Effects of inert dilution on flickering motions of jet diffusion flames

Low calorific value residual gases are generated in many industrial processes. Typically, these waste gases consist of high percentage of inert gas such as nitrogen and carbon dioxide. They are worthy of recovering to be utilised as low-cost fuels or supplemental fuels in thermal units for achieving savings of energy consumption, reducing waste gas emissions and reducing costs. However, owing to the special fuel properties, many technical problems, such as slow burning velocity and combustion instabilities, remain to be solved in utilising these low calorific value residual gases. In this study, we aimed at investigating instability of jet diffusion flames caused by the buoyancy effects called 'flickering motion', especially with an emphasis on the effect of inert dilution. A mixture of methane and nitrogen was used to obtain the flames. The results showed that the flickering frequency in the high velocity range (from 30 to 120 cm/s) was lower than that in the low velocity range (from 10 to 30 cm/s). In the high velocity range, the flickering frequency was about 11 Hz, which was nearly independent of the flow velocity and gas composition. However, in the low velocity range, the flickering frequency decreased with an increase in flow velocity, and the flickering frequency decreased with increasing methane concentration. The flickering frequencies at low velocities were in the range of about 11-15 Hz. The relation between Strouhal number (St) and Froude number (Fr) is St 8 Fr-m, and the value of m is between 0.495 and 0.550.