Abstract: 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.

Key words: low-pressure twin-fluid water mist, droplet size, water mist flux, droplet velocity, water mist momentum