Time-resolved Temperature Research Articles

MCBTE: A variance-reduced Monte Carlo solution of the linearized Boltzmann transport equation for phonons

MCBTE solves the linearized Boltzmann transport equation for phonons in three-dimensions using a variance-reduced Monte Carlo solution approach. The algorithm is suited for both transient and steady-state analysis of thermal transport in structured materials with size features in the nanometer to hundreds of microns range. The code is portable and integrated with both first-principles density functional theory calculations and empirical relations for the input of phonon frequency, group velocity, and mean free path required for calculating the thermal properties. The program outputs space- and time-resolved temperature and heat flux for the transient study. For the steady-state simulations, the frequency-resolved contribution of phonons to temperature and heat flux is written to the output files, thus allowing the study of cumulative thermal conductivity as a function of phonon frequency or mean free path. We provide several illustrative examples, including ballistic and quasi-ballistic thermal transport, the thermal conductivity of thin films and periodic nanostructures, to demonstrate the functionality and to benchmark our code against available theoretical/analytical/computational results from the literature. Moreover, we parallelize the code using the Matlab Distributed Computing Server, providing near-linear scaling with the number of processors. Program summaryProgram Title:MCBTECPC Library link to program files:https://doi.org/10.17632/rzn86w7t3p.1Developer's repository link:https://github.com/abhipath90/MCBTECode Ocean capsule:https://codeocean.com/capsule/7196257Licensing provisions: GPLv3Programming language: MATLABNature of problem: Calculation of time- and space-dependent temperature and heat flux profiles, and frequency-resolved effective thermal conductivity in structured systems where heat is carried by phonons.Solution method: Solution of linearized Boltzmann transport equation for phonons, variance-reduced Monte Carlo approach.Additional comments including restrictions and unusual features: Runtime about 1 to 10 hours on a personal computer.

Laser-induced fluorescence thermometry of supercritical CO2 flows inside a micro-channel

This work demonstrates a thermometric technique using laser-induced fluorescence (LIF) in supercritical carbon dioxide flows in a micro-channel. Rhodamine 6G was used as a temperature-sensitive fluorescent dye. The flow conditions were at a pressure of 7.9 MPa and temperature in the range of 23°-90°C. 2D spatial distributions and time-resolved temperature profiles were obtained at this high pressure. Measured LIF signals showed close relations to the temperatures obtained from resistance temperature detectors.

Lumped Element Model for Thermomagnetic Generators Based on Magnetic SMA Films.

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature.

Top-level physics requirements and simulated performance of the MRSt on the National Ignition Facility.

The time-resolving Magnetic Recoil Spectrometer (MRSt) for the National Ignition Facility (NIF) has been identified by the US National Diagnostic Working Group as one of the transformational diagnostics that will reshape the way inertial confinement fusion (ICF) implosions are diagnosed. The MRSt will measure the time-resolved neutron spectrum of an implosion, from which the time-resolved ion temperature, areal density, and yield will be inferred. Top-level physics requirements for the MRSt were determined based on simulations of numerous ICF implosions with varying degrees of alpha heating, P2 asymmetry, and mix. Synthetic MRSt data were subsequently generated for different configurations using Monte-Carlo methods to determine its performance in relation to the requirements. The system was found to meet most requirements at current neutron yields at the NIF. This work was supported by the DOE and LLNL.

A validated thermal and biological model for predicting algal productivity in large scale outdoor cultivation systems

Proper assessment of the sustainability of algal products requires an understanding of algal growth rate with regional and temporal resolution. This study developed a bulk growth model that utilizes geospatial data, reactor geometry inputs, and a maximum of six strain-specific parameters to predict algal productivity with temporal and regional resolution. The model requires local weather data to calculate the time-resolved culture temperature which is combined with a biological model based on maximum theoretical carbon fixation to predict the time-resolved algal concentration. Validation of the model against experimental data illustrates an average accuracy of 0.27% ± 5.32% when comparing algal concentration at harvest over 45 days of cultivation data for Galdieria sulphuraria 5587.1, and Galdieria sulphuraria Soos grown in photobioreactors (PBRs) in Mesa, AZ (33.4152° N, 111.8315° W). For open raceway ponds (ORPs), an accuracy of 0.9% ± 2.35% was achieved when comparing algal concentration at harvest over 50 days of cultivation data for Chlorella vulgaris, Desmodesmus intermedius, Nannochloropsis oceanica grown in Atlanta, GA (33.7490° N, 84.3880° W) and Mesa, AZ (33.4152° N, 111.8315° W). The validation trials spanned three seasons (winter, spring, and summer) to ensure model accuracy throughout the year, and concentration between harvests was modeled with hourly resolution. The validated model is used to assess the productivity at several case-study locations in the United States with Chlorella vulgaris, grown in open raceway ponds and Galdieria sulphuraria grown in vertical flat panel photobioreactors. The model has been published as an open-source tool, in an effort to increase the fidelity of future studies that rely on outdoor algae cultivation for feedstock production and allow for time resolved results for future techno-economic and life cycle assessments.

REGALS: a general method to deconvolve X-ray scattering data from evolving mixtures.

Mixtures of biological macromolecules are inherently difficult to study using structural methods, as increasing complexity presents new challenges for data analysis. Recently, there has been growing interest in studying evolving mixtures using small-angle X-ray scattering (SAXS) in conjunction with time-resolved, high-throughput or chromatography-coupled setups. Deconvolution and interpretation of the resulting datasets, however, are nontrivial when neither the scattering components nor the way in which they evolve are known a priori. To address this issue, the REGALS method (regularized alternating least squares) is introduced, which incorporates simple expectations about the data as prior knowledge, and utilizes parameterization and regularization to provide robust deconvolution solutions. The restraints used by REGALS are general properties such as smoothness of profiles and maximum dimensions of species, making it well suited for exploring datasets with unknown species. Here, REGALS is applied to the analysis of experimental data from four types of SAXS experiment: anion-exchange (AEX) coupled SAXS, ligand titration, time-resolved mixing and time-resolved temperature jump. Based on its performance with these challenging datasets, it is anticipated that REGALS will be a valuable addition to the SAXS analysis toolkit and enable new experiments. The software is implemented in both MATLAB and Python and is available freely as an open-source software package.

A multi-channel x-ray temporal diagnostic for measurement of time-resolved electron temperature in cryogenic deuterium-tritium implosions at OMEGA.

Electron-temperature (Te) measurements in implosions provide valuable diagnostic information, as Te is unaffected by residual flows and other non-thermal effects unlike ion temperature inferred from a fusion product spectrum. In OMEGA cryogenic implosions, measurement of Te(t) can be used to investigate effects related to time-resolved hot-spot energy balance. The proposed diagnostic utilizes five fast-rise (∼15 ps) scintillator channels with distinct x-ray filtering. Titanium and stepped aluminum filtering were chosen to maximize detector sensitivity in the 10 keV-20 keV range, as it has been shown that these x rays have similar density and temperature weighting to the emitted deuterium-tritium fusion neutrons. Initial data collected using a prototype nosecone on the existing neutron temporal diagnostic demonstrate the validity of this diagnostic technique. The proposed system will be capable of measuring spatially integrated Te(t) with 20 ps time resolution and <10% uncertainty at peak emission in cryogenic DT implosions.

Influence of sample temperature on nanosecond laser-induced Cu plasma spectra

There are many experimental methods to increase the emission intensity of laser-induced breakdown spectroscopy (LIBS). Among these methods, pre-heating the target is a simple and effective method to improve LIBS signal intensity. This study used Nd: YAG nanosecond laser pulse to ablate Cu target and produce Cu plasma, and time-resolved spectra of the Cu plasma were measured at different Cu temperatures. The measurement suggested that the higher the target temperature, the stronger the time-resolved spectra of the Cu plasma, and the higher the time-resolved electron temperature and density. Next, the morphology of ablation craters was observed by microscope at different target temperatures. As the Cu target was heated, the melting area in the center of ablation crater became larger, which indicated that more mass targets were ablated to produce plasma, thus producing stronger spectral signal. Finally, the thermal behavior of Cu at different target temperatures was simulated by one-dimensional heat conduction equation. The simulated results show that the temperature of the Cu under laser irradiation increased with the increase of initial temperature of the Cu, and the melting depth also increased, which was consistent with the observed results of Cu target morphology.

Spectral dynamics and gas-phase oxidation of laser-produced plutonium plasmas

Pu gas-phase oxidation and Pu oxide bands identified with Pu I spectral modeling and time-resolved excitation temperature of Pu plasma.

Time-resolved temperature and velocity field measurements in gas turbine film cooling flows with mainstream turbulence

Gas turbine film cooling strategies must provide adequate cooling performance under high levels of mainstream turbulence. Detailed information about the structure, dynamics and transport of the cooling films is needed to understand the flow physics and develop suitable numerical simulation tools. Here, we study film cooling flows in a wind tunnel with mainstream turbulence generated by an active turbulence grid. Gas temperature and velocity fields are measured using a laser-imaging method based on thermographic phosphor tracer particles. By replacing the previously used tracer BaMgAl10O17:Eu2+ with ZnO, significant gains in accuracy and precision could be achieved. The increased sensitivity (~ 1%/K) of ZnO led to a threefold improvement in the single-shot, single-pixel temperature precision to ± 5 K. The smaller particle size (dp,v ~ 600 nm) and agglomerated nanoparticle structure also reduced the tracing response time to ~ 5 µs allowing accurate tracking of turbulent fluctuations approaching 10 kHz. Moreover, no uncertainty arising from multiple scattering effects were observed using ZnO particles in this enclosed wind tunnel geometry at an estimated average seeding density of 2 × 1011 particles/m3. Time-average, fluctuation and single-shot temperature–velocity fields are presented for two mainstream turbulence levels (overline{u}^{prime}/{overline{u}}_{mathrm{m}} = 7% and 14%) and two momentum ratios (IR = 4.7 and 9.3) at a fixed density ratio of 1.55. These flow conditions produce a cooling jet which is detached from the surface. High main flow turbulence causes faster mixing with the surrounding hot gas, increasing the wall-normal spreading of the cooling jet. The instantaneous flow fields show that mainstream turbulence has a significant effect on the shear layer velocity fluctuations and consequently on the streamwise and wall-normal turbulent heat flux, which is derived from the simultaneously acquired temperature–velocity data. We found that high mainstream turbulence reduces the heat flux away from the wall, suggesting that mainstream turbulence can act to diminish cooling performance. Sets of instantaneous measurements recorded at a 6 kHz repetition rate also reveal the dynamic interactions between the main flow turbulence and the cooling jet. These findings and the recorded data can be used to advance turbulence modelling for numerical simulations.Graphic abstract

High-speed filtered Rayleigh scattering thermometry in premixed flames through narrow channels

Abstract High-speed filtered Rayleigh scattering at 10 kHz repetition rate for time-resolved temperature measurements in premixed flames propagating in mm-high channels is demonstrated for the first time. The instrument relies on a pulse burst laser with a 200 ns temporal width, and a CCD camera operated in subframe burst gate mode to achieve high signal to noise ratio despite the limited optical access. A 10-inch-long iodine cell acts as a molecular filter and the laser, which is seeded using an external cavity diode laser and operated in a temporally stretched mode, is wavelength-tuned to the peak of a strong iodine absorption line. A CCD camera, operated in the sub-frame burst gating mode with the help of an external slit configuration, is used as the detector to achieve improved camera noise performance. Filtered Rayleigh scattering temperature measurements in premixed flat flames stabilized over a McKenna burner indicate an instrument precision of 3%. Temperature measurements in the products region are within 3% of measurements obtained with conventional Rayleigh scattering. The system's capabilities are demonstrated through time-resolved temperature measurements in a premixed methane-air flame propagating in a 1.5 mm-high rectangular channel designed to study flame quenching in flame arrestors. Surface scattering from the optical windows and the channel surfaces is successfully suppressed and time-resolved temperature profiles are obtained for both quenching and no-quenching events.

Acquisition of kHz-frequency two-dimensional surface temperature field using phosphor thermometry and proper orthogonal decomposition assisted long short-term memory neural networks

A reconstruction technique of kHz time-resolved two-dimensional (2D) surface temperature field was achieved with the discrete point measurements and low sampling rate 2D thermographic phosphor (TP) thermometry measurements using a long short-term memory (LSTM) based artificial intelligence framework. The 2D surface temperature field of a 350 °C plate with a 2.5 Hz swing cooling jet was measured using TP thermometry at a sampling rate of 20 Hz. At the same time, high-frequency thermocouples with a sampling rate of 1 kHz were recorded for the construction of LSTM neural networks training and for validation. The 20 Hz 2D surface temperature field was analyzed with proper orthogonal decomposition to acquire the energy modes and model coefficients. The mode coefficients are then trained with the discrete but high-frequency time-resolved temperature information from the thermocouple by LSTM to acquire the time-resolved mode coefficients. Finally, the high-frequency time-resolved 2D surface temperature field is obtained by reconstructing modes and the time-resolved coefficients. The reconstructed result shows that the current technique can obtain high time-resolved and spatially resolved 2D surface temperature fields very well.

Time-Resolved Analysis of SF6 Arc Plasmas in a Laboratory Model Chamber for Circuit Breaker

This article presents time-resolved analysis of SF6 arc plasma generated in a laboratory-scale model chamber prepared for studying the characteristics of a circuit breaker. During the arc discharge, the optical emission spectra are measured by a high-speed spectrometer under various operating conditions. The time-resolved electron temperature and density values are determined using the well-known Boltzmann plot and Stark broadening methods, respectively, for the measured Cu I, S II, and F I emission lines. Moreover, the temporal evolution of the electrical conductivity of the arc plasma is deduced from the measured electron temperature and density values, depending on the operating conditions. In our experimental conditions, it is revealed that the electrical conductivity of arc plasmas is mainly determined by Coulomb (electron–ion) collisions as opposed to electron–neutral collisions due to the much higher Coulomb collision frequency compared to the electron–neutral collision frequency. Both the electron temperature and density were found to increase when the discharge current is increased at a fixed gas pressure, resulting in an increase of the electrical conductivity. When the gas pressure inside the model chamber is increased, the electron density is increased as well, but the electrical conductivity does not change significantly due to the slight change in the electron temperature. The present study will be helpful for investigating the performance capabilities of circuit breakers because the electrical conductivity, the most important parameter to guarantee successful circuit breaking at points near the current-zero, is determined by the temperature and density of the electrons.

Rise time-based phosphor thermometry using Mg4FGeO6:Mn4+

The rise time method is a time-resolved temperature measurement technique based on phosphorescence. Its short measurable time and high uncertainty limit its application. In this study, a long-term excitation on the millisecond level was used on the phosphor Mg4FGeO6:Mn4+ to realize a low uncertainty rise-time-based phosphor thermometry. A 385 nm UV-LED was used as excitation light and a high-speed camera was used as the detector. The results of calibration show that Mg4FGeO6:Mn4+ has a long rise time from 1140 ms to 2920 ms which is directly related to the temperature and has a low uncertainty of 1.2% in the temperature range of 293 K to 623 K. High absolute sensitivity of 3.02 μs K−1 was found at 623 K, and it increased with the decrease of temperature, reaching 7.65 μs K−1 at 293 K. To evaluate the effort of the excitation light on the rise time, the rise time of Mg4FGeO6:Mn4+ was measured with different excitation time and excitation energy. The results show that the rise time of Mg4FGeO6:Mn4+ is stable with different excitation times and decreases a little with the increase of the excitation energy. Finally, the rise time-based phosphor thermometry using Mg4FGeO6:Mn4+ was successfully applied on a 2D temperature measurements of jet impingement cooling. This is the first time of using MFG with a long-term excitation as the rise time-based phosphor thermometry. The high sensitivity and low-uncertainty demonstrate that Mg4FGeO6:Mn4+ has excellent performance for rise-time-based phosphor thermometry.

The dynamics of a non-premixed rotating detonation engine from time-resolved temperature measurements

The structure and dynamics of a hydrogen-air rotating detonation engine (RDE) are described based on 100-kHz laser absorption spectroscopy measurements of water temperature at four simultaneous locations within the detonation channel. The analysis focuses on the evolution of the flowfield over a 200 ms period for three separate air mass flow rate cases. Two-dimensional unwrapped visualizations of the temperatures show a flowfield structure containing regions with the detonation front, combustion products, oblique shock, and refilling reactants, qualitatively agreeing with previous simulations and experiments. A major conclusion is that water from the combustion products is measured throughout all space and time in the RDE, including near the injector, implying the presence of performance loss processes such as burning upstream of the detonation wave or the back recirculation of combustion products with fresh fuel–air. By analyzing the elevated temperatures of the reactants during the refill process, one estimation for the mass fraction of combustion products in the reactants is as high as 20–30% on average. This product mass fraction is found to be inversely proportional to the bulk air mass flow rate and decreases as time progresses. This indicates these non-ideal processes are more significant closer to RDE ignition for poorer performing operating conditions. For the largest air mass flow case, water temperatures near the nominally cold plenum conditions likely corroborate the presence of a recirculation region on the RDE inner body. Analysis of inter- and intra-cycle temperature dynamics further support non-ideal processes occurring behind the detonation wave and during the refill process. As a whole, the data indicates that the RDE performance is better as time progresses away from ignition or for higher air mass flow rates. These data are also important for comparison with numerical models.

Time-resolved electron temperature and density of spark discharge assisted femtosecond laser-induced breakdown spectroscopy

In laser-induced breakdown spectroscopy (LIBS), spark-discharge (SP) is an effective auxiliary method to improve spectral emission intensity, and can effectively affect plasma temperature and electron density. In traditional LIBS technology, nanosecond laser is usually used as excitation source. Compared with nanosecond laser, femtosecond (fs) pulse laser has many advantages in the interaction between laser and matter. The SP can also enhance the emission intensity of fs-LIBS. However, the discussion on plasma temperature and electron density of SP assisted fs-LIBS is rare. In this study, we measured time-resolved spectra of fs-LIBS with the SP. By using the Boltzmann plot and the Stark broadening, we calculated time-resolved plasma temperatures and electron densities for different laser energies at different discharge voltages. We hope that this study can provide a better understanding for SP assisted fs-LIBS.

Time-resolved size, velocity, and temperature statistics of aluminum combustion in solid rocket propellants

The study of time-resolved aluminum combustion mechanisms is essential for understanding the deflagration of ammonium perchlorate-based metalized solid rocket propellants. In order to gain further insight into the performance of solid propellants, spatially and temporally resolved aluminum agglomerate particle dynamics are obtained by combining digital in-line holography (DIH) and two-color imaging pyrometry with high-speed acquisition at up to 20 kHz. Holography is used to find the size, three-dimensional position, and three-dimensional velocity evolution of agglomerates over time. Then, the temperature of individual particles is extracted from the high-speed imaging pyrometry. This diagnostic technique not only produces joint size, position, velocity, and temperature statistics over time, but also captures the combustion histories for thousands of particles per experiment. For the first time, these spatial and temporal dynamics of individual aluminum particulates are examined while they travel away from the propellant surface. Initial results demonstrate how aluminum agglomerates of similar size exhibit varying initial acceleration but similar steady-state velocities. Average velocity also decreases as particle size increases, which is consistent with viscous flow dynamics of small particles in convective flow. As they move further away from the propellant surface, large agglomerates also show a convergence to an average projected temperature between the melting point of aluminum oxide and the boiling point of aluminum. The average projected particle surface temperatures were measured to be 2494 ± 231 K. The method outlined in this work demonstrates a new capability for gathering the evolution of joint statistics for aluminum agglomerates in solid-rocket propellants. Future applications of this technique can be used to evaluate the detailed combustion mechanisms of existing or new propellant formulations.

Thermal dynamic imaging of mid-infrared quantum cascade lasers with high temporal–spatial resolution

The time-resolved (TR) temperature profile of actively biased mid-infrared quantum cascade lasers (MIR QCLs) was measured by using charge-coupled-device (CCD)-based thermoreflectance microscopy (TRM) with an ultrafast temporal resolution of 50 ns and a high spatial resolution of 390 nm. Based on the measured TR two-dimensional (2D) temperature profiles, the heat generation and dissipation dynamics within the lasers have been investigated. It is found that the active-region temperature increases quickly to a peak value (up to ∼100 °C above ambient room temperature) within 500 ns upon pulsed current injection of 6 A. The heat dissipation to the top and bottom cladding layers of the device is initially comparable, yet it evolves differently with time. Within 1–2 μs, the heat dissipation to the top cladding is substantially reduced and most of the heat is drained to the substrate through the bottom cladding layer. This constrained heat dissipation results in the elevated temperature in the active region, leading to thermal quenching of lasing operation, which is confirmed by experimental light–current–voltage measurement and theoretical thermal modeling. The TRM is an enabling tool for probing internal thermal dynamics of many active electronic and photonic devices, particularly for those needing special heat and thermal arrangement.

Surface Temperature Measurements of Burning Solid Propellants Using Phosphor Thermography

In recent years, phosphor thermography has received increased attention as an alternative to other common temperature measurement techniques (e.g., thermocouples, pyrometry). This technique has been successfully used for surface temperature measurements on coated surfaces (e.g., turbine blades, furnace walls) and, in some cases, mixed with a reacting material. However, this technique has yet to be used for measuring the surface temperature of burning energetic materials (e.g., propellants, explosives). Adapting this technique to the study of energetic materials would enable temporally and spatially resolved surface temperature measurements that also lack some of the shortcomings of conventional methods. This work investigates the feasibility of using phosphor thermography to measure the surface temperature of burning nitrocellulose, an energetic material used extensively in single-, double-, and triple-base propellant formulations. Using phosphor thermography, two-dimensional time-resolved surface temperature measurements were obtained across the surface of a burning nitrocellulose pellet. The average surface temperatures measured with this approach () were in good agreement with previously published measurements for nitrocellulose obtained with embedded thermocouples (). This work demonstrates the viability of using phosphor thermography for measuring the surface temperature of burning solid propellants as well as other types of energetic material compositions.

Measurement of three-dimensional flow structure and transient heat transfer on curved surface impinged by round jet

In this study, the transient heat transfer characteristics of a circular cylinder surface cooled by an impinging air jet were measured through thermographic phosphor thermometry. Time-resolved surface temperature fields were visualized under constant heat flux conditions. After the jet impingement on the circular cylinder, the curved three-dimensional (3D) wall jet developed toward both spanwise and streamwise directions, followed by most of the fluid moving along the curved surface due to the Coanda effect. Therefore, the high heat transfer region was elongated toward the streamwise direction. The impinging angle was established to be a dominant factor in heat transfer characteristics. The heat transfer was improved with an increase in the impinging angle, as the cooling area expanded toward the streamwise direction. To discuss the transient heat transfer characteristics, time-resolved two-dimensional Nusselt number distributions were obtained according to the varied impinging angles and Reynolds numbers. The secondary peak of heat transfer was not clearly observed due to the stabilizing effect of curvature on the wall jet. To understand the heat transfer phenomenon, the 3D flow structures of the jet on the curved surface were obtained using time-resolved volumetric particle tracking velocimetry (PTV). The velocity vectors and streamwise vortices were visualized according to the impinging angle and analyzed in three-dimensions. The 3D curved wall jet possessed a big counter-rotating streamwise vortex structure in the upper region of the attached wall jet on the cylinder wall. With an increase in the impingement angle, the streamwise vortex developed longer at the same Reynolds number. The 3D flow structure and heat transfer distribution were found to demonstrate similar tendencies.