Effects of Water Vapor Dilution on the Laminar Burning Velocity and Markstein Length of Ammonia/Water Vapor/Air Premixed Laminar Flames

Since the Industrial Revolution, large amounts of ammonia have been synthesized by the Haber–Bosch process and used as fertilizer and chemical materials. Nitrogen fixation by industrial processes has contributed to the development of human society. However, a serious concern about the nitrogen cycle has been pointed out from the viewpoint of the planetary boundary. The planetary boundary quantifies the allowable limit to maintain a sustainable environment. This nitrogen cycle, which includes the limit of the biogeochemical flow boundary, was suggested to exceed the limits and to pose a serious environmental pollution issue. Therefore, a nitrogen cycle in which nitrogen compounds are collected from the environment and released into the atmosphere has recently been proposed. In this cycle, ammonia–water needs to be processed appropriately. In addition to the standpoint of the planetary boundary, ammonia–water is focused as a fuel that is able to be handled more safely. To design combustion systems that use ammonia–water, its fundamental combustion characteristics need to be elucidated. The objective of the present study is to clarify the effects of water vapor dilution on the laminar flame characteristics of ammonia/water vapor/air premixed flames. A propagating flame in a constant-volume chamber was observed to determine the laminar burning velocity and Markstein length for ammonia/water vapor/air premixed flames for various water vapor dilution ratios. The laminar burning velocity decreased and the Markstein length increased with an increasing water vapor dilution ratio. The effects of water vapor dilution on the laminar burning velocity for ammonia flames were also investigated by numerical calculations. The numerical results show that the decrease in the temperature is the dominant factor of the decrease in the laminar burning velocity with an increase in the water vapor dilution ratio. This study contributes to the understanding of the effects of water vapor dilution on the laminar flame characteristics of ammonia flames up to elevated pressure conditions.