An Experimental And Numerical Investigation On The Interaction Of Water Mist And 2-D Counterflow Diffusion Flame

Fire safety is a pivotal aspect in the design of spacecraft while clean and high efficient fire extinguishment technology is an important component in fire safety design. Water mist, with its merits of high efficiency, no pollution and low water damage, is a potential candidate for the next generation fire extinguishers used in the spacecraft. The indoor environment in spacecraft is relatively complicate and delicate. In order to reduce water damage and improve the suppression efficiency of water mist, it is significant for screening out appropriate chemical additives that can be used to improve the overall capabilities of water mist. However, the addition of additives may influence the spray characteristics and change the suppression mechanisms for the interaction of water mist and the flame. Consequently, it is necessary to study on the influences of gravity and chemical additives on the interaction process of water mist and the counterflow diffusion flame.2-D counterflow diffusion flame is easy to characterize and it shares some similarities with the electrical wire fire under forced convection. This study designs a platform for the interaction of water mist and flame based on the Tsuji burner flame, k-s turbulence model is used to simulate the flow field of the contraction section, the results of which are used to testify the uniformity of the air velocity near the inlet of the contraction chamber. The platform includes four main components:supply and transportation system for the oxidants and gas fuel, characterization system, and data collection and remote control system. The experimental apparatus can be used to control the water mist droplet distribution, concentration, additive types and the flame stretch rate. Besides, it is also capable of measuring the flame temperature, water mist droplet distribution and velocity field. These functions enable it to be a suitable tool for evaluating the water mist suppression efficiency and mechanisms.In the second part of the study, the interaction of counterflow 2-D methane diffusion flame and mono-dispersed water mist is simulated numerically under normal and microgravity environment with an emphasis on the influences of water mist on the flame near the stagnation point. The simulation results show that, under different gravities, the buoyancy force is the main factor that can influence the flame surface and temperature. For the enveloped flame, the flame extinction near the stagnation point is the key step for the flame transformation from enveloped flame to wake flame. The release of water mist significantly lowers the flame temperature near the stagnation point while most of the water mist deviates from the stagnation streamline and flow downstream. With the increase of droplet diameter, the influences of gravity and buoyancy force on the droplet motion increases, and the differences of residence time and maximum flame temperature under normal and microgravity environment increase. However, these differences can be controlled within 10% for the water mist with droplet diameter below 20 p,m. In order to produce meaningful experimental results that can be extended to the microgravity environment, it is advised to conduct the screening experiments with small droplet diameter, high concentration of water mist under low air velocity.In the third part of the study,9 additives are selected as the water mist additives because of their low toxicity, environmental friendly and economically viable characteristics. The critical flame stretch rate is used as a criteria for evaluating the suppression efficiency. Besides, the flame temperature and the morphology of the residual particle formed during the interaction of additive-containing water mist and flame are used to qualitatively analyze the suppression efficiency of additive-containing water mist. The experimental results show that, for the potassium-containing water mist, the suppression efficiency from high to low is C2O4K2>C2H3O2>KCl KHCO3>K3PO4. For the phosphorous-containing water mist, the suppression efficiency follows the order K3PO4>NH4H2PO4>H3PO4 under low concentration; but the suppression efficiency of H3PO4 surpasses NH4H2PO4 under high concentration. For the iron-containing additives, FeCl2 can inhibit the flame while FeSO4 can promote the flame. The suppression efficiency of chemical additives can depend on the following factors:1) the chemical nature of the additives; 2) the residence time controlled by the flow condition and 3) the formation of residual particle.This study introduces a platform for studying the water mist and 2-D counterflow diffusion flame and lay a foundation for 1) the application of water mist in spacecraft, 2) screening out high efficiency water mist additives under normal gravity and 3) the investigation of suppression mechanisms.