Aerodynamic levitation of laser-heated solids in gas jets

Solid spheres were aerodynamically levitated in gas jets and laser heated to temperatures above 2000 K. Stable levitation in a supersonic jet from a 0.081-cm nozzle was demonstrated with 0.03–0.20 g, 0.24–0.47-cm-diam specimens at a height between 0.7–2.0 cm above the nozzle and ambient pressures between 1.1–18 Torr. A model of supersonic jet levitation accurately predicts height vs pressure over the full range of conditions that were investigated. The efficiency with which jet momentum is converted into levitation force decreases with the jet:specimen diameter ratio and jet Reynolds number. The rate of jet spreading with distance from the nozzle deduced from levitation experiments agrees with that measured by pitot tube traverses of the jet. Pitot tube pressure measurements also reveal a transition from laminar to turbulent supersonic jet flow at nozzle Reynolds numbers just above the maximum values at which stable levitation is observed. Laser heating reduces the jet momentum required for levitation at a given height and increases levitation stability. In experiments with subsonic jets, the required jet momentum flow rate exceeds the specimen weight by ∼2/CD, where CD is the specimen drag coefficient at its terminal free-fall speed under the ambient conditions. Exploratory studies of laser-heated liquid levitation are also reported.