Let 1

Introduction
1-1 Introduction
Increasing concern over the fossil fuel shortage and pollution of air, and the requirement for alternative fuels for Internal Combustion Engines (ICEs) have been a major concern for the researchers. The need for sustainable energy systems has led researchers to re-evaluate the combustion process and the prospects of fuels. For these reasons, research into the properties of fuel and its constituent chemicals is as relevant as ever. The combustion of medium chain length hydrocarbons, oxygenates and other petrochemicals are still not very well understood, particularly the kinetics of reactions that may involve hundreds of reactants, forming a vast number of intermediate species.
Combustion is defined as "rapid oxidation, producing heat, or both light and heat, moreover, slow oxidation accompanied by relatively little heat and no light". Combustion can occur in either a flame or non-flame mode.
It is hard to define flames precisely, but the most relevant definition of flames is " visible chemical component undergoing highly exothermic chemical reaction takes place in a small zone with the evolution of heat". The flame speed is the measured rate of expansion of the flame front in a combustion reaction. Flame speed is typically measured in m/s.
The flame can be classified into two categories according to the propagation speed that is detonation is a supersonic compression wave in which chemical reaction is produced by compressive heating. Detonation waves proceed at ranging from 1 to 4 km/s. They cannot occur in the conventional fuel–air mixtures employed in gas turbine combustors, but the possibility could arise in situations where oxygen injection is employed to facilitate ignition and engine acceleration. Currently, pulse detonation engines are of interest to the military and these engine combustors employ detonation waves. The other type is deflagration is the low velocity typically, expansion wave (subsonic) in which chemical reaction is produced by the diffusion of mass and heat. Deflagration waves in hydrocarbon fuel-air mixtures normally propagate at velocities below 1 m/s. All the flame processes that occur in gas turbine combustors fall within this category.
The reaction zone is usually called the flame zone, flame front or reaction wave. Within the flame zone, rapid reactions occur and light is frequently, but not always, emitted from the flame. This phenomenon is determined by the role of chemical and physical procedures.
Flames can be produced in two radically different ways depending upon how the reactants are brought together, premixed and non-premixed.
In the premixed flame, the oxidizer has been mixed with the fuel before it reaches the flame front. The reaction creates a thin flame front as all of the reactants are readily available.
In a diffusion flame (non-premixed) the reactants are initially separated, and the reaction occurs only at the interface between the fuel and oxidizer, where mixing and reaction both take place. It can also be defined as the flame in which the oxidizer combines with the fuel by diffusion. As a result, the flame speed is limited by the rate of diffusion. Diffusion flames tend to burn slower and to produce more soot than premixed flames because there may not be sufficient oxidizer for the reaction to complete.
Researchers pointed out two types of flames are linked to the gaseous mixtures and each type of these flame can be subdivided into two type according to the regimes of flow to laminar and turbulent. In practical devices that involve flowing fluids, turbulent flows are more frequently encountered than laminar flow; but, perhaps more importantly understanding laminar flames is necessary prerequisite to study turbulent flames. Unlike a laminar flame, which has a flame velocity depends uniquely on the thermal and chemical properties of mixture, while turbulent flame depends on the character of the flow, as well as properties of mixture.
In fact, a spherical flame can propagate either outwardly (positive) or inwardly ( negative). Additionally, it can either propagate in space, and exhibit a flame speed, or be stationary (zero).
The outwardly propagating flame, or explosion, is centrally ignited. It is suitable to quantify the burning velocity. An outwardly orientated stationary flame can be generated by a radially inward flow of unburned gas through a porous external spherical wall to a smaller central region from which the burned gases are removed.
The inwardly propagating flame, or implosion, is the instantaneous ignition around a spherical outer boundary. The practical initiation of this type would be unachievable; however, it can be studied computationally. It is assumed that no flow takes place on the unburned gas. An inwardly orientated stationary flame is more practically feasible and can be created by the outward flow of unburned gas from a spherical, porous burner.
1-2 Physics or Chemistry?
The subject of combustion embraces both physics and chemistry. In the present context, physics is taken to include heat transfer, mass transfer, thermodynamics, gas dynamics, and fluid dynamics. In many practical combustion devices, physical processes are much more limiting to combustion performance than chemical processes.
In general, chemical processes are important mainly for their influence on pollutant emissions and, in aircraft combustors, on lean light off and lean blowout limits at high altitudes. However, at most operating conditions, the main interest lies not so much on the limits of combustion as on the structure, heat-release