| End states lying in the broad range of vapor, liquid, and mixture fluid regions were experimentally produced with Iso-Octane (2,2,4 Trimethylpentane) in order to study the behavior of the finite-amplitude waves and rapid phase changes in the general vicinity of the critical point. Rapid phase changes were achieved behind a reflected shockwave at the shock-tube end wall by shock compression of the upstream vapor state. Measurements of shock velocity, pressure, and temperature were done together with extensive photographic observations of the final state through a sapphire window at the end of the shock-tube observation chamber. Shock calculations were performed by Rankine-Hugoniot relation and the Hobbs equation of state modified by the use of special, near-critical terms--HGK terms. It is found that (i) measured pressures agree with the prediction of the Hobbs equation of state with HGK modification together with the classical shock-discontinuity theory (i.e. the Rankine-Hugoniot equations), (ii) phase changes produced by the shock compression of a high molecular substance are in general accordance with the prediction by the equilibrium model.; Photographic observations of the end states show a rich variety of two-phase vortex rings, depending on initial conditions and shock strength. On the lipid saturation boundary, vortex rings are seen to be well defined far from the critical point, but are more diffuse and turbulent as the apparent critical point is approached. On the vapor saturation boundary, condensation occurs in a normal fashion for weak compression, but an interesting ray structure is observed in the condensation as the shock strength becomes stronger. The diameter of the smallest vortex ring seen through a macroscopic photography is in the order of tens of microns. |
