Small angle neutron scattering setup for high pressure measurements at IBR-2

Abstract

The experiments which were carried out showed the possibility of using a titanium-zirconium high pressure cell for small angle neutron scattering. We report on some curves measured by SANS with the help of this setup to illustrate the capabilities of the method.     

Small angle neutron scattering and small angle X-ray scattering studies of platinum-loaded carbon foams

The morphology of carbon nanofoam samples comprising platinum nanoparticles dispersed in the matrix was characterized by small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) techniques. Results show that the structure of pores of carbon matrix exhibits a mass (pore) fractal nature and the average radius of the platinum particles is about 2.5 nm. The fractal dimension as well as the size distribution parameters of platinum particles varies markedly with the platinum content and annealing temperature. Transmission electron micrographs of the samples corroborate the SANS and SAXS results.      

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURN^TM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH₄/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.     

X-ray inelastic scattering from small particles of graphite

The spectrum of inelastically scattered X-ray radiation from small particles of graphite shows energy losses which are due to volume and surface plasmons, as in the foil case. Surface plasmons from spherical particles (SPS) have not been observed. It is shown that graphite is inappropriate in the study of SPS using X-ray scattering experiments.     

Particle-size measurements in flames using light scattering and dissymmetry ratios

Particle-size measurements have been performed above a flat-flame burner using C₂H₆/O₂ and C₂H₄/O₂ mixtures and the assumption that a self-preserving distribution (s.p.d.) obtains for spherical particles. The absence of significant deviations from sphericity for the particles has been verified. Dissymmetry ratios were measured for parallel polarized light from an argon-ion laser at 4880 Å. The measured results are compared with previously obtained data for CH₄/O₂ mixtures and show differences in the rates and amounts of particulate growths, with the growth curves for all three gases leveling off at about 40–50 mm above the burner rim. The rates of particulate growths downstream of the flame are ordered as follows in the gas mixtures: C₂H₄/O₂>C₂H₆/O₂>CH₄/O₂.     

Small angle X-ray scattering by spherical particles of Polystyrene and Polyvinyltoluene

The high intensity, high resolution multiple reflection diffractometer has been applied to make accurate measurements of the small angle X-ray scattering of eight different Dow Chemical latexes with stated diameters 2.051, 1.305, 0.814, 0.557, 0.365, 0.264, 0.126, and 0.088 μm. Using thin dry samples, the measured intensities of diffraction extrema with orders higher than the third were found to agree well with the simple Rayleigh-Gans theory which neglects multiple scattering as well as interparticle interference. Orders below the fourth showed a distinct effect of interparticle interference. This effect could be demonstrated to vanish when a liquid solution sample was used instead of a dry one. Scattering curves of thick dry samples had strongly decreased contrast between maxima and minima, an effect which is known to be due to multiple scattering. The following particle diameters were determined from the measured scattering curves: 2.102±0.013 μm, 1.313±0.008 μm, 0.823±0.004 μm, 0.552±0.002 μm, 0.352±0.002 μm, 0.254±0.002 μm, 0.119±0.001 μm and 0.078±0.001 μm. The discrepancies between these and the stated sizes are larger for the smaller particles. It is believed that the X-ray measurements are more accurate than the stated sizes which are based on measurements in the electron microscope. Therefore, as secondary standards, the diameter obtained from the X-ray measurements should be used.     

1D high-speed Rayleigh measurements in turbulent flames

Time-correlated sampling of quantities in transient combustion processes requires high-speed imaging at repetition rates in the order of typical flame-inherent frequencies. The present study demonstrates the feasibility of temperature measurements in turbulent flames along a line at 10 kHz using Rayleigh scattering. High signal intensities were gained using an 80 W Nd:YAG laser for excitation in combination with an optimized combination of an achromatic lens, an objective lens and a CMOS camera. This allowed achieving signal-to-noise ratios up to 140 at 10 kHz repetition rates. The experimental setup and data processing aspects are described as well as details on the system characteristics are given. Temperature measurements of the DLR-A jet flame with a Reynolds number of 15.200 were compared with high-quality conventional 10 Hz simultaneous Raman/Rayleigh data. The data showed excellent agreement highlighting the reliability of the here demonstrated technique.     

LES study of turbulent ethanol spray flames using CSE coupled with non-adiabatic chemistry tables

Conditional Source-term Estimation (CSE) is applied to three turbulent ethanol spray flames (EtF3, EtF6, and EtF8) in Large Eddy Simulation (LES). The objectives of this paper are to include the heat losses due to spray evaporation and gas radiation in the chemistry tabulation, assess the impact of these changes on the temperature and droplet statistics, and evaluate the performance of LES-CSE for the selected flames. The profiles of gas temperature, spray velocity, velocity root mean square (rms) and droplet size distribution are well reproduced in the simulations compared to available experimental data. Temperature underpredictions near the centreline are observed, in particular, at locations closer to the jet exit for flames with lower jet velocity. A wider flame is predicted in EtF8 compared to the experiment and regions of local extinction are visible. The use of non-adiabatic chemistry library results in a noticeable improvement in the temperature predictions near the peak locations, especially for flames with higher velocity and closer to the jet exit. The heat losses due to evaporation are larger than those from radiation, confirming the importance of including the evaporation effects in the chemistry tables. The droplet velocity is well predicted, except for EtF8 where an underprediction is observed far downstream. The velocity rms is slightly underpredicted at some locations, probably due to the simple stochastic model used. Overall, LES-CSE with non-adiabatic chemistry tables successfully captures the gas-spray quantities in the selected flames.     

Particle-size measurements in flames using light scattering; comparison with diffusion-broadening spectroscopy

Particle-size measurements have been performed above a flat-flame burner using CH₄/O₂ mixtures. Size estimates were made on the assumption that particle sizes are defined by the self-preserving distribution (s.p.d.). Desymmetry ratios were measured for parallel and perpendicularly polarized argonion-laser radiation at 4880 A. These ratios for the assumed s.p.d. yield mean radii when the Mie theory is applied to spherical particles for a known value of the complex index of refraction (m = 1.57-0.44 i).The measured results are found to be in good agreement with our earlier studies using diffusion-broadening spectroscopy.     

用小角X射线散射研究纳米粒子的粒度分布

采用小角X射线散射中的对数高斯分布函数的方法，确定了纳米粒子的粒度分布。通过理论分析、计算，对氧化锌纳粒子进行实验测试。结果表明，氧化锌纳米粒子的平均半径尺寸为3．34nm，分布的标准偏差为1．35。     

Experimental measurements of geometric properties of turbulent stratified flames

Combustion under stratified conditions is common in many systems. However, relatively little is known about the structure and dynamics of turbulent stratified flames. Two-dimensional imaging diagnostics are applied to premixed and stratified V-flames at a mean equivalence ratio of 0.77, and low turbulent intensity, within the corrugated flame range. The present results show that stratification affects the mean turbulent flame speed, structure and geometric properties. Stratification increases the flame surface density above the premixed flame levels in all cases, with a maximum reached at intermediate levels of stratification. The flame surface density (FSD) of stratified flames is higher than that of premixed flames at the same mean equivalence ratio. Under the present conditions, the FSD peaks at a stratification ratio around 3.0. The FSD curves for stratified flames are further skewed towards the product side. The distribution of flame curvature in stratified flames is broader and more symmetric relative to premixed flames, indicating an additional mechanism of curvature generation, which is not necessarily due to cusping. These experiments indicate that flame stratification affects the intrinsic behaviour of turbulent flames and suggest that models may need to be revised in the light of the current evidence.     

In situ soft X-ray absorption spectroscopy of flames

The feasibility of in situ soft X-ray absorption spectroscopy for imaging carbonaceous species in hydrocarbon flames is demonstrated using synchrotron radiation. Soft X-rays are absorbed by core level electrons in all carbon atoms regardless of their molecular structure. Core electron spectroscopy affords distinct advantages over valence spectroscopy, which forms the basis of traditional laser diagnostic techniques for combustion. In core level spectroscopy, the transition linewidths are predominantly determined by the instrument response function and the decay time of the core–hole, which is on the order of a femtosecond. As a result, soft X-ray absorption measurements can be performed in flames with negligible Doppler and collisional broadening. Core level spectroscopy has the further advantage of measuring all carbonaceous species regardless of molecular structure in the far-edge region, whereas near-edge features are molecule specific. Interferences from non-carbon flame species are unstructured and can be subtracted. In the present study, absorption measurements in the carbon K-edge region are demonstrated in low-pressure (P _total = 20–30 Torr) methane jet flames. Two-dimensional imaging of the major carbonaceous species, CH₄, CO₂, and CO, is accomplished by tuning the synchrotron radiation to the respective carbon K-edge, near-edge X-ray absorption fine structure (NEXAFS) transitions and scanning the burner.     

Hydroxyl space-time correlation measurements in partially premixed turbulent opposed-jet flames

A high repetition-rate, two-point, time-resolved, laser-induced fluorescence technique is used to perform simultaneous two-point OH time-series measurements in a series of turbulent opposed-jet partially premixed flames with varying fuel-side equivalence ratio and bulk Reynolds number. Time scales of OH in these flames have previously been reported; however, the extension to two-point detection permits measurements of new spatial and temporal statistics previously unavailable in such flames. In particular, the simultaneous OH time series are used here to compute spatial and temporal autocorrelation functions. Filtered OH length scales (l_r,OH), corresponding to radial OH fluctuations in turbulent stagnation flames, are obtained from the spatial autocorrelation function, including their variation across the stagnation plane. In general, maximum OH fluctuations occur at the stagnation plane, thus minimizing the OH integral length scale at the axial location of peak OH. For all flames of this study, trends in OH length scale follow those of axial time scale (τ_I,OH). For flames with constant Re, l_r,OH decreases with less partial premixing. However, this change in integral length scale appears to be more significant for flames at lower Re in comparison to those at higher Re. Similar to OH integral time scales, for flames with the same fuel composition, l_r,OH decreases with increasing Re. Moreover, fuel-lean mixtures appear to be more sensitive to Re variations as compared to fuel-rich mixtures. PACS 47.70.Pq; 32.50.+d; 47.27.wg     

Nitric oxide concentration measurements in atmospheric pressure flames using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

We report the application of electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) for measurements of nitric oxide concentration ([NO]) in three different atmospheric pressure flames. Visible pump (532 nm) and Stokes (591 nm) beams are used to probe the Q-branch of the Raman transition. A significant resonance enhancement is obtained by tuning an ultraviolet probe beam (236 nm) into resonance with specific rotational transitions in the (v’=0, v”=1) vibrational band of the A²Σ⁺–X²Π electronic system of NO. ERE-CARS spectra are recorded at various heights within a hydrogen-air flame producing relatively low concentrations of NO over a Hencken burner. Good agreement is obtained between NO ERE-CARS measurements and the results of flame computations using UNICORN, a two-dimensional flame code. Excellent agreement between measured and calculated NO spectra is also obtained when using a modified version of the Sandia CARSFT code for heavily sooting acetylene-air flames (φ=0.8 to φ=1.6) on the same Hencken burner. Finally, NO concentration profiles are measured using ERE-CARS in a laminar, counter-flow, non-premixed hydrogen-air flame. Spectral scans are recorded by probing the Q₁ (9.5), Q₁ (13.5) and Q₁ (17.5) Raman transitions. The measured shape of the [NO] profile is in good agreement with that predicted using the OPPDIF code, even without correcting for collisional effects. These comparisons between [NO] measurements and predictions establish the utility of ERE-CARS for detection of NO in flames with large temperature and concentration gradients as well as in sooting environments. PACS 07.88.+y; 42.62.Fi; 42.65.Dr     

The performance of in situ adaptive tabulation in computations of turbulent flames

This paper presents a detailed characterization of the local and global errors associated with the in situ adaptive tabulation (ISAT) algorithm, which is used in conjunction with a transported PDF method. Calculations of a non-premixed turbulent methane/air piloted jet flame (Sandia flame D) using a skeletal chemical mechanism were performed using ISAT coupled with the computational fluid dynamics (CFD) code FLUENT. The three strategies implemented in ISAT for the growing of the ellipsoids of accuracy (EOAs) are discussed, and the cumulative distribution function (CDF) of the local error is presented for each of the three growing strategies. Computations are also performed to characterize the global error in the ISAT/PDF calculation. The computations used to characterize the global error were performed in parallel to achieve substantial savings in computational time.

In general the local error is well controlled, but there is a small probability of relatively large errors. Results from the investigation suggest that large retrieve errors are due to the region of accuracy (ROA) being non-convex, where the ROA is the connected region for which the error does not exceed the error tolerance, ?_tol. The global error in ISAT is found to be small compared to statistical error for ?_tol ≤ 10^?4, and is found to vary linearly with ?_tol.     

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.     

Kirchhoff's and Planck's radiation laws for small particles in scattering atmospheres or flames

Size and shape effects that modify Planck's radiation law for small enclosures and the emission cross section for small particles do not affect the validity of Kirchhoff's radiation laws when applied to macroscopic collections of small particles such as in scattering atmospheres or flames.