Research on the Formation Characteristics of Fog and Frost on Optical Windows of Unsealed Equipment Compartments in Aircrafts

In this study, a numerical method for the formation and dissipation of fog and frost is established using the Eulerian multiphase flow liquid film model. In this approach, the formation and dissipation of fogging and frosting layers is directly determined by the saturation of the water vapor surface, and it does not depend on any empirical coefficients. Additionally, Buck’s formula is used to determine the saturation vapor partial pressure, which is applicable for a relatively wide temperature range (−50 °C to 10 °C). This numerical method was validated by the existing experimental data about fogging and frosting, and afterwards the fogging and frosting processes on the optical observation window in the aircraft are further analyzed for three typical working conditions, namely the ground, the fixed-altitude, and the high-altitude descent. The calculation results show that, under the ground working condition, the maximum thickness of the fog layer on the outer surface of the optical window can completely reach the millimeter level within one hour, and the average thickness of the frost layer can reach the sub-millimeter level, which is one order of magnitude smaller compared to under the ground working condition. Under the high-altitude descent working condition, by setting the fixed wall temperature boundary condition on the outer surface of the glass, it is found that in extreme cases, the maximum thickness of the frost layer on the inner wall of the glass can reach the sub-millimeter level within one hour. The research conclusions provide effective basic data support for the subsequent design of anti-fogging and defrosting devices under flight conditions.