Study On The Fire Extinguishing Efficiency Of Water Mist Produced By Effervescent Atomizer

As one of Halon potential alternatives, water mist has been widely investigated around the world because of its high suppression effectiveness, non-toxicity, environmental friendliness and competitively price. The traditional water mist nozzle included pressure jet nozzles, impingement nozzles and twin-fluid nozzles. All of them have more or less disadvantages for fire suppression. Therefore, many research institutes and corporations are taking up with innovations in mist generation. Effervescent atomization is a method of twin-fluid atomization that involves bubbling a small amount of gas into the liquid before it is ejected from the atomizer. The technique of bubbling gas directly into the liquid stream inside the atomizer body is essentially different from other methods of twin-fluid atomization (either internal or external mixing) and leads to significant improvements in performance in terms of smaller drop sizes and/or lower injection pressures. Furthermore, the amount of atomizing gas required is considerably less than what is employed in all other twin-fluid atomization techniques. Effervescent atomization has been used successfully in a number of applications since its inception over ten years ago. Now it has been used in gas turbine combustors, consumer products, furnaces and boilers, internal combustion (IC) engines and incinerators, but has been rarely applied in fire suppression.First, two kinds of effervescent atomizers were designed for following atomization mechanism research and fire suppression experiments based on characteristics of effervescent atomization and requirements for good fire suppression effectiveness. Then water mist generation systems were developed. The flux characteristics of the effervescent atomizers were researched and the relationship of water flowrate with GLR and gas injection pressure was got. The liquid discharge coefficient are influenced by GLR, injection pressure and atomizer geometry.A set of experiments were conducted to visualize the two-phase flow pattern inside the atomizer and the flow structure outside the atomizer with a high speed camera. The results show that there are three regimes of the two-phase flows inside the discharge orifice, one is bubbly flow, another is annular flow while the other is the intermittent flow between them. A bubbly-intermittent flow transition criterion can be obtained by using the drift flux model with appropriate value for the maximum void fraction to take into account the entrance effect. The intermittent-annular flow transition criterion was predicted by modeling the annular flow along with the void fraction relationship, based on the drift flux model. In our experiments, the value of the maximum void fraction is 0.7 for bubbly-intermittent flow transition criterion and 0.784 for intermittent-annular flow transition criterion. In the bubbly flow regime, after discharging from the orifice the bubbles exploded and shattered the liquid sheath to small droplets, and the liquid ligaments between the bubbles would form relatively larger droplets. In the annular flow regime, a large amount of gas in discharged through the core portion of the nozzle exit orifice while the liquid is discharged in an annular shape. Hence, the liquid annulus is disintegrated into fine spray drops by the expansion of the gas and the shear stress. And the spray in the intermittent flow regime shows the mixed behavior of the bubbly and the annular flow regimes. Because of the transition of the flow regime inside the atomizer, the effervescent spray shows unsteadiness which corresponds with the flow regime.The spray characteristics (droplet size and velocity distribution, cone angle and flux density) of water mist produced by effervescent atomizer were measured under different conditions by a LDV/PDA (Laser Doppler Velocimetry/Phase Doppler Anemometry) system and other methods. The influence of injection pressure, GLR and atomizer geometry to the spray characteristics was analyzed. A SMD correlation encompassing all those internal flow regimes based on the Lund et al.'s primary atomization model was proposed, and the correlation represents the most of the measured data within ±20%.A series of fire suppression experiments were performed with the effervescent atomizer under local application and total flooding application. The flame shape, the temperature of the flame, fuel surface and the compartment, and the concentration of O₂, CO₂, CO were measured. Through the experiment results the fire extinguishing mechanisms of effervescent water mist with different kinds of atomizing gas were analyzed. And the influence of atomizer operation conditions, fuel type, ventilation conditions and atomizing gas type to fire suppression effectiveness were researched. The results can provide theoretic guidance for extending application of effervescent atomization in fire suppression technology.The comparative fire suppression experiments of water mist produced by pressure jet nozzles showed that the effervescent atomizers have the better effectiveness than the traditional ones under either local application or total compartment application. And the fire suppression effectiveness will greatly improved with the halocarbon fire extinguishing agent or inert gas as the atomizing gas when suppressing the fire which is sheltered from barrier. Therefore, it has a good future that applied effervescent atomizer in fire suppression technology.