## 3 Results
### The Potential Causes of the Observations
#### 3.2.1 The Potential Causes of Observations of Case 01  
The pressure contour and velocity vector field of the Slit 01 case at HAB 0.1 \(m\) at the selected instant of time, are presented in Figures 28 and 29. The figures show that the air at the monitoring HAB is flowing from high pressure to low pressure region, i.e., drawn from surrounding to the enclosed space via entraining slit. This observation generally agrees well with the commonly agreed flaming behaviour in the fire scenario. To be more specific, a fire in the compartment is expected to grow in direct proportion to available oxygen. In a typical compartment fire, air carrying oxygen is often observed to be drawn from outside into the base of the fire through openings in the building, e.g., doors, windows, roof. The opening of the slit in the current case acting as the source of entraining air promotes the combustion process, and at the same time facilitates the movement of combustible gas mixture, and the subsequently involved flame structure, heat and smoke, from higher pressure areas towards lower pressure areas accessible openings. The pressure contour combined with velocity vector field also indicates the relative vortex core location and the approximate size of the spinning reacting flow. It can be seen from the figures that, the size of the swirling reacting flow, as well as the rotating radius, of the fire whirl of the Slit 01 case is relatively small when compared with that of the Slit 02 case shown in from Figures 32-35. It is suggested that the core structure of the formulated fire whirl is constrained in a confined space, and is rotating in a relatively small radius with respect to the fuel pan. The revolution orbit of the fire whirl with respect to the fuel source is obtained by transferring the location of the vortex of all time instant of interest and plotting into Figure 30. In addition, the incoming velocity of the slit of the monitoring HAB, at the corresponding time instant, is presented in Figure 31. The location of the fire whirl core is indicated by the \(\boldsymbol{\Theta}\) in various colour, in both figures. Figure 28: Pressure contour of the domain at 0.1 m HAB, at five timesteps of the Slit 01 case selected based on representative time instants associated with Slit 01 (anti-clockwise from top-right with increase in time). The contours indicate the fire whirl core location and collectively illustrate a full circle of orbital revolution of the whirling reacting flow with respect to fuel source. Figure 29: Vector field plot of the domain at 0.1 m HAB, at five timesteps of the Slit 01 case selected based on representative time instants associated with Slit 01 (anti-clockwise from top-right with increase in time). The plots indicate the fire whirl core location and collectively illustrate a full circle of orbital revolution of the whirling reacting flow with respect to fuel source. From Figure 30, it can be seen the anti-clockwise revolution orbit of the fire whirl of the Slit 01 case, starting from, and ending with, is not in a shape of a regular circle. It appears the vortex core location deviates from domain centerline as it approaches the entrainment slit, and restores to the near fuel source location as it moves away from entrainment slit. To be more specific, for the fire whirl originated from the fuel pan centre, the rotating radius can be defined as the distance between the vortex core and the fuel pan centre. It can be observed that, from time instant of \(t=47.25\) s () to \(t=47.45\) s (), the fire whirl is rotating anti-clockwise from northern wall region to western wall region and moves away from the air entrainment slit. The rotating radius decreases significantly. While from time instant of \(t=47.66\) s () followed by \(t=47.73\) s () and to \(t=47.81\) s (), the fire while sweep from near eastern wall region towards near northern wall region and moves towards the air entrainment slit, the rotating radius increases significantly.