Paul E. DesJardin

Mechanistic Model for Aluminum Particle Ignition and Combustion in Air

A mechanistic model for the ignition and combustion of an isolated aluminum particle burning in air is presented. The model consists of two stages, ignition and combustion. In the ignition stage, melting and heterogeneous surface reactions (HSR) are assumed to occur until a predefined transition temperature of the oxide is attained. In the combustion stage, a quasi-steady state diffusion flame is assumed, and a new conserved scalar formulation is presented that accounts for the deposition of metal oxide on the surface of the molten aluminum. A system of nonlinear ordinary differential equations that describes each stage self-consistently with the gas-phase analysis is developed. Representative results are presented for a range of ambient temperature conditions and compared to experimental measurements. Predictions of overall burn rates, particle velocity, and flame radius show good agreement with experimental data. Also discussed is the extension of the conserved scalar approach to include a more generalized oxidizing environment as well as HSR from nitride reactions during the quasi-steady burning stage.

Modeling of particle compressibility and ignition from shock focusing

Two-phase shock-driven reacting flow simulations are conducted to determine the post-detonation shock-focusing ignition and burning of aluminum particle mixtures. A model for aluminum particles that accounts for material compressibility from shock heating and expansion is presented. The Lagrangian description of the particles is incorporated into an Eulerian description of the gas phase resulting in a fully compressible, two-way coupled simulation. Simulations are conducted of an isolated explosive located near a corner to promote ignition of the particles from shock focusing. Parametric studies are conducted to determine the effects of equivalence ratio, particle size, and charge placement, on the post-detonation pressure and impulse. Results highlight the importance of the timing and position of the shock focusing event relative to the local mixture equivalence ratio that results in an optimal equivalence ratio which maximizes impulse for the geometry considered.

A Dilute Spray Model for Fire Simulations: Formulation, Usage and Benchmark Problems

The focus of this work is to develop a two-phase spray model for application to unsteady fire simulation for the dispersion of dilute liquid fuel or fire suppressant sprays. The model is based on a stochastic separated flow (SSF) approach for which droplet transport equations are integrated in time to account for mass, momentum and energy transfer to the liquid phase. Turbulence models for parcel and sub-parcel droplet dispersion, spray breakup and spray collision are also developed and implemented. Two-way coupling between the liquid spray and the gas phase is accomplished through a numerical sub-cycling procedure. A strategy plan for spray model verification and validation is summarized and results presented for selected cases. The problems examined thus far indicate that the approach provides a robust and accurate means to solve dispersed phase spray transport problems for application to fire phenomena.

Towards a Mechanistic Model for Aluminum Particle Combustion

A relatively simple mechanistic model for the combustion of an aluminum particle in air is presented. The model assumes combustion to occur in two stages. In the first stage, phase transition and heterogeneous surface reactions take place until the melting temperature of the oxide is reached. In the second stage, a quasi-steady state diffusion flame is established allowing for the use of commonly employed flame sheet approximations. Modified Ranz-Marshall and standard drag correlations for a sphere are used to describe the unsteady heating, mass loss rate, and drag of the particle, with the surrounding gas. A system of non-linear ordinary differential equations are formulated and numerically integrated in time for predictions of particle mass, temperature and velocity with, and without, the effects of heterogeneous combustion. Results indicate that, within the assumptions of the current model, the effects of heterogeneous combustion have a significant impact on the overall particle burn time and temperature history for gas temperatures ranging from 1500 to 2500 K. At higher particle Reynolds number, and for temperatures greater than 2500 K, the effects of heterogeneous combustion are not as important and an ignition criterion based on the oxide melting temperature may be sufficient.© 2003 ASME

A Modeling Investigation of Suppressant Distribution from a Prototype Solid-Propellant Gas-Generator Suppression System into a Simulated Aircraft Cargo Bay

One new technology for fire suppressant distribution in total-flooding applications is the solid-propellant gas-generator (SPGG) technology. This article presents experimental and modeling studies of one such prototype system in order to better understand observations in the testing of this system. This particular SPGG system generates fine particles that act to suppress any fire in conjunction with inert gases also generated in the SPGG system. Initial conditions for the simulations are obtained from the available measurements of the prototype system. The modeling provides key information related to the distribution of the particles and their potential effectiveness as a fire suppressant. The primary variable in the SPGG design as identified in the initial measurements, also presented here, was the particle size, with typical particle sizes being measured at 2 and 15 µm. The key modeling result is that there is a tradeoff between the most uniform distribution of particles and the available surface-to-volume ratio for chemical suppression. Information is also provided regarding the thermal dissipation from the SPGG system.

Effect of Surface Catalysis on Measured Heat Transfer in an Expansion Tunnel Facility

T HE effect of gas/surface interaction in the form of surface catalytic recombination occurring in hypersonic flows as the reacting gas behind the bow shock dissociates can have a significant impact on the resulting heat transferred to the surface. Recombination that occurs either in the boundary layer or locally at the surface increases the net level of heat transferred to the surface due to the chemical energy released from the recombination process. As vehicle speed and shock layer temperature increases, the component of heat transfer due to catalytic recombination can become a dominant portion of the total heat load on the surface. To understand the measured heat transfer rate in a ground test experiment, the catalytic phenomena contributing to the heat transferred to the test article must be characterized or found through other means to validate computational fluid dynamic (CFD) simulations or extrapolate the ground test measurements to flight. A number of studies have been made in recent years regarding measurements made in reflected shock tunnels addressing the observed level of catalytic heating in air, carbon dioxide, and nitrogen environments [1–3]. Inmany cases, measured heat transfer rates have been observed to be consistent with very high efficiencies of recombination or nonphysical processes on the surface. One of the main conclusions from these works is that it is unclear if the observations about surface catalysis in reflected shock tunnel environments are in someway linked to the residual nonequilibrium, excitation, or chemical state of the freestream test gas (or at least inadequate description of it using current, state-of-the-art methods) [4]. The catalytic response observed in such tests is suspicious because it is not in line with general expectations from literature.

Near-Surface Carbon-Dioxide Tunable Diode Laser Absorption Spectroscopy Concentration Measurements in Hypervelocity Flow

Measurements of carbon-dioxide concentration are made in the LENS-XX expansion tunnel at the Calspan—University at Buffalo Research Center to investigate the effects of surface catalysis in hypersonic flows. A tunable diode laser absorption spectroscopy setup probes the P36e CO2 absorption line of the ν1+ν3 combination band at 2.7153u2009u2009μm. Numerical simulations are computed with data-parallel line relaxation using the specified reaction efficiency surface catalysis model. Absorption measurements adjacent to the surface of an aluminum cylinder correlate well with simulations indicating low catalytic efficiency at the surface. The velocity, density, freestream temperature, specific enthalpy, and stagnation point pitot pressure of the run are 4.6u2009u2009km/s, 1.53u2009u2009g/m3, 1328.4xa0K, 11.75u2009u2009MJ/kg, and 31.6xa0kPa, respectively. Simulation of the absorption spectrum is performed using a nonhomogeneous line-by-line code using the high-resolution transmission molecular absorption database for spectroscopic parameters. Input...

Near-wall modeling for vertical wall fires using one-dimensional turbulence

This paper presents the simulation of an idealized vertical wall fire using one-dimensional turbulence (ODT) modeling. Near wall gas-phase molecular processes of conduction, gas-phase and shoot reactions, and radiative heat transfer are treated exactly while the effects of turbulent mixing processes are modeled using ODT triplet mapping stirring events that allow the effects of turbulence-chemistry-radiation interactions to be examined. Transport equations for species and temperature are solved using an operator splitting algorithm method that employs a Crank-Nicholson scheme for diffusion/conduction, and the LSODE library to integrate the numerically stiff chemical source terms. Radiative heat transfer is accounted by using a two-flux model. Results are presented for the evolution of turbulent wall fires, with and without the effects of turbulent mixing. The use of the ODT model is shown to capture a laminar to turbulent flow transition resulting in enhanced heat transfer to the wall.Copyright

Emission measurements from high enthalpy flow on a cylinder in the LENS-XX hypervelocity expansion tunnel

The LENS-XX expansion tunnel was used to study high enthalpy flow on a cylindrical test object. The emission from the bow shock region is investigated with a UV-Visible imaging spectrometer for altitudes above 70 km and velocities above 5 km/s. In the LENS-XX facility we have been able to measure the emission from the bow shock for approximately 1 millisecond. In addition flow velocity using a microwave and a Doppler shift technique is reported.

Near-Surface Nitric Oxide Concentration Measurement in the LENS-XX Expansion Tunnel Facility

A high-resolution tunable diode laser absorption spectrometer setup has been constructed and installed in the LENS-XX hypervelocity expansion tunnel at CUBRC. Previous studies in the literature indicate that the heterogeneous recombination of atomic nitrogen and oxygen into nitric oxide (NO) is an important catalytic pathway in hypervelocity flows. Concentration measurements of NO along the surface of a 4” long, 2” diameter stainless steel cylinder are made using the fundamental vibrational-rotational absorption feature in the mid IR region located at 1835.56 cm. These near-surface measurements are used to describe the catalytic reactivity of the gas-surface interface. Data traces obtained using both a single-element and multi-element detector are discussed.