Single-shot Temperature Measurements Research Articles

Simultaneous thermometry and velocimetry for a shock tunnel using homodyne and heterodyne detection

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.

Broadband ultrafast-laser-absorption imaging in the ultraviolet for spatiotemporally resolved measurements of temperature and CN

This Letter describes the development and implementation of an ultraviolet and broadband ultrafast-laser-absorption-imaging (UV-ULAI) diagnostic for one dimensional (1D) imaging of temperature and CN via its B2Σ+←X2Σ+ absorption bands near 385 nm. The diagnostic was demonstrated by acquiring single-shot measurements of 1D temperature and CN profiles in HMX flames at a repetition rate of 25 Hz. Ultrashort pulses (55 fs) at 800 nm were generated using a Ti:Sapphire oscillator and then amplification and wavelength conversion to the ultraviolet were carried out using an optical parametric amplifier and frequency doubling crystals. The broadband pulses were spectrally resolved using a 1200 l/mm grating and imaged on an EMCCD camera to obtain CN absorbance spectra with a resolution of ≈0.065 nm and a bandwidth of ≈4 nm (i.e., 260 cm–1). Simulated absorbance spectra of CN were fit to the measured absorbance spectra using non-linear curve fitting to determine the gas properties. The spatial evolution of gas temperature and CN concentration near the burning surface of an HMX flame was measured with a spatial resolution of ≈10 μm. 1D profiles of temperature and CN concentration were obtained with a 1-σ spatial precision of 49.3 K and 4 ppm. This work demonstrates the ability of UV-ULAI to acquire high-precision, spatially resolved absorption measurements with unprecedented temporal and spatial resolution. Furthermore, this work lays the foundation for ultraviolet imaging of numerous atomic and molecular species with ultrafast time resolution.

Sensitive and single-shot OH and temperature measurements by femtosecond cavity-enhanced absorption spectroscopy.

In many low-temperature plasmas (LTPs), the OH radical and temperature represent key properties of plasma reactivity. However, OH and temperature measurements in weakly ionized LTPs are challenging, due to the low concentration and short lifetime of OH and the abrupt temperature rise caused by fast gas heating. To address such issues, this Letter combined cavity-enhanced absorption spectroscopy (CEAS) with femtosecond (fs) pulses to enable sensitive single-shot broadband measurements of OH and temperature with a time resolution of ∼180 ns in LTPs. Such a combination leveraged several benefits. With the appropriately designed cavity, an absorption gain of ∼66 was achieved, enhancing the actual OH detection limit by ∼55× to the 1011 cm-3 level (sub-ppm in this work) compared with single-pass absorption. Single-shot measurements were enabled while maintaining a time resolution of ∼180 ns, sufficiently short for detecting OH with a lifetime of ∼100 μs. With the broadband fs laser, ∼34,000 cavity modes were matched with ∼95 modes matched on each CCD pixel bandwidth, such that fs-CEAS became immune to the laser-cavity coupling noise and highly robust across the entire spectral range. Also, the broadband fs laser allowed simultaneous sensing of many absorption features to enable simultaneous multi-parameter measurements with enhanced accuracies.

Ultrafast-laser-absorption spectroscopy in the mid-infrared for single-shot, calibration-free temperature and species measurements in low- and high-pressure combustion gases.

This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented. The accuracy of the diagnostic was validated at 1000 K and pressures up to 40 bar in a heated-gas cell before being applied to characterize the spatiotemporal evolution of temperature and CO in AP-HTPB and AP-HTPB-aluminum propellant flames at pressures between 1 and 40 bar. The results presented here demonstrate that ULAS in the mid-IR can provide high-fidelity, calibration-free measurements of gas properties with sub-nanosecond time resolution in harsh, high-pressure combustion environments representative of rocket motors.

Combustion for aircraft propulsion: Progress in advanced laser-based diagnostics on high-pressure kerosene/air flames produced with low-NOx fuel injection systems

The challenges in designing high-performance aeronautical combustion systems have not changed significantly over the years, but the increase of stringent regulations and the need to tackle rising fuel prices require new sophisticated analysis processes. To address this concern, the theme of this study was cast in terms of the ability of improving the performances of advanced non-intrusive instrumentation in high-pressure turbulent reactive two-phase flows to enhance the combustion efficiency and reduction of pollutants of new innovative low-NOx injection systems fed with liquid kerosene. In the first part, a Lean Premixed injection system designed for helicopter engines was first studied using the Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) and Planar laser-induced fluorescence (PLIF) of OH, kerosene vapor and NO. These diagnostics were applied individually or in combination for custom-made solutions for accessing to detailed information on the spatial distributions of velocity, droplets size, OH, kerosene and NO molar fractions in pressure conditions ranging from 0.4 to 1.8 MPa. Complementary large-Eddy simulations of these flames were performed using the PCM-FPI tabulated chemistry approach in conjunction with a polydisperse Euler–Lagrangian approach. Comparison of simulations with experiments highlighted the importance of the size distribution of the droplets injected into the flow on the mutual predictions of the aerodynamic field, the structure of the flame and its anchorage point as well as the spatial distribution of the fuel and OH species. The second part of this work was aimed to study the advantages and limitations of the chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) diagnostic for performing time-series of single-shot temperature measurements on an aircraft multi-point injection system developing two-phase flames under realistic pressure conditions.

In situ nozzle reservoir thermometry by laser-induced grating spectroscopy in the HELM free-piston reflected shock tunnel

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.

Assessment of single-shot temperature measurements by thermally-assisted OH PLIF using excitation in the A2Σ+–X2Π (1-0) band

The present paper reports on the assessment of single-shot 2D temperature measurements by thermally assisted OH PLIF for the excitation of the A2Σ+–X2Π (1–0) band. The temperature sensitivity of the ratio between the emission intensity of the (2–0) band and (0–0), (1–1) bands is numerically tested by using a LASKIN software package for the transitions Q2(7), Q1(8), R1(14), and P1(2). It is found to be the most efficient for Q1(8), for which the ratio almost linearly depends on the temperature in the range of 1200–2200 K. Fluorescence quenching by oxygen has a minor effect on the results. The capability of the temperature measurements by the thermally assisted OH PLIF has been experimentally tested for a laminar premixed methane/air flame based on comparison with the results of two-line and line-scanning PLIF thermometry. The method is also applied for a partially premixed turbulent swirling flame in a model gas-turbine combustor. The method allows to detect hot regions in turbulent flames.

Single-Shot Temperature Measurements in a Scramjet Combustor Using Thermally Assisted Laser-Induced Fluorescence

The knowledge of the temperature in the flowfield of the scramjet is critical to the understanding of its performance. This study represents the application of an optical diagnostic technique, thermally assisted laser-induced fluorescence, to the temperature measurements in the combustor of a supersonic combustion engine. The experiments were conducted in the T4 shock tunnel using a scramjet model. Experimental data were obtained by focusing a laser beam into the combustor and exciting the OH radical. A detailed numerical model that simulates all radiative and collisional processes of relevance with the appropriate system of differential equations was developed for the analysis of the influence of energy transfer processes on the fluorescence signal. The population distributions achieved with numerical modeling of the LIF process validated the approach proposed to deduce temperatures. By evaluating the scramjet LIF spectra with a full spectral fit, a temperature distribution across the combustor width was obtained. The experimental results were compared with the computational fluid dynamics simulations of the combustion process.

Phosphor thermometry at 5 kHz rate using a high-speed fiber-optic spectrometer

A simple kHz-rate spectral phosphor thermometry technique with sub-millisecond temporal resolution for contactless point measurements of short duration transient surface temperature changes was developed by employing a compact high-speed fiber-optic spectrometer, a high repetition rate pulsed green laser, and a fast decaying thermographic phosphor. Temperature changes were determined from the temperature-dependent emission spectra of the YAG:Ce phosphor using a spectral slope method. The precision of single-shot temperature measurements was better than 4% for temperatures up to 700 K. An application of the technique for time-resolved measurements of local transient temperature changes at 5 kHz during repetitive long CO2 laser pulse heating is presented. The measured temperature transients are well-predicted using a two-dimensional axisymmetric heat transfer model.

Technique developments and performance analysis of chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering combustion thermometry.

This work characterizes the state of the art in the analysis of high-repetition-rate, ultrafast combustion thermometry using chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS). Several key aspects of the CARS spectroscopy system are described, including: (1)the ultrafast laser source, (2)use of the frequency-doubled idler versus signal from the optical parametric amplifier, (3)the geometry constraints for phase matching, and (4)spectral fitting for single-shot temperature measurements. A frequency-dependent instrument response function (IRF) for the detection system was modeled as a variable-width Gaussian and implemented through a frequency convolution of synthetic spectra. Proper accounting of the IRF increased spectral fitting performance in the high-frequency region where signal oscillations are weaker and narrower. Aggregated data from 25 system performance assessments taken over four months yielded accuracy and precision of 2.7% and ±3.5% for flame temperatures, and 9.9% and ±6.1% at room temperature, using the commonly reported method. A new processing technique, based on the statistical method of maximum likelihood, was implemented for turbulent flames where strong fluctuations in expected temperatures necessitate use of multiple temperature calibrations. Results from multiple sets of laser parameters are combined to generate an error-weighted temperature from the top-performing calibrations. A testing procedure was designed to characterize system performance when the range of expected temperatures is unknown, simulating the random temperature field of a highly turbulent flame. Accuracy error of the CPP fs-CARS system increased in this more-stressing test at all temperatures, but precision was significantly affected only at room temperature. System stability is characterized, and the contribution from shot-to-shot laser fluctuations on measurement precision is quantified. Finally, the near-adiabatic and steady assumptions for the Hencken burner calibration flame are examined in an axial scan; significant deviations from ideal behavior were observed only at heights of more than four diameters above the burner surface.

Potential of two-line atomic fluorescence for temperature imaging in turbulent indium-oxide-producing flames

The applicability of two-line atomic fluorescence (TLAF) for temperature imaging in an indium-based flame spray pyrolysis (FSP) process is demonstrated using a single tunable optical parametric oscillator (OPO) to generate the required excitation wavelengths consecutively. Single-shot images of the detected fluorescence signals demonstrate that the signal levels in the flame are suitable for evaluation of temperature and verify the capability and potential of the measurement technique directly during particle formation without additional indium seeding. Qualitative averaged two-dimensional temperature distributions in the FSP flame are presented, showing the influence of varying sheath gas flow rates on the resulting temperature distribution. With the addition of a second OPO and detection system, the two fluorescence signals acquired consecutively in this work could be obtained simultaneously and enable spatio-temporally resolved single-shot temperature measurements in flame synthesis processes of indium-containing nanoparticles.

Thermometry and cooling of a Bose gas to 0.02 times the condensation temperature

Ultracold gases promise access to many-body quantum phenomena at convenient length and time scales. However, it is unclear whether the entropy of these gases is low enough to realize many phenomena relevant to condensed matter physics, such as quantum magnetism. Here we report reliable single-shot temperature measurements of a degenerate $^{87}$Rb gas by imaging the momentum distribution of thermalized magnons, which are spin excitations of the atomic gas. We record average temperatures as low as $0.022(1)_\text{stat}(2)_\text{sys}$ times the Bose-Einstein condensation temperature, indicating an entropy per particle, $S/N\approx0.001\, k_B$ at equilibrium, that is well below the critical entropy for antiferromagnetic ordering of a Bose-Hubbard system. The magnons themselves can reduce the temperature of the system by absorbing energy during thermalization and by enhancing evaporative cooling, allowing low-entropy gases to be produced within deep traps.

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering temperature and concentration measurements using two different picosecond-duration probes

A hybrid fs/ps pure-rotational CARS scheme is characterized in furnace-heated air at temperatures from 290 to 800 K. Impulsive femtosecond excitation is used to prepare a rotational Raman coherence that is probed with a ps-duration beam generated from an initially broadband fs pulse that is bandwidth limited using air-spaced Fabry-Perot etalons. CARS spectra are generated using 1.5- and 7.0-ps duration probe beams with corresponding coarse and narrow spectral widths. The spectra are fitted using a simple phenomenological model for both shot-averaged and single-shot measurements of temperature and oxygen mole fraction. Our single-shot temperature measurements exhibit high levels of precision and accuracy when the spectrally coarse 1.5-ps probe beam is used, demonstrating that high spectral resolution is not required for thermometry. An initial assessment of concentration measurements in air is also provided, with best results obtained using the higher resolution 7.0-ps probe. This systematic assessment of the hybrid CARS technique demonstrates its utility for practical application in low-temperature gas-phase systems.

Pr:YAG temperature imaging in gas-phase flows

In this study, a new thermographic phosphor for planar gas-phase thermometry is investigated. The thermographic phosphor used is composed of trivalent praseodymium (Pr3+) ions doped into a yttrium aluminum garnet (YAG) host. Spectrally-resolved emission data were taken in a furnace for temperatures up to 1,300 K. The emission spectra were used to develop a temperature measurement strategy utilizing a non-equilibrium population ratio. The developed temperature measurement technique was demonstrated in a turbulent heated air jet for exit temperatures ranging from 300 to 750 K. The results demonstrate the promise of the Pr:YAG phosphor for obtaining high-precision single-shot temperature measurements in gas-phase flows.

One-dimensional single-shot thermometry in flames using femtosecond-CARS line imaging

We report single-laser-shot one-dimensional thermometry in flames using femtosecond coherent anti-Stokes Raman scattering (fs-CARS) line imaging. Fs-CARS enables high-repetition-rate (1-10 kHz), nearly collision-free measurement of temperature and species concentration in reacting flows. Two high-power 800 nm beams are used as the pump and probe beams and a 983 nm beam is used as the Stokes beam for CARS signal generation from the N2Q-branch transitions at ∼2330 cm(-1). The probe beam is frequency-chirped for single-laser-shot imaging. All three laser beams are formed into sheets and crossed in a line which forms the probe region. The resulting 1D line-CARS signal at ∼675 nm is spatially and spectrally resolved and recorded as a two-dimensional (2D) image. Single-shot temperature measurements are demonstrated in flat-field flames up to temperatures exceeding 2000 K, demonstrating the potential of fs-CARS line imaging for high-repetition-rate thermometry in turbulent flames. Such measurements can provide valuable data to validate complex turbulent-combustion models as well as increase the understanding of the spatio-temporal instabilities in practical combustion devices such as modern gas-turbine combustors and augmentors.

Temperature and pressure imaging using infrared planar laser-induced fluorescence

A new diagnostic technique for measurements of temperature and pressure distribution in gaseous flows has been developed. The technique, based on infrared planar laser-induced fluorescence (IR-PLIF), is applicable to all IR-active species. A simple two-line excitation approach is used for measurements of temperature, while pressure measurements utilize online excitation on one rotational line and offline excitation on another. A demonstration of the technique in a supersonic underexpanded jet of 30% CO2 and 70% N2 was performed, and the results are, to the best of our knowledge, the first demonstration of temperature and pressure imaging using IR-PLIF. The developed diagnostic shows potential for single-shot two-dimensional measurements of temperature and pressure in gaseous flows.

Measurements of mean and fluctuating temperature in an underexpanded jet using electrostrictive laser-induced gratings

Instantaneous temperature measurements were obtained in an underexpanded jet using electrostrictive laser-induced gratings. Evaluation of the technique under static, low-pressure conditions provided a baseline uncertainty or precision for single-shot temperature measurements of 4.4% of the local mean temperature, which represents the minimum detectable temperature fluctuation. The underexpanded jet was operated at a nozzle pressure ratio of 2.39 and a fully expanded jet Mach number of 1.19. Data were acquired along the centerline and over two radial traverses through the shear layer. Mean temperature data agree well with expectations, describing the shock-cell structure and the compressible shear layer. The growth in shear-layer width with downstream distance can be identified in the mean and fluctuating temperature measurements. Temperature fluctuations are near the baseline detection limit in the jet core and surrounding ambient air, and reach a maximum in the shear layer. The temperature fluctuation measurements compare well with previous computational and experimental work, confirming the application of the technique to a turbulent, supersonic flow.

Dual-pump dual-broadband CARS for exhaust-gas temperature and CO 2–O 2–N 2 mole-fraction measurements in model gas-turbine combustors

Application of dual-pump, dual-broadband (DPDB) coherent anti-Stokes Raman scattering (CARS) for the measurement of temperature and multiple-species mole fractions is presented for the first time in a liquid-fueled combustor of practical interest. In this system pure rotational transitions of O 2–N 2 and the ro-vibrational transitions of CO 2–N 2 are probed using two narrowband pump beams, a broadband pump beam, and a broadband Stokes beam. This technique permits highly accurate temperature measurements at both low and high temperatures as well as mole-fraction measurements of two molecules with respect to N 2 from each laser shot. Single-shot measurements of temperature and mole-fraction ratios of CO 2/N 2 and O 2/N 2 in the exhaust stream of a swirl-stabilized, JP-8-fueled, model gas-turbine combustor are presented for equivalence ratios ranging from 0.45 to 1.0. Agreement between mean rotational and ro-vibrational temperatures is within ∼3%, and mean measurements of CO 2/N 2 and O 2/N 2 mole-fraction ratios are within ∼15% of equilibrium theory. To illustrate the ability of the current measurement system to track multiple scalar statistics in turbulent reacting flows, histograms and scatter plots of temperature and species mole fractions are presented within the potential-core and turbulent-shear-layer regions of the exhaust stream.

Scalar length scales and spatial averaging effects in turbulent piloted methane/air jet flames

The effects of spatial averaging in measurements of scalar variance and scalar dissipation in three piloted methane/air jet flames (Sandia flames C, D, and E) are investigated. Line imaging of Raman scattering, Rayleigh scattering, and laser-induced CO fluorescence is applied to obtain simultaneous single-shot measurements of temperature, the mass fractions of all major species, and mixture fraction, ξ, along 7-mm segments. Spatial filters are applied to ensembles of instantaneous profiles to quantify effects of spatial averaging on the Favre mean and variance of mixture fraction and scalar dissipation at several locations in the three flames. The radial contribution to scalar dissipation, χ r = 2 D ξ (∂ ξ/∂ r) 2, is calculated from the filtered instantaneous profiles. The variance of mixture fraction tends to decrease linearly with increasing filter width, while the mean and variance of scalar dissipation are observed to follow an exponential dependence. In each case, the observed functional dependence is used to extrapolate to zero filter width, yielding estimates of the “fully resolved” profiles of measured quantities. Length scales for resolution of scalar variance and scalar dissipation are also extracted from the spatial filtering analysis and compared with length scales obtained from spatial autocorrelations. These results provide new insights on the small scale structure of turbulent jet flames and on the spatial resolution requirements for measurements of scalar variance and scalar dissipation.

Investigation of a dynamic diffusion flame of H 2 in air with laser diagnostics and numerical modeling

Detailed studies of flame–vortex interactions play a vital role in improving our understanding of turbulent combustion. A combined experimental and numerical study was conducted on a low-speed, buoyant, jet diffusion flame of hydrogen in air to investigate the vortex–flame interaction and the effects of preferential diffusion on the flame's structure. A time-dependent, axisymmetric mathematical model with detailed transport processes and a chemical-kinetics mechanism was used to simulate the dynamics of the flame. Single-shot measurements of temperature and the concentrations of molecular hydrogen (H 2), the pollutant nitric oxide (NO), atomic oxygen (O), atomic hydrogen (H), and the hydroxyl radical (OH) were made using optical techniques such as coherent anti-Stokes Raman scattering, degenerate four-wave mixing, and planar laser-induced fluorescence. Temperature and mole fractions of different species are presented in two-dimensional contour maps and compared with the numerical predictions. The model predicted the behavior of the experimentally observed dynamic flame quite well, including variations in temperature and molar concentrations of fuel and tracer species such as H, OH, and NO. Discrepancies in the concentration of O atoms were also noted.