Background: Fire whirl is an extreme fire behaviour in wildland fires, and an essential factor for its formation is the surrounding generating eddy. No systematic experimental study has been conducted on natural fire whirls with varying heights of the initial generating eddy.

Aims: The aim of this research was to provide a comprehensive experimental study on the effect of initial generating eddy height on fire whirl formation and flame characteristics.

Methods: The experiments were conducted in a fixed-frame facility with varying channel wall height (representing the initial generating eddy height). A 20-cm-diameter propane burner (10.0-100.0 kW in heat release rate) was used.

Key results: The critical channel wall height for fire whirl formation decreases with the heat release rate. The mean flame height grows remarkably with initial generating eddy height for large heat release rates, but it varies only slightly at relatively small heat release rates.

Conclusions: The formation of fire whirl depends on the initial generating eddy height, rotational strength, and heat release rate. A flame height correlation of the fire whirl is obtained by considering the initial generating eddy height.

Implications: This work provides a basis for improving the prediction accuracy of natural fire whirls in wildland fires.