A new contactless technique is presented for the detection of micron-sized insulating particles in the flow of an electrically conducting fluid. A transverse magnetic field brakes this flow and tends to become entrained in the flow direction by a Lorentz force, whose reaction force on the magnetic-field-generating system can be measured. The presence of insulating particles suspended in the fluid produce changes in this Lorentz force, generating pulses in it; these pulses enable the particles to be counted and sized. A two-dimensional numerical model that employs a moving mesh method demonstrates the measurement principle when such a particle is present. Two prototypes and a three-dimensional numerical model are used to demonstrate the feasibility of a Lorentz force particle analyzer (LFPA). The findings of this study conclude that such an LFPA, which offers contactless and on-line quantitative measurements, can be applied to an extensive range of applications. These applications include measurements of the cleanliness of high-temperature and aggressive molten metal, such as aluminum and steel alloys, and the clean manufacturing of semiconductors.

A Lorentz force particle analyzer (LFPA) is a device for the contactless measurement of micron-sized particles in electrically conducting fluids. The LFPA is based on the measurement of either force or torque changes acting upon a magnetic-generating system when the particle passes near a magnetic field. In this paper, we first formulate the theory of the LFPA using a magnetic dipole with a magnetic field penetrating a particle-laden thin fluid flow layer. The disturbed electromagnetic quantities in the presence of a particle are analytically solved using an approach analogous to that for potential flow, providing a relationship between the measured physical quantities, i.e., force, torque and induced magnetic field, and the particle size. A detailed investigation of physical properties and operating parameters provides a rational framework for predicting the sensitivity of LFPAs in laboratory experiments and in industrial practice.

A Lorentz force particle analyzer (LFPA) is a device for the contactless measurement of micron-sized particles in an electrical conductor. Calibration is vital for the successful application of an LFPA. In this article, we first show that the presence of particles that possess the same size but are located at different positions will lead to variations of their measured quantity; therefore, the LFPA requires calibration. Then, two numerical models are developed to solve this problem. For the case of a wire, a Halbach-array magnetic system with 24 segments replaces the cubic permanent magnet (PM) to generate a homogeneous magnetic field in the working area. Three identical sensors are arranged around the wire at azimuthal intervals of 2 π /3 and at certain distances (double the size of the magnetic system) in the axial direction, and the resultant values of the calibration deviation coefficient ( η ) smaller than 6% except for the case of edge. For the case of a thin sheet, we create an arc-like boundary at the front of the cubic PM to reduce the gradient of the magnetic flux density along the through-thickness direction. Two sensors are placed at the two sides of the thin sheet, and η ⩽ 10% is obtained. These results imply that the numerical models presented here are simple, robust and reliable methods for the numerical calibration of an LFPA and may become a low-cost alternative to the expensive full-scale calibration achieved by the experiment.