Usually, depending on the initial mixing of fuel and oxidizer, combustion process is classified as either non-premixed 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.

Our micro-gravity experiment on a round SF₆ liquid jet issued into an otherwise stagnant N₂ gas at room temperature showed that the Taylor instability excited immediately downstream of the nozzle exit disappeared at pressures exceeding a certain value (7.6 MPa). In the present paper, the mathematics and physics of Taylor instability are fully explored to disclose various aspects overlooked in the conventional atomization theory. Then, the analysis is extended to analyze the Taylor instability property for a liquid jet with near-critical-mixing surface. The following are found. The growth rate is not crucially affected by the thermodynamic state of the liquid surface. Instead, the emergence of Taylor instability crucially relies on the value of surface tension because Taylor instability can not be excited when the jet speed exceeds the speed of capillary wave which propagates upstream from the disturbed region.