Titan Montgolfiere balloon analysis and design using computational fluid dynamics simulations

In this paper a multiple-layer heated balloon is considered for future Titan missions. We describe computational fluid dynamics (CFD) studies aimed at predicting the buoyancy for double-, triple-, and quadruple-walled balloons, and determining the sensitivity to the location of an external heat source. The buoyancy predictions from CFD show that the effective gap conductivity is higher than what is predicted by engineering correlations. Direct and large-eddy simulations (DNS/LES) are carried out for an idealized concentric spherical annulus with isothermal walls in order to investigate the source of the discrepancy and the resulting data is then used to design a new correlation. Results for multiple-layer balloons show the allowable scientific payload grows rapidly with increasing number of walls. Next, the location of the heat source is analyzed, and we consider locations internal and external to the balloon. It is found that regardless of its location, an externally located heat source generates too much wasted heat and balloon performances is degraded substantially. A second design study deals with the external payload (gondola) and its effect on the balloon filling (blockage effects). It is found that even though important reductions in filling rates take place, it does not affect the overall descent rate, primarily due to the low gravity on Titan..