Prechamber Hot Jet Ignition of Ultra-Lean [H.sub.2]/Air Mixtures: Effect of Supersonic Jets and Combustion Instability.
One demand manufacturers of gas engines are facing is to meet ever-stringent legislation on emissions including oxides of nitrogen [1,2,3,4]. Ultra-lean operation of internal combustion engines can reduce NOx emissions and also improve their thermal efficiency . However, ignition of ultra-lean fuel/air mixtures has many challenges. For example, misfires can occur in the engine as a result of poor ignition. Such misfires and difficulties in ignition can lead to cycle-to-cycle variability, rough operation, reduction in efficiency, and increase in unburned hydrocarbon emissions - none of which are desirable [6-7].
An approach that can potentially solve this problem is to use a hot turbulent jet to ignite the lean mixture instead of a conventional electric spark [8-9]. The hot turbulent jet is generated by burning a small quantity of stoichiometric or near-stoichiometric fuel/air mixture in a separate small volume called the prechamber. The higher pressure resulting from prechamber combustion pushes the combustion products into the main chamber in the form of a hot turbulent jet, which then ignites the mixture in the main chamber. Compared to a conventional spark plug, the hot jet has a much larger surface area over which ignition can occur. Hot jet ignition has the potential to enable the combustion system to operate near the fuel's lean fammability limit, leading to ultra-low emissions.
Turbulent hot jet ignition, however, is a complicated phenomenon. There are very few computational and experimental studies on the fundamental mechanisms involving the complex coupling between turbulent mixing and chemical reactions. Several interrelated chemical and physical processes are involved. For example, the jet containing hot combustion products penetrates into the lean mixture, providing a high-temperature environment for mixing and ignition. It may also contain active radicals such as H, O and OH, which initiate chain-branching reactions. We can expect that the radicals which are important for ignition chemistry, the mixing process between the hot exhaust and the fresh lean mixtures, turbulence, and strain rate all affect the ignition process.
Among the very limited studies we could find in the literature [10,11,12,13,14,15,16], Chen et al.  is among the first to study turbulent hot jet ignition. It was found that the formation of the large-scale eddy structure of turbulent jets is triggered first by vortex pairs; these eddy structures dominate the external features of the jet.
Yamaguchi et al.  investigated the effect of nozzle diameter in a divided chamber bomb. Their results show that a smaller nozzle diameter resulted in "well-dispersed burning", the larger nozzle diameter resulted in "flame kernel torch ignition", and the largest nozzle diameter enabled laminar flame to pass through the nozzle. Sadanandan et al.  studied ignition of hydrogen/air mixtures by a hot jet using a high-speed laser Schlieren and OH-Planar Laser Induced Fluorescence (PLIF) techniques. They observed no OH radicals at the nozzle exit and speculated that possible heat loss through the nozzle walls had a strong influence.
Attard et al.  performed single cylinder experiment using turbulent jet for wide range of dilution while still maintaining adequate combustion stability. Toulson et al.  tested spark ignition combustion with natural gas and propane fuels across varying speed and load points at several air to fuel ratios varying from stoichiometric to lean in a single cylinder optical engine. Flame propagation of different fuels were studied at varying engine load conditions. There studies were limited mainly to measuring performance parameters in engine. They do not provide much insight about the physics of jet ignition. Gentz et al.  studied the effect of nozzle diameter and number of nozzles for combustion of a premixed propane/air mixture initiated by a turbulent jet ignition system using a Rapid Compression Machine. They studied the jet spread and jet Reynolds number effect on ignition. Shah et al. [22, 23] carried out CFD simulations to characterize hot jet in inert environment.
These limited studies have shown the great potential of the concept of hot jet ignition for extra-lean combustion. Our knowledge, however, is far from complete. Contradictory results exist. For example, there is disagreement on where ignition most likely occurs: Sadanandan et al.  observed ignition occurring near the tip of the jet, whereas Elhsnawi et al.  observed ignition occurring on the lateral sides of the jet. More importantly, no studies have examined the effect of nozzle geometry and the effect of supersonic jets on ignition characteristics. Furthermore, combustion instability in gas engines under ultra-lean operation conditions has not been extensively discussed.
Motivated by these, we developed an experiment that uses a dual chamber design (a small prechamber resided within the big main chamber) to understand the fundamental mechanisms of turbulent hot jet ignition. The primary focus was to find out the physics of jet ignition and governing time scales and length scales affecting ignition behavior. Both high-speed Schlieren photography and OH Chemiluminescence were applied to visualize the jet penetration and ignition processes in the main chamber. Infrared imaging and Schlieren PIV (SPIV) were used to characterize the thermal and velocity fields of the hot jet. The present paper focuses on the ignition mechanisms, the effect of supersonic jets on lean ignition limits, as well as combustion instability. The results show substantial decrease in the lean limit using supersonic jets. Moreover supersonic jets result in smaller ignition delay time. Supersonic jets create a high temperature zone at the downstream of shock structures. This region initiates ignition in main combustion chamber. Lastly, combustion instability becomes severe at the lean limit conditions. Instability modes are mainly focused at two frequencies, 180 Hz which is the natural frequency of the combustor and 2400Hz, longitudinal mode of flame propagation.
The experimental setup is schematically shown in Figure 1 (a) and (b). A small volume, 100cc cylindrical stainless steel (SS316) prechamber was attached to the rectangular (10" x 6" x 6") carbon steel (C-1144) main chamber. The main chamber to prechamber volume ratio was 100:1.
A stainless steel orifice plate with various nozzle designs (as shown in Figure 1 (c)) separated both chambers. Jet ignition characteristics of [H.sub.2]/air for six different nozzle designs (straight, convergent and convergent-divergent (C-D)) were studied. Nozzle dimensions are tabulated in Table 1 and schematically shown in Figure 1(c).
A thin, 0.001" thick aluminum diaphragm isolated both chambers with dissimilar equivalence ratios from mixing. A provision was made to heat up the fuel/air mixture in both chambers up to 600 K using built-in heating cartridges (Thermal Devices, FR-E4A30TD) inserted into the main chamber side and bottom walls. For the present paper, all tests were done at room temperature 300 K. The mixture in the prechamber was ignited by an electric spark created by a 0-40kV capacitor discharge ignition (CDI) system. An industrial grade double Iridium Bosch spark plug was attached at the top of the prechamber. The transient pressure histories of both chambers were recorded using high resolution ~ 5kHz Kulite (XTEL-190) pressure transducers combined with NI-9237 signal conditioning and pressure acquisition module via by LabVIEW software. Two K-type thermocouples were positioned at the top and bottom of the main chamber to ensure uniform temperature lengthwise thus avoiding natural convection or buoyancy effect. A 1" thick polymer insulation jacket was wrapped around the prechamber and main chamber to minimize heat loss. Fuel (industrial grade [H.sub.2]) and air were introduced separately to the main chamber using the partial pressure method. Unlike the main chamber where fuel and air mixed in the chamber itself, fuel/air in the prechamber was premixed in a small stainless steel mixing chamber (2.5 cm diameter, 10 cm long) prior going into prechamber.
After an electric spark ignites the fuel/air mixture in the prechamber, prechamber pressure starts to rise. Because the volume of the prechamber is very small and the initial flow field is quiescent, combustion in it is very likely to occur through the propagation of a laminar flame. Once prechamber pressure reaches the rupture pressure of aluminum diaphragm, the diaphragm bursts and the pressure difference between pre and main chamber results in a transient compressible jet with large density ratio with respect to the relatively cold fuel/air mixture in the main chamber. The jet further penetrates into the main chamber and could possibly ignite the mixture in the main chamber under favorable conditions. The jet properties, such as temperature, mean and fluctuating velocities are largely influenced by the prechamber combustion process, orifice geometry.
An accurate assessment of diaphragm rupture time is required in order to calculate precise ignition delay. Ignition delay is defined as the time required from the time of diaphragm rupture to the instant of main chamber ignition. A series of tests were conducted to find out when and at what conditions the thin aluminum diaphragm will rupture. A potential difference of 5V was applied using National Instrument DAQ module (NI-9263) to the aluminum diaphragm via two thin copper wires touching the diaphragm. As the diaphragm ruptured, copper wires lost contact with aluminum. As a result, voltage dropped sharply marking the event of rupture. The rupture time is defined as the time interval between the completion of the electric spark in the prechamber and the rupture of the diaphragm. It was found the rupture time for [H.sub.2]/air mixtures to be 2.6[+ or -]0.1 milliseconds, consistent for all test conditions.
High-Speed Schlieren and OH* Chemiluminescence Imaging
A customized trigger box synchronized with the CDI spark ignition system sent a master trigger to two high-speed cameras for simultaneous Schlieren and OH* chemiluminescence imaging. The main chamber was installed with four rectangular (5.5" x 3.5" x 0.75") quartz windows (type GE124) on its sides for optical access. High quality UV transparent (85% UV transmission at 240 nm) quartz windows were used. One pair of the windows was used for ztype Schlieren system. Z-type Schlieren system positions light source, mirrors, test section and camera in a "Z" shape as shown in Figure 1. Another pair was selected for simultaneous OH* chemiluminescence measurement. The high-speed Schlieren technique was used to visualize the evolution of the hot jet as well as the ignition process in the main chamber. The system consisted of a 100W (ARC HAS-150 HP) mercury lamp light source with a condensing lens, two concave parabolic mirrors (6" diameter, focal length 1.2 m), and a high-speed digital camera (Vision Research Phantom v7). Schlieren images were captured with a resolution of 800 x 720 pixels with a framing rate up to 12000 fps.
The simultaneous high-speed OH* chemiluminescence measurement provided a better view of the ignition and flame propagation processes. A high-speed camera (Vision Research Phantom v640) camera, along with video-scope gated image intensifier (VS4-1845HS) with 105 mm UV lens, were utilized to detect OH* signals at very narrow band 386 [+ or -] 10 nm detection limit. The intensifier was externally synced with the camera via high-speed relay and acquired images at the same frame rate (up to 12,000 fps) with the Phantom camera. A fixed intensifier setting (gain 65,000 and gate width 20 microseconds, aperture f8) was used all through.
Hot Wire Pyrometry (HWP) and Infrared (IR) Imaging
The Hot Wire Pyrometry (HWP) technique provides a time resolved temperature field along a line during jet propagation. Planar time-dependent radiation intensity measurements of the flame were acquired using an infrared camera (FLIR SC6100) with an InSb detector. The view angle of the camera was aligned perpendicular to the flame axis (50 cm from the burner center to the camera lens) such that the half view angle of the camera is less than 10 deg. The radiation intensity detected by each pixel of the camera focal plane array can be approximated by a parallel line-of-sight because of the small view angle. The spatial resolution is 0.2x0.2 [mm.sup.2] for each pixel. The band pass filter was used to measure the radiation intensity of [H.sub.2]O (2.58 [+ or -] 0.03 [micro]m).
Schlieren Particle Image Velocimetry (SPIV)
In Schlieren PIV (SPIV) method a turbulent flow field containing turbulent eddies serve as PIV particles. These self-seeded successive Schlieren images with short time delay, [DELTA]t can be correlated to find instantaneous velocity field information. Due to path integrated nature of Schlieren an inverse Abel transformation is required to find true velocity field. A z-type Herschellian high-speed Schlieren system was used for Schlieren PIV. The Schlieren system consisted of a 100 Watt mercury arc lamp (Q series, 60064-100MC-Q1, Newport Corporation, Model 6281) light source with a condensing lens assembly (Q Series, F/1, Fused Silica, Collimated, 200-2500 nm), two concave parabolic mirrors (6" diameter, aperture f/8, effective focal length 1219.2 mm), a knife-edge, an achromatic lens (f = 300 mm) to collimate the light, a beam splitter (1" cube, Thorlabs PBS251) and two identical high speed CCD cameras (v711, Vision Research Phantom). Utilization of two high speed cameras lie in precise controlling of the inter-frame delay, [DELTA]t. A small [DELTA]t is essential in order to resolve high exit jet velocity, [U.sub.0].
RESULTS AND DISCUSSIONS
Results and discussions are divided into following subsections. At first, lean limit, ignition delay for different type of nozzles are discussed, followed by jet ignition mechanism using simultaneous Schlieren and OH* chemiluminescence imaging. Next supersonic jet characteristics, shock structures at jet exit and qualitative temperature field measurements are presented using simultaneous Schlieren and IR imaging. Radiation intensity field from infrared diagnostics and temperature profile near jet exit was measured using Hot Wire Pirometry (HWP). Lastly combustion instability characteristics near lean limit are reported.
Effect of Nozzle Geometry on Lean Limit and Ignition Delay
One of the main goals was to understand the effect of nozzle geometry on the lean ignition limit in the main chamber. Six different nozzles were tested; their dimensions are summarized in Table 1. The lean limit for each nozzle was found by gradually reducing the fuel-equivalence ratio of the main chamber until ignition cannot occur. Note the fuel/air equivalence ratio of the prechamber mixture was fixed to 1 for all cases, whereas the fuel/air equivalence ratio of the main chamber mixture was varied. Figure 2 and Table 2 show the lean limit ([[phi].sub.limit]) of the main chamber mixture for six nozzles. As can be seen, [[phi].sub.limit] extends for supersonic nozzles compared to its straight counterpart. Out of the four supersonic nozzles we tested, nozzle 4 and nozzle 5 showed lowest lean limit, [[phi].sub.limit] = 0.22 and 0.23 respectively.
Supersonic nozzles also show a lower ignition delay time, [[tau].sub.ignition] compared to its straight counterpart. The ignition delay, [[tau].sub.ignition] as a function of the equivalence ratio, [phi] is plotted in Figure 3. As mentioned earlier, ignition delay is the time required for the initiation of main chamber ignition from the diaphragm rupture. Ignition delay is smaller for supersonic nozzles 4 and 5 compared to straight nozzles 1 and 2. Figures 4 and 5 show the ignition processes for C-D nozzles with area ratio 4 and 9 respectively. [[phi].sub.limit] for these two nozzles are 0.22 and 0.23 respectively, and are the smallest compared to other nozzles.
The high-speed Schlieren technique enables visualization of the jet penetration process, as well as the ignition and subsequent turbulent flame propagation processes in the main chamber. High-speed OH* chemiluminescence helps to identify the flame front location and the ignition mechanism (whether the hot jet coming out from the prechamber is a jet of hot combustion products or a jet of flames). For all ultra-lean cases, ignition occurred via jet ignition mechanism (a jet consists of hot combustion products only, which ignited the ultra-lean mixture in the main chamber). In our previous studies with main chamber equivalence ratio near stoichiometric to stoichiometric ([phi] = 0.75 - 1) we observed ignition occurred via hot flame jet. Unlike a hot jet of combustion products, a flame jet produces a jet full of wrinkled turbulent flames and active radicals. Initial pressure, temperature, equivalence ratio along with geometric factors like prechamber volume, orifice diameter, spark position affect ignition behavior and regulates ignition mechanisms. At lower equivalence ratio and lower pressure hot jet ignition is predominant. A higher initial temperature leads to flame jet ignition.
Figures 4 to 9 show the time sequence of simultaneous Schlieren (top) and OH* Chemiluminescence (bottom) of the ignition process for different nozzles at the lean-limit condition. Figures 4 and 5 represent jet ignition by straight nozzles of 1.5 and 3 mm diameters respectively. Initially shock structures are visible for a little while, but these structures diminish quickly and ignition starts afterwards at the absence of any shock structures. As for convergent and C-D nozzles, exit jet contains straight/diamond shock structures as seen from Figures 6 to 9 and ignition starts at the presence of shock structures. With increase in area ratio, [A.sub.e]/[A.sub.t] jet width at exit increases as well. It is speculated that the shock structures in the supersonic jets increase static temperature behind the shocks, which may increase the ignition probability of the main-chamber mixture and thus reduce the lean limit. This is depicted by IR imaging which will be discussed in the following section.
Figure 10 shows simultaneous planar time-dependent radiation intensity measurements and high-speed Schlieren imaging to capture shock structure of turbulent hot jets. These images represent the time-instance just before the ignition occurs in main chamber. The main chamber was filled with only air (non-reactive) to characterize the hot jet. Qualitative understanding of the temperature field can be obtained from the infrared images. Perhaps the most significant feature of these infrared images is the presence of shock structures for converging and C-D nozzles at the instance of main chamber ignition. Unlike convergent or C-D nozzles, straight nozzles do not show any shock structures just before ignition. Besides shock structures a high temperature zone at the downstream of stock structure was observed for convergent and C-D nozzles. This is due to the fact that static temperature rises after each shock and creates a high temperature zone. The location and width of the zone varies for different nozzles. But the much interesting fact lies in the ignition pattern of these nozzles. For all these nozzles ignition starts from this high temperature zones.
The jet temperature at a location that is 4 mm downstream of the nozzle exit was measured using Hot Wire Pyrometry (HWP) technique. The results are shown in Figure 11 (a) and (b). Nozzles 4 and 5 exhibit higher temperatures at the centerline than the other nozzles. Figure 12 reveals the radiation intensity along the jet centerline in stream wise direction. For straight nozzles (nozzle 1 and 2), the radiation intensity drops in a monotonic fashion, indicating the temperature of the jet keep decreasing as a result of mixing between the hot jet and the cold ambient mixture. However, for nozzles 3, 4 and 5, the measured radiation intensity first fluctuates near the nozzle exit due to the presence of shocks, for which the static temperature increases downstream of the shock. It then increases rapidly at a location further downstream, indicating establishment of a higher temperature zone at that location. Resulted ignition of the main chamber lean mixture was observed to take place at this high temperature zone for nozzles 3, 4 and 5. In other words, this high temperature zone downstream of the nozzle exit is responsible for reducing the lean limit of the main chamber mixture by using a supersonic nozzle.
Combustion Instability in Main Chamber
Unstable flame propagation due to strong presence of combustion instability was observed at ultra-lean conditions, [phi] ranging from 0.22 to 0.4. This, however, was not observed for near-stoichiometric main-chamber mixtures in our previous study. The instability becomes more and more severe as the fuel/air equivalence ratio of the main chamber mixture reduces at lean-burn conditions. Oscillating flames were observed from both high-speed Schlieren imaging as well as oscillating pressure data. Reduction in stability of flame propagation limit is a major setback that not only affects performance via unstable combustion dynamics but also affect the structural integrity of the engine. Controlling such instability, active or passive, requires adequate knowledge about the fundamental mechanism such as different types of instability modes, perturbation energy, and frequencies.
Figure 13 shows the pressure histories in the prechamber and the main chamber for C-D nozzle 4 at lean limit, [phi] = 0.22. At the onset of spark prechamber pressure rises due to combustion of the stoichiometric mixture in the prechamber. Increased pressure in the prechamber creates a transient pressure difference, [DELTA]P(t) responsible for driving the hot combustion products through the orifice as a turbulent jet. Once this hot jet ignites the main chamber, main chamber pressure increases and eventually may lead to oscillation. Combustion instability was found only at lean-burn conditions and was independent of nozzle type, geometry or jet characteristics such as jet velocity, fluctuation etc. Main chamber fuel/air equivalence ratio is the only factor instability depends on.
Essentially an acoustic disturbance grows in time and affects the chamber pressure which in turn affects the heat release rate and flame propagation speed [20-21]. The pressure wave generated from combustion reflects from the combustor wall and interacts with the flame front. A vigorously burning flame has a much higher flame speed and stability that prevents it to get influenced from reflected pressure waves. With decrease in fuel/air ratio, at lean limit flame speed decreases significantly. This allows the reflected pressure waves longer time to interact with different modes of combustion process. Fast Fourier Transform (FFT) of the pressure data reveals two instable frequency modes, 180 Hz and 2380 Hz respectively, as shown in Figure 14. Modal analysis of the structure alone shows its natural frequency at 180 Hz and higher order (seventh) mode near 2400 Hz. This higher order mode is associated with vibrations of the structure in longitudinal direction which is also the direction of flame propagation. This behavior could well be attributed to the coupling between weak heat release rate at lean limit ([phi] ~0.22 - 0.4) and strong acoustic modes of the combustors that produces instable frequency near 2400 Hz.
Lastly, high-speed Schlieren images of flame propagation and corresponding velocity fields are plotted in Figure 15 for three different phases (a), (b) and (c) of pressure oscillation cycle. Phase (a) represents the crest (highest point) and phase (c) represents trough (lowest point) of the wave, while phase (b) denotes the equilibrium point. It has been seen from simultaneous Schlieren and OH* Chemiluminescence imaging that oscillating heat release is in phase with pressure perturbation, which satisfes the Rayleigh criteria. Rayleigh criteria can be expressed as,
[mathematical expression not reproducible] (3)
where T is the period of oscillation, p'(t) is the fluctuation in pressure, q'(t) is the fluctuation in heat release rate and R is the Rayleigh's index. A positive Rayleigh's index indicates an amplification of the pressure oscillation due to the fluctuating heat release rate while a negative Rayleigh's index denotes a dampening of the pressure oscillations.
The velocity field in Figure 15 obtained from Schlieren PIV (bottom) shows how the direction of flame propagation changes in one pressure perturbation cycle. Flame moves inward at phase (a) and outward in phase (c). Red arrows denote the global movement of the flame surface. This oscillating nature of flame propagation changes flame dynamics and burning rate, as well as the structural life of the combustor.
The present paper investigated the ignition characteristics of ultra-lean [H.sub.2]/air mixtures using a supersonic hot jet generated by prechamber combustion. High-speed Schlieren, OH* chemiluminescence and IR imaging were applied to visualize the jet penetration and ignition processes. Schlieren PIV was used to measure the velocities and fluctuations of the transient jet. A parametric study was performed comparing different supersonic and straight nozzles. Flame instability at the lean-burn limits was discussed.
A vital finding is the extension of lean limit, [[phi].sub.limit] and lower ignition delay of the lean [H.sub.2]/Air mixture in the main chamber by using a supersonic nozzle than a straight nozzle. Ignition in the main chamber was achieved for [[phi].sub.limit] = 0.22 using a supersonic nozzle. Simultaneous Schlieren and OH* Chemiluminescence results show ignition initiates from the side surface of the hot jet. Due to the presence of shock structures at the exit of supersonic jet, supersonic jet exit temperature is higher than straight nozzles. Increase in the static temperature behind the shocks thus escalates ignition probability, which is the main reason that the lean limit can be further reduced. Moreover, converging and C-D nozzles create a high temperature zone downstream of shocks responsible for initiation of ignition. This is a key piece of information. This could help us better control the ignition location and ignition delays and design a better prechamber for lean combustion.
However, combustion instability arises at ultra-lean conditions. An acoustic disturbance grows in time and affects the chamber pressure which in turn affects the heat release rate and flame propagation speed. This creates an oscillating flame propagation. Combustion instability occurred at two different frequencies, 180 Hz is the natural frequency mode of the combustor and 2400 Hz is the longitudinal flame propagation mode. Flame oscillations along longitudinal direction were measured using Schlieren PIV over a single pressure oscillation cycle. Further detailed investigation of combustion instability will be conducted in future. Currently hydrogen was chosen for this study for its simple chemistry. In the future fuels more relevant to natural gas engines will be studied.
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Sayan Biswas and Li Qiao
Associate Professor, School of Aeronautics and Astronautics
Engineering, Purdue University
Table 1. Dimensions of nozzles used in turbulent hot jet ignition experiment. Nozzle # Type [d.sub.inlet] (mm) [d.sub.throat](mm) 1 Straight 1.5 1.5 2 Straight 3 3 3 Convergent 3 - 4 C-D 3 1.5 5 C-D 3 1.5 6 C-D 3 1.5 Nozzle # [d.sub.exit](mm) [A.sub.e]/[A.sub.t] 1 1.5 2 3 - 3 1.5 - 4 3 1 5 4.5 9 6 7.5 16 Table 2. Lean limit, [phi]limit of different nozzles. Nozzle # [[phi].sub.limit] 1 0.34 2 0.31 3 0.29 4 0.22 5 0.23 6 0.29
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|Author:||Biswas, Sayan; Qiao, Li|
|Publication:||SAE International Journal of Engines|
|Date:||Sep 1, 2016|
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