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Ignition dynamics of a pulse detonation igniter in a supersonic cavity flameholder
Combustion and Flame 215 (2020) 376–388
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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Ignition dynamics of a pulse detonation igniter in a supersonic cavity flameholder Daniel Cuppoletti a,∗, Timothy Ombrello b, Campbell Carter b, Stephen Hammack b, Joseph Lefkowitz c a b c
University of Cincinnati, Cincinnati, OH, USA U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA Technion - Israel Institute of Technology, Haifa, Israel
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 25 January 2020 Accepted 27 January 2020 Available online 29 February 2020 Keywords: Detonation Ignition Scramjet Cavity Supersonic PLIF
a b s t r a c t The dynamics of using a pulse detonation wave to ignite a supersonic cavity flameholder was investigated. Fueling conditions ranging from lean to rich were investigated. Simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence, and chemiluminescence imaging were used to elucidate the fluid dynamics and combustion physics throughout the transient disruption of the ignition event. For moderate and rich fueling conditions, the ignition event stabilized combustion away from the detonation plume until the detonation products were fully exhausted. Ignition of leaner conditions required a lowlevel flame kernel to persist in the cavity step until the recirculating cavity flow field was re-established. The initial ignition mechanism was shown to be primarily driven by hot products from the detonation plume shedding into the low velocity region near the aft facing step in the cavity, followed by propagation and stabilization of a flame to a favorable region of the cavity. Quenching of cavity combustion at lean overall cavity mixtures was shown to be due to temporary backpressuring of cavity fuel injectors and cavity evacuation from the detonation pressure impulse. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Achieving ignition and stable combustion in high-speed airbreathing combustors is a challenge due to short residence times and transient fluid dynamics involved [1]. The flow-through time for an entire scramjet engine is on the order of milliseconds. Flame stabilization has been addressed extensively through the use of cavities, struts, and other devices that create locally reduced velocity with increased residence times. The continuous recirculation of hot combustion products and radicals provides a sustained energy addition to the primary combustor flow. Ignition is often achieved through energy deposition by electrical or chemical means, or some combination of both such as fueled torches. Capacitive spark or plasma discharge [2,3] and various types of combustion devices create a local elevation in temperature that initiates an energy cascade for sustained heat release to the remainder of the fueled system. System level considerations often govern decisions on ignition devices. While electrical spark discharge can be simpler, a fueled
∗
Corresponding author. E-mail addresses: [email protected] (D. Cuppoletti), [email protected] (T. Ombrello), [email protected] (C. Carter), [email protected] (S. Hammack), [email protected] (J. Lefkowitz).
torch can provide greater energy deposition with lower electrical energy requirements through coupling with chemical heat release. The localized nature of spark ignition coupled with the low propagation speed of deflagration can affect the ignition limits and ignition probability in combustors with short residence times. Recently, a pulse detonation igniter has been developed and demonstrated in a supersonic flow, employing the strong transient combustion and high-pressure impulse provided by the outflow [2,4–6]. Pulse detonation (PD), also referred to as pressure gain combustion, provides a unique energy deposition method capable of influencing a large region over a short time frame. This increased volumetric influence of the PD ignition device can improve the probability of ignition. Ombrello et al. [4] studied the ignition process of a PD igniter compared to spark discharge, utilizing high frame rate shadowgraph and CH∗ chemiluminescence imaging. For fueling conditions near stoichiometric in the cavity, the detonation products ignited the cavity more rapidly than spark discharge and burning persisted over the first few milliseconds. Depending on the fueling conditions, combustion was either sustained, quenched, re-established after a delay, or stabilized within the cavity. If steady-state combustion did not stabilize in the cavity after the initial consumption of fuel in the cavity, then it was considered a failed ignition event. This paper describes
https://doi.org/10.1016/j.combustflame.2020.01.030 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
D. Cuppoletti, T. Ombrello and C. Carter et al. / Combustion and Flame 215 (2020) 376–388
Nomenclature D M p, P Q T t 0 ∞ d f
Diameter of PD tube Mach number Pressure Volumetric Flow Rate Temperature time Freestream Condition Ambient Condition Detonation Condition Fuel Condition Equivalence Ratio
the details of the initial ignition and flame stabilization process in further detail to provide insight into the dynamic process of transferring a detonation wave to a deflagrating cavity mixture. The current study combines simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence (PLIF), and CH∗ chemiluminescence imaging to investigate the ignition process of a PD igniter for cavity fueling conditions ranging from lean to rich. All three diagnostic methods have different utility for understanding the flowfield and combustion processes involved during the ignition transient. The combination of all three diagnostics provides an improved understanding of the cavity ignition mechanisms. 2. Methods The experiments were conducted in Research Cell 19, a directconnect supersonic wind tunnel at Wright-Patterson Air Force Base. The test section is 15.24 cm wide and has an entrance height of 5.08 cm with a 2.5◦ divergence of the lower wall where the cavity resides, as shown in Fig. 1. The cavity spans the tunnel width and has a 1.65 cm depth and a 22.5◦ closeout ramp. A shear layer forms over the cavity with recirculating flow in the cavity and weak oblique shocks from the streamline deflections due to the shear layer recompression on the closeout ramp. The configuration has been well studied experimentally and computationally [7–10]. The exit of the PD igniter exhausts normal to the cavity floor and
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was located 2.5 cm aft of the cavity step and 1.9 cm from centerline. Two pressure transducers were utilized on the lower surface facility centerline: P1 located 2.96 cm aft of the cavity step, and P2 located 3.46 cm aft of the cavity ramp closeout. The cavity was fueled with C2 H4 from eleven injectors in the aft ramp. This study focused on cavity fueling and ignition only, with a global equivalence ratio of global < 0.016 for all conditions tested. Injecting fuel directly into the cavity decouples the cavity fueling from entrainment through the cavity shear layer (i.e. no upstream fueling was utilized). All tests were conducted with free-stream conditions at Mach number M = 2, total temperature T0 = 589 K, and total pressure P0 = 483 kPa. The PD igniter had a volume of 49 cm3 with an inner diameter of 10.2 mm. The PD reactants, C3 H8 and N2 O, were injected at the head end of the PD tube, and the tube was overfilled for all experiments. Overfilling the PD tube ensures that a detonation is supported all of the way to the exhaust plane into the cavity. The excess reactants in the cavity from overfilling had negligible effect on the cavity ignition process [2]. The mixture was ignited at the head end near the reactant injection location and exhausted into the cavity through a 7.62 mm diameter orifice. For direct cavity fueling, the operational range of the cavity at the given Po and To tested is approximately 30–260 standard liters per minute (slpm) corresponding to the lean and rich blowout limits of the cavity. For these conditions the cavity can sustain heat release by cycling hot products and reactants within the cavity. The fueling rate for ignition differs from the operational range and is strongly dependent on the energy deposition method. Two separate studies are presented in this paper. The first study utilized multiple pulses of the PD igniter at 10 Hz with an unfueled cavity and cavity fueling at 68 slpm, which is near the lean ignition limit for a single pulse. The second study focused on investigation of a single PD igniter pulse for five different fueling conditions shown in Table 1 alongside the operational pressure drop (Pf = Pf uel − Pcavity ) for the fuel injectors. The injector pressure drop is discussed further in Section 3.2. Fuel flow rates are identified as the metrics for comparison since the equivalence ratio varies spatially and temporally due to turbulent mixing within the cavity. Wall-based extractive sampling via a Fourier Transform Infrared (FTIR) spectrometer in the cavity step region indicated equivalence ratios varying from = 1.0 for 68 slpm to = 2.5 for 200 slpm, and has been confirmed through other experimental
Fig. 1. Experimental layout and basic cavity flowfield features.
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Fig. 2. Imaging fields-of-view showing schlieren volume (gray box), OH PLIF field-of-view (blue plane) directly over the PD centerline, and CH∗ chemiluminescence imaging elevated field-of-view (red cone). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Fueling conditions for single pulse ignition studies. Condition
Qf [slpm]
Pf [kPa]
1 2 3 4 5
200 95 75 70 55
138 46.5 31.0 26.9 17.2
and computational investigations [11–16]. The 200 slpm condition is rich throughout the cavity, and burning tends to anchor near the aft ramp [17]. The 95, 75, and 70 slpm conditions are referred to as “intermediate” conditions since the cavity fueling is favorable for stable combustion and the ignition dynamics are similar for these conditions. The 70 slpm is slightly above the lean ignition limit for the PD igniter, while the 55 slpm condition is below the lean ignition limit. It should be noted that ignition and flame stabilization can be achieved at 55 slpm and lower when other energy deposition devices and configurations are used, such as spark discharge [2,18]. 2.1. Diagnostics Schlieren, chemiluminescence, and OH PLIF imaging were acquired simultaneously at fs = 40 kHz frame rate (see Fig. 2 for fields of view). Schlieren imaging used a collimated Hg-Xe light source passed spanwise across the tunnel with a horizontal knife edge at the focal point to highlight vertical density gradients. The exposure time for schlieren imaging was texp = 1 μs. Frames were captured with a Photron SA-Z camera. Chemiluminescense was captured with an image intensifier (LaVision HS-IRO) coupled to a Photron SA-Z. The exposure time was texp = 10 μs. A 431 nm bandpass filter with 17-nm full-width at half-maximum transmission (Semrock FF01-434/17) was used to target CH∗ emission. For C2 H4 combustion, the emission is primarily from CH∗ with a minor secondary contribution from C2 ∗ [3]; however, it should be noted that the detonation event contains very high broadband emission intensities during the first few milliseconds. Details of the imaging fields-of-view are shown in Fig. 2. The OH PLIF system utilized a diode-pumped solid-state (DPSS) Nd:YVO4 laser (EdgeWave Innoslab HD40II-ET), frequency doubled to produce 150 W at 532-nm with a pulse repetition rate of 40 kHz. The DPSS laser beam pumped a high-power tunable dye laser (Sirah CREDO) that contained a solution of Rhodamine
590 dye (Exciton) in ethanol. The 566-nm dye-laser beam was frequency doubled using two barium borate (BBO) crystals. More detailed information regarding the laser system configuration can be found in Hammack et al. [19]. For the current measurements, the laser power was 4 W (100 μJ per pulse) at 283 nm, controlled by limiting the 532-nm pump power entering the dye laser. The 283-nm excitation beam was reflected upwards and then horizontally over the top of the test section. The beam was formed into a sheet using a plano-concave cylindrical lens and a plano-convex spherical lens. The sheet was approximately 70 mm by 0.25 mm at the waist at the probe volume (in the cavity), with the major dimension aligned in the tunnel streamwise direction (see Fig. 2) directly over the PD exhaust. Fused-silica windows were used for both the top tunnel wall and the side walls. Before sheet formation, the beam passed through an angled thin fused-silica window. A beam sample was passed over a small burner-stabilized premixed CH4 /air flame and then on to a UV diffuser and photodiode. OH LIF in the flame was focused into a 0.1-m monochromator and was measured by an attached photomultiplier tube. The assembly allowed for calibration and monitoring of both laser wavelength and laser pulse energy throughout the test. OH radicals were excited using the Q1 (7) transition of the A2 + − X 2 (1,0) band (λex = 282.93 nm in air). The transition was chosen for its strength and the temperature insensitivity of the ground state. Emission from the transitions in the A2 + − X 2 (1,1) and (0,0) bands were recorded using a Photron SA-Z with an attached image intensifier (LaVision HS-IRO) gated at 100 ns. Collection optics included a 100 mm, f/2.8 UV lens (Sodern Cerco) fitted with a high-transmission ( > 80% at 310 nm) bandpass filter (Laser Components GmbH). To accommodate the schlieren setup, the PLIF camera and HS-IRO were rotated away from perpendicular to the laser sheet and equipped with a Scheimpflug adaptor (LaVision) to maintain focus of the image plane. The intensity scales for each diagnostic results are consistent across this paper unless otherwise noted. For OH PLIF, the intensity scale is [0–30 0 0] counts. The CH∗ chemiluminescence intensity varies greatly from near the camera saturation value during the detonation to low levels during steady state burning. For this reason, the natural log of the intensity is shown for the images and integrated signals, ranging from [5.0–8.3]. Chemiluminescence and OH PLIF are presented with false color scale to assist in resolving features. The images are not modified other than in color scale and overlaying with lines indicating surface locations. Frame-integrated intensities are presented as filtered signals with
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Fig. 3. OH LIF image of (a) the detonation plume in an unfueled cavity at t = 0.05 ms after detonation showing integration region of interests and (b) profiles of intensity through the plume.
Fig. 4. Evolution of pressure and temperature in the PD plume for LIF analysis and (b) normalized intensity ratio (Imd /Ipl ) of analysis compared to measured data.
arbitrary units. Savitzky–Golay filtering introduces a time lag O(μs) for the impulse rise time while drastically improving the signal quality for comparisons. White lines in the OH PLIF and CH∗ chemiluminescence outline the cavity location in the images. The vertical edge of the cavity step is just outside of the field-of-view for the OH PLIF images, resulting in no line shown on the left side of the images. 2.2. Interpretation of imaging diagnostics The primary goal of the current study was to provide spatiotemporally resolved measurements of the cavity ignition process with the PD ignition device. The three simultaneous imaging diagnostics provide greater insight into ignition and detonation phenomena than a single diagnostic. Since schlieren is a line-ofsight measurement, the density gradients from shock waves, turbulence, fuel injection, and combustion are superimposed on the density gradients from the detonation, making it difficult to decouple the various effects in the flowfield. On the other hand, schlieren imaging enables visualization of the detonation interacting with the cavity shear layer and freestream flow. The blow-
down time of the PD igniter can be inferred from the secondary shock generated from the plume interaction with the freestream, and the fuel jet dynamics during the detonation event can be visualized. The frame-integrated chemiluminescence and OH PLIF intensity provide a qualitative metric of combustion intensity. The analysis focused on order of magnitude changes in intensity indicating burning or flame quenching. Integrated chemiluminescence was indicative of combustion heat release within the cavity. Integrated OH PLIF intensity is indicative of ground state OH species in plane with the PD igniter, and presents a two-dimensional visualization of the flame and in this case the detonation plume. Discrepancies between integrated chemiluminescence and OH PLIF indicated reaction zone three-dimensionality. Additionally, OH is produced near the flame front and exists as a product while CH∗ chemiluminescence is indicative of combustion heat release rate, and therefore exists mainly near the primary reaction zone. Challenges for data interpretation included reflections from the tunnel surfaces, dependence of the PLIF signal on local pressure (through collisional line-broadening and line-shifting), and a contribution from C2 ∗ chemiluminescence (in addition to CH∗ emission).
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Fig. 5. Instantaneous flowfield images for 95 slpm fueling condition. The scales are the same for all OH PLIF and chemiluminescence images.
The OH PLIF measurement visualized OH in the detonation plume and from cavity combustion at a plane centered on the PD igniter exit. Fluorescence of OH is dependent on temperature and pressure, which vary greatly throughout the detonation transients. Figure 3 shows an instantaneous OH PLIF image at t = 0.05 ms after detonation into an unfueled cavity. Axial and spanwise profiles illustrate a higher OH PLIF intensity in the Mach disk and plume shear layer in comparison to the core of the plume. It is assumed that OH concentration after the detonation is uniform. The cavity is unfueled and there is no additional OH production from cavity burning, therefore, it stands to be reasoned that the intensity variation in the OH images is predominately a function of the
local variations in the temperature and pressure. The velocity in the plume shear layer is reduced as it mixes with the cavity fluid, subsequently recovering a greater portion of the total temperature. The fluid downstream of the Mach disk is at elevated temperature and pressure due to the normal shock. To establish confidence that measured intensity was predominantly from OH fluorescence as opposed to emission from the PD plume, a measurement with the laser sheet placed 2 PD diameters (19 mm) from the PD centerline was conducted for the same conditions. The emission from the PD plume was 4–5 times lower in intensity than in-plane fluorescence measurement, confirming that the signal is predominately from OH fluorescence.
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Fig. 6. Integrated normalized intensity for 95 slpm fueling condition. Lines indicate instantaneous images in Fig. 5.
In order to determine the OH PLIF dependence on the transient temperature and pressure of the detonation event, OH PLIF signal dependence was calculated including the Boltzmann fraction dependence for the N = 7 ground state, the laser-transition overlap integral, and the electronic quenching, using cross-sections from Tamura et al. [20]. The calculation was conducted for a range of temperatures and pressures expected in the plume upstream (Ipl ) and downstream (Imd ) of the Mach disk, as estimated from measurements and compressible flow theory and annotated in Fig. 3a. Pressure measurements at the exit of the PD igniter combined with assumptions of Chapman–Jouguet conditions after the detonation and compressible flow equations were used to approximate the time-dependent conditions in the plume and Mach disk. The conditions in the plume are thus approximated as constant pressure (61.7 kPa) with varying temperature, and the conditions above the Mach disk are approximated as constant temperature (3070 K) with varying pressure, as illustrated in Fig. 4a. These conditions were used to estimate the time dependence of the relative OH PLIF signal in the core of the plume and after the Mach disk. The results of this LIF analysis are compared with the measured ratio of OH PLIF signal intensity across the Mach disk in the detonation plume Imd /Ipl in Fig. 4b. Imd /Ipl ranges from an initial value of 4 down to 1 as the detonation blows down from a supersonic effluent condition to a subsonic condition with no Mach disk. The intensity ratios from the LIF analysis correspond reasonably well to the OH PLIF measurements, building confidence that the OH PLIF signal intensities are globally indicative of the temperature and pressure variation of the product species. For example, the profile of OH across the plume, shown in Fig. 3b, shows increased fluorescence intensity in the plume shear layers which is strongly correlated to higher temperature of the OH present as the fluid velocity decreases and kinetic energy is recovered. Similar observations of the distribution of PD plume temperature with infrared measurements of the PD igniter were made by Ombrello et al. [3]. 3. Results A primary observation of this work is that the ignition process is primarily driven by the following: 1. Initial fuel distribution in the cavity. 2. Fluid dynamics and thermodynamics of the detonation plume. 3. The magnitude and extent of disruption to the cavity flow and fuel supply.
The results presented in this section are along the lines of these observations. Regarding the fuel distribution in the cavity, this is a function of the fueling method and the fluid dynamics within the cavity. Direct fueling from the aft ramp has been studied extensively to understand the local fuel distribution and flowfield within the cavity. Hammack et al. [9] utilized 10 kHz OH PLIF to visualize flame structure and dynamics for direct cavity fueling from lean to rich conditions. Peterson et al. [10,14] and Hassan et al. [15] investigated various numerical methods to compute the non-reacting flowfield and fuel distribution. Tuttle et al. [17] conducted particle image velocimetry measurements of the non-reacting and reacting flowfields. The cavity sustains a bulk recirculating flow with a small secondary recirculation in the base region of the cavity step. Additionally, comparison of streamlines in various spanwise planes shows a level of three-dimensionality with some swirl in the cavity step region [10]. Peterson showed that for 56 slpm fuel injection conditions, the equivalence ratio is = 1 near the fuel injectors along the bottom of the cavity and lean throughout the majority of the cavity. McGann et al. [18] investigated the fuel distribution for this condition using nanosecond gated laser-induced breakdown spectroscopy, with results in line with those of Peterson et al. [10] for lean conditions. For a higher fueling rate of Q f = 99 slpm, the boundary of = 1 spans from the edge of the cavity step down to the upper edge of the fuel injectors. This results in rich conditions in the cavity step and lean conditions along the lower edge of the cavity shear layer. Chemiluminescence measurements from Ombrello et al. [3] validated that the flame stabilizes in the lean to stoichiometric regions, shifting from the cavity step region to the shear layer and aft portion of the cavity ramp as fuel rate increases. Instantaneous images of the cavity ignition process is shown in Fig. 5 for Q f = 95 slpm. These snapshots span the entire cavity ignition sequence, for a total of 10 to 20 ms depending on the fueling condition. The shock wave produced by the detonation decoupling once it emerges into the cavity and the subsequent disruption are shown in the first two times (rows). The OH PLIF images reveal the detonation plume structure and OH production from the cavity flame. The image sequences show the flame spread and stabilization along the cavity step. At 8 ms, the OH emission is very low and the flame is bifurcated to either side of the detonation exhaust as indicated by the chemiluminescence. At 12 ms, the cavity contained a stable recirculating flowfield and steady burning. Careful examination of the schlieren images indicated approximately 8 ms for the pulse detonation to complete
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Fig. 7. Cavity ignition sequence OH PLIF during 0 < t < 1 ms for 200, 75, & 55 slpm fueling conditions. Refer to Fig. 5 for scale bars.
the exhaust process. The exhaust time is relatively constant across all conditions within +/− 1 ms. The frame-integrated OH PLIF and CH∗ traces for Q f = 95 slpm are shown in Fig. 6. Frame-integrated traces are a global representation of the cavity ignition process and are useful for analyzing the temporal nature of the transient ignition process. The dashed lines indicate the time locations of the image sequences in Fig. 5. The traces show incipient ignition followed by decreased OH PLIF intensity until approximately 8 ms after which the OH intensity rapidly increases. Incipient ignition is determined by additional signal intensity as compared to an unfueled condition. The chemiluminescence intensity shows a steady decrease until 8 ms and then stabilizes to quasi-steady levels, which indicates there is always significant burning within the cavity after incipient ignition.
3.1. Incipient ignition with pulse detonation Figures 7 and 8 compare instantaneous image sequences of the incipient ignition event for three fueling conditions. Six frames are shown through the first 1 ms after the detonation emerges into the cavity. The 200, 75, and 55 slpm conditions are shown for comparison. The three fueling rates correspond to a variety of fueling conditions, including near the rich ignition limit (200 slpm), an intermediate condition (75 slpm), and a condition (55 slpm) that is below the lean ignition limit of 68 slpm. For the first 0.1 ms, all three conditions exhibit the same OH PLIF intensity with minimal differences in the instantaneous snapshots. The highest intensity signal is anchored in and above the shear layer due to the Mach disk, and convected downstream by the cross flow. The chemiluminescence images show globally higher intensity for the 200 slpm condition, indicating a higher heat release rate, as more
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Fig. 8. Cavity ignition sequence chemiluminescence during 0 < t < 1 ms for 200, 75, & 55 slpm fueling conditions. Refer to Fig. 5 for scale bars.
fuel is burned in the cavity. As the underexpanded plume from the detonation relaxes, the Mach disk recedes into the cavity, and the diameter of the plume is reduced. The coupling of the turbulent shear layer with the detonation plume forces shedding of high temperature fluid laterally outward from the plume. This high temperature fluid ignites a favorable fuel/air mixture in the region surrounding the plume as seen in the chemiluminescence images. For the 200 slpm condition, the flame spreads across the center of the cavity. At the 75 slpm fueling rate, an ignition kernel is convected towards and established near the cavity step, resulting in flame spreading and quasi-steady burning within the cavity. At the 55 slpm condition, however, the OH PLIF only exists within the detonation plume, and no additional OH production is observed in the cavity step region, indicating that the fuel concentration in the cavity step is not sufficiently high to propagate an ignition kernel in the cavity step region and sustain reaction in the cavity. The lean ignition limit with the PD igniter is at a higher fueling rate than with less disruptive ignition methods, such as
electrical spark discharge. To investigate the higher lean ignition limit, detonation pulses were delivered at 10 Hz to the unfueled cavity and at 68 slpm, which is a fueling condition characterized by intermittent ignition behavior. The transient process of incipient ignition was investigated through image sequences and frame-integrated plots of the OH PLIF and chemiluminescence signals. The initial ignition sequence in Fig. 9 compares OH PLIF of the unfueled cavity with failed and successful ignition of the Q f = 68 slpm fueling condition. Images from the first 0.1 ms appear nearly identical for all conditions as the images are dominated by OH from the detonation products. At 0.25 ms, OH has spread to the base of the cavity and the wake downstream of the plume, indicating burning of C2 H4 in the cavity. For failed ignition, the sequence shows a few localized spots of OH that persist near the cavity step. For successful ignition, high-intensity OH PLIF signal appears at the cavity step 2 ms after the detonation. The result indicates increased heat release that spreads as the detonation
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Fig. 9. Instantaneous OH images for the unfueled cavity 68 slpm fueling condition with failed cavity ignition (No Ignition) and successful cavity ignition (Ignition). Fueling at 68 slpm is on the cusp of the lean ignition limit. Refer to Fig. 5 for scale bars.
interacts with a favorable fuel/air mixture, achieving ignition and steady state burning for this lean condition. The OH PLIF highlights the underexpanded jet plume structure that has expanded outside of the detonation tube in the cavity floor. While a Mach disk generally forms at some location in an underexpanded jet, the Mach disk location in the PD plume is affected by the impedance of the freestream flow and the jet plume and is fixed at the cavity shear layer location for the first 0.5 ms until the detonation pressure relaxes and the plume recedes. This is confirmed by comparison with plume measurements of the detonation exhausting into a quiescent environment [21] in which the detonation plume expands to a greater extent. The OH PLIF images clearly show the structure and trajectory of the detonation plume. The detonation plume shear layer transports OH radicals upstream and downstream of the detonation exhaust location. The OH radicals in the plume shear layer have a higher fluorescence level than the center of the detonation plume, indicating higher temperature
as the fluid has recovered a portion of the stagnation temperature. The highly coupled process for ignition requires transport of high-temperature radicals from the detonation plume shear layer and adequate local equivalence ratio to propagate a flame into the cavity for incipient ignition. Figure 10 shows the frame-integrated intensity of OH PLIF and chemiluminescence for the unfueled, failed, and successful ignition events for the Q f = 68 slpm condition. The OH PLIF trace for the failed ignition event follows the unfueled trace with a minor secondary increase around 2 ms as a small amount of fuel is engaged by the detonation plume, but the cavity flame does not stabilize as indicated by the chemiluminescence intensity decreasing to zero. In successful ignition cases, the OH PLIF trace shows multiple increases and decreases of signal in the first 8 ms, indicating shedding of high-temperature radicals that engage the cavity mixture, generating bursts of additional OH. The OH PLIF intensity decreases as the detonation products exhaust from the
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Fig. 10. Integrated intensity of OH and chemiluminescence measurements for the unfueled cavity compared to failed and successful ignition with cavity fueling of 68 slpm.
Fig. 11. Integrated intensity of OH PLIF in the cavity step region (Ist ) as indicated in Fig. 3a for unfueled, Q f = 68 slpm, and Q f = 95 slpm fueling conditions.
tube and then increase steadily after 10 ms. The chemiluminescence intensity is much greater as the initial detonation emerges, and decreases to a minimum around 3 ms, followed by a steady increase from 3 to 8 ms. The discrepancies between decreases and increases in the OH and chemiluminescence signals are related to the exhaust of the detonation products and are discussed further in a subsequent section. To illustrate the shedding process, Fig. 11 shows the integrated OH PLIF signal intensity in the cavity step region Ist as indicated in Fig. 3a. The unfueled condition shows that a small portion of OH is present from the initial detonation plume shear layer. For failed ignition at Q f = 68 slpm, an increase in OH signal is observed, but it is insufficient to sustain combustion. For successful ignition at Q f = 68 slpm, multiple OH production events are observed as high-temperature radicals are transported into the cavity step region, eventually resulting in flame propagation and stabilization. The OH shedding events are most apparent for the Q f = 68 slpm condition as the fueling conditions are fluctuating around the lean ignition limit. For Q f = 95 slpm, smaller scale fluctuations in OH intensity are observed as more fuel is engaged during the incipient ignition event, however, the shedding process is still observed in the first 2 ms. For this condition, the OH signal decreases as the flame stabilizes away from the OH PLIF plane during the PD exhaust. 3.2. Detonation impulse and interaction Previously, Ombrello et al. [2] hypothesized that quenching of cavity burning could be due to backpressuring of fuel injectors in
the cavity. Static pressure measurements at fs = 3.9 kHz within the cavity and downstream of the aft ramp confirm this hypothesis. During the initial detonation event spanning from 0 < t < 2 ms, the overpressure from the plume penetrates the shear layer and drives fluid from the cavity, as observed from the schlieren image sequences in Fig. 12. The following mass injection and heat release in the cavity causes the bulk pressure within the cavity to increase, further driving fluid from the cavity and disrupting the recirculating cavity flowfield. This phenomenon is schematically illustrated in Fig. 13 and quantitatively shown by the pressure traces in the cavity (P1 ) and downstream of the ramp (P2 ) in Fig. 14. During the detonation impulse, the pressure increases approximately 10% within the cavity. The lifting of the shear layer from the high momentum detonation plume combined with the favorable pressure gradient drives fluid from the cavity. The pressure-driven acceleration of flow from the cavity is suggested by a decrease in pressure just aft of the cavity ramp (P2 ). This evacuation behavior was also noted in the computations of Ombrello et al. [2], where simulations showed the cavity integrated mass fraction of C2 H4 decreased to 20% of the initial value over a 2 ms period. For the 200 slpm fueling condition, the pressure rise is 50% greater, which is attributed to greater initial heat release in the cavity. The direct cavity fueled configuration is exposed to the increased cavity pressure from the detonation wave impulse as it expands into the cavity. Figure 15 shows the change in fuel pressure (Pd = Pf uel − P 1max ) normalized to the initial fuel pressure drop (Pf ) across the fuel injectors, at the peak of the detonation impulse, for each fueling condition. The detonation overpressure results in a 40% reduction in pressure drop across the fuel injectors for the 55 slpm condition with a decreasing effect as the fuel flow increases (with higher injector pressure drop). For the lower fuel conditions (55 and 70 slpm), the backpressuring of the fuel injectors temporarily reduces the fuel concentration in the cavity for 1–2 cavity cycle periods, which is long enough to inhibit cavity ignition. The combination of the cavity evacuation and the backpressuring of the fuel injectors is responsible for initial suppression in cavity burning observed in the measurements. These results demonstrate the sensitivity of cavity ignition to the impedance of the fueling system and integration method of the PD igniter. 3.3. Global cavity ignition The frame-integrated OH PLIF and chemiluminescence for five fueling conditions ranging from rich to lean are shown in Fig. 16. Comparison of the five conditions shows that ignition of the cavity can exhibit very different behavior with respect to incipient ignition, quenching, and re-establishment of the flame. For the three fueling conditions: 70, 75, and 95 slpm, the flame quenching time after initial ignition varies with the higher fueling conditions
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Fig. 12. Schlieren snapshots showing evacuation of the cavity during the detonation overpressure.
Fig. 13. Schematic of cavity flowfield during detonation overpressure and backpressure of fuel injection.
Fig. 14. Pressures in the cavity floor (P1 ) and downstream of the ramp (P2 ) during the detonation event depicting the pressure gradient driving flow out of the cavity.
resulting in more secondary burning from 2 to 10 ms. In the chemiluminescence signal for fueling conditions 75 and 95 slpm, there is always some level of heat release in the cavity throughout the ignition process. For the 70 slpm fueling rate, near the lean ig-
Fig. 15. Magnitude of fuel injector backpressuring from the detonation normalized to the pressure drop for each fueling condition.
nition limit, the combustion is almost entirely quenched (i.e. only a very low level of chemiluminescence is still detectable). For the 70 slpm fueling condition, the weak flame kernel propagates to fill the cavity at approximately 7 ms when the PD exhaust process is complete. This illustrates how the incipient ignition event seeds a flame kernel that must persist during the disruptive blow down
Fig. 16. Integrated intensity for ignition of 200-55 slpm fueling conditions. Dashed gray lines indicate image times from Fig. 17.
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Fig. 17. Full cavity ignition sequence for 200-55 slpm fueling conditions showing OH and chemiluminescence. Refer to Fig. 5 for scale bars.
of the PD exhaust in order to propagate and stabilize a flame in the cavity. For the highest fueling condition, 200 slpm, the OH PLIF signal indicates local quenching in the measurement plane, while the chemiluminescence signal indicates continuous burning at all times in the cavity. The OH signal does not increase again until approximately 12 ms, approximately 4 ms after the PD exhaust process is complete. This is due to bifurcation of the flame as shown in Fig. 17 as the flame is locally quenched around the PD exhaust plume. The 15–to 20 ms delay until the flame is re-established for the 200 slpm condition is due to the flame preference to anchor near the cavity ramp, requiring the PD exhaust process to complete before the flame can propagate and stabilize.
Instantaneous OH PLIF and chemiluminescence images for the full ignition sequence of all five fueling conditions are shown in Fig. 17. These simultaneous image sequences reinforce the observations in Fig. 16. The initial detonation pulse engages a large portion of the cavity fuel/air mixture as it expands within the cavity. For the 200, 95, and 75 slpm fueling rates, combustion spreads laterally in the cavity, while for the 70 and 55 slpm fueling rates combustion is quenched within the first few milliseconds. Sustaining a kernel of high temperature products supplied with enough reactants to persist through the PD exhaust period is necessary for combustion to re-establish. In the case of higher fueling rates, the flame bifurcates on either side of the detonation plane.
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For the intermediate conditions, the flame appears to anchor in the cavity step region. For the 70 slpm fueling rate, a very weak kernel persists in the cavity until the PD exhaust is complete and the quasi-steady cavity combustion is re-established. Of course, the fuel must continue to enter the cavity and mix in a region that is exposed to the stabilized reaction. For the 55 slpm condition, there is insufficient fuel concentration to sustain an ignition kernel, due to temporary backpressuring of the fuel injectors as described above, and combustion in the cavity is quenched. 4. Summary This work provided temporally and spatially resolved measurements of pulse detonation ignition of a scramjet cavity flameholder fueled with C2 H4 at a Mach 2 freestream condition. Simultaneous 40-kHz schlieren, chemiluminescence, and OH PLIF imaging provided unprecedented insight into the ignition mechanism of high-temperature detonation products coupled with a recirculating fuel/air mixture within the cavity. It was found that ignition of the cavity mixture was dependent on the shedding of high-temperature fluid through the exhaust plume shear layer laterally into the cavity mixture. For the configurations tested, a large quantity of energy was exhausted through the cavity shear layer into the unfueled supersonic freestream. The dynamics of the detonation plume interacting with the cavity shear layer resulted in shedding of high-temperature eddies into the cavity. Two mechanisms that inhibited cavity ignition were identified. The first was suppression of cavity fueling for 1–2 cavity cycle periods (2–3 ms) for low fueling rates that occurs due to backpressuring of the fuel injectors. This process is the reason for the higher lean ignition limit for pulse detonation ignition compared to spark ignition. The second mechanism that inhibited cavity ignition was local combustion quenching during and due to the PD exhaust process of the detonation products. This process bifurcated the flame and disrupted the cavity recirculation flowfield (up to 12 ms) to the extent that the heat release was not sustained beyond the exhaust period. The results presented show that a combination of appropriate fueling conditions with the integration method of the pulse detonation igniter are closely linked to achieving successful ignition in a supersonic cavity flameholder. When executed appropriately, pulse detonation can provide robust ignition across a broader range of fueling conditions than electrical discharge devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the support of the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base for the facilities and financial support for the research. The authors would also like to acknowledge the financial support of the National Research Council (NRC) Research Associateship Pro-
gram from the National Academies of Sciences, Engineering, and Medicine. The authors would like to thank Paul Gross and Bill Terry (Innovative Scientific Solutions, Inc.) for their assistance in preparation and execution of the tunnel tests. References [1] E.T. Curran, W.H. Heiser, D.T. Pratt, Fluid phenomena in scramjet combustion systems, Annu. Rev. Fluid Mech. 28 (1996) 323–360, doi:10.1146/annurev.fl.28. 010196.001543. [2] T.M. Ombrello, C.D. Carter, C.J. Tam, K.Y. Hsu, Cavity ignition in supersonic flow by spark discharge and pulse detonation, Proc. Combust. Inst. 35 (2) (2015) 2101–2108, doi:10.1016/j.proci.2014.07.068. [3] T. Ombrello, D.L. Blunck, M. Resor, Quantified infrared imaging of ignition and combustion in a supersonic flow, Exp. Fluids 57 (9) (2016), doi:10.1007/ s00348- 016- 2210- 0. [4] T. Ombrello, C. Carter, J. McCall, F. Schauer, A. Naples, J. Hoke, H.K. Y., Enhanced mixing in supersonic flow using a pulse detonator, J. Propuls. Power 31 (2) (2015) 654–663, doi:10.2514/1.B35316. [5] T. Ombrello, S. Peltier, C. Carter, Effects of inlet distortion on cavity ignition in supersonic flow, 53rd AIAA Aerospace Sciences Meeting, Kissimmee, FL (2015), doi:10.2514/6.2015-0882. [6] J.D. Miller, S.J. Peltier, M.N. Slipchenko, J.G. Mance, T.M. Ombrello, J.R. Gord, C.D. Carter, Investigation of transient ignition processes in a model scramjet pilot cavity using simultaneous 100 khz formaldehyde planar laer-induced fluorescence and ch∗ chemiluminescence imaging, Proc. Combust. Inst. 36 (2017) 2865–2872, doi:10.1016/j.proci.2016.07.060. [7] M.R. Gruber, A.S. Nejad, New supersonic combustion research facility, J. Propuls. Power 11 (5) (1995) 1080–1083. [8] M.R. Gruber, R.A. Baurle, T. Mathur, K.Y. Hsu, Fundamental studies of cavity-based flameholder concepts for supersonic combustors, J. Propuls. Power 17 (1) (2001) 146–153. [9] S.D. Hammack, T. Lee, K.Y. Hsu, C.D. Carter, High-repetition-rate oh planar laser-induced fluorescence of a cavity flameholder, J. Propuls. Power 29 (5) (2013) 1248–1251, doi:10.2514/1.B34756. [10] D.M. Peterson, E. Hassan, S.G. Tuttle, M. Hagenmaier, C.D. Carter, Numerical investigation of a supersonic cavity flameholder, 52nd AIAA Aerospace Sciences Meeting, AIAA, National Harbor, MD (2014). [11] D. Cuppoletti, T.M. Ombrello, K. Rein, Cavity ignition dependence on pulse detonation impulse parameters, AIAA Aerospace Sciences Meeting, AIAA (2018), doi:10.2514/6.2018-1358. [12] T.M. Ombrello, N.S. Okhavat, M. Rhoby, Measurements of scramjet fueling conditions, 56th AIAA Aerospace Sciences Meeting, AIAA, Kissimmee, FL (2018), doi:10.2514/6.2018-1360. [13] M. Rhoby, T.M. Ombrello, N.S. Okhovat, A.M. Hansen, K.C. Gross, Infrared hyperspectral imaging diagnostics of fueling strategy for scramjets, J. Propuls. Power 34 (6) (2018) 1391–1400, doi:10.2514/1.B36939. [14] D.M. Peterson, E.A. Hassan, Hybrid rans/les of combustion in a supersonic cavity flameholder at mach 2 and mach 3, 56th AIAA Aerospace Sciences Meeting, AIAA, Kissimmeee, FL (2018), doi:10.2514/6.2018-1144. [15] E.A. Hassan, D.M. Peterson, K. Walters, E.A. Luke, Dynamic hybrid reynoldsaveraged navier stokes/large-eddy simulation of a supersonic cavity, J. Propuls. Power 32 (6) (2016) 1343–1352, doi:10.2514/1.B36132. [16] H. Do, C.D. Carter, Q. Liu, T.M. Ombrello, S. Hammack, T. Lee, K.-Y. Hsu, Simultaneous gas density and fuel concentration measurements in a supersonic combustor using laser induced breakdown, Proc. Combust. Inst. 35 (2015) 2155– 2162, doi:10.1016/j.proci.2014.07.043. [17] S.G. Tuttle, C.D. Carter, K.Y. Hsu, Particle image velocimetry in a nonreacting and reacting high-speed cavity, J. Propuls. Power 30 (3) (2014) 576–591, doi:10. 2514/1.B34974. [18] B. McGann, C.D. Carter, T.M. Ombrello, S. Hammack, T. Lee, D. H., Gas property measurements in a supersonic combustor using nanosecond gated laserinduced breakdown spectroscopy with direct spectrum matching, Proc. Combust. Inst. 36 (2017) 2857–2864, doi:10.1016/j.proci.2016.06.089. [19] S. Hammack, C. Carter, C. Wuensche, T. Lee, Continuous hydroxyl radical planar laser imaging at 50 khz repetition rate, Appl. Opt. 53 (23) (2014) 5246–5251, doi:10.1364/AO.53.005246. [20] M. Tamura, P.A. Berg, J.E. Harrington, J. Luque, J.B. Jeffries, G.P. Smith, D.R. Crosley, Collisional quenching of ch(a), oh(a), and no(a) in low pressure hydrocarbon flames, Combust. Flame 114 (1998) 502–514, doi:10.1016/ S0 010-2180(97)0 0324-6. [21] D. Cuppoletti, T. Ombrello, K. Rein, Energy coupling mechanism for pulse detonation ignition of a scramjet cavity, Proc. Combust. Inst. (2018), doi:10.1016/j. proci.2018.08.005.
Contents lists available at ScienceDirect
Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Ignition dynamics of a pulse detonation igniter in a supersonic cavity flameholder Daniel Cuppoletti a,∗, Timothy Ombrello b, Campbell Carter b, Stephen Hammack b, Joseph Lefkowitz c a b c
University of Cincinnati, Cincinnati, OH, USA U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA Technion - Israel Institute of Technology, Haifa, Israel
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 25 January 2020 Accepted 27 January 2020 Available online 29 February 2020 Keywords: Detonation Ignition Scramjet Cavity Supersonic PLIF
a b s t r a c t The dynamics of using a pulse detonation wave to ignite a supersonic cavity flameholder was investigated. Fueling conditions ranging from lean to rich were investigated. Simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence, and chemiluminescence imaging were used to elucidate the fluid dynamics and combustion physics throughout the transient disruption of the ignition event. For moderate and rich fueling conditions, the ignition event stabilized combustion away from the detonation plume until the detonation products were fully exhausted. Ignition of leaner conditions required a lowlevel flame kernel to persist in the cavity step until the recirculating cavity flow field was re-established. The initial ignition mechanism was shown to be primarily driven by hot products from the detonation plume shedding into the low velocity region near the aft facing step in the cavity, followed by propagation and stabilization of a flame to a favorable region of the cavity. Quenching of cavity combustion at lean overall cavity mixtures was shown to be due to temporary backpressuring of cavity fuel injectors and cavity evacuation from the detonation pressure impulse. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Achieving ignition and stable combustion in high-speed airbreathing combustors is a challenge due to short residence times and transient fluid dynamics involved [1]. The flow-through time for an entire scramjet engine is on the order of milliseconds. Flame stabilization has been addressed extensively through the use of cavities, struts, and other devices that create locally reduced velocity with increased residence times. The continuous recirculation of hot combustion products and radicals provides a sustained energy addition to the primary combustor flow. Ignition is often achieved through energy deposition by electrical or chemical means, or some combination of both such as fueled torches. Capacitive spark or plasma discharge [2,3] and various types of combustion devices create a local elevation in temperature that initiates an energy cascade for sustained heat release to the remainder of the fueled system. System level considerations often govern decisions on ignition devices. While electrical spark discharge can be simpler, a fueled
∗
Corresponding author. E-mail addresses: [email protected] (D. Cuppoletti), [email protected] (T. Ombrello), [email protected] (C. Carter), [email protected] (S. Hammack), [email protected] (J. Lefkowitz).
torch can provide greater energy deposition with lower electrical energy requirements through coupling with chemical heat release. The localized nature of spark ignition coupled with the low propagation speed of deflagration can affect the ignition limits and ignition probability in combustors with short residence times. Recently, a pulse detonation igniter has been developed and demonstrated in a supersonic flow, employing the strong transient combustion and high-pressure impulse provided by the outflow [2,4–6]. Pulse detonation (PD), also referred to as pressure gain combustion, provides a unique energy deposition method capable of influencing a large region over a short time frame. This increased volumetric influence of the PD ignition device can improve the probability of ignition. Ombrello et al. [4] studied the ignition process of a PD igniter compared to spark discharge, utilizing high frame rate shadowgraph and CH∗ chemiluminescence imaging. For fueling conditions near stoichiometric in the cavity, the detonation products ignited the cavity more rapidly than spark discharge and burning persisted over the first few milliseconds. Depending on the fueling conditions, combustion was either sustained, quenched, re-established after a delay, or stabilized within the cavity. If steady-state combustion did not stabilize in the cavity after the initial consumption of fuel in the cavity, then it was considered a failed ignition event. This paper describes
https://doi.org/10.1016/j.combustflame.2020.01.030 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
D. Cuppoletti, T. Ombrello and C. Carter et al. / Combustion and Flame 215 (2020) 376–388
Nomenclature D M p, P Q T t 0 ∞ d f
Diameter of PD tube Mach number Pressure Volumetric Flow Rate Temperature time Freestream Condition Ambient Condition Detonation Condition Fuel Condition Equivalence Ratio
the details of the initial ignition and flame stabilization process in further detail to provide insight into the dynamic process of transferring a detonation wave to a deflagrating cavity mixture. The current study combines simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence (PLIF), and CH∗ chemiluminescence imaging to investigate the ignition process of a PD igniter for cavity fueling conditions ranging from lean to rich. All three diagnostic methods have different utility for understanding the flowfield and combustion processes involved during the ignition transient. The combination of all three diagnostics provides an improved understanding of the cavity ignition mechanisms. 2. Methods The experiments were conducted in Research Cell 19, a directconnect supersonic wind tunnel at Wright-Patterson Air Force Base. The test section is 15.24 cm wide and has an entrance height of 5.08 cm with a 2.5◦ divergence of the lower wall where the cavity resides, as shown in Fig. 1. The cavity spans the tunnel width and has a 1.65 cm depth and a 22.5◦ closeout ramp. A shear layer forms over the cavity with recirculating flow in the cavity and weak oblique shocks from the streamline deflections due to the shear layer recompression on the closeout ramp. The configuration has been well studied experimentally and computationally [7–10]. The exit of the PD igniter exhausts normal to the cavity floor and
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was located 2.5 cm aft of the cavity step and 1.9 cm from centerline. Two pressure transducers were utilized on the lower surface facility centerline: P1 located 2.96 cm aft of the cavity step, and P2 located 3.46 cm aft of the cavity ramp closeout. The cavity was fueled with C2 H4 from eleven injectors in the aft ramp. This study focused on cavity fueling and ignition only, with a global equivalence ratio of global < 0.016 for all conditions tested. Injecting fuel directly into the cavity decouples the cavity fueling from entrainment through the cavity shear layer (i.e. no upstream fueling was utilized). All tests were conducted with free-stream conditions at Mach number M = 2, total temperature T0 = 589 K, and total pressure P0 = 483 kPa. The PD igniter had a volume of 49 cm3 with an inner diameter of 10.2 mm. The PD reactants, C3 H8 and N2 O, were injected at the head end of the PD tube, and the tube was overfilled for all experiments. Overfilling the PD tube ensures that a detonation is supported all of the way to the exhaust plane into the cavity. The excess reactants in the cavity from overfilling had negligible effect on the cavity ignition process [2]. The mixture was ignited at the head end near the reactant injection location and exhausted into the cavity through a 7.62 mm diameter orifice. For direct cavity fueling, the operational range of the cavity at the given Po and To tested is approximately 30–260 standard liters per minute (slpm) corresponding to the lean and rich blowout limits of the cavity. For these conditions the cavity can sustain heat release by cycling hot products and reactants within the cavity. The fueling rate for ignition differs from the operational range and is strongly dependent on the energy deposition method. Two separate studies are presented in this paper. The first study utilized multiple pulses of the PD igniter at 10 Hz with an unfueled cavity and cavity fueling at 68 slpm, which is near the lean ignition limit for a single pulse. The second study focused on investigation of a single PD igniter pulse for five different fueling conditions shown in Table 1 alongside the operational pressure drop (Pf = Pf uel − Pcavity ) for the fuel injectors. The injector pressure drop is discussed further in Section 3.2. Fuel flow rates are identified as the metrics for comparison since the equivalence ratio varies spatially and temporally due to turbulent mixing within the cavity. Wall-based extractive sampling via a Fourier Transform Infrared (FTIR) spectrometer in the cavity step region indicated equivalence ratios varying from = 1.0 for 68 slpm to = 2.5 for 200 slpm, and has been confirmed through other experimental
Fig. 1. Experimental layout and basic cavity flowfield features.
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Fig. 2. Imaging fields-of-view showing schlieren volume (gray box), OH PLIF field-of-view (blue plane) directly over the PD centerline, and CH∗ chemiluminescence imaging elevated field-of-view (red cone). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Fueling conditions for single pulse ignition studies. Condition
Qf [slpm]
Pf [kPa]
1 2 3 4 5
200 95 75 70 55
138 46.5 31.0 26.9 17.2
and computational investigations [11–16]. The 200 slpm condition is rich throughout the cavity, and burning tends to anchor near the aft ramp [17]. The 95, 75, and 70 slpm conditions are referred to as “intermediate” conditions since the cavity fueling is favorable for stable combustion and the ignition dynamics are similar for these conditions. The 70 slpm is slightly above the lean ignition limit for the PD igniter, while the 55 slpm condition is below the lean ignition limit. It should be noted that ignition and flame stabilization can be achieved at 55 slpm and lower when other energy deposition devices and configurations are used, such as spark discharge [2,18]. 2.1. Diagnostics Schlieren, chemiluminescence, and OH PLIF imaging were acquired simultaneously at fs = 40 kHz frame rate (see Fig. 2 for fields of view). Schlieren imaging used a collimated Hg-Xe light source passed spanwise across the tunnel with a horizontal knife edge at the focal point to highlight vertical density gradients. The exposure time for schlieren imaging was texp = 1 μs. Frames were captured with a Photron SA-Z camera. Chemiluminescense was captured with an image intensifier (LaVision HS-IRO) coupled to a Photron SA-Z. The exposure time was texp = 10 μs. A 431 nm bandpass filter with 17-nm full-width at half-maximum transmission (Semrock FF01-434/17) was used to target CH∗ emission. For C2 H4 combustion, the emission is primarily from CH∗ with a minor secondary contribution from C2 ∗ [3]; however, it should be noted that the detonation event contains very high broadband emission intensities during the first few milliseconds. Details of the imaging fields-of-view are shown in Fig. 2. The OH PLIF system utilized a diode-pumped solid-state (DPSS) Nd:YVO4 laser (EdgeWave Innoslab HD40II-ET), frequency doubled to produce 150 W at 532-nm with a pulse repetition rate of 40 kHz. The DPSS laser beam pumped a high-power tunable dye laser (Sirah CREDO) that contained a solution of Rhodamine
590 dye (Exciton) in ethanol. The 566-nm dye-laser beam was frequency doubled using two barium borate (BBO) crystals. More detailed information regarding the laser system configuration can be found in Hammack et al. [19]. For the current measurements, the laser power was 4 W (100 μJ per pulse) at 283 nm, controlled by limiting the 532-nm pump power entering the dye laser. The 283-nm excitation beam was reflected upwards and then horizontally over the top of the test section. The beam was formed into a sheet using a plano-concave cylindrical lens and a plano-convex spherical lens. The sheet was approximately 70 mm by 0.25 mm at the waist at the probe volume (in the cavity), with the major dimension aligned in the tunnel streamwise direction (see Fig. 2) directly over the PD exhaust. Fused-silica windows were used for both the top tunnel wall and the side walls. Before sheet formation, the beam passed through an angled thin fused-silica window. A beam sample was passed over a small burner-stabilized premixed CH4 /air flame and then on to a UV diffuser and photodiode. OH LIF in the flame was focused into a 0.1-m monochromator and was measured by an attached photomultiplier tube. The assembly allowed for calibration and monitoring of both laser wavelength and laser pulse energy throughout the test. OH radicals were excited using the Q1 (7) transition of the A2 + − X 2 (1,0) band (λex = 282.93 nm in air). The transition was chosen for its strength and the temperature insensitivity of the ground state. Emission from the transitions in the A2 + − X 2 (1,1) and (0,0) bands were recorded using a Photron SA-Z with an attached image intensifier (LaVision HS-IRO) gated at 100 ns. Collection optics included a 100 mm, f/2.8 UV lens (Sodern Cerco) fitted with a high-transmission ( > 80% at 310 nm) bandpass filter (Laser Components GmbH). To accommodate the schlieren setup, the PLIF camera and HS-IRO were rotated away from perpendicular to the laser sheet and equipped with a Scheimpflug adaptor (LaVision) to maintain focus of the image plane. The intensity scales for each diagnostic results are consistent across this paper unless otherwise noted. For OH PLIF, the intensity scale is [0–30 0 0] counts. The CH∗ chemiluminescence intensity varies greatly from near the camera saturation value during the detonation to low levels during steady state burning. For this reason, the natural log of the intensity is shown for the images and integrated signals, ranging from [5.0–8.3]. Chemiluminescence and OH PLIF are presented with false color scale to assist in resolving features. The images are not modified other than in color scale and overlaying with lines indicating surface locations. Frame-integrated intensities are presented as filtered signals with
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Fig. 3. OH LIF image of (a) the detonation plume in an unfueled cavity at t = 0.05 ms after detonation showing integration region of interests and (b) profiles of intensity through the plume.
Fig. 4. Evolution of pressure and temperature in the PD plume for LIF analysis and (b) normalized intensity ratio (Imd /Ipl ) of analysis compared to measured data.
arbitrary units. Savitzky–Golay filtering introduces a time lag O(μs) for the impulse rise time while drastically improving the signal quality for comparisons. White lines in the OH PLIF and CH∗ chemiluminescence outline the cavity location in the images. The vertical edge of the cavity step is just outside of the field-of-view for the OH PLIF images, resulting in no line shown on the left side of the images. 2.2. Interpretation of imaging diagnostics The primary goal of the current study was to provide spatiotemporally resolved measurements of the cavity ignition process with the PD ignition device. The three simultaneous imaging diagnostics provide greater insight into ignition and detonation phenomena than a single diagnostic. Since schlieren is a line-ofsight measurement, the density gradients from shock waves, turbulence, fuel injection, and combustion are superimposed on the density gradients from the detonation, making it difficult to decouple the various effects in the flowfield. On the other hand, schlieren imaging enables visualization of the detonation interacting with the cavity shear layer and freestream flow. The blow-
down time of the PD igniter can be inferred from the secondary shock generated from the plume interaction with the freestream, and the fuel jet dynamics during the detonation event can be visualized. The frame-integrated chemiluminescence and OH PLIF intensity provide a qualitative metric of combustion intensity. The analysis focused on order of magnitude changes in intensity indicating burning or flame quenching. Integrated chemiluminescence was indicative of combustion heat release within the cavity. Integrated OH PLIF intensity is indicative of ground state OH species in plane with the PD igniter, and presents a two-dimensional visualization of the flame and in this case the detonation plume. Discrepancies between integrated chemiluminescence and OH PLIF indicated reaction zone three-dimensionality. Additionally, OH is produced near the flame front and exists as a product while CH∗ chemiluminescence is indicative of combustion heat release rate, and therefore exists mainly near the primary reaction zone. Challenges for data interpretation included reflections from the tunnel surfaces, dependence of the PLIF signal on local pressure (through collisional line-broadening and line-shifting), and a contribution from C2 ∗ chemiluminescence (in addition to CH∗ emission).
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Fig. 5. Instantaneous flowfield images for 95 slpm fueling condition. The scales are the same for all OH PLIF and chemiluminescence images.
The OH PLIF measurement visualized OH in the detonation plume and from cavity combustion at a plane centered on the PD igniter exit. Fluorescence of OH is dependent on temperature and pressure, which vary greatly throughout the detonation transients. Figure 3 shows an instantaneous OH PLIF image at t = 0.05 ms after detonation into an unfueled cavity. Axial and spanwise profiles illustrate a higher OH PLIF intensity in the Mach disk and plume shear layer in comparison to the core of the plume. It is assumed that OH concentration after the detonation is uniform. The cavity is unfueled and there is no additional OH production from cavity burning, therefore, it stands to be reasoned that the intensity variation in the OH images is predominately a function of the
local variations in the temperature and pressure. The velocity in the plume shear layer is reduced as it mixes with the cavity fluid, subsequently recovering a greater portion of the total temperature. The fluid downstream of the Mach disk is at elevated temperature and pressure due to the normal shock. To establish confidence that measured intensity was predominantly from OH fluorescence as opposed to emission from the PD plume, a measurement with the laser sheet placed 2 PD diameters (19 mm) from the PD centerline was conducted for the same conditions. The emission from the PD plume was 4–5 times lower in intensity than in-plane fluorescence measurement, confirming that the signal is predominately from OH fluorescence.
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Fig. 6. Integrated normalized intensity for 95 slpm fueling condition. Lines indicate instantaneous images in Fig. 5.
In order to determine the OH PLIF dependence on the transient temperature and pressure of the detonation event, OH PLIF signal dependence was calculated including the Boltzmann fraction dependence for the N = 7 ground state, the laser-transition overlap integral, and the electronic quenching, using cross-sections from Tamura et al. [20]. The calculation was conducted for a range of temperatures and pressures expected in the plume upstream (Ipl ) and downstream (Imd ) of the Mach disk, as estimated from measurements and compressible flow theory and annotated in Fig. 3a. Pressure measurements at the exit of the PD igniter combined with assumptions of Chapman–Jouguet conditions after the detonation and compressible flow equations were used to approximate the time-dependent conditions in the plume and Mach disk. The conditions in the plume are thus approximated as constant pressure (61.7 kPa) with varying temperature, and the conditions above the Mach disk are approximated as constant temperature (3070 K) with varying pressure, as illustrated in Fig. 4a. These conditions were used to estimate the time dependence of the relative OH PLIF signal in the core of the plume and after the Mach disk. The results of this LIF analysis are compared with the measured ratio of OH PLIF signal intensity across the Mach disk in the detonation plume Imd /Ipl in Fig. 4b. Imd /Ipl ranges from an initial value of 4 down to 1 as the detonation blows down from a supersonic effluent condition to a subsonic condition with no Mach disk. The intensity ratios from the LIF analysis correspond reasonably well to the OH PLIF measurements, building confidence that the OH PLIF signal intensities are globally indicative of the temperature and pressure variation of the product species. For example, the profile of OH across the plume, shown in Fig. 3b, shows increased fluorescence intensity in the plume shear layers which is strongly correlated to higher temperature of the OH present as the fluid velocity decreases and kinetic energy is recovered. Similar observations of the distribution of PD plume temperature with infrared measurements of the PD igniter were made by Ombrello et al. [3]. 3. Results A primary observation of this work is that the ignition process is primarily driven by the following: 1. Initial fuel distribution in the cavity. 2. Fluid dynamics and thermodynamics of the detonation plume. 3. The magnitude and extent of disruption to the cavity flow and fuel supply.
The results presented in this section are along the lines of these observations. Regarding the fuel distribution in the cavity, this is a function of the fueling method and the fluid dynamics within the cavity. Direct fueling from the aft ramp has been studied extensively to understand the local fuel distribution and flowfield within the cavity. Hammack et al. [9] utilized 10 kHz OH PLIF to visualize flame structure and dynamics for direct cavity fueling from lean to rich conditions. Peterson et al. [10,14] and Hassan et al. [15] investigated various numerical methods to compute the non-reacting flowfield and fuel distribution. Tuttle et al. [17] conducted particle image velocimetry measurements of the non-reacting and reacting flowfields. The cavity sustains a bulk recirculating flow with a small secondary recirculation in the base region of the cavity step. Additionally, comparison of streamlines in various spanwise planes shows a level of three-dimensionality with some swirl in the cavity step region [10]. Peterson showed that for 56 slpm fuel injection conditions, the equivalence ratio is = 1 near the fuel injectors along the bottom of the cavity and lean throughout the majority of the cavity. McGann et al. [18] investigated the fuel distribution for this condition using nanosecond gated laser-induced breakdown spectroscopy, with results in line with those of Peterson et al. [10] for lean conditions. For a higher fueling rate of Q f = 99 slpm, the boundary of = 1 spans from the edge of the cavity step down to the upper edge of the fuel injectors. This results in rich conditions in the cavity step and lean conditions along the lower edge of the cavity shear layer. Chemiluminescence measurements from Ombrello et al. [3] validated that the flame stabilizes in the lean to stoichiometric regions, shifting from the cavity step region to the shear layer and aft portion of the cavity ramp as fuel rate increases. Instantaneous images of the cavity ignition process is shown in Fig. 5 for Q f = 95 slpm. These snapshots span the entire cavity ignition sequence, for a total of 10 to 20 ms depending on the fueling condition. The shock wave produced by the detonation decoupling once it emerges into the cavity and the subsequent disruption are shown in the first two times (rows). The OH PLIF images reveal the detonation plume structure and OH production from the cavity flame. The image sequences show the flame spread and stabilization along the cavity step. At 8 ms, the OH emission is very low and the flame is bifurcated to either side of the detonation exhaust as indicated by the chemiluminescence. At 12 ms, the cavity contained a stable recirculating flowfield and steady burning. Careful examination of the schlieren images indicated approximately 8 ms for the pulse detonation to complete
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Fig. 7. Cavity ignition sequence OH PLIF during 0 < t < 1 ms for 200, 75, & 55 slpm fueling conditions. Refer to Fig. 5 for scale bars.
the exhaust process. The exhaust time is relatively constant across all conditions within +/− 1 ms. The frame-integrated OH PLIF and CH∗ traces for Q f = 95 slpm are shown in Fig. 6. Frame-integrated traces are a global representation of the cavity ignition process and are useful for analyzing the temporal nature of the transient ignition process. The dashed lines indicate the time locations of the image sequences in Fig. 5. The traces show incipient ignition followed by decreased OH PLIF intensity until approximately 8 ms after which the OH intensity rapidly increases. Incipient ignition is determined by additional signal intensity as compared to an unfueled condition. The chemiluminescence intensity shows a steady decrease until 8 ms and then stabilizes to quasi-steady levels, which indicates there is always significant burning within the cavity after incipient ignition.
3.1. Incipient ignition with pulse detonation Figures 7 and 8 compare instantaneous image sequences of the incipient ignition event for three fueling conditions. Six frames are shown through the first 1 ms after the detonation emerges into the cavity. The 200, 75, and 55 slpm conditions are shown for comparison. The three fueling rates correspond to a variety of fueling conditions, including near the rich ignition limit (200 slpm), an intermediate condition (75 slpm), and a condition (55 slpm) that is below the lean ignition limit of 68 slpm. For the first 0.1 ms, all three conditions exhibit the same OH PLIF intensity with minimal differences in the instantaneous snapshots. The highest intensity signal is anchored in and above the shear layer due to the Mach disk, and convected downstream by the cross flow. The chemiluminescence images show globally higher intensity for the 200 slpm condition, indicating a higher heat release rate, as more
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Fig. 8. Cavity ignition sequence chemiluminescence during 0 < t < 1 ms for 200, 75, & 55 slpm fueling conditions. Refer to Fig. 5 for scale bars.
fuel is burned in the cavity. As the underexpanded plume from the detonation relaxes, the Mach disk recedes into the cavity, and the diameter of the plume is reduced. The coupling of the turbulent shear layer with the detonation plume forces shedding of high temperature fluid laterally outward from the plume. This high temperature fluid ignites a favorable fuel/air mixture in the region surrounding the plume as seen in the chemiluminescence images. For the 200 slpm condition, the flame spreads across the center of the cavity. At the 75 slpm fueling rate, an ignition kernel is convected towards and established near the cavity step, resulting in flame spreading and quasi-steady burning within the cavity. At the 55 slpm condition, however, the OH PLIF only exists within the detonation plume, and no additional OH production is observed in the cavity step region, indicating that the fuel concentration in the cavity step is not sufficiently high to propagate an ignition kernel in the cavity step region and sustain reaction in the cavity. The lean ignition limit with the PD igniter is at a higher fueling rate than with less disruptive ignition methods, such as
electrical spark discharge. To investigate the higher lean ignition limit, detonation pulses were delivered at 10 Hz to the unfueled cavity and at 68 slpm, which is a fueling condition characterized by intermittent ignition behavior. The transient process of incipient ignition was investigated through image sequences and frame-integrated plots of the OH PLIF and chemiluminescence signals. The initial ignition sequence in Fig. 9 compares OH PLIF of the unfueled cavity with failed and successful ignition of the Q f = 68 slpm fueling condition. Images from the first 0.1 ms appear nearly identical for all conditions as the images are dominated by OH from the detonation products. At 0.25 ms, OH has spread to the base of the cavity and the wake downstream of the plume, indicating burning of C2 H4 in the cavity. For failed ignition, the sequence shows a few localized spots of OH that persist near the cavity step. For successful ignition, high-intensity OH PLIF signal appears at the cavity step 2 ms after the detonation. The result indicates increased heat release that spreads as the detonation
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Fig. 9. Instantaneous OH images for the unfueled cavity 68 slpm fueling condition with failed cavity ignition (No Ignition) and successful cavity ignition (Ignition). Fueling at 68 slpm is on the cusp of the lean ignition limit. Refer to Fig. 5 for scale bars.
interacts with a favorable fuel/air mixture, achieving ignition and steady state burning for this lean condition. The OH PLIF highlights the underexpanded jet plume structure that has expanded outside of the detonation tube in the cavity floor. While a Mach disk generally forms at some location in an underexpanded jet, the Mach disk location in the PD plume is affected by the impedance of the freestream flow and the jet plume and is fixed at the cavity shear layer location for the first 0.5 ms until the detonation pressure relaxes and the plume recedes. This is confirmed by comparison with plume measurements of the detonation exhausting into a quiescent environment [21] in which the detonation plume expands to a greater extent. The OH PLIF images clearly show the structure and trajectory of the detonation plume. The detonation plume shear layer transports OH radicals upstream and downstream of the detonation exhaust location. The OH radicals in the plume shear layer have a higher fluorescence level than the center of the detonation plume, indicating higher temperature
as the fluid has recovered a portion of the stagnation temperature. The highly coupled process for ignition requires transport of high-temperature radicals from the detonation plume shear layer and adequate local equivalence ratio to propagate a flame into the cavity for incipient ignition. Figure 10 shows the frame-integrated intensity of OH PLIF and chemiluminescence for the unfueled, failed, and successful ignition events for the Q f = 68 slpm condition. The OH PLIF trace for the failed ignition event follows the unfueled trace with a minor secondary increase around 2 ms as a small amount of fuel is engaged by the detonation plume, but the cavity flame does not stabilize as indicated by the chemiluminescence intensity decreasing to zero. In successful ignition cases, the OH PLIF trace shows multiple increases and decreases of signal in the first 8 ms, indicating shedding of high-temperature radicals that engage the cavity mixture, generating bursts of additional OH. The OH PLIF intensity decreases as the detonation products exhaust from the
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Fig. 10. Integrated intensity of OH and chemiluminescence measurements for the unfueled cavity compared to failed and successful ignition with cavity fueling of 68 slpm.
Fig. 11. Integrated intensity of OH PLIF in the cavity step region (Ist ) as indicated in Fig. 3a for unfueled, Q f = 68 slpm, and Q f = 95 slpm fueling conditions.
tube and then increase steadily after 10 ms. The chemiluminescence intensity is much greater as the initial detonation emerges, and decreases to a minimum around 3 ms, followed by a steady increase from 3 to 8 ms. The discrepancies between decreases and increases in the OH and chemiluminescence signals are related to the exhaust of the detonation products and are discussed further in a subsequent section. To illustrate the shedding process, Fig. 11 shows the integrated OH PLIF signal intensity in the cavity step region Ist as indicated in Fig. 3a. The unfueled condition shows that a small portion of OH is present from the initial detonation plume shear layer. For failed ignition at Q f = 68 slpm, an increase in OH signal is observed, but it is insufficient to sustain combustion. For successful ignition at Q f = 68 slpm, multiple OH production events are observed as high-temperature radicals are transported into the cavity step region, eventually resulting in flame propagation and stabilization. The OH shedding events are most apparent for the Q f = 68 slpm condition as the fueling conditions are fluctuating around the lean ignition limit. For Q f = 95 slpm, smaller scale fluctuations in OH intensity are observed as more fuel is engaged during the incipient ignition event, however, the shedding process is still observed in the first 2 ms. For this condition, the OH signal decreases as the flame stabilizes away from the OH PLIF plane during the PD exhaust. 3.2. Detonation impulse and interaction Previously, Ombrello et al. [2] hypothesized that quenching of cavity burning could be due to backpressuring of fuel injectors in
the cavity. Static pressure measurements at fs = 3.9 kHz within the cavity and downstream of the aft ramp confirm this hypothesis. During the initial detonation event spanning from 0 < t < 2 ms, the overpressure from the plume penetrates the shear layer and drives fluid from the cavity, as observed from the schlieren image sequences in Fig. 12. The following mass injection and heat release in the cavity causes the bulk pressure within the cavity to increase, further driving fluid from the cavity and disrupting the recirculating cavity flowfield. This phenomenon is schematically illustrated in Fig. 13 and quantitatively shown by the pressure traces in the cavity (P1 ) and downstream of the ramp (P2 ) in Fig. 14. During the detonation impulse, the pressure increases approximately 10% within the cavity. The lifting of the shear layer from the high momentum detonation plume combined with the favorable pressure gradient drives fluid from the cavity. The pressure-driven acceleration of flow from the cavity is suggested by a decrease in pressure just aft of the cavity ramp (P2 ). This evacuation behavior was also noted in the computations of Ombrello et al. [2], where simulations showed the cavity integrated mass fraction of C2 H4 decreased to 20% of the initial value over a 2 ms period. For the 200 slpm fueling condition, the pressure rise is 50% greater, which is attributed to greater initial heat release in the cavity. The direct cavity fueled configuration is exposed to the increased cavity pressure from the detonation wave impulse as it expands into the cavity. Figure 15 shows the change in fuel pressure (Pd = Pf uel − P 1max ) normalized to the initial fuel pressure drop (Pf ) across the fuel injectors, at the peak of the detonation impulse, for each fueling condition. The detonation overpressure results in a 40% reduction in pressure drop across the fuel injectors for the 55 slpm condition with a decreasing effect as the fuel flow increases (with higher injector pressure drop). For the lower fuel conditions (55 and 70 slpm), the backpressuring of the fuel injectors temporarily reduces the fuel concentration in the cavity for 1–2 cavity cycle periods, which is long enough to inhibit cavity ignition. The combination of the cavity evacuation and the backpressuring of the fuel injectors is responsible for initial suppression in cavity burning observed in the measurements. These results demonstrate the sensitivity of cavity ignition to the impedance of the fueling system and integration method of the PD igniter. 3.3. Global cavity ignition The frame-integrated OH PLIF and chemiluminescence for five fueling conditions ranging from rich to lean are shown in Fig. 16. Comparison of the five conditions shows that ignition of the cavity can exhibit very different behavior with respect to incipient ignition, quenching, and re-establishment of the flame. For the three fueling conditions: 70, 75, and 95 slpm, the flame quenching time after initial ignition varies with the higher fueling conditions
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Fig. 12. Schlieren snapshots showing evacuation of the cavity during the detonation overpressure.
Fig. 13. Schematic of cavity flowfield during detonation overpressure and backpressure of fuel injection.
Fig. 14. Pressures in the cavity floor (P1 ) and downstream of the ramp (P2 ) during the detonation event depicting the pressure gradient driving flow out of the cavity.
resulting in more secondary burning from 2 to 10 ms. In the chemiluminescence signal for fueling conditions 75 and 95 slpm, there is always some level of heat release in the cavity throughout the ignition process. For the 70 slpm fueling rate, near the lean ig-
Fig. 15. Magnitude of fuel injector backpressuring from the detonation normalized to the pressure drop for each fueling condition.
nition limit, the combustion is almost entirely quenched (i.e. only a very low level of chemiluminescence is still detectable). For the 70 slpm fueling condition, the weak flame kernel propagates to fill the cavity at approximately 7 ms when the PD exhaust process is complete. This illustrates how the incipient ignition event seeds a flame kernel that must persist during the disruptive blow down
Fig. 16. Integrated intensity for ignition of 200-55 slpm fueling conditions. Dashed gray lines indicate image times from Fig. 17.
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Fig. 17. Full cavity ignition sequence for 200-55 slpm fueling conditions showing OH and chemiluminescence. Refer to Fig. 5 for scale bars.
of the PD exhaust in order to propagate and stabilize a flame in the cavity. For the highest fueling condition, 200 slpm, the OH PLIF signal indicates local quenching in the measurement plane, while the chemiluminescence signal indicates continuous burning at all times in the cavity. The OH signal does not increase again until approximately 12 ms, approximately 4 ms after the PD exhaust process is complete. This is due to bifurcation of the flame as shown in Fig. 17 as the flame is locally quenched around the PD exhaust plume. The 15–to 20 ms delay until the flame is re-established for the 200 slpm condition is due to the flame preference to anchor near the cavity ramp, requiring the PD exhaust process to complete before the flame can propagate and stabilize.
Instantaneous OH PLIF and chemiluminescence images for the full ignition sequence of all five fueling conditions are shown in Fig. 17. These simultaneous image sequences reinforce the observations in Fig. 16. The initial detonation pulse engages a large portion of the cavity fuel/air mixture as it expands within the cavity. For the 200, 95, and 75 slpm fueling rates, combustion spreads laterally in the cavity, while for the 70 and 55 slpm fueling rates combustion is quenched within the first few milliseconds. Sustaining a kernel of high temperature products supplied with enough reactants to persist through the PD exhaust period is necessary for combustion to re-establish. In the case of higher fueling rates, the flame bifurcates on either side of the detonation plane.
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For the intermediate conditions, the flame appears to anchor in the cavity step region. For the 70 slpm fueling rate, a very weak kernel persists in the cavity until the PD exhaust is complete and the quasi-steady cavity combustion is re-established. Of course, the fuel must continue to enter the cavity and mix in a region that is exposed to the stabilized reaction. For the 55 slpm condition, there is insufficient fuel concentration to sustain an ignition kernel, due to temporary backpressuring of the fuel injectors as described above, and combustion in the cavity is quenched. 4. Summary This work provided temporally and spatially resolved measurements of pulse detonation ignition of a scramjet cavity flameholder fueled with C2 H4 at a Mach 2 freestream condition. Simultaneous 40-kHz schlieren, chemiluminescence, and OH PLIF imaging provided unprecedented insight into the ignition mechanism of high-temperature detonation products coupled with a recirculating fuel/air mixture within the cavity. It was found that ignition of the cavity mixture was dependent on the shedding of high-temperature fluid through the exhaust plume shear layer laterally into the cavity mixture. For the configurations tested, a large quantity of energy was exhausted through the cavity shear layer into the unfueled supersonic freestream. The dynamics of the detonation plume interacting with the cavity shear layer resulted in shedding of high-temperature eddies into the cavity. Two mechanisms that inhibited cavity ignition were identified. The first was suppression of cavity fueling for 1–2 cavity cycle periods (2–3 ms) for low fueling rates that occurs due to backpressuring of the fuel injectors. This process is the reason for the higher lean ignition limit for pulse detonation ignition compared to spark ignition. The second mechanism that inhibited cavity ignition was local combustion quenching during and due to the PD exhaust process of the detonation products. This process bifurcated the flame and disrupted the cavity recirculation flowfield (up to 12 ms) to the extent that the heat release was not sustained beyond the exhaust period. The results presented show that a combination of appropriate fueling conditions with the integration method of the pulse detonation igniter are closely linked to achieving successful ignition in a supersonic cavity flameholder. When executed appropriately, pulse detonation can provide robust ignition across a broader range of fueling conditions than electrical discharge devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the support of the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base for the facilities and financial support for the research. The authors would also like to acknowledge the financial support of the National Research Council (NRC) Research Associateship Pro-
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