气液流量对低压双流体细水雾雾场特性的影响

由于对民机货舱中不同气液流量的低压双流体细水雾研究较少，且对单个喷头雾通量和雾动量的定量、定性分析不够完善，还需设计开展实验进行更深入的研究。参考美国联邦航空管理局（简称“FAA”）制定的相关标准，设计并搭建飞机货舱低压双流体细水雾实验平台，主要包括FAA全尺寸飞机货舱、低压双流体细水雾系统。采用的喷头与常规的喷头有所不同，喷出的细水雾是一个圆形平面，具有优良的弥散性和滞空性。实验中采用马尔文粒径分析仪结合Spraytec软件测量不同气液流量下的细水雾雾滴粒径，发现当气体流量从250 L/min增加至350 L/min、液体流量保持0.5 L/min不变时，细水雾雾滴粒径从130 μm降低至95 μm；当液体流量分别为0.75 L/min和1.0 L/min时，粒径大小分别从161 μm降至110 μm，从201 μm降至142 μm。根据美国防火协会的标准，采用量杯收集法测量得到不同气液流量下的雾通量，发现单独增加气体流量或液体流量，其雾通量都会增加。当气体流量为350 L/min、液体流量为1.0 L/min时，雾通量达到最大值，为0.255 L/（min·m2）。利用粒子图像测速仪的高速摄像机拍摄喷雾照片，测量雾滴速度，结果显示，在3种不同的液体流量工况下，随着气体流量从250 L/min增加至350 L/min，对应的雾滴速度均近似以0.04的增长率上升，最小值为8.5 m/s，最大值为16.0 m/s。在假设细水雾雾滴形状为球状时，雾动量关系式由雾滴质量与速度的乘积得到。选取雾滴速度最大值代入公式，得到单个雾滴在不同气液流量下的雾动量变化曲线，发现当气体流量为250 L/min、液体流量为1.0 L/min时，单个雾滴动量达到最大值，为3.6×10-8 kg·m/s。通过实验研究不同气液流量对低压双流体细水雾雾场特性的影响，得出以下结论：当液体流量不变，气体流量从250 L/min以25 L/min的变化率增加至350 L/min时，雾滴粒径和雾动量逐渐减小，而雾滴速度和雾通量逐渐增加；当气体流量不变，液体流量从0.5 L/min以0.25 L/min的变化率增加至1.0 L/min时，雾滴粒径、雾动量和雾通量均逐渐增大，而雾滴速度逐渐减小。对于单个雾滴，雾动量取决于雾滴粒径的大小；而对于喷头喷出的所有雾滴，雾滴速度决定了整体雾动量的大小。今后课题将进行细水雾灭火实验，探究达到最佳灭火效果的气液流量。结论可用于飞机货舱细水雾灭火系统的设计与改进，为飞机防火系统一些参数的设置提供了实验和理论基础。

Effect of gas-liquid flow rate on field characteristics of low-pressure twin-fluid water mist

Since there are few studies on low-pressure twin-fluid water mist with different gas-liquid flow rates in civil aircraft cargo tanks, and the quantitative and qualitative analysis of single nozzle mist flux and mist momentum is not perfect, it is necessary to design and carry out experiments for further research. Referring to the relevant standards formulated by the Federal Aviation Administration (FAA), a low-pressure twin-fluid water mist experimental platform for aircraft cargo tanks was designed and built, mainly including the FAA full-scale aircraft cargo tanks and low-pressure twin-fluid water mist system. The nozzle was different from the conventional nozzle. The spray of water mist is a circular plane with excellent dispersion and stagnation. Malvin particle size analyzer combined with Spraytec software was used to measure the droplet size of water mist under different gas-liquid flow rates. It was found that when the gas flow rate increased from 250 to 350 L/min and the liquid flow rate remained 0.5 L/min, the droplet size decreased from 130 to 95 μm. When the liquid flow rates were 0.75 and 1.0 L/min, the particle sizes decreased from 161 to 110 μm and 201 to 142 μm, respectively. According to the standards of the National Fire Protection Association, the water mist flux under different gas-liquid flow rates was measured by the cup collection method. It was found that the water mist flux increased with the increase of gas flow or liquid flow alone.When the gas flow rate was 350 L/min and the liquid flow rate was 1.0 L/min, the water mist flux reached the maximum value of 0.255 L/(min·m2). The velocity of droplet was measured by using the high speed camera of Particle Image Velocimetry. The results showed that when the gas flow rate increased from 250 to 350 L/min under three different liquid flow conditions, the corresponding droplet velocity increased approximately with the growth rate of 0.04 with a minimum value of 8.5 m/s and a maximum value of 16 m/s. Assuming that the shape of water mist droplets was spherical, the relationship of mist momentum was obtained by the product of droplet mass and velocity. The maximum value of droplet velocity was selected and substituted into the formula to obtain the variation curve of mist momentum of single droplet under different gas-liquid flow rates. When the gas flow was 250 L/min and the liquid flow was 1.0 L/min, the momentum of single droplet was the maximum as 3.6×10-8 kg.m/s. The influence of gas and liquid flow on the mist feild characteristies of low pressure twin fluid water mist was studied by experiments. The results showed that when the liquid flow rate was constant and the gas increased from 250 to 350 L/min with a gradient of 25 L/min, the droplet size and momentum decreased gradually, while the droplet velocity and flux increased gradually. When the gas flow rate was constant and the liquid increased from 0.5 to 1.0 L/min with a gradient of 0.25 L/min, the droplet size, momentum and flux increased gradually, while the droplet velocity decreased gradually. For a single droplet, the droplet momentum depended on the droplet size ; for all the droplets emitted by the nozzle, the droplet velocity determined the overall fog momentum. In the future, water mist fire extinguishing experiment will be carried out to explore the gas-liquid flow rate to achieve the best fire extinguishing effect. The conclusion can be used for the design and improvement of low pressure water mist fire extinguishing system in aircraft cargo compartment, which provides experimental and theoretical basis for the development of water mist system driven by low pressure.