[Chemistry Class Notes] Explosions Pdf for Exam

To define explosion, it is the acceleration of the reaction, induced either by an increase in temperature or by increasing the length of the reaction chain, is what causes the transition from any combustion to an explosion.

An explosion is also explained as a rapid expansion in the volume associated with an extremely vigorous outward release of energy, generally with the generation of high temperatures and the release of high-pressure gases.

Types of Explosions

There are two types of explosions, where the first is a thermal explosion, and the second is a chain explosion.

Thermal Explosions

The thermal explosion theory is based on the idea that gradual heating increases the rate at which heat is released by the reaction (heat explosion) until it exceeds the rate at which heat is lost from the field. At the given pressure and the given composition of the mixture, the explosion will take place at a particular ignition temperature, which may be determined from the calculations of heat gain and heat loss.

During the induction time, the thermal explosion hypothesis accounts for the fuel consumption and temperature increase. At sufficiently high rates of consumption, the explosion will not take place.

Chain-Branch Reactions

It follows from the branched-chain reaction theory that there is a limit to explosion or ignition without a temperature rise. In this case, the so-called chain explosion will take place when the probabilities of chain branching and the termination are equal. However, most fires are chain-thermal in nature (it means both the chain auto-acceleration and heat accumulation contribute to explosion – heat explosion).

Detonation

The front area flame moves at supersonic speed, and the transition from laminar to turbulent flow produces a shock wave, which accounts for the reaction’s progressive acceleration. The amount of increase in temperature because of the compression in the shock wave will result in the mixture’s self-ignition, and detonation sets in.

The shock wave-combustion zone complex produces the detonation wave. And, the detonation varies from normal combustion in its ignition mechanism and supersonic velocity of 2–5 km/sec for gases and 8–9 km/sec for solid and liquid explosives.

Special Aspects

The emission of light is a combustion’s characteristic feature. Visible, ultraviolet, and infrared bands of molecules and atomic lines are generally noticed in flame spectra. In addition, continuous spectra from radical, atom, and ion recombination or incandescent particles are commonly observed. The thermal energy of gas (thermoluminescence) and the chemical energy emitted in exothermic elementary reactions are the sources of flame radiation (which is chemiluminescence).

In a Bunsen burner which is fed with enough amount of air, up to 20% of the reaction heat is released as infrared energy and less than 1% as ultraviolet and visible radiation, the infrared being mostly the thermoluminescence. At the same time, the radiation from the inner Bunsen flame cone in the visible and ultraviolet regions represents chemiluminescence.

Applications

A few uses of flame and combustion phenomena are categorized under five general heads, where a few are given below:

Explosive detonation and combustion are commonly used in a variety of employment, with the ultimate purpose of a mechanical explosion or action. The practical explosive applications are based on the theory of their detonation and combustion. The combustion of condensed explosives takes place mostly in the gaseous phase due to their sublimation, evaporation, or decomposition and is treated in terms of the theory of gaseous combustion that provides for the burning velocity, its dependence on pressure and temperature, and the parameters that determine the combustion regime and the explosive’s nature.

The above figure is the representation of the test explosion, which happened at White Sands Missile Range in New Mexico in 1985.

• In Internal-Combustion Engines

These comprise different engines, turbojets, ramjets, and gas turbines. In general, the Otto engine operates with a mixture that is compressed in a cylinder by a piston. The mixture is ignited with a spark shortly before the piston reaches the tip, and the flame propagates at a normal rate through the unburned mixture by raising the pressure and pushing the piston. There is a maximum amount of compression for any of the mixture compositions and engine designs.

The above figure represents the internal combustion engine of an automobile.

The diesel engine, given above, operates with a fuel spray injected into the cylinder of the engine as liquid droplets, which mix with air by turbulent diffusion and then evaporate. At the engine’s normal operations, the temperature of compressed air is high enough for the self-ignition of the fuel.

The products of combustion of solid, liquid, or gaseous propellants in the rockets are ejected from the combustion chamber via (de Laval) nozzle at higher velocity. The kinetics of chemical processes knowledge in the nozzle is important to determine the required thrust. And, the thrust decreases with the combustion product’s increasing mean molecular weight. Mixtures of high heat of combustion and low molecular weight, thus, are used for rockets.