Effects of diluents on the lifted flame characteristics in laminar nonpremixed coflow propane jets

The effects of diluents including CO2, N2, and He on the lifted flame characteristics of laminar coflow propane jets are experimentally investigated by measuring their liftoff heights, HL, and visualizing flow fields with oil mist. For lifted flames at a specified diluent mole fraction, XD, HL increases in the order of He, N2, CO2 when the diluent is added to the coflow stream. However, the order of HL becomes opposite when the diluent is added to the fuel stream. In addition, HL increases with increasing XD for all cases. From the visualization of nonreacting flow fields, it is found that the negative buoyancy represented by the density difference between the fuel jet and the coflow induces a stagnation flow near the fuel nozzle by decelerating the fuel jet. As such, the lifted flame is found to be stabilized further upstream with increasing negative buoyancy. In addition to the buoyancy effect, the effects of diluents on HL via the edge flame speed of the lifted flames, Se, are estimated by evaluating the laminar burning velocity, SL0. Among the dilution, thermal, and chemical effects, the dilution effect is found to be dominant in reducing SL0 for all cases, while the chemical effect is negligible. Finally, a correlation for HL is formulated using the ratios of the positive to negative buoyancy and the fuel jet velocity, U0, to SL0, which shows a satisfactory agreement with the experimental data.