Although discrete flame spread is a common phenomenon in the practical fire scenarios, such as a group of burning trees under bushfire, research gaps still exist to address the flame spreading criteria and the related spread characteristics. Therefore, through this study, a group of birch rods with different lengths (denoted by l within 60–100 mm) and spacings (S, 1–9 mm) were analyzed experimentally and theoretically. It was known that the critical criterion of discrete flame spreading is determined by the l and S, where a criterion of l≥3d (sample thickness) could predict the critical spacing between the separated birch rods. Theoretical models were also developed to predict the global flame spread rate and mass loss rate under various S and l. The predictions based on the newly developed models agree reasonably well with those experimental data. Global flame spread rate, mass loss rate and flame height increase first and then decrease along with a bigger S, where the dimensionless flame height shows a piecewise exponential relationship with the dimensionless heat release rate. The research outcomes of this study provide a theoretical basis for the fire risk evaluation of those discrete fire spread.
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