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Stalcup, Erik JamesNumerical Modeling of Upward Flame Spread and Burning of Wavy Thin Solids
Master of Sciences, Case Western Reserve University, EMC - Aerospace Engineering
Flame spread over solid fuels with simple geometries has been extensively studied in the past, but few have investigated the effects of complex fuel geometry. This study uses numerical modeling to analyze the flame spread and burning of wavy (corrugated) thin solids and the effect of varying the wave amplitude. Sensitivity to gas phase chemical kinetics is also analyzed. Fire Dynamics Simulator is utilized for modeling. The simulations are two-dimensional Direct Numerical Simulations including finite-rate combustion, first-order pyrolysis, and gray gas radiation. Changing the fuel structure configuration has a significant effect on all stages of flame spread. Corrugated samples exhibit flame shrinkage and break-up into flamelets, behavior not seen for flat samples. Increasing the corrugation amplitude increases the flame growth rate, decreases the burnout rate, and can suppress flamelet propagation after shrinkage. Faster kinetics result in slightly faster growth and more surviving flamelets. These results qualitatively agreement with experiments.

Committee:

James T'ien (Committee Chair); Joseph Prahl (Committee Member); Yasuhiro Kamotani (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

modeling;simulation;numerical modeling;combustion;computational combustion;direct numerical simulation;flame spread;burning;wavy;corrugated;fire dynamics simulator;FDS;fuel structure;fuel geometry;complex geometry;cardboard;

Ferkul, Paul VincentA model of concurrent flow flame spread over a thin solid fuel
Doctor of Philosophy, Case Western Reserve University, 1993, Mechanical Engineering
A numerical model is developed to examine laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flows. The model provides a more precise fluid-mechanical description of the flame by incorporating an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point. Parabolic equations are used to treat the downstream flame, which has a higher flow Reynolds number. The parabolic and elliptic regions are coupled smoothly by an appropriate matching of boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Steady spread with constant flame and pyrolysis lengths is found possible for thin fuels and this facilitates the adoption of a moving coordinate system attached to the flame with the flame spread rate being an eigenvalue. Calculations are performed in purely forced flow in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Both quenching and blowoff extinction are observed. The results show that as flow velocity or oxygen percentage is reduced, the flame spread rate, the pyrolysis length, and the flame length all decrease, as expected. The flame standoff distance from the solid and the reaction zone thickness, however, first increase with decreasing flow velocity, but eventually decrease very near the quenching extinction limit. The short, diffuse flames observed at low flow velocities and oxygen levels are consistent with available experimental data. The maximum flame temperature decreases slowly at first as flow velocity is reduced, then falls more steeply close to the quenching extinction limit. Low velocity quenching occurs as a result of heat loss. At low velocities, surface radiative loss becomes a significant fraction of the total combustion heat release. In addition, the shorter flame length causes an increase in the fraction of conduction downstream compared to conduction to the fuel. These heat losses lead to lower flame temperatures, and ultimately, extinction. This extinction mechanism differs from that of blowoff, where the flame is unable to be stabilized due to the high flow velocity.

Committee:

James T'ien (Advisor)

Keywords:

model concurrent flow flame spread over thin solid fuel

Sheng-Yen, HsuFlame Spread and Extinction Over Solids in Buoyant and Forced Concurrent Flows: Model Computations and Comparison with Experiments
Doctor of Philosophy, Case Western Reserve University, 2009, EMC - Fluid and Thermal Engineering

A detailed three-dimensional model for steady flame spread over thin solids in concurrent flows is used to compare with existing experiments in both buoyant and forced flows. This work includes (1) several improvements in the quantitatively predictive capability of the model, (2) a sensitivity study of flame spread rate on input parameters, (3) introduction of flame radiation into the buoyant-flow computations and (4) quantitative comparisons with two sets of buoyant upward spread experiments using cellulosic samples and a comparison with forced downwind spread tests using wider cellulosic samples. In additional to sample width and thickness, the model computation and experimental comparison cover a substantial range of environmental parameters such as oxygen percentage, pressure, velocity and gravity that are of interest to the applications to space exploration.

In the buoyant-flow comparison, the computed upward spread rates quite favorably agree with the experimental data. The computed extinction limits are somewhat wider than the experimental limits based on only one set of older test data (the only one available). Comparison of the flame thermal structure (also with this set of older data) shows that the computed flame is longer and there is structure difference in the flame base zone. This is attributed to the sample cracking phenomenon near the fuel burnout, a mechanism not treated in the model. Comparison in forced concurrent flows shows that the predicted spread rates are lower than the experimental ones if the flames are short but higher than the experimental ones if the flames are long. It is believed that the experimental flames may have not fully reached the steady states at the end of 5-second drop.

The effect of gas-phase kinetic rate on concurrent flame spread rates is investigated through the variation of the pre-exponential factor. It is found that flames in forced flow are less sensitive to the change of kinetics than flames in buoyant flow; and narrow samples are more sensitive to the change of kinetics compared with wide samples. The rate of chemical kinetics affects the flame spread rates primarily through two mechanisms: the amount of un-burnt fuel vapors escaping the reaction zone and the induced velocity variation through flame temperature change in the case of the buoyant flames.

Committee:

James S. T'ien, PhD (Advisor); Yasuhiro Kamotani, PhD (Committee Member); Chih-Jen (Jackie) Sung, PhD (Committee Member); Chung-Chiun Liu, PhD (Committee Member); Gary A. Ruff, PhD (Committee Member); David Urban, PhD (Committee Member); Sandra L. Olson, PhD (Committee Member)

Subjects:

Engineering; Mechanical Engineering

Keywords:

model comparison with experiments; concurrent flame spread; buoyant upward spread; chemical kinetics; incomplete combustion; pressure limit; extinction; reaction order; Damkohler number; diffusion flame

Kulis, Michael J.Concentration Measurements During Flame Spread Through Layered Systems in Terrestrial and Microgravity Environments
Doctor of Philosophy, University of Akron, 2008, Chemistry

A diode laser system was utilized to obtain spatially and temporally resolved absorption spectra during flame spread through non-homogenous fuel-air mixtures. The diode laser was wavelength-modulated in order to increase detection sensitiviy of the system. Gas concentrations were determined from the absorption spectra using regression models.

Two fundamentally different systems were studied in this work. Methane concentrations were determined from absorption measurements in a buoyant plume of methane and air formed in a vertical low speed flow tunnel. Methanol and water concentrations were determined from absorption measurements in a system in which a non-uniform methanol-air layer was formed by evaporation along a horizontal gallery floor. In both systems, the layered mixture was ignited and measurements were obtained as the flame propagated. Measurements in the gallery were made in both terrestrial and microgravity environments. Thus, the ability to measure gas concentration using a system that meets the severe restrictions of microgravity research was demonstrated.

Committee:

David Perry, PhD (Advisor); Fletcher Miller, PhD (Advisor); James Hardy, PhD (Committee Member); Jun Hu, PhD (Committee Member); Matthew Espe, PhD (Committee Member); Rex Ramsier, PhD (Committee Member)

Subjects:

Chemistry

Keywords:

flame spread; diode laser; layered systems; microgravity

Jiang, Ching-BiauA model of flame spread over a thin solid in concurrent flow with flame radiation
Doctor of Philosophy, Case Western Reserve University, 1995, Mechanical Engineering
A numerical model is developed to examine steady laminar flame spread and extinction over a thin solid in concurrent flows with flame radiation. The fluid mechanical description in the model includes the elliptic momentum, energy and species equations with a one-step second-order finite rate Arrhenius reaction. The multidimensional nature of radiation field, involving gray absorbing, emitting and nonscattering media (CO2 and H2O), is simulated by the S-N discrete ordinates method. A simplified thermally thin solid phase treatment assumes a zeroth-order pyrolysis relation and includes radiative interaction between the surface and gas phase. Computations are performed for purely forced flow in zero gravity using the oxygen percentage and free stream velocity as parameters. Selected results are presented showing the detailed flame profile, flow structure, and flame spread characteristics. A flammability boundary is determined, which consists of two branches. The low-speed quenching branch is due to radiative losses from both the gas phase and solid surface, and the high-speed blowoff branch is due to inadequate flow residence time. The effect of gas radiation on flame behavior is examined by comparison with results that neglect gas radiation. The results indicate that the influence of gas radiation is important both in the gas phase and at the solid surface. In a low-speed flow, the flame temperature decreases, the flame size shrinks, and the flame spread rate is lowered when gas radiation is included. For high-speed flow, gas-phase radiation cools the flame, but the radiative heat flux feedback is increased in the solid pyrolysis region, increasing the fuel vaporization rate. This in turn results in a higher spread rate for these flames when compared with computed results that neglect gas radiation. With gas radiation, quenching occurs at a higher flow velocity, but the blowoff limit is essentially the same when compared to the model predictions without gas radiation

Committee:

James T'ien (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Flame spread; Concurrent flow; Flame radiation

Hsu, Sheng-YenFlame Spread and Extinction Over Solids in Buoyant and Forced Concurrent Flows: Model Computations and Comparison with Experiments
Doctor of Philosophy, Case Western Reserve University, 2009, EMC - Fluid and Thermal Engineering

A detailed three-dimensional model for steady flame spread over thin solids in concurrent flows is used to compare with existing experiments in both buoyant and forced flows. This work includes (1) several improvements in the quantitatively predictive capability of the model, (2) a sensitivity study of flame spread rate on input parameters, (3) introduction of flame radiation into the buoyant-flow computations and (4) quantitative comparisons with two sets of buoyant upward spread experiments using cellulosic samples and a comparison with forced downwind spread tests using wider cellulosic samples. In additional to sample width and thickness, the model computation and experimental comparison cover a substantial range of environmental parameters such as oxygen percentage, pressure, velocity and gravity that are of interest to the applications to space exploration.

In the buoyant-flow comparison, the computed upward spread rates quite favorably agree with the experimental data. The computed extinction limits are somewhat wider than the experimental limits based on only one set of older test data (the only one available). Comparison of the flame thermal structure (also with this set of older data) shows that the computed flame is longer and there is structure difference in the flame base zone. This is attributed to the sample cracking phenomenon near the fuel burnout, a mechanism not treated in the model. Comparison in forced concurrent flows shows that the predicted spread rates are lower than the experimental ones if the flames are short but higher than the experimental ones if the flames are long. It is believed that the experimental flames may have not fully reached the steady states at the end of 5-second drop.

The effect of gas-phase kinetic rate on concurrent flame spread rates is investigated through the variation of the pre-exponential factor. It is found that flames in forced flow are less sensitive to the change of kinetics than flames in buoyant flow; and narrow samples are more sensitive to the change of kinetics compared with wide samples. The rate of chemical kinetics affects the flame spread rates primarily through two mechanisms: the amount of un-burnt fuel vapors escaping the reaction zone and the induced velocity variation through flame temperature change in the case of the buoyant flames.

Committee:

James S. T'ien, PhD (Advisor); Yasuhiro Kamotani, PhD (Committee Member); Chih-Jen (Jackie) Sung, PhD (Committee Member); Chung-Chiun Liu, PhD (Committee Member); Gary A. Ruff, PhD (Committee Member); David Urban, PhD (Committee Member); Sandra L. Olson, PhD (Committee Member)

Subjects:

Engineering; Mechanical Engineering

Keywords:

model comparison with experiments; concurrent flame spread; buoyant upward spread; chemical kinetics; incomplete combustion; pressure limit; extinction; reaction order; Damkohler number; diffusion flame

Tseng, Ya-TingThree-Dimensional Model of Solid Ignition and Ignition Limit by a Non-Uniformly Distributed Radiant Heat Source
Doctor of Philosophy, Case Western Reserve University, 2011, EMC - Mechanical Engineering

An unsteady three-dimensional numerical model has been built to study ignition, flame decay, and flame growth over a composite solid sample fuel upon non-uniformly distributed radiant heating. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed.

The model is applied to a vertically-oriented sample in a gravitational field similar to the configuration of NASA-STD-6001 Test #1. The sample material is a thin composite solid that consists of 50% combustible and 50% inert. The ignition source is a radiant beam heater with heat flux obeying a Gaussian spatial distribution. No external ignition pilot is introduced.

The computed results provide a detailed sequence of the ignition events including the first appearance of the reaction kernel, the spread and decay of the initial premixed flames, and eventually the formation of an anchoring solid diffusion flame. By varying the radiant heating rate, two ignition modes are identified: reaction initiated at the surface at low heating rates, and reaction initiated in the gas phase at high heating rates. Computed ignition boundaries yield the critical heating rate for ignition, as well as the minimum total energy for solid ignition as a function of the heating rate. Parameters varied in the computations include the shape of the radiant heat source, sample thickness, pressure, and gravity.

Committee:

James S. T'ien (Committee Chair); Iwan Alexander (Committee Member); Yasuhiro Kamotani (Committee Member); David Schiraldi (Committee Member); Gary A. Ruff (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Ignition; Ignition limits; flame spread; flame decay; flame growth; fire growth; material flammability; radiant ignition source

Kleinhenz, Julie EliseFlammability and Flame Spread of Nomex® and Cellulose in Space Habitat Environments
Doctor of Philosophy, Case Western Reserve University, 2006, Aerospace Engineering
In the enclosed environment of a manned space habitat, fire safety becomes a major concern. Though various combinations of pressure and oxygen can be used to support human habitation, the bulk of flammability testing for materials is done at standard atmospheric conditions. This study focuses on the flammability and flame spread in these potential space habitat environments. Pressures were kept below 1 atm, Oxgyen/Nitrogen mixtures between 21% and 30% O2, and gravity levels of 1 g/ge (Earth), 0.38 g/ge (Martian), and 0.16 g/ge (Lunar) were used. Two materials were examined. Part 1 of this report discusses Kimwipes®, a thin tissue paper, which was better suited for the reduced gravity experiments. A scaling relation that predicts upward spread rate as a function of pressure, gravity, and width, is introduced and experimentally verified. Nomex® fabric, a practical material commonly used in the space program, is the focus of Part 2. Nomex® has a lower oxygen limit for upward spread than for downward spread. A unique phenomenon was observed during experimentation, whereby the continually elongating flames broke off in the middle into two smaller upward propagating flames. One flame would then extinguish, and the remaining flame would repeat this cycle. The process was documented in detail and is believed to be a function of a two stage pyrolysis occurring in the fuel.

Committee:

James T'ien (Advisor)

Keywords:

Flame spread; flammability; combustion; Nomex

Endo, MakotoNumerical modeling of flame spread over spherical solid fuel under low speed flow in microgravity: Model development and comparison to space flight experiments
Doctor of Philosophy, Case Western Reserve University, 2016, EMC - Mechanical Engineering
Flame spread over solid fuel presents distinctive characteristics in reduced gravity, especially when the forced flow velocity is low. The lack of buoyancy allows a blue, dim flame to sustain where the induced velocity would otherwise blow it off. At such low velocities, a quenching limit exists where the soot content is low and the effect of radiative heat loss becomes important. The objective of this study is to establish a high fidelity numerical model to simulate the growth and extinction of flame on solid fuels in a reduced gravity environment. The great importance of the spectral dependency of the gas phase absorption and emission were discovered through the model development and therefore, Statistical Narrow-Band Correlated-k (SNB-CK) spectral model was implemented. The model is applied to an experimental con figuration from the recent space experiment, Burning And Suppression of Solids (BASS) project conducted aboard the International Space Station. A poly(methyl methacrylate) (PMMA) sphere (initial diameter of 2cm) was placed in a small wind tunnel (7.6cm x 7.6cm x 17cm) within the Microgravity Science Glovebox where flow speed and oxygen concentration were varied. Data analysis of the BASS experiment is also an important aspect of this research, especially because this is the first space experiment that used thermally thick spherical samples. In addition to the parameters influencing the flammability of thin solids, the degree of interior heat-up becomes an important parameter for thick solids. For spherical samples, not only is the degree of internal heating constantly changing, but also the existence of stagnation point, shoulder, and wake regions resulting in a different local flow pattern, hence a different flame-solid interaction. Parametric studies using the numerical model were performed against (1) chemical reaction parameters, (2) forced flow velocity, (3) oxygen concentration and (4) amount of preheating (bulk temperature of the solid fuel). Flame Spread Rate (FSR) was used to evaluate the transient effect and maximum flame temperature, standoff distance and radiative loss ratio were used to evaluate the spontaneous response of the gas phase to understand the overall response of the burning solid fuel. After evaluating the individual effect of each parameter, the efficacy of each parameter was compared. Selected results of this research are: [1] Experimental data from BASS and numerical simulation both showed that within the time period between ignition until the flame tip reaches the shoulder of the sample, the flame length and time have almost a linear relation. [2] Decreasing forced flow velocity increases the radiative loss ratio whereas decreasing oxygen mole fraction decreases the radiative loss ratio. This fi nding must be considered in the effort to replicate the behavior of flame spread over thick solid fuels in microgravity on earth. [3] Although the standoff distance will increase when the forced flow velocity is decreased as well as when the oxygen mole fraction is decreased, the forced flow velocity has a much stronger effect on the standoff distance than the oxygen mole fraction. [4] Unlike the previous two comparisons, the effect of forced flow velocity and oxygen mole fraction on the maximum flame temperature was at similar level, reduction of either parameter would result in lowering the maximum flame temperature. [5] The effect of preheating on the flame spread rate becomes stronger when either the oxygen flow rate or forced flow velocity becomes larger. Depending on which element is more important, we can distinguish oxygen flow rate driven flame spread from preheating driven flame spread. Findings of this research are being utilized in the design of the upcoming space experiment, Growth and Extinction Limits of solid fuel (GEL) project. This research is supported by the National Aeronautics and Space Administration (NASA). This work made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University and the Ohio Supercomputer Center.

Committee:

James S. T'ien (Committee Chair); Yasuhiro Kamotani (Committee Member); Fumiaki Takahashi (Committee Member); Erkki Somersalo (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

Numerical modeling; flame spread; solid fuel; spherical fuel; microgravity; combustion; space experiment; NASA; GEL; SoFIE; SNB-CK; Radiation; Heat transfer; Finite Element; FEM; FDS; Microgravity Experiment; NASA-STD-6001; BASS; SIFI; FSR; axisymmetric;