Study On Safety Design And Leakage Detection Technology Of The Aircraft Pneumatic Duct System

High temperature pneumatic duct system of the aircraft is bled from the engines, APU andground responsible for the anti-icing system,environment control system,hydraulic system and so on.The duct system is routed throughout the airframe, wings and engine pylon, the accidental rupture cannot only make pressure drop dramatically in the ducts, but also lead to the malfunction of thecorresponding system and the equipments around it. So the reliability and durability of the ducts arevery important for flight safety. The performances of the heat transfer and fluid flow, thermal stresscompensation methods and overheat detection technology are conducted in this paper via theoreticalderivation, numerical and experimental simulation. The thesis has the following major contents:(1) mechanism of the duct system and selection of the design parameters.The space layout of the pneumatic duct system of ARJ21-700is presented. According to theposition of the shut-off valve in the duct system, the pneumatic duct system is divided into fivesubsystems: bleed system, anti-ice system, trim system, pack discharge system and piccolo system.The mechanism of various subsystems and their accessories are introduced. And then, the attributeparameters of the duct system are given. The multilayer insulations design is adopted in this paper andits computing method is derived. Finally, the calculation conditions are determined by the air bleedway, flight acceleration, design criterion and environment temperature, which provide the boundaryconditions for the further calculation.(2)Study on the heat transfer and fluid flow characteristics of the internal and externalducts.The fluid network theory and the finite volume method are adopted to analysis the internal andexternal heat transfer and flow characteristics of the duct system. The temperature, pressure and flowrate of each node in the pipeline as well as the external temperature field of the duct system areobtained. The research results show that the rupture with small flow rate has less effect on thetransport characteristics, but its influence on the neighboring environment temperature requiresattention. A better PID control logic and switching characteristics of the valve can improve thepressure pulsation of the duct system. Furthermore, the total heat transfer of the cabin air increaseswith the duct in down location and diameter.The space layout of the duct system needs to consider itsheat loss and the heat load transferring to the cabin. For a given duct system, optimized layout of the duct can be achieved by establing the proper optimal object functions with heat loss of the duct andthe thermal load transferring to the cabin.(3)Stress and displacement safety design of the duct system with finite element method.By analyzing the flight state and safe assessment methods, the stress analysis, compensation andoptimization method of the aircraft duct system is developed by combination with beam element, shellelement and solid element model. Meanwhile, the beam element model and shell element model ofthe duct system with straight pipe bend, flange, ball joint, sliding seal and rod is derived and therelevant key part is modeled by solid element model. Finally, the influencing factors such as safetydesign criterion, the flight acceleration, temperature, pressure of the air and the layout of theaccessories on the stress distribution of the duct system are investigated.(4) Stress and displacement safety design of the bleed system and its influence factorsTaken bleed system of ARJ21-700as an example, the stress analysis, compensation, checkingand optimization work are conducted by the method mentioned above. In the process of compensation,some design ideas such as load-bearing design firstly, displacement constraints and stress releasewith alternate compensation rule, stress compensation by duct unit dividing method, symmetricalarrangement of the slide joint are pointed out. Meanwhile,the layout of the duct system is optimizedbased on the angle of deflection of the ball joint.Finally, fatigue properties of the duct system arestudied by Miner fatigue damage theory and rain-flow counting method.(5)Leakage detection technology of the high temperature pneumatic duct system.Based on the study of the duct overheat detector of commercial aircraft in existence, threemodified overheat detection systems based on heat-sensitive property material and combinationalcircuits with simple structure, high reliability and global positioning capabilities are designed, whichfill a strategic gap in duct overheat detection region. Also, the overheat detection experiment platformis built. The influences of the overheat length and internal air temperature on the system response timeare conducted. Meanwhile,based on the three-dimension flow field simulation of the leakage byFLUENT, the influences of pressure,temperature,thickness of the insulation,the deflector size andthe leakage position on the detection system are investigated. Finally, the computing method of flowrate of eyelet leak is derived and the detection layout is optimized according to the present results.