Probing Phase Transitions with Supersonic Beams - A Joint Experimental and Theoretical Study

Pulsed, supersonic beams of propane have been investigated by mass-resolved time-of-flight measurements as a function of source pressure, covering sub- and supercritical expansion conditions. The experimental observation of a pronounced change in the terminal flow velocity is explained in terms of a thermodynamic model that is capable of describing the expansion of gases, liquids, and supercritical fluids; in particular, it allows the treatment of the vapor-liquid phase boundary and the critical point. Its major prediction is a distinct pressure dependence of the mean terminal flow velocity that is caused by condensation both in the stagnation reservoir and during the jet expansion. The agreement with experimental data is excellent.