AN EXPERIMENTAL-STUDY OF REFLECTED LIQUEFACTION SHOCK-WAVES WITH NEAR-CRITICAL DOWNSTREAM STATES IN A TEST FLUID OF LARGE MOLAR HEAT-CAPACITY

Near-critical states have been achieved downstream of a liquefaction s hock wave, which is a shock reflected from the endwall of a shock tube . Photographs of the shocked test fluid (iso-octane) reveal a rich var iety of phase-change phenomena. In addition to the existence of two-ph ase toroidal rings which have been previously reported, two-phase stru ctures with a striking resemblance to dandelions and orange slices hav e been frequently observed. A model coupling the flow and nucleation d ynamics is introduced to study the two-wave system of shock-induced co ndensation and the liquefaction shock wave in fluids of large molar he at capacity. In analogy to the one-dimensional Zeldovich-von Neumann-D oring (ZND) model of detonation waves, the leading part of the liquefa ction shock wave is a gasdynamic pressure discontinuity (Delta approxi mate to 0.1 mu m, tau approximate to 1 ns) which supersaturates the te st fluid, and the phase transition takes place in the condensation rel axation zone (Delta approximate to 1-10(3) mu m, tau approximate to 0. 1-100 mu s) via dropwise condensation. At weak to moderate shock stren gths, the average lifetime of the metastable state, tau proportional t o 1/J, is long such that the reaction zone is spatially decoupled from the forerunner shock wave, and J is the homogeneous nucleation rate. With increasing shock strength, a transition in the phase-change mecha nism from nucleation and growth to spinodal decomposition is anticipat ed based on statistical mechanical arguments. In particular, within a complete liquefaction shock the metastable region is entirely bypassed , and the vapour decomposes inside the unstable region. This mechanism of unmixing in which nucleation and growth become one continuous proc ess provides a consistent framework within which the observed irregula rities can be explained.