Experiments of the Prototype for a Stratospheric Balloon-borne Heat Transfer Laboratory

Balloon-borne scientific missions are gaining interest in recent years due to their potential to study the Earth, its atmosphere and the outer space with lower costs and risks than a launched space mission. Furthermore, the instruments on-board these balloons generally are subjected to less harsh requirements which are easier to accomplish. However, to a greater or lesser extent the interaction with the stratospheric thermal environment needs to be evaluated. Usually, at stratospheric level the convection has a minor effect over the surfaces and could be considered negligible but even if it is not the dominant heat transfer mechanism, it could be relevant for critical instruments and it makes necessary to assess the heat transfer convection over the surfaces of interest in order to reach the optimum thermal design. A stratospheric balloon-borne heat transfer laboratory has been designed for the purpose of estimating the heat transfer lost by convection from flat plates at stratospheric altitudes and at the balloon ascent phase. The prototype for in-flight experiments, consisting of heated aluminium flat plates, has been built and tested on ground at controlled pressures for estimating the free convection heat transfer and to know the Rayleigh number and Knudsen number ranges in which the flat plates will operate since at stratospheric pressures (10 to 30 mbar expected) the atmosphere has a significant level of rarefaction and the usual heat transfer correlations for flat plates do not account for rarefied gas conditions. Although the in-flight experiments are expected to be close to continuum flow condition, the on-ground experiments were carried out up to transition flow in order to assess the relative importance of the convection compared with the radiation or conduction heat transfer mechanisms. The present experimental results will help to validate the flight thermal model for analyses and to correlate the in-flight experiments.