Experimental Study of Traversing Hot-Jet Ignition of Lean Hydrocarbon-Air Mixtures in a Constant-Volume Combustor

Transcription

1 Paper # 070IC-0320 Topic: Internal Combustion and Gas Turbine Engines 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Experimental Study of Traversing Hot-Jet Ignition of Lean Hydrocarbon-Air Mixtures in a Constant-Volume Combustor Prasanna Chinnathambi Abdullah Karimi Manikanda Rajagopal Razi Nalim Mechanical Engineering, Indiana University-Purdue University, Indianapolis, IN 46202, USA A constant-volume combustor is used to investigate ignition by a traversing jet of reactive hot gas, in support of combustion engine applications that include novel constant-volume wave rotor, combustion gas turbines and pre-chamber IC engines. The combustor rig consists of two combustion chambers: a rotating pre-chamber that issues a traversing hot jet, and a stationary main chamber with long aspect ratio similar to a wave rotor combustor. In the pre-chamber, a fuel-rich mixture is ignited using an electric spark, generating high pressure that results in breaking a diaphragm and discharge of a hot gas jet through a nozzle. The traversing nozzle sweeps the jet of hot gases across one end of the main chamber containing fuel-lean mixture at room temperature and pressure, with complex jet penetration, entrainment, and vortex dynamics. Two different jet traverse speeds were used, with traverse time fixed at 40.6 ms and 8.1 ms, which correspond to pre-chamber rotation rate of 150 rpm and 750 rpm, respectively. The overall equivalence ratio of the main chamber mixture was maintained in the lean range at equivalence ratios of 0.4, 0.6 and 0.8. The pre-chamber uses rich ethyleneair mixtures with equivalence ratio of 1.1 for all the test cases. Ignition delay is measured by high-speed videography and image processing. Ignition delay time for methane-air mixtures is typically a few milliseconds, while ethylene-air mixtures exhibited less than a millisecond delay time. Blended methane-hydrogen fuel exhibited shorter ignition delay than methane, as well as faster apparent flame propagation speed. Supporting computational studies suggest that both chemical kinetics and mixing between the transient jet and the combustible mixture affect ignition delay. 1. Introduction A jet of hot reactive gas employed as an ignition source finds application in internal combustion engines implementing lean burn technology (Toulson et al., 2010, Attard et al., 2012, Dober and Watson, 1999), pulsed detonation engines (Lieberman et al., 2002) and wave rotor combustors (Wijeyakulasuriya et al., 2010, Matsutomi et al., 2010, Perera, 2010). Chemically active radicals and fast turbulent mixing in the jet create an explosion that is more energetic than a spark (Dober and Watson, 1999), allowing rapid ignition of lean mixtures. Further, the penetrating and distributed nature of ignition can overcome mixture non-uniformity and accelerate combustion. By enabling lean stratified mixtures, peak gas temperature is constrained, heat losses to the walls are reduced, and pollutant emissions can be mitigated. Such ignition is of particular interest for wave rotor combustors, for which a similar constant-volume combustor (CVC) experiment has been previously reported (Perera, 2010, Bilgin et al., 1998). In this CVC, the premixed lean mixture is ignited using a hot jet produced by combustion of fuel-rich mixture ignited using a spark in a prechamber. A combustible mixture can be ignited by an inert gas jet or reactive gas from another combustion source. Hot-jet ignition involves complex flow phenomena such as vortex evolution, jet mixing, and turbulence generation. Much of the classical literature on jet ignition was concerned with avoiding ignition in mines, and typical experiments used inert hot jets, at low velocities into quiescent mixtures. Wolfhard (1958) observed that nitrogen and carbon dioxide have similar minimum jet temperature for ignition, while argon and helium require higher jet temperature. Fink and Vanpée (1975) developed an overall rate expression for ignition of methane- and ethane-air mixtures by low-velocity hot inert gas jets. Cato and Kuchta (1966) experimented with laminar hot air jets and identified jet base temperature, jet dimensions, composition of the combustible mixture, and jet velocity, as ignition determining factors. Vanpée and Wolfhard (1959) found correlations between the ignition temperature of the jet (below which ignition fails) and the limit flame temperature of the successful flame, for several fuels. The presence of reactive species in the jet influences ignition kinetics. In the present study, the composition of the chemically active jet is kept fixed for all cases, using ethylene and

2 air of equivalence ratio (ϕ) 1.1 for the pre-chamber mixture. A high-speed compressible transient jet in a confined volume is usually accompanied by shock wave formation due to both jet initiation and combustion pressure rise, leading to subsequent interactions that change the flame shapes. The ignition delay time for a jet-ignited CVC may be defined as the time from jet initiation to the occurrence of rapid, visible, and pressure-generating heat release (Perera, 2010). This includes time for physical and chemical processes, unlike purely chemical ignition delay time reported in shock tube and rapid compression studies (Davidson and Hanson, 2004). Bilgin (1998) developed a CVC with long aspect ratio and square cross-section, representing a wave rotor channel. The CVC chamber mixture is ignited by a jet of hot combustion products from a separately fueled pre-chamber that could be spun to cause the jet to traverse across one end of the CVC (Bilgin et al., 1998). The relative motion reproduces the action of a channel in a wave-rotor combustor and the pre-chamber may be representative of a previously combusted channel supplying hot gas. Bilgin proposed a correlation between the Damköhler number and ignition of a fuel-air mixture in the CVC. Perera (2010) carried out experiments on the same CVC test rig for three different fuels methane, ethylene, and propane by varying the equivalence ratios in the pre-chamber and the CVC chamber. The pre-chamber was set stationary and centered on the channel cross-section in these tests. The ignition delay time and the ignitability limits for both lean and rich mixtures were investigated for all the three fuels in the CVC chamber, for fixed operating conditions. Variation in ignition delay time was observed for fuels with different pre-chamber equivalence ratios and nozzle geometry. Expectedly, methane exhibited the highest ignition delay time while ethylene mixtures had the lowest ignition delay time. The dependence of ignition delay time on CVC equivalence ratio was interesting and as yet not fully explained. The laminar flame speed of methane (major constituent in natural gas) is lower than that of the other hydrocarbon fuels due it its higher activation energy and this effect is most significant under lean conditions (Ren et al., 2001). The combustion characteristics of lean methane mixtures can be enhanced by blending with a highly reactive gas such as hydrogen (Dagaut and Nicolle, 2005). At high temperature, the concentration of H radicals due to presence of H 2 induces a fast initiation reaction: H + CH 4 CH 3 + H 2 (Ju and T, 1995 ). Maxson et al. (1991) investigated a pulsed jet combustion (PJC) system by introducing hot jets as a single stream or multiple streams into a quiescent mixture by evaluating its ability to reliably ignite and sustain rapid combustion in otherwise slow-burning lean mixtures. Attard et al. (2010) reviewed turbulent jet ignition systems for pre-chamber sparkignition engines. The pre-chamber mixture is well controlled and reliably spark-ignited, producing a reactive hot jet that acts as a distributed ignition source for the main CVC mixture. Boretti and Watson (2009 ) studied jet ignition in conjunction with direct injection to ignite the main chamber stratified mixtures. The pre-chamber is either spark ignited or autoignited by contact with a hot glow plug. Efficient combustion was achieved for different fuel mixtures ranging from near stoichiometric to extremely lean. Tarzhanov et al. (2006) investigated hot detonation products to detonate stagnant propane-air mixtures and found that detonation initiation depends on the initial volume concentrations, mass fraction of hot detonation products, and the energy deposited from the detonation products. Mayinger et al. (1999) derived correlations between the induction time (ignition delay time), the mixing time of the jet, and the adiabatic auto ignition time for the fuel-air mixtures. The main objective of the current study is to understand the process of ignition and behavior of the traversing hot jet in a confined volume containing lean fuel-air mixture. The combustion chamber employs an optically accessible rectangular window with high-speed imaging to capture luminosity emitted in the wavelength range of nm during the combustion process. The videography sheds light on the fluid dynamic behavior of the jet, its penetration and ignition zones as the jet traverses through and across the quiescent mixture. Two traverse rates are used, m/s and 4.91 m/s, corresponding to traverse times of 40.6 and 8.1 ms, and rotation rates of 150 rpm and 750 rpm. The study further reports the lean ignitability and ignition delay times for three gaseous fuels; methane, hydrogen-enriched methane, and ethylene. The effect of slow and moderate jet traverse speed was compared for different fuel mixtures, while keeping the nozzle geometry and hot-jet composition fixed. 2. Experiment Design The experiment to examine the stationary and traversing hot jet ignition processes in a long rectangular constant-volume combustor (CVC) used a rig with rotating and structural components inherited from the work of Bilgin (1998) and Perera 2

3 (2010). The rig includes the main CVC chamber and a pre-chamber (Fig. 1). The CVC is a stationary rectangular channel of square cross-section with side dimensions of 39.9 mm (1.57 inches) and 406 mm (16.0 inches) long. The pre-chamber is a cylindrical chamber with an internal volume of m 3 (51 cubic inches), mounted on a shaft that can be rotated by an electric motor via a belt drive. A nozzle delivers hot gas from the pre-chamber to one end of the main chamber. The mixtures in the respective chambers are isolated during filling by an aluminum diaphragm of thickness mm (0.003 inches), which is ruptured during jet delivery (Fig. 1). The diaphragm is scored to facilitate and precisely control rupture. The main chamber is placed flush against the pre-chamber before the start of each experiment with the help of a sealing ring assembly provided near the entrance of the main chamber (Fig. 1). Details of the setup and preparation for a stationary jet experiment are explained by Perera (2010). Fueling of the chambers with the desired amount of fuel-air mixture was performed using the partial pressure method and the fueling system described by Perera (2010). The main chamber and pre-chamber are each filled with quiescent fuel-air mixtures at room temperature and atmospheric pressure. For the current study ethylene of 99.5% purity supplied by Praxair was used to produce ethylene-air mixtures for fuelling the pre-chamber. A conventional spark-ignition system is employed in the pre-chamber to develop the combustion torch jet. A capacitive discharge ignition (MSD 6AL) system is used in conjunction with an ignition coil (MSD Blaster 2) to produce the high voltage and current required for the spark plug. The pressure rise in the prechamber causes rupture of the scored aluminum diaphragm delivering the jet through the convergent nozzle having exit diameter of 6.0 mm. The hot jet enters the main chamber at one end, traversing across the open end from the upper wall to the lower wall. The main chamber is equipped with dynamic pressure transducers and also provides visual access through optical windows comprising the two long sides of the main chamber. For structural reasons the leading edge of the main chamber window is located 47.0 mm (1.85 inches) away from the entrance. Videographic data recorded using Vision Research Phantom v9.0 high-speed camera was used to study the transient jet and complex ignition and combustion phenomena in the main chamber. Operating at 10,000 frames per second the camera sensor is exposed for a duration of μs every 100 μs. The reason for using a high exposure time was due to the importance of exposing the sensor to the slightest illumination possible. (a) Hot-jet ignition rig (b) Pre-chamber (c) Nozzle insert and Diaphragm assembly Figure 1: Major components of hot-jet ignition experimental rig 3. Measurements and Discussion Diaphragm Rupture Time Analysis In order to ensure that the jet flow is initiated immediately before or near the start of traverse of the CVC, it is necessary to trigger the pre-chamber spark at a precisely known instant during a cycle of rotation, and to know the precise duration from spark ignition to diaphragm rupture in the pre-chamber. This diaphragm rupture time is important in this study as it 3

4 defines incipient jet injection, and is the start of the ignition delay period. Diaphragm rupture time reported by Perera ( 2010) for stationary pre-chamber was re-evaluated with ethylene mixture at ϕ = 1.1 while the pre-chamber was spinning at 150 rpm. The camera was placed at a safe distance of about 5 m in front of the rotating pre-chamber, and the images were recorded at a resolution that encompasses the entire pre-chamber within the frame. Perpendicular laser targets were used to identify the axis of the pre-chamber, as the experiments were performed in dark conditions. The video was captured at the frame rate of 6400 frames per second (limited by the set resolution) with the lens aperture set to an f-number of 1:1.8 and the exposure time of 153 μs. The diaphragm rupture time is defined as the elapsed time between the trigger pulse that corresponds to initiation of spark in the pre-chamber and the first appearance of luminous zone in the captured images. A series of tests were performed and a mean value of the diaphragm rupture time was found as 15.4 ms with a standard deviation of ±0.3ms. This corresponds to 13.2 degrees of rotation at 150 rpm, and 69.1 degrees of rotation at 750 rpm. Detection of Ignition and Ignition Delay Time Ignition of a combustible mixture has been defined in various ways by different researchers. Radiation of visible light was observed as ignition in other hot jet ignition research conducted by Wolfhard (1958), Vanpée and Wolfhard (1959), Fink and Vanpée (1975) and Cato and Kuchta (1966). Among various experiments, the definition of ignition delay time varies as much as the definition of ignition. Some of the variations are due to the measuring equipment and methods used, and some due to the physical variations of the experimental facility. In shock-ignition experiments the ignition time is measured from the instant when the shock wave reflects at the closed end of the driven section to the instant when combustible mixture appears to ignite, with shock arrival measured by pressure transducers (Baker and Skinner, 1972, Tarzhanov et al., 2006) and ignition determined using photomultipliers (Baker and Skinner, 1972, Davidson and Hanson, 2004) or the emission of specific species (Davidson and Hanson, 2004, Tarzhanov et al., 2006). Radiation from combustion products including the species and radicals from oxidation of methane, ethylene, and propane in shock tube experiments is emitted in the wavelength range of nm. Soot radiation is emitted in the visible and infrared wavelength ranges (Grosshandler, 1993, Baukal, 2000). For the current study ignition delay time is defined as elapsed time between the diaphragm rupture event and the detection of luminous region having intensity above a specified threshold limit in main chamber (Perera, 2010). The high-speed camera captures luminosity emitted in the wavelength range of nm which corresponds to emissions by soot radiation during the combustion process. Image Processing Video images of the jet in the main CVC chamber taken through a side window were captured at a resolution of at frame rate of 10,000 per second (time interval between frames is 100 μs) with the lens aperture set to an f-number of 1:1.8 and the exposure time of μs and EDR (Extreme Dynamic Range TM ) exposure of 0 μs. EDR can be used to adjust exposure on pixel level to reduce over-exposure at certain regions due to bright spots. Setting EDR exposure time to 0 μs, the exposure level at the time of triggering the camera remains unchanged for the entire series of captured images. An image processing code was written and applied using the Sobel edge-detection technique (Parker, 1997) to provide optimal edge detection for the current study. The high-speed images from each test were analyzed frame-byframe for luminosity value at each pixel of the frame. The pixel values of the 8-bit monochrome camera output vary from 0 to 255. The threshold value for identification of ignition was set at 50% of the full-scale signal. The threshold value was chosen due to increased chemical activity for traversing cases near the nozzle region. The pixel luminosity value, number of pixels above the threshold per frame, and the location of pixels above the threshold were tracked on each frame. The number of pixels and growth of the pixel area is tracked for subsequent frames to detect ignition. Two cases of jet ignition with significantly different jet structure and behavior will be presented first to illustrate the range of ignition activity. The original grey scale images as captured using the high-speed camera and binary images processed using the code for ignition detection is compared for each case. In Fig. 2 images are presented for methane mixture at ϕ = 0.6 and traverse time of 40.6 ms. Ignition zones were identified at three different locations visible in the 4.8 ms frame of the processed image. It is observed that these ignition zones continue to grow rapidly over the next 0.3 ms. This distributed source of ignition at multiple regions in the main chamber is an important characteristic of jet ignition and has implications for the rate of combustion (Attard et al., 2012). A second set of images corresponding to ethylene mixture at ϕ = 0.4 and a traverse time of 40.6 ms is presented in Fig. 3. the maximum luminosity observed in the images is lower than that of methane mixture with ϕ = 0.6 (Fig. 2). The existence of ignition regions above the threshold limit can be better discerned in the processed images compared to the original images. As the ignition delay time is significantly different for the two fuels, the images have been presented over different time scales. The default criterion for successful ignition was set at 50% threshold limit in this study but the outcome of successful ignition was 4

5 not changed even though the threshold level was increased to 80%. However, ignition delay time measurements were made using a 10% threshold as chosen by (Perera, 2010) to enable comparison with other stationary pre-chamber cases. Figure 4 shows high speed video images that are processed for edge detection indicating the boundary of the reaction zone. Edge detection is required to precisely define the boundary in the original images and can be further used to extract information on flame structures and apparent flame velocity from the captured images. Original Images (Grey Scale) Processed Images (Binary) Figure 2: Identification of ignition zones and subsequent growth (fuel: methane, = 0.6, hot-jet traverse time 40.6 ms). Ignition zones are circled at t=4.8ms Original Images (Grey Scale) Processed Images (Binary) Figure 3: Identification of ignition zones and subsequent growth (fuel: ethylene, = 0.4, hot-jet traverse time 40.6 ms) 5

6 Original Images (Grey Scale) Processed Images (Binary) Figure 4: Flame edge detection by image processing for hydrogen-enriched methane mixture, = 0.8 for pre-chamber traverse time of 40.6 ms Effect of Jet Traverse Speed on Ignition The objective of the current study was to identify ignition locations and to examine the effect of hot-jet traverse speeds on the ignition delay time for lean fuel-air mixtures in the CVC. The ignition delay in the CVC chamber can be affected by several parameters. The mass injected into the CVC chamber before the ignition would be lower for low jet traverse time. This could affect the ignition delay in the CVC chamber. However, the jet traverse speed time for the tests discussed in this work is much higher than the ignition delay time in the CVC chamber and hence the difference in injected mass would not have any significant effect for speeds considered. The jet penetration and the fluid dynamics of mixing are different for different initial jet locations. For low traverse speed (40.5ms traverse time), the jet remains attached to the wall of the CVC chamber and later impinges on the lower wall producing vortices. A range of ignition delay times between 0.6 and 4.8 ms was observed. Methane-air mixtures exhibited the highest ignition delay time while ethylene mixtures had the lowest ignition delay time. Hydrogen-enriched methane mixtures shows ignition delay times that falls between the other two fuels. For mixtures with ϕ = 0.4, ignition onset was identified close to the nozzle location irrespective of the type of fuel accompanied with longer ignition delay times. The ignition delay time obtained from the experimental study is listed in Table 1 for various fuels. The data of Perera (2010) is included for convenience, but may not be directly compared due to differences in mass injection rates and injection conditions. It is generally observed that faster jet traverse (in 8.1 ms) results in more rapid ignition than the slower jet traverse (40.6 ms). The ignition jet images for ethylene mixture (ϕ = 0.8) for the two traverse speeds are presented in Fig. 5. It is observed that for fixed equivalence ratio the location of the ignition zones for the two cases depends upon the jet penetration at different traverse speeds. A similar trend has been observed for all the fuels considered in the study. Due to the extended ignition delay time for ϕ = 0.4 and increased exothermic activity along the 6

7 jet for ethylene air mixtures the images in Fig. 6 reveals the nature of jet penetration as it traverses through the main chamber at different speeds. It can be observed that 40.6 ms, the jet barely moves from its initial location and behaves much like a stationary offset jet, which initially travels along the wall and later impinges on the bottom wall. At a fixed time (e.g., t = 0.5 ms), the jet traversing at high speed penetrates twice as far as the jet traversing at low speed. Fuel Equivalence Ratio, Table 1: Measured Ignition Delay Time Slower traverse (40.6 ms) Ignition Delay Time (ms) Faster traverse (8.1 ms) Stationary (Perera 2010) C 2 H 4 CH 4 H 2 -CH Figure 5: Ignition for ethylene, =0.8 mixtures, for hot-jet traverse time of 40.6 ms (left) and 8.1 ms (right). Red border indicates frame of ignition detection. The ignition zones for pure methane and hydrogen-enriched methane have been compared for mixture ratio of ϕ = 0.8 and jet traversing in 40.6 ms in Fig. 7. It is noticed from figure that for both the fuels, the ignition zone tends to move towards the bottom wall of the CVC chamber, where ignition occurs. For the case of methane mixture with ϕ = 0.6 and traverse duration of 40.6, ignition region is located at the center-axis of the chamber (Fig. 2). Fig. 7 also indicates that the flame propagation is faster in the hydrogen-enriched methane mixture as compared to pure methane-air combustion. The results corresponding to pure methane and hydrogen-enriched methane mixture (ϕ = 0.8 and spin rate at 750 rpm) are 7

8 presented in Fig. 8. It is observed that the location of the ignition zone was consistent for pure methane and hydrogen blended methane mixtures for fixed equivalence ratio and traverse speed, but the ignition delay time was lower Figure 6: Traversing jet penetration before ignition for ethylene =0.4 for hot-jet traverse time of 40.6 ms (left) and 8.1 ms (right) Figure 7: Traversing jet ignition of methane =0.8 (left) and hydrogen-enriched methane =0.8 (right) for hot-jet traverse time of 40.6ms 8

9 Figure 8: Comparison of ignition and subsequent flame propagation for methane =0.8 (left) and hydrogen-enriched methane =0.8 (right) for hot-jet traverse time of 8.1 ms Shock-Flame Interaction In the confined volume of the CVC, the injection of the hot jet and rapid combustion gives rise pressure waves. The resulting shock wave reflects at the opposite end of the chamber and returns to interact with the incipient flame. The process of shock interaction with a propagating premixed flame is encountered in various physical processes ranging from deflagration to detonation transition to supernovas. The understanding of shock-flame interaction is essential for promoting faster combustion reactions in novel combustion devices such as steady or pulsed detonation engines (Kailasanath, 2003, Akbari and Nalim, 2006) or wave rotor combustors (Nalim and Pekkan, 2003, Nalim, 1999). In wave-rotor combustor the flame is expected to be within the corrugated flame regime (Warnatz et al., 1996), where the major effect is produced by the flame area increase following the shock-flame interaction. Shock-flame interaction leads to a significant increase in total energy release rates; as a consequence, the overall reaction rate increases due to the baroclinic vorticity production (Kilchyk, 2009, Kilchyk et al., 2011). The high-speed video images from experiments corresponding to combustion of ϕ = 0.8 ethylene mixture and jet traversing speed of (8.1 ms) is presented in Fig. 9. Due to the interaction of the propagating flame with the shock wave returning from the main CVC chamber end, the flame front deforms into a mushroom like shape. Numerical analysis of the shock-flame interaction and the reaction rate increase for different traverse speeds has been carried out in another work (Karimi et al., 2013). Predicted results indicate that shock-flame interaction causes significant increase in reaction rates. The reaction rate increase has been observed to be caused by both flame length/surface increase due to deformation and kinetic amplification (Kilchyk, 2009). 9

10 Figure 9: shock flame interaction observed for ethylene =0.8 (right) for hot-jet traverse time of 8.1 ms. 3. Conclusions Traversing jet ignition tests were carried out for ethylene, methane and hydrogen-enriched methane mixtures with the jet traversing the end of a long constant-volume combustor in 40.6 ms and 8.1 ms. The fuel-air mixtures were successfully ignited for a range of lean equivalence ratios (ϕ = 0.4, 0.6 and 0.8) in the CVC chamber, while keeping the pre-chamber fuel-air ratio fixed (ethylene at ϕ = 1.1). It is observed that for the same fuel-air mixtures, the lean ignitability limit measured using a hot-jet is well below the ignitibility limit by spark ignition methods. By using a quiescent mixture in the main CVC chamber, the fluid dynamic behavior of the traversing jet was observed clearly. For the fuels and equivalence ratios considered in the study, the ignition delay time generally was lower for the faster traverse jet (8.1 ms) compared to the slower traverse jet (40.6 ms), the latter behaving essentially as an offset stationary jet.. For methane at ϕ = 0.8, the difference was as much as 2.5ms. The observation supports the fact that ignition was controlled by the traversing nature of the jet across the chamber. Hydrogen enrichment of the methane mixture expectedly decreased the ignition delay times and increased the consumption rate of the CVC mixture compared to pure methane. Criterion for detection of ignition and measurement of ignition delay times can further be improved by gathering dynamic pressure data in the main CVC chamber. The flame deformation caused by shock-flame interaction is also clearly observed from the high speed video images. Acknowledgement: The authors would like to thank Mr. Kevin Murphy for fabrication and assembly of the main chamber, fueling system, and other rig components, Mr. Indika Perera for design of rig components and instrumentation, Mr. Michael David for assessing and programming the edge detection technique, and the following for improvements to the rig electrical systems and safety:.mr. Kenneth Lee, Mr Kyung Hoon Bang and Ms. Golnaz Mortazavi. 10

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