Effects of Water Vapor and CO2 Addition on Combustion Characteristics for Lean Hydrocarbon-Air Mixtures under Normal and Microgravity Conditions

Experiments on combustion of extremely lean mixtures in the vicinity of flammability limits have acquired importance from the viewpoint of development of new kinds of combustion systems having low fuel consumption and low emissions. Furthermore, the determination of combustion characteristics for vapor addition under normal gravity and microgravity is very important for control of safety engineering in the space. In this work, experiments have been carried out with extremely lean, quiescent hydrocarbon-air mixtures to examine the effects of water vapor and CO2 addition on combustion characteristics under normal gravity and microgravity conditions. The microgravity technique achieved in a freely falling chamber is employed because the realizations of symmetrical flame propagations in a tube are impossible. Experimental condition for the initial mixtures corresponds to room temperature and 0.1MPa and the fuels used are methane and propane of 99.9% purity, respectively. The experimental results in this study are as follows. : 1) The water vapor addition for low oxygen concentration of propane-air mixtures effectively increases the flame speed; 2) Effects of water vapor addition on flame speed depend on a fuel property; 3) It is possible to control the combustion behavior by water vapor addition in the mixtures under low oxygen concentration.