Rapid Measurement Of Moisture Content In Insulation Layers Of Commercial Aircrafts

Porous insulation layers in aircraft walls may acquire moisture when cruising at high altitudes.The accumulated moisture increases aircraft's weight,reduces thermal and sound insulation performance,corrodes envelope structure,and induce risks of electronic device failure.The moisture content in insulation layers must be accurately measured for appropriate maintenance of the materials.Because of low density and hydrophobic property,measuring moisture content in insulation blankets is more challenging than measuring moisture in high-density and hydrophilic porous materials.The thermal method has been developed to measure moisture contents in soils and sponge blocks,based on the additive law of the volumetric heat capacity(the product of density and specific heat).The moisture content was inferred from the difference in the volumetric heat capacity before and after moisture acquisition.However,the conventional thermal method is only applicable to an infinite test domain.There is still lack of an accurate and rapid method and a test device for measuring moisture content in insulation blanket layers with a finite thickness.This investigation adopted computational fluid dynamics(CFD)simulation to investigate the effects of a finite test domain(thickness)and the associated boundary heat loss on moisture measurement.Insulation material of various thicknesses,bounded by two solid plates,was modeled to imitate the porous insulation layers in aircraft wall.The thickness and thermal properties of the solid plates are the same as the metallic shell and lining.The convective heat loss to the surrounding air from both solid plates was considered.The over-measured moisture content due to a finite geometric domain was regressed into formulas,by means of which over-measured moisture content was subsequently deduced.In order to verify the accuracy of the above remedial strategy,the corrected moisture content was then compared with that obtained by the gravimetric weighing using a digital precision balance.For rapid measurement of moisture content in insulation blanket,a portable moisture detector was developed,which included a monitor and a dual-needle probe.The monitor powered the heating needle for constant heating and collected the generated transient temperature rises.The moisture content was instantly solved based on the temperature responses' database,which took advantage of the linear relationship between the temperature response and the heat generation rate of the heating needle.The two volumetric heat capacities,which corresponded to the closest matching between the measured and predicted temperature rises,were averaged based on the matching degree to output the final moisture content.The results show that significant over-measurement of moisture content can occur for the insulation material with a thickness less than 45 mm.The smaller is the insulation layer thickness,the more was the over-measured moisture content.With the remedial strategy,the averaged moisture content values for a number of repetitive measurements were close to those from the gravimetric weighing.The deviation between both was less than 1.0 kg H2O/m3,which should meet the requirements for engineering applications.However,the measured moisture contents have poor repetition,especially for the insulation blankets with high moisture content(for the case of 25 kg H2O/m3).A favorable feature of the portable moisture detector is that it took only 100 s for probe heating and temperature sensing and less than 0.5 s for moisture interpretation.