Cavity Flameholder Research Articles

Gas property measurements in a supersonic combustor using nanosecond gated laser-induced breakdown spectroscopy with direct spectrum matching

Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene (C 2 H 4 ) fueling are simultaneously measured using nanosecond gated (10 ns) laser-induced breakdown spectroscopy (n-LIBS). Emission spectra from the laser-induced plasma over a range (550 nm–830 nm) containing multiple emission lines of O, H, N, and C are captured and used to quantify the gas conditions. A direct spectrum matching (DSM) process is employed for the gas property quantification in which an unknown spectrum is matched to a spectrum from a database, constructed through n-LIBS analysis of well-known gas conditions. A back-scattering collection method is implemented to allow for measurements using a single optical access point. Fuel mole fraction and gas density are mapped within a cavity flameholder on planes parallel to the Mach 2 freestream flow above the cavity. With C 2 H 4 injection from the closeout ramp of the cavity toward the front step, a high fuel mole fraction region appears near the front step where density and flow velocity are low, and thus favorable for cavity flame ignition. The fuel mole fraction at a location in the cavity is found to be nearly proportional to fueling rate; the gas density, on the other hand, is not affected by the fueling rate, under the typical operating conditions.

Numerical Modeling of Quasi-DC Plasma-Assisted Combustion for Flame Holding Cavity

ABSTRACTThe quasi-DC discharge plasma across the backward facing step of a flame-holding cavity can be used to improve the flame stabilization of the cavity in a scramjet combustor. The effects of plasma filaments on the main flow structures, cavity drag, mass exchange rate, cavity oscillating characteristics, stagnation pressure loss, and combustion efficiency are numerically investigated based on the dominant thermal blocking mechanism. The results show that the cavity shear layer fluctuates dramatically due to the ‘cutting’ effect of plasma, which leads to obvious changes in the fuel mixing and burning in the combustor, and the pressure distribution near the rear edge of the cavity. Some symmetrical structures in the shape of ‘branch pipes’ are observed, which represent the distribution of product water. The periodical producing of them can be attributed to the movement of the cavity shear layer. Compared with the no plasma case, the cavity drag coefficient overall increases 11.3%, yet the mass exchange rate rises sharply over 10 times. The sound pressure level of the cavity representative points is raised by the plasma, and the first dominant frequency of pressure oscillation at the monitor point on the rear edge is twice the plasma actuation frequency. Although the stagnation pressure loss increases, the plasma shows benefits in energy producing that outweigh the energy consumed by the plasma actuator.

Parametric study of combustion oscillation in a single-side expansion scramjet combustor

As a promising candidate for future air-breathing systems, the viability and efficiency of scramjet propulsion is challenged by a variety of factors including the combustion oscillation in scramjet combustor. A series of comparative experiments focusing on the combustion oscillation issue has been carried out in the present work. The obtained experimental results show that as the global equivalence ratio increases, the combustion oscillation becomes more regular and frequent which is the most intensive in the vicinity of the fuel jet and the periodic combustion oscillation is more possible when the injectors and flame-holding cavity are mounted on the expansion-side wall. In order to avoid the combustion oscillation in scramjet combustor, distributed injection scheme is an effective method which can induce two parts interacting stable flame. In addition, the results reveal that the varying fuel including hydrogen, ethylene and kerosene with different chemical kinetics has a significant effect on the reaction process in scramjet combustor, which can result in stable combustion, periodic oscillation and failed ignition respectively on the same operating condition of this paper. We believe that the present work is helpful to the designing of scramjet propulsion device.

Visualization of Simulated Fuel–Air Mixing in a Dual-Mode Scramjet

Nitric oxide planar laser-induced fluorescence measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of nitric oxide and nitrogen was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flameholder. Nitric oxide planar laser-induced fluorescence signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements using ethylene fuel. A theoretical study of the selected nitric oxide rotational transitions was performed to obtain a laser-induced fluorescence signal that was linear with the nitric oxide mole fraction and approximately independent of the pressure and temperature.

超音速流における衝撃波干渉するキャビティー保炎器下流の燃焼メカニズム

The objective of this study is to explore the effects of incident shock waves on combustion downstream of a cavity flameholder. The images of flame chemiluminescence showed that the flame looks extinguished downstream of an incident shock wave, as well as OH-PLIF measurements indicated higher OH concentration upstream of the incident shock wave, while lower OH concentration downstream of the incident shock wave. The results of two-dimensional numerical simulations for reacting flows showed that the incident shock wave generates a strong recirculation zone and reverse flows, and the main flow curved around the zone, corresponding well to the flame structure seen by experiments. In addition, it was found that about 90% of the injected H2 was converted to H2O at the rear edge of the calculated wall domain when an incident shock wave was introduced, which means that cavity-stabilized flames is not extinguished by the incident shock wave but burnt out there, thus the fuel was almost fully consumed. That is to say, the incident shock wave enhances chemical reactions because the generated recirculation zone and low speed region increases residence time and increased turbulent kinetic energy enhances the mixing.

Fuel–Air Mixing Experiments in a Directly Fueled Supersonic Cavity Flameholder

In the present study, planar laser-induced acetone fluorescence was used to quantify the streamwise concentration distribution of an ethylene fuel surrogate (acetone-seeded nitrogen) injected directly into the recirculation region of a cavity flameholder embedded in a nonreacting supersonic airflow. The effect of injection configuration and fueling rate were studied. Parallel injection from the cavity front wall resulted in an ultra-fuel-rich recirculation region (beyond the upper flammability limit of an ethylene–air mixture) with a mixing efficiency that degraded with increased fueling rate. Downstream angled injection from the cavity floor yielded a substantial improvement in terms of the spatial uniformity and flammability of the local mixture. Its mixing efficiency was observed to be independent of fueling rate. The superior mixing ability of the latter injection configuration has the potential to provide increased cavity flameholder operability through a broader range of ignition and blowout limits and more flexibility in terms of ignitor placement than conventional direct-fueling schemes. Further investigation of its performance over a wider range of reacting conditions is warranted.

From ignition to stable combustion in a cavity flameholder studied via 3D tomographic chemiluminescence at 20 kHz

This work reports the study of the ignition processes in a Mach-2 cavity combustor based on three-dimensional (3D) measurements at 20 kHz. The 3D measurements were obtained by a combination of tomographic chemiluminescence and fiber-based endoscopes. Measurements of 3D flame and flow properties were reported under two fueling conditions of the combustor. The properties included 3D volume, surface area, shape factor, and 3D3C (three-dimensional and three-component) velocity of the ignition kernel. These results clearly distinguished the ignition stage from the stable combustion stage of the combustor and enabled the determination of a transition time to quantify both stages. The analysis of the change of the ignition kernel's shape, when combined with the 3D3C velocity measurements, also illustrated flame-flow interactions in the cavity combustor. These results demonstrated the utility of the 3D diagnostics to overcome some of the limitations of established planar diagnostics and to resolve the dynamics of high-speed combustion devices both spatially and temporally.

Direct spectrum matching of laser-induced breakdown for concentration and gas density measurements in turbulent reacting flows

A direct spectrum matching method for laser-induced breakdown spectroscopy is proposed to simultaneously measure gas density and concentration in turbulent reacting environments with improved measurement accuracy. The breakdown spectrum recorded in the target flow is directly matched with a spectrum out of a database consisting of various emission spectra recorded under well-defined conditions in a range of gas density and composition. It is shown that the wavelength, intensity and line width of the atom/ion emission lines in the spectrum indicate atom composition and gas density that are independent of parent molecular species in the target flow. Once a matching spectrum (within 550–830 nm containing O, H, N, and C lines) in the database of a known gas condition is found, the concentration and gas density at the location of the breakdown can be accurately derived. A 532-nm Nd:YAG laser with 10-Hz pulse repetition rate is used to induce breakdown in fuel/air mixtures in a variable pressure combustion chamber to build the spectrum database.In addition, it is used in a cavity flameholder of a model supersonic combustor to measure the gas density and concentration fields in a turbulent reacting environment. All the measurements are completed within 100 ns after laser firing, before breakdown affects the flow and the fast evolving environment alters the breakdown spectrum.

Effects of thermally cracked component of n-dodecane on supersonic combustion behaviors in a scramjet model combustor

Supersonic combustion ramjet technology using liquid hydrocarbon fuels is invaluable due to great benefits of the energy density per unit volume and handling. This study carried out experiments to investigate the effects of the components in thermally cracked n-dodecane on ignition and supersonic combustion behavior at supersonic speeds (Mach 2.0) and at stagnation temperatures between 1800 K and 2400 K in a supersonic model combustor with a cavity flame holder. Components of the thermally cracked n-dodecane at 1.0 MPa in a flow reactor were experimentally measured by using a gas chromatograph. Based on the measured components, surrogate fuels consisting of main components of cracked n-dodecane, hydrogen, methane, ethane and ethylene were defined, and fractions of the component was varied at less than 10%. Ignition and supersonic combustion behaviors were tested for single component and surrogate fuels. In all cases where supersonic combustions were achieved, ignitions were firstly observed in the boundary layer at the expanding cross section zone. The flames, then, propagated upstream and were held at the cavity. Two combustion modes, jet wake stabilized and cavity stabilized flames were observed based on the distribution of rise in pressure and OH chemiluminescence. In cases of pure fuel injections, ignitions for ethylene injections could be observed at a stagnation temperature of 1800 K. On the other hand, ignitions for methane and ethane injections could be observed above a stagnation temperature of 2200 K. Ignitions for surrogate fuel injections were also investigated. These ignition limits were also validated by a simple estimation of chemical reaction and residence times. It was also found that an increase in fuel temperature improved the ignition limits although the effect of the dynamic pressure ratio between the fuel injection and the main flow was rather limited in the present study.

Numerical study of combustion and convective heat transfer of a Mach 2.5 supersonic combustor

In this paper, characteristics of combustion and convective heat transfer of a supersonic combustor at two fuel/air equivalence ratios of 0.9 and 0.46 were numerically studied. The numerical method of Favre averaged Navier–Stokes simulation with SST k-ω turbulence model and a multiple-step reaction mechanism of ethylene is introduced. The inlet Mach number of the combustor is 2.5 and inlet total temperature is 1650K, corresponding to Mach 6 flight conditions. Ethylene is injected at two locations upstream of a flame-holding cavity. The numerical method was validated by comparing the present results of wall pressures and heat fluxes to experiments and theoretical analysis. It is found that, due to injection of fuel at the bottom wall, fuel/air mixing and combustion occurs mainly in the vicinity of the bottom wall. High non-symmetry in distributions of the bottom and the top wall heat fluxes is observed. Peaks of wall heat flux at different locations and at varied fuel/air equivalence ratios are identified, which are caused respectively by effect of cavity and by shock structure formed upstream of the injection points. It is also found that heat flux peaks are strongly related to the reaction step of CO→CO2, contributing to major heat releasing.

Experimental Investigation of Reacting Flow Characteristics in a Dual-Mode Scramjet Combustor

AbstractIn this work, a hydrogen-fueled dual-mode scramjet combustor was investigated experimentally. Clean and dry air was supplied to the combustor through a Mach 2 nozzle with a total temperature of 800 K and a total pressure of 800 kPa. The high enthalpy air was provided by an electricity resistance heater. Room temperature hydrogen was injected with sonic speed from injector orifices vertically, and downstream the injector a tandem cavity flame holder was mounted. Except wall pressure profiles, velocity and temperature profiles in and at exit of the combustor were also measured using hydroxyl tagging velocimetry (HTV) and tunable diode laser absorption spectroscopy (TDLAS), respectively. Results showed that combustion occurred mainly at the bottom side of the combustor. And there were also an extreme disparity of the velocity and temperature profiles along the

Heat Flux Measurements in a Scramjet Combustor Using Embedded Direct-Write Sensors

Embedded heat flux sensors fabricated using direct-write technology have been developed and tested within a direct-connect gaseous hydrocarbon-fueled scramjet combustor under Mach 5 flight conditions. The sensors employed a multilayer thermopile architecture fabricated via the additive direct-write thermal spray process, which enables their integration directly onto the components to be tested. The sensors were embedded within a 0.50-mm-thick (0.020-in.-thick) thermal barrier coating on test plugs that were inserted into the tunnel for testing. Fuel equivalence ratios and dynamic pressures were varied from to 1.0 and and 48 kPa (500 and 1000 psf), respectively, with increases in the global heat flux found to depend on the tunnel location. Heat flux values ranging from 30 to (26 to ) were measured downstream of the cavity flame holder, 20 to (18 to ) in the cavity flame holder, and 20 to (18 to ) upstream of the flame holder. Direct-write sensors reported heat flux trends that were consistent with separate area-averaged calorimetric measurements, and the local nature of the direct-write sensors enabled the detection of subtle transient phenomena not captured by calorimetry.

Experimental investigations on combustion characteristics of a cavity pilot augmentor of the turbine-based combined cycle engine

A cavity flame holder has been a very promising novel concept to be applied to the augmentor of a turbine-based combined cycle (TBCC) engine as it offers improvements in ignition performance, altitude relight, lean blowout limit, operating range, as well as combustion efficiency compared to conventional bluff body flame holders. The present paper describes the detailed designs of a cavity pilot augmentor and provides a laboratory-scale test rig. A series of experiments has been conducted to investigate the combustion characteristics of the test rig under atmospheric pressure. Experimental results show that for the bypass inlet temperature 343–473 K and inlet Mach number 0.25–0.40, ignition equivalence ratio and ignition pressure oscillation obtained is between 0.94–1.72 and 1.1–2.4% respectively, and lean blowout equivalence is between 0.42 and 0.92, indicating good ignition and lean blowout performances; as the total fuel-to-air ratio increases, the combustion efficiency decreases in the main at both afterburner operating mode and ramjet operating mode, while the increase of the bypass inlet Mach number or temperature is beneficial to high combustion efficiency, especially as inlet temperature over 423 K, and the combustion efficiency got in present experiment is between 70% and 95%. This paper demonstrates the feasibility for the cavity pilot flame holder to be applied to a realistic augmentor of TBCC engine preliminarily and provides reference for TBCC augmentor design.

Large-eddy/Reynolds-averaged Navier–Stokes simulation of cavity-stabilized ethylene combustion

In this study, a hybrid large-eddy/Reynolds-averaged Navier–Stokes (LES/RANS) method is used to simulate ethylene combustion inside a cavity flameholder. The cavity flameholder considered is Configuration E of University of Virginia’s Scramjet Combustion Facility, which consists of a Mach 2 inlet nozzle, a constant-area isolator, a combustor, and an extender, through which the exhaust gases are vented to the atmosphere. To increase the fuel-residence time, a cavity is fitted along the upper wall inside the combustor section of the flameholder. The configuration has the capability of injecting ethylene through a series of ports located upstream of and inside the cavity along the upper wall the combustor. In the simulations, ethylene combustion is modeled using a 22-species ethylene oxidation mechanism. Also, a synthetic eddy method is used to introduce turbulence at the inflow plane of the flameholder. For an equivalence ratio of 0.15, a cavity stabilized flame is predicted. Predictions are compared with line-of-sight temperature, water column-density, water mole-fraction, CO column-density, and CO2 column-density measurements at three stations within and downstream of the cavity. Agreement with experiment is generally good within the cavity. Downstream of the cavity, the simulations predict higher temperatures near the wall. Analysis of the flame structure predicted by the LES/RANS method indicates that the flame propagates into a stoichiometric to fuel-rich mixture near the cavity. Flame angles captured in the simulation are in close agreement with those predicted through classical premixed turbulent flame-speed estimates. Further downstream, the flame structure is non-premixed in character, and near complete conversion of CO to CO2 is observed by the time the flame reaches the combustor exit.

Mixing-related low frequency oscillation of combustion in an ethylene-fueled supersonic combustor

The present work investigated combustion instabilities inside an ethylene-fueled scramjet combustor mounted on a Mach 2.1 direct-connect test facility with an inflow stagnation temperature of 846K. Effects of fueling schemes on the combustion stability characteristics were examined. The experimental results suggest that the oscillation modes correlate with mixing status closely. For the cases with a quasi-steady thermal throat or stable shock trains, flame fluctuation exists in a mode of thermo-acoustic type oscillation with a broad frequency range. For the cases with a transient thermal throat, if a fuel/air premixed region from the injection to the cavity flameholder exists, the cavity pilot flame could reignite the fuel/air mixture and undergo a process similar to deflagration–detonation transition (DDT). This process couples with the flame quenching upstream of the injection location, and a DDT-type low frequency oscillation can be formed.

English

Reacting flow field of a cavity-based hydrogen-fuelled supersonic combustion ramjet (scramjet) combustor has been explored numerically. 3-D RANS equations are solved alongwith k-e turbulence model and infinitely fast rate kinetics for combustion. The flow field inside the flame holding cavity and its effect on the mixing and reaction are explored in detail by performing simulations of two different combustors (with and without cavities) for which elaborate surface pressure measurements are available for reacting and non-reacting flows. Simulations capture all the finer details of the flow field and good match between computed surface pressures experimental values for different equivalence ratios forms the basis of further analysis. The cavity of the combustor behaves as an open cavity for both non-reacting and reacting flow-even though the flow patterns inside the cavity are quite different for both the cases. Flame-holding cavities are seen to augment mixing and reaction in small sized combustors. Defence Science Journal, Vol. 64, No. 5, September 2014, pp.417-425, DOI:http://dx.doi.org/10.14429/dsj.64.5191

Multispecies Midinfrared Absorption Measurements in a Hydrocarbon-Fueled Scramjet Combustor

Spatially resolved laser absorption measurements of CO, , and within an ethylene-fueled direct-connect model scramjet combustor are presented. The sensors employ a variety of laser sources at midinfrared wavelengths to provide access to fundamental vibrational band absorption transitions for each species. Both scanned-wavelength-modulation spectroscopy and scanned-wavelength direct-absorption are used, with particular attention paid to employing these methods in a manner that accounts for expected nonuniformities in temperature and composition throughout the combustor. Results for product temperatures and column densities offer insight on the ongoing combustion process downstream of fuel injection throughout the combustion-product plume, and on the significant temporal variations in the combustor. Additional tests measure the temperature and concentration of in the cavity flameholder during a flame extinction event, which gives an upper bound of the cavity residence time. These measurements are the first use of these midinfrared wavelengths for measurements in a scramjet combustor, and this work introduces an important new tool for characterizing hydrocarbon combustion in scramjet engines.

Ignition and supersonic combustion behavior of liquid ethanol in a scramjet model combustor with cavity flame holder

This study carried out experiments to investigate the combustion behavior of ethanol at Mach 2.0 and stagnation temperatures between 1800 and 2200K using a supersonic combustor with a cavity flame holder. The distribution of droplet diameters in fuel sprays was measured by a Laser Diffraction Spray Analyzer to estimate the vaporization time of fuel in the combustor. CH chemiluminescences and Schlieren photographs were measured by a video camera to identify the combustion mode and to measure the penetration height of fuel in the supersonic flow. Both gaseous and liquid ethanol injections were tested to examine the effect of the fuel vaporization process. The minimum equivalence ratio of injected fuel to air in the main flow and the minimum stagnation temperature for successful combustion were derived based on Weibull distribution statistics. Two combustion modes, intensive and transient, were identified based on the distribution of a rise in pressure and the CH chemiluminescence emissions over the cavity. Supersonic combustion was observed above the stagnation temperature of 2200–2000K for liquid and gaseous ethanol injections. Compared with previous research, the reactivity of ethanol was located between the levels of ethane and ethylene. The ignition limits of the stagnation temperature for gaseous and liquid ethanol injections were also estimated and validated by calculating fuel breakup, vaporization, residence times and chemical reaction time. A fuel penetration height over 10mm resulted in no supersonic combustion for the equivalence ratio of 1.0, although this enhanced fuel and air mixing.

Simultaneous gas density and fuel concentration measurements in a supersonic combustor using laser induced breakdown

Laser-induced breakdown is used for quantitative gas property measurements (gas density and ethylene fuel concentration) in a cavity flameholder in a supersonic crossflow. A plasma is produced by a focused laser beam (Nd:YAG, 532nm) in the cavity to measure gas properties at the location of the plasma and to ignite cavity flames. Plasma energy (PE), defined by the laser pulse energy absorbed/scattered in the plasma, and plasma emission spectra are recorded for estimating gas density and species concentration, respectively. To obtain correlations of PE vs. gas density and emission spectra vs. fuel concentration, calibration experiments are conducted using a variable-pressure (0–900mbar)/temperature (300–900K) chamber and a Hencken burner installed in a variable-pressure (50–900mbar) combustion chamber. Total measurement time is sufficiently short, ∼80ns after laser arrival at the plasma region, to capture the high intensity portion of the emission and to minimize effects of plasma displacement (in the high-speed flow). Specifically, the laser pulse energy incident and transmitted (through the plasma) are measured, and the plasma emission spectra are captured during a 50-ns gate, after an approximate 30-ns time delay (relative to onset of emission from the plasma volume) to avoid strong background emission from the plasma.

Velocimetry Measurements of a Scramjet Cavity Flameholder with Inlet Distortion

Particle image velocimetry measurements were made along the center plane of a scramjet cavity flameholder to analyze simulated inlet flow distortion in the direct-connect test environment. Mach 3 nonreacting tests examined an oblique shock impinging upon locations in- and upstream of the cavity, including cases with wall-normal air injection upstream of the cavity to simulate fuel injection. Addition of flow distortion altered the size and shape of the primary recirculation region within the cavity by deflecting the bounding shear layer: the recirculation region was compressed by shock impingement upstream of the cavity, and shock impingement on the cavity itself expanded it. Air injection upstream of the cavity thickened the shear layer and produced a stronger effect on velocity direction than magnitude, preventing the formation of a large-scale recirculation region in two of the three shock locations studied. Flow distortion and upstream air injection both increased flow unsteadiness, with the greatest increases occurring in the shear layer and above the cavity closeout ramp. Additionally, results suggest the formation of spanwise secondary flow patterns that may account for flow nonuniformities observed in prior studies. This work presents the first velocimetry characterization of a scramjet cavity flameholder under distorted-flow conditions.