Advancements in nonclassical gas dynamics

Shock waves can be formed in all states of matter, be it in the single- or multi-phase condition, when the substance is subjected to a rapid change of state, e.g., a sudden pressure variation. In the case of shock waves in vapors and gases, because of the fact that the developed theory is often based on the ideal-gas equation of state, it is commonly accepted that shock waves can be only of the compressive type, i.e., in the direction of flow, the gas experiences an abrupt increase of pressure whilst undergoing a supersonic-to-subsonic transition. Studies in the early 1940s and 1970s by Bethe, Zel’dovich and Thompson have shown however, that it is in principle possible, from a theoretical viewpoint, to deliberately create expansion shock waves, provided that the correct substance and initial states are chosen. The caloric property that is key in determining the nature of shock waves (and therefore the admissibility of expansion shock waves) is referred to as the fundamental derivative of gas dynamics and the implications of its value, namely when it is negative in so-called BZT fluids, have been studied extensively from a theoretical perspective. Experimental evidence of expansion shock waves in single-phase vapors is however unavailable. The motivation of this work stems from this lack of experimental evidence. The goal of the research project is to first identify BZT fluids. Once this is done, the aim is to develop, design, build and commission a setup which can be used to generate and study nonclassical expansion shock waves in molecularly complex BZT candidates. Finally, an unsteady nonclassical expansion shock wave is to be created and measured. The achievements of this project are: i) the identification of siloxane fluids as BZT candidates and ii) the building of a novel high-temperature flexible asymmetric shock tube with a fast-opening custom-designed valve. Cascading from this, new theory regarding nonclassical dense-gas dynamics has been developed which is very useful in the study of organic Rankine cycle turbogenerators which can benefit from the use of BZT fluids and their effects