Study On Moisture Accumulation In Thermal Insulation Subjected To Dynamic Boundary Conditions During Aircraft Flight

Porous materials are widely used because of excellent heat and insulation performances,but it can easily acquire moisture,when subjected to changes of environmental parameters.The accumulated moisture could further cause many negetive effects including corrosion,degradation of thermal insulation performance,and weight gain.Usually,the damage is not easy to detect,because moisture accumulation is a slow process.Compared with this,the covered insulation of aircrafts always undergoes in the sharp change temperature and pressure environment during flight,which will result in large difference in moisture transfer and phase changes in porous materials between flight and routine conditions.Therefore,it is very necessary to reveal flow characteristics,distribution of temperature,rule of phase change of moisture in the insulation of aircraft and also to quantify the accumulated moisture.Based on the above research background,this thesis took the insulation of aircraft as a research object,furthermore,momentum equations by combing Brinkman-Forch-Heimer Extended Darcy with Boussinesq model were built based on volume averaging assumptions and thermodynamic equilibrium theory to described nonlinear flow characteristic during the sharp change temperature process and simulated the phase changes among vapor,liquid and ice.Then,a natural drying experiment was performced to verify the model accuracy under large temperature difference conditions.Meanwhile,the model applicability was verified through a humidifying experiment which placing the dry polyurethane foam in saturated air environment.With the help of reduced-scale mockup,this thesis acheived an experiment to simulate the real flight and evaluated the applicability and accuracy of the model under low-pressure condition through comparing the final residual quantity between experiment data and the simulation result.In consideration of 0.8atm pressure of cabin environment,the influence of pressure on moisture transfer in porous materials was analyzed by qualitative and quantitative methods.The crucial factors affected on moisture accumulation were determined by the validated model under 11 different flight scenarios including different altitude,relative humidity,temperature,relative humidity of airport and flight time duration.According to existing aircraft outer wall structure,a channel was developed between lining layer and insulation material,and the natural convection form of moisture migration was improved by the forced convection form which effectively achieved the drying effect.Finally,the results show that the developed model provided a good agreement with the experiment data during drying process under normal conditions,and the nonlinear distribution of temperature and moisture were mainly presented at initial stage of test.The predicted variation ratio of moisture were consistent with their observed values during sharp temperature change and humidification process.According to qualitative and quantitative analysis on the influence of the main parameters in the mathematical model under low pressure(0.8atm),moisture accumulation process in the insulation was simulated during once flight,the results showed that the low pressure resulted in a decrease of the vapor flow rate,and also the enhancement of its diffusion effect.Furthermore,this would reduce moisture accumulation(decreased by 15%),and keep a similar variation trend of moisture with routine conditions.The moisture accumulation simulation results under different flight scenarios showed that the cruise altitude(aircraft surface temperature)and cabin relative humidity would be two key parameters determining the moisture accumulation rates,the higher altitude and relative humidity(10～30%)would cause more moisture accumulation in insulation.Besides,moisture accumulation mainly occured in crusing stage,and approximately presented a direct ratio relationship between moisture accumulation and cruising time.However,the influence of airport relative humidity on moisture accumulation could be ignored.In addition,a improvement scheme model was developed to reduce moisture accumulation under the high moisture content in insulation.It also shown that the scheme could improve dehumidifying effect more than 70-90%.Based on the existing air-conditioning system of aircraft and the layout of insulation,the improvement scheme will be considered feasible.