Study On Neutral-Plane Location In Shafts And Stairwell In High-rise Buildings

There are more factors causing fire and more vertical channels resulting in the rapid spread of fire smoke in high-rise building, therefore the the fire risk of high-rise building is greater. Different parts of high-rise buildings positions differently relative to the neutral-plane, the gas flow in different ways, so the specific smoke control strategy will be determined according to the location of neutral plane. Thereby, study on smoke movement in vertical channels of high-rise building, especially the neutral-plane location in these channels under the chimney effect, is of great significant for fire safety. The neutral-plane in shafts and stairwells with different opening conditions were studied in this article.For the neutral-plane position in shaft, Klote model and two zone models have been developed, however, the two models didn't fully consider the heat loss between the fire smoke and the shaft walls. It shows that there are some limitations in these two models. As for the neutral-plane location in straiwell has been reported rarely.Through theoretical analysis, considering the heat loss between smoke and shaft walls, multi-zone model and continuous model were proposed for determining the neutral plane location in shaft under a continuous side opening and a continuous side opening with a upper opening of the shaft. Numerical study and experiments were carried out to validate the models.The following conclusions were obtained in the verification process:a) The deviation of Klote model was biggest; In any conditions, Klote model values higher than the simulation and experiment results; The deviation of two zone model was bigger; The deviation of Klote model and two-zone model for higher shafts was relatively smaller.b) Neutral surface height changes little with the fire HRR, more with the opening condition. With the HRR increases, the neutral-plane location decreaces slightly; with the top opening area increases, the neutral-plane location increases significantly. Under the continuous side opening, the neutral-plane maintains below the half of shaft height; when there is a upper opening, the neutral-plane may higer than 1/2 height of the shaft.c) When the area of upper opening isn't very big, the deviations of continuous model and multi-zone model are small.When WH(?)≈0, the multi-zone model couldn't calculation the neutral-plane height very precisely, howevre, the continuous model could.Experiments in a 1/3 model of a stairwell and simulations were carried out in order to investigate the neutral-plane location in stairwell under different opening conditions. The following conclusions were obtained:a) Under the conditions of each floor opening and each floor over the fire-floor opening in stairwell, the position of neutral-plane was higher; Under the conditions of adjacent floors opening in stairwell, the position of neutral-plane was lower; Under the condition of top opening, strong chimney effect was produced.b) The neutral-plane does not exist for overall stairwell, and small neutral surface exists for disordered floors in stairwell separately, when the fire was in first floor and a side continuous opening in stairwell. And a number of "Shaft" small cycles were formed between in and out the stairwell by the side opening. With the increased in fire power, the number of disordered floors increased, also the number of small cycles increased.c) There was a a continuous side opening in stairwell, and the fire HRR retained the same. With the location of fire raised, the range of air flow into the stairwell increased, the number of disordered floors decreased, the number of small cycles also decreased.