This methodhas also been employed in a series of experiments where multiple equidistant fire whirls weregenerated between two vertical screens that were both parallel to a propane line fire and movingin opposite directions. There are other mechanical methods that can leadto the generation of whirling flames, including the use of air curtains and tangentially orientedblowers at the fuel surface.One advantage of these techniques is that restrictive walls are not necessarily needed, which enableseasier experimental probing. However, maintaining the flow symmetry inside the domain is morechallenging than with other methods.While the differences among on-source fire whirls formed in enclosed configurations shownin Figure 4 are minimal, some differences are observed when the fuel source is changed betweena liquid pool fire, a gas burner, or solid combustibles. For poolfires and solid combustibles, the velocity of the gasified vapor is minimal,unlike a gas burner, which may impart additional momentum in the axial direction of the firewhirl, similar to a swirling jet. In a liquid pool fire, the swirl establishes an Ekman-like boundarylayer over the fuel surface, which draws the flame sheet closer to the fuel surface . This provides more heat to the fuel surface and increases the fuel evaporation and burningrate, subsequently entraining more air (this is discussed in more detail in Section 5.3). There aredifficulties characterizing fire whirls over liquid pools, such as the precession of the whirl aroundthe enclosure, uneven heating, and subsequent variations in the heat-release rate. Experimentally,using a gas burner can be ideal, as the fuel flow rate (fuel’s Froude number 4) can be set to aconstant value during experimentation, decoupling heat release from flow conditions and enablingvariations over a wide range. Several works have used gas burners to better understand the empiricalrelationships between the flame height, ambient swirl, and heat-release rate and have observedthat the diameter of the fire whirls generated is somewhat smaller than those generated from liquidpools