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AME514.ignition.review

AME514.ignition.review - FLAME IGNITION Paul D Ronney...

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1 FLAME IGNITION Paul D. Ronney Department of Aerospace and Mechanical Engineering University of Southern California, Los Angeles, CA 90089 Abstract Current understanding of the initiation of deflagration waves from electric sparks and other concentrated sources of energy is reviewed, emphasizing understanding obtained since the seminal works by Lewis and von Elbe. The primary focus is on gas-phase phenomena but ignition of aerosols and dust suspensions is also discussed. Initially, a phenomenological approach to used to describe basic ignition phenomena. Then, starting from a simplified set of conservation equations for a one- dimensional system with spherical symmetry, the phenomenological approach is refined using recent theory based on activation energy asymptotics. These models, and computational extensions, are compared with experimental observations. Influences of the state of the combustible mixture, the characteristics of the ignition source, and the flow environment on these ignition processes are examined. It is shown that simple diffusive-thermal models are adequate to qualitatively describe many of these influences, especially when Lewis number effects are included. Applications to combustion systems of practical importance, e.g. internal combustion engines and fire safety considerations, are discussed and topics requiring further study are identified. A ME5 14 FALL 200 4 INCOMPLETE DRAFT VERSION NOT FOR ATTRIBUTION
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2 TABLE OF CONTENTS NOMENCLATURE ............................................................................................................ 3 1. INTRODUCTION .......................................................................................................... 4 2. BASIC CONCEPTS OF FLAME IGNITION ..................................................................... 2 2.1 Phenomenological description of Lewis and von Elbe .......................................... 2 2.2 Estimates of the minimum ignition energy .......................................................... 2 3.0 MATHEMATICAL THEORY OF FLAME IGNITION ...................................................... 5 3.1 The conservation equations for flame initiation ................................................. 5 3.2 Solutions using activation energy asymptotics .................................................... 7 3.2.1 Steady solutions ................................................................................. 7 3.2.2 Stability of the stationary solutions and application to ignition ....... 10 3.2.3 Time-dependent solutions ................................................................... 11 3.3 Numerical Solutions .......................................................................................... 12 3.4 Detailed versus simplified chemical kinetics and transport ............................... 12 3.5 Multidimensional effects .................................................................................. 13 4.0 DYNAMICS OF IGNITION PROCESSES ...................................................................... 15 4.1 Sub-critical ignition kernels .............................................................................. 15 4.2 Transition from ignition to steady propagation ................................................... 16 5.0 EFFECTS OF THE STATE OF THE COMBUSTIBLE MIXTURE ....................................... 17 5.1 Pressure ............................................................................................................ 17 5.3 Stoichiometry ................................................................................................... 17 5.4 Additives ......................................................................................................... 18 6.0 EFFECTS OF THE CHARACTERISTICS OF THE ENERGY DEPOSITION SOURCE ....... 18 6.1 Volume ............................................................................................................. 18 6.2 Duration ........................................................................................................... 19 6.3 Method of energy deposition .............................................................................. 20 6.3.1 Spark discharges ............................................................................... 20 6.3.2 Thermal energy versus radicals .......................................................... 20 6.3.3 Lasers ................................................................................................ 20 7.0 EFFECT OF FLOW ENVIRONMENT ............................................................................ 21 7.1 Mean flow and mean strain ............................................................................... 21 7.2 Turbulence ........................................................................................................ 21 9.0 IGNITION OF HETEROGENEOUS MIXTURES ............................................................ 21 9.1 Aerosols ........................................................................................................... 21 9.3 Non-premixed systems ...................................................................................... 22 10.0 PRACTICAL APPLICATIONS .................................................................................... 22 11.0 CONCLUSIONS ......................................................................................................... 23 REFERENCES .................................................................................................................... 23
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3 NOMENCLATURE A Pre-exponential factor in reaction rate expression C p Specific heat at constant pressure D Mass diffusivity d q Quenching distance E a Activation energy E ign Ignition energy E min Minimum ignition energy H Heat release per unit mass of fuel H chem Chemical enthalpy release before flame extinguishment k Thermal conductivity Le Lewis number P Pressure q Scaled ignition power (Eq. 15)
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