Hsin-Yi Shih

A comparison of extinction limits and spreading rates in opposed and concurrent spreading flames over thin solids

Flame-spread phenomena over thin solids are investigated for purely forced-opposing and concurrent flows. A two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically. In the first part of the paper, flammability limits and spread rates in opposed flow are presented, using oxygen percentage, free-stream velocity, and flow-entrance length as parameters. The comparison of the flammability boundaries and spread-rate curves for two different entrance lengths exhibits a cross-over phenomenon. Shorter entrance length results in higher spread rates and a lower oxygen-extinction limit in low free-stream velocities, but lower spread rates and a higher oxygen-extinction limit in high free-stream velocities. The entrance length affects the effective flow rate that the flame sees at the base region. This affects the radiation loss and gas residence-time in an opposing way to cause the cross-over. Radiation also affects the energy balance on the solid surface and is in part responsible for the solid-fuel non-burn-out phenomenon. In the second part of the paper, a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties. While the spread rate in concurrent spread increases linearly with free-stream velocity, the spread rate in opposed flow varies with free-stream velocity in a non-monotonic manner, with a peak rate at an intermediate free-stream velocity. At a given free-stream velocity, the limiting oxygen limits are lower for concurrent spread, except in the very low free-stream-velocity regime, where the spreading flame may be sustainable in opposed mode and not in concurrent mode. The cross-over disappears if the two spread modes are compared using relative flow velocities with respect to the flames rather than using free-stream velocities with respect to the laboratory.

Model calculation of steady upward flame spread over a thin solid in reduced gravity

Steady upward flame spread in two-dimensional laminar flow over a thin solid is solved numerically in reduced gravity based on a combustion model recently formulated for concurrent flows. This flame spread model is more sophisticated than most previous ones because it avoids the boundary layer approximation and uses full elliptic Navier-Stokes equations. In addition, two-dimensional flame radiation treatment is accomplished by using the S-N discrete ordinates method. The emitting gas media are carbon dioxide and water vapor, the combustion products. Computed flow and flame structure are presented. The details of the flame stabilization zone near the solid burnout is resolved. Downstream flame is found to deviate from the self-similar boundary layerscaling relation. The effect of gravity level is studied. Flame length and spread rate increase approximately linearly with gravity level. A low-gravity flame quenching limit is predicted. Gaseous flame radiation is found to be important for flame structure, flame dimension, and extinction limit. Flame radiative feedback is an essential part of the solid surface energy balance. However, predicted flame spread rates have similar magnitudes as those computed by the model neglecting flame radiation.

Modeling concurrent flame spread over a thin solid in a low-speed flow tunnel

A three-dimensional model of concurrent flame spread over a thin solid in a low-speed flow tunnel in microgravity was formulated and numerically solved. In a parametric study varying the flow velocity, oxygen level, and tunnel and solid fuel widths, two distinctive types of flame behavior were noted. In high-oxygen-percentage and/or higher-speed flows, the flames were long and far away from the quenching limit. In such cases, three-dimensional effects were dominated by heat loss to the wall in the downstream portion of the flame. In low-oxygen and low-speed flows, the flames were short and in the region near the quenching limit. These near-limit flames were controlled by the oxygen supply rate. Oxygen-side diffusion in the cross-wind direction became a dominant mechanism exhibiting large effects on the narrow three-dimensional flames. A number of trend reversals on spread rates and extinction limits were discovered for these nearlimit flames. Aided by the detailed flame profiles obtained in the computations, an explanation for the reversal phenomena is offered in the paper.

A three-dimensional model of steady flame spread over a thin solid in low-speed concurrent flows

Athree-dimensional model of a steady concurrent flame spread over a thin solid in a low-speed flowtunnel in microgravity has been formulated and numerically solved. The gas-phase combustion model includes the full Navier-Stokes equations for the conservation of mass, momentum, energy and species. The solid is assumed to be a thermally thin, non-charring cellulosic sheet and the solid model consists of continuity and energy equations whose solution provides boundary conditions for the gas phase. The gas-phase reaction is represented by a one-step, second-order, finite-rate Arrhenius kinetics and the solid pyrolysis is approximated by a one-step, zeroth-order decomposition obeying an Arrhenius law. Gas-phase radiation is neglected but solid radiative loss is included in the model. Selected results are presented showing detailed three-dimensional flame structures and flame spread characteristics. In a parametric study, varying the tunnel (solid) widths and the flow velocity, two important three-dimensional effects have been investigated, namely wall heat loss and oxygen side diffusion. The lateral heat loss shortens the flame and retards flame spread. On the other hand, oxygen side diffusion enhances the combustion reaction at the base region and pushes the flame base closer to the solid surface. This closer flame base increases the solid burnout rate and enhances the steady flame spread rate. In higher speed flows, three-dimensional effects are dominated by heat loss to the side-walls in the downstream portion of the flame and the flame spread rate increases with fuel width. In low-speed flows, the flames are short and close to the quenching limit. Oxygen side diffusion then becomes a dominant mechanism in the narrow three-dimensional flames. The flame spreads faster as the solid width is made narrower in this regime. Additional parametric studies include the effect of tunnelwall thermal condition and the effect of adding solid fuel sample holders.

Upward Flame Spread Over Thin Solids in Partial Gravity

Experiments to observe upward and downward flame spread and extinction over a thin solid fuel in partial-gravity environments were conducted in an aircraft flying parabolic trajectories. In the upward spreading case, flames with constant lengths and steady spread rates were observed using narrow fuel samples in reduced pressures. The upward flame spread rates and the flame and pyrolysis lengths increased linearly with the gravity level. The proportionality constants, however, increased with pressure and sample width. For comparison, downward spreading tests were also conducted using the same reduced-pressure atmospheres needed to obtain steady flames in the upward spreading case. In downward spreading, the steady spread rates and the flammability boundary exhibited a non-monotonic dependence on gravity. This behavior is attributed to competition between finite-gas-phase residence times in the flame stabilization zone and radiative heat losses from the flame. Throughout the accessible range of partial gravity, the upward spreading flames propagated at higher speeds than the downward spreading flames and the fuel is more flammable in the upward spread direction. A three-dimensional concurrent-flow flame-spreading model, originally developed for forced flows in a duct at microgravity, was reformulated and numerically solved for buoyant flow. The numerical flame spread simulation provides detailed flame structure including gas flow and temperature fields, oxygen and fuel transport, and solid temperature and thickness distributions and predicts the essential three-dimensional features observed for the narrow, reduced pressure flames

An Experimental Study of Upward Flame Spread and Interactions Over Multiple Solid Fuels

Upward flame spread and flame interactions over multiple solid fuels are experimentally studied, and the effects of flame interactions on the flame spreading rates are analyzed. Flame spreading characteristics and spreading rates are measured and compared for six different geometric arrangements of thin solids at different solid width and separation distance between solids. The flame spread rate increases as the separation distance between the parallel solids decreases because of the flow channeling effect and radiation interactions, which reaches the maximum at an intermediate separation distance and then decreases as the separation distance becomes smaller due to the flow resistance and limited thermal expansion. To compare the six types of solid geometry studied, the highest flame spread rate is enclosure type of solids, followed by ⊓-shaped solids, four parallel solids, two parallel solids, L-shaped solids, and single solid.

Solid fuel combustion experiments in microgravity using a continuous fuel dispenser and related numerical simulations

The conventional way of determining the flammability characteristics of a material involves a number of tedious single-sample tests to distinguish flammable from non-flammable conditions. A novel test device and fuel configuration has been developed that permits multiple successive tests for indefinite lengths of thin solid materials. In this device, a spreading flame can be established and held at a fixed location in front of optimized diagnostics while continuous variations of test parameters are made. This device is especially well-suited to conducting experiments in space (e.g. aboard the International Space Station) where the limited resources of stowage, volume, and crew time pose major constraints. A prototype version of this device was tested successfully in both a normal gravity laboratory and during low-gravity aircraft trials. As part of this ongoing study of material flammability behavior, a numerical model of concurrent-flow flame spread is used to simulate the flame. Two and three-dimensional steady-state forms of the compressible Navier-Stokes equations with chemical reactions and gas and solid radiation are solved. The model is used to assist in the design of the test apparatus and to interpret the results of microgravity experiments. This paper describes details of the fuel testing device and planned experiment diagnostics. A special fuel, developed to optimize use of the special testing device, is described. Some results of the numerical flame spread model are presented to explain the three-dimensional nature of flames spreading in concurrent flow and to show how the model is used as an experiment design tool.

Modeling wall influence on solid-fuel flame spread in a flow tunnel

Concurrent-flow flame spread over a thin solid in a low-speed flow tunnel is investigated theoretically to support a proposed space experiment. By modifying a previous flame-spreading model, valid for unbounded domain, the effects of flow confinement due to finite tunnel height and the radiative interaction between the tunnel wall and the flame and solid fuel are studied. Computed results show that, as tunnel height is decreased, the flow is accelerated to a higher velocity in the downstream due to thermal expansion. This presses the flame closer to the solid fuel, and increases the heat conduction rate to the solid, the flame length, and the spread rate. When the channel height became too small, however, conductive heat loss to the wall became substantial, which reverses the trend and decreases the flame length. Radiative interaction between the tunnel wall and the flame system is found to be a strong function of the wall radiation reflectivity. When the wall reflectivity is increased, the total radiative loss from the flame system (including the solid fuel) is decreased. This substantially increases the flame length and spread rate and hence becomes an important parameter in the experimental design. (Author)

Flame spread and interactions in an array of thin solids in low-speed concurrent flows

Flame spread in an array of thin solids in low-speed concurrent flows was investigated and numerical solved. A previous steady, two-dimensional flame-spread model with flame radiation was employed and adapted in this work. The flame structures of spreading flames between parallel solids were demonstrated and some of the features were presented, including flow channelling effect and flame radiation interactions. The channelling effect is caused by flow confinement by the presence of the other solids; the flows through the hot combustion gases are accelerated downstream drastically. Radiation interactions between flames and solids contributed to a less heat-loss system, and radiation re-absorption by flames resulted in a larger flame with higher temperature, which increased the conductive heat fluxes to the solids and flame spread rate. Consequently, the extinction limit for the interacting flames is extended beyond the low-speed quenching limit for a single flame. The influence of the separation distance on the flame spread rate was also studied, which exhibits a non-monotonic behaviour. At larger separation distance, the flame spread rate increases with decreasing the separation distance owing to the channelling effect and radiation interactions. However, at very small separation distance, the flame spreading rate decreases with decreasing the distance because of the limited space for thermal expansion and flow résistance between solids.

Model Analysis of Syngas Combustion and Emissions for a Micro Gas Turbine

The purpose of this study is to investigate the combustion and emission characteristics of syngas fuels applied in a micro gas turbine, which is originally designed for a natural gas fired engine. The computation results were conducted by the commercial CFD software STAR-CD, where the three-dimension compressible k-e model for turbulent flow and PPDF (Presumed Probability Density Function) model for combustion process were constructed. As the syngas are substituted for methane, the total heat input from the blended fuels and the fuel flow rates are varied with syngas compositions and syngas substitution percentages. The computed results presented the syngas substitution effects on the combustion and emission characteristics at different syngas percentages (up to 80%) for two typical syngas compositions and the conditions where syngas applied at fixed heat input were examined.Results showed the flame structures varied with different syngas substitution percentages. The high temperature regions were dense and concentrated on the core of the primary zone for H2-rich syngas, and then shifted to the sides of the combustor when syngas percentages were high. The NOx emissions decreased with increasing syngas percentages, but NOx emissions are higher at higher hydrogen content for the same syngas percentage. The CO2 emissions also decreased at 10% syngas substitution, but then increased as syngas percentage increased. Only using H2-rich syngas could produce less carbon dioxide. The detailed flame structures, temperature distributions, and gas emissions of the combustor were presented and compared. The exit temperature distributions and pattern factor were also discussed. Before syngas fuels are utilized as an alternative fuel for the micro gas turbine, further experimental testing are needed as the CFD modeling results provide a guidance for the improved designs of the combustor.Copyright