Air Stream Channel Along Commercial Airplane Fuselage To Provide Thermal Insulation

Modern commercial airplanes cruise at a typical altitude about 11,000 m where the outside air can be as low as -65℃. To keep the cabin in an appropriate temperature condition, a good thermal insulation shall be installed to the airplane fuselage. The insulation structure of an airplane is mainly composed of shell, fiberglass blanket and lining. However, the porous structure of fiberglass insulation can absorb moisture easily, especially when the aircraft lands at a humid airport. The insulation structure experiences large temperature difference during airplane cruise, which may result in condensation and even icing of the moisture therein. But after an airplane lands, the frozen moisture melts. The cyclic phase change of moisture does great harms to airplanes.In order to reduce moisture gain into the insulation, one may have to reduce the amount of insulation or develop a new insulation structure. This thesis proposes an air channel between the shell and the lining surfaces to insulate airplane fuselage by air stream barrier. At the bottom of the aircraft fuselage, hot and dry air from engine compressor is supplied in. When air sweeps the air channel, the hot air heats the interior cabins and simultaneously extracts moisture out of the porous insulation. Then the cooled air is finally delivered into the passenger cabin via the overhead ceiling diffusers. Using this design, the engine bleed air can be cooled down by the air channel while at the same time providing thermal insulation to the interior cabins and maintaining the insulation structure in a dry status. To verify the system design and evaluate its performance, a computational fluid dynamics (CFD) program is applied to investigate such an air channel system for a single-aisle aircraft cabin. The CFD solution domain includes the shell, insulation, ventilation channel, lining and interior passenger and cargo cabins. On the outer shell surface, thermal boundary conditions are set into the combination of free stream convection and radiation, so the outdoor static air temperature and mean sky radiation temperature are specified. Then the RNG k-s model is applied to approximate the turbulence effects within the channel and inside cabins. After coupling heat conduction within the solid domain and thermo-flow within the air channel and cabins, temperature distribution in the whole domain is solved and then heat insulation performance of the air channel can be evaluated. To validate the CFD modeling, a partial aircraft cabin mockup is constructed and placed into a psychrometric chamber that is cooled to about -19℃. Many Pt100 resistance-wire probes are used to measure temperature distribution on the outer shell surface, across the air channel, on both internal and external surfaces of the lining.The results show that the CFD modeling is able to accurately capture both velocity and temperature profiles across the air channel that are comparable to the measurement data obtained in a partial aircraft cabin mockup. The air stream channel is effective to insulate an airplane. On benefiting from the heating effect by the channel passage, the draught complaints by passengers seated neighboring to windows can be much alleviated using this system. However, if there is no fiberglass insulation, hot air within the channel may lead overheating of the cargo compartment, so the cargo compartment may have to be insulated to prevent from being overheated. A closer analysis of the channel passage shows temperature profiles therein are asymmetric. If the channel surface temperatures are highly different, it also leads to asymmetric velocity profiles. The convective heat transfer coefficient on the cold surface presents a trend of decreasing first and then increasing gradually along the streamwise direction. In the entry region, due to dense fluid near the cold surface, the fluid boundary layer thickness increases quickly; then the fluid develops into turbulent status and boundary layer is restrained. As the main flow near the cold surface is opposed by the denser fluid gravity, this leads to stronger fluid turbulence and therefore turbulent diffusion of energy transport is enhanced, which aids the convective heat transfer with the cold channel surface.