Generation and Measurement of Rarefaction Shock Waves in Dense Vapors

Gas dynamics is a mature science, however some of its questions are still unanswered. In 1942 Hans Bethe treated the general theory of shock waves. In 1946, Yakov Zeld’ovich, independently addressed the same subject. Both scientists highlighted the possibility that, for some fluids in thermodynamics states close to the vapor-liquid critical point and featuring large heat capacity, rarefaction shock waves may form. This is impossible for any other fluid, including air or steam. In the 1970s, Philip Thompson refined the theory and initiated the systematic study of nonclassical gas dynamics, followed by many scientists, but experimental evidence remained elusive. This article documents the discovery of rarefaction shock waves, and therefore of nonclassical gas dynamics, by means of experiments carried out with a special Ludwig tube filled with the dense vapor of siloxane D6, a silicon oil, at high temperature and pressure. The pressure discontinuity was measured and the speed of the shock wave is supersonic, as prescribed by theory. Experimental results are repeatable and comply with several theoretical requirements. It can be conjectured that the existence of rarefaction shock waves may be exploited in several industrial processes, like the production of micro-particles using the organic fluid as solvent, the design of shock-less micro organic Rankine cycle turbines, and of turbo-expanders for large industrial heat pumps. Moreover, this knowledge further substantiates studies about astrophysical plasmas. This outcome is the cornerstone for future experimental investigations on other nonclassical phenomena, like compression fans, mixed-wave fields, effects on boundary layers, and unconventional mixed waves in vapor-liquid flows.