Stephen D. Tse

Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atmospheres

Recognizing that previous experimental studies on constant-pressure, outwardly propagating, Spherical flames with imaging capability were limited to pressures less than about 5 atm, and that pressures within internal combustion engines are substantially higher, a novel experimental apparatus was designed, to extend the environmental pressure to 60 atm. Results substantiate previous observations of the propensity of cell formation over the flame surface due to hydrodynamic and diffusive-thermal instabilities and provide convincing evidence that wrinkled flame is the preferred mode of propagation in hydrogen/air mixtures in environments with pressures above only a few atmospheres. It is further shown that, by using helium as the diluent, and by reducing the oxygen concentration of the combustible, diffusional-thermal instability can be mostly suppressed and the hydrodynamic instability delayed. Stretch-free laminar flame speeds were subsequently determined for such smooth flames up to 20 atm and were compared with the calculated values, allowing for detailed chemistry and transport.

Outward propagation, burning velocities, and chemical effects of methane flames up to 60 ATM

Using a specially designed high- and constant-pressure combustion chamber, the propagation and morphology of spark-ignited expanding spherical methane flames were imaged using schlieren cinematography and a high-speed digital camera. Stretch-free laminar burning velocities were subsequently determined for methane/air flames up to 20 atm and methane/oxygen/helium flames up to 60 atm. Computational simulation using GRI-MECH 3.0 showed satisfactory agreement with the experimental data up to 20 atm, and moderate deviation for pressures above 40 atm. Markstein lengths, global activation energies, and overall reaction orders were also determined as functions of pressure, with the latter two parameters exhibiting non-monotonic behavior caused by the changeover from H-O2 to HO2 chemistry similar to that of the explosion limits of homogeneous hydrogen/oxygen mixtures.

Flame synthesis of aligned tungsten oxide nanowires

Aligned single-crystal WO2.9 nanowires are grown directly from tungsten substrates at high rates using a flame synthesis method. The nanowires have diameters of 20–50nm, lengths >10μm, coverage density of 109–1010cm−2, and growth rates >1μm∕min. Growth occurs by the vapor-solid mechanism, with local gas-phase temperature (∼1720K) and chemical species (O2, H2O, and H2) strategically specified at the substrate for self-synthesis. Advantages of this synthesis method are reduced processing times, absence of necessity for substrate pretreatment or catalysts, scalability for large-area surface coverage, high purity and yield of oriented nanowires, and continuous processing conditions.

Controlling mechanisms in the transition from smoldering to flaming of flexible polyurethane foam

Experiments have been conducted to study the controlling mechanisms involved in the transition from smoldering to flaming of a porous combustible material, flexible polyurethane foam, with an air flow forced across one of the porous-fuel surface. The experiments are performed in a small-scale, vertical, combustion wind tunnel, with the air flow forced upward and parallel to an exposed foam surface that forms one of the walls of the tunnel test section. Smolder is initiated at the bottom of the sample and propagates upward in the same direction as the forced and buoyancy-induced flows. The resulting smolder is therefore two-dimensional and forward. Thermocouple measurements of the foam interior, along with visual observations, when compared with schlieren interferometry images of the gas phase at the porous-fuel/air-flow interface, indicate that transition to flaming occurs inside the hot char region below the smokler front and not at the inteface. An innovative ultrasonic imaging, technique is employed to track the evolution of the char permeability in real time, evincing that the char continues to react and increase substantially in permeability long after the primary smolder front has passed. The ongoing heterogeneous reactions in the char region result in the formation of large voids that provide locations for the onset of homogeneous gas-phase reactions. Furthermore, the higher permeability of the char favors the flow of oxidizer, as well as pyrolysis vapors produced by the primary smolder reaction, into the char interior, which, in conjunction with the reduced heat losses, leads to the onset of a homogeneous gas-phase ignition. This process results in the transition from slow smoldering to fast, exothermic, gas-phase reactions, which rapidly engulfs the entire fuel sample in flames.

Optically accessible high-pressure combustion apparatus

The design and operation of a novel optically accessible high-pressure combustion apparatus is presented. The apparatus provides optical access for the direct observation of the morphology and development of premixed reaction fronts at elevated pressures. A chamber-in-chamber design with an innovative connecting system allows for safe, constant-pressure measurements, alleviating the extreme overpressures encountered in high-pressure combustion processes within closed bombs. Auxiliary design features include gap-adjustable electrodes for spark ignition and ports for jet stirring. As a result, the apparatus is well suited for the study of laminar premixed flames, flame instabilities, turbulent flames, and detonations. Results from the study of centrally ignited hydrogen and methane fuels in oxygen-inert mixtures up to 60 atm initial pressure demonstrate the suitability of the apparatus for high-pressure combustion experiments.

Microgravity Burner-Generated Spherical Diffusion Flames: Experiment and Computation

Abstract Microgravity experiments were conducted in the 2.2-s drop-tower facility at the NASA Glenn Research Center to study the transient response of the burner-generated spherical diffusion flame caused by its initial displacement from the steady-state position. The experiment involved issuing H2/CH4/inert mixtures of constant fuel mass flow rates from a bronze, porous, 1.27-cm-diameter, spherical burner into atmospheric air. The experimental results on the flame trajectory were found to agree well with those obtained through fully transient computational simulation with detailed chemistry and transport, and appropriate initial conditions. Furthermore, although steady-state behavior should exist for such flames, the experimental and computational results indicated that it cannot be reached within the 2.2-s microgravity duration for the fuels and mass-flow rates tested. To assess the role of radiation on the flame dynamics and extinction, computations were performed without radiation, with radiation employing the optically thin approximation, and with radiation utilizing a detailed emission/absorption statistical narrow band (SNB) model. The computation showed that while the influence of radiative heat loss on the position of the flame is small, proper consideration of radiative effects is crucial in assessing the state of flame extinction. Specifically, while all simulations of the experimental cases studied incorporating radiative heat loss revealed that the flame extinguishes well before the attainment of steady state, simulations accounting for gaseous reabsorption of radiative emissions were required to adequately represent the experiments in terms of extinction time, with the optically thin simulations predicting premature extinction during the flame expansion process. Effects of heat loss to the porous burner were also examined, and the lack of correspondence between the visible flame luminosity and flame strength, as related to flame temperature and heat release rate, was noted.

A computational study of oscillatory extinction of spherical diffusion flames

Abstract The transient behavior of burner-supported spherical diffusion flames was studied in the transport-induced limit of low mass flow rate and the radiation-induced limit of high mass flow rate which characterize the isola response of flame extinction. Oscillatory instability was observed near both steady-state extinction limits. The oscillation typically grows in amplitude until it becomes large enough to extinguish the flame. The oscillatory behavior was numerically observed using detailed chemistry and transport for methane (50%CH 4 /50%He into 21%O 2 /79%He) and hydrogen (100% H 2 into 21%O 2 /79%He) diffusion flames where the fuel was issued from a point source, and helium was selected as an inert to increase the Lewis number, facilitating the onset of oscillation. In both methane and hydrogen flames, the oscillation always leads to extinction, and no limit cycle behavior was found. The growth rate of the oscillation was found to be slow enough under certain conditions to allow the flame to oscillate for over 450 s, suggesting that such oscillations can possibly be observed experimentally. For the hydrogen flames, however, the frequency of oscillation near the transport-induced limit is much larger, approximately 60 Hz as compared to 0.35 Hz for the methane flame, and the maximum amplitude of temperature oscillations was about 5 K. The distinctively different structures of the hydrogen and methane flames suggest that while both instabilities are thermal-diffusive in origin, oscillations in the hydrogen flames resemble those of premixed flames, while oscillations in the methane flames are non-premixed in character.

Role of dipole–dipole interaction on enhancing Brownian coagulation of charge-neutral nanoparticles in the free molecular regime

In contrast to van der Waals (vdW) forces, Coulombic dipolar forces may play a significant role in the coagulation of nanoparticles (NPs) but has received little or no attention. In this work, the effect of dipole-dipole interaction on the enhancement of the coagulation of two spherically shaped charge-neutral TiO(2) NPs, in the free molecular regime, is studied using classical molecular dynamics (MD) simulation. The enhancement factor is evaluated by determining the critical capture radius of two approaching NPs for different cases of initial dipole direction with respect to path (parallel∕perpendicular) and orientation with respect to each other (co-orientated∕counterorientated). As particle diameter decreases, the enhancement of coagulation is augmented as the ratio of dipole-dipole force to vdW force becomes larger. For 2-nm TiO(2) NPs at 273 K, the MD simulation predicts an average enhancement factor of about 8.59, which is much greater than the value of 3.78 when only the vdW force is considered. Nevertheless, as temperature increases, the enhancement factor due to dipole-dipole interaction drops quickly because the time-averaged dipole moment becomes small due to increased thermal fluctuations (in both magnitude and direction) of the instantaneous dipole moment.

An application of ultrasonic tomographic imaging to study smoldering combustion

An ultrasonic imaging technique has been developed and applied to examine smoldering combustion within a permeable medium. The technique provides information about local permeability variations within a smoldering sample, which can, in turn, be interpreted to visualize the propagation of the smolder reaction. The method utilizes the observation that transmission of an ultrasonic signal through a porous material increases with increasing permeability. Since a propagating smolder reaction leaves behind a char that is higher in permeability than the original (unburnt) material, ultrasonic transmission can be employed to monitor smolder progress. Additionally, the technique allows observation of the evolution of the char (i.e., material left by the smolder reaction), which, in certain circumstances, can continue to increase in permeability, due to secondary reactions (either oxidative or pyrolytic in nature). Experiments have been conducted, applying the technique to smoldering combustion in a two-dimensional geometry with line-of-sight imaging. For axisymmetric configurations, tomographic techniques have been implemented, providing three-dimensional mappings of the smolder front, as well as visualization of the smolder process itself. The results have furthered the understanding of two-dimensional smolder, and have been especially informative in identifying the controlling mechanisms leading to the transition from smoldering to flaming combustion.

Combined Flame and Electrodeposition Synthesis of Energetic Coaxial Tungsten-Oxide/Aluminum Nanowire Arrays

A nanostructured thermite composite comprising an array of tungsten-oxide (WO2.9) nanowires (diameters of 20-50 nm and lengths of >10 μm) coated with single-crystal aluminum (thickness of ~16 nm) has been fabricated. The method involves combined flame synthesis of tungsten-oxide nanowires and ionic-liquid electrodeposition of aluminum. The geometry not only presents an avenue to tailor heat-release characteristics due to anisotropic arrangement of fuel and oxidizer but also eliminates or minimizes the presence of an interfacial Al2O3 passivation layer. Upon ignition, the energetic nanocomposite exhibits strong exothermicity, thereby being useful for fundamental study of aluminothermic reactions as well as enhancing combustion characteristics.