Optical particle sizing for in situ measurements Part 1

Abstract

A particle sizing counter suitable for in situ measurements in two-phase flows is presented in a two-part sequence. The technique employs near forwardscatter from the focus of a He-Ne laser beam, together with pulse-height analysis of the signals from individual particles. A novel and essential feature of the technique is a numerical inversion scheme to unfold the dependence of the scattered signals on particle trajectory through the measurement volume. This feature allows the capability of truly in situ measurements with a working space of 50 cm between optical elements. The inversion procedure is performed by an on-line computer or microprocessor unit and uses a prior calibration with monodisperse aerosols of known size. As presently configured, the instrument has a demonstrated capability of determining size distributions in the 1-30-microm diam range, at concentrations up to ~10(5) cm(-3) in flows of temperatures up to 1600 K. The measured dependence of response on particle diameter agrees well with calculations from the Mie scattering theory. It is anticipated that the technique can be extended to cover particle diameters in the 0.5-50-microm range with concentrations up to 10(6) cm(-3). Adaptation to measurements of absorbing and irregular particles can be achieved by a straightforward calibration technique. Part 1 describes the trade offs in the optical design and develops the numerical inversion scheme. Part 2 discusses experimental measurements at ambient conditions and combustion temperatures (1600 K). An assessment of the accuracy of the technique is also presented.