The push toward the miniaturization of electro-mechanical devices and the resulting need for micro-power generation (milli-watts to watts) with low-weight, long-life devices has led to the recent development of the field of micro-scale combustion. The concept behind this new field is that since batteries have low specific energy, and liquid hydrocarbon fuels have a very high specific energy, a miniaturized power generating device, even with a relatively inefficient conversion of hydrocarbon fuels to power would result in increased lifetime and/or reduced weight of an electronic or mechanical system that currently requires batteries for power. In addition to the interest in miniaturization, the field is also driven by the potential fabrication of the devices using Micro Electro Mechanical Systems (MEMS) or rapid prototyping techniques, with their favorable characteristics for mass production and low cost. The micro-power generation field is very young, and still is in most cases in the feasibility stage. However, considering that it is a new frontier of technological development, and that only a few projects have been funded, it can be said that significant progress has been made to date. Currently there is consensus, at least among those working in the field, that combustion in the micro-scale is possible with proper thermal and chemical management. Several meso-scale and micro-scale combustors have been developed that appear to operate with good combustion efficiency. Some of these combustors have been applied to energize thermoelectric systems to produce electrical power, although with low overall efficiency. Several turbines/engines have also been, or are being, developed, some of them currently producing positive power, also with low efficiency to date. Micro-rockets using solid or liquid fuels have been built and shown to produce thrust. Hydrogen-based micro size fuel cells have been successfully developed, and there is a need to develop reliable reformers (or direct-conversion fuel cells) for liquid hydrocarbons so that the fuel cells become competitive with batteries. In this work, some of the technological issues related to meso and micro-scale combustion and the operation of thermochemical devices for power generation will be discussed. Some of the systems currently being developed will be presented and described.

This paper presents a generalized pyrolysis model that can simulate the gasification of noncharring, charring, and intumescent materials, as well as smoldering in porous media. Separate conservation equations are solved for gaseous and condensed phase mass and species, solid phase energy, and gas-phase momentum. An arbitrary number of gas-phase and condensed-phase species can be accommodated, each having its own temperature-dependent thermophysical properties. The user may specify any number of solid to gas, solid to solid, or solid + gas to solid + gas reactions of any order. Both in-depth radiation transfer through a semi-transparent medium as well as radiation transport across pores are considered, and melting is modeled using an apparent specific heat. All volatiles generated inside the solid escape to the ambient with no resistance to flow unless the pressure solver is invoked to solve for the pressure distribution in the solid, with the resultant flow of volatiles calculated according to Darcy’s law. Similarly, the user may invoke a gas-phase convective-diffusive solver that determines the composition of the volatiles, including diffusion of species from the ambient into the solid. Thus, in addition to calculating the mass-flux of volatiles escaping from the solid, the actual composition of the vapors can be predicted. To aid in determining the required material properties, the pyrolysis model is coupled to a genetic algorithm that can be used to estimate the required input parameters from bench-scale fire tests, thermogravimetric analysis, or a combination thereof. Model predictions are compared to experimental data for the thermo-oxidative decomposition of a non-charring solid (PMMA) and the thermal pyrolysis of a charring solid (white pine), as well as the gasification and swelling of an intumescent coating, and finally smoldering in polyurethane foam. The predictive capabilities of the model are shown to be generally quite good.

The ignition of and flame spread over solid fuels is of fundamental importance to the field of fire safety. Knowing how, why, and when a material will ignite informs how dangerous a materials use may be. Luckily, there have been no fatal spacecraft based fires beyond the tragedy of the Apollo 1 mission in January 1967. And baring the February 1997 fire aboard the Russian Mir Space Station, there have been very few spacecraft fires in the decades since. This fact can be primarily attributed to the extraordinary caution exercised in design, planning, use of materials, and rigorous fire safety testing. To that end, the effects of environmental variables and material properties on the time to ignition of and opposed flow flame-spread rate over cast cylindrical thermoplastic rods has been investigated. The stated goal of this work being to assess the importance of environmental variables and experimental parameters on the time to ignition or flame spread of a common laboratory thermoplastic, and to gain a better understanding of the lower bounds of material flammability in both 1g and micro-gravity environments.

In the case of time to ignition over cast PMMA rods it is found that clear PMMA rods exhibit longer times to ignition than do black PMMA rods for similar experimental conditions. Additionally, mass flux at ignition, as determined during time to ignition experiments, does not exhibit a discernible trend as a function of external radiant heat flux given the available experimental data and corresponds very well to the theoretically predicted range of mass fluxes.

As a part of the BASS-II campaign of micro-gravity combustion experiments conducted aboard the ISS, it is seen that increasing oxygen concentration or opposed flow velocity acts to increase the flame-spread rate for all three rod diameters within the range of environmental variable values tested.

In conjunction with the BASS-II experiments, ground based experiments were conducted to investigate the effects of oxygen concentration, external radiant heating, and sample diameter on flame spread over cast black and clear PMMA rods under earth standard gravity. Similar to the micro-gravity BASS-II experiments, it was found that flame-spread rate increases with increasing oxygen concentration or eternal radiant heat flux, but increased with decreasing sample diameter. It was also found that with the use of external radiant heating, the effective LOI, or oxygen concentration at which sustained flame-spread was possible, could be reduced.

In comparing the BASS-II micro-gravity flame-spread results to those obtained in 1g, it is clear that flame-spread in micro-gravity is faster if one accounts for the fact that the flow velocities tested in both cases are near the lower bound of what are feasible or relevant flow velocities in each case. Similar trends in flame-spread rate with sample diameter, oxygen concentration, and flow velocity (beyond the natural convection break-point in 1g) were observed, but for the tested conditions, flame-spread in micro-gravity is categorically faster than in 1g.

Lastly, numerical modeling of flame-spread over cast PMMA rods as a function of ambient oxygen concentration, external radiant heating, and gravitational acceleration was undertaken with NIST’s FDS. FDS does effectively model increases in flame-spread rate with increasing externally applied radiant heating (at 21 percent oxygen by volume), as well as an increase in flame-spread rate with an increase in ambient oxygen concentration, both for 1g and micro-gravity conditions. Yet, the magnitude of the flame-spread rates calculated from these simulations is approximately an order of magnitude greater than the experimental results for both 1g or micro-gravity conditions. The exact cause of this difference is hypothesized to be attributable to a combination of the numerical mesh resolution and the solid and gas phase kinetic parameters employed. Additionally, in all cases investigated the numerical simulations correctly predicted the fact that micro-gravity flame-spread was faster than flame-spread under earth standard gravity.

This work investigates the effects of external radiation, ambient pressure and microgravity on the flammability limits of fire-resistant (FR) materials. Future space missions may require spacecraft cabin environments different than those used in the International Space Station, 21%O2, 101.3kPa. Environmental variables include flow velocity, oxygen concentration, ambient pressure, micro or partial-gravity, orientation, presence of an external radiant flux, etc. Fire-resistant materials are used in astronauts, firefighter, and racecar driver suits, cable harnesses, airplane components, etc. However, their fire resistant characteristics, including flammability limits may depend on the environmental conditions and require further study.

The addition of an external radiant flux is able to extend the flammability limits of materials. Based on the results, there is a Limiting Oxygen Concentration (LOC) above which flame spread occurs without the aid of an external radiant flux. Below this critical oxygen concentration flame spread may occur if there is an external radiant flux large enough to allow the fabric to continue the pyrolysis process. Experiments with four different kinds of fabrics showed that FR material content affects the value of the minimum external radiant flux for flame spread. As FR material content increases, so does the value of the minimum external radiant flux.

Regarding the effect of ambient pressure, pressures above 70 kPa result in small changes in the LOC. However, as ambient pressure drops below 70 kPa reducing ambient pressure results in an increase in the LOC. This flammability behavior can be phenomenologically explained in terms of factors such as heat transfer between the flame and unburned solid, buoyancy induced flows, and chemical kinetics. A reduction in ambient pressure decreases the heat transfer from the flame to the unburned fuel while at the same time reducing buoyancy induced flows and the associated heat losses. Examining the Flame/No-Flame spread boundaries in terms of the ambient pressure and oxygen partial pressure revealed a nearly linear relationship between p and pO2. The reduction in pO2 as a function of p suggests that is possible for materials to exhibit significantly different flammability behavior when following a constant oxygen partial pressure curve, like for example normoxic atmospheres similar to those suggested for future spacecraft.

The combination of microgravity and/or addition of an external radiant flux are able to extend the flammability limits of ETFE insulated wires. Under no external radiant flux microgravity conditions resulted in a 6% decrease in the Limiting Oxygen Concentration (LOC) for opposed flame spread. When microgravity conditions were combined with an external radiant flux of 25 kW/m2 the decrease in the LOC was 12%. Microgravity limits heat losses, making possible to reduce the oxygen concentration and flame temperature while maintaining a critical solid decomposition rate. External radiation is able to further compensate for reductions in flame temperature, resulting in a noticeable reduction between the normal gravity and microgravity LOC. The normal gravity and microgravity LOC values of ETFE insulated wires were also compared to those of polyethylene (PE) insulated wires. This comparison showed that microgravity results in a bigger change in the LOC of ETFE than PE. This observation is explained in terms of thermal parameters, critical mass flux and chemical kinetic effects.

Adoption of testing methodologies similar to those described in the present work can produce flammability maps that provide more information than Pass/Fail methods while presenting a clearer picture of the flammability of materials. The results of this work are important given that the flammability of materials is routinely tested without considering the effect of environmental variables, which according to the results presented in this dissertation, may not be indicative of the absolute flammability limits of materials.

The existence of interactions between flames and electric fields has been known for quite some time and has received experimental and theoretical study over the years but remain an active topic of research in the combustion community. The present work specifically investigates the sensitivity of premixed flames electric fields. Prior work has demonstrated that electric fields may be used to enhance and control combustion, but full potential and range of applications for this effect have not been explored. Multiple theories have been presented to explain the process, but there is not yet a truly complete understanding of how electric fields fundamentally change the process of combustion. This work serves to explore new applications of electric-flame interactions and to provide experimental measurements to support the development of detailed theoretical models.

Thermo-diffusive and acoustic instabilities in freely propagating flames can trigger the formation of wrinkled flames and turbulence, which may or may not be desirable in different scenarios. Electric fields present a means to interact with such instabilities either by generating direct hydrodynamic forces ('ionic wind') or by modifying the rate of combustion. Experiments performed on downward-propagating hydrocarbon flames have demonstrated that electric fields can be applied to excite or suppress existing instabilities and control the onset of turbulence. Numerical models following from these experiments indicate that subtle spatial variations in chemical reactivity can achieve similar control of inherently unstable flames.

A practical limitation of combustion is the phenomenon of quenching, where heat losses to the surroundings extinguish a flame. The enhancement achieved by electric fields is shown to dramatically increase flame propagation speed and reduce quenching in methane-air flames, which may permit the miniaturization of combustion-based power generation systems. In the course of these experiments, flames were found to exhibit different behavior depending on the direction of the applied field. This result is consistent with a proposed model of ion transport in laminar flames, although the experiments have shown a greater degree of enhancement than predicted.

Ultimately, modeling the electrical aspects of combustion should be based on detailed accounting of the ion species present in flames. To support the ongoing development of detailed ion-chemistry mechanisms, a non-intrusive microwave interferometer for use in shock tube studies of combustion kinetics was developed. This was used to measure the formation and consumption of free electrons in shock-induced combustion. The range of equivalence ratios tested and the variety of hydrocarbon fuels used provide a rich dataset. Comparison of these results to complementary chemical kinetics simulations have been used to validate proposed improvements to existing mechanisms.

The ignition of natural combustible material by hot metal particles is an important fire ignition pathway by which wildland and wildland-urban-interface spot fires are started. There are numerous cases reported of wild fires started by clashing power-lines or from sparks generated by machines or engines. Similarly there are many cases reported of fires caused by grinding, welding and cutting sparks. Up to this point, research on hot particle spot fire ignition has largely focused on particle generation and transport. A small number of studies have examined what occurs after a hot particle contacts a natural fuel bed, but until recently the process remained poorly understood.

This work describes an investigation of the effect of particle size, temperature and thermal properties on the ability of hot particles to cause flaming ignition of cellulosic fuel beds. Both experimental and theoretical approaches are used, with a focus on understanding the physics underlying the ignition process. For the experimental study, spheres of stainless steel, aluminum, brass and copper are heated in a tube furnace and dropped onto a powdered cellulose fuel bed; the occurrence of flaming ignition or lack thereof is visually observed and recorded. This procedure is repeated a large number of times for each metal type, varying particle diameter from 2 to 11 mm and particle temperature between 575 and 1100 degrees C. The results of these experiments are statistically analyzed to find approximate ignition boundaries and identify boundary trends with respect to the particle parameters of interest. Schlieren images recorded during the ignition experiments are also used to more accurately describe the ignition process. Based on these images, a simple theoretical model of hot particle spot fire ignition is developed and used to explore the experimental trends further. The model under-predicts the minimum ignition temperatures required for small spheres, but agrees qualitatively with the experimental data. Model simulations identify the important physics controlling ignition for different sized particles and clarify many of the experimental trends.

The results show a hyperbolic relationship between particle size and temperature, with the larger particles requiring lower temperatures to ignite the cellulose than the smaller particles. For very small spheres, the temperature required for ignition is very sensitive to particle size, while for very large spheres, ignition temperature shows only a weak dependence on that variable. Flaming ignition of powdered cellulose by particles < 11 mm in size requires particle temperatures of at least 600C. Ignition has not been observed for 2 mm particles at temperatures up to 1100C, but the statistical analysis indicates that ignition by particles 2 mm and smaller may be possible at temperatures above 950C. No clear trend is observed with particle metal type, but copper particles require slightly higher ignition temperatures and seem more sensitive to experimental variation, likely due to their relatively high thermal conductivity. High-speed Schlieren images taken during the ignition experiments show that once particles land, they volatilize the powdered cellulose and the fuel vapor diffuses out into the surrounding air. Ignition occurs in the mixing layer between the vapor and the air, either during the initial expansion of the pyrolyzate away from the particle, or after a stable plume of volatiles has formed.

Modeling results indicate that in the large-particle, high-conductivity limit, the particle's surface temperature remains close to its impact temperature over the timescales of ignition. As a result, particle thermal properties are unimportant and ignition occurs when heat generation in the mixing layer overcomes losses to the surrounding air. When the large-particle limit does not apply, the particle cools upon impact with the fuel bed. In addition to the losses to the surrounding air, the reaction zone experiences losses to the cooling particle and must generate a larger amount of heat for ignition to occur. Because cooling is so important, the initial bulk energy is more useful than impact temperature for predicting ignition by smaller particles. Along those lines, the additional heat of melting available to molten particles helps to resist particle cooling; as such, molten aluminum particles 3.5-7 mm in diameter can ignite at lower temperatures than solid particles of the same size with similar thermal properties. Decreasing volumetric heat capacity does increase minimum ignition temperature somewhat, but this effect is reduced for larger particles. Emissivity does not appear to have a significant effect on ignition propensity, suggesting that, over the timescales of ignition, radiation heat transfer is small relative to other modes of particle heat loss.

Over half of the world's population, approximately 3 billion people, rely on biomass fuels (wood, charcoal, dung, and agricultural residues) and liquid fuels such as kerosene in cook stoves to meet basic needs for cooking and heating. Most cook stoves in use operate indoors without sufficient ventilation, have low efficiencies, and produce products of incomplete combustion. Indoor air pollution of toxic gases and soot aerosols from household energy use in developing countries cause health problems for families, disproportionately affects women and children, and in aggregate, has significant effects on public health. Additionally, aggregate emissions of black carbon and greenhouse gases from the combustion of household fuels in cook stoves adversely affect the environment and pose a serious threat to global climate change.

This work looks at two components of household energy use in developing countries: 1) cook stove technology, and 2) assessing indoor concentrations of pollutants to determine personal exposure for public health. In this dissertation, the formulation of two computational fluid dynamics (CFD) models of the heat transfer, buoyant turbulent fluid flow, and combustion within a cook stove is described. Then, a model for indoor concentrations of pollutants in a room is described, and the model is applied to questions relevant to indoor air pollution and public health.

In this work, an enthalpy-temperature hybrid method is proposed for the numerical solution of generalized phase change problems, and applied to the prediction of polymer pyrolysis and ignition. The basic idea of this method is to treat both enthalpy and temperature as independent variables, and to solve the conservation equations and the constitutive equations (enthalpy-temperature relations) simultaneously. The formula of the enthalpy-temperature relations are not necessary the same for different phases, but can be chosen independently according to the characteristics of physical problems and the convenience of numerical analysis for each respective phase. Therefore this method applies to the problems regardless of the form of the constitutive equations. It overcomes the difficulty or even impossibility encountered in the traditional enthalpy-temperature method, of which either enthalpy or temperature must be consistently and explicitly expressed as a function of the other over all the phases. The method is first applied to a one-dimensional classical freezing problem for method demonstration and verification. It is found that the numerical results of temperature history and the position of phase change interface agree well with the analytic solution existing in the literature. The method is then applied to the numerical simulation of the pyrolysis and ignition of a composite material with a polymer as the matrix and fiberglass as the filling material. Three models of oxygen distribution in the molten layer are considered to explore the melting and oxygen effects on the polymer pyrolysis. Numerical calculation shows that high oxygen concentrations in the molten layer enhance the pyrolysis reaction, resulting in a larger amount of pyrolysate, but in lower surface temperatures of the sample. It also shows that distribution of oxygen in the molten layer has a strong effect on pyrolysate rate, and therefore on ignition and combustion of polymers. Comparison with available experimental data indicates that a model of oxygen distribution in the molten layer that is limited to a thin layer near the surface describes best the ignition process for a homogeneously blended polypropylene/fiberglass composite.