Stereo PIV measurements in fire whirls
Fire whirls are standing vortex structures that often form in forest fires. Here, we report an experimental study of a laboratory burner fire whirl generated using the fixed-frame method, with a particular focus on the velocity field associated with the whirl. Even under controlled laboratory conditions, the velocity field is not easily characterized. Fire whirls are inherently unsteady, which tends to smear time averages, and precession of the core can cause additional spatial smearing. In our experiments, we use particle image velocimetry to overcome these challenges, and report instantaneous snapshots of the flow field inside and outside the core, as well as ensemble-averaged results that take into account the unsteady motion of the whirl axis. We find the ensemble-averaged velocity field closely resembles a Burgers vortex for the upper 60% of the whirl. Near the base region (within one burner diameter of the base), a double ring structure is seen which is likely made up of two vortex rings. Also, entrainment into the whirl appears to be dominated by the radial flow in the base region.
The assistance of Mr. Timothy Bennett in helping with the design and construction of the experimental apparatus used in this study is greatly appreciated. Part of this work was supported by Princeton University’s Andlinger Center for Energy and the Environment.
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