An experimental study and analysis of lift-off length in inclined nonpremixed turbulent jet flames

The lifted flame behavior of inclined turbulent jets, considering the relative angle between the fuel jet momentum and flame buoyancy was investigated experimentally by varying the inclination angle of non-premixed fuel jets. Variations of lift-off length from the flame base to the nozzle exit was quantified ex-perimentally with nozzles of various diameters (2, 3, and 5 mm) and inclination angles (range of -90 & DEG; to 90 & DEG;). The data was analyzed based on the experimental finding of upstream preheating effect depending on inclination angles. Major findings are as follows: (1) The lift-off length ( h ) increases linearly with the increase in initial fuel jet velocity (ue) at a fixed inclination angle. The proportionality slope K of the lin-ear relationship between h versus ue decreases appreciably with jet inclination angle for the negatively inclined flames; while for the positively inclined flames, the lift-off length decreases relatively weakly. (2) Physical analysis on the flow characteristics of inclined jets was conducted, and the preheating ef-fect was proposed based on the combustion behaviors, especially for the negatively inclined jet flames. The preheating temperatures of unburned fuel/air mixtures at the flame base and nozzle exit were ex-perimentally quantified, revealing that the negatively inclination angle can have a significant influence on the preheating temperatures. (3) Based on the proposed preheating mechanism, a physical model ac-counting for the effect of jet inclination angle was developed to quantify the lift-off length of inclined jet flames. The proposed model successfully represented lift-off lengths at all the experimental conditions with various inclination angles and nozzle diameters. The present findings provide new data set and a reasonable physical model for lifted flame behavior of inclined turbulent jet flames, revealing the effect of the relative angle between fuel jet momentum and flame buoyancy.& COPY; 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.