Alfred E. Frey

Bio: Alfred E. Frey is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Laminar flame speed & Flame spread. The author has an hindex of 2, co-authored 2 publications receiving 236 citations.

A theory of flame spread over a solid fuel including finite-rate chemical kinetics

Abstract: A theory for the steady-state flame spread over a thermally thin solid fuel is developed in this study. The model considers a laminar diffusion flame in a uniform opposed flow and includes the two-dimensional, elliptic, gas-phase energy, and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics. The unsteady, solid-fuel equations neglect heat conduction ahead of the flame but include transient heating and Arrhenius pyrolysis kinetics and are coupled to the quasisteady gas phase. The equations are solved in the laboratory coordinate system. In this study the two-dimensional distributions of temperature and species are obtained, including the low reactivity zone in the flame region. The solid-fuel surface profiles indicate a region of almost uniform temperature (vaporization temperature) during pyrolysis for some parameter values; however, the value is not universally constant for the fuel and does depend on the ambient parameters (pressure, oxygen mass fraction, and magnitude of opposed velocity). The dependence of the flame-spread rate on the ambient parameters is investigated, and qualitative agreement is obtained with experimental results in the near-extinction-limit region. Quantitative agreement with experimental data depends on the choice of parameter values, especially the gas-phase kinetics model parameters, which are generally unknown. The flame-extinction limits due to increased opposed velocity, reduced pressure, and reduced ambient oxygen mass fraction are all obtained in the results calculated from this theory.

Near-limit flame spread over paper samples

Abstract: In this study the near-limit characteristics of a spreading flame are considered. Flame spreading rates and temperature profiles are measured as extinction conditions are approached. The flame is extinguished by increasing the heat loss, reducing the total pressure, or reducing the oxygen mole fraction in the environment. The gas phase temperature profiles are obtained with fine-wire thermocouple probes. The flame spreading results show that the power-law correlations of McAlevy and Magee [3] do not remain valid near the extinction limit. In all cases the slope of the Log (flame spread rate) vs. Log (total pressure) curves increase and approach vertical at extinction. Differences in vertical and horizontal flame spreading are discussed. The flame temperature profiles are examined for a near-limit flame, but the total pressure level is the only parameter changed. In the near-limit flame the maximum flame temperature is reduced slightly, but the flame is enlarged in physical size greatly. It is observed that near the pyrolysis front, heat transfer forward in the gas phase and normal to the fuel surface are of the same order of magnitude.

Modeling and simulation of combustion processes of charring and non-charring solid fuels

Abstract: Some of the progress that, owing to modeling and numerical simulation, has been made to the understanding of chemical and physical processes, which occur during combustion of solid fuels, is presented. The first part of the review deals with thermal degradation processes of charring (2ood and, in general, cellulosic materials) and non-charring (poly-methyl-methacrylate) materials. Gas-phase combustion processes (ignition, flame spread and extinction) are discussed in the second part of the review. Solid fuel degradation has been described by kinetic models of different complexity, varying from a simple one-step global reaction, to multi-step reaction mechanisms, accounting only for primary solid fuel degradation, and to semi-global reaction mechanisms, accounting for both primary solid degradation and secondary degradation of evolved primary pyrolysis products. Semi-global kinetic models have been coupled to models of transport phenomena to simulate thermal degradation of charring fuels under ablation regime conditions. The effects of bubble formation on the transport of volatiles during thermal degradation of non-charring fuels, described through a one-step global reaction, have also been modeled. On the contrary, very simplified treatments of solid phase processes have been used when gas phase combustion processes are also simulated. On the other hand, the latter have also always been described through one-step global reactions. Numerical modeling has allowed controlling mechanisms of ignition and flame spread to be determined and the understanding of the interaction between chemistry and physics during thermal degradation of solid fuels to be improved. However, the chemical processes are not well understood, the few kinetic data are in most cases empirical and variations of solid properties during degradation are very poorly known, so that even the most advanced models do not in general give quantitative predictions.

Controlling Mechanisms of Flame Spread

Abstract: Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...

Thermographic phosphors for thermometry: A survey of combustion applications

Abstract: Temperature is a fundamental thermodynamic parameter used to describe physical, chemical and biological processes. In combustion as in many other applications, knowledge about temperature plays a substantial role in helping to maintain an efficient and clean environment Being able to measure temperature accurately in combustion and in fire-related applications is important for giving a better understanding of heat transfer phenomena and improving existing models. In the present review paper a method based on the spectroscopy of inorganic luminescent materials is described and exemplified in experiments related to combustion. The method involves the use of thermographic phosphors which enable remote temperature diagnostics to be performed with a high degree of sensitivity and accuracy. The technique is superior to those based on thermocouples and pyrometry, particularly in the vicinity of flames and when the measured surface is subjected to random movements. Several phosphor materials suitable for temperature probing are described. The application of thermographic phosphors to temperature measurements in one-point and in two-dimensions in flame spread scenarios, and in pyrolysis experiments involving different construction materials and polymers are described. Many thermographic phosphors have the property of being insensitive to variations in pressure up to 1 GPa. This property extends the use and development of thermographic thermometry to other domains, such as internal combustion engines. The temperature has been measured in a point and in two-dimensions inside the combustion chamber. The complex procedures required to implement the use of thermocouples on moving objects inside an engine make thermocouples an expensive choice. It also limits the possibilities of altering the measurement locations and thereby also complicating the investigation of different engine geometries and components. Thermographic phosphors have also been employed in gas turbine applications. Temperature probing in the afterburner of a full-size aircraft engine is described with the aim to study the effects of various engine loads on the wall temperature. Furthermore, the application of thermographic phosphors to study the temperature of droplets in relation to sprays is described. In spray dynamics, temperature is a crucial parameter for gaining an understanding of atomisation, evaporation and heat convection from the surrounding gases. Finally the application of thermographic phosphors for gas temperature measurement by seeding the particles into a gas flow is described together with the challenges associated with seeding the particles for in-situ flame measurements. (C) 2010 Published by Elsevier Ltd. (Less)

A theory of flame spread over a solid fuel including finite-rate chemical kinetics

Abstract: A theory for the steady-state flame spread over a thermally thin solid fuel is developed in this study. The model considers a laminar diffusion flame in a uniform opposed flow and includes the two-dimensional, elliptic, gas-phase energy, and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics. The unsteady, solid-fuel equations neglect heat conduction ahead of the flame but include transient heating and Arrhenius pyrolysis kinetics and are coupled to the quasisteady gas phase. The equations are solved in the laboratory coordinate system. In this study the two-dimensional distributions of temperature and species are obtained, including the low reactivity zone in the flame region. The solid-fuel surface profiles indicate a region of almost uniform temperature (vaporization temperature) during pyrolysis for some parameter values; however, the value is not universally constant for the fuel and does depend on the ambient parameters (pressure, oxygen mass fraction, and magnitude of opposed velocity). The dependence of the flame-spread rate on the ambient parameters is investigated, and qualitative agreement is obtained with experimental results in the near-extinction-limit region. Quantitative agreement with experimental data depends on the choice of parameter values, especially the gas-phase kinetics model parameters, which are generally unknown. The flame-extinction limits due to increased opposed velocity, reduced pressure, and reduced ambient oxygen mass fraction are all obtained in the results calculated from this theory.

Theory of opposed-flow flame spread

Abstract: A critical, historical review of the flame spread literature is given, beginning with the first systematic studies of opposed-flow flame spread. Important modeling effects are described, including qualitative, simplified, μg and comprehensive numerical modeling. A brief discussion of subjects with the potential for further development is also given. Although this review focuses on flame-spread theory the emphasis is on the logical development, not the detailed mathematics.