Studies On Stable Character Of Smoke Layer Under Sprinkler Spray In Fires

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.