Velocity measurements of gas escaping a particle bed during shock-driven expansion

To understand the behavior of gas escaping a rapidly decompressed particle bed, an experimental study is performed in a cylindrical (D = 41 mm) glass vertical shock tube containing a densely packed particle bed. The bed is comprised of spherical glass beads. Two sets of beads are used, with median diameters of 67.5 and 254.5 $\mu$m. The volume fraction of the glass beads is approximately 60%. High-speed pressure sensors capture the shock wave and expansion wave fronts. Optical measurements based on particle image velocimetry (PIV) are developed to examine the velocity of gas initially above the bed as well as gas initially within the interstices of the particle bed using both quantitative and qualitative visualization techniques. For above-bed gas flow analysis, passive tracer particles are seeded above the bed, whereas for interstitial gas measurements, lightweight but non-passive particles are mixed into the upper layers of the bed itself. Development of this technique to optically measure interstitial escape flow is utilized herein to measure the gas rise velocity in response to variation in bead diameter, with faster gas velocities observed as bead diameter increases. For the experiments performed herein, an initial acceleration of the gas velocity is observed at the earliest stages of particle bed decompression, whereas the gas velocity begins to decelerate between 1.25 and 2.25 ms of the estimated arrival of the expansion wave at the particle bed.