Thermal Behavior And Its Dynamic Effects On Stratospheric Aerostats

Stratospheric aerostats which have inherent capabilities of low speed and long duration flight are attractive candidates for pre-alarming monitoring, navigation and telecommunications research. The thermal characteristics of stratospheric aerostats are much different from that of spaceflight and high speed aircraft, which relate to forced convection, natural convection, and the complicated coupled effect with thermal radiation. These thermal characteristics will influence the rising characteristics, flight control and skin properties of the stratospheric aerostat to a certain extent. Accordingly, the thermal characteristics and its effects become the common basic matter among the many key technologies of stratospheric aerostats. So far, our knowledge on thermal characteristic mechanisms, temperature distributions and its dynamic effects on stratospheric aerostats are far from the requirement of developing new technologies in aerostat fields. Therefore, it is important to investigate the thermal and dynamic characteristics of stratospheric aerostats. Based on this knowledge, the following problems of stratospheric aerostats were investigated: thermal radiation properties of the skin, heat transfer experiment of the aerostat model, the coupled heat transfer characteristics of the skin, environment and filled gas, and thermal dynamic effects.The infrared emissivity data of a skin material was obtained by an experiment measure firstly. Based on the translucent medium radiation transfer characteristics and the apparent radiation data, the surface emissivity and medium absorption coefficient was obtained by the least square method. In order to obtain the thermal response characteristics of stratospheric aerostats, an experiment apparatus was set up, and the transient temperature distributions of both skin and inner gas were obtained under the irradiation of a solar simulator and various airflow conditions.By analyzing the thermal environment characteristic of stratosphere and the heat transfer mechanism of aerostats,the transient coupled thermal models of aerostats were established. After verification, the transient thermal response of the stratospheric aerostat in rising and station-keeping processes was simulated, and the influences of the skin radiative properties and the thermal environmental factors on temperature changes were analyzed. A mathematic model for natural convection inside the ballonet was established by integrating the knowledge of experiment and the coupled boundary conditions. By combining FLUENT software with programs for calculating thermal properties and boundary conditions, numerical simulation of the natural convection inside the double-ellipsoid aerostat at float conditions was carried out. Then the influences of the skin radiative properties and the double ballonets on the natural convection were analyzed. By analyzing the results, the local convective heat flux and the convective heat transfer coefficients were also obtained. A correlation was presented for the local heat transfer coefficient by analyzing the numerical results.Based on the investigation on the coupled heat transfer characteristics of stratospheric aerostats, the effects of the nonuniform temperature distributions of the filled gas on the center of mass and the moment of stratospheric aerostats at float conditions were investigated. The thermal properties changes produced by the temperature difference of skin and the airflow as well as the effects of thermally driven flow on the buoyancy lift were simulated through a thermal dynamic model.By the investigations in this thesis, the coupled heat transfer mechanisms and the transient thermal characteristics of stratospheric aerostats during the rising and station-keeping processes are comprehensively understood. The thermal characteristics data of the skin and the filled gas under typical aviate conditions are accumulated, and the dynamic effects produced by coupled heat transfer are established. The conclusions are valuable for the thermal control, flight dynamics control and design, and the development of the skin materials of stratospheric aerostats.