Analysis and experiment on air-ejecting for improving wind speed of pneumatic extinguisher

Abstract: 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.