Study On The Wire Insulation Ignition Evolution Characteristics At Early Stage Under Weakly Buoyancy Environment

To guarantee the fire safety in manned spacecraft, it is pretty significant that researches about the ignition evolution characteristics of wire insulation at early stage in microgravity. However, the microgravity experimental time is too short in the ground based microgravity facilities and the cost for space experiment is pretty expensive. Thus we have to conduct experiments in normal gravity to simulate the microgravity effects on ignition. In this thesis, it has been carefully discussed that how to build effective weakly buoyancy environment to simulate microgravity effects. Then researches have been investigated about early fire evolution at ignition initiation stage of one classical non-metal material, wire insulation, under weakly buoyancy in the ground. Based on varied microgravity experimental simulation methods, the wire insulation has been ignited by the overload current and its evolution characteristics at the ignition initiation stage have been focused on, that is, the wire insulation pre-ignition temperature variations, the smoke transportation characteristics and soot production distribution characteristics of wire insulation. Then the corresponding results in microgravity have been predicted in details. The main content of the thesis is as follows:At the beginning, in chapter 2, ground experimental methods for microgravity combustion simulation were summarized in details, including their simulation principle, availability and shortages. Then lots of improvement have been put forward, especially one new simulation method based on the conventional narrow channel method. Besides, simulation principle of the new method, called low pressure narrow channel method, has been introduced in details. The new method is supposed to be accomplished in investigating the evolution characteristics at fire initiation stage of the thermally thick fuel in weakly buoyancy environment.In chapter 3, firstly, one simple mode of wire insulation pre-ignition temperature variations was proposed. Then, the mathematical result was obtained, showing the effect of weakly buoyancy on pre-ignition temperature variations. Then, effects of pressure, oxygen concentration, environmental composition, current, wire insulation species and thickness have been studied with the low pressure method. Results showed that the temperature change rate, the balanced temperature increased with the pressure decrease, while the time for arriving at the balance temperature decreased. This proved that the low pressure method has effectively prohibited the buoyancy convection and limited the natural convection heat transfer.Besides, the narrow channel height and species of the narrow channel wall were investigated with the narrow channel method. It was found that the temperature change rate, the balanced temperature increased with the the narrow channel height decrease, while the time for arriving at the balance temperature again decreased, proving the effectiveness of the narrow channel method. In addition, the weaker wall heat storage and smaller wall heat conductivity material should be applied and the narrow channel was supposed to put in the horizontal direction, which could prohibit the buoyancy more effective. Finally, the effectiveness of low pressure narrow channel method has been confirmed and its applicability to the simulation of wire insulation pre-ignition temperature variations has also been provided.In chapter 4, firstly, one simple mode of the smoke small group transportation characteristics of wire insulation was proposed. Then the smoke small group moving track and injection angle has been discussed, the effect of pressure was introduced and the results in microgravity was predicted. Then, effects of pressure, environmental composition, oxygen concentration, current, wire insulation species and thickness have been studied with the low pressure method. It was found that in the pure nitrogen environment, with the pressure decrease, the crack mode of wire insulation changed as follows:heat expanding mode, smoke injecting mode, bubbling mode, showing the increasing pyrogenation of wire insulation. Besides, the beginning time for smoke injection or bubbling decreased and smoke injection angle increased with the pressure decrease, which indicated the buoyancy convection has been prohibited. In the air composition or pure oxygen environment, the amount of oxygen decreased with the pressure decrease, leading to less oxidation of wire insulation, so that the beginning time for smoke injection increased. Again, the smoke injection angle increased with the pressure decrease, showing the effectiveness of low pressure method. Furthermore, in the fixed oxygen environment, with the nitrogen decrease, the oxidation of wire insulation increased, which provided one new improvement to the low pressure method:the weakly buoyancy environment could be built by only reducing the amount of nitrogen so that the chemical reaction would not be suppressed in the low pressure environment. In addition, the beginning time for smoke injection and the amount of smoke increased obvious with the overload current; thus the value of current in the working equipment must be checked timely for fire safety. Finally, the wire insulation species had a great influence on the smoke transportation characteristics at the initiation stage, thus more stable materials of wire insulation should be chose.Besides, the narrow channel height and the direction of narrow channel wall were investigated with the narrow channel method. Results showed with the narrow channel height decrease, the smoke transportation of wire insulation changed as follows:dispersing mode, gathering-dispersing mode, gathering mode. And the smoke moving characteristics were similar with that in microgravity in the narrow channel with 10mm-15mm height. Furthermore, the narrow channel, put in the horizontal direction, could prohibit the buoyancy more effective and obtain the similar smoke transportation characteristics in microgravity, when compared with the vertical direction. Finally, results showed that the smoke transportation characteristics were similar with that in microgravity in the low pressure narrow channel with 10mm-21.4 mm height, indicating the effectiveness of the low pressure narrow channel method.In chapter 5, firstly, the principle of laser extinction method and the method for calculation soot volume fraction were proposed. Then one Matlab program has been created to obtain the soot volume fraction in the smoke of the wire insulation. And one full-court soot volume fraction measure experimental system has been built and the whole calculation method has been checked. Then based on the method, effects of environmental composition, pressure, nitrogen on soot volume fraction distribution have been studied. In the air composition environment, it was found that the soot volume fraction decreased firstly and then increased later with the pressure decrease. In the same pressure environment, the soot volume fraction became the largest in the pure oxygen case, for the extent of the wire insulation oxidation increased obviously in this case. This showed that we could apply the lower pressure but higher oxygen concentration method to predict the soot volume fraction distribution in microgravity. Then, in the fixed oxygen environment, with the nitrogen decrease, the soot volume fraction of wire insulation increased, which showed that the weakly buoyancy environment could be built by only reducing the amount of nitrogen. Besides, with pressure decrease, the distribution of wire insulation soot volume fraction became more similar to that in microgravity. Finally, SEM images of the soot particle from classical cases were obtained. It was found that the mean particle diameter of the soot particle increased with the pressure decrease, which followed the trend in microgravity.In conclusion, variation characteristics at ignition initiation stage of wire insulation under weakly buoyancy were obtained, the corresponding results in microgravity were predicted and many useful improvements were made in the microgravity simulation methods in this thesis. This work is supposed to provide not only a large number of basic datum but also valuable theory guidance for the evaluation of material ignition characteristics under microgravity and fire monitor in manned spacecraft in the future.