【作者】 黄鑫；

【导师】 廖光煊；

【作者基本信息】 中国科学技术大学 ， 安全技术及工程， 2007， 博士

【摘要】 细水雾作为哈龙替代品之一，由于灭火高效、无毒、价格低廉和环境友好等优良特性，在全球范围内受到普遍重视并得到广泛研究。传统的细水雾发生方法都有着或多或少的缺点和局限性，因此国内外的很多研究机构正致力于开发研究新型的细水雾发生方法，以充分发挥细水雾灭火技术的优越性，使其能在更广泛的领域内得到应用。气泡雾化技术作为一种新型的内混式双流体雾发生技术具有工作压力低、雾化效果好、用气量少以及简单可靠等众多优点，它已经在内燃机、燃气轮机、工业锅炉、废液焚化炉和制药等方面得到了广泛的应用，但在灭火方面的应用仍属于空白。本文根据气泡雾化技术的特点及有效灭火对细水雾雾场特性的要求，首先研究发展了基于气泡雾化的细水雾发生方法，并构建了气泡雾化细水雾发生的模拟实验系统。然后使用所设计的系统研究了气泡雾化喷嘴的流量特性，并通过拟合分析求出了液体流量与GLR（气液质量流量比）和气体注入压力之间的数学关系。实验中发现气泡雾化喷嘴的流量系数主要受到气液流量比、注入压力以及喷嘴结构等的影响。利用高速摄影的方法研究了气泡雾化喷嘴内部的两相流型和喷嘴外部的雾场结构及液相破碎过程，分析了气泡雾化的机理。结果表明，随着液体流量的减小和GLR的增大，气泡雾化喷嘴内部的两相流动从稳定的泡状流态变化到不稳定的过渡态，再到相对稳定的环状流态。选取合适的最大截面含气率α_max值，可用漂移流动模型来获得泡状流态和过渡态的转换标准。而基于漂移流动模型，使用截面含气率和流动的关系来模拟环状流动可以获得过渡态和环状流态的转换标准。对于我们的实验，泡状流态至过渡态的α_max=0.7，过渡态至环状流态的α_max=0.784。当喷嘴内部为泡状流动时，气泡爆破将周围液膜击碎成小液滴，而两层气泡之间的带状液膜则破碎为大液滴。当喷嘴内部为环状流动时，连续喷出的气体在喷口附近很短的距离内由于膨胀和剪切作用将液膜破碎成液丝和液滴。过渡态流动下，喷雾则间歇显示出泡状流态和环状流态下的模式。由于喷嘴内部流型的转变，气泡雾化细水雾存在不稳定的现象，其变化规律与两相流型是对应的。作为气泡雾化细水雾灭火性能研究的基础，测量了不同工况下气泡雾化细水雾的四个雾特性参数，其中应用激光多普勒分析仪测量细水雾的平均粒径和三维平均速度分布，应用激光片光和CCD成像法测量细水雾的雾化锥角，使用收集法测量雾通量的分布，讨论分析了工作压力、气液流量比和喷嘴结构对这四个雾特性参数的影响。根据气泡雾化喷嘴内部两相流型变化导致的雾滴形成机理的不同对Lund等人预测雾滴粒径的模型进行了改进，以能在更宽的GLR范围内对雾滴粒径进行预测，模型计算结果与实验数据的偏差在±20％内。使用自行设计的气泡雾化喷嘴进行了对防火保护对象的局部释放工况和全淹没工况下的灭火有效性实验，测量了灭火过程中火焰形态，火焰、油面、空间温度和O₂、CO₂、CO浓度的动态变化过程，研究了不同雾化气体与细水雾耦合作用下的灭火机理，并具体对喷嘴工作参数、燃料类型、喷嘴类型、通风状况以及雾化气体类型等因素对于气泡雾化细水雾抑制熄灭火灾有效性的影响规律和定量关系进行了分析，为气泡雾化技术在新一代灭火系统的推广应用提供理论指导。通过与压力式喷头产生的单流体细水雾的灭火有效性比较实验，表明了无论在局部释放工况还是全淹没工况下，气泡雾化细水雾均显示出比单流体细水雾更为高效的灭火性能，并且在有障碍物的场合使用卤烃气体和惰性气体辅助雾化会极大的改善灭火效果。因此，气泡雾化技术在清洁高效灭火系统中有着广阔的应用前景。更多还原

【Abstract】 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.更多还原