Sejin Kwon

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Abstract Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H2/O2/N2 mixtures at values of flame stretch up to 7600 s−1, and existing measurements for C3H8/O2/N2 mixtures at values of flame stretch up to 900 s−1. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H2/O2/N2 mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.

Design, fabrication and testing of a catalytic microreactor for hydrogen production

A catalytic microreactor for hydrogen production was fabricated by anisotropic wet etching of photosensitive glass, which enables it to be a structure with high tight tolerance and high aspect ratio. As a reactor structure, a microchannel was used for improving heat and mass transfer in the reactor. The primary fuel source is methanol for a mobile device. Endothermic catalytic steam reforming of methanol was chosen for producing gaseous hydrogen. The Cu-based catalyst, Cu/ZnO, was prepared by the co-precipitation method and coated on the surface of the microchannel for methanol steam reforming. An overall microfabrication process was established for a MEMS-based catalytic microreactor. The fabricated reactor has a volume of 1.8 cm3 including the volume of the reaction chamber 0.3 cm3 and produced dry reformate with high hydrogen content, 73%. The hydrogen flow was 4.16 ml min−1, which can generate a power output of 350 mWe for a fuel cell.The page numbers of this article were corrected on 24 July 2006. The corrected electronic version is identical to the print version.

Flame surface properties of premixed flames in isotropic turbulence: Measurements and numerical simulations

Abstract An experimental and theoretical investigation of free turbulent premixed flames propagating in isotropic turbulence at neutrally stable preferential diffusion conditions is described. Experiments were limited to the wrinkled thin laminar flamelet regime and involved mixtures of hydrogen, air, and nitrogen ignited within a fan-stirred combustion chamber. Measurements included flame tomography for flame surface statistics and two-point laser velocimetry for unburned gas turbulence properties. Flame surface properties were numerically simulated using a two-dimensional flame propagation algorithm combined with statistical time series simulation of unburned gas velocities along the flame surface. Measurements showed progressively increasing flame radius fluctuations, flame surface fractal dimensions, and turbulent/laminar flame perimeters with increasing mean frame radius. The rate of increase of these properties all increased with increasing turbulence intensities relative to laminar flame speed. Simulated flame properties duplicated these trends but underestimated the effects of turbulence—a deficiency mainly attributed to the limitations of a two-dimensional simulation. Extension of the method to a three-dimensional simulation, to obtain a more definitive evaluation of the simulation, appears to be computationally feasible.

Preferential Diffusion Effects on the Surface Structure of Turbulent Premixed Hydrogen/Air Flames

Abstract An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. It was found that flame surface area (and thus the local turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder, with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Finall...

Turbulent Premixed Hydrogen/Air Flames at High Reynolds Numbers

Abstract Measurements of mean and fluctuating reaction progress variable and streamwise velocities are reported for turbulent premixed hydrogen/air flames burning at relatively high Reynolds numbers (up to 1800. based on streamwise r.m.s. velocity fluctuations and integral length scales). A round-jet geometry was used with the flame surrounded by a hot combustion-gas environment at atmospheric pressure. Mixing-limited combustion was achieved: values of r.m.s. velocity fluctuations, normalized by the laminar burning velocity (u/SL), exceeded 15 in some instances. Test conditions included fuel-equivalence ratios of 0.3-3.6 and burner exit Reynolds numbers (based on exit diameter) of 7000-40000 with fully-developed turbulent pipe flow at the burner exit. It was found that effects of diffusive-thermal (preferential-diffusion) phenomena were important, for both stable (fuel-equivalence ratios greater than 1.8) and unstable conditions, even at the present high Reynolds numbers; for example, flame surfaces were ...

Heat transfer and quenching analysis of combustion in a micro combustion vessel

Combustion phenomena in a millimeter scale combustor subjected to an intense heat loss were theoretically investigated. Although there has been much research on micro combustion devices, some basic questions on the combustion processes in such devices are yet to be answered. Two of the most prominent questions are the lower limit of the combustor size, and the combustion efficiency in a millimeter size combustor. As the combustor is reduced to a scale comparable to the laminar flame thickness, the existing theoretical model is not adequate for the prediction of the combustion process inside such a volume. On the other hand, many measurement techniques developed for macro scale combustion phenomena cannot be applied due to the size limitation further complicating the situation. In the present study, a simple theoretical analysis partially relying on experimental data is proposed. The analysis is based on the existing model of the flame propagation in a macro scale closed vessel. While the effect of heat loss to the wall is insignificant and ignored in a macro scale vessel, it is a decisive factor in a micro scale combustor. A heat loss model was derived from the measured data and the governing equation of conservation of mass and energy is integrated with constitutive thermodynamic relationships between gas properties. Calculations were compared with the measured data on quenching and resulted in a satisfactory agreement despite the simple nature of the proposed model.

Fabrication and test of a MEMS combustor and reciprocating device

We propose the concept of a microelectromechanical system (MEMS) scale reciprocating device powered by burnt gas and we have fabricated its prototype to investigate the applicability of the concept in a microscale power generator. In this investigation, combustion in a microscale chamber was studied using thermodynamic principles to estimate theoretical power output. We have carried out an evaluation of relevant subsystems and fabrication processes to realize the heat engine in such a small scale for further development of similar devices. A variable depth microcombustion chamber was built in-house to test the combustion characteristics in a small volume. Measurements include pressure transition after ignition and high-speed flame visualization. Test conditions include combustion chamber depth around the flame quenching distance, below which combustion theory predicts no burning. By analyzing the measured data, the combustion efficiency and available work were obtained and used for the design of a reciprocating MEMS device. The results of the combustion measurements required that the chamber height be 1 mm or more for stable ignition and flame propagation. Based on these findings of the microcombustion experiment, we formulated a MEMS fabrication process and made a reciprocating device. The device has a combustion chamber with a volume of 1 mm3, and the cross section of the cylinder has a rectangular shape with a height of 1 mm and a width of 2 mm. Photosensitive glass was chosen as a structural material. A thick photoresist mold and electroplating were used for constructing the overall structure. A single stroke experiment with hydrogen as a fuel was recorded by a high-speed digital video camera showing piston displacement at reasonable speeds.

Scaling and Evaluation of Pt/Al2O3 Catalytic Reactor for Hydrogen Peroxide Monopropellant Thruster

A scaling methodology of hydrogen peroxide monopropellant thruster is described. As the decomposition process of the hydrogen peroxide on the surface of catalyst bed is extremely complex, empirical method was taken for design purposes. A small-scale thruster was fabricated and important design parameters, including temperature at different locations of the catalyst bed, were measured. Based on the measurement, the catalyst bed size as a function of the propellant flow rate was estimated. Using the scaling methodology, a catalyst bed configuration for a thruster capable of delivering 50 N was estimated. The thruster built on this design produced 42 N at sea level and specific impulse of 123 s.

Viscous potential flow analysis of capillary instability with heat and mass transfer

We carry out the linear viscous-irrotational analysis of capillary instability with heat transfer and phase change. We consider the cylindrical interface shared by two viscous incompressible fluids enclosed by two concentric cylinders. In viscous potential flow, viscosity enters the model through the balance of normal stresses at the interface. We write the dispersion relation from the stability analysis for axisymmetric disturbances in terms of a set of dimensionless numbers that arise in this phase change problem. For the film boiling condition, plots depicting the effect of some of these parameters on the maximum growth rate for unstable perturbations and critical wavenumber for marginal stability are presented and interpreted. Viscous effects of a purely irrotational motion in the presence of heat and mass transfer can stabilize an otherwise unstable gas–liquid interface.

Flame/stretch interactions in laminar and turbulent premixed flames

The flame/stretch interactions of laminar and turbulent premixed flames are considered both experimentally and computationally. Potentially strong effects of flame/stretch interactions due to preferential-diffusion phenomena within practical turbulent premixed flames were suggested by experiments and numerical simulations of spherical outwardly propagating laminar premixed flames. These considerations were limited to conditions where ignition disturbances, pressure variations, intrinsic unsteadiness of propagating spherical flames, and radiative heat losses were small. Flame reactants consisting of H 2 /O 2 /N 2 and several light hydrocarbon/air mixtures were studied for various fuel-equivalence ratios and pressures of 0.5-4.0 atm at normal temperature (298±3K). The measurements and predictions yielded several interesting results, as follows: Flame response to stretch was linear using a local conditions hypothesis to define characteristic flame length and time scales, yielding constant Markstein numbers for given flame conditions; effects of stretch were surprisingly strong with up to 100 percent variations of laminar burning velocities resulting from rather modest stretch rates well below extinction conditions (i.e., Karlovitz numbers less than 0.5); there was a progressive tendency for greater ranges of unstable preferential-diffusion conditions (negative Markstein numbers) as pressures were increased for all reactant mixtures studied; and several contemporary detailed treatments of multicomponent transport and chemical reaction mechanisms yielded reasonably good predictions of laminar burning velocities and their sensitivity to flame stretch due to preferential-diffusion effects. The predictions suggest that the strong sensitivity of the present flames to stretch is mainly caused by preferential diffusion of light radicals and stable species relative to typical stable reaction products and heat, with increased preferential-diffusion instability at elevated pressures resulting from reduced radical concentrations in the reaction zone due to increased radical recombination rates. The potential practical importance of flame/stretch interactions was examined by considering the properties of strongly turbulent premixed flames. These measurements involved premixed H 2 /O 2 /N 2 and C 3 H 8 /air flames propagating in the thin wrinkled flamelet regime within isotropic turbulence. Test conditions included unstable, near-neutral, and stable flames with respect to effects of preferential-diffusion. The experiments yielded several interesting observations, as follows: 1) Rates of turbulent flame propagation progressively decreased as flame stability with respect to preferential-diffusion effects increased even through unstretched laminar burning velocities and turbulence properties were the same; 2) Distortion of the flame surfaces by turbulence as the flames grew caused their fractal dimensions to progressively increase from a value of 2.0, appropriate for a smooth surface, to asymptotic values in the range 2.3-2.4, irrespective of preferential-diffusion stability conditions; 3) Other parameters characterizing the extent of distortion of the flame surfaces showed no tendency to approach asymptotic values for available observation times, however, raising questions about the existence of steady turbulent flame propagation properties for the present test conditions; and 4) The extent of flame surface distortion progressively increased at a given flame diameter, but decreased at a given time of propagation, as preferential-diffusion stability was progressively increased even though unstretched laminar burning velocities and turbulence properties were the same. These flame/stretch interactions in turbulent flames can be explained by noting that stable (unstable) preferential-diffusion conditions tend to retard (enhance) distortion of the flame surface by turbulence for outwardly propagating spherical turbulent premixed flames in much the same way that preferential-diffusion effects interact with small disturbances to yield either stable (unstable) flames for nonturbulent conditions.