| Portable pneumatic extinguisher is an effective device which 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 extinguisher, a new method to weaken air velocity attenuation by increasing the flow rate using an air ejector is proposed and investigated in this study. This research belongs to the category of subsonic air ejector.Firstly, tow types of turbulivity of air jet’a’has been expressed by the non-uniformity of flow velocity distributions on. It’s necessary to use this factor for the calculation of air velocity where1.5meters downstream from the outlet cross-section of mixing chamber which defined as u1.5. Experimental and the CFD (Computational Fluid Dynamics) methods are applied to investigate the influence on performance of the air ejector. Four parameters are characterized:converging angle of entraining chamber θ; diameter of the mixing chamber Dm; the nozzle position (NXP) S and lenth of mixing chamber Lm.Up to241different models are established and meshed by Gambit2.3and then simulated and calculated by Fluent6.3with the turbulence model of RNG k-epsilon. Consequently,241different results containing flow rate of nozzle outlet cross-section Gp, entrainment flow rate Ge, flow rate of mixing chamber outlet cross-section Gm, air velocity of mixing chamber outlet cross-section uc are acquired. Based on these data, the response surfaces for u1.5, Lc and Ac which are used to investigate the interaction between Dm,θ, S and Lm to u.2.5and entrainment ratio have been established. The results indicate that the parameters Dmand θ have great influence on u1.5, Lc and Ac. It is also demonstrated that the interaction between Dm,θ and Lm is significant. However, parameter S gives a relative delicate influence on u1.5, Lc and Ac, and also the interaction with the other parameters is weak. In addition, to find out the mechanism of influence on u1.5, Lc and Ac, the pressure field and velocity field of every single factor of Dm,9, S and Lm have been investigated and compared as well. It indicates that high value of θ could create a relatively significant negative pressure zone in the mixing chamber, which requires a larger Dm to provide sufficient air input, as a consequence u1.5and entrainment ratio increase accordingly. Because of the negative pressure zone nearby nozzle outlet, the effective power of centrifugal fan and engine are increased both. The optimum value of θ is25.3°, when Dm equals to144mm, Lm equals to900mm, S equals to-60mm, u1.5could reach the maximum value. The maximum u1.5Lc and Ac which is gained by experiments using an ejecting pneumatic extinguisher, is32.6m/s,2.32m and0.060m2respectively, which exceeds the traditional pneumatic extinguisher8.31%,9.95%,62.2%. Meanwhile, the value of u1.5could keep adding up as Dm continues to increase, but the range of Dm has been limited by design and practical applicability of portable pneumatic extinguisher.For the MTE pneumatic ejecting extinguisher, Up to486different models are established and meshed by Gambit2.3and then simulated and calculated by Fluent6.3with the turbulence model of Spalart-Allmaras. Consequently,486different results containing flow rate of nozzle outlet cross-section Gp, entrainment flow rate Ge, flow rate of mixing chamber outlet cross-section Gm, air velocity of mixing chamber outlet cross-section uc are acquired. Based on these data, the response surfaces for u1.5, Lc and Ac which are used to investigate the interaction between Dm,9, S and Lm to u2.5and entrainment ratio have been established. The results indicate that the parameters Dmand9have great influence on u1.5, Lc and Ac. It is also demonstrated that the interaction between Dm,θ and Lm is significant. However, parameter S and9gives a delicate influence on M1.5, Lc and Ac. In addition, to find out the mechanism of influence on m1.5, Lc and Ac, the pressure field and velocity field of every single factor of Dm,θ, S and Lm have been investigated and compared as well. Because of the negative pressure zone nearby nozzle outlet, the effective power of centrifugal fan and engine are increased both. The optimum value of6is20°, when Dm equals to80mm, Lm equals to700mm,S equals to0, u1.5could reach the maximum value. The maximum u1.5, Lc and Ac which is gained by experiments using an ejecting pneumatic extinguisher, is42.7m/s,3.16m and0.091m2respectively. Meanwhile, the micro turbine pneumatic ejecting extinguisher has been designed and manufactured. Grassland fire extinguishing experiment has been carried out and it has been proved that the extinguishing effect of MTE pneumatic ejecting extinguisher is much better than portable pneumatic extinguisher. |
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