Theoretical Model And Experimental Study Of Smoke Neutral Plane

Opening is the main channel for mass and energy exchange between building and outside environment, while the neutral plane is the main factor affecting the smoke flow through openings. The smoke flow through openings determines the fire development within the room; on the other hand, it is also the main reason for smoke spreading in buildings out of fire room. Therefore, it is of great significance to study the neutral plane in order to prevent smoke spread in a fire.By theoretical analysis, experiments and numerical simulation, this paper focused on the following aspects about the neutral plane:the burning characteristics and stability of fire source in experiment; calculation method for interface height of smoke layer; the shaft neutral plane calculation model considering the temperature and opening width distribution; neutral plane calculation model and the critical condition to suppress overflow for vertical opening of carbin with exhaust. The specific work of the paper includes:The burning character of pool fire and its stability under water-bath protection was studied. By the experiments, it is found that the radiation feedback of gasoline with a bright flame, accounts for84.6%to the total energy feedback, while31.9%for the methanol. Therefore, the burning rate of gasoline is more sensitive to flame disturbance; while the burning rate is more stable with water-bathing, the mean value decreased within10%. Thus, in order to get a more stable fire source, water-bathing protection was given to the pool fire in experiments of this paper.A new method to calculate interface height of smoke layer was developed. Based on objective temperature data, the within-class variance of smoke and air under different temperature threshold was compared to obtain the optimal threshold, and the two methods also effectively exclude subjectivity and empiricism during the threshold selection process. The experiment results showed that the model can eliminate the adverse effects of temperature oscillation to the smoke layer. When gas temperature was relatively lower with little gradient, this method can be effective for the smoke layer height calculation of the entire fire process, and provide a better partition interface location prediction for zone model than other methods.Reasons for shaft temperature’s decline were investigated, and a new neutral plane calculation model considering the temperature and opening width distribution was proposed. According to the heat transfer calculation equations for internal pipe flow, heat loss to the wall was carried out by scale analysis. And the results showed that in shafts with long-time smoke travelling, wall heat loss to the wall should be taken into consideration and a temperature calculation formula by exponential distribution was obtained for shaft. Based on this formula, iterative algorithm is proposed to calculate the influence of the cold air entrainment from the lower part of the shaft neutral plane. A universal neutral plane calculation model for shaft was developed, which considered the opening width, temperature distribution and the opening size in the shaft top. Through1/8size small-scale experiments, it was showed that the neutral plane height calculated by the model was in little variance when compared with experiment results, generally within5%, better than the other existing models.Neutral plane calculation model and the critical condition to suppress overflow for vertical opening of carbin with exhaust were studied. Zone model and continuous model were developed to calculate the neutral plane in vertical opening of carbin with exhaust and a formula was derived to estimate critical exhaust rate to suppress overflow and mass flow rate flowing into the burning room though openings. It is found by numerical simulation that the indoor temperature is mainly affected by the fire power, while the opening flows was dominated by the density difference due to temperature difference. The smoke exhaustion amount, opening width and fire power are important factors affecting the overflowing-inhibition condition, which have a great impact on the opening flow state. Full-size experimental study found that:under the fire condition of exhaust cabins, the indoor temperature approached stability at about200s, and smoke sank to floor. The temperature rise at different heights approximates linearly distribution in the cabin, and temperature changed little under the same fire power. The study also found that the critical exhaust rate increased respectively with the opening width and fire power. Zone model and continuous model were examined by both full-size experiments and numerical simulation. It is found that both of these two models can calculate the neutral plane height, critical exhaust rate, mass inflow through the opening with good accuracy. Zone model is easy-handling, but compared with the continuous model, its calculation result is relatively worse, the calculated critical exhaust rate is relatively lower, as the mass inflow through the opening is higher.