Optimal Halbach Magnet Array Design for Portable NMR Targeting Multiphase Flow Metering Applications

Abstract

In this paper, a Halbach array-based nuclear magnetic resonance device for multiphase flow measurement is suggested. The design approach used simultaneously a 3-D finite-element method (FEM)-based software combined with particle swarm optimization algorithm. The goal of the design is to generate a relatively intense and highly homogenous magnetic field inside the target sensing area using a compact and lightweight magnet array. Simulation results on a device consisting of 12 Halbach arrays, each consisting of 12 cuboid permanent magnet elements of size 20 mm $\times20$ mm $\times46.5$ mm size, indicate that a highly homogenous magnetic field distribution of 0.890 T maximal intensity and 606 ppm homogeneity could be achieved within a probe cross section of 40 mm diameter when the Halbach arrays are distant from each other by a distance of 4 mm. This is adequate for the desired application while it leads to a light and compact overall Halbach array of 21.6 kg weight and 600 mm length. Experimental validation which was done using newly constructed two Halbach arrays of cuboid and trapezoid magnet elements, respectively, indicates a good match with FEM simulations. Furthermore, sensitivity analyses were performed to identify significant design variables for further optimization.