay of the flamelets is completely consistent with the DNS data [10]. The random appearance, quenching, and reappearance of autoignition spots in a statistically steady situation are possibly a new experimental finding. It should be noted that the quenching observed in Fig. 3 involves flame disappearance rather than pre-ignition kernels that failed to ignite as observed by DNS [15], as only fully fledged combustion could be visualized in these images. 3.3. Autoignition lengths Calculation of ÆLignæ and LRMS was done by integrating over the PDFs compiled by processing many images such as those in Fig. 2. Figure 4 shows typical PDFs for high and low Tair cases, and comparison with average OH chemiluminescence images. It is clear that, for low Tair autoignition occurs farther from the injector, but also that the PDFs rise sharply at the injector-side. This allows an unambiguous definition of the min-
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Fig. 4. Two-dimensional PDFs of autoignition location and corresponding average OH chemiluminescence. Axial intensity profiles are also shown. (A) Uair = Ufuel = 26 m/s, Tair = 1010 K, and Y H2 ¼ 0:13; (B) Same but Tair = 1000 K.
imum autoignition length Lmin, which could have been similarly defined in terms of the average OH. The PDFs show a smooth tail at long distances from the injector, which is consistent with the fuzzy boundary between the ÔRandom SpotsÕ and ÔNo IgnitionÕ regimes. Figure 5 shows the effects of Tair, Uair, Ufuel, Y H2 , and Y CH4 on the autoignition lengths. Autoignition shifted downstream with decreasing Tair, increasing Uair, and increasing Ufuel for all conditions tested. Comparisons between Figs. 5A and B or C and D show that Lmin was about 60–70% of the mean, ÆLignæ (although this is partly a reflection of the ÔcontaminationÕ of the PDF from flamelets evolving from the ignition spots). When the two velocities were equal, Figs. 5A and B, the measurements only covered a small range of variables. Within this range, it was confirmed that relatively small quantities of methane in hydrogen ðY CH4 ¼ 0:05; 0:07; and 0:10Þ and the degree of dilution ðY H2 ¼ 0:13 and 0:26Þ do not seem to have as significant an effect on autoignition as the velocities and temperatures, similar to the results of [4]. More extensive experiments with unequal jet and co-flow velocities, Figs. 5C and D, show that the trends are non-linear, and that the axial location increases sharply as Tair decreases, which explains why the boundary between the ÔNo IgnitionÕ and the ÔRandom SpotsÕ regimes is not well defined. In contrast, for every one of the curves shown, flashback would have occurred for a temperature higher than the hottest point plotted by about 1–2 K, which shows that the boundary between the ÔRandom SpotsÕ and the ÔFlashbackÕ regime is sharper. In addition to the observed difference between Lmin and ÆLignæ, the randomness of the autoignition location can be quantified with LRMS. LRMS was large (up to 40% of the mean), which is an over-estimation because the PDFs include contributions from the short-lived propagating flamelets, as previously discussed. However, the wide
Fig. 5. Mean and minimum autoignition lengths as a function of Tair. (A) ÆLignæand (B) Lmin for Uair = Ufuel and various Y H2 and Y CH4 ; (C) ÆLignæand (iv) Lmin for varying Uair, Ufuel but fixed Y H2 ¼ 0:13 and Y CH4 ¼ 0. In (A and B), markers are empty for Y CH4 ¼ 0 and filled for Y CH4 6¼ 0. In (C and D), markers are for Ufuel: d (40 m/s), + (70 m/s), · (100 m/s), and * (120 m/s). In all four subplots, lines are for Uair: solid (20 m/s), dashed (26 m/s), dashed dot (28 m/s), and dotted (32 m/s).
spread of autoignition locations was also captured and confirmed in many high-speed films that do not have this problem (not shown). Hence, there seems to be a real effect of the turbulent flow in the spatial distribution of the ignition kernel, although the unavoidable fluctuations of Tair, which were not measured here, must also make a contribution that should be elucidated. 3.4. Autoignition delay times Mean and minimum (Æsignæ and smin) residence times until the point of autoignition could be defined simply as ÆLignæ/Uair and Lmin/Uair. However, this does not account for the velocity of the mixture in the jet, which can be a factor of 2–5 times higher than Uair. In addition, many of our data show autoignition at close distances to the injector nozzle. Hence, the jet momentum cannot be ignored. An improved attempt at a suitable definiˆ as tion can be made based on a mean velocity U ˆ and smin = Lmin/U ˆ . For smin: in Æsignæ = ÆLignæ/U smin ¼
Lmin ¼ ^ U
Z 0
Lmin
dx ; U ðxÞ
ð1Þ
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with U(x) being the mean centreline velocity in the jet. For jets in turbulent co-flows with comparable velocities, the normalized axial centreline velocity air can be described by UUðxÞU ¼ F ðdxeff Þ, where deff ¼ fuel U air qfuel 12 dð qair Þ . The functional relation F ðdxeff Þwas obtained from [22]. It was checked in a limited number of cases, with Pitot tube and hot wire measurements, and good agreement was found. The densities of both streams were calculated from their known composition, and the measured Tair and Tfuel. Figure 6 shows the results for the residence times from injection to the minimum autoignition length (smin). Similar trends were found for Æsignæ. The residence time was hence of the same order as the turbulent timescale. The effect of increasing Tair was a decrease in smin, as expected. On the other hand, we note that, for a given Tair, the data for different Ufuel but for the same Uair collapse approximately on a single band, while the bands of curves for different Uair do not. Thus, it can be concluded that the residence time until the point of autoignition was mostly affected by Uair, and consequently the background turbulence intensity, and not Ufuel or the bulk straining of the flow, at least within the experimental envelope that was investigated. This suggests that the phenomena are not simply kinetically controlled, but that the turbulence and mixing play a role in determining the location of autoignition. The fact that for the same Tair, a higher Uair results in delayed autoignition, reveals
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a delaying effect of the air stream on autoignition, consistent with [8]. Significant effects of air flow velocity and turbulence were also observed for a more complex chemistry and geometry in the experiments of [23], where a liquid fuel spray was perpendicularly injected into a heated air stream, with the air velocity being lower than the fuel injection velocity. Here, it was found that as the turbulence was strengthened, the autoignition delay increased in the region above 1300 K. It is possible that the higher Uair result in higher v, which in turn delay autoignition. Measurements of n and tests with different turbulence promoting grids and injector diameters must be performed to understand these observations better (e.g., changing the perforated hole size while keeping its blockage the same should not affect the turbulence intensity but will directly affect the integral length scale, while changing the injector diameter will not affect the background turbulence, but will affect the mixing patterns prior to autoignition). Measurements of n will also assist the interpretation of the ÔNo IgnitionÕ regime, as it is not clear at present if this lack of ignition is because nMR disappears due to excessive mixing, or because the chemistry becomes too slow relative to the turbulence thus revealing a critical condition. Figure 6 can also be considered as an Arrhenius plot and used to define effective activation temperatures (Tact). The slopes range from about 25,000 to 40,000 K in the Æsignæ plots, and 60,000– 110,000 K for smin. These values are higher than those found in uniform hydrogen–air mixtures (e.g., experimental data at 1 bar [24] show a Tact of about 30,000 K at the same temperature range). Furthermore, the Arrhenius plots show deviations from straight lines, and they may also depend on the velocity. The data do not span a wide enough range to draw solid conclusions, but it seems that a single Tact cannot be defined. Turbulent mixing can cause non-linearity in Arrhenius plots even with one-step chemistry [17], and hence the present data suggest that the pre-ignition reactions have been influenced by the turbulence. However, this conclusion must be re-examined with other fuels that show a less complex kinetic behaviour than hydrogen, where thermal feedback may not be a prerequisite for autoignition [2].
4. Conclusion
Fig. 6. Residence times defined by Eq. (1) as a function of Tair for Uair = Ufuel (A) and Uair„Ufuel (B). Symbols as in Fig. 5.
It has been shown that it was possible to achieve a variety of non-premixed autoignition phenomena, with a jet of (diluted) hydrogen or hydrogen/methane mixture injected into a turbulent co-flow of hotter air. Four ÔregimesÕ were identified: ÔNo Ignition,Õ ÔRandom Spots,Õ ÔFlashback,Õ and ÔLifted FlameÕ. The boundary between ÔRandom SpotsÕ and ÔFlashbackÕ was very sharp (i.e., spanned 1–2 K 2air temperature for a given
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set of air and injection velocities), while between ÔNo IgnitionÕ and ÔRandom SpotsÕ it could only be defined within a range of about 5 K in air temperature. In the statistically stable ÔRandom SpotsÕ regime, independent kernels appeared and were advected with the flow before they disappeared. Flashback was found to be promoted by a decrease in air and/or injection velocities. Optical measurements of autoignition lengths and subsequent analyses showed that, inside the tested range, autoignition was relatively insensitive to changes in hydrogen dilution and the presence of traces of methane. Both minimum and mean autoignition lengths increased by a decrease in air temperature or an increase in either air or injection velocities. In an attempt to quantify the autoignition delay time, a residence time until autoignition location was defined in terms of the decay of the centreline velocity of the jet within the co-flow. This residence time approximately collapsed the data within each band of similar air velocities, more so for the case where the jet and co-flow velocities were equal. The autoignition delay at the same air temperature increased with increasing air velocity. This suggests that the present phenomena are not only chemically controlled and that turbulent mixing delays autoignition.
References [1] A. Lin˜an, Acta Astronautica 1 (1974) 1007–1039. [2] T.G. Kreutz, C.K. Law, Combust. Flame 104 (1–2) (1996) 157–175. [3] T.G. Kreutz, C.K. Law, Combust. Flame 114 (3–4) (1998) 436–456. [4] C.G. Fotache, T.G. Kreutz, C.K. Law, Combust. Flame 110 (4) (1997) 429–440.
[5] C.G. Fotache, C.J. Sung, C.J. Sun, C.K. Law, Combust. Flame 112 (3) (1998) 457–471. [6] C.G. Fotache, T.G. Kreutz, D.L. Zhu, C.K. Law, Combust. Sci. Technol. 109 (1995) 373–393. [7] J.D. Blouch, C.J. Sung, C.G. Fotache, C.K. Law, Proc. Combust. Inst. 27 (1998) 1221–1228. [8] J.D. Blouch, C.K. Law, Combust. Flame 132 (3) (2003) 512–522. [9] J.D. Blouch, J.-Y. Chen, C.K. Law, Combust. Flame 135 (3) (2003) 209–225. [10] E. Mastorakos, T.B. Baritaud, T.J. Poinsot, Combust. Flame 109 (1-2) (1997) 198–223. [11] H.G. Im, J.H. Chen, C.K. Law, Proc. Combust. Inst. 27 (1998) 1047–1056. [12] R. Hilbert, D. Thevenin, Combust. Flame 128 (1–2) (2002) 22–37. [13] S. Sreedhara, K.N. Lakshmisha, Proc. Combust. Inst. 28 (2000) 25–34. [14] S. Sreedhara, K.N. Lakshmisha, Proc. Combust. Inst. 29 (2002) 2051–2059. [15] T. Echekki, J.H. Chen, Combust. Flame 134 (3) (2003) 169–191. [16] P. Domingo, L. Vervisch, Proc. Combust. Inst. 26 (1996) 233–240. [17] E. Mastorakos, A. Pirez da Cruz, T.B. Baritaud, T.J. Poinsot, Combust. Sci. Technol. 125 (1997) 243–282. [18] E. Mastorakos, R.W. Bilger, Phys. Fluids 10 (1998) 1246–1248. [19] E. Mastorakos, A.M.K.P. Taylor, J.H. Whitelaw, Combust. Flame 91 (1) (1992) 55–64. [20] H.G. Im, J.H. Chen, Combust. Flame 119 (4) (1999) 436–454. [21] R. Cabra, T. Myhrvold, J.-Y. Chen, R.W. Dibble, A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29 (2003) 1881–1888. [22] Y. Sakai, T. Watanabe, S. Kamohara, T. Kushida, I. Nakamura, Int. J. Heat Fluid Flow 22 (2001) 227–236. [23] Y. Mizutani, T. Takada, in: ICDERS, 17th, 1999. [24] S.A. Bozhenkov, S.M. Starikovskaia, A.Yu Starikovskii, Combust. Flame 133 (1–2) (2003) 133–146.
Comments Antonio Cavaliere, University of Naples, Italy. Has the noise in the movie shown during the presentation been analyzed in frequency? Is this frequency correlated to the extinction/ignition process and if so, in which way? Reply. Analysis of the pressure signal from these experiments has been done, but was not presented in this paper. The ignition and extinction processes are both evident in the pressure traces. The frequency of occurrence of auto-ignition (fign) correlates very closely with the auto-ignition length (Lign), so that decreased Lign results in significant increase in fign. These results are being prepared for publication. d
Peter Jansohn, Paul Scherrer Institute, Switzerland. Does the conclusion ‘‘ignition delay times increase with higher velocity i.e., turbulent velocity fluctuations’’ hold
true for both types (co-flow with same velocity, jet flow with (strongly) different velocities for air vs. fuel) of experimental conditions? Reply. Yes, in both cases and keeping everything else the same, the ignition delay time increases monotonically with the co-flow air velocity for all conditions tested. d
Jacqueline H. Chen, Sandia National Laboratories, USA. How is the ignition delay time affected by the temperature of the air co-flow? In particular, how near are you to the cross-over temperatures, Tc on average and are the ignition kernels affected more by the local scalar dissipation rate due to the proximity of the kernel temperature to this critical temperature Tc where branching and recombination rates are the same?
C.N. Markides, E. Mastorakos / Proceedings of the Combustion Institute 30 (2005) 883–891 Reply. The effect of the air co-flow temperature (Tair) on the ignition delay time (tign) is presented in Fig. 6 of the paper. Tc for hydrogen/air chemistry is approximately 925 K at 1 atm [1]. The range of Tair in our experiments is between 945 and 1010 K, so it is not appreciably higher. The initial temperature of the most reactive mixture fraction is expected to be slightly lower than Tair because the fuel stream is colder by about 50– 300 K. The effect of the scalar dissipation rate in the locality of the auto-ignition kernels has not been examined here. We cannot be sure whether at the higher velocities, when tign is larger, this is indeed caused by higher dissipation rates at the ignition locations, but relevant measurements are in progress.
Reference [1] B. Pellett, Chinitz, Review of Air Vitiation Effects on Scramjet Ignition and Flameholding Combustion Processes, 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (2002). d
Peter A.M. Kalt, University of Adelaide, Australia. The temperature of the fuel jet is not controlled in
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the same way as the ambient air. Changes in jet velocity will accordingly change the mass flux of colder fuel, and the local temperature. Furthermore, auto-ignition may be occurring in regions of steep local temperature gradients. The correlation between jet velocity and auto-ignition height could be strongly coupled to these issues of dilution with ‘‘cold’’ fuel. For example, the lift-off height in the Cabra/Dibble vitiated coflow burner ([20] in paper) is sensitive to fluctuations in temperature as small as 5–10 K. Please comment. Reply. Evidence from analysis and DNS of autoignition in non-premixed flows suggests that ignition occurs at the ‘‘most reactive’’ mixture fraction ([10] in paper), which is determined by the chemistry and the two stream temperatures. Because the most reactive mixture fraction is very lean for the present conditions (detailed chemistry calculations show that it is less than 0.05), the initial, ‘‘frozen,’’ temperature of the fluid particles that are likely to auto-ignite is very little affected by the fuel temperature. We chose to present the data in terms of air temperature for this reason. The sensitivity to air temperature is very strong, as Fig. 6 of the paper shows.