Investigation of macroscopic nearcritical fluid phenomena by applying laser-induced thermal acoustics

Abstract

The political and social aspiration to reduce greenhouse gases together with increasing energy demands are driving the development of new sustainable energy solutions. To achieve long term sustainability both innovative energy sources and improvements in efficiency are essential. Higher process efficiencies have been achieved by raising combustion pressures, reaching values that now exceed the critical pressures of the injected fuels. However, for an efficient and stable combustion a profound understanding of the processes prior to the combustion, such as fluid injection, disintegration and subsequent evaporation is essential. Unfortunately, the fundamental changes in fluid behaviour at near- to supercritical conditions leading to the observed fluid phenomena are not yet fully understood. Besides fluid injection, supercritical fluids themselves have been identified as an innovative path for an efficient energy conversion and heat transfer processes. The Brayton cycle using supercritical carbon dioxide as operating fluid, supercritical water as coolant and process fluid in nuclear reactors, or the application of supercritical methane as new 'green' fuel in rocket propulsion are just a few examples. Laser-induced thermal acoustics (LITA) has been identified as a promising diagnostic tool in near- to supercritical fluid research. The latter is based on the capability to acquire speed of sound data as well as to resolve thermal and acoustic attenuation in both pure fluids and complex mixing processes, such as evaporation and jet disintegration. Moreover, based on the non-linear pressure dependencies, LITA becomes increasingly more effective in high-pressure environments. By analysing the frequency domain of recorded signals, speed of sound data can be directly determined without any equation of state or additional modelling approaches. Furthermore, acoustic damping rates and thermal diffusivities can be acquired by an analytical expression for the temporal-domain of the signals. By combining both evaluations with suitable analytic expressions for the thermodynamic properties, mixing states, such as local mixing temperatures and concentrations, can be determined. Moreover, since in complex fluids at near- to supercritical conditions acoustic damping is related to both sound dispersion and volume viscosity, important insights into the physics of supercritical fluids are provided. Concordantly, the purpose of this thesis is to apply LITA in the investigation of macroscopic fluid phenomena at nearcritical to supercritical fluid conditions. This includes the following major research objectives. First, the significance of volume viscosities in complex fluids at dense gas conditions as well as the dependency of acoustic damping on mixing states are assessed. Second, the feasibility of time-resolved LITA measurements under complex flow conditions is evaluated. To achieve these objectives an experimental test facility has been designed, which enables investigations at high pressure and high temperature conditions in both pure fluids and complex mixing processes. Moreover, the laser-induced thermal acoustics setup of the ITLR has been optimised for high pressure investigations. Also a new high-speed LITA system with an adjustable measurement volume has been developed. Furthermore, a new post-processing methodology capable of analysing both the frequency and time-domain of the signal has been developed and validated. With the developed system and routines investigations in carbon dioxide, nitrogen, and binary mixtures at gas and gas-like states have been conducted to assess acoustic attenuation and volumes viscosities. Additionally, a jet mixing process has been studied to characterise the LITA arrangement and to evaluate the dependency of acoustic damping on mixing concentration. At last, to assess the feasibility of transient LITA measurements in turbulent, physically complex flow conditions and to further characterise the evaporation process, time-resolved LITA measurements have been performed in the wake of a free falling droplet evaporating in a supercritical atmosphere.