No abstract

Quartz lamp heaters and hypersonic wind tunnel are currently applied in thermal assessment of heat resistant materials and surface of aircraft. However, it is difficult to achieve precise heat flux distribution by quartz lamp heaters, while enormous energy is required by a large scale hypersonic wind tunnel. Electron beam can be focused into a beam spot of millimeter scale by an electromagnetic lens and electron-magnetically deflected to achieve a rapid scanning over a workpiece. Moreover, it is of high energy utilization efficiency when applying an electron beam to heat a metal workpiece. Therefore, we propose to apply an electron beam with a variable speed to establish a novel method to realize various non-uniform heat flux boundary conditions. Besides, an electron beam thermal assessment equipment is devised. To analyze the feasibility of this method, an approach to calculate the heat flux distribution formed by an electron beam with variable-speed scanning is constructed with beam power, diameter of the beam spot and dwell duration of the electron beam at various locations as the key parameters. To realize a desired non-uniform heat flux distribution of the maximum gradient of 1.1 MW/m3, a variable-speed scanning strategy is constructed on basis of the conservation of energy. Compared with the desired heat flux, the maximum deviation of the scanned heat flux is 4.5% and the deviation in the main thermal assessment area is less than 3%. To verify the method, taking the time-average scanned heat flux as the boundary condition, a heat transfer model is constructed and temperature results are calculated. The experiment of variable-speed scanning of an electron beam according to the scanning strategy has been carried out. The measured temperatures are in good agreement with the predicted results at various locations. Temperature fluctuation during the scanning process is analyzed, and it is found to be proportional to the scanned heat flux divided by volumetric heat capacity, which is applicable for different materials up to 3.35 MW/m2. This study provides a novel and effective method for precise realization of various non-uniform heat flux boundary conditions.

No abstract

Development of thermal assessment technology is crucial to the security assessment of hypersonic vehicles. A strategy of high-precision reconstructions of nonuniform heat flux and temperature field characteristics of a hypersonic vehicle surface by a scanning electron beam is constructed, and a fraction frequency and random scanning mode that can significantly reduce the temperature fluctuation of materials is proposed. The reconstruction effects in this scanning mode are successively investigated in the HIFiRE-5b flight condition and the reentry condition of the capsule of Apollo 6, showing that the reconstruction accuracies of the heat flux and temperature fields are within 2 and 5%, respectively, and the temperature history is accurately reconstructed with an amplitude of temperature fluctuation below 11 K. This research is expected to promote the electron beam thermal assessment to reconstruct the thermal boundary conditions of hypersonic vehicles more realistically.

Aerogel coating (AC) is an emerging constructional material to achieve ambitions to reduce our carbon footprint. However, common ACs, which are usually formulated by straightforward filler–matrix composites, suffer from unsatisfying thermal-insulating performance due to the low porosity and polymer intrusion into the filler. These can be solved by applying an easy foaming process to the ACs, where meanwhile the intrusion issue can be relented by particles decoration on the inner surface of bubbles. Herein, an architected foamy AC by melamine polymer and silica aerogel particles is demonstrated as a proof-of-concept. The high-porosity foamy AC can achieve much lower thermal conductivity and less heat release as well as super­hydrophobicity and excellent fire resistance. The coating can be air-sprayed on surfaces regardless of the materials and shapes, followed by natural drying. It provides an economical drop-in measure to enhance the energy conservation and fire safety of various buildings.