On the potential and challenges of laser-induced thermal acoustics for experimental investigation of macroscopic fluid phenomena

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 Abstract

Pressure measurement in gas flows using laser-induced grating lifetime

“…A less complex non-linear technique that does not require fitting to a spectrally-resolved signal is laser-induced grating scattering (LIGS), also known as laser-induced thermal acoustics (LITA) [16][17][18]. LIGS/LITA experiments measure the speed of sound in a gas directly and models of the LIGS signal evolution can be used to determine transport properties such as the acoustic damping rate and thermal diffusivity, of key interest for studies of fundamental fluid physics [19]. Previous studies have demonstrated the capabilities of the LIGS technique for temperature measurements, which can readily achieve sub-1 % precision and accuracies of order 1 % [20][21][22][23][24].…”

Collaborative Research on Droplet Dynamics Under Extreme Ambient Conditions

“…Spontaneous Raman scattering [2] and combined laser induced fluorescence and phosphorescence [36] enabled the measurement of gas composition in the wake of an evaporating droplet, gas temperatures and, for the case of acetone, mean droplet temperatures. Laser-induced thermal acoustics has been extended to enable the measurement of the acoustic damping rates in the supercritical region of pure fluids [56]. Based on these measurements, the importance of bulk viscosity in the liquid-like supercritical region was assessed and closure models for describing momentum transport and dissipation effects in supercritical fluids were evaluated.…”

Experimental Investigations of Near-critical Fluid Phenomena by the Application of Laser Diagnostic Methods

“…A detailed description of the optical arrangement used for the presented investigations as well as the spatial resolution of approximately 312 mm in diameter and less than 2 mm in length in the x-direction can be found in the work of Steinhausen et al [28]. For a theoretical description of the generation of laser induced gratings the reader is referred to Cummings et al [4], Schlamp et al [26] as well as Stampanoni-Panariello et al [27].…”

“…As descripted in our previous work [28] the evaluation of LITA signals proposed by Schlamp et al [26] enables us after careful calibration to extract the speed of sound c s , the acoustic damping rate as well as the thermal diffusivity D T from the shape of the LITA signal. Using the assumptions presented by Steinhausen et al [28] the time-dependent diffraction efficiency (t) of a detected LITA signal shows the following dependencies…”

“…Argon is chosen as the fluid for calibration because of two reasons. First, argon shows non-resonant fluid behaviour leading to a simplified model of the LITA signal independent from the thermal diffusivity (see [26,28]). Second, for gaseous argon the volume viscosity is negligible compared to the shear viscosity (see [13,15]).…”