*3.1. Analysis of Smoke Spread in Atrium Fire*

A large amount of smoke and heat generated during a fire will form a hot smoke stream. The flow direction of the smoke is often the direction of the fire spread, and the flow speed of the smoke is often the fire spreading speed [18]. The smoke will gradually collect over the atrium, and the smoke layer will continue to settle, which will continuously reduce visibility, affect the visual range of evacuees and then affect the evacuation speed.

According to the principle of fire dynamics, the development of fire goes through three stages named accelerated combustion, stable combustion and the extinguishing stage. The spread of smoke in each stage is also different. In the stage of accelerated flame combustion, fire smoke is generated and continues to spread upwards and accumulates, reaching the ceiling and continuing to spread around. The fire has developed into a stable combustion stage, and a large amount of smoke generated before has accumulated in the ceiling and formed a stable smoke layer, which continues to settle. Until the extinguishing phase, smoke is continuously generated and fills the entire atrium. Figure 6 shows the schematic diagram of smoke spread under working Cases 1~5.

It is not difficult to see from Figure 6 that the smoke rises first under the action of thermal buoyancy, and after reaching the ceiling, it begins to spread horizontally until it fills the ceiling and spreads into the ring corridor. Due to the restrictions of the walls on the east and west sides of the atrium, the smoke begins to fill downward, and then the smoke continues to be generated. At 300 s, it has basically filled the atrium, and the smoke layer in the atrium begins to settle slowly, with a clear boundary with the cold air layer.

Comparing Cases 1, 3 and 4, we find that different heat release rates will make a significant difference in the speed of smoke spread; under the condition of a large heat release rate, the smoke spreading rate is greater than that of the small heat release rate, and the thickness of the stable smoke layer is also significantly thicker. It is not difficult to understand that the greater the heat release rate is, the more obvious the thermal buoyancy effect is, so the stronger the winding effect is. Therefore, it can reach the roof earlier, gather, and begin to settle. When the natural smoke exhaust (Cases 1, 2) and mechanical smoke exhaust (Cases 4, 5) are turned on, the concentration and thickness of the smoke layer will be significantly reduced. This is because the opening of the smoke exhaust vent makes the smoke continuously pump outside, reducing the concentration of the smoke layer and making the thickness of the smoke layer decrease. From this point of view, the smoke exhaust effect of the large atrium is good, and it is very necessary to set up a reasonable smoke exhaust system.
