Combustion

Abstract

The fundamental principles of combustion mechanisms are of great interest to scientists and engineers. Contemporary key issues are the reduction of fuel consumption, the reduction of pollutant emissions and fire safety. The launching of space craft relies exclusively on combustion engines and manned spaceflight systems are subject to stringent fire safety requirements. This paper shows that the science of combustion can derive important benefits from experiments under microgravity conditions.

In the first section it is argued that combustion processes are in the main driven by physical processes, such as mass and heat transport, as much as by the chemical ones. It is noted that combusting flows are characterized by very large temperature gradients in the gas phase which induce significant “mean buoyancy” effects under gravity conditions, and also by a “fluctuating buoyancy” when a flame is turbulent.

In the second section some of the limitations of the present understanding of combustion processes are reviewed. The relevancy of conducting experiments with negligible natural convection is then discussed. It is concluded that the most valuable means for reducing natural convection is to use reduced gravity conditions.

The third section is devoted to some of the applications of microgravity conditions in the combustion field. Two typical examples are presented to illustrate these applications: single fuel droplet burning and flame spread along a solid surface. Results of relevant experiments are presented which show the feasibility of combustion tests using microgravity conditions and the advantages of eliminating natural convection for conducting well-defined model experiments.