Application of a pulsed laser for soot measurements in premixed flames

A pulsed Nd: YAG laser has been used to perform scattering/extinction measurements in flat, premixed methane/oxygen flames to determine soot particle sizes, number concentrations and soot volume fractions. A discussion is given on accuracies in these determinations with respect to experimental and theoretical parameters. Absorptions were measured through a referencing method which yielded a single-pulse relative standard deviation in the normalized signal of 0.7%. The incident laser fluence was kept below 0.1 J/cm² in the focusing point in the flame to avoid soot vaporization effects. Interference in the measurements from polyaromatic hydrocarbons (PAH) is also discussed.     

Dual-pump dual-broadband coherent anti-Stokes Raman scattering in reacting flows

A dual-pump, dual-broadband coherent anti-Stokes Raman scattering system for simultaneous measurements of temperature and concentrations of N2, O2, and CO2 in reacting flows is demonstrated. In this system pure rotational transitions of N2-O2 and rovibrational transitions of N2-CO2 are probed simultaneously with two narrowband pump beams, a broadband pump beam, and a broadband Stokes beam. The main advantage of this technique is that it permits accurate temperature measurements at both low and high temperatures as well as concentration measurements of three molecules.     

Computer screen as a programmable light source for visible absorption characterization of (bio)chemical assays

Visible absorption features suitable for color recognition and micro-plate reading of a standard bioassay are performed by the combination of a computer screen used as a programmable light source and a web camera as detector. The method provides in this way a highly available platform for 'home tests' or 'self-tests', where the requirement is to monitor well defined assays and the use of economical instrumentation is advantageous.     

On the sensitivity of rotational CARS N2 thermometry to the Herman–Wallis factor

Purely rotational spectral signals of coherent anti‐Stokes Raman scattering (CARS) from nitrogen molecules are studied as a function of the vibration–rotation interaction that weakens the rigid rotor approximation under which the dominant terms of the Raman cross section are calculated. The effect of the vibration–rotation interaction is quantified by means of the Herman–Wallis (HW) factor, and different approaches to its determination are evaluated in terms of their relative contribution to the CARS intensity and thermometric measurements made in a fuel‐rich hydrocarbon flame. Known HW factors are contrasted with more complete expressions of recent derivation, and it is found that relative line strength adjustments amount to about a few percent. Such differences result in temperature corrections of less than 1%. This value should be considered for the definition of the ideal thermometric accuracy of the technique but it is of minor importance in comparison with other sources of uncertainty (e.g. Raman line widths) that emerge from the complexity typical of reactive gas mixtures. Copyright © 2011 John Wiley & Sons, Ltd.     

Analog low-loss full-range phase modulation by utilizing a V-shaped switched ferroelectric liquid-crystal cell in reflective mode

We have studied the analog (V-shaped switching) mode in ferroelectric liquid crystals in reflective mode for analog phase modulation applications. We have found that several combinations of cell thicknesses and input polarization states exist for which near-lossless analog phase modulation with a range of approximately 2pi rad is obtained, and we demonstrate one such combination experimentally. Despite a slight deviation from the ideal conditions, e.g., the tilt angle was 38 degrees instead of the desired 45 degrees , virtually pure 1.6pi rad phase modulation was obtained; the measured values agree very well with our numerical simulations of the real device.     

Effect of atomization on surface oxide composition in 316L stainless steel powders for additive manufacturing

The initial oxide state of powder is essential to the robust additive manufacturing of metal components using powder bed fusion processes. However, the variation of the powder surface oxide composition as a function of the atomizing medium is not clear. This work summarizes a detailed surface characterization of three 316L powders, produced using water atomization (WA), vacuum melting inert gas atomization (VIGA), and nitrogen atomization (GA). X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy analyses were combined to characterize the surface state of the powders. The results showed that the surface oxides consisted of a thin (~4 nm) iron oxide (Fe₂O₃) layer with particulate oxide phases rich in Cr, Mn, and Si, with a varying composition. XPS analysis combined with depth-profiling showed that the VIGA powder had the lowest surface coverage of particulate compounds, followed by the GA powder, whereas the WA powder had the largest fraction of particulate surface oxides. The composition of the oxides was evaluated based on the XPS analysis of the oxide standards. Effects of Ar sputtering on the peak positions of the oxide standards were evaluated with the aim of providing an accurate analysis of the oxide characteristics at different etch depths.