David C. Walther

Oxidizer Flow Effects on the Flammability of Solid Combustibles

A study is presented on the effect of oxidizer flow characteristics on the piloted ignition and opposed flame spread of a slab of PMMA heated by an external radiant flux. The objective is to establish the basis for a potential new lest method that may be used to determine the flammability performance of solid materials in terms of their ignition delay, critical heat flux for ignition, and flame-spread rate, for varied oxidizer flow conditions. The proposed Forced-flow Ignition and flame-Spread Test (FIST) follows the concepts of the LIFT (ASTM E 1321-93), and thus consists of a combination of ignition delay and flame spread experiments as a function of an externally applied radiant flux, but incorporates controlled forced convection as the predominant mechanism for the gas-phase transport of heat and mass. PMMA slabs were tested to assess the applicability of the methodology, and results are presented for radiant fluxes ranging from 0 to 35 kW/m2, forced oxidizer flow velocities of 1.0, 1.75, and 2.5 m/s. and natural convection. Results of the variation of ignition delay as a function of free stream oxygen concentration (18 to 45%) are also presented. Following the LIFT methodology, the ignition delay and flame spread data are used to assemble flammability diagrams for PMMA at different oxidizer flow velocities. Additional natural convection tests were performed in the LIFT apparatus with the purpose of establishing a base line for comparison of results. It is shown that the resulting FIST and LIFT flammability diagrams are similar. However, the diagrams as well as the parameters extracted from them depend on the oxidizer flow velocity, resulting in families of flow-dependent flammability diagrams. As a result of the use of forced convection, it is considered that the FIST is suitable for characterizing solid material flammability in mixed regimes ranging from natural to predominantly forced convection, and at different oxygen concentrations. This could provide a means to more accurately rank the relative flammability of solid combustible materials that would be used in environments where the oxidizer flow differs from air in natural convection, such as areas with significant air currents, vitiated environments, or microgravity applications (space based facilities).

Space shuttle based microgravity smoldering combustion experiments

Results from four microgravity smoldering combustion experiments conducted aboard the NASA Space Shuttle are presented in this work. The experiments are part of the NASA funded Microgravity Smoldering Combustion (MSC) research program, aimed to study the smolder characteristics of porous combustible materials in a microgravity environment. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The microgravity smolder experiments reported here have been conducted to investigate the propagation of smolder through a polyurethane foam sample under both diffusion driven and opposed forced flow driven smoldering. The present experiments, although limited, are unique in that they provide the only available information about smolder combustion in microgravity in sample sizes large enough to allow the self-propagation of the smolder reaction throughout the sample length. Two quiescent tests at ambient oxygen concentrations of 35% and 40% and two opposed forced flow tests with air as oxidizer, were conducted aboard the NASA Space Shuttle (STS-69 and STS-77 missions). The MSC data are compared with normal-gravity data to determine the effect of gravity on smolder, and are used to verify present theoretical models of smolder combustion. It is found that for the present test conditions, the microgravity opposed flow smolder reaction temperatures, propagation velocities, toxic compound production and reaction extent lie between those of normal-gravity upward and downward tests. Thermogravimetric analysis shows little effect of gravity on the kinetics of the smolder process in these cases. Neither of the two quiescent, microgravity cases resulted in self-sustained smolder propagation, whereas the normal-gravity downward cases propagated vigorously. The difference in these results shows that gravity has a significant effect on smolder combustion, at least for the sample size tested. Correlation of the forced flow smolder velocity data with a heat transfer based model, indicates that simplified heat transfer models of smolder propagation can effectively describe vigorous smolder, away from limiting conditions such as extinction and flaming.

Numerical analysis of piloted ignition of polymeric materials

A numerical model is developed to analyze piloted ignition of polymeric materials exposed to an external radiant heat flux in a convective oxidizer flow. The model considers the coupled thermo-chemical processes that take place both in the condensed and gas phases. Condensed phase processes include oxidative and thermal pyrolysis, phase change, and in-depth radiation absorption. Heat and mass transport in the gas phase are described by the reactive boundary layer conservation equations. Ignition is determined by the onset of thermal runaway in the gas phase. PMMA is used as the testing material, and ignition delay are predicted and compared with experimental data for different external heat fluxes and flow velocities. The numerical model is used to explore the ignition controlling mechanism and the critical conditions at ignition under various external heat fluxes and different airflow velocities. It is also used to provide an ignition criterion that could simplify the theoretical description of the ignition process. It is found that for a given flow velocity the pyrolysate mass flux at ignition on the sample surface is independent of the external heat flux, whereas the sample surface temperature at ignition varies with the external heat flux. Therefore, if the oxidizer flow velocity is a fixed parameter, it is possible to adopt a critical pyrolysate mass flux as an accurate ignition criterion. The use of this criterion allows the use of a simplified model to predict piloted ignition that solely considers the condensed phase.

The Effect of Buoyancy on Opposed smoldering

An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the smolder front. These data were compared with opposed-forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground-based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the ultrasound imaging system (UIS). The UIS measurements were used as a second means of providing smolder propagation velocities as well as to obtain permeabilities of the reacting samples. Results of forced-flow testing in normal gravity were compared to results in microgravity at a range of ambient pressures and forced flows. Results indicate that a critical oxidizer mass flux of roughly 0.5 to 0.8 g/m2s is required in normal gravity for a self-sustaining propagation in this configuration. In microgravity tests, self-sustained smolder propagation was observed at a significantly lower oxidizer mass flux of 0.30 g/m2s. Analysis suggests that the removal of buoyancy-induced heat losses in microgravity allows for self-sustained propagation at an oxidizer mass flux below the critical value observed in normal-gravity testing. Normal-gravity tests also show that the smolder propagation velocity is linearly dependent on the total oxidizer mass flux in an oxidizer-limited regime. Pressure effects on the chemical kinetics of a smolder reaction are inferred by comparison of normal-gravity and microgravity tests and believed to be only weakly dependent on pressure (∼P 1/3).

SMOLDER IGNITION OF POLYURETHANE FOAM: EFFECT OF OXYGEN CONCENTRATION

Experiments have been conducted to study the ignition of both forward and opposed smolder of a high void fraction, flexible, polyurethane foam in a forced oxidizer flow. Tests are conducted in a small scale, vertically oriented, combustion chamber with supporting instrumentation. An electrically heated Nichrome wire heater placed between two porous ceramic disks, one of which is in complete contact with the foam surface, is used to supply the necessary power to ignite and sustain a smolder reaction. The gaseous oxidizer, metered via mass flow controllers, is forced through the foam and heater. A constant power is applied to the igniter for a given period of time and the resulting smolder is monitored to determine if smolder is sustained without the assistance of the heater, in which case smolder ignition is considered achieved. Reaction zone temperature and smolder propagation velocities are obtained from the temperature histories of thermocouples embedded at predetermined positions in the foam with junctions placed along the fuel centerline. Tests are conducted with oxygen mass fractions ranging from 0.109 to 1.0 at a velocity of 0.1 mm/s during the ignition period, and 0.7 or 3.0 mm/s during the self-sustained propagation period. The results show a well-defined smolder ignition regime primarily determined by two parameters: igniter heat flux, and the time the igniter is powered. These two parameters determine a minimum igniter/foam temperature, and a minimum depth of smolder propagation (char), which are conditions required for ignition to occur. The former is needed to establish a strong smolder reaction, and the latter to reduce heat losses from the incipient smolder reaction to the surrounding environment. The ignition regime is shifted to shorter times for a given igniter heat flux with increasing oxygen mass fraction. A model based on concepts similar to those developed to describe the ignition of solid fuels has been developed that describes well the experimental ignition results.

The effect of fiberglass concentration on the piloted ignition of polypropylene/fiberglass composites

This work examines the piloted ignition of a blended polypropylene fiberglass (PP/GL) composite material exposed to an external radiant flux in a forced convective flow of air. The effect of glass concentration on the ignition delay and critical heat flux for ignition at different external radiant fluxes and a fixed airflow velocity are determined experimentally. The effect of the airflow velocity on the ignition delay for a fixed fiberglass concentration is also stud-ied. The experiments are conducted in an apparatus (Forced Ignition and Spread Test) previously developed by the authors to study environmental effects on the piloted ignition of solid combustibles. The results of the study provide information about the effects of fiberglass concentration on the relative flammability of composite materials. It is found that the ignition delay and critical heat flux for ignition are functions of the fiberglass concentration in the composite, increasing as the glass concentration is increased. This is due primarily to the increase in the thermal inertia of the material by the fiberglass addition. The data are also used to develop an ignition diagram in terms of external heat flux and composite glass concentration that determines, for a given external heat flux, the critical concentration of fiberglass beyond which the PP/GL composite material does not ignite. This diagram is particularly useful for fire safety design purposes. The results for the effect of the flow velocity on the ignition delay show that the flow velocity affects the ignition delay particularly at values near the critical heat flux for ignition, increasing its value as the flow velocity is increased.

Microgravity Ignition Delay of Solid Fuels in Low-Velocity Flows

Experiments have been performed in microgravity and normal gravity to determine the effects of low-velocity airflows on the piloted ignition delay of solid fuels. Natural convection prevents material testing at the low oxidizer velocities encountered in space facilities (∼0.1 m/s); thus, it is necessary to conduct these tests in reduced gravity. Tests have been conducted with two types of fuels, polymethylmethacrylate (PMMA) and a polypropylene/glass fiber composite, aboard the NASA KC-135 aircraft, under air velocities below those induced by natural convection. The short reduced gravity period (∼25 s) provided by the aircraft limits the testing to high external fluxes (∼30 kW/m 2 ) so that the ignition delay times are shorter than the microgravity time. In normal gravity, the ignition delay and critical heat flux for ignition decrease as the forced-flow velocity decreases, until they reach minimum values that are limited by natural convection. The microgravity data indicate that ignition delay is further reduced as the air velocity is lowered. A theoretical model is used to predict the ignition delay for PMMA at low flow velocities in microgravity. The model predicts that the critical heat flux for ignition at the flow conditions expected in space facilities could be as much as half the value measured in normal gravity. The results are important because they imply that, in space facilities, ignition may occur more easily than in normal gravity. If the results are confirmed by long-term microgravity testing, they may have important implications for the fire safety design of space facilities.

Development and characterisation of small-scale rotary engines

This paper describes the development and characterisation of small-scale rotary engines with displacements in the range of 781500 mm for portable applications in the range of 10200 W of power output. Small-scale combustion engines present a number of research challenges including manufacturing tolerances, sealing, thermal management, ignition, combustion efficiency and porting. Four engines have been characterised using a custom test bench and show an increase in performance due to design changes that mitigate the challenges associated with small-scale engines. The volumetric power density has been increased from 11 W/cm in a 348 mm engine operating with a supercharged hydrogen/air mixture to 22 W/cm in a 1500 mm engine operating with naturally aspirated liquid hydrocarbon fuel. The thermal efficiency has also been increased from 0.2 to 4%. Continued improvements in sealing, thermal management, combustion efficiency and friction reduction will allow further increases in engine performance.

Small-scale smoldering combustion experiments in microgravity

Results from small-scale experiments of the smolder characteristics of a porous combustible material (flexible polyurethane foam) in microgravity and normal gravity are presented. The microgravity experiments were conducted in the Spacelab Glovebox on the USML-1 mission of the Space Shuttle Columbia, June/July 1992, and represent the first smolder experiments ever conducted under extended periods of microgravity. The use of the Glovebox limited the size of the fuel sample that could be tested and the power available for ignition but provided the opportunity to conduct such experiments in space. Four tests were conducted, varying the igniter geometry (axial and plate) and the convective environment (quiescent and forced). A series of comparative tests was also conducted in normal gravity. Measurements conducted included temperature histories at several locations along the fuel sample, video recording of the progress of the smolder, and postcombustion char and gas composition analyses. The results of the tests showed that smolder did not propagate without the assistance of the igniter, primarily because of heat losses from the reaction to the surrounding environment. In microgravity, the reduced heat losses caused by the absence of natural convection resulted in only slightly higher temperatures in the quiescent microgravity test than in normal gravity but a dramatically larger production of combustion products in all microgravity tests. Particularly significant is the proportionately larger amount of carbon monoxide and light organic compounds produced in microgravity, despite comparable temperatures and similar char patterns. This excessive production of fuel-rich combustion products may be a generic characteristic of smoldering polyurethane in microgravity, with an associated increase in the toxic hazard of smolder in spacecraft.

Leakage Flow Analysis for a MEMS Rotary Engine

An internal leakage flow analysis is presented for a MEMS fabricated rotary engine in order to establish design parameters for micro engine sealing systems. This research is part of the MEMS Rotary Engine Power System (REPS) group effort to develop a portable power system based on an integrated generator and Wankel rotary internal combustion engine. In order to have acceptable system efficiency, it is necessary to suppress internal leakage and thereby maintain a critical level of compression ratio. There are two inherent leakage paths in rotary engines, which result in blowby and reduced compression ratio: leakage around the apexes of the rotor and leakage across the rotor faces. These sealing issues arise due to the large pressure gradients, which occur along these leakage paths in the combustion chamber. It is the aim of this work to examine the effects of reduced scale on both traditional and novel rotary engine apex sealing mechanisms. In contrast to the macro scale, viscous forces have an increased importance in micro scale engines since Re~.01. A simplified Poiseuille-Couette flow model has been developed to analyze the leakage flows of rotary type engines. Since the Reynolds number for the MEMS REPS is extremely small, the model assumes that the flow is laminar, viscous, incompressible, and steady with air as the working fluid. The model indicates that if a 1 μm gap can be maintained between the housing and moving parts (rotor apexes and faces), leakage flows at expected engine operation speeds will only reduce the compression ratio from 8.3:1 to 6.1:1 so long as the rotation speed is greater than 10,000 rpm. It is doubtful that a traditional or simple micromachine design will yield such a gap and therefore several novel, integrated sealing approaches are under investigation. The model will determine design specification for one of these approaches, an integrated cantilever flexure apex. In conjunction with the theoretical model, a scaled engine experiment at the macro scale is used to verify the modeling effort. The scaling of the experiment complies with Reynolds scaling and ensures that Hele-Shaw flow within the leakage paths is maintained. The experiment does not operate as a functional engine, rather the experiment is designed to maintain a precise clearance between the rotor and housing. In order to preclude additional pressure driven flow effects, an electric motor is used to spin the rotor and simulate the rotation expected due to the combustion pressure acting on the rotor face.Copyright