Modeling of temperature profiles under continuous tube-flow microwave and steam heating conditions

Mathematical models were developed based on perfectly mixed flow (PMF), pis ton flow with heat diffusion (PFHD) and laminar flow (LF) approaches to pre dict liquid temperature history under continuous tube-flow microwave and st eam heating conditions. Two helical glass coils placed inside domestic micr owave ovens (one coil in each of the two 700 W capacity ovens) or in a stea m cabinet were used for heating and a spiral condenser at the exit was used for cooling. Transient and steady state mean temperatures of the fluid wer e experimentally measured at the exit and were compared with predictions fr om the mathematical models for both systems. The residence time, velocity d istribution as well as temperature profiles, along the radius and the lengt h of the tubes, were computed using the models. The PFHD and the LF models better described temperature profiles during the initial transient period, while the PMF model showed a better agreement with experimental data during steady state conditions. The occurrence of secondary turbulence in the hel ical coil (associated with high Dean numbers) was believed to be responsibl e for reducing the radial temperature gradients and achieving close to "per fectly mixed piston flow" situation. A relatively larger temperature gradie nt across the radius was observed under microwave heating conditions than u nder steam heating conditions.' The time-temperature effects were integrate d to predict the lethality at selected temperatures and pow rates for both continuous-flow thermal processing.