MAGNETO-FLUID-DYNAMIC ENERGY CONVERSION SYSTEMS FOR AEROSPACE APPLICATIONS

The study concerns the application of Magnetohydrodynamics (MHD) to the Space flights. This field puts a number of challenging issues due both to the severe service conditions and to the launch requirements. Space vehicles are subject to radiations and are hit by high-energy particles; during the launch, the vehicle undergoes very strong vibrations, which can cause severe damages to the equipment with moving parts, such as pistons and gears, making them unsuitable for Space missions. Furthermore, the minimization of mass and volume of the on board devices is a critical aspect of the design, particularly for the energy conversion equipment, such as electric power generators, active cooling devices and propulsion systems. Any static energy conversion technology, with a high ratio power/volume and power/weight and that can adapt its shape to the vehicle, is suitable for space applications. Due to the static nature, MHD energy converters withstand the vibrations during the take-off, can work at high temperatures and, due to the limited mechanical stress during the functioning, they can be manufactured by using light materials, reducing the load. This thesis deals with two aspects of the MHD energy conversion in Space: the energy supply of the on board electrical devices and the cruise propulsion. In PART I two concepts of MHD induction generators are proposed aiming overcoming the main drawbacks of classic MHD generators, but holding all their advantages. The first is a Flowing MHD generator; a mono-dimensional Model is solved in Simulink® environment performing preliminary dimensioning and performance estimation of the generator. The second is the TA-MHD generator, a quasi-static converter combining thermoacoustics (TA) and MHD. First, a theoretical study has been developed in order to perform a coarse sizing and performance estimation for a set of parameters; then, a series of FEM simulations allowed a preliminary assessment of the behavior of the charges subject to an acoustic wave and an electric field. The charges segregated inside the capacitor’s plates, move forward and backward due to the acoustic wave and distribute in the whole cross section without sticking to the walls. The results show that the charge carriers can give rise to an alternate electric current in the fluid, which the energy conversion process is based on. The Multi-objective Tabu Search (MO-TS) algorithm has been customized in order to investigate the optimal design of the MHD section of the generator. 2D and 3D optimization problems have been solved considering as objective functions, first the power to be maximized and the voltage to be minimized, and then adding the mass to be minimized. The solutions confirm the suitability of the TA-MHD generator for aerospace but shows different optimal points, depending on the specific application. PART II deals with the MagnetoPlasmaSail (MPS), a Space propulsion concept exploiting the solar wind energy. A FEM analysis of the sail formation and inflation is conducted. The results, even if the simulation parameters are right different from the real ones, is a first step in the comprehension of the phenomenon and create the premises for a dimensionless analysis. The Magnetic Reynolds number is much greater than one, so the magnetic field is in frozen conditions (its field lines are strongly advected by the plasma). Also the Interaction Parameter is much larger than one, so that the fluid streamlines will be heavily modified by the magnetic field. Under optimum conditions this will lead to the plasma confinement inside the magnetic bubble, reinforcing the magnetic field, and allowing excellent fuel efficiency. The FEM simulation confirms the expected behavior of the magnetic field interacting with the plasma: the magnetosphere formation process is qualitatively shown as its interaction with the solar wind. The introduction of non-dimensional parameters allows extending the results to different orders of magnitude.