On the potential and challenges of laser-induced thermal acoustics for experimental investigation of macroscopic fluid p
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RESEARCH ARTICLE
On the potential and challenges of laser‑induced thermal acoustics for experimental investigation of macroscopic fluid phenomena Christoph Steinhausen1 · Valerie Gerber1 · Andreas Preusche2 · Bernhard Weigand1 · Andreas Dreizler2 · Grazia Lamanna1 Received: 5 June 2020 / Revised: 19 October 2020 / Accepted: 22 October 2020 © The Author(s) 2020
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
List of symbols
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Latin characters AP1,P2 Complex amplitudes of the acoustic waves (−) AT Complex amplitudes of the thermal grating (−)
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B Amplitude of harmonic oscillation (−) C Amplitude of harmonic(oscillation ) (−) DT Thermal diffusivity m2 s−1 Eexc Pulse energy ( of the excitation ) beams kg m2 s−2 I Signal intensity (−) P1,2 Complex parameter to compute 𝛹 (t) ; P1,2 = AP1,P2 𝛴P1,P2 (−) ( ) Pint Power of the interrogation beam kg m2 s−3 T Complex parameter to compute 𝛹 (t) ; T = AT 𝛴T (−) Tch Fluid temperature in measurement chamber (K) Tmix Local mixing temperature (K) U𝛩 Dimensionless modulation depth of thermalization grating (−) UeP Dimensionless modulation depth of electrostriction grating (−) ( ) cp Specific isobaric heat m2 s−2 K−1 ( capacity ) cs Speed of sound m s−1 f Focal length (m) j Indicator related to fluid behaviour; j = 1 : resonant; j = 2 : non-resonant ( ) (−) p Pressure kg m−1 s−2 q Magnitude of (the grating vector; ) q = 2𝜋∕𝛬 m−1 ( ) s Specific entropy m2 s−2 K−1 t Time (s) t0 Time of laser pulse (s) v Fluid velocity component ( −1 ) in y-direction m s w Fluid velocity component ( ) in z-direction m s−1 xFl Local mole fraction of a fluid in a mixture
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