Search

The mechanic performance of ejecting pneumatic fire extinguisher is determined by the following parameters: air speed at funnel exit, outfire range and ejecting stability; and the second and third parameters depend on the fire extinguishing agent pellet's velocity on funnel exit and the amount of fire extinguishing agent pellets detained in the funnel when extinguisher is working. In this research, the CFD-DEM coupling model was adopted to simulate the working process of ejecting pneumatic fire extinguisher, and the type of 6MF-30 pneumatic extinguisher was used to test the reliability of the coupling model. The funnel throat angle X1, the adding position of fire extinguishing agent X2, and the funnel length X3 were selected as the influencing factors, and total 23 experiments were conducted by the simulation model under the quadratic orthogonal rotation design. The virtual tunnel was designed according to the tunnel equipped to 6MF-30 pneumatic extinguisher. The virtual fire extinguishing agent pellets were found on the EDEM interface, coupled to the original CFD by Lagrangian model. The pretest results showed that this model had strong convergence which was proved by its stable airflow performance when running at the time of 0.2-0.25 s, and stable pellet number when running at 0.34 s. The air velocity at exit calculated by the CFD-DEM coupling model was similar to the theoretical value. It could be found that the number of aggregating virtual pellets detained in the region between the air tunnel and the fire extinguishing agent's channel was consistent with the result of research by other scholars. The simulating experiment data were exported for analyzing. The average velocity in axial direction on each point at funnel exit was read by Fluent, and after subtracting the average value 197.550, the deviation data of the 23 experiments were got and selected as the outlet wind velocity (index 71). The max virtual pellet velocities in axial direction monitored by EDEM within 0-0.4 s of all experiments were selected as the spouting velocity (index Y2). The vertical coordinates of every virtual pellet at 0.37, 0.38, 0.39, 0.4 s in all experiments were read by EDEM, and 'countif' function was used to calculate the number of virtual pellet located in the tunnel (as assessment index Y3) when working. After the significant test and the variance analysis, the regression equation building was done by using SAS9.1. The result showed that the fire extinguishing agent pellet's velocity on funnel exit and the amount of fire extinguishing agent pellets detained in the funnel were significantly correlated to the three factors (P<0.0001, P<0.001 respectively), but the air speed at funnel exit had no correlation with these factors. By using response surface method combined with weighting method, the optimal aggregative index could be obtained under the condition that the funnel throat angle was 20°, the adding position of fire extinguishing agent was 140 mm and the funnel length was 700 mm. Under this condition, the efficiency of ejecting pneumatic fire extinguisher was mostly improved. The virtual test designed by the optimized parameters showed that the fire extinguishing agent pellet's velocity on funnel exit increased to 41.24 m/s, and the number of fire extinguishing agent pellets detained in the funnel decreased to 209. The verify test of the physical prototype indicated that the optimized parameters could improve the working effect of ejecting pneumatic fire extinguisher, which was manifested in extending the outfire range by 2.15 m, reducing the possibility of reburning and shortening the outfire time by 1.7 s. [ABSTRACT FROM AUTHOR]

A portable pneumatic extinguisher is an effective device that has been widely used for forest and grassland fire extinguishing in China. In order to enhance the effective range and rate of discharge of portable pneumatic extinguishers, a new method to weaken air velocity attenuation by increasing the flow rate using an air ejector was proposed and investigated in this study. This research belongs to the category of subsonic air ejector. First, the turbulivity of air jet "c" has been updated to the value of 0.106 by experiments. It is necessary to use this factor for the calculation of air velocity where 2.5 m downstream from the centrifugal fan which defined as u2.5. Experimental and the CFD (Computational Fluid Dynamics) methods are applied to investigate the influence on the performance of the air ejector. Three parameters were characterized: converging angle of entraining chamber θ; diameter of the mixing chamber Dm and the nozzle position (NXP) S. Up to 240 different models were established and meshed by Gambit 2.3, and then simulated and calculated by Fluent 6.3 with the turbulence model of RNG k-epsilon. Consequently, 240 different results containing the flow rate of nozzle outlet cross-section mp, the entrainment flow rate me, the flow rate of the mixing chamber outlet cross-section mc, and the air velocity of a mixing chamber outlet cross-section uc were acquired. Based on this data, the response surfaces for u2.5 and entrainment ratio that are used to investigate the interaction between Dm, θ and S to u2.5 and entrainment ratio were established. The results indicated that the parameters Dm and θ have a great influence on u2.5 and entrainment ratio. It was also demonstrated that the interaction between Dm and θ is significant. However, parameter S gave a relatively delicate influence on u2.5 and entrainment ratio, and also the interaction with the other parameters was weak. In addition, to find out the mechanism of interaction between Dm and θ, three groups of different pressure fields have been investigated and compared as well. It indicates that a high value of θ (when θ < 36.25°) could create a relatively significant negative pressure zone in the mixing chamber, which requires a larger Dm to provide sufficient air input, and as a consequence u2.5 and entrainment ratio increase accordingly. Because of the negative pressure zone nearby the nozzle outlet, the effective power of the centrifugal fan and engine are both increased. The optimum value of θ is 36.25°, and when Dm equals to 144 mm, u2.5 could reach the maximum value. The maximum u2.5, which was gained by experiments using an ejecting pneumatic extinguisher, was 36.4 m/s. Meanwhile, the value of u2.5 could keep adding up as Dm continued to increase, but the range of Dm has been limited by the design and practical applicability of the portable pneumatic extinguisher. [ABSTRACT FROM AUTHOR]