Modern large-scale commercial buildings are moving towards the direction of large-scale, multi-functional, holistic development, set of commercial, catering, amusement, culture, leisure and other functions in one, the formation of industry-rich building complex. Due to the large scale of the building, complex functions, and the pursuit of architectural integrity and continuity of commercial business, modern large-scale commercial buildings, especially large-scale above ground commercial building area is often far more than 20,000 ITI. in order to control the situation of a large number of underground commercial connected together, often using firewalls, fire compartments, sunken plazas, etc., fire protection separation, in order to ensure that the integrity of the commercial and create an air-conditioned environment, many sunken plazas are usually closed, and the fire is usually closed, and the fire is not a problem. In order to ensure the overall commercial and create an air-conditioned environment, many sunken plazas are usually closed, and in case of fire, the roof natural smoke exhaust windows are opened to exhaust smoke, so as to create the effect equivalent to semi-outdoor, and this special design also brings new problems to the overall smoke evacuation of the building. Sunken plaza using natural smoke exhaust, and sinking plaza adjacent to the ground semi-closed commercial street using mechanical smoke exhaust, as the commercial street directly to the sunken plaza, adjacent areas of the two different smoke exhaust systems are bound to affect each other, how to effectively carry out the design of the smoke exhaust mode, optimize the location of the exhaust port, improve the efficiency of the smoke exhaust in case of fire, to create a safe environment for evacuation of people, in-depth demonstration and analysis needs to be carried out. The author will analyze a specific case.

1 building profile and the problem

A certain city sub-center project is a joint development of four large-scale integrated commercial plots, underground***3 layers, of which the first and second floors are commercial, with a total area of nearly 170,000m, the third floor is a car garage, the plots are interconnected, the commercial scale is large, multi-functional, high density of personnel, and the internal traffic flow is complex during normal operation, while the building design needs to be The building design needs to take the safety of evacuation of people in case of emergency, convenient and reasonable use of functions, and beautiful and environmentally friendly building as the basic design requirements. Underground commercial **** set up 13 sunken squares, combined with firewalls, commercial streets and other separation measures to separate the underground commercial into nine fire blocks with an area of less than 20,000Tn. The huge volume of the project forms a densely populated underground commercial neighborhood, which can be compared to the concept of underground commercial city. Taking one of the sunken plaza underground second floor as an example, the sunken plaza adopts the roof and high side windows for natural smoke exhaust, and the second floor of the underground commercial street adopts mechanical smoke exhaust, and the sunken plaza is proposed to be separated from the smoke wall for smoke prevention, the section is shown in Figure i. Difficulties in the design of this project: when starting the mechanical smoke exhaust of the underground commercial street, the lower part of the sunken plaza will make up the wind to the underground commercial street, and the make-up airflow will reduce the effect of the natural smoke exhaust in the sunken plaza. Therefore, how to effectively carry out the smoke separation between the two, optimize the arrangement of the mechanical smoke vent in the underground commercial street, reduce its impact on the natural smoke exhaust effect of the sunken plaza, as well as both start, the sunken plaza of the smoke effect of how to be demonstrated and analyzed.

2 Underground commercial street fire smoke exhaust simulation and optimization

2.1 Fire Scenario Design

In the sunken plaza area, most of the sunken plaza's bottom or lower layers have a certain area of the floor covering area at each level, the floor covering area is also part of the sunken plaza, which is referred to as "sunken plaza corridor zone". ". Taking the sunken plaza as an example, in order to investigate the smoke removal strategy of the corridor area, the smoke removal effect of the sunken plaza under the two situations of setting up mechanical smoke removal in the corridor area and utilizing the natural smoke removal window in the upper part of the sunken plaza is studied, and two working conditions are set up, i.e., l, 2. In order to investigate the effect of the distance of the nearest smoke exhaust outlet of the underground commercial street from the smoke barrier between the underground commercial street and the sunken plaza on the natural smoke removal of the sunken plaza, the working conditions are set up, i.e., 3-5, which are shown in Table 1. As shown in Table 1. Table 1 sunken plaza corridor area smoke strategy research working conditions table sunken plaza underground commercial street nearest smoke outlet from the commercial street working conditions fire scale and sunken plaza between the smoke blocking corridor area smoke exhaust smoke plumbing wall distance / m sunken plaza phase mechanical smoke, hole fire l neighboring restaurant fire, and the edge of the corridor area 8MW with a smoke wall does not open the sunken plaza phase to take advantage of the sunken plaza 2 neighboring restaurant fire, the natural smoke exhaust window exhaust smoke in the fire The edge of the area is not blocked not open 8MW smoke hanging wall using the sunken plaza underground commercial street natural smoke exhaust window exhaust fire when 310 fire.

2. 2 commercial street and the sunken plaza between the smoke wall settings

Figures 4 and 5 show the case 1 and case 2 sunken plaza vertical cross-section of the fire visibility comparison and temperature vector map comparison. It can be seen, the sunken plaza ring corridor with mechanical smoke exhaust (ring corridor and the opening between the installation of smoke walls) and no fire science and technology January 2o12, Volume 3l, Issue 1 mechanical smoke exhaust (ring corridor and the opening between the smoke wall), the natural smoke exhaust of the sunken plaza can be better to spread to the sunken plaza smoke exhaust; but in Case 1, due to the formation of the ring corridor mechanical smoke exhaust of the forced air convection of the air in the sunken plaza, the natural exhaust air convection of the sunken plaza can be better than in Case 1, because the ring corridor mechanical smoke exhaust. However, in Case 1, due to the mechanical smoke corridor formed by the forced airflow on the natural exhaust air convection in the sunken plaza has a certain degree of disturbance, resulting in part of the smoke was "dragged" down, slowing down its rise in the sunken plaza in the rate of smoke exhaust, and the sunken plaza corridor no smoke, can be guaranteed through the sunken plaza of the exhaust of the sunken plaza for a longer period of time of the lower part of the clear layer of air. Comprehensive analysis of conditions 1, 2, it is recommended that the sunken plaza corridor area is not set up mechanical smoke exhaust, are using the top of the sunken plaza natural smoke window smoke exhaust, can achieve effective smoke exhaust.

2.3 Underground commercial street smoke vent and smoke wall distance

Figures 6 and 7 show the comparison of fire visibility and temperature vector diagrams for conditions 3, 4, 5 over the vertical section of the underground commercial street and sunken plaza, respectively. As can be seen from the figure: due to less combustible materials in the underground commercial street, the fire scale is small, in the underground commercial street under the role of mechanical smoke exhaust, coupled with the underground commercial street and the sunken plaza set up between the 2m high smoke barrier, in the case of the three conditions of 3 to 5, the smoke can be controlled in the underground commercial street within 1200S, only a small amount of smoke spread to the sunken plaza. It can be seen from Fig. 7: under the premise that the smoke exhaust system in the underground commercial street can control the smoke well, when the horizontal distance of the nearest exhaust port from the smoke barrier is 10m and l5FO_, the forced convection formed by the mechanical smoke exhaust makes the air in the sunken plaza flow to the pedestrian street, which affects the efficiency of the exhaust of the sunken plaza, and it is not conducive to the rapid exhaust of a small amount of smoke that spreads to the sunken plaza; while when the nearest exhaust port When the horizontal distance of the nearest smoke vent is increased to 20i71 from the smoke barrier wall, the airflow disturbance of the mechanical smoke exhaust system of the underground commercial street to the sunken plaza is obviously reduced, and most of the air in the sunken plaza forms a good convection through the upper smoke vent, and when a small amount of smoke spreads to the sunken plaza, it can also be quickly discharged through the top smoke vent window and the high side window. Vector diagram of the vertical section of the fire temperature of the comprehensive analysis of conditions 3, 4, 5, it is recommended that the nearest smoke vent in the underground commercial street from the horizontal distance between the underground commercial street and the sunken plaza of the smoke wall is not less than 20ITI, but should not be greater than the code of the farthest 30m requirements.

3Conclusion

For a mega underground commercial building project, through fire dynamics simulation, the smoke exhaust program of the sunken plaza ring corridor area and the arrangement of the mechanical smoke vent in the underground commercial street adjacent to the sunken plaza are demonstrated. The results show that:

(1) the lowest or lower floors of the sunken plaza have a certain "sunken plaza corridor area" on each floor, if the corridor area is set up for mechanical smoke exhaust, the forced airflow formed by the mechanical smoke exhaust has a certain degree of perturbation to the natural smoke exhaust air convection in the sunken plaza, resulting in part of the smoke is "dragged", and the smoke is "dragged". "dragged" down. Therefore, it is recommended that the sunken plaza corridor area are utilized on the top of the sunken plaza natural smoke exhaust window, which can achieve effective smoke exhaust.

(2)When the horizontal distance between the nearest smoke vent in the underground commercial street and the smoke-blocking vertical wall from the underground commercial street is 2O~30rrl, the mechanical smoke exhaust system of the underground commercial street significantly reduces the airflow disturbance of the sunken plaza, and most of the air in the sunken plaza forms a good convection through the upper smoke vent. Even if a small amount of smoke spreads to the sunken plaza, it can be quickly discharged through the top smoke vent and high side windows.

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