【作者】 李开源；

【导师】 霍然；

【作者基本信息】 中国科学技术大学 ， 安全技术及工程， 2008， 博士

【摘要】 水喷淋系统是常见的建筑消防设施。火灾发生时,从喷头喷出的液滴一方面能抑制燃烧,冷却火场烟气并防止火灾蔓延,另一方面却能改变火场内烟气的流动状态,造成烟气层失稳,引起烟气拥塞,使得上部烟气迅速填充建筑下部空间,并对建筑内其他消防系统特别是排烟系统的效果产生影响。针对此类问题,本文对火灾烟气层在水喷淋及排烟系统作用下的行为特性规律进行了研究,分析了水喷淋和烟气层相互作用的力学机理,进而对喷淋作用下采用的排烟模式及其有效性进行了探讨。首先从理论推导和实验两个方面,对水喷淋作用下烟气层失稳现象进行了研究。通过对喷淋启动后,喷淋覆盖区内拖曳力分布进行计算,在前人的基础上,提出以喷淋覆盖区内最大单位面积拖曳力和最大单位面积浮力之比D₀/B₀作为烟气层稳定性判据,并通过实验对此判据进行了验证。实验结果表明,与前人的结论（Bullen和Zhang）相比,D₀/B₀能更好的与实验结果相符合。当D₀/B₀小于1时,烟气层保持稳定状态,喷淋区内烟气—空气之间的交界面清晰,当D₀＞B₀时,烟气层失去稳定性,出现拥塞射流,其长度以喷头正下方即喷淋覆盖区中心最长,向外逐渐缩短,呈倒置的钟型。水喷淋有可能导致原先稳定在建筑上部的烟气层发生失稳,使得上部烟气发生向下运动,引起拥塞射流。喷淋作用下火灾烟气层的拥塞射流长度与火源功率、烟气层厚度及喷头工作压力等因素有关。本文在烟气层稳定性的基础上,以动能定理为理论依据,建立了一种预测喷淋作用下拥塞射流长度的数学模型,得到了射流长度与喷淋压力、烟气层厚度和温度的相互关系。而后通过全尺寸喷淋实验,将理论计算结果与与实验进行了对比,通过对比得到,失稳后,喷淋覆盖区拥塞射流长度随喷淋压力p_d的增加而增加,数学模型计算值与实验值符合得较好。p_d增加,喷淋对烟气的降温能力趋于不变,而烟气拥塞射流的长度则随p_d的增大近似线性增加,当喷淋区烟气失去稳定性后,加大喷淋压力仅能小幅降低烟气温度,却显著降低了喷淋覆盖区的烟气高度,增加了对火场人员的威胁。研究了水喷淋系统和排烟系统共同作用时的相互影响,以及多系统共同作用时烟气的流动特性问题。针对喷头近域水平自然排烟问题,在实验的基础上,根据伯努利定理,提出了预测自然排烟速度的概念模型。通过模型实验,研究了该模型的适用性,同时,对水喷淋和排烟共同作用下烟气层稳定性问题进行了研究。通过实验发现,喷淋压力p_d,增加,排烟口的烟气流速减小,当压力增至某一特定值时,排烟速度降为0,出现排烟失效。排烟速度模型计算值和实验实测值吻合较好,能够反映实验规律。排烟和喷淋同时发挥作用时,排烟的影响主要体现在降低烟气层的温度上,在排烟未失效的情况下,适度的排烟可以将烟气层高度控制在挡烟垂壁下边缘以上,而过度排烟则可导致防烟分区内烟气温度的急剧下降,使得烟气层的整体稳定性降低。在自然排烟的基础上,对近域水平机械排烟在水喷淋作用下的排烟效果进行了研究。通过实验分析了在喷头近域进行排烟的必要性,由于自然排烟的不可靠性,在喷头近域必须采用机械排烟作为替代设备。在喷淋作用下,喷头近域机械排烟速度将减小,但并不会出现排烟失效。随着工作压力p_d的增加,平均排烟速度趋于一极值,这一速度值的大小为喷淋区无燃烧烟气时排烟风机在排烟口能产生的最大气流速度。因而,在喷头近域采用机械排烟系统是可靠的。本文最后对利用数值模拟的方法对水喷淋与火源和烟气共同发生作用时火源功率的变化以及此条件下烟气层和喷淋的相互作用进行了研究。以此为依据,验证了CFD方法研究火场内水喷淋作用的可行性。结果表明,随着喷淋流量（工作压力）的增加,到达火源表面的水量（喷淋密度）增大,喷淋的燃烧抑制能力增强,火源功率的衰减幅度逐渐增加。火区内烟气失稳以及失稳后的烟气高度均与火源功率有关,流量不变,火源功率降至某一值,烟气层出现失稳,烟气层高度则随火源功率的减小而降低,功率越小,烟气层越低,以欧拉-拉格朗日法为基础的数值模拟技术,对水喷淋在火场中的相关问题,有较强的适用性。更多还原

【Abstract】 Recently,sprinkler system is installed in the buildings with large inner space.The sprinkler spray out of the nozzles may suppress the fire and cool the smoke as to prevent the fire spreading.But on the other hand,the spray may also cause the instability of the smoke layer and affect another fire fighting system such as the smoke venting opening and the smoke exhausting system.In this dissertation, interaction of the smoke layer and the sprinkler spray was analyzed.The behavior of the smoke layer under sprinkler spray was observed by experiments.The effect of sprinkler spray on the smoke venting and the mechanical smoke exhausting system was studied.The instability of the smoke layer under sprinkler spray was studied by theory and experiments firstly.Based on the calculation of the drag force distribution in the spray region,ratio of the initial drag force and the initial buoyancy force directly below the sprinkler D₀/B₀ was expressed as a new criterion for smoke layer stability.Experiments results showed that the smoke layer remained stable when D₀/B₀ was less than 1,and a downward smoke plume which represented the instability of the smoke layer formed when the D₀/B₀ ratio was more than 1. Compared to Bullen and Zhang,D₀/B₀ was more suitable for the experiment phenomenon.The instability of the smoke layer would cause the smoke logging plume.Based on the theoretical analysis,the length of the smoke logging plume was predicted by a mathematical model on the interaction of the smoke layer with sprinkler spray.For validating the model,several experiments were carried out.Results showed that the predictions,including the critical condition and the length of the plume,by the mathematical model agreed well with that observed and measured in the experiments. The length of the downward plume was shown to increase with the sprinkler operating pressure by an approximately linear correlation.In this thesis,effect of the sprinkler spray on the horizontal smoke venting which is located on the adjacency of the sprinkler was studied.Stability of the smoke layer under the sprinkler spray with smoke venting was compared with the situation without smoke venting.A conception model was developed for velocity of the horizontal smoke venting under the sprinkler spray.The model was deduced by the Bernoulli’s equation.Experiments had been carried out for validating the conception model.The results showed that the smoke venting velocity was affected by both of the buoyant effect of the hot layer and the drag effect of sprinkler spray.Smoke can be exhausted by venting only when the buoyant effect is stronger than the drag effect.Velocity of the smoke venting decreased as the sprinkler operating pressure p_d increased.And no smoke flowed out under certain operating pressure,which represents the failure of the smoke venting.The velocity calculated by the conception model agreed well with that observed and measured in the experiments.Effect of the smoke venting was mainly on decreasing the smoke temperature when the venting was not failure.So stability of the smoke layer would become very weak if the smoke venting effect was very strong in the spray region.And moderate smoke venting was necessary for containing the smoke within the curtained smoke compartment.Based on analysis for smoke venting under sprinkler spray,the effect of the sprinkler spray on the mechanical smoke exhausting system was experimental studied. The necessity of smoke exhausting or venting in the adjacency of the sprinkler was proposed by experiment firstly.The mechanical smoke exhausting system was chosen as substitute of the smoke venting as the unreliability of smoke venting failure.The experiments results showed that velocity of the mechanical smoke exhausting decreased a bit when the sprinkler was operating.The velocity of the mechanical smoke exhausting decreased to a certain value as the sprinkler operating pressure p_d increased,and no failure was found under the spray.So the mechanical smoke exhausting system is reliable in the adjacency of the sprinkler compared to the smoke venting.Numerical simulation is applied to investigate the layer height of hot smoke and the development of the HRR under sprinkler spray.Based on the simulation result, effect of the spray and the fire source on the descent character of the hot smoke is analyzed.The driving force of the smoke descent is concluded by comparing the developments of the layer height under different fire source condition.The study shows that increasing the flow of the sprinkler,the decay extent of the HRR increased, and the descent effect to the hot smoke of the spray enhanced,the descent region extends and the layer height decreased.The smoke descent results from the synergistic effect of the descent effect of the spray and the buoyant effect.The layer height decreases with the decreasing of the HRR after the smoke descent arising.The temperature of the smoke decreased with increasing of the sprinkler operating pressure.The CFD technology of Eulerian-Lagrangian is fit for simulating the sprinkler spray phenomenon in the building fire.更多还原