ABSTRACT
In order to understand the complex processes involved during combustion, it is very important to measure the concentration of various species as they are used and generated during this process. But what do we mean by “species”? The species in a combustion system consists of atoms, molecules, radicals, and ions. We know that molecules are the uncharged species with paired electron spins. Examples of molecules are monoatomic (He, Ar), diatomic (CO, H2), and polyatomic species (CH4, C2H6, C3H8, NH3, etc.). Species with unpaired electron spins are called radicals. For example, species such as O, H, OH, CH, and CH3 are radicals. Monatomic molecules that are paired can be labeled as atoms. These radicals are more reactive in nature than stable molecules due to the presence of unpaired electrons that can cause the chemical reactions to be faster. As a
result, the lifetimes of radicals are quite short due to higher reactivity. This is mostly confined to the high-temperature zones of combustion. If molecules are charged, they are called ions. For example, H+ and O-are ions. Generally ions will be more reactive than radicals but their concentrations are too negligibly small to contribute to the chemical conversion of fuel and oxidizer into its product. The determination of species, both stable molecules and radicals, are quite important in the reaction scheme used in the simulation of a combustion system. The utilities of species in a reaction scheme can be divided into three categories: reactants, intermediate, and products. We know that any atom, radical, and molecule can be termed as an intermediate species if its concentration attains a peak value across the flame. The products can include atoms, radicals, and molecules although molecules are the more desirable product in a combustion system.
