Laser-induced incandescence for soot diagnostics at high pressures

The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signal's proportionality to soot volume faction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed.     

Forward-illumination light-extinction technique for soot measurement

A forward-illumination light-extinction (FILE) soot volume fraction measurement technique was developed and tested. By using a camera and a point light source in front of the flame and a diffuser behind the flame, with this technique one can achieve a two-dimensional soot concentration measurement with only one window when one is studying confined combustion. The line-of-sight quantitative soot volume fraction is obtained by calculation of the reflected light intensity with or without the presence of soot cloud. Verification of this technique was accomplished by measurement of an axisymmetric ethylene diffusion flame. The field distribution obtained by Abel inversion is presented and matched well with previous point measurements. The FILE technique has high time resolution when a high-speed camera and a copper vapor laser are adopted. All these advantages of FILE make it suitable for line-of-sight integrated, two-dimensional soot distribution of transient combustion, e.g., in the case of in-cylinder Diesel combustion.     

Two-dimensional imaging of soot volume fraction in laminar diffusion flames

A technique for acquiring two-dimensional soot-volume-fraction measurements in laminar flames has been demonstrated. The technique provides a map of very low noise concentration over a range of wavelengths (250-1100 nm). A noise level of 0.0007 in extinction and a spatial resolution of 30-40 mum for soot concentration were achieved with an arc lamp source that was filtered to provide greater spatial coherence and a CCD detector. The broadband arc lamp source also allowed us to avoid the added noise resulting from speckle with coherent laser sources. Beam steering, due to refractive-index gradients in the flame, was measured and compared with theoretical predictions. The optical arrangement to minimize the effect of beam steering is described. As a result the beam steering had no effect on the soot measurements in the flames examined. Flame-transmission maps obtained with this system in an ethylene/air laminar diffusion flame are presented. Tomographic analysis from use of an Abel inversion of the line-of-sight data to obtain radial profiles of soot concentration is described.     

Two-dimensional imaging of soot volume fraction by the use of laser-induced incandescence

A recently developed laser-induced incandescence technique is used to make novel planar measurements of soot volume fraction within turbulent diffusion flames and droplet flames. The two-dimensional imaging technique is developed and assessed by systematic experiments in a coannular laminar diffusion flame, in which the soot characteristics have been well established. With a single point calibration procedure, agreement to within 10% was found between the values of soot volume fraction measured by this technique and those determined by conventional laser scattering-extinction methods in the flame. As a demonstration of the wide range of applicability of the technique, soot volume fraction images are also obtained from both turbulent ethene diffusion flames and from a freely falling droplet flame that burns the mixture of 75% benzene and 25% methanol. For the turbulent diffusion flames, approximately an 80% reduction in soot volume fraction was found when the Reynolds number of the fuel jet increased from 4000 to 8000. In the droplet flame case, the distribution of soot field was found to be similar to that observed in coannular laminar diffusion flames.     

Passive directional discrimination in laser-Doppler anemometry by the two-wavelength quadrature homodyne technique

We report a method for passive optical directional discrimination in laser-Doppler anemometers. For this purpose frequency-shift elements such as acousto-optic modulators, which are bulky and difficult to align during assembly, have traditionally been employed. We propose to use a quadrature homodyne technique to achieve directional discrimination of the fluid flow without any frequency-shift elements. It is based on the employment of two laser wavelengths, which generate two interference fringe systems with a phase shift of a quarter of the common fringe spacing. Measurement signal pairs with a direction-dependent phase shift of +/- pi/2 are generated. As a robust signal-processing technique, the cross-correlation technique is used. The principles of quadrature homodyne laser-Doppler anemometry are investigated. A setup that provides a constant phase shift of pi/2 throughout the entire measurement volume was achieved with both single-mode and multimode radiation. The directional discrimination was successfully verified with wind tunnel measurements. The complete passive technique offers the potential of building miniaturized measurement heads that can be integrated, e.g., into wind tunnel models.     

Optical patternation: a technique for three-dimensional aerosol diagnostics

A novel technique based on optical patternation is described for three-dimensional diagnostic studies of aerosols used in analytical spectroscopies. The aerosol is illuminated with a thin laser light sheet to capture images of the fluorescence and Lorenz-Mie light-scattering signals from the aerosol field with a charge-coupled detector. These measurements allow for the rapid and nonintrusive elucidation of two-dimensional spray structures, planar mass distributions, and spatial droplet size distributions. The ratio of the fluorescence image to the Lorenz-Mie image is then utilized to construct a spatially resolved map of the volume-to-surface area mean of the aerosol (Sauter mean diameter). Three-dimensional maps of spray structure, mass distribution, and droplet size distribution are obtained for the entire aerosol field by image stacking. The technique is applied to the measurement of the droplet size over the aerosol field at distances of 5-30 mm from the nebulizer tip where droplet sizes ranged from 6 to 12 microm for a direct injection high efficiency nebulizer used in inductively coupled plasma spectrometries.     

A calibration-independent laser-induced incandescence technique for soot measurement by detecting absolute light intensity

Laser-induced incandescence (LII) has proved to be a useful diagnostic tool for spatially and temporally resolved measurement of particulate (soot) volume fraction and primary particle size in a wide range of applications, such as steady flames, flickering flames, and Diesel engine exhausts. We present a novel LII technique for the determination of soot volume fraction by measuring the absolute incandescence intensity, avoiding the need for ex situ calibration that typically uses a source of particles with known soot volume fraction. The technique developed in this study further extends the capabilities of existing LII for making practical quantitative measurements of soot. The spectral sensitivity of the detection system is determined by calibrating with an extended source of known radiance, and this sensitivity is then used to interpret the measured LII signals. Although it requires knowledge of the soot temperature, either from a numerical model of soot particle heating or experimentally determined by detecting LII signals at two different wavelengths, this technique offers a calibration-independent procedure for measuring soot volume fraction. Application of this technique to soot concentration measurements is demonstrated in a laminar diffusion flame.     

Two-dimensional measurement technique for birefringence vector distributions: measurement principle

The two-dimensional measurement principle for a birefringence vector distribution in transparent materials is analyzed. The system nonuniformity that results from the system components makes the two-dimensional measurement principle quite different from that of a point-measurement method, and the measurement principle requires a two-dimensional analysis. A pulsed optical phase modulation is employed to simplify the two-dimensional mathematical analysis. As a result, concepts are proposed of the system function that characterizes the system nonuniformity that results from the system components and of the intrinsic function that is related to the birefringence vector distribution in a birefringent sample. The influence of the system nonuniformity on the two-dimensional measurement is eliminated by measurement of the intrinsic function, whereas its two values allow for the mathematical separation of the birefringence vector components. The effectiveness of the two-dimensional analysis is illustrated by measurement of a birefringence vector distribution, which is induced by an internal stress distribution in a poly(methyl methacrylate) plate, owing to the photoelastic effect.     

Optical emission spectroscopic study for diagnostics in high gravity DC-plasma CVD diamond growth

The effect of gravity on materials processing is rather complicated, and a large number of considerably unexpected results due to high gravity or sometimes centrifugation have been reported. The authors have been conducting high gravity diamond thin film growth by the DC-plasma CVD method up to 100 G. The effects of high gravity appeared in the nucleation site density, growth rate, grain size, uniformity, film coverage and morphology. With the increase in gravity, the nucleation site density, growth rate, grain size were all found to be increased, the uniformity and film coverage were improved, and {100} facet became dominant rather than {111} facet. To understand the deposition mechanism in these effects observed in high gravity, a diagnostic apparatus for spectroscopic study of the optical emission from the DC-plasma was developed, and the OES (optical emission spectroscopy) apparatus has been employed in high gravity CVD experiments. The influence of gravity on the most active species in the plasma, such as H, CH, C₂ was discussed together with the results from the Raman spectroscopy and X-ray diffraction study.     

Calculation of optical thicknesses of magnesium emission spectral lines for diagnostics of laser-induced plasmas

In this work, plasma characterization by laser-induced breakdown spectroscopy (LIBS) has been investigated. We propose a method based on the calculation of the optical thicknesses of emission spectral lines in the framework of a homogeneous optically thick plasma in local thermodynamic equilibrium (LTE). In this approach, self-absorption is taken into account to retrieve the optically thin intensities and plasma characterization is achieved. The developed procedure is applied to magnesium (Mg) lines measured from plasmas generated in air at atmospheric pressure from calcium hydroxide samples using an infrared Nd:YAG laser. The influence of laser irradiance on both plasma shape and emission intensity was studied to select the most suitable experimental conditions. Spectral lines of Mg I-II were measured and analyzed for different laser energies, delay times, and concentrations of the analyte. In each case, the plasma temperature, the electron density, and the parameters Nl were determined, without employing curves-of-growth. The results obtained showed the practical usefulness of the method to provide valuable information in LIBS experiments.     

Two-color laser-induced incandescence (2C-LII) technique for absolute soot volume fraction measurements in flames

A two-color version of the laser-induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in flames. By using a calibrated tungsten ribbon lamp, soot peak temperatures were measured as a function of fluence at several locations in an ethylene diffusion flame by using a steeply edged laser beam profile. Above a certain fluence threshold, peak temperatures were tightly distributed just above 4000 K independent of the particle size and number density. Radial profiles of soot volume fraction were obtained and compared (not calibrated) with results from the laser extinction technique. Good agreement showed the validity of the 2C-LII technique at a controlled fluence.     

金属疲劳断裂的声发射检测技术

疲劳断裂是金属结构的主要失效形式,通过金属疲劳断裂时声发射特征参数的提取,建立了声发射特征参数和裂纹扩展速率之间的关系,由试样的三点弯曲疲劳试验,证明采用声发射技术监测疲劳裂纹的扩展,不仅与疲劳裂纹扩展的变化规律相似,而且能实时的捕捉到疲劳裂纹的产生。     

Pulsed digital holography for high-speed contouring that uses a two-wavelength method

A two-wavelength method for a fast shape measurement by use of a pulsed ruby laser is presented. The wavelength change is produced by alteration of the distance between the plates of the laser's output etalon. One plate of the etalon is mounted on a vibrating piezoelectric element; this allows a fast wavelength change. Two holograms at different wavelengths are recorded in a few microseconds by use of a CCD. The holograms are reconstructed digitally, and the wave-front phase is calculated. The shape is obtained by subtraction of the phases of the wave fronts recorded at different wavelengths. Environmental disturbances at low frequencies, such as air turbulence, vibrations, and object drift, have no influence on the measurement. Experimental results are presented.     

Two-dimensional measurement technique for birefringence vector distributions: data processing and experimental verification

A two-dimensional measurement method for a birefringence vector distribution differs from a point-measurement method not only in the two-dimensional system nonuniformity but also in the system reliability. The previously proposed intrinsic vector allowed for the elimination of the influence of the system nonuniformity, whereas the two-dimensional system reliability is ensured by both an online diagnosis technique and an image lock-in processing. It is revealed that the measured intrinsic vector is relevant not only with the birefringence vector distribution in a sample but also with the natural birefringence vector distributions that exist in the optic components. The complete solution from the measured intrinsic vector results in a bidirectional vector for the desired birefringence vector distribution. The correctness of the two-dimensional measurement principle is examined by means of a comparison of the measured data with that calculated from a finite-element analysis based on the photoelastic effect.     

Holographic interferometry using two-wavelength holography for the measurement of large deformations

Visible holographic interferometry is generally too sensitive for the measurement of large deformations. We present a holographic method that permits an increase in the range of measurable deformations. It requires the use of two different wavelengths, λ(1) and λ(2), and two holograms in series. We develop the theoretical basis of a method that permits the obtention of an interferogram as if a longer equivalent wavelength, λ(eq) = λ(1)λ(2)/|λ(1) -λ(2)|, were used. The method is experimentally tested by use of a setup that can be easily converted into a classical single-wavelength holographic interferometer, permitting comparison of the interferograms of the same deformation produced with both methods. Significant results are presented.     

Two-dimensional surface profile imaging technique based on a double-grating frequency shifter

A two-dimensional surface profile imaging technique that uses a low-coherence heterodyne interferometer is proposed. A double-grating frequency shifter was used in a tandem interferometer to provide the achromatic frequency shift for low-coherence light. A chopper, together with a processing circuit, was implemented to modulate the interference fringes. The surface profile was measured from the interference fringes taken by a CCD camera using a five-step method. The uncertainty in the displacement measurement is 0.34 microm for a displacement range of 43 microm. The surface profile of a glass sample with low effective reflectivity was acquired.     

Digital imaging technique for optical emission spectroscopy of a hydrogen arcjet plume

A digital imaging technique has been developed for optical emission spectroscopy measurements of a 1.6-kW hydrogen arcjet plume. Emissions from the Balmer α and β transitions of excited atomic hydrogen were measured with a computer-controlled red-green-blue color CCD detector with and without line-centered bandpass interference filters. A method for extending the effective dynamic range of the detector was developed, whereby images obtained with a wide range of exposure times are combined to form a single composite nonsaturated map of the plume emission structure. The line-of-sight measurements were deconvoluted to obtain the true radial intensity distribution with an inverse Abel transformation. Analysis of the inverted measurements indicates that the upper levels of the Balmer α and β transitions are not thermalized with the electrons in the plasma. The local thermodynamic equilibrium assumption fails for this plasma, and the electron temperature is not equivalent to the apparent excitation temperature obtained when a Boltzmann energy distribution is assumed for the atomic hydrogen excited states.