Validation of the Analytical Approximation for Induction Heating Ionic Solutions Using COMSOL Multiphysics®
New methods of generating high-frequency magnetic fields of tens or even hundreds of millitesla at frequencies exceeding 2 MHz are allowing important new applications for induction heating. These include heating low conductivity ionic solutions in situations where electric or electromagnetic fields (e.g. microwaves) cannot be used.
Induction heating can be described by a numerical approximation. However, experimental validation is difficult, and so the numerical approximation has remained untested. In particular, the coefficient describing the geometry of the sample being heated was questionable. Furthermore, limits for the conductivity of the heated sample and frequency response were unknown. Finally, is was unknown whether a term for the complex permittivity ought to be used with materials such as ionic solutions that are partially magnetically “transparent”.
The AC/DC Module from COMSOL Multiphysics® has been used to validate the numerical approximation and calculate accurate geometrical coefficients for spheres, and cylinders and sheets at various orientations, at a range of frequencies from 2 MHz to 40 MHz. A uniform magnetic field volume was generated by simulating a pair of Helmholtz coils and then using this for heating saline of various concentrations. Initial simulation work was unsuccessful because strong temperature gradients were set up within the sample. However, setting the thermal conductivity of the saline to a very high value overcame this. Experiments were then conducted at 2 MHz to measure the heating rates of well-stirred saline samples and compared to the COMSOL Multiphysics® simulation results, and good agreement was found if electrical conductivity alone, as opposed to the complex permittivity, was used.
The COMSOL Multiphysics® work was then extended to include a full sweep of parameters including frequency, geometry (shape, orientation and size), electrical conductivity and input power. It was found that the coefficients for the induction heating numerical approximation used up until now were incorrect. In addition, the length to diameter ratio of cylinders orientated perpendicular to the magnetic field gave an unexpected additional term. The study has greatly aided us in our induction heating applications, and helped drive engineering ideas. This paper gives details of the simulation used, the methodology and the results.
This was my first foray into finite element modelling. The ease of use of the COMSOL Multiphysics® Model Builder, together with its intuitive approach, meant that I was able to complete this work with no formal training. Furthermore, the post-processing within COMSOL Multiphysics® gave me new insights into the workings of induction heating.
