Abstract
Mixing and evaporation processes play an important role in fluid injection and disintegration. Laser-induced thermal acoustics (LITA) also known as laser-induced grating spectroscopy (LIGS) is a promising four-wave mixing technique capable to acquire speed of sound and transport properties of fluids. Since the signal intensity scales with pressure, LITA is effective in high-pressure environments. By analysing the frequency of LITA signals using a direct Fourier analysis, speed of sound data can be directly determined using only geometrical parameters of the optical arrangement no equation of state or additional modelling is needed at this point. Furthermore, transport properties, like acoustic damping rate and thermal diffusivity, are acquired using an analytical expression for LITA signals with finite beam sizes. By combining both evaluations in one LITA signal, we can estimate mixing parameters, such as the mixture temperature and composition, using suitable models for speed of sound and the acquired transport properties. Finally, direct measurements of the acoustic damping rate can provide important insights on the physics of supercritical fluid behaviour.Graphic
Highlights
Fluid injection, disintegration, and subsequent evaporation are of high importance for a stable and efficient combustion
The purpose of this study is to present the calibration and validation processes needed for the extraction of speed of sound data, acoustic damping rates, as well as thermal diffusivities using Laser-induced thermal acoustics (LITA) with a spatial resolution with an order of magnitude O 10−1 mm in diameter and O 100 mm in length in a high-pressure and high-temperature environment for resonant and non-resonant fluids
The uncertainty analysis of the operating conditions as well as the Fourier analysis of the LITA signal, the calibration, and validation of the grid spacing is performed according to the Guide to the expression of uncertainty in measurement by the Joint Committee for Guides in Metrology (2008)
Summary
Disintegration, and subsequent evaporation are of high importance for a stable and efficient combustion. The region between the critical isotherm and the Widom line, which is characterized by the maximum in specific isobaric heat capacity, is identified as liquid-like It preserves large densities and sound dispersion (Simeoni et al 2010; Bencivenga et al 2009), while exhibiting the molecular structure of a gas (Santoro and Gorelli 2008). It is important to point out that the current macroscopic description of supercritical states is mainly focused on the selection of accurate equation of states The latter are capable to describe the continuous fluid transformation in terms of density changes and the singularities in terms of some physical properties (heat capacity, isothermal compressibility, etcetera) across the Widom line. This approach, may not be sufficient for a correct description of the dynamical behaviour of supercritical fluids, as currently suggested by the microscopic investigations
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