Numerical and Experimental Analysis of Power Reflection and Thermal Homogeneity in a Continuous Flow Microwave Heater for Liquid Foods

Abstract

Abstract

A continuous flow microwave heater with a hexagonal cavity (2450 MHz) and a helical tube with 5.5 turns was experimentally and numerically analyzed for the heating of green coconut water (GCW) and a simulated fruit juice/pulp (viscous, pseudoplastic) to pasteurization temperatures. The phenomena of electromagnetism, heat transfer, and laminar flow were coupled in a multiphysics model that was numerically solved using the finite element method. Simulations showed that secondary flow and a larger power absorption in the first half of the tube improved thermal homogenization along it. The polygonal shape of the cavity provided a more disperse and uniform electric field distribution in comparison with circular and elliptical geometries. Experiments were performed at 1.0 L/min with two power levels (3.0 and 4.2 kW). Cavity showed excellent impedance matching with an experimental power reflection of 1% without external tuning. Absorption energy efficiency (magnetron to fluid) was 89 ± 3% for GCW and 96 ± 6% for the model food. Regarding homogeneity at the outlet, a temperature deviation of only 0.2 °C was predicted and measured for the GCW. For the model food, actual deviations were higher (2.2 and 4.2 °C) due to the viscosity, but acceptable.