Hydrocarbon pool fires can cause serious fire accidents. This study focused on the flame-merging behavior and pulsation frequency of annular hydrocarbon pool fires with various inner and outer diameters. Experiments were performed with annular pool fires having outer diameters (D) in the range of 0.15-0.55 m and inner diameters (d) in the range of 0.05-0.45 m. The fuel used was n-heptane. The flame-merging probability and pulsation frequency of annular pool fires with various inner and outer diameters were examined. With the development of the pool fire, the initially produced "annular flame" finally merged. According to the dimensional analysis, a fitted correlation of the piece function with different heat-release rates and annular-pool sizes was proposed for describing the flame merging and pulsation frequency, and the proposed correlation in this work is suitable for no wind conditions. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.