A finite element model for simulating temperature distributions in rotating food during microwave heating

With considering the continuous rotation of the turntable during microwave heating, a three-dimensional computer model based on FEM was successfully developed to predict time-dependent temperatures distributions of food sample. The slide interface between rotating and stationary part were specially assigned and treated, as the disconnections of the nodes occur during rotating which may cause the electromagnetic analysis result in error. On this basis, temperature was estimated by coupling electromagnetic and heat transfer analysis, whereas the node coordinate and dielectric properties of sample were updated with time steps, and the heat generations were renewed according to these parameters.     

Using mobile metallic temperature sensors in continuous microwave assisted sterilization (MATS) systems

The goal of this study was to evaluate suitability of using mobile metallic temperature sensors in continuous microwave assisted sterilization (MATS) systems. A computer simulation model using the finite difference time domain method was developed to study the influence of microwave field on the accuracy of mobile metallic temperature sensors, ELLAB, in a MATS system in which food packages with embedded sensors traveled on a conveyor belt. Simulation results indicated that the metallic temperature sensors did not change the overall heating patterns within food samples. But when a metallic temperature sensor was placed in parallel to the electric field component within microwave cavities the field intensity had intense singularity at the sensor tip and caused localized overheating. The electric field singularity adjacent to the tip of the metallic temperature sensor can be avoided by placing the sensor perpendicular to the electric field component. The simulated heating patterns and temperature profiles were verified with experimental results. It was evident from both simulated and experimental results that the metallic temperature sensor could be used to capture temperature profile in a MATS system when placed in a suitable orientation.     

A computational model for calculating temperature distributions in microwave food applications

By heating volumetrically, microwave processes have several advantages over conventional heating processes. The main advantage is the increase of process rates and thus improving the quality of microwave heated, dried, pasteurised or sterilised products. However, to ensure product safety and thus satisfy regulatory bodies, temperature distributions have to be as uniform as possible.A new simulation approach has been developed, based on a user-friendly interface coupling two commercial software packages, to model time-dependent temperature profiles of arbitrarily shaped microwave treated products in three dimensions. Simulations have shown uneven temperature distributions when products were exposed to uncontrolled microwave applications.Using this model, hot and cold spots in the products may be simulated to test appropriate microwave treatment control strategies. A further development of this approach, being one-of-a-kind to date, describes a feedback-control loop in the simulation which helps optimising microwave processes by ensuring minimal time/temperature treatments and uniform temperature distributions. Thus microwave power pulse programs can be developed and tested in a model before being implemented in a real microwave system. Validation of the model was performed using non-invasive magnetic resonance imaging (MRI). Verification results showed that simulated data agrees well with the measured data in discrete locations (heating curves) as well as the temperature data throughout the samples.

Industrial relevance

The presented simulation approach calculates 3D temperature distribution as a function of time and thus allows for the determination of hot and cold spots in the products. With this, appropriate microwave treatment control strategies can be tested.A further development of this approach, being one-of-a-kind to date, describes a feedback-control loop in the simulation which allows for optimising microwave processes by ensuring minimal time/temperature treatments and uniform temperature distributions. Thus microwave power pulse programs can be developed and tested in a model before being implemented in a real microwave system.With this approach the main advantage of microwave applications, the increase of process rates due to the volumetric heating can be utilised and at the same time the quality of the treated product can be optimised and product safety can be ensured by improving temperature uniformity. Furthermore, regulatory bodies can be satisfied.     

Simulation of heating uniformity in a heating block system modified for controlled atmosphere treatments

Reliable and repeatable insect thermal mortality data rely on the performance of a heating device. Computer simulation has been widely used to optimize structures, design parameters and process conditions. To improve the temperature uniformity of a heating block system (HBS), a computer model was developed using finite element software COMSOL. Good agreement was obtained between the simulated and experimental block surface temperatures at three positions of the HBS and three heating rates. The validated computer model was further used to predict the effects of heating rates, the position of test insects and the addition of gases on the block and air temperature distributions. Simulation results showed that increasing heating rate reduced heating uniformity. The position of test insects in the treatment chamber largely affected their heating rate, with a position closer to the surface of the heat block providing a better temperature match between test insects and the HBS. When gas was added, block temperatures within the treatment chamber, particularly near the gas inlet, were influenced by gas speeds, temperatures and the gas channel design. The heating uniformity in the treatment chamber of the HBS was improved by heating the gas before adding it to the HBS, by routing the gas channel through the heating block to preheat the gas, and by using a relatively slow gas speed. The simulation results demonstrated that the validated computer model could be a reliable tool to evaluate the heating performance of the HBS for studying insect thermal death kinetics and optimize treatment conditions for the HBS when modified to include controlled atmospheres.     

Development of a computer simulation model for processing food in a microwave assisted thermal sterilization (MATS) system

The microwave assisted thermal sterilization computer simulation model (MATS-CSM) was developed to improve the previous computer simulation model for the microwave assisted thermal sterilization (MATS) system. Development of the new MATS-CSM included determination of optimum heating time step, evaluation of electromagnetic field distribution and the resulting heating pattern in food, and experimental validation of heating patterns. It was determined that the minimum number of discretization that would not cause immediate divergence of the EM-heat transfer solution was 32 steps corresponding to 97 mm and 5.6 s of displacement and heating time for every step, respectively. Furthermore, this study successfully demonstrated the symmetrical electromagnetic field distribution between top and bottom microwave entry ports and a staggered electric field pattern from one cavity of the MATS to the next. In addition, MATS-CSM confirmed that incorporating heat diffusion in the simulation model reduces the difference in hot spot and cold spot temperature by 65%. It also confirmed that water circulation reduces the edge heating effect, as observed in experiments. The heating pattern generated by MATS-CSM was verified experimentally through a chemical marker method. Based on the percent areal cross section of the weighted average temperature, there were no noticeable differences between the heating zones generated by the MATS-CSM and by the chemical marker method. The percent areal cross section of the cold area 1, cold area 2, and hot area by MATS-CSM were 35%, 25%, and 40%, respectively, and the cold area 1, cold area 2, and hot area by chemical marker method were 35%, 30%, and 35%, respectively.     

Evaluation of convective heat transfer coefficient between fluids and particles in suspension as food model systems for natural convection using two methodologies

This study was conducted to evaluate the convective heat transfer coefficient between Newtonian and non-Newtonian fluids and food particles inside of a glass vessel and to assess the effect of controlled variables: convection phenomena, heating temperature, immersion fluid, and different food particles. The coefficient was evaluated from the sample temperature as a function of the heating time by two approaches: a lumped parameter method and an analytical methodology. High values corresponded to heating in Newtonian fluids (70–181, 57–248 W/m² K), and lower corresponded in non Newtonian fluids (31–169, 28–167 W/m² K). Similarly, major heat transfer coefficient corresponded to the heating of mushrooms (32–110, 28–132 W/m² K), followed by tomatoes (30–181, 35–183 W/m² K) and potatoes (31–148, 34–248 W/m² K), respectively. Fluid properties were more important than the thermal properties of the particles. This coefficient was higher at 85 °C in both types of fluids. Both methods generated satisfactory values, but the analytical approach showed more accuracy.