Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Abstract: Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm3 was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.

Preferential Diffusion Effects on the Surface Structure of Turbulent Premixed Hydrogen/Air Flames

Abstract: An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. It was found that flame surface area (and thus the local turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder, with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Finall...

A model study of the interaction of multiple turbulent diffusion flames

Abstract: This research program has shown that a valid model for studies of mass fires can be produced using multiple jets of gaseous fuels. The basic requirement is that the fuel jets produce turbulent diffusion flames which are buoyancy controlled. A specific operating range where this requirement is met was found for this model. A number of flame arrays have been studied in which the dimensionless groups of (array-flame height/single-flame height), number of jets, source-shape factor, and [flame spacing/(fuel-flow rate) 2/5 ] were the important variables. Assuming the flame to be a series of point sources, only (array-flame height/single-flame height) and [flame-spacing/(fuel-flow rate) 2/5 ] showed a strong correlation. This correlation is shown to exist both analytically and experimentally, but not to the same extent. The experimental data showed no consistent effect of jet number or flame array. The difference between the analytical and experimental trends is accredited to a mutual aspirating effect of one flame upon another which decreases the effective spacing between the jets. A good correlation was also obtained, considering the flame as a continuous area fire, between the dimensionless flame height based on the extent of the fire area and the total fuel-flow rate.