To investigate the mechanism of fire whirls, small scaled experiments in a vertical square shaft were conducted in the laboratory. It was observed that whirling flame can be obviously divided into three regions along flame height according to flame characteristics. In the rotating coordinates, the momentum equation of the rotating flame under the fire whirl steady state was obtained through the analysis of physical phenomena. The further analysis shows that the maximum axial-velocity is dependent on the temperature difference between whirling flame center and hot air outside the flame, and on the length of the third flame region mainly driven by buoyancy. In addition, the correlation of the maximum circumferential and axial velocity is found, which is dependent on fuel pool radius and the length of the third flame region.%为进一步研究火旋风机理，在实验室条件下进行了小尺寸方形竖槽道内的火旋风实验.通过观察实验现象，发现在该槽道内形成的旋转火焰在竖直方向上有明显的分界，根据火焰特征将其在高度上分为3个区域.在匀速旋转坐标系下，对该物理现象进行了深入分析，得出火旋风稳定时旋转火焰的动量方程，进一步分析得到旋转火焰最大轴向速度取决于旋转火焰中心和外围热空气的温度差以及旋转火焰顶端的第3区域火焰长度.同时还得到旋转火焰最大周向速度与最大轴向速度的关系，其取决于油盆半径和旋转火焰顶端主要受浮力作用的火焰长度.
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