Master of Sciences (Engineering), Case Western Reserve University, 2020, EMC - Aerospace Engineering

Microgravity experiments are performed to study wind-assisted flame spread over discrete fuel elements. Ultra-thin cellulose-based fuel segments are distributed uniformly in a low-speed flow and flame spread is initiated by igniting the most upstream fuel segment. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. Flame propagation across a gap only occurs when a burning fuel segment, before it burns out, ignites the subsequent segment. During this process, gaps between samples reduce the fuel load, increasing the apparent flame spread rate and decreasing the heat transfer between adjacent segments. The reduction in heat transfer decreases the solid burning rate. In this study, sample segment length, gap size, and imposed flow velocity are varied to study the impacts on burning characteristics, including propensity of flame spread, flame spread rate, and solid burning rate. Detailed profiles of the transient flame spread process are also presented.

Committee: YaTing Liao (Advisor); Paul Barnhart (Committee Member); Brian Maxwell (Committee Member); Sandra Olson (Committee Member); Paul Ferkul (Committee Member) Subjects: Aerospace Engineering; Engineering

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 n (open full item for complete abstract)

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

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 n (open full item for complete abstract)

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

Master of Sciences, Case Western Reserve University, 2019, EMC - Mechanical Engineering

Solar panels installed on residential rooftops have gained popularity because of their economic efficiency as well as environmental friendliness. However, this type of installation affects the fire performance on the rooftop and introduces new fire concerns. To gain a fundamental understanding of this process, a gas burner fire over an inclined surface (90 cm wide and 440 cm long) with and without a parallel top panel is numerically simulated using FireFOAM. Cases with three different inter-panel distances (6.25 cm, 12.5 cm, and 25 cm) and a case without the top panel are simulated. The results show that the top panel has multiple effects on the fire behaviors. First, it reflects and increases the radiative heat input on the bottom panel. Second, it confines the flow between the two panels and forces the flame to stay close to the bottom panel, increasing the conductive heat flux onto the bottom panel. Third, the top panel reduces the fresh air (oxygen) that enters the fire domain. When the equivalence ratio drops below one, this effect impedes the gas-phase reaction and decreases the gas-phase temperature, and hence the heat flux on the lower panel. As a result of these effects, the flame temperature and heat flux on the lower panel first increases and then decreases when the inter-panel distance decreases. Physics-based mitigation strategies are proposed and shown effective to alleviate the adverse effects of the solar panels on the rooftop fire performance.

Committee: Ya-ting Liao (Committee Chair); James T'ien (Committee Member); Fumiaki Takahashi (Committee Member) Subjects: Mechanical Engineering

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

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. The (open full item for complete abstract)

Committee: James T'ien (Advisor) Subjects: