flame merging; buoyant flames; flame characteristics


The purpose of this study was to investigate the merging behavior of small-scale buoyant flames that might be representative of flames from a leaf in a shrub. Zirconia felt pads soaked in n-heptane were suspended on thin rods and spaced both horizontally and vertically. Time-dependent video images from flames from two-pad and three-pad configurations were analyzed to determine merging probability, combined flame characteristics (height, area, and width), and changes in burn time. Correlations of these combined flame characteristics were developed based on horizontal and vertical spacing between the pads. Merging probability correlated with an exponential function that was quadratic in horizontal and/or vertical spacing. Flame heights corrected for vertical inter-pad spacing showed a maximum increase of 50% over single flame heights, and were correlated with an exponential decay function. Flame areas increased by a maximum of 34%, but on average were relatively constant. Corrected flame widths for the merged flames increased by as much as 55% in some configurations, but decreased by up to 73% in other configurations. Burn times for upper pads decreased when there was no horizontal spacing. The limited flame growth observed in these non-overlapping configurations in the horizontal dimension imply that overlapping configurations seem to be necessary for significant flame growth.

Original Publication Citation

Fletcher, T. H., D. Haycock, S. Tollefsen, and D. O. Lignell, “Merging of Horizontally- and Vertically-separated Small-scale Buoyant Flames,” Fire, 4(3), 51 (2021). DOI: 10.3390/fire4030051

Document Type

Peer-Reviewed Article

Publication Date







Ira A. Fulton College of Engineering and Technology


Chemical Engineering

University Standing at Time of Publication

Full Professor