| The temperature in the aircraft engine nacelle is very high,and there are many complex fuel oil lines and hydraulic oil lines,these flammable and explosive materials pose a huge threat to the safe flight of the aircraft.Once fire occurs in the engine nacelle while flying,serious casualties and huge financial loss will be caused if the fire extinguishing system fails to suppress the fire timely and effectively.Therefore the reliability of the fire extinguishing system is very important to the flight safety.At present,Halon 1301 is used in most aircraft engine nacelles.The spatial distribution of fire extinguish agent is the key evaluation indicator of the reliability and design rationality of the fire extinguishing system.The release time of agent is very short,therefore the development of fast-response and high-precision measurement equipment of fire extinguishing agent is meaningful to the the airworthiness certification and independent design of aircraft engine nacelle fire extinguishing system in China.Until now,the two kinds of FAA certified Halon sensors are both based on differential-pressure method.These sensors use vacuum pump to sample,and the probes are directly mounted inside the nacelle,which influences the flow filed and the distribution of the agent.Such kind of measurement method is invasive.In addition,sampling line will increase the response time of the sensors.The gas sensing techniques based on infrared absorption have the advantages of fast response,low interference and high precision.It can realize the real-time measurement of the trace gas,and has been widely used in the atmospheric monitoring,industrial process control and combustion diagnosis.Halon 1301 has strong absorption in the mind-infrared region.In this thesis,to achieve higher accuracy and faster response time,we studied two kinds of concentration measurement techniques of aircraft fire extinguishing agent based on infrared absorption.Moreover,an aircraft engine nacelle simulation experimental platform was designed and set up.The release and measurement experiment of Halon 1301 was also carried out in the engine nacelle simulator,which is the first time for the fire extinguishing agent sensor to be tested in the dynamic environment in China.The research results can provide theoretical and technical support for the further improvement and the future application of the measuring system in the real engine nacelle.The main research contents are as follows:1.In this thesis,the non-dispersed infrared(NDIR)prototype was improved based on the previous research.The optical path was simplified,and the dual-used chassis was designed which can achieve open-path measurement method;the concentration measurement of Halon substitute pentafluoroethane(HFC-125)was realized by the improved sensor,the influence of humidity was studied through experiment;the influence of temperature was analyzed from principle model,the temperature correction algorithm was proposed for the first time and evaluated by experiment.2.In this thesis,a concentration measurement system of Halon 1301 was developed based on tunable mid-infrared laser.First,the calculation model of the aircraft fire extinguishing agent's volume concentration was established based oin Lambert-Beer Law;the infrared absorption spectrum of Halon 1301 was analyzed in detail,the absorption peak near 8280 nm was targeted,the quantum cascade laser(QCL)and mercury cadmium telluride(MCT)detector were selected accordingly;the measurement system was designed and set up based on an the QCL;the absorption of Halon 1301 in the scanning range of the QCL was measured through experiment,the optimum working temperature,modulation range of the QCL and the suitable absorption length which can satisfy the measurement requirement were obtained;according to the specific absorption of Halon 1301,the concentration inversion algorithm was designed and the data acquisition and processing program was written.In addition,some key performance indexes of the system such as measurement precision,long-term stability and response time were also analyzed by experiment.3.The application of the measurement system in this paper is not limited to Halon 1301,but can be extended to other gaseous extinguishing agents which contain carbon-fluorine bond.In this thesis,the infrared absorption spectrums of Halon substitutes—pentafluoroethane(HFC-125),heptafluoropropane(HFC-227ea)and hexafluoropropane(HFC-236fa)were measured and the feasibility of the application of the existing system on the measurement of the three kinds of fire extinguishing agent was also analyzed;the absorption of these three extinguishing agent in the scanning range of the QCL was studied by experiment,and the optimum measuring conditions were obtained by experiments;the perfonmance indexes of the system when applied to the measurement of HFC-125,HFC-227ea and HFC-236fa were also analyzed by experiment,respectively.4.In this paper,an aircraft engine nacelle simulation experimental platform was designed and built,the performance of the measurement system based on the tunable mid-infrared laser was evaluated,and the measurement system was improved accordingly.The platform can provide a flow field dynamic environment which is similar with the actual application scenarios in engine nacelle,and it can realize the quantitative filling and spraying of the fire extinguishing agent.Moreover,the measuring section of the simulator was combined with the measurement system,the open-path and non-invasive measurement was obtained.The results of the experiment in the simulator show that the system has a worse performance in dynamic environment,which is caused by the rapid changes of the agent in high-speed flow field and the high-speed air flow.To solve these problems,we modified the measurement system.In addition,experiments under different spraying pressure,air speed,the mass of the released agent and injection direction were carried out,the measurement results show good agreement with the previous full-scale fire-extinguishing experiment. |
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