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# Pressure traces recorded at xLITA\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$x_{\mathrm{LITA}}$$\end{document} for He-Ar/Ar for p5≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p_5 \approx$$\end{document} 5, 20 and 43 bar

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Non-resonant laser-induced thermal acoustics (LITA), a four-wave mixing technique, was applied to post-shock flows within a shock tube. Simultaneous single-shot determination of temperature, speed of sound and flow velocity behind incident and reflected shock waves at different pressure and temperature levels are presented. Measurements were perfor...

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... Further challenges for the measurement technique are the extremely high temperature and pressure as well as the very short measurement time in the nozzle reservoir of shock tunnels, which can be overcome by optical measurement techniques. Homodyne laserinduced thermal grating spectroscopy (LITGS, also called resonant LIGS or resonant LITA) needs no seeding particles and offers the possibility of temperature measurements with a high temporal resolution [3][4][5][6][7] under single-shot conditions. In addition, only a small optical access is required for this method, which is particularly advantageous for measurements under high temperatures and pressures. ...
... In the literature, different values can be found for x and y, which also differ between the procedures electrostrictive (LIEGS, also called non-resonant LIGS or non-resonant LITA) and LITGS. In the work of Förster et al. [7], the value 2 is given for x and -3.4 for y for LIEGS. Schlamp et al. [8] report, deviating from this, y = −3 (fluid at rest) or y = −4.25 (flowing fluid). ...
... A more recent work on measurements with laser-induced gratings can be found in Förster [7]. The experimental setup used is basically based on those of Schlamp et al. [16] and Hemmerling et al. [17]. ...
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At our institute a piston-driven shock tunnel is operated to investigate structures of space transportation systems under reentry and propelled flight conditions. For temperature measurements in the nozzle reservoir under single-shot conditions, laser-induced thermal grating spectroscopy is used to date to measure the speed of sound of the test gas. The temperature then can be calculated from this data. The existing experimental setup has already been successfully used to measure flows up to an enthalpy of 2.1 MJ/kg. Since conducting the experiments is extremely time-consuming, it is desirable to extract as much data as possible from the test runs. To additionally measure the velocity of the test gas, the test setup was extended. Besides, extensive improvements have been implemented to increase the signal-to-noise ratio. As the experiments can be conducted much faster at the double-diaphragm shock tube of the institute without any restrictions on the informative value, the development of the heterodyne detection technique is carried out at this test facility. A series of 36 single-shot temperature and velocity measurements is presented for enthalpies of up to 1.0 MJ/kg. The averaged deviation between the measured values and the values calculated from the shock equations of all measurements related to the average of the calculated values is 2.0% for the Mach number, 0.9% for the velocity after the incident shock and 4.8% for the temperature after the incident shock.
... The shock tube was spatially fixed in a foundation such that no recoil occurred to induce deflection and laser beam misalignment throughout the experiment. In a similar small-sized conventional, spatially fixed shock tube, Förster et al. [28] presented simultaneous single-shot temperature and velocity measurements by non-resonant heterodyne LIGS. Experiments in nitrogen and argon at shock Mach numbers 1.67-1.96 ...
... Alternatively, a so-called heterodyne approach uses a second interrogation beam as local oscillator (i.e., reference beam) not passing the measurement volume but being mixed with the signal beam from the measurement volume prior to detection. Interference of both signal and reference beams enables additional quantification of the local bulk fluid velocity (single component) from the oscillation beat frequency and Doppler shift-thus enabling direct, simultaneous measurement of local flow Mach number via sound speed and fluid velocity; compare Förster et al. [28]. The present study solely applies the homodyne approach as quantification of fluid residual velocity components in the high-vorticity, post-reflected shock state 5 is not considered of interest. ...
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Experimental determination of test gas caloric quantities in high-enthalpy ground testing is impeded by excessive pressure and temperature levels as well as minimum test timescales of short-duration facilities. Yet, accurate knowledge of test gas conditions and stagnation enthalpy prior to nozzle expansion is crucial for a valid comparison of experimental data with numerical results. To contribute to a more accurate quantification of nozzle inlet conditions, an experimental study on non-intrusive in situ measurements of the post-reflected shock wave stagnation temperature in a large-scale free-piston reflected shock tunnel is carried out. A series of 20 single-shot temperature measurements by resonant homodyne laser-induced grating spectroscopy (LIGS) is presented for three low-/medium-enthalpy conditions (1.2–2.1 MJ/kg) at stagnation temperatures 1100–1900 K behind the reflected shock wave. Prior limiting factors resulting from impulse facility recoil and restricted optical access to the high-pressure nozzle reservoir are solved, and advancement of the optical set-up is detailed. Measurements in air agree with theoretical calculations to within 1–15%, by trend reflecting greater temperatures than full thermo-chemical equilibrium and lesser temperatures than predicted by ideal gas shock jump relations. For stagnation pressures in the range 9–22 MPa, limited influence due to finite-rate vibrational excitation is conceivable. LIGS is demonstrated to facilitate in situ measurements of stagnation temperature within full-range ground test facilities by superior robustness under high-pressure conditions and to be a useful complement of established optical diagnostics for hypersonic flows.
... Three different atmospheres, namely nitrogen with a purity of 99.999 % , argon with a purity of 99.998 % , and carbon dioxide with a purity of 99.995 % , are studied. The optical setup is adapted from the one described in Baab et al. (2016) and Förster et al. (2015). ...
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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
... A radial piston pump or a pressurized fluid reservoir provides the required injection pressures depending on the desired NPRs in the experiments. A more detailed description of the test facilities is given in [43,36,44]. ...
... For a detailed description of the optical arrangement the reader is referred to Förster et al. [44] and Baab et al. [36]. Here, only the essential information are given. ...
Article
The present work provides an overview on the possible phase transitions associated with supercritical fluid injection and a detailed evaluation of the mixing process between injectant fluid and quiescent ambience. The experiments cover superheated liquid disintegration, pseudo-boiling transition and single-phase jets under different nozzle pressure ratios. Pseudo-boiling effects emerge when rapid, subsequent changes in pressure/temperature interact with the non-linear behavior of thermodynamic response functions across the Widom line. The associated density fluctuations cause a significant increase in the scattering cross section and may lead to thermo-convective instabilities. Our analysis of the mixing process demonstrates the limited applicability of the adiabatic mixing model, which is often restricted to short residence times even in highly turbulent jets (Re=O(105)). Specifically, our findings show the importance of considering all coupled transport processes in the analysis of mixing problems at high pressures, in particular in presence of large mass concentration and temperature gradients.
... Besides the coherence of the LIGS signal and the simple data analysis, the main advantages are that the signal quality of LIGS increases with increasing pressure and that potential absorbers for the generation of a thermal grating occur during shock compression at high temperatures. LIGS has already been successfully used in several shock-related [9,10] and shock-tube-related [4,11] investigations in recent years. The highest levels of pressure and temperature, which have been reported to be investigated with LIGS in gaseous media and flames so far, are 14 MPa [12] and around 2300 K [13,14], respectively. ...
... In total, four of six measurements show better results for the alternative reference value. Not shown here is the comparison of LIGS temperature with a reference value calculated from measured pressure under the assumption of isentropic change as applied in [11,26]. The values determined for isentropic change were roughly in the middle between T ref and T p , and the overall deviation for the six measurements was not smaller than the deviation to T p . ...
... Another aspect is the limits imminent to the LIGS technique itself. For LIEGS (non-resonant LITA), a correlation of about p 2 T −3 for the signal strength can be found in the literature [11,16,33]. For the resonant LITGS technique used in this work, the dependency of signal strength on pressure and temperature is predicted to be p 4 T −6 for low densities and p −2 T −0.6 for high densities [33]. ...
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Free-piston shock tunnels are ground-based test facilities allowing the simulation of reentry flow conditions in a simple and cost-efficient way. For a better understanding of the processes occurring in a shock tunnel as well as for an optimal comparability of experimental data gained in shock tunnels to numerical simulations, it is highly desirable to have the best possible characterization of the generated test gas flows. This paper describes the final step of the development of a laser-induced grating spectroscopy (LIGS) system capable of measuring the temperature in the nozzle reservoir of a free-piston shock tunnel during tests: the successful adaptation of the measurement system to the shock tunnel. Preliminary measurements were taken with a high-speed camera and a LED lamp in order to investigate the optical transmissibility of the measurement volume during tests. The results helped to successfully measure LIGS signals in shock tube mode and shock tunnel mode in dry air seeded with NO. For the shock tube mode, six successful measurements for a shock Mach number of about 2.35 were taken in total, two of them behind the incoming shock (p $$\approx$$ 1 MPa, T $$\approx$$ 600 K) and four after the passing of the reflected shock (p $$\approx$$ 4 MPa, T $$\approx$$ 1000 K). For five of the six measurements, the derived temperatures were within a deviation range of $$6\%$$ to a reference value calculated from measured shock speed. The uncertainty estimated was less than or equal to $$3.5\%$$ for all six measurements. Two LIGS signals from measurements behind the reflected shock in shock tunnel mode were analyzed in detail. One of the signals allowed an unambiguous derivation of the temperature under the conditions of a shock with Mach 2.7 (p $$\approx$$ 5 MPa, T $$\approx$$ 1200 K, deviation $$0.5\%$$, uncertainty $$4.9\%$$).
... The technique has also been successfully applied to calibration of Two-Colour Planar Laser Induced Fluorescence imaging of temperature distributions in a firing engine [10] . Recent developments have also included applications in shock tubes and high-speed fuel injection jets and mixing [11,12] . ...
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Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS) in a motored reciprocating compression machine to simulate diesel engine operating conditions. A portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 1064 nm and the fourth harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser induced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substitute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used to measure temperatures in both combusting and non-combusting cycles with good agreement with the results of LITGS measurements which had a single-shot precision of ± 15 K and standard error of ± 1.5 K. The accuracy was estimated to be ± 3 K based on the uncertainty involved in the modified equation of state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously resolved and temperatures of 2300 ± 100 K, typical of flames, were recorded in individual cycles. The results confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion under compression-ignition conditions.
... ment and data acquisition system is sketched in Fig. 3 . A detailed characterization of the shock tube facility can be found e.g. in Förster et al. (2015) . ...
Article
In this study, n-hexane was compressed beyond six times its critical pressure and discharged into argon at subcritical pressure (with respect to the injectant). The injection temperature systematically varied from sub- to supercritical values to investigate near-critical disintegration phenomena of retrograde jets. Here, the ratio of the pressure in the nozzle reservoir and chamber was always above 14 leading to highly-expanded injections. We analyzed the breakup process in terms of combined shadowgraphy and light scattering measurements. Close to the critical point, different physical mechanisms have been observed. Their occurrence is predominantly determined by the expansion process resulting from the thermodynamic conditions in nozzle reservoir and chamber in combination with thermodynamic properties of the injectant. The experimental images for near- but subcritical injection temperatures imply that a fluid in liquid state expands in the nozzle and atomizes downstream of it. For sufficiently low backpressures, high liquid superheats trigger rapid vapor formation across a thin transition layer inside the nozzle that leads to a choked two-phase flow. A thermodynamic model that assumes a discontinuous phase transition layer and uses metastable fluid properties provides a physical explanation of the resulting underexpanded two-phase disintegration downstream of the nozzle exit. An increase to supercritical injection temperatures increases the compressibility of the fluid within the nozzle. An isentropic flow analysis showed that this triggers choking at thermodynamic states in the supercritical pressure regime resulting in the discharge of a sonic single-phase fluid. The expansion within the Mach barrel can lead to a supersaturated fluid state at near-critical temperature. In this case, a sharp phase transition front established approximately half a nozzle diameter downstream of the exit. We defined a dimensionless parameter to characterize the two-phase extent in underexpanded jets with near-critical phase transition based on initial injection conditions and retrograde fluid properties. We deduced the axial extent of the two-phase region from light scattering signals for a wide parameter range and demonstrate that it features a clear dependency upon the proposed parameter, which demonstrates its feasibility.
... The optical arrangement is illustrated in Fig. 4. Note again that it is adapted from earlier versions of the setup described in Förster et al. (2015) and Baab et al. (2016). The description here is, therefore, limited to the essentials like the wavelengths of the excitation ( = 1064 nm ) and interrogation beams ( = 532 nm ). ...
... The corresponding constant of proportionality is found in a calibration experiment using a system with known thermodynamic properties. The calibration procedure is identical to the one in Förster et al. (2015), where the effect of uncertainties from the calibration is found to be within 1.3%. One of the important features of LITA is that this characteristic frequency is also found if the LITA is heavily deteriorated by scattering events caused for instance by inhomogeneous density fields along the beam path lengths and turbulence. ...
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We report a comprehensive speed of sound database for multi-component mixing of underexpanded fuel jets with real gas expansion. The paper presents several reference test cases with well-defined experimental conditions providing quantitative data for validation of computational simulations. Two injectant fluids, fundamentally different with respect to their critical properties, are brought to supercritical state and discharged into cold nitrogen at different pressures. The database features a wide range of nozzle pressure ratios covering the regimes that are generally classified as highly and extremely highly underexpanded jets. Further variation is introduced by investigating different injection temperatures. Measurements are obtained along the centerline at different axial positions. In addition, an adiabatic mixing model based on non-ideal thermodynamic mixture properties is used to extract mixture compositions from the experimental speed of sound data. The concentration data obtained are complemented by existing experimental data and represented by an empirical fit.
... This link is provided by a simple scaling constant providing a significantly simpler calibration procedure. In recent studies, we proved the effectiveness of LITA for quantitative speed of sound measurements in high-pressure atmospheres [37] and the turbulent far-field zone of extremely underexpanded jets [38]. ...
... We determined Λ from frequency measurements in pure nitrogen at ambient conditions for which the speed of sound is well-known. The uncertainties from this calibration procedure can be considered to be within 1.3 % as it was found in Förster et al. [37]. ...
Article
This study is the first to provide a comprehensive speed of sound database for multi-component jet mixing at high pressure. It serves as a unique reference for numerical simulations of mixture preparation processes in future liquid rocket engines and internal combustion engines. We performed quantitative speed of sound measurements in jet mixing zones for five configurations with well-defined experimental conditions. The database covers three different injectant fluids (two alkanes and a fluoroketone) that were brought beyond their critical temperature and pressure prior to injection and discharged into cold nitrogen at supercritical pressure (with respect to the pure injectant properties). Here, we chose the conditions such that subsonic jets were obtained and re-condensation due to cooling of the injected fluid was prevented. Hence, we provide speed of sound data for single-phase jet mixing for three different binary systems. Quantitative data are presented along the jet centerline with sufficiently high spatial resolution to properly resolve the axial decay. In addition, two radial profiles at a position close to the nozzle allow for an assessment of the transversal mixing characteristics. The experimental speed of sound data show consistent trends, which corroborate that mixture effects are correctly resolved in the measurement.
... Challenges arising from measurements in a convergent-divergent nozzle originate from strong flow gradients resulting in a loss of signal quality. Förster et al. [1] recently compared LITA with well-established laser-based techniques (tunable diode laser absorption spectroscopy, TDLAS, laser-induced fluorescence, LIF, and coherent anti-Stokes Raman spectroscopy, CARS) on their application to shock-heated flows. They point out that a unique feature of LITA is that signal analysis is independent on its intensity since the speed of sound and Mach number can be determined solely from a frequency analysis. ...
... The optical setup used for the experiments is similar to the one recently used at our facilities [1,5,6,16]. Figure 6 outlines all relevant components of the setup. ...
... Pressure and temperature dependency The LITA signal intensity is known to be a function of static pressure and temperature [1,23,24]. As a consequence, for a given total flow condition, the signal intensity decreases with increasing Mach number. ...
Article
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Non-resonant laser-induced thermal acoustics (LITA) was applied to measure Mach number, temperature and turbulence level along the centerline of a transonic nozzle flow. The accuracy of the measurement results was systematically studied regarding misalignment of the interrogation beam and frequency analysis of the LITA signals. 2D steady-state Reynolds-averaged Navier–Stokes (RANS) simulations were performed for reference. The simulations were conducted using ANSYS CFX 18 employing the shear-stress transport turbulence model. Post-processing of the LITA signals is performed by applying a discrete Fourier transformation (DFT) to determine the beat frequencies. It is shown that the systematical error of the DFT, which depends on the number of oscillations, signal chirp, and damping rate, is less than $$1.5\%$$ for our experiments resulting in an average error of $$1.9\%$$ for Mach number. Further, the maximum calibration error is investigated for a worst-case scenario involving maximum in situ readjustment of the interrogation beam within the limits of constructive interference. It is shown that the signal intensity becomes zero if the interrogation angle is altered by $$2\%$$. This, together with the accuracy of frequency analysis, results in an error of about $$5.4\%$$ for temperature throughout the nozzle. Comparison with numerical results shows good agreement within the error bars.