Flame spread over solid fuel in a partially premixed atmosphere -the role of fuel diffusion to flame front

摘要

Usually, depending on the initial mixing of fuel and oxidizer, combustion process is classified as either nonpremixed or premixed burning. Although it may be useful to consider these two modes in theoretical manner, some examples show the intermediate state or coupling modes. For example, at the leading edge of a lifted flame, the "triple" flame could be observed by partial premixing between fuel and oxidizer. Recently, it has been recognized that partially premixing is a key technique to reduce pollutant emission from the diesel combustion chamber. Thus, fundamental understanding of partially premixed phenomena is needed in the variety of combustion conditions. In this study, we investigate the flame spread over solid fuel in a partially premixed atmosphere, which may occur under poorly ventilated conditions in fire, forming the combustible mixtures of oxygen and fuel vapors. The downward flame spread is examined in a duct with opposed low-speed flow to change the conditions of atmosphere. Hydrogen, methane, and propane are added in the ambient air to observe the partially premixing effect. The fuel concentration is kept below the lean flammability limit. Results show that, in the partially premixed atmosphere, the flame spread rate is increased, with larger high temperature region. Pyrolysis region is also expanded. This could be explained by the fuel diffusion to the leading flame edge to increase the heat release rate, causing a larger amount of combustible fuel vapors.