Dynamic Light Scattering (DLS) for the Characterization of Working Fluids in Chemical and Energy Engineering

The present habilitation thesis summarizes and reviews the research activities performed at the Department of Engineering Thermodynamics (LTT) of the University of Erlangen-Nuremberg during the past two decades on the field of thermophysical property research by dynamic light scattering (DLS) for the characterization of working fluids in chemical and energy engineering. These activities have been carried out as a continuation of the work performed at the Ruhr-University of Bochum (RUB), where Prof. Leipertz was active in this field for more than ten years before he founded the LTT-Erlangen in 1989. Besides own research activities, which are documented and provided in detail through the attached contributions published as papers in several international scientific journals [1-25] or as a book chapter [26], also previous theoretical and experimental work done by different research groups accepted worldwide is reviewed. It is worth mentioning that the main contributions to progress in the field of DLS for a reliable determination of thermophysical properties certainly originate from the groups at RUB and LTT-Erlangen.
In the following, at first an introduction is given pointing out the importance of the thermophysical properties of fluids for technical applications as well as the motivation for their characterization by DLS. Here, also the continuous progress in the field of DLS for the measurement of thermophysical properties of fluids is given in retrospect, covering the first steps since helium lasers became available in the 1960s and until today. This first section, however, deals with DLS from the bulk of fluids and with the application of this method to fluid surfaces, or in a more general formulation to phase boundaries, also called surface light scattering (SLS). The latter technique could be brought to a level of quality, which makes it nearly unique in its application for viscosity and surface tension measurements. This has been achieved nearly solely by the research activities at LTT-Erlangen during the last ten years.
In the second and third section of this habilitation thesis, the methodological principles of DLS and its experimental realization for a reliable determination of thermophysical properties of fluids are presented. In the fourth section, measurement examples are presented for the variety of thermophysical properties accessible both by DLS from the bulk of fluids and by SLS, i.e., thermal and/or mutual diffusivity, dynamic viscosity, speed of sound, and sound attenuation and dynamic or kinematic viscosity, surface tension, and interfacial tension, respectively. Here, also limitations of the method regarding the thermodynamic state and the accuracy will be discussed in detail.
Finally, a compilation of thermophysical property data obtained until now at LTT-Erlangen for specific working fluids in chemical and energy engineering is given. The objects of investigation cover industrial standard reference fluids for thermal conductivity [2, 27] and viscosity [12, 16, 22, 24], alternative refrigerants including pure partially halogenated chlorofluorocarbons (HCFCs) [6, 28, 29, 30], pure hydrofluorocarbons (HFCs) [6, 8, 15, 18, 28-33] and their mixtures [9, 11, 14, 17, 21, 28], and hydrofluoroethers (HFEs) [37], a newly developed working fluid for organic rankine cycles (ORC) [19, 20], pure ionic liquids (ILs) [25] and their mixtures with dissolved substances [23, 38]. While refrigerant mixtures of technical interest [9, 11, 28] have been investigated at a given composition, special interest has also focused during the last years on the study of thermophysical properties of refrigerant mixtures in dependence on their composition [14, 17, 21] as a means of developing and improving prediction models. Besides offering a significant contribution to a reliable database for different working fluids, DLS from bulk fluids [5, 36] and SLS [28] also provide answers to fundamental questions regarding the critical behavior of pure fluids and fluid mixtures.