The laser extinction method (LEM) is particularly suitable for measuring particle sizes in flames because this method, which is based on the Beer–Lambert law, is non-intrusive and easy to implement. In the LEM, the interpretation of the extinction data is usually developed under the assumption that light extinction due to scattering is a result of the superposition of single scattering by individual particles; however, this could be violated for flames with dense concentrations of particles in which multiple scattering could occur. Quantifying the effect of multiple scattering under general conditions is still a formidable problem. In this work, we carried out a series of careful measurements of the laser extinction using standard particles of various known sizes, number densities and optical path lengths, all under the condition that the acceptance angle of the detector was limited to nearly zero. Combined with a four-flux model, we quantitatively analyzed the effect of multiple scattering on the size measurement using the LEM. The results show that the effect of multiple scattering could be ignored when the optical thickness is less than two under strict restrictions on the detector acceptance angle. Guided by this, the size distribution of an alumina (Al2O3) particle sample was measured by the LEM with dual wavelengths. Parameterized distributions were solved with the help of graph plotting, and the results compared well with the measurement from the Malvern particle size analyzer. The same method was then used to measure the particle size distribution in the plume of a solid rocket motor (SRM). The use of an off-axis parabolic mirror in the experimental setup could suppress the jitter of light passing through the SRM plume, and the particle size in the plume of the measured SRM was in the order of microns.
We propose a laminar-jet-discharging method to measure the interfacial tension of deformable surfaces such as soap bubbles. This method avoids the need to measure the small pressure difference inside and outside a soap bubble in a static state. By allowing the air in a soap bubble to discharge in a laminar flow state through a long tube, we show that the surface tension of the soap bubble is proportional to the rate of change of the fourth power of the bubble radius. Experimentally, we verify that this linear relationship is valid over a long period and thus can be measured with cameras at slow recording rates. Our method offers a straightforward and accurate way to measure the surface tension of soap bubbles using easy-to-obtain devices. In addition, we propose a new development of the pendant drop method based on the silhouette of the drop, which does not require any pressure transducer or computation of the second-order derivatives of the drop profile and hence is easier to implement and less sensitive to the accuracy of the drop profile determination. For pure liquids, results from the new pendant drop method compare well with standard values. This method is thus used to verify the laminar-jet-discharging measurements.
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