Chih-Jen Sung

Structure, aerodynamics, and geometry of premixed flamelets ☆

Abstract Recent advances in the understanding of the structure, dynamics, and geometry of laminar premixed flames under the influence of stretch, as manifested by aerodynamic straining, flame curvature, and flame/flow unsteadiness, are reviewed and presented in a tutorial manner. The discussion first treats the flame as a structureless surface which propagates into the fresh mixture with a constant velocity—the laminar flame speed, and the phenomena of cusp formation and volumetric burning rate augmentation through flame wrinkling are demonstrated. It is then shown that by considering the effects of stretch on the flame structure, and by allowing for mixture nonequidiffusion, the flame responses, especially the flame speed, can be quantitatively as well as qualitatively modified. By using the stretch-affected flame speed, we then describe the phenomena of cusp broadening, of tip opening of the Bunsen flame, and of the intrinsic hydrodynamic, body-force and diffusional—thermal modes of flamefront cellular instabilities. Additional topics covered include forced and intrinsic oscillatory flame dynamics, and quantitative extraction of the global flame parameters represented by the activation energy, the Markstein length, and the Lewis number.

Dynamics of weakly stretched flames: quantitative description and extraction of global flame parameters

Generalized expressions for the flame response to weak stretch rate variations were derived based on an integral analysis. Together with values of the laminar flame speed, laminar flame thickness, and the one-step overall reaction order and activation energy determined from the computational results of the one-dimensional planar flame, these expressions for the stretched flames were then used to correlate the computational results of the spherical outwardly propagating, spherical inwardly propagating, and counterflow hydrogen/air and propane/air flames. These correlations yielded the laminar flame speeds through linear extrapolation to zero stretch rate, the Markstein lengths representing the sensitivity of the flame response to stretch rate, and the flame Lewis number. Furthermore, it is shown that the extracted Markstein lengths and Lewis numbers from the three flame configurations are largely the same for given equivalence ratio and system pressure, and that these Lewis numbers also agree well with those predicted from the two-reactant flame theory of Joulin and Mitani. The feasibility of a priori quantitative determination of stretch effects on laminar premixed flames is suggested.

Skeletal mechanism generation for surrogate fuels using directed relation graph with error propagation and sensitivity analysis

A novel implementation for the skeletal reduction of large detailed reaction mechanisms using the directed relation graph with error propagation and sensitivity analysis (DRGEPSA) is developed and presented with examples for three hydrocarbon components, n-heptane, iso-octane, and n-decane, relevant to surrogate fuel development. DRGEPSA integrates two previously developed methods, directed relation graph-aided sensitivity analysis (DRGASA) and directed relation graph with error propagation (DRGEP), by first applying DRGEP to efficiently remove many unimportant species prior to sensitivity analysis to further remove unimportant species, producing an optimally small skeletal mechanism for a given error limit. It is illustrated that the combination of the DRGEP and DRGASA methods allows the DRGEPSA approach to overcome the weaknesses of each, specifically that DRGEP cannot identify all unimportant species and that DRGASA shields unimportant species from removal. Skeletal mechanisms for n-heptane and iso-octane generated using the DRGEP, DRGASA, and DRGEPSA methods are presented and compared to illustrate the improvement of DRGEPSA. From a detailed reaction mechanism for n-alkanes covering n-octane to n-hexadecane with 2115 species and 8157 reactions, two skeletal mechanisms for n-decane generated using DRGEPSA, one covering a comprehensive range of temperature, pressure, and equivalence ratio conditions for autoignition and the other limited to high temperatures, are presented and validated. The comprehensive skeletal mechanism consists of 202 species and 846 reactions and the high-temperature skeletal mechanism consists of 51 species and 256 reactions. Both mechanisms are further demonstrated to well reproduce the results of the detailed mechanism in perfectly-stirred reactor and laminar flame simulations over a wide range of conditions. The comprehensive and high-temperature n-decane skeletal mechanisms are included as supplementary material with this article.

Determination of laminar flame speeds using digital particle image velocimetry: Binary Fuel blends of ethylene, n-Butane, and toluene

The atmospheric laminar flame speeds of mixtures of air with ethylene, n -butane, toluene, ethylene- n -butane, ethylene-toluene, and n -butane-toluene were experimentally and computationally investigated over an extended range of equivalence ratios. Binary fuel blends with 1:1, 1:2, and 2:1 molar ratios were examined. Experimentally, the laminar flame speeds were determined using digital particle image velocimetry (DPIV). Since the use of DPIV enables the mapping of the two-dimensional flow field adhead of the flame, the reference speed based on the minimum axial velocity point as well as the imposed strain rate can be identified simultaneously. The latter can now be unambiguously determined by the radial velocity gradient at the minimum velocity point. By systematically varying the imposed strain rate, the corresponding laminar flame speed was obtained through nonlinear extrapolation to zero strain rate. The associated experimental accuracy of the DPIV measurements was also assessed and discussed. Computationally, the laminar flame speeds were simulated for all single-component fuel/air and binary fuel blend/air mixtures with a detailed kinetic model. Comparison of experimental and computed flame speeds shows generally good agreement. A semiempirical mixing rule was developed. The mixing rule which requires only the knowledge of the flame speeds and flame temperatures of the individual fuel constituents, is shown to provide acurate estimates for the laminar flame speeds of binary fuel blends under the conditions tested.

A RAPID COMPRESSION MACHINE FOR CHEMICAL KINETICS STUDIES AT ELEVATED PRESSURES AND TEMPERATURES

A rapid compression machine (RCM) has been designed and fabricated for the purpose of chemical kinetics studies at elevated pressures and temperatures. The present RCM is pneumatically driven and hydraulically actuated and stopped. Stroke of the machine varies from 7 to 10 inches and clearance is also adjustable. Compression ratio of up to 21 can be obtained. The optically-accessible reaction chamber is equipped with sensors for the measurements of pressure and temperature. In addition, a rapid sampling apparatus is incorporated in the reaction chamber for determining species concentration at specific post-compression time. A deliberately machined crevice on the cylindrical surface of the piston has been optimized, using STAR-CD CFD package, in order to suppress the formation of the roll-up vortex and provide a homogeneous core of reaction mixture. Temperature mapping using planar laser induced fluorescence of acetone shows that roll-up vortex is indeed suppressed by using the present creviced piston. Experiments with either inert gases or reactive mixtures demonstrate the reproducibility of pressure traces. Compression process is also shown to be very rapid and free from any significant mechanical vibrations. Measurements show that highly repeatable compressed conditions of up to 50 bar and greater than 1000 K can be obtained. A numerical model accounting for heat loss is also developed to simulate the RCM data. This work documents the design and operation of the present RCM as well as establishes its suitability for combustion studies.

Augmented reduced mechanisms for NO emission in methane oxidation

Abstract A previously derived 12-step, 16-species augmented reduced mechanism (ARM), based on GRI-Mech 1.2, was shown to be comprehensive for methane oxidation at the levels of global response as well as detailed flame structure. The present study updates and extends this effort by basing the reduction on the recently released GRI-Mech 3.0 and by including the description of NOx formation. Specifically, by assuming all the nitrogen-containing species (except N 2 ) are in steady state, an equivalent 12-step ARM was developed. Subsequently, a 14-step and a 15-step ARMs were derived to account for NO formation. The 14-step ARM is basically the 12-step ARM plus two more steps that respectively describe the thermal, prompt, and nitrous oxide mechanisms, and the prompt mechanism. Further inclusion of NH 3 -related reactions yielded the 15-step ARM. A 17-step ARM was also developed to account for the additional emissions of nitrogen dioxide and nitrous oxide. It is shown that, by including such optimum numbers of non-steady-state intermediates for the various mechanisms the present ARMs exhibit good to excellent performance in predicting a wide range of combustion phenomena under extensive thermodynamic parametric variations.

Heat Transfer of Aviation Kerosene at Supercritical Conditions

The heat transfer characteristics of China no. 3 kerosene were investigated experimentally and analytically under conditions relevant to a regenerative cooling system for scramjet applications. A test facility developed for the present study can handle kerosene in a temperature range of 300-1000 K, a pressure range of 2.6-5 MPa, and a mass How rate range of 10-100 g/s. In addition, the test section was uniquely designed such that both the wall temperature and the bulk fuel temperature were measured at the same location along the flowpath. The measured temperature distributions were then used to analytically deduce the local heat transfer characteristics. A 10-component kerosene surrogate was proposed and employed to calculate the fuel thermodynamic and transport properties that were required in the heat transfer analysis. Results revealed drastic changes in the fuel flow properties and heat transfer characteristics when kerosene approached its critical state. Convective heat transfer enhancement was also found as kerosene became supercritical. The heat transfer correlation in the relatively low-fuel-temperature region yielded a similar result to other commonly used jet fuels, such as JP-7 and JP-8, at compressed liquid states. In the high-fuel-temperature region, near and beyond the critical temperature, heat transfer enhancement was observed; hence, the associated correlation showed a more significant Reynolds number dependency.

Laminar flame speeds of preheated iso-octane /O2/N2 and n-heptane /O2/N2 mixtures

Laminar flame speed measurements are carried out for premixed i so-octane/air and n-heptane/air mixtures under conditions of atmospheric pressure, equivalence ratios ranging from 0.7 to 1.4, and unburned mixture temperatures of 298, 360, 400, and 470 K using the counterflow flame technique. These experiments employ the digital particle image velocimetry technique to characterize the two-dimensional flow field upstream of the flame. As such, the reference stretch-affected flame speed and the imposed stretch rate can be simultaneously determined. By systematically varying the imposed stretch rate, the corresponding laminar flame speed is obtained by linearly extrapolating to zero stretch rate. In addition, the effect of nitrogen dilution level on the laminar flame speed is investigated by varying the nitrogen molar percentage in the oxidizer mixture from 78.5 to 80.5%. These results are further used for the determination of overall activation energies at different equivalence ratios. The experimental laminar flame speeds are subsequently compared with the computed values using two iso-octane reaction mechanisms and two n-heptane reaction mechanisms available in the literature, followed by discussion and sensitivity analysis.

Investigation of Vaporized Kerosene Injection and Combustion in a Supersonic Model Combustor

Injection and combustion of vaporized kerosene was experimentally investigated in a Mach 2.5 model combustor at various fuel temperatures and injection pressures. A unique kerosene heating and delivery system, which can prepare heated kerosene up to 820 K at a pressure of 5.5 MPa with negligible fuel coking, was developed. A three-species surrogate was employed to simulate the thermophysical properties of kerosene. The calculated thermophysical properties of surrogate provided insight into the fuel flow control in experiments. Kerosene jet structures at various preheat temperatures injecting into both quiescent environment and a Mach 2.5 crossflow were characterized. It was shown that the use ofvaporized kerosene injection holds the potential of enhancing fuel-air mixing and promoting overall burning. Supersonic combustion tests further confirmed the preceding conjecture by comparing the combustor performances of supercritical kerosene with those of liquid kerosene and effervescent atomization with hydrogen barbotage. Under the similar flow conditions and overall kerosene equivalence ratios, experimental results illustrated that the combustion efficiency of supercritical kerosene increased approximately 10-15% over that of liquid kerosene, which was comparable to that of effervescent atomization.

On the structure of nonsooting counterflow ethylene and acetylene diffusion flames

The structures of ethylene/oxygen/nitrogen and acetylene/oxygen/nitrogen diffusion flames in the counterflow configuration were investigated experimentally and computationally. The temperature and major species concentration profiles were measured with spontaneous Raman scattering. The experimental situations were computationally simulated with detailed reaction mechanisms and transport properties. The kinetic mechanism was based on GRI-Mech, with modifications to predict more closely the adiabatic flame speeds of ethylene/air and acetylene/air mixtures, and with additional description of higher hydrocarbon formation and oxidation up to C6 species. The numerical predictions were found to be in reasonably good agreement with the experiment. Both experimental and computational results indicate that acetylene is the major intermediate species in the ethylene flame, having a significant influence on the heat release, overall fuel destruction, and molecular mass growth. The reaction pathways leading to benzene formation in these flames were examined computationally, with the goal of achieving a better understanding of soot nucleation in diffusion flames.