Moisture Accumulation In Commercial Aircraft Walls

Modern commercial airplanes operate in an extreme environment.For example,at a high cruising altitude,the atmospheric air temperature is extremely low.In such a circumstance,there are large temperature gradients and water vapor pressure gradients across the insulation layers.A net migration of water vapor towards the inner aircraft shell surface can occur.Subject to the cabin humidity and thus the dew-point temperature,the water vapor may condense into liquid water or even freeze into ice.The accumulated moisture not only degrades the insulation performance,but also leads to microorganism growth.The microbes can corrode the aircraft structure and worsen the cabin air quality.Moisture condensation is the reuslt of heat and mass transfer,and can be more complicated when accompanying with variation of cabin air pressure during a flight.It is necessary to understand mechanisms of moisture accumulation in aircraft walls and analyze the factors that affect the condensation and accumulation rates.Then appropriate measures might be taken to minimize the moisture accumulation in aircraft walls.A reduced-scale mockup of a sectional aircraft cabin was constructed to mimic the moisture accumulation during a flight.The mockup was composed of shell,insulation layers,lining sheet,heat and humidity control system,ventilation system and so on.The inner shell surface was covered with insulation layers to resist heat loss out of the mockup.Inside the mockup,there are a heater,a humidifier and two mixing fans to maintain appropriate temperature and humidity conditions.Two symmetric slot openings on the top of the mockup were used to draw the air into the cabin for ventilation.The internal air was exhausted out of the mockup by a fan below the mockup.The ambient environment of the mockup was controlled by an altitude psychrometric chamber.During tests,the internal and external environment was monitored by temperature and humidity test instruments.A digital precision balance was used to weigh the mass change of insulation layers after a simulating flight.The amount of condensed moisture in the mockup was projected to Boeing 737 airplanes by analogy.In order to sort out the key parameters contributing the moisture accumulation,the variable control method was adopted to design experimental tests.The considered variables include the air parameters for the ambient environment,such as seasons and flight altitude,those for the cabin environment,such as cabin pressure,temperature and relative humidity,those for aircraft wall materials,such as metallic shell,composite shell,and insulation blankets contained in flim bags or not.The results reveal that the amount of moisture accumulation in the wall increases tremendouly when the wrapped film is removed.The maximum moisture accumulation in a B737 aircraft can be up to 14 kg after a flight of five hours.The flight altitude and the cabin air relative humidity are the two key parameters affecting the moisture accumulation,while the seasons,cabin air pressure and temperature play relatively weak roles.With increase of flight altitude or cabin relative humidity,a significant increase of water vapor pressure difference across the insulation layers can be resulted,which leads to more condense moisture in aircraft walls.In addition,a shell material with a larger thermal conductivity and a serious damage to the wrapping film of insulation layers can also cause more moisture condensation.