Rotational Temperature Profiles Research Articles

Rotational and Gas Temperature Measurements for He-C Plasma: Application to Heterogeneous Carbon Nanotubes Synthesis

Plasmas containing carbon cover a wide range of application, either using graphite electrodes or carbon containing plasma gas. The study of the C2 molecule is an interesting way to get information on moderate temperature part of the discharge. In the case of heterogeneous carbon nanotubes (CNTs) synthesis by electric arc, reasonable yield can be achieved with substitution of boron and/or nitrogen atoms by optimizing synthesis parameters. This is based on the knowledge of the plasma and gas temperatures in the synthesis chamber. While electrodes material is vaporized through the arc, nanotubes formed in the growth zone a few centimeters away, when temperature decreases down to 1200 °C–1500 °C. Molecular temperature is particularly relevant since it corresponds to the outer part of the arc, in contact with the growth zone. In this article, Abel inversion is used on side-on integrated spectral profiles for C2(0,0) Swan band, in order to get radial rotational temperature profiles. In addition, gas temperature measurements are performed using thermocouples in the arc chamber. Results show that a strong plasma temperature gradient is associated with favorable synthesis conditions for carbon-bore-nitrogen nanotubes. Gas thermal homogeneity is reached in the chamber after about 30 s, mainly through convection processes.

First Detection and Thermal Characterization of Terminator CO2Ice Clouds With ExoMars/NOMAD

AbstractWe present observations of terminator CO2ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20°S and 20°N) and Southern midlatitudes at dawn (45°S and 55°S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2column profiles in 16 of 26 cases. Small particles (<0.5 μm) prevail at dusk, while water ice likely provides most source nuclei at dawn. Clouds east of Hellas are found to be dominantly nucleated on surface‐lifted dust. CO2ice is sometimes detected in unsaturated air together with dust nuclei at dawn, suggesting ongoing sublimation. Depending on latitude and local time, the interplay between particle precipitation and the lifetime of temperature minima (i.e., cold pockets) determines CO2ice properties.

Evaluation of particle-based continuum methods for a coupling with the direct simulation Monte Carlo method based on a nozzle expansion

This paper investigates three different particle-based continuum methods, the ellipsoidal statistical Bhatnagar-Gross-Krook (ESBGK) and Fokker-Planck (ESFP) methods and the Low Diffusion (LD) method, for a coupling with the direct simulation Monte Carlo (DSMC) method. After a short description of the methods and their implementation, including the coupling concept for the LD-DSMC, simulation results of a nozzle expansion are compared with available experimental measurements and a DSMC simulation. Excellent agreement between ESBGK, ESFP, and DSMC can be observed in the throat of the nozzle, while the LD method fails to predict the correct velocity, temperature, and density profile. Further downstream, only the DSMC and the coupled ESBGK/ESFP-DSMC simulations are able to reproduce the measured rotational temperature profiles. A performance comparison shows the possible computational savings of a coupled ESBGK/ESFP-DSMC simulation, where a speedup of four orders of magnitude can be observed compared to a regular DSMC simulation.

The importance of thermal dissociation in CO2 microwave discharges investigated by power pulsing and rotational Raman scattering

The input power of a CO2 microwave plasma is modulated at kHz rate in scans of duty cycle at constant average power to investigate gas heating dynamics and its relation to dissociation efficiency. Rotational temperature profiles obtained from rotational Raman scattering reveal peak temperatures of up to 3000 , while the edge temperature remains cold (500 ). During the plasma ‘OFF’-period, the gas cools down convectively, but remains overall too hot to allow for strong overpopulation of vibrational modes (2200 in the core). Fast optical imaging monitors plasma volume variations and shows that power density scales with peak power. As dissociation scales with observed peak rotational temperature, it is concluded that thermal processes dominate. A simple 0D model is constructed which explains how higher power density favors dissociation over radial energy transport. Thermal decomposition is reviewed in relation to quenching oxygen radicals with vibrationally excited CO2, to reflect on earlier reported record efficiencies of 90%.

A rotational Raman study under non-thermal conditions in pulsed CO2–N2 and CO2–O2 glow discharges

This work employs in situ rotational Raman spectroscopy to study the effect of N2 and O2 addition to CO2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO2 and 25%, 50% and 75% of N2 or O2 admixture, in a 5–10 ms on-off cycle, 50 mA plasma current and 6.7 mbar total pressure. The rotational temperature profile is not affected by adding N2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O2 and N2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N2 containing mixtures show that CO2 conversion factor α increases from 0.15 to 0.33 when the content of N2 is increased from 0% to 75%, demonstrating the potential of N2 addition to enhance the vibrational pumping of CO2 and its beneficial effect on CO2 dissociation. Furthermore, the influence of admixtures on CO2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O2 leads to a decrease of α, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O2.

Advanced heat transfer analysis of continuously variable transmissions (CVT)

The presented paper focuses on heat transfer analysis of rubber-belt continuously variable transmissions (CVT). The huge advantage of this system is the continuous change of the transmission ratio without interrupting the torque output. The moderate efficiency of CVTs due to belt deformation and frictional forces, however, leads to increased thermal loads. Especially the belt life span suffers under high temperatures. The numerical prediction of the resulting heat distribution at critical load cases is of key interest. In current literature it has hardly been investigated due to the complexity of the system. The numerical model introduced in this work is able to conduct time efficient heat transfer analysis within an enclosed CVT by using computational fluid dynamics (CFD). The transient process is transferred to a quasi-steady-state case reducing the computational time drastically. A new method to compute rotational symmetric temperature profiles for non-rotating pulleys has been developed. As a result, the surface temperatures of each component can be computed accurately. Measurements, conducted on an engine test rig, confirm the numerical results. The presented model can be applied to evaluate design changes and to reduce peak temperatures and hence increase product reliability. Moreover, the presented method offers a huge advantage for further transient processes which can be represented by a steady-state case and focus on heat transfer analysis.

Microwave plasma-assisted ignition and flameholding in premixed ethylene/air mixtures

In this study, a 2.45 GHz microwave source and a surfatron were used, coupled with a T-shaped quartz combustor, to investigate the role of a nonthermal microwave argon plasma jet on the plasma-assisted ignition and flameholding of a premixed ethylene/air mixture. A modified U-shaped plot of the minimum plasma power required for ignition versus fuel equivalence ratio was obtained, whereby the plasma power required for plasma-assisted ignition decreased with increase in fuel equivalence ratios in the range 0.2–0.6, but for fuel equivalence ratios of 0.7 and above, the plasma power required for ignition remained fairly constant throughout. It was observed that leaner fuel/air mixtures were more sensitive to heat losses to the surrounding and this sensitivity decreased with increase in the fuel equivalence ratio. Comparison with results obtained from previous studies suggested that the mixing scheme between the plasma and the premixed fuel/air mixture and the energy density of the fuel used played an important role in influencing the minimum plasma power required for ignition with the effect being more pronounced for near stoichiometric to rich fuel equivalence ratios (0.7–1.4). Flame images obtained showed a dual layered flame with an inner white core and a bluish outer layer. The images also showed an increased degree of flameholding (tethering of the flame to the combustor orifice) with increase in plasma power. The concurrency of the dual peaks in the emission intensity profiles for OH(A), CH(A), C2(d), and the rotational temperature profiles obtained via optical emission spectroscopy along with the ground state OH(X) number density profiles in the flame using cavity ringdown spectroscopy led to the proposal that the mechanism of plasma-assisted flameholding in ethylene/air flames is predominantly radical dependent with the formation of an inner radical rich flame core which enhances the ignition and stabilization of the surrounding coflow.

Measuring the Distribution and Excitation of Cometary CH3OH Using ALMA

AbstractThe Atacama Large Millimeter/submillimeter Array (ALMA) was used to obtain measurements of spatially and spectrally resolved CH3OH emission from comet C/2012 K1 (PanSTARRS) on 28-29 June 2014. Detection of 12-14 emission lines of CH3OH on each day permitted the derivation of spatially-resolved rotational temperature profiles (averaged along the line of sight), for the innermost 5000 km of the coma. On each day, the CH3OH distribution was centrally peaked and approximately consistent with spherically symmetric, uniform outflow. The azimuthally-averaged CH3OH rotational temperature (Trot) as a function of sky-projected nucleocentric distance (ρ), fell by about 40 K between ρ= 0 and 2500 km on 28 June, whereas on 29 June,Trotfell by about 50 K between ρ =0 km and 1500 km. A remarkable (~50 K) rise inTrotat ρ = 1500-2500 km on 29 June was not present on 28 June. The observed variations in CH3OH rotational temperature are interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling, and heating through Solar irradiation, but collisional and radiative non-LTE excitation processes also play a role.

Laser Diagnostics of Plasma in Synthesis of Graphene-Based Materials

Rotational temperature profiles of H2 in a microwave plasma chemical vapor deposition (MPCVD) reactor were measured via coherent anti-Stokes Raman scattering (CARS) spectroscopy. The temperature was found to increase with reactor pressure, plasma generator power, and distance from the deposition surface. At 10 Torr, the measured temperature range was approximately 700–1200 K while at 30 Torr it was 1200–2000 K under the conditions studied. The introduction of CH4 and N2 to the plasma increased the rotational temperature consistently. These findings will aid in understanding the function of the chemical composition and reactions in the plasma environment of these reactors which, to date, remains obscure.

Multi-scale study of condensation in water jets using ellipsoidal-statistical Bhatnagar-Gross-Krook and molecular dynamics modeling.

Homogeneous water condensation and ice formation in supersonic expansions to vacuum for stagnation pressures from 12 to 1000 mbar are studied using the particle-based Ellipsoidal-Statistical Bhatnagar-Gross-Krook (ES-BGK) method. We find that when condensation starts to occur, at a stagnation pressure of 96 mbar, the increase in the degree of condensation causes an increase in the rotational temperature due to the latent heat of vaporization. The simulated rotational temperature profiles along the plume expansion agree well with measurements confirming the kinetic homogeneous condensation models and the method of simulation. Comparisons of the simulated gas and cluster number densities, cluster size for different stagnation pressures along the plume centerline were made and it is found that the cluster size increase linearly with respect to stagnation pressure, consistent with classical nucleation theory. The sensitivity of our results to cluster nucleation model and latent heat values based on bulk water, specific cluster size, or bulk ice are examined. In particular, the ES-BGK simulations are found to be too coarse-grained to provide information on the phase or structure of the clusters formed. For this reason, molecular dynamics simulations of water condensation in a one-dimensional free expansion to simulate the conditions in the core of a plume are performed. We find that the internal structure of the clusters formed depends on the stagnation temperature. A larger cluster of average size 21 was tracked down the expansion, and a calculation of its average internal temperature as well as a comparison of its radial distribution functions (RDFs) with values measured for solid amorphous ice clusters lead us to conclude that this cluster is in a solid-like rather than liquid form. In another molecular-dynamics simulation at a much lower stagnation temperature, a larger cluster of size 324 and internal temperature 200 K was extracted from an expansion plume and equilibrated to determine its RDF and self-diffusion coefficient. The value of the latter shows that this cluster is formed in a supercooled liquid state rather than in an amorphous solid state.

The Efficiency of Inelastic Collision Models with the Variable Sphere Molecular Model in the Direct Simulation Monte Carlo Method

The variable sphere (VS) molecular model for the inverse power law (IPL) potential is combined with the statistical inelastic cross section (SICS) and Borgnakke-Larsen (BL) models and the transport coefficients are estimated using the Monte Carlo integration technique for Wang-Chang-Uhlenbeck (WCU) theory. The VS model with SICS (VS-SICS) and BL (VS-BL) models are applied to the simulation of the normal shock wave structure in a nitrogen gas using the direct simulation Monte Carlo (DSMC) method, and then compared with the results of VSS model combined with the SICS model (VSS-SICS) and BL model (VSS-BL). Structures of number density, rotational temperature profiles, and rotational energy distribution in the VS-SICS and VS-BL models shock wave agree well with those of the VSS-SICS and VSS-BL models. The VS model demonstrated applicability to the inelastic molecular collision model.

A new, versatile, direct-current helium atmospheric-pressure glow discharge

A novel direct current glow discharge sustained in helium at atmospheric pressure has been developed. Current–voltage behavior and spectroscopic characteristics strongly suggest that the system operates in the glow regime, in spite of the high pressure. The diffuse and extremely stable discharge is typically operated within a voltage range of 300–900 volts (in the current-controlled mode) and at currents ranging over tens to hundreds of milliamps. Spatially resolved spectroscopic measurements of some selected species are presented. Rotational temperature profiles were calculated using the OH emission spectrum, yielding values in the positive column ranging from 1300 to 1600 K.

Direct simulation Monte Carlo study of rotational nonequilibrium in shock wave and spherical expansion of nitrogen using classical trajectory calculations

The rotational nonequilibrium in a normal shock wave and in a steady spherical expansion is studied for nitrogen using the direct simulation Monte Carlo method coupled with classical trajectory calculations. The intermolecular potential for nitrogen is taken as an accurate potential energy surface constructed by van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)] and readjusted by Cappelletti et al. [Mol. Phys. 93, 485 (1998)]. The shock wave results are compared with the rotational distribution and the number density and rotational temperature profiles measured by Robben and Talbot [Phys. Fluids 9, 653 (1966)]. The agreement is generally good except at extremely low upstream temperatures, where the classical-mechanical treatment may be inadequate. The spherical expansion results are compared with the rotational distribution and the number density and rotational temperature profiles measured by Marrone [Phys. Fluids 10, 521 (1967)] and Coe et al. [Phys. Fluids 23, 706 (1980)] along the axis of a low density free jet expansion. It is shown that the rotational distribution gradually departs from a Boltzmann distribution farther downstream with an overpopulation in the higher rotational levels, in good agreement with the measured spectral data except near the orifice, where the spherical source expansion analogy may be inapplicable. The rotational temperature obtained by a Boltzmann fit at the low rotational levels or defined as the mean rotational energy agrees satisfactorily, except near the orifice, with the experimental rotational temperatures determined from the spectral data.

Investigation of the nitrogen shock wave structure on the basis of trajectory calculations of the molecular interaction

The shock wave structure in a diatomic gas is investigated using the direct statistical simulation (Monte-Carlo) method. The energy exchange between translational and rotational degrees of freedom (TR-exchange) is calculated by solving the dynamic problem of the interaction between rigid-rotator molecules within the framework of classical mechanics. The density profiles calculated are compared with the experimental data and on this basis the nitrogen rotational relaxation time is estimated. The possibility of using simplified intermolecular interaction models, namely, the variable-diameter sphere model employed together with a phenomenological consideration of the TR-exchange, is studied. Gasdynamic parameter profiles in the shock wave are analyzed. Simple approximations of the velocity gradient and translational and rotational temperature profiles are obtained on the basis of a parametric calculation of the shock wave structure. This makes it possible approximately to describe the gasdynamic parameter profiles in terms of elementary functions.

Spatial rotational temperature and emission intensity profiles in silane plasmas

The spatial variation of the rotational temperature of the 0-0 band of the A transition of SiH in radio frequency pure silane discharges is investigated by utilizing a method of improved accuracy for the simulation of the emission spectrum. Furthermore, the variation of the rotational temperature as a function of the pressure at the maximum intensity position of the interelectrode space is examined. The results are used to extract information about the relative variations of the electron energy distribution function and the effective electron density in the interelectrode space. Thus, it is considered that there is a sensitive relation of the observed high rotational temperatures to the mean electron energy, provided that the electron energy distribution function does not change. More specifically, a decrease of the rotational temperature is observed on moving away from the powered electrode sheath region and a similar decrease is also detected with increasing pressure. This behaviour is compared with the spatial emission intensity profiles. The variation of the space-integrated emission intensity under the various discharge conditions, related to the modification of the effective electron density, is also presented.

Construction of Collision Model of Diatomic Molecules. 3rd Report. Improvement of Collision Model.

At 1st and 2nd reports, the Danamic Molecular Collision (DMC) model of diatomic molecules which is based on the molecular dynamics is constructed. The equilibrium state, transport coefficient at certain temperature, and the density profile of normal shock wave can be calculated well by this model. However, it becomes clear that this model can not calculate the property of the distribution of energy of the MD simulation, the temperature dependency of transport coefficient and the distance between density and rotational temperature profile of normal shock wave. For this reason, the DMC model is improved to calculate these values accurately in this report. First, we introduce the new potential parameter obtained from the potential which is based on the ab-initio calculation. Second, the collision cross section which is based on kinetic theory is introduced to calculate the temperature dependence of transport coefficient well.

Spectroscopic Investigation of Nitromethane Flames

AbstractNitromethane (CH3NO2) Pool Fire Flames have been investigated analysing both, the radiation emitted from intermediate combustion radicals (diatomic molecules) in the ultraviolet (UV) and visible (VIS) spectral range, and the near infrared spectra dominated by broad water bands. Comparing the UV/VIS spectra to calculated band profiles, rotational and vibrational temperatures (Trot, Tvib) of OH, NH and CN have been determined. At local thermal equilibrium Trot and Tvib must be equal. The determined temperatures show that the OH and NH emissions originate from pure thermal excitation (Trot = Tvib = 2300 K–2380 K) but the CN emissions show additionally a chemical excitation indicated by different vibrational and rotational temperatures in the Boltzman factor (Trot = 2100 K, Tvib = 4300 K). For a more detailed study of the flame, the emission of OH radicals has been investigated more extensively by monitoring a two‐dimensional rotational temperature and emissivity profile. In cooler flame regions, the observed spectra are dominated by the near infrared radiation emitted from stable combustion products like water and carbon dioxide. Therefore, NIR spectra (1000 nm to 2500 nm) have been compared to broad band profiles calculated with a self‐developed code basing on the data of the ‘Handbook of Infrared Radiation of Combustion Gases’. The flame temperatures obtained by this method range from 1800 K to 1900 K. The results are correlated to flames of methane and nitrogen oxide which emit similar spectra indicating similar reaction mechanisms in the gaseous phase.

Diagnostics and modeling of strained fuel-rich acetylene/oxygen flames used for diamond deposition

Low pressure (35 torr) strained acetylene/oxygen flames similar to those used for diamond chemical vapor deposition are studied using laser-induced fluorescence (LIF) and mass spectrometry. Relative OH concentration and OH rotational temperature profiles are obtained by LIF, while absolute mole fractions of C 2H 2, CH 4, H 2, and CO are measured at the substrate by mass spectrometry. The results of the diagnostics are compared with the predictions of a one-dimensional strained flame code. The agreement between model prediction and experiment is generally good and usually within experimental uncertainties.

Quenching Effects on Laser-Fluorescence Measurements of OH Rotational Temperature in H2/O2/Ar Flames†

Abstract The OH rotational temperature profiles in a series of low-pressure (76-torr) H2/O2/Ar flames (Φ = 0.88,0.98,1.22,1.37,and 1.50) were measured via laser-induced fluorescence(LIF) from the A 2Σ−X 2Π transition in the OH molecule. The temperature measurements were obtained from temporally integrated two-line fluorescence from the OH (1,1) band at 314.5 nm with excitation in the OH (1,0) band from either the Q1 (3)or Q1(7)line. Time-resolved fluorescence measurements of the quenching rates for these transitions were used to correct the two-line fluorescence signals for viriation in fluorescence yield. The two-line fluorescence temperature measurement technique was calibrated with fluorescence excitation spectra in the R1 branch of the (1,0) band, which were also corrected for varaition in quenching. The rotational-level-dependent quenching rates decreased as much as 25% as the excited-state rotational level increased from N′ = 4 to N′ = 11. Over the range of equivalence ratios that were studied, the inclusion of the quenching correction in the analysis lowers the measured rotational temperatures by as much as 150 K, compared to an analysis that assumed a constant fluorescence yield. Neglecting rotational-level-dependent quenching can introduce a systematic error in OH LIF temperature measurements as muchas 10% in the temperature range from 1000 to 1500 K.

Shock-tube analysis of argon influence in Titan radiative environment

The effects of argon addition, in the range of 0-20% in a N2-CH4 mixture on the nonequilibrium radiation emitted behind a normal shock wave, have been investigated in a free-piston-driven shock tube. The intensity of spontaneous emission, for the B2S+ —» X 2S+ electronic transition of CN molecules, is measured at a shock velocity of 5700 m/s propagating in a 200-Pa test gas mixture. Rotational and vibrational temperature profiles in the shock layer are obtained by matching three spectral lines simultaneously recorded in the A*> = 0 band with theoretical spectra calculations. The results show that the nonequilibrium radiation overshoot weakly increases with argon addition, whereas the equilibrium intensity value is not affected. The characteristic relaxation time of radiation is also affected so as to be reduced by argon addition. The vibrational relaxation time for CN molecules is also determined from the temperature profiles, but the accuracy is difficult to assess since the temperatures are weakly dependent functions of ratios of intensities.