OH Planar Laser-induced Fluorescence Imaging Research Articles

Effect of water content in ethanol on flame front wrinkling and curvature in a spark-ignition engine using OH PLIF imaging

The production of ethanol fuel involves separating water and ethanol by distillation and dehydration in an energy intensive process. To reduce the carbon footprint, hydrous ethanol is seen as a promising alternative. This study focused on the effect of water content in ethanol on flame development in a single-cylinder optical spark-ignition engine. Ethanol with 0%, 4% and 10% water content per volume was used with Port Fuel Injection (PFI) and Direct Injection (DI) for homogeneous and heterogeneous mixture preparation. Tests were also conducted with iso-octane for comparison. Flames captured by Planar Laser Induced Fluorescence (PLIF) of OH were spatially filtered to remove scales larger than the flame size, then the flame brush was quantified in terms of thickness, crossing frequency between instantaneous and filtered contours, and curvature. Increasing water content resulted in slower flame growth, with 10% hydrous exhibiting marginally higher flame speed than iso-octane. PFI led to faster flame growth and thicker flame brushes than DI at the same timing after ignition. When the brush thickness was normalised by flame radius, all hydrous fuels and iso-octane exhibited similar values with PFI, whilst anhydrous ethanol exhibited the thickest normalised brush. The alcohols maintained similar normalised thicknesses with DI and iso-octane exhibited the thickest normalised brush. When flames of the same radii were considered with PFI at the same timing, the fuels with sequentially lower laminar burning velocity were associated with larger crossing frequency. For PFI, increasing water content resulted in decreased flame roundness irrespective of size, with the 4% hydrous and iso-octane exhibiting similar values. With DI, anhydrous ethanol flames were rounder than the 4% hydrous when averaged over all cycles, but marginally rounder than the 10% hydrous and iso-octane. Increasing water content widened the curvature distribution and increased the mean with PFI. Flame roundness decreased with increase in mean curvature and Lewis number for both PFI and DI.

Investigation of the high-temperature field distribution characteristics for a multi-jet burner by OH-PLIF and coherent anti-Stokes Raman spectroscopy

The construction of a high-temperature gas calibration source is of great significance since it can provide an effective high-temperature experimental environment for, e.g. verifying high-temperature measurement techniques and studying high-temperature combustion mechanisms. Here, we try to obtain a high-temperature gas field on a multi-jet burner by controlling the gas supplies to it. We use OH planar laser-induced fluorescence (OH-PLIF) to observe the compositional uniformity of the field and coherent anti-Stokes Raman scattering (CARS) to investigate the temperature uniformity of the field. We find from OH-PLIF images that the distribution of OH between the adjacent jets becomes more uniform with the increasing flow rate of CH4, and the flow rate of the co-flow N2 around jets also affects the uniformity of OH distribution. The measured temperature distribution by CARS is consistent with the OH distribution. At the jet outlet location, the temperature distribution had a periodic variation and gradually became more uniform with the height increased from the jet outlet. We find that the flow rate of CH4 and co-flow N2 and the radiative heat transfer rate play an important role in temperature distribution for the multi-jet burner. Also, the results show that a wide range of temperatures can be constructed by regulating the recipe of the gas supplies, and the highest temperature achieved in this work is 2457 K.

Experimental study on combustion and thermal characteristics of impinging premixed flames for low heating value gas (LHVG) fuels

The combustion and thermal characteristics of impinging premixed flames for low heating value gases (LHVGs) composed of methane, propane, and nitrogen are experimentally investigated. For the experiment, a combustor with an impingement plate is used to study the flame stability regions based on flame shapes and visualize OH radical distribution. The OH radical distribution is observed via OH* chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF). Thermal characteristics are investigated by measuring the temperature distribution of the plate and the heat transfer rate into the cooling water. The results show that the flame stability region narrows as the heating value of the fuels decreases. Also, as the equivalence ratio increases (i.e., »1), the flame stability region is expanded. The OH radical intensity of LHVGs is observed to be lower than that of methane. The OH-PLIF images show where OH radicals are more distributed in the flame. The temperature distribution of the plate is determined by the flame shape according to the heating value, and the heat transfer rate into the water has a linear tendency to the fuel's thermal power. Based on these findings, LHVGs can be utilized without system improvement within the flame stability range in the impinging flame system.

Effects of doping ceria on flame quenching in a narrow channel with zirconia-based functional coatings

Wall coating modification has been a potential solution to organize the combustion process in space-constrained system. For this purpose, CeO2 doping effects on the flame stability of premixed methane/air mixtures confined within an adjustable-gap narrow channel with coating ZrO2-based ceramics are experimentally investigated. A series of characterization means are utilized to analyze these prepared composite coatings. Results elaborate that the catalytic activities of CeO2-ZrO2 solid solution correlate with the ratio of CeO2 dopant in bulk. Phase structural stabilities can be strengthened through increasing the CeO2 content, which is due to the significant increase in the ratio of tetragonal phase to monoclinic phase of ZrO2. X-ray photoelectron spectroscopy and H2 temperature-programmed reduction analysis outcomes show that the concentration of formed oxygen vacancies increases significantly with richer CeO2 phase doped, which promotes the exaltation of lattice oxygen activity and fluidity. Flame quenching tests demonstrate that the wall surface temperature dependence of the quenching distance is changed substantially by doping CeO2 into the ZrO2 coating wall, eliminating the negative temperature effect observed for a pure ZrO2 case at the medium temperature ranges of 673–773 K. The cases of higher-CeO2-content doping provide a lower flame quenching distance, especially at slightly fuel-rich conditions, which arises from the enhancement of the storage and release functions of highly active oxygen ions through Ce4+/Ce3+ redox cycles. Comparing of OH planar laser-induced fluorescence images near the extinction limit indicates that in addition to flow rate and air–fuel ratio, different coating materials also have dramatic effects on the post flame characteristic and flame oscillation. In general, this paper not only provides a good overview of the chemical interaction between the wall and flame, but also makes an implication for the design of high-stability small-scale combustors by reasonably manufacturing this type of CeO2-ZrO2 coating functional materials.

Difference in Thermo-Acoustic Instability Frequency Between Partially and Fully Premixed Flames

The difference in thermo-acoustic instability frequency was analyzed between partially premixed (PP) and fully premixed (FP) flames in a methane-fueled gas turbine combustor. The instability frequencies of both PP and FP flames shifted to high frequencies as the equivalence ratio increased. However, the instability frequencies of two flames were different at some equivalence ratio conditions, despite the same air and fuel flow rates. The convection time was introduced to determine the reason for the difference, and values of two flames were calculated quantitatively using OH-planar laser-induced fluorescence imaging and particle image velocimetry. The PP flames had a longer than the FP flames, and incomplete premixing of fuel and air in PP flames was determined to be the cause of this result. A low-order network model was adopted to confirm that whether the difference in between PP and FP flames can cause different instability frequencies. The network model well-predicted that two flames featured different instability frequencies owing to the difference in despite the same equivalence ratio. In conclusion, the PP flames had longer than FP flames owing to incomplete premixing of fuel and air, and it consequently contributed to the difference in instability frequency.

Modal Decomposition Analysis of Combustion Instability Due to External Perturbation in a Mesoscale Burner Array

AbstractIn this work, the growth regime of combustion instability was studied by analyzing 10 kHz OH planar laser-induced fluorescence (PLIF) images through a combination of dynamic mode decomposition (DMD) and spectral proper orthogonal decomposition (SPOD) methods. Combustion instabilities were induced in a mesoscale burner array through an external speaker at an imposed perturbation frequency of 210 Hz. During the transient growth phase of combustion instability, 10 kHz OH PLIF imaging was employed to capture spatially and temporally resolved flame dynamics. Increased acoustic perturbations prevented flame reignition in the central recirculation zone and eventually led to the flame being extinguished inward from the outer burner array elements. Coherent modes and their growth rates were obtained from DMD spectral analyses of high-speed OH PLIF images. Positive growth rates were observed at the forcing frequency during the growth regime. Coherent structures, closely associated with thermoacoustic instability, were extracted using an appropriate SPOD filter operation to identify mode structures that correlate to physical phenomena such as shear layer instability and flame response to longitudinal acoustic forcing. Overall, a combination of DMD and SPOD was shown to be effective at analyzing the onset and propagation of combustion instabilities, particularly under transient burner operations.

Effects of hydrogen enhancement on mesoscale burner array flame stability under acoustic perturbations

Partial substitution of hydrocarbon fuel with hydrogen can effectively improve small-scale combustion system stability and performance, potentially opening the way for novel compact power generation and/or propulsion systems in the future. In this study, the effects of hydrogen enhancement between 0% and 40% hydrogen volumetric fractions in methane fuel were experimentally observed in a mesoscale burner array subjected to external acoustic perturbations. The mesoscale burner array utilizes an array of swirl-stabilized burner elements and their interactions with neighboring elements to improve the overall flame stability and simultaneously reduces the combustor length scale. OH∗ chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) were used to image various hydrogen-enriched flames at an equivalence ratio of 0.7, subjected to transverse acoustic perturbations at 320 Hz. Two acoustic modes were imposed by controlling the phase difference between two speakers perturbing the flow. OH∗ chemiluminescence images exhibited flame length scale reduction, leading to a denser flame array. Also, flame arrays with higher hydrogen enrichment were found to be more robust against transverse acoustic perturbations, demonstrated by reduced fluctuations in the global heat release rate. OH-PLIF images showed that flames with higher hydrogen enrichment initiated V- to M-shaped flame shape transition even under fuel lean conditions, thereby improving the combustion stability. OH-PLIF images were also used for flame stability analysis through spectral proper orthogonal decomposition (SPOD). The SPOD analysis showed hydrogen enrichment diminished flame fluctuation structures under fuel lean operation.

OH, PAH, and sooting imaging in piloted liquid-spray flames of diesel and diesel surrogate

Comparison of the flame structure, polycyclic aromatic hydrocarbons (PAH), and soot formation characteristics of commercial diesel and a diesel surrogate is investigated using planar laser-induced fluorescence (PLIF) of OH and PAH at 283 nm, and laser-induced incandescence (LII) of soot at 1064 nm. The diesel and diesel surrogate flames present a similar overall appearance, with a “dark core” region of fine droplets exiting the injector and a light blue outer flame near the base. Hot soot regions appear downstream, indicated by bright yellowish flame luminosity. Comparison of simultaneously acquired single-laser-shot PLIF images of OH and PAH indicates that smaller PAH reside mostly in the central regime in the ~20–60% height of the flame, sandwiched between the flame fronts. In contrast, larger PAH and soot particles are formed further downstream. Soot-LII signals are present only in the upper ~40% height of the flame for both the fuels. The distribution of mean PAH-PLIF and soot-LII signals along the flame height is also found to be similar for both fuels, except diesel is shown to produce approximately 1.5× more soot than the diesel surrogate. Based on the analysis of probability distributions, the maximum OH-PLIF signals in both fuels have similar radial and axial spatial distributions, while maximum PAH-PLIF signals occur at a higher axial location for the diesel surrogate. However, for the diesel surrogate, maximum soot-LII signals are frequently appeared at lower heights. Therefore, exploration of more appropriate diesel surrogates may be necessary to closely mimic the PAH and soot formation behaviors of diesel fuel. The present study shows that the combined optical diagnostic techniques and the statistical analysis can be used for studying commercial diesel and diesel surrogates, and thus establish a robust framework for detailed studies of chemical kinetics, combustion dynamics, and soot formation in liquid hydrocarbon spray combustion.

Megahertz-rate OH planar laser-induced fluorescence imaging in a rotating detonation combustor.

Megahertz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor for the first time, to the best of our knowledge. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355 nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 µJ/pulse at 1 MHz and 150 µJ/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH-PLIF measurements and in the number of pulses over previous megahertz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

Effect of incident laser sheet orientation on the OH-PLIF imaging of detonations

This study aims to investigate the effect of laser sheet orientation on the OH-planar laser-induced fluorescence (OH-PLIF) imaging of a detonation wave and to identify the potential benefit of using a transverse laser orientation, compared to the conventional frontal orientation. In this study, we developed a new laser-induced fluorescence (LIF) model, based on a preexisting one, to include a more fundamental calculation of the absorption lineshape and the effects of the laser orientation on the fluorescence signal. One- and two-dimensional simulations are used to validate this new LIF model for both laser orientations using OH-PLIF images from the literature. The effect of the two laser orientations on detonation imaging is investigated numerically on a two-dimensional OH field. Based on these results, the frontal orientation seems more suitable to collect information near the detonation front, while the transverse laser orientation can give more quantitative information far from the front and only for short optical paths. With the present laser configuration and experimental conditions, both laser orientations present the same limitation with a strong laser energy absorption for long optical paths. While these first results show a qualitative agreement with experimental data, new experimental data are required to quantitatively validate this promising transverse laser orientation and confirm its relevance for OH-PLIF imaging of detonations.

Effects of Flame Behaviors on Combustion Noise from Lean-Premixed Hydrogen Low-Swirl Flames

This experimental study explored the influence of global/local flame behaviors on direct combustion noise produced by a lean-premixed gaseous low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten kilohertz chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuations, respectively, whereas 2-D particle image velocimetry (2-D PIV) measurements at 4 Hz were applied to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of . Spectral proper orthogonal decomposition of the chemiluminescence and OH-PLIF images revealed that the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, whereas the secondary peak was attributed to periodically generated vortical flame structures near the downstream side of the flame boundary. The 2-D PIV results suggest that vortical flame structures are likely generated by the interaction between the flame and the inner/outer shear layers.

Iron Contamination in High-Enthalpy Test Facilities: OH PLIF Imaging Considerations

Thermometry measurements were performed in a scramjet combustor using thermally-assisted laser-induced fluorescence. Experimental data were obtained through laser-induced fluorescence where a laser beam was focused into the combustor and the OH () transition at 283.55 nm was excited. Measurements in the 360 nm spectral region resulted in detection of a strong fluorescence line. Investigation of this phenomenon led to the conclusion that when the OH line is chosen for excitation, another line of a different species that lies very close to is also unintentionally excited. The second species has been identified as iron, Fe I. The assumption is that the erosion of the shock tunnel walls introduced iron in the flow that is excited with the laser when exciting OH. Thus, it is very important to ensure that this is avoided in future. Instead, another OH excitation line should be used for OH excitation line in planar laser-induced fluorescence (PLIF) experiments where iron may potentially also be present in the flow. Alternatively, fluorescence from this iron line can be circumvented by proper filtering of the PLIF. A proposal is made to turn the iron contamination into advantage by using iron present in the non-combusting flow for PLIF.

Emission characteristics of turbulent non-premixed ammonia/air and methane/air swirl flames through a rich-lean combustor under various wall thermal boundary conditions at high pressure

This study is dedicated to understanding the combustion and emission characteristics of turbulent non-premixed ammonia (NH3)/air and methane (CH4)/air swirl flames in a rich-lean gas turbine−like combustor at high pressure under various wall thermal boundary conditions. In this study, the emission characteristics of both flames were obtained through numerical simulations using large−eddy simulations with the finite-rate chemistry technique. In addition, for NH3/air flames, simultaneous NO and OH planar laser−induced fluorescence (PLIF) images were acquired in order to qualitatively verify the numerical results. The results show that the minimum NO emission could be obtained when the primary zone global equivalence ratio (ϕglobal/pri) was 1.1, irrespective of the wall thermal condition, using a rich-lean combustor in NH3/air flames, whereas in CH4/air flames, the maximum NO emission was obtained with a ϕglobal/pri value of 1.0. Moreover, in NH3/air flames, the local NO concentration is largely dependent on the local OH concentration, whereas in CH4/air flames, the local NO concentration is largely dependent on the local temperature. The NO−OH correlation in NH3/air flames was experimentally verified using simultaneous NO and OH PLIF images. Additionally, it was found that, in NH3/air flames, the wall heat losses due to the combustor wall cooling greatly affected the NH3 oxidation and led to significant emissions of unburnt NH3, although lower NO emission resulted from the combustor wall cooling. This was primarily because of the lower OH concentration level in the flame region owing to the wall heat losses.

Statistical Analysis of Flame Oscillation Characterization of Oxy-Fuel in Heavy Oil Boiler Using OH Planar Laser-Induced Fluorescence

The present work investigated the flame structures and oscillations of oxy-fuel combustions in a heavy oil boiler using OH planar laser-induced fluorescence imaging. Combustion instabilities, such as flame oscillation and combustion fluctuation, can assess the performance of an industrial burner in the boiler. The peak position variation in OH concentration was associated with the change of the reaction zone that corresponded with the fluctuation of the heat-release zone in the combustion chamber, which provides a valuable reference for the design of the combustion chamber. The experimental results suggest that the phenomenon of stratified flame combustion is related to the characteristic of flame oscillation. The substitution of N2 with CO2 will not significantly influence the flame oscillation frequency but increases the number of flame surface. As O2 concentration increased in the O2/CO2 atmosphere, the phenomenon of stratified flame combustion disappeared, and the flame presented an island-like structure. The bimodal oscillation of the combustion center was demonstrated by means of the probability density method; CO2 played a role in the extension of the combustion center. The combustion fluctuation of inner regions was quantitatively described; CO2 could maintain interregional stabilization to some extent. Compared with traditional measurement methods, PLIF technology has great advantages in evaluating burner performance and optimizing the design of the combustion chamber.

Comprehensive Combustion Stability Analysis Using Dynamic Mode Decomposition

Combustion stability is an important consideration for many energy systems because of its impact on performance and efficiency. Flame dynamics that govern combustion stability are often complex and difficult to resolve, particularly from experimental data. However, recent advances in postprocessing techniques, such as dynamic mode decomposition (DMD), have partially enabled flame dynamic analysis. This study aims to provide a comprehensive measure of combustion stability through a detailed investigation of coherent structures and their energy contents, frequencies, and growth factors from DMD. These results are then correlated to underlying physics and chemistry that drive flame dynamics. Three different data sets were analyzed in this study. Numerically constructed images and OH-planar laser induced fluorescence (OH-PLIF) images of laminar flames were used to characterize stable flame dynamics. OH-PLIF images of acoustically perturbed swirl-stabilized turbulent flames were used to characterize oscillator...

Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram

This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity (DT=u′Lx) to the molecular diffusivity within the preheat layer (D*=SLδF,L). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

Multiple optical diagnostics on effect of fuel stratification degree on reactivity controlled compression ignition

Reactivity controlled compression ignition (RCCI) was investigated on a light-duty optical engine under different fuel stratification degrees, using multiple laser diagnostic techniques. The engine was run at a speed of 1200rpm and under a load of about 7bar gross IMEP. To form different fuel stratification degrees, the direct-injection timings of n-heptane were changed, while the port-injection timings of iso-octane was kept constant. The fuel/air equivalence ratio and primary reference fuel (PRF) number were quantified by the fuel-tracer planar laser-induced fluorescence (PLIF) under non-combusting condition. The results indicated that with retarding n-heptane injection timing from −90° CA ATDC (RCCI-90 case) to −10°CA ATDC (RCCI-10 case), regions of higher fuel concentration and reactivity moved downstream to the edge of combustion chamber before high-temperature heat release (HTHR) phase. Time-resolved natural combustion luminosity imaging and single-shot OH PLIF imaging indicated that RCCI-10 case presented a staged combustion process that an auto-ignition first happened in the region of high reactivity around the combustion chamber and then another auto-ignition process took place in the region of low reactivity in the central part of the combustion chamber. The staged combustion feature involved in RCCI combustion could result in lower combustion pressure-rise rate. PLIF images of formaldehyde showed that formaldehyde first formed during low-temperature heat release (LTHR) phase in the regions where n-heptane resided. With retarding n-heptane injection timings, both formaldehyde and OH PLIF images presented more stratified distribution, and the consumption of formaldehyde and formation of OH processes got slower. OH PLIF images indicated that HTHR phase of RCCI could extend to the central part of combustion chamber. In the low-load LTC conceptual model proposed by Musculus et al. (2013), no HTHR happened and UHC formed in the central part of combustion chamber. This meant that RCCI could have less UHC emission than LTC in theory.

Proper Orthogonal Decomposition for Analysis of Plasma-Assisted Premixed Swirl-Stabilized Flame Dynamics

The primary aim of this paper is to establish the effectiveness of microwave plasma discharges to improve combustor flame dynamics and stability through minimizing heat release fluctuations. A continuous, volumetric, direct coupled, nonequilibrium, atmospheric microwave plasma discharge was applied to a swirl-stabilized premixed methane-air flame to minimize combustion instabilities. Proper orthogonal decomposition (POD) is used to postprocess data and extract information on flame dynamics that are usually lost through classical statistical approaches. POD analysis carried out on OH planar laser-induced fluorescence images reveals that even at coupled plasma powers corresponding to less than 5% of the thermal power output, significant improvement in mean energy content of flames (~23%) was observed. The corresponding decrease in heat release fluctuations resulted in improved combustor flame dynamics and flame stability, which was found to be in good agreement with acoustic pressure measurements. In the presence of plasma discharge, an effective decoupling between the flame oscillations and the fluid unsteadiness was established due to the differences in flame stabilization mechanisms resulting in up to 47% reduction in root-mean-square pressure fluctuations. Thus, effective fluid-acoustic decoupling in addition to the accelerated combustion chemistry due to the nonthermal effects of plasma led to significantly improved combustor dynamics, namely, decreased heat release and pressure fluctuations.

Combustion effects of a staged transverse jet and pulsed detonation in supersonic crossflow

The combustion effects of a H2/O2 pulsed detonation (PD) staged downstream of a primary transverse H2 jet in supersonic crossflow (JICF) was investigated. A Mach 2.35 crossflow representative of Mach 7 flight combustor entry conditions (T∞=1330K, P∞=40kPa) was considered. The jet-to-crossflow momentum flux ratio (Jj) of the primary jet was varied between 0.5 and 5.0 to span different stabilization modes. The PD provides a short-duration, high temperature and momentum, radical-rich plume of gas to the wake of the primary transverse jet. The temporal evolution of the interaction between the two plumes was studied using high-speed schlieren and OH* chemiluminescence imaging. Phase-locked OH planar laser-induced fluorescence imaging was used to identify the instantaneous reaction zone along the center-plane at various times of the blowdown process. It was found that the interaction of the two plumes promotes earlier ignition and release of heat. The presence of a high momentum flux ratio transverse jet increases the effective blowdown period (EBP) over which OH* is exhausted; thus increasing the duration of the effects of the PD on the JICF. Lastly, the penetration of the reacting layer into the crossflow was observed to increase with the staged PD present as compared to cases without it.

Experimental investigations of coaxial injectors in a laboratory-scale rocket combustor

An investigation is performed through experiments of a laboratory-scale optically accessible combustor equipped with a single-element shear coaxial injector. Various optical diagnostics approaches, such as hydroxyl (OH) planar laser-induced fluorescence (OH-PLIF), high-speed imaging and infrared imaging, are simultaneously used to determine the OH radical spatial distribution, flame instantaneous fluctuations, and temperature fields outside the combustor nozzle. This study emphasizes the combustion characteristics and flame stabilizations of two propellant combinations under the same thrust level, including gaseous hydrogen/gaseous oxygen (GH2/GO2) and gaseous methane/gaseous oxygen (GCH4/GO2). Moreover, to validate a pressure-scaling criterion, different pressure conditions are also considered. Based on the results of the high-speed and OH-PLIF images, combustion similarities are evident under different chamber pressures for each propellant couple. The two kinds of flame are seen to be stabilized in the vicinity of the oxidizer injector lip but the anchor region in the GCH4/GH2 cases is found to change dynamically near the lip. The lift-off GCH4/GO2 flame exhibits evident unsteady combustion phenomenon in the combustor. Experimental data indicates that GCH4/GO2 flame stabilization is hard to achieve compared to the GH2/GO2 flame. Furthermore, the GCH4/GO2 flame has a greater heat releasing intensity and gentler reaction zone distribution. This indicates that relative to the GH2/GO2 flame, the GCH4/GO2 flame causes major issues with regard to heat protection of the inner wall in the combustor. Meanwhile, according to the infrared images, the GCH4/GO2 plume has a lower outlet core temperature and larger temperature gradient, which is an advantage for employment in future space engines. This study, identifying preliminary behavior differences between the GH2/GO2 and GCH4/GO2 flames, will be applicable for future engine design.