arXiv subjects

Forman A. Williams

Publications and source records attributed to Forman A. Williams.

6 recordsLinked to original sources

arXiv2024

Near-limit H$_2$-O$_2$-N$_2$ combustion in nonpremixed counterflow mixing layers

Numerical computations employing detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.

physics.flu-dyn↗

arXiv2024

Influences of stoichiometry on steadily propagating triple flames in counterflows

Most studies of triple flames in counterflowing streams of fuel and oxidizer have been focused on the symmetric problem in which the stoichiometric mixture fraction is $1/2$. There then exist lean and rich premixed flames of roughly equal strengths, with a diffusion flame trailing behind from the stoichiometric point at which they meet. In the majority of realistic situations, however, the stoichiometric mixture fraction departs appreciably from unity, typically being quite small. With the objective of clarifying the influences of stoichiometry, attention is focused on one of the simplest possible models, addressed here mainly by numerical integration. When the stoichiometric mixture fraction departs appreciably from $1/2$, one of the premixed wings is found to be dominant to such an extent that the diffusion flame and the other premixed flame are very weak by comparison. These curved, partially premixed flames are expected to be relevant in realistic configurations. In addition, a simple kinematic balance is shown to predict the shape of the front and the propagation velocity reasonably well in the limit of low stretch and low curvature.

physics.flu-dyn↗

arXiv2024

A model for the constant-density boundary layer surrounding fire whirls

This paper investigates the steady axisymmetric structure of the cold boundary-layer flow surrounding fire whirls developing over localized fuel sources lying on a horizontal surface. The inviscid swirling motion found outside the boundary layer, driven by the entrainment of the buoyant turbulent plume of hot combustion products that develops above the fire, is described by an irrotational solution, obtained by combining Taylor's self-similar solution for the motion in the axial plane with the azimuthal motion induced by a line vortex of circulation $2 πΓ$. The development of the boundary layer from a prescribed radial location is determined by numerical integration for different swirl levels, measured by the value of the radial-to-azimuthal velocity ratio $σ$ at the initial radial location. As in the case $σ=0$, treated in the seminal boundary-layer analysis of Burggraf et al. (Phys. Fluids, 1971), the pressure gradient associated with the centripetal acceleration of the inviscid flow is seen to generate a pronounced radial inflow. Specific attention is given to the terminal shape of the boundary-layer velocity near the axis, which displays a three-layered structure that is described by matched asymptotic expansions. The resulting composite expansion, dependent on the level of ambient swirl through the parameter $σ$, is employed as boundary condition to describe the deflection of the boundary-layer flow near the axis to form a vertical swirl jet. Numerical solutions of the resulting non-slender collision region for different values of $σ$ are presented both for inviscid flow and for viscous flow with moderately large values of the controlling Reynolds number $Γ/ν$. The velocity description provided is useful in mathematical formulations of localized fire-whirl flows, providing consistent boundary conditions accounting for the ambient swirl level.

physics.flu-dyn↗

arXiv2024

Experimental and Computational Investigation of the Influence of Ethanol on Auto-ignition of n-Heptane in Non-Premixed Flows

Experimental and computational investigations are carried out to elucidate the influence of ethanol addition on n-heptane auto-ignition in counterflows. An axisymmetric stream of air, temperature gradually increased, is directed onto the surface of an evaporating pool of a liquid fuel. The air-stream temperature at auto-ignition is measured at various strain rates for n-heptane, ethanol, and various n-heptane/ethanol mixtures. Critical conditions for auto-ignition are predicted employing San Diego Mechanism for both fuels and fuel mixtures, and the results are compared with measurements. Measurements and predictions show that low-temperature chemistry (LTC) plays a significant role in promoting auto-ignition of n-heptane at low strain rates, but there is insufficient residence time at high strain rates for LTC to take place, so auto-ignition is promoted by high-temperature chemistry. Experimental and computational results show addition of ethanol inhibits LTC of n-heptane. To identify the responsible elementary steps, computations are performed to identify those dominate O2 consumption and contribute to the temperature rise in the reaction zone for n-heptane and n-heptane/ethanol mixtures at low strain rates. For n-heptane, O2 is consumed primarily by the low-temperature steps that result in ketohydroperoxide; the temperature rise is produced by subsequent LTC steps. For the mixtures, a key step consuming O2 is O2 + CH3CHOH = HO2 + CH3CHO, and the heat release occurs through the classical high-temperature reaction mechanism. Thus, the inhibition of auto-ignition that is observed to occur when ethanol is added to n-heptane arises from the competition for O2 between this step and the LTC addition of O2 to the heptyl radical and to the radical arising from the subsequent isomerization, for n-heptane.

physics.chem-ph↗

arXiv2022

A Multipurpose Reduced Mechanism for Ethanol Combustion

New multipurpose skeletal and reduced chemical-kinetic mechanisms for ethanol combustion are developed, along the same philosophical lines followed in our previous work on methanol. The resulting skeletal mechanism contains 66 reactions, only 19 of which are reversible, among 31 species, and the associated reduced mechanism contains 14 overall reactions among 16 species, obtained from the skeletal mechanism by placing \ce{CH3CHOH}, \ce{CH2CH2OH}, \ce{CH3CO}, \ce{CH2CHO}, \ce{CH2CO}, \ce{C2H3}, \ce{C2H5}, \ce{C2H6}, \ce{S-CH2}, \ce{T-CH2}, \ce{CH4}, \ce{CH2OH}, \ce{CH3O}, \ce{HCO}, and \ce{O} in steady state. For the reduced mechanism, the steady-state relations and rate expressions are arranged so that computations can be made sequentially without iteration. Comparison with experimental results for autoignition, laminar burning velocities, and counterflow flame structure and extinction, including comparisons with the 257-step, 54-species detailed San Diego Mechanism and five other mechanisms in the literature, support the utility of the skeletal and reduced mechanisms, showing, for example, that, in comparison with the San Diego mechanism, they decrease the computational cost by 70\% and 80\%, respectively. Measures of computation times and of extents of departures from experimental values are defined and employed in evaluating results. Besides contributing to improvements in understanding of the mechanisms, the derived simplifications may prove useful in a variety of computational studies.

physics.chem-ph↗

arXiv2022

Minimum Ignition Energy of Methanol-Air Mixtures

A method for computing minimum ignition energies for gaseous fuel mixtures with detailed and reduced chemistry, by numerical integration of time-dependent conservation equations in a spherically symmetrical configuration, is presented and discussed, testing its general characteristics and accuracy. The method is applied to methanol-air mixtures described by a 38-step Arrhenius chemistry description and by an 8-step chemistry description based on steady-state approximations for reaction intermediaries. Comparisons of predictions with results of available experimental measurements produced reasonable agreements and supported both the robustness of the computational method and the usefulness of the 8-step reduction in achieving accurate predictions.

physics.chem-ph↗