Numerical Study On Characteristic And Extinguishment Of Water Mist In Microgravity

With the development of manned space technology, it becomes an important research topic to investigate the fire safety in the manned spacecraft. Now, Halon extinguishing agent and carbon dioxide extinguishing agent are used to extinguish fire in spacecraft, but both of them have some drawbacks. Water mist technology has been developed and regarded as a promising substitute since it is clean, effective, stable and nontoxic. So it is necessary to investigate characteristics of water mist and its fire extinguishing in microgravity. In this paper, a numerical simulation method is adopted to study the dynamics of water drop impact on solid surface, water mist centerline velocity characteristics and effectiveness of water mist fire extinguishing, and discuss the related effect factors to reveal the extinguishing mechanism of the water mist in microgravity.A VOF model is adopted to simulate the drop deformation process after impact on normal temperature and hot solid surface in microgravity. The effect of droplet initial diameter, impact velocity and solid surface temperature on the drop spreading deformation process is studied. The results indicate that the cooling effect of water mist on the hot solid surface is good. With the droplet impact velocity constant, the spreading factor is larger for the drop with the smaller initial impact diameter, while the drop with larger diameter rebounds after impact on the solid surface. It can enhance the cooling effect of water drop on the solid surface to reduce the diameter within a certain range. With the droplet impact velocity difference, the deformation process the drop impact on solid surface is different, and the cooling mechanism is also different.The PDPA device in FDS is used to measure the centerline velocity of water mist with various initial velocities with the change of the spray distance, and the polynomial of the velocity change is obtained in microgravity. With the increase of the spray distance, the velocity of water mist declines in microgravity, which is in accord with the theoretical analysis. The approximate expression of the centerline velocity decline trend is the quadratic polynomial of the spray distance, and its coefficients can be indicated by polynomials of the initial velocity.Fire Dynamic Simulator is adopted to investigate effectiveness of water mist fire extinguishing in microgravity. The effect of water mist diameter, velocity and flow rate on the fire suppression is studied. The fire extinguishing efficiency is better for the finer water mist within a certain range. As for the water mist velocity, the performance of fire suppression is affected slightly in microgravity. The results on the effect of water flow rate show that the flow rate should be higher than a critical value to suppress the fire effectively. But the fire extinguishing efficiency cannot be improved significantly by increasing the water mist flow rate beyond a certain threshold value. The evaporation cooling is the dominantly mechanisms of fire suppression in microgravity.In this paper, these results can direct the experiment study on fire suppression of water mist in microgravity. At the same time, it can provide some valuable information for the use of the water mist as a fire-extinguishing agent and the design of the water mist fire extinguishing system in spacecraft.