Premixed flame

A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge (also called pre-mixture) of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughout a homogeneous stoichiometric premixed charge, the combustion process once initiated sustains itself by way of its own heat release. The majority of the chemical transformation in such a combustion process occurs primarily in a thin interfacial region which separates the unburned and the burned gases. The premixed flame interface propagates through the mixture until the entire charge is depleted. The propagation speed of a premixed flame is known as the flame speed (or burning velocity) which depends on the convection-diffusion-reaction balance within the flame, i.e. on its inner chemical structure. The premixed flame is characterised as laminar or turbulent depending on the velocity distribution in the unburned pre-mixture (which provides the medium of propagation for the flame). A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge (also called pre-mixture) of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughout a homogeneous stoichiometric premixed charge, the combustion process once initiated sustains itself by way of its own heat release. The majority of the chemical transformation in such a combustion process occurs primarily in a thin interfacial region which separates the unburned and the burned gases. The premixed flame interface propagates through the mixture until the entire charge is depleted. The propagation speed of a premixed flame is known as the flame speed (or burning velocity) which depends on the convection-diffusion-reaction balance within the flame, i.e. on its inner chemical structure. The premixed flame is characterised as laminar or turbulent depending on the velocity distribution in the unburned pre-mixture (which provides the medium of propagation for the flame). Under controlled conditions (typically in a laboratory) a laminar flame may be formed in one of several possible flame configurations. The inner structure of a laminar premixed flame is composed of layers over which the decomposition, reaction and complete oxidation of fuel occurs. These chemical processes are much faster than the physical processes such as vortex motion in the flow and, hence, the inner structure of a laminar flame remains intact in most circumstances. The constitutive layers of the inner structure correspond to specified intervals over which the temperature increases from the specified unburned mixture up to as high as the adiabatic flame temperature (AFT). In the presence of volumetric heat transfer and/or aerodynamic stretch, or under the development intrinsic flame instabilities, the extent of reaction and, hence, the temperature attained across the flame may be different from the AFT. For a one-step irreversible chemistry, i.e., ν F F + ν O O 2 → P r o d u c t s {displaystyle u _{F}{ m {{F}+ u _{O}{ m {{O}_{2} ightarrow { m {Products}}}}}}} , the planar, adiabatic flame has explicit expression for the burning velocity derived from activation energy asymptotics when the Zel'dovich number β ≫ 1. {displaystyle eta gg 1.} The reaction rate ω {displaystyle omega } (number of moles of fuel consumed per unit volume per unit time) is taken to be Arrhenius form, where B {displaystyle B} is the pre-exponential factor, ρ {displaystyle ho } is the density, Y F {displaystyle Y_{F}} is the fuel mass fraction, Y O 2 {displaystyle Y_{O_{2}}} is the oxidizer mass fraction, E a {displaystyle E_{a}} is the activation energy, R {displaystyle R} is the universal gas constant, T {displaystyle T} is the temperature, W F   &   W O 2 {displaystyle W_{F} & W_{O_{2}}} are the molecular weights of fuel and oxidizer, respectively and m   &   n {displaystyle m & n} are the reaction orders. Let the unburnt conditions far ahead of the flame be denoted with subscript u {displaystyle u} and similarly, the burnt gas conditions by b {displaystyle b} , then we can define an equivalence ratio ϕ {displaystyle phi } for the unburnt mixture as Then the planar laminar burning velocity for fuel-rich mixture ( ϕ > 1 {displaystyle phi >1} ) is given by