diffusionflames - MECH 6120 Combustion Notes on Diffusion...

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MECH 6120: Combustion Notes on Diffusion Flames 1 Introduction In diffusion flames, the fuel and oxidizer are not mixed together prior to ignition. Instead, they mix during the combustion process. The fuel and oxidizer may initially be in different phases, such as liquid fuel and gaseous oxidizer. In this case the combustion process is called heterogeneous . When the reactants initially have the same phase, the combustion process is called homogeneous . From the previous notes, we know that the flame speed of a premixed flame is related to the reaction rate. In a more general sense, we would say that the adiabatic flame speed for premixed flames is kinetically controlled – in that the reaction rate (determined from chemical kinetics) ultimately governs the speed at which a flame will propagate through the mixture. In diffusion flames the concept of a flame speed looses its significance (or usefulness). This is because a diffusion flame is governed by diffusion of fuel and oxidizer as well as chemical kinetics. If the chemical reaction rates are fast compared to the diffusion rates, the combustion process is said to be diffusion controlled , since the slowest step (diffusion in this case) will be the ‘bottleneck’ which ultimately determines the overall combustion rate. Likewise, if diffusion rates are fast compared to chemical reaction rates, the combustion process is kinetically controlled – as in the case of premixed flames. This is not to say that diffusion is unimportant in premixed flames – because diffusion of heat and radical species upstream does cause the flame to propagate through the mixture. However, the combustion process in premixed flames is, again, not dependent upon the diffusion mixing of fuel and oxidizer prior to combustion. We will study two types of diffusion flames; namely a homogeneous flame (e.g., a gaseous fuel jet burning in air) and a heterogeneous flame (e.g., a fuel droplet burning in a diesel engine). The governing differ- ential equations that apply to both problems are the same, but the boundary conditions are different. In the heterogeneous (or two-phase) problem we must take into account that there is mass transfer from one phase to another, and this complicates the problem somewhat. air fuel fuel O 2 flame products products Figure 1: A typical jet diffusion flame 2 Jet Diffusion Flames Basic Properties As the name implies, a jet diffusion flame is formed from a jet of a gaseous fuel flowing into an oxidizing environment. A diagram of a typical configuration is shown in Fig. 1. As will be shown in the notes, the flame in a diffusion flame (i.e., the region where the reaction is taking place) is a relatively thin ‘sheet’, and it exists where the fuel and the oxidizer meet in stoichiometric proportions. In this sense, gas diffusion flames have a ‘hollow’ structure; a candle flame, for example, may appear to us to be a homogeneous ‘blob’ of hot gases, but actually the flame exists only as a thin surface that surrounds the gases.
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