Laser Rayleigh Scattering Research Articles

Experimental determination of the most probable mixing states for auto-ignition of transient fuel injection into hot, vitiated coflows

The current study focuses on characterizing the auto-ignition process from turbulent fuel injection into a high-temperature, vitiated environment using the jet-in-hot-coflow (JHC) burner configuration. High-speed (10-kHz acquisition rate) optical and laser-based diagnostics are used to identify ignition kernel formation and to determine the most probable mixture fraction and temperature conditions that directly facilitate ignition. Four operating cases are studied that present variations in coflow temperature, jet velocity, and fuel type. High-speed OH* chemiluminescence (CL) imaging is used to obtain data on the spatial position and delay time (following fuel injection) of the formation of the first auto-ignition kernels. Over the limited conditions tested, the ignition delay times and heights show a strong sensitivity to temperature and fuel type, but only a mild sensitivity to jet velocity. High-resolution, kHz-rate laser Rayleigh scattering (LRS) is used to provide simultaneous mixture fraction and temperature measurements prior to and at the onset of ignition. High signal-to-noise ratios (SNR >200) enable reliable measurements of mixture fraction at ultra-lean conditions that seemingly promote auto-ignition. A statistical characterization of the most probable mixture fraction values leading to ignition (ξig) is presented and compared with calculated values of the most reactive mixture fraction, ξMR, which has been identified previously from theory and simulations as the parameter governing auto-ignition. Results show that very lean conditions near ξMR are preferentially encountered, but the probability density function (PDF) of ξig is described by a near-exponential distribution, spanning values across the flammability limits. Some limitations to the current methodologies for determining ξig are discussed, which are due primarily to the fact that the discretely sampled mixture fraction data is inherently asynchronous with the ignition event itself.

Application of Planar Laser Rayleigh Scattering for Measurement of Gas Temperature Distributions in Effusion Jet Cooled Panels Exposed to High Temperatures

Abstract This experimental study examines the use of planar laser Rayleigh scattering to measure instantaneous gas temperature distributions at different heights above the surface of an effusion cooled plate. An experimental test rig was used to model combustor conditions with a bulk crossflow temperature of 1500 K. Carbon dioxide was used as coolant at multiple blowing ratios ranging from 1.12 to 11.1. A “temperature-pegging” methodology was used to process Rayleigh light scattering images to create high resolution and accurate temperature images at heights of 2, 2.75, and 3.5 mm above the surface of a prototypical effusion plate. Measured temperature distributions were used to calculate root-mean-square (RMS) distributions, and were also converted to film effectiveness maps based on the upstream crossflow gas and effusion coolant temperatures. It is found that the film cooling region spreads upstream with increasing effusion jet blowing ratio parameter. The RMS deviation of gas temperatures over each measurement plane shows that the RMS fluctuations are low inside and outside the effusion film, but are high near the film edge. At a given height above the effusion panel, the RMS fluctuations decrease in the film region with increasing blowing ratio. Film effectiveness follows similar trends with high film effectiveness region expanding with increasing effusion jet blowing ratios.

Quantitative 2D thermometry in turbulent sooting non-premixed flames using filtered Rayleigh scattering.

This paper demonstrates the application of a thermometry method in turbulent sooting non-premixed flames using filtered Rayleigh scattering (FRS). Fuel tailoring is used to establish a specific C2H2-based fuel mixture such that temperature can be determined accurately by a single FRS measurement over the entirety of mixture fraction space, or equivalently, for all relevant thermo-chemical states. Evaluation is performed in a hierarchy of flows to establish measurement precision and accuracy. Initial assessments in a series of heated fuel mixtures; non-sooting, near-adiabatic flat flames; and laminar non-premixed sooting flames show accuracy of the approach over a full range of expected temperatures and high single-shot measurement precision (e.g., 65<SNR<80) for temperatures between 1900 and 2200 K. Subsequently, 2D temperature measurements are evaluated in a turbulent non-premixed sooting flame. For the current fuel mixture, the local mixture-averaged Rayleigh scattering cross section is nearly constant over all composition space, and thus traditional laser Rayleigh scattering (LRS) is used as a measurement standard in the absence of soot. The results show excellent agreement between the proposed FRS thermometry approach and LRS in non-sooting regions. In the presence of soot, the proposed FRS-based approach shows no signs of interference. In addition, 2D temperature imaging shows high SNR (60-74) over all temperature conditions. Thus, we believe the proposed methodology successfully provides a high-resolution 2D temperature method under turbulent sooting flame conditions.

Combined microwave and laser Rayleigh scattering diagnostics for pin-to-pin nanosecond discharges

In this work, the temporal decay of electrons produced by an atmospheric pin-to-pin nanosecond discharge operating in the spark regime was measured via a combination of microwave Rayleigh scattering (MRS) and laser Rayleigh scattering (LRS). Due to the initial energy deposition of the nanosecond pulse, a variation in the local gas density occurs on the timescale of electron decay. Thus, the assumption of a constant collisional frequency is no longer applicable when electron number data are extracted from MRS measurements. To recalibrate MRS measurements throughout the electron decay period, temporally resolved LRS measurements of the local gas density were performed over the event duration. The local gas density was calculated to be 30% of the ambient level during the later stages of electron decay, and it recovers at about 1 ms after discharge. A shock front traveling approximately 500 m/s was additionally observed. Coupled with plasma volume calibration via temporally resolved intensified charge-coupled device imaging, the corrected decay curves of the electron number and electron number density are presented with a measured peak electron number density of 4.5 × 1015 cm−3 and a decay rate of ∼(0.1–0.35) × 107 s−1. A hybrid MRS and LRS diagnostic technique can be applied for a broad spectrum of atmospheric-pressure microplasmas where a variation in the gas number density is expected due to energy deposition in the discharge.

Optimization of Filtered Rayleigh Scattering for the Measurement of Pressure and Temperature

ABSTRACT The stand-off measurements of temperature and pressure are important for a variety of physical and engineering processes. Filtered Rayleigh scattering (FRS) configurations that enhance and suppress sensitivity to temperature are modeled and demonstrated using premixed, atmospheric methane-air Hencken burner generating temperatures up to 2100 K. For a characteristic flame gas mixture, modeling of the FRS signal using a molecular iodine filter shows that conditions exist for which the signal has a greater dynamic range or does not change significantly with respect to high gas temperatures. Laser Rayleigh scattering (LRS) measurements coupled with nonadiabatic flame simulations validate the temperatures, while FRS measurements are compared with the model predictions for different observation angles and iodine cell conditions. Results are discussed in the context of making direct measurements of gas pressure at high temperatures and within turbulent flames.

Quantitative planar temperature imaging in turbulent non-premixed flames using filtered Rayleigh scattering.

The paper presents results demonstrating the application of filtered Rayleigh scattering (FRS) for quantitative temperature measurements within turbulent non-premixed jet flames. Through targeted fuel tailoring, i.e., the selection of a specific fuel mixture, temperature measurements are made under non-premixed fueling conditions with a single FRS measurement. For this to be feasible, the instantaneous measured FRS signal is uniquely proportional to the local temperature for all thermo-chemical states of the targeted fuel-oxidizer system. Simulated results using laminar, counterflow flame calculations show that for select CH4/H2/Ar fuel mixtures issuing into air, a unique relationship between the local FRS signal and temperature is achieved for all mixture fraction values over a full range of operating conditions from near-equilibrium to near-extinction flames. Furthermore, for the selected FRS-optimized fuel, the local mixture-averaged Rayleigh scattering cross section is nearly constant from fuel to oxidizer to products. Thus, traditional laser Rayleigh scattering (LRS) can be used to determine the temperature as a "standard" to which to compare and assess the FRS-based temperature results. Simultaneous LRS-FRS temperature measurements from Re=10,000, 20,000, and 30,000 turbulent non-premixed jet flames are presented that show good agreement between the two techniques in terms of instantaneous temperature fields and statistical quantities at various spatial locations within the flame. These results provide confidence that the current approach of FRS thermometry allows for accurate temperature measurements within the selected set of turbulent non-premixed flames.

Investigation of the Combustion Front Structure during Homogeneous Charge Compression Ignition Combustion via Laser Rayleigh Scattering Thermometry

Investigation of the Combustion Front Structure during Homogeneous Charge Compression Ignition Combustion via Laser Rayleigh Scattering Thermometry

Shearlet-based detection of flame fronts

Identifying and characterizing flame fronts is the most common task in the computer-assisted analysis of data obtained from imaging techniques such as planar laser-induced fluorescence (PLIF), laser Rayleigh scattering (LRS), or particle imaging velocimetry (PIV). We present a novel edge and ridge (line) detection algorithm based on complex-valued wavelet-like analyzing functions -- so-called complex shearlets -- displaying several traits useful for the extraction of flame fronts. In addition to providing a unified approach to the detection of edges and ridges, our method inherently yields estimates of local tangent orientations and local curvatures. To examine the applicability for high-frequency recordings of combustion processes, the algorithm is applied to mock images distorted with varying degrees of noise and real-world PLIF images of both OH and CH radicals. Furthermore, we compare the performance of the newly proposed complex shearlet-based measure to well-established edge and ridge detection techniques such as the Canny edge detector, another shearlet-based edge detector, and the phase congruency measure.

Experimental investigation of nozzle aspect ratio effects on underexpanded hydrogen jet release characteristics

Most experimental investigations of underexpanded hydrogen jets have been limited to circular nozzles in an attempt to better understand the fundamental jet-exit flow physics and model this behaviour with pseudo source models. However, realistic compressed storage leak exit geometries are not always expected to be circular. In the present study, jet dispersion characteristics from rectangular slot nozzles with aspect ratios from 2 to 8 were investigated and compared with an equivalent circular nozzle. Schlieren imaging was used to observe the jet-exit shock structure while quantitative Planar Laser Rayleigh Scattering was used to measure downstream dispersion characteristics. These results provide physical insight and much needed model validation data for model development.

Investigation of differential diffusion in turbulent jet flows using planar laser Rayleigh scattering

A series of laser Rayleigh-scattering experiments has been performed to investigate the effects of differential molecular diffusion in turbulent nonreacting jet flows. A turbulent jet of a mixture of Freon and H{sub 2} exiting into coflowing air was studied at various Reynolds numbers. In laminar flow, Rayleigh scattering clearly showed H{sub 2} diffusing ahead of Freon. In turbulent flow, the instantaneous Rayleigh images showed differential diffusion at the many interfaces between jet fluid and entrained air. Yet, ensemble averages of instantaneous images showed no average diffusion of H{sub 2} ahead of Freon.

218 平面レーザーレーリー散乱法を用いたパルス管冷凍機内の温度場の可視化

For the pulse tube refrigerators, it is difficult to understand the refrigeration phenomena in experimental approach because the gas condition in pulse tube refrigerators is high pressure and low temperature which arc fluctuating. To measure the temperature under the above condition, we adopted Planar Laser Rayleigh Scattering Method (PLRS method) using excimer laser (wavelength 193nm). It is possible to measure the gas temperature field without contact with the gas. In order to evaluate the possibility using PLRS method, we took the following basic approach. We measured the Rayleigh scattering intensity in the metal tube with quartz glass windows for several static pressure (0.1, 0.2, 0.4, 0.7MPa). The linearity error between Rayleigh scattering intensity and pressure is 15.4% in a laser shot measurement, 1.3% in the average of 10 laser shots.

中真空領域の標準真空計及び実用真空計にレーザー光のレーリー散乱を用いる可能性に関する研究

中真空領域の標準真空計及び実用真空計にレーザー光のレーリー散乱を用いる可能性に関する研究

Microscopic Structure of Turbulent Diffusion Flames.

Microscopic structure of turbulent diffusion flames is studied by a time-resolved planar video picture system obtained by a laser Rayleigh scattering (LRS) method and a single-point LRS measurement. The microscopic temperature is measured by using a two-dimensional LRS signal and image processing. Coaxial turbulent diffusion flames at moderate Reynolds numbers, which exhibit typical diffusion flame structures, are formed on laboratory-scale burners. It is found that the flame can be divided into four typical regions characterized by the distribution of macroscale of temperature fluctuations. These four regions are visualized by the two-dimensional LRS pictures. The turbulent heat transfer mechanisms in these four regions are discussed in terms of the two-dimensional LRS and the power spectral density of temperature fluctuations, in terms of one-point LRS. A cluster of temperature inhomogeneity is observed by cluster analysis in Regions I and III. It is found that different structures of microscopic inhomogeneity in the mixing region of Taylor's dissipation length scale appear, corresponding to the characteristics of mixing and combustion mechanisms in each region.

Simultaneous multi-point temperature measurements by Laser Rayleigh Scattering in turbulent diffusion flames.

The instantaneous spatial temperature profile is measured by the Laser Rayleigh Scattering method using a detector which has 512 photodiode elements in one line. It was applied to two typical turbulent diffusion flames with different degrees of turbulence. The results obtained are as follows. (1) A comparatively large scale of temperature fluctuation dominates the temperature fluctuation intensity in turbulent diffusion flames. (2) The average and instantaneous maximum temperature tends to decrease when the Re number increases, and consequently the turbulence seems to increase.

Laser Rayleigh Scattering Studies on Potential Structural Units, Organometallic Substrates, Part I.

Article Laser Rayleigh Scattering Studies on Potential Structural Units, Organometallic Substrates, Part I. was published on January 1, 1986 in the journal Zeitschrift für Physikalische Chemie (volume 267O, issue 1).

Laser Rayleigh Scattering from Macromolecular Solutions

Light scattering has been an important technique in the study of dilute solutions of both synthetic and biological macromolecules for many years. Determination of the angular and concentration dependence of the scattered intensity gives a measure of weight-average molecular weight, solution virial coefficients and, for larger molecules, radius of gyration. Lasers, by virtue of their high intensity and directionality, have extended the range of concentration and angles which may be conveniently used in these “classical” experiments [1,2].

Including methods for the study of refractive index fields in combustion and aerodynamicsF. J. Weinberg. London: Butterworths, 1963. x+251 pp. (illus.). 55s

Most experimental investigations of underexpanded hydrogen jets have been limited to circular nozzles in an attempt to better understand the fundamental jet-exit flow physics and model this behaviour with pseudo source models. However, realistic compressed storage leak exit geometries are not always expected to be circular. In the present study, jet dispersion characteristics from rectangular slot nozzles with aspect ratios from 2 to 8 were investigated and compared with an equivalent circular nozzle. Schlieren imaging was used to observe the jet-exit shock structure while quantitative Planar Laser Rayleigh Scattering was used to measure downstream dispersion characteristics. These results provide physical insight and much needed model validation data for model development.