Nonclassical gasdynamics

Abstract

The study of compressible fluid dynamics has traditionally focused on flows of gases whose thermodynamic properties are related according to the ideal-gas law. In this context, for a long time, it has been demonstrated that shock waves in gases are exclusively of the compressive type, characterized by a discontinuity across which the fluid pressure rapidly increases. However, in the 1940s, world-renowned scientists such as Bethe and Zel’dovich put forth the theoretical possibility of the formation of rarefaction shock waves (RSWs) if the fluid is a high molecular complexity compound and its thermodynamic state is that of a dense vapour. Across such a RSW, the flow experiences an abrupt drop in pressure. The theoretical concept of RSWs became more known and gained widespread acceptance when further investigations were conducted in the 1970s by Thompson and his colleagues. They introduced the fundamental derivative of gasdynamics Γ, the caloric fluid property that determines the nature of shock waves that can occur in a flow depending on the molecular complexity of the fluid and its thermodynamic state. To acknowledge the significant contributions of the aforementioned scientists to this field of fluid mechanics, fluids that possess the unique characteristic of theoretically admitting RSWs due to the occurrence of thermodynamic states featuring Γ < 0 in the dense-vapour phase are referred to as BZT fluids. The branch of fluid dynamics dealing with fluid flows that might display characteristics that are radically different or even opposite to those of classical gas dynamics is aptly called nonclassical gasdynamics.

Despite extensive theoretical knowledge of nonclassical gas dynamics, which includes rarefaction shock waves (RSWs), there is still a lack of compelling experimental evidence supporting their existence. The motivation for the research documented in this dissertation is two-fold: firstly, it is crucial to conduct experiments that can provide empirical validation of nonclassical gas dynamics, with a specific focus on observing RSWs, which have proven elusive in previous attempts. Secondly, performing accurate measurements of fluid properties in the dense-vapour thermodynamic regime has the potential to improve the thermodynamic models of BZT fluids or fluids made of complex organic molecules in general. This in turn can contribute to a more accurate characterisation of flows in practical applications that involve these fluids, such as turbine flows in Organic Rankine Cycle (ORC) systems or compressors in high temperature heat pumps.

This research work aimed to provide conclusive experimental evidence for the existence of nonclassical expansion shock waves in the flows of a candidate BZT fluid, siloxane D6. For this purpose, two novel test facilities namely the Asymmetric Shock Tube for Experiments on Rarefaction Waves (ASTER) and the Organic Vapour Acoustic Resonator (OVAR) have been conceived, developed, designed, built and commissioned at TU Delft. Relevant theoretical studies were performed to complement the experimental observation of nonclassical effects. Novel measurements of fluid properties in the nonclassical gasdynamic region of the candidate BZT fluid were executed, the outcomes of which are useful for the improvement and the optimisation of thermodynamic models for this fluid.