Numerical modeling of melt velocity and thermal distributions during aircraft high-gravity arcs

Abstract

Centrifuge crystal growth experiments under 2–6 gravity have produced crystals with microstructure indicative of diffusion controlled growth. A current hypothesis for this phenomenon is that Coriolis and gravity gradient forces produced by the centrifugal motion can effectively damp convective flows. Our research has included the study of solidification during KC-135 aircraft parabolic maneuvers which produce alternate periods of 25 s low gravity and 45 s of high (1.8 g 0) gravity. We, thus, examined the question of how the Coriolis and gravity gradient forces during high gravity maneuvers compare to that for common centrifuges. Microstructural examination of samples solidified during high gravity arcs has revealed no evidence of convection dampening. As a first approximation, we model the high gravity arc as a centrifuge with radius of 20,480 ft and angular speed of 0.318 rpm. Scaling analysis indicates that the Coriolis and gravity gradient expected on the aircraft high gravity arc are less than that for the centrifuges by a factor of 100. Detailed Navier-Stokes analysis of the fluid flow and thermal fields during solidification of aluminum and Cd-Te in KC-135 high gravity show that convective flows of about 1 mm/s are induced. The thermal field, however, is only slightly modified by the convection. Coriolis and gravity gradient during solidification in KC-135 high gravity arcs, even at accelerations that have been shown to produce significant convective flow dampening in some centrifuge sytems, are shown to have no significant influence on the melt thermal and flow fields. The KC-135 high gravity arc could, thus, be advantageously utilized for experiments where separation of centrifugal acceleration and Coriolis acceleration is desirable.