Physical & Computational Modeling of Heat Transfer for Titan Montgolfiere Balloons
MetadataShow full item record
Saturn's moon, Titan, is attracting intense scientific interest, leading in turn to wide interest in exploring it with Aerobots, balloons or airships. Their function would be similar to Mars Rovers but instead of moving laboriously on wheels, they would float freely from location to location. A Montgolfier, "Hot Air Balloon" appears to be an attractive type of Aerobot for Titan. To design a Montgolfiere it is essential to know the temperature of the lifting gas, and so in turn it is necessary to quantify heat transferred between the craft and its surroundings. Heat transfer for existing balloons is well understood. However, Titan's gravity and especially atmospheric conditions are radically different from those in which balloons have ever been flown so there is great uncertainty in predictions of heat transfer rates. In particular, thermal radiation accounts for most heat transfer for existing balloons, but over Titan, heat transfer will be almost entirely by convection. The authors are conducting a two-pronged investigation of this heat transfer. Nott is making measurements on working physical model balloons at Titan temperatures while the Caltech team is developing detailed computational fluid dynamics models. The overall objective is to create accurate computational simulations of Titan balloon heat transfer. Once validated with data from practical experiments, these mathematical models will be very useful, allowing quick evaluation of any candidate designs for a Titan balloon. The experimental work is being done in the "The Titan Sky Simulator" which has an open interior approximately 4.5 meter tall and 2.5 meters square. It is being operated at about 95K and working prototypes of Titan balloons are being flown inside it. This very low temperature, corresponding to Titan's surface, leads to a high gas density even though the Simulator operates at 1 atmosphere pressure. Computational models simulate laminar and turbulent free convection around simple balloon geometries, by directly solving the coupled equations of motion (Navier-Stokes) and energy equation for laminar and turbulent flows. Estimates show that under relevant conditions, a Boussinesq approximation can be used, wherein the fluid is assumed nearly incompressible and the buoyancy force is proportional to temperature fluctuations. The equations are solved using two different algorithms, and in each case, insensitivity of the results to grid spacing and domain size are demonstrated. The models currently focus on steady-state results for stationary balloons for direct comparison with the experiments, considering a variety of cases with different balloon size, power inputs, and single- versus double-walled designs. In addition, experimental and computational results for the temperature distribution and net buoyancy will be compared to JPL system-level models that utilize empirical heat-transfer coefficients. This work is supported by the NASA Jet Propulsion Laboratory with Dr. Jeffrey Hall as Program Manager.