Webster's dictionary provides a useful starting point for a definition of combustion as "rapid oxidation generating heat, or both light and heat; also, slow oxidation accompanied by relatively little heat and no light." For our purposes, we will restrict the definition to include only the rapid oxidation portion, since most practical combustion devices belong in this realm.
This definition emphasizes the intrinsic importance of chemical reactions to combustion. It also emphasizes why combustion is so very important: combustion transforms energy stored in chemical bonds to heat that can be utilized in a variety of ways. Throughout this blog, we illustrate the many practical applications of combustion.
COMBUSTION MODES AND FLAME TYPES
Combustion can occur in either a flame of nonflame mode, and flames, in turn, are categorized as being either premixed flames or nonpremixed (diffusion) flames. The difference between flame and nonflame modes of combustion can be illustrated by the processes occurring in a knocking spark-ignition engine (Fig. 1)
In fig. 1a, we see a thin zone of intense chemical reaction propagating through the unburned fuel-air mixture. the thin reaction zone is combustion. As the flame moves across the combustion space, the temperature and pressure rise in the unburned gas. Under certain conditions (Fig. 1b), rapid oxidation reactions occur at many locations within the unburned gas, leading to very rapid combustion throughout the volume. This essentially volumetric heat release in an engine is called autoignition, and the very rapid pressure rise leads to the characteristic sound of engine knock. Knock is undesirable, and a recent challenge to engine designers has been how to minimize the occurrence of knock while operating with lead-free gasolines (the discovery that tetraethyl lead reduces knock, made by Thomas Midgley in 1921, allowed engine compression ratios to be increased, and thereby improved efficiency and power). In compression-ignition or diesel engines, however, autoignition initiates the combustion process by design.
the two classes of flames, premixed and nonpremixed of diffusion, are related to the state of mixedness of the reactants, as suggested by their names. In a premixed flame, the fuel and the oxidizer are mixed at the molecular level prior to the occurrence of any significant chemical reaction. The spark-ignition engine is an example where premixed flames occur. Contrarily, in a diffusion flame, the reactants are initially separated, and reaction occurs only at the interface between the fuel and oxidizer, where mixing and reaction both take place. An example of a diffusion flame is a simple candle. In practical devices, both types of flames maybe present in various degrees. Diesel-engine combustion is generally considered to have significant amounts of both premixed and nonpremixed or diffusion burning. The term "diffusion" applies strictly to the molecular diffusion of chemical species, i.e., fuel molecules diffuse toward the flame from one direction while oxidizer molecules diffuse toward the flame from the opposite direction. In turbulent nonpremixed flame, turbulent convection mixes the fuel and air together on a macroscopic basis. Molecular mixing at the small scales, i.e., molecular diffusion, then completes the mixing process so that chemical reactions can take place.
 |
| Figure 1. (a) Flame and (b) nonflame modes of combustion in a spark-ignition engine. |
the two classes of flames, premixed and nonpremixed of diffusion, are related to the state of mixedness of the reactants, as suggested by their names. In a premixed flame, the fuel and the oxidizer are mixed at the molecular level prior to the occurrence of any significant chemical reaction. The spark-ignition engine is an example where premixed flames occur. Contrarily, in a diffusion flame, the reactants are initially separated, and reaction occurs only at the interface between the fuel and oxidizer, where mixing and reaction both take place. An example of a diffusion flame is a simple candle. In practical devices, both types of flames maybe present in various degrees. Diesel-engine combustion is generally considered to have significant amounts of both premixed and nonpremixed or diffusion burning. The term "diffusion" applies strictly to the molecular diffusion of chemical species, i.e., fuel molecules diffuse toward the flame from one direction while oxidizer molecules diffuse toward the flame from the opposite direction. In turbulent nonpremixed flame, turbulent convection mixes the fuel and air together on a macroscopic basis. Molecular mixing at the small scales, i.e., molecular diffusion, then completes the mixing process so that chemical reactions can take place.
No comments:
Post a Comment