Millimeter-Wave Interferometry for Opaque Particle-Laden Flows

A novel method to measure the concentration of particles in optically opaque particle-laden flows is presented. This method is based on the principle of millimeter wave interferometry, using a fully-integrated frequency modulated continuous-wave (FMCW) radar operating between 77 and 81 GHz to measure path-integrated particle concentrations between the radar and a reflector. The instrument is capable of quantitative, high-speed (20 kHz) path-integrated concentration measurements in dispersed multiphase flows with concentrations one to two orders of magnitude higher than those at reach with state-of-the-art optical methods. The interferometer was demonstrated and calibrated for path-integrated number concentrations up to <tex-math notation="LaTeX">(4.36 ± 0.24) 10⁸~m⁻²</tex-math> using glass microspheres with a mean diameter of <tex-math notation="LaTeX">109.2~μ m</tex-math> . Two independent measurements of particle size distribution (PSD) were performed using X-ray microtomography and dry sieving. The calibration setup relied on high-resolution particle shadowgraphy applied to individual thin particle streams and used multistreams superposition to reproduce large optical depths in a controlled particle-air mixture. The instrument exhibited excellent linearity and low error during the calibration, with a phase shift-to-number concentration slope of <tex-math notation="LaTeX">(1.378 ± 0.043) 10⁻⁷~m²</tex-math> , validating the measurement concept and paving the way for practical applications. The leading uncertainties are discussed, providing guidelines for exploiting the measurement concept without necessarily performing a direct calibration.