# Research on smoke control for a fire in an underground shopping center, based on a smoke barrier and a mechanical smoke evacuation system

### Introduction to the underground mall model

Sections 9, 10, 11 and 12 of Fuxin City Underground Commercial Street, which includes an underground shopping mall, are located in the middle of Fuxin Underground Commercial Street. His article mainly simulates a fire in this underground mall. The effective use space of the mall is 90m long, 18.4m wide and 3m high19. There are two exits on each side of the mall and each exit is 4.4m wide and 2.7m high, and each store is 6.6m wide. The schematic diagram of the underground shopping mall model is shown in Fig. 1. In order to simplify the model, a staircase leading above the ground is established only for the upper left exit, while the other three exits are assumed to be directly connected to the ground. level.

### Fire source and monitoring point setting

In order to study the flow of fire smoke near the exit and the flow of fire smoke in the middle of the corridor with a high risk coefficient, two representative locations of the fire source are selected with reference to the structure plane of the building, which are respectively left and middle of the corridor, and the size of the fire source is 2.0 m × 2.0 m, as shown in Fig. 1. According to the reference20.21 and the grid cell size of the geometric model of this simulation, smoke temperature below 60°C and visibility above 10 m at the characteristic human eye height of 1.75 m are selected as standards for safe evacuation. According to this standard, a total of 7 monitoring points (A, B, C, D, E, F and G) are placed at an interval of 11 m in the hallway of the mall, which are 1.75 m from the ground. and the monitoring of the points (J and K) are fixed at the level of the outlet channel to monitor the law of evolution of the smoke parameters. The fire source growth stage heat release rate curve is t2 typical, the fire load density is set at 635 MJ/m2the fire growth coefficient is 0.125, the maximum heat release rate is set at 5MW, and the growth time is 200s22.

### Grid Settings

The size of the space to be divided into grids is an underground shopping center with a size of 90 m × 18 m × 3 m and a stairwell of 18 m × 6 m × 10 m. The characteristic diameter formula D* from the fire source is23

$$D^{*} = left( {frac{{dot{Q}}}{{rho_{infty } c_{p} T_{infty } sqrt g }}} right)^ {{2}/{5}},$$

(1)

where: (dot{Q})—Fire source heat release rate, kW; (c_{infty})—Density of ambient air, kg m−3; (c_{p})—Specific heat of air at constant pressure, kJ (kg K)−1; (Tinfty)— ambient air temperature, K; (g)—Gravitational acceleration, 9.81 ms−2.

If the ambient temperature is 293 K, the characteristic diameter of the 5 MW focus will be 1.84 m. In order to ensure the accuracy of the simulation, the ratio of (D^{*}) grid cell size must be between 4 and 1623. The larger the ratio, the finer the simulation results and the longer the simulation time. Accuracy and time consumption of the simulation are comprehensively considered. The model is divided into three parts in which the number of grid cells is set to 270×50×8, 54×15×8 and 54×15×18, and the size of the grid cells is approximately 0.33 m × 0.4 m × 0.38 m, which is the same as the grid cell size. The ratio is between 4.6 and 5.6, which can meet the precision requirements.

### Boundary condition

The smoke vent boundary condition is set to exhaust and the smoke vent start mode is set to start after the temperature controller responds. The ambient temperature is 293 K and the limit at the output is set to “Open”.

### simulation time

Depending on the construction area and exit width, and in accordance with regulations in24 and25the ground floor area conversion coefficient is 0.7 and the average staff density is 0.8 person/m2. The average walking speed of people is 1.1m/s, the average effective value of reducing the width of stairs and exits is 0.3m, the fire detection time is 30s, and the average response time of people is 120 s. The approximate results of the calculations show that the evacuation time of people in the worst case does not exceed 530 s. Therefore, the simulation time is set to 600 s.