Combustion of various hydrocarbons is one of the most significant, if not the most significant sources of power in the modern world. In power stations, rockets, aircraft, cars and other land vehicles, and ships, chemical energy stored in hydrocarbons is burnt to provide a source of heat and ultimately power. In most cases, the hydrocarbon concerned is a fluid, a liquid or a gas. Hence the fluid dynamics of combusting fluids is an important area of investigation. It is also one of the most complex areas of investigation, as it combines rapid chemical reaction, heat release and change of properties (for example density) with fluid dynamics, usually in the form of turbulent fluid flow.
Combustion is a chemical reaction between a fuel, usually a mixture of hydrocarbons, and an oxidant, usually air. This process is typically very complex, involving many specific reactions - in the case of combustion of methane in air more than 40 elementary reactions have been identified. The reactions proceed rapidly and are accompanied by a release of heat (which is of course the idea). Possible effects on the flow of the combustion process are as follows :
Conversely the flow can affect the progress of the combustion, typically by controlling the rate at which the various components involved in the combustion are supplied to the combustion zone. The flame produced by the burner for a domestic cooker is a good example - as the flow rate of gas to the burner increases so does the rate of combustion and thus the heat released. The effect of the flow can be at a smaller scale as well. Turbulence can act to increase the rate of supply of reactants to the combustion zone, thus enhancing combustion in a turbulent flow. However if the turbulence level becomes too high it can disrupt combustion completely, leading to flame extinction.