Validation of Thermal Concepts for a Solar Powered High-Altitude Platform based on Models and Experiments

Kurzfassung

High-altitude platforms are a currently intensively discussed method for performing Earth observation tasks. These tasks range from disaster monitoring and the provision of telecommunications services to continuous global monitoring. With the aim of being able to make statements about these areas, the German Aerospace Center is currently developing a highaltitude platform. The area of operation of the DLR high-altitude platform is in the stratosphere at an altitude of more than 20 kilometers with an operating time of longer than one month. Due to the harsh environmental conditions in the stratosphere, which are characterized by low air density, low outside temperature and large differences in solar radiation between day and night, the avionics devices accommodated in the fuselage, that are required for the safe control of the high-altitude platform, can suffer from overheating. Thermal management plays an important role in estimating the temperatures in the fuselage of the high-altitude platform. A first step in the process of establishing thermal management is the creation of thermal mathematical models depicting the occurring heat transfer processes in a modeling and simulation environment. Subsequently, validation experiments are performed on a fuselage section to validate the accuracy of the thermal mathematical models, focusing on the internal heat transfer processes inside the fuselage of the high-altitude platform. In addition, measurements are performed that take into account the influence of external parameters, such as solar radiation and wind speed. The validation of the numerical models by means of experiments is carried out by comparing measured data with simulation results. In the course of the validation experiments, measurements are performed with various internal heat sources, ranging from an infrared heater to self-constructed avionics dummies. With their help, critical parameters such as the Nusselt number or the effective heat transfer area are determined, as a comparison of measurement and simulation indicates. Furthermore, the influence of varying external parameters, including solar radiation and wind speed, on the thermal mathematical models is validated.