CHARACTERISTICS OF MICROSCALE DIFFUSION FLAMES

摘要

This paper describes an investigation of microscale diffusion flames, which are potential sources of energy for future micro power generators. Of particular interest was the extinction process, as it would have direct bearing on the minimum size and power of the devices in which such flames would be used. Propane diffusion flames stabilized on top of vertical stainless steel hypodermic tubes were studied. Since intrusive probes could not be used at small scales, flame visualization and chemiluminescence measurements were used to investigate the extinction process. The causes of extinction and blowoff were elucidated by correlating measured microflame behavior with the predictions of a simple jet flame model. The flame lengths predicted by the model were in good agreement with observations. Moreover, provided with one experimental parameter, the flame standoff distance, the model properly predicted the dependence of the flame shape upon the fuel flow rate. The model also predicted that at fuel flow rates near extinction, the fuel jet completely premixes with air within the standoff distance to form a combustible mixture just upstream of the flame, suggesting that near extinction the fuel burns in a flat premixed flame. The model accurately determined the fuel flow rate at quenching by equating the predicted flame length to the measured standoff distance. Finally, experiments conducted under this study showed that heating the surrounding air significantly reduces the fuel flow rate at extinction.