Gennaro Cuccurullo

Effect on orange juice of batch pasteurization in an improved pilot-scale microwave oven.

The effects on orange juice batch pasteurization in an improved pilot-scale microwave (MW) oven was evaluated by monitoring pectin methyl-esterase (PME) activity, color, carotenoid compounds and vitamin C content. Trials were performed on stirred orange juice heated at different temperatures (60, 70, 75, and 85 degrees C) during batch process. MW pilot plant allowed real-time temperature control of samples using proportional integrative derivative (PID) techniques based on the infrared thermography temperature read-out. The inactivation of heat sensitive fraction of PME, that verifies orange juice pasteurization, showed a z-value of 22.1 degrees C. Carotenoid content, responsible for sensorial and nutritional quality in fresh juices, decreased by about 13% after MW pasteurization at 70 degrees C for 1 min. Total of 7 carotenoid compounds were quantified during MW heating: zeaxanthin and beta-carotene content decreased by about 26%, while no differences (P < 0.05) were found for beta-cryptoxanthin in the same trial. A slight decrease in vitamin C content was monitored after MW heating. Results showed that MW heating with a fine temperature control could result in promising stabilization treatments.

IR temperature measurements in microwave heating

In this paper a technique for the evaluation of the dielectric constant of a sample placed inside a microwave oven and confined in a cylindrical box is proposed. The box acts as a waveguide so that a simple model for the propagating wave can be assumed. Since traditional techniques for temperature measurements cannot be applied in microwave heating, the IR thermography shows to be an useful tool for measuring the sample surface temperature. The measure of the surface temperature evolution in the sample along with application of a simple analytical model allows to obtain the dielectric constant of the sample as a function of chemical composition, temperature and frequency. Preliminary results are presented and discussed with reference to pure water.

An Analytical Approximation for Continuous Flow Microwave Heating of Liquids

Both a numerical and an analytical models were developed to simulate temperature profiles in continuous laminar pipe flow during microwave heating. Fully developed velocity and thermally developing conditions were assumed. The numerical solution was obtained by first solving Maxwell equations and then by coupling them with the energy balance for the flowing fluid. On the other hand, the same problem was solved analytically under the simplifying assumption foreseeing uniform heat generation inside the pipe. With the aim of reducing computational efforts, numerical and analytical results were compared in order to investigate conditions for which the two models allowed to recover the same temperature patterns. Thus, it has been shown that suitable conditions can be found for which the simplified analytical model can lead to an easy way to predict the heat transfer through the pipe.

Temperature field for radiative tomato peeling

Nowadays peeling of tomatoes is performed by using steam or lye, which are expensive and polluting techniques, thus sustainable alternatives are searched for dry peeling and, among that, radiative heating seems to be a fairly promising method. This paper aims to speed up the prediction of surface temperatures useful for realizing dry-peeling, thus a 1D-analytical model for the unsteady temperature field in a rotating tomato exposed to a radiative heating source is presented. Since only short times are of interest for the problem at hand, the model involves a semi-infinite slab cooled by convective heat transfer while heated by a pulsating heat source. The model being linear, the solution is derived following the Laplace Transform method. A 3D finite element model of the rotating tomato is introduced as well in order to validate the analytical solution. A satisfactory agreement is attained. Therefore, two different ways to predict the onset of the peeling conditions are available which can be of help for proper design of peeling plants. Particular attention is paid to study surface temperature uniformity, that being a critical parameter for realizing an easy tomato peeling.

Some critical remarks about the radiative heat transfer in air frame cavities according to EN ISO 10077-2

Thermal performances of windows frames are established, in Europe, by the international standard UNI EN ISO 10077-2:2012. The standard introduces an equivalent thermal conductivity for air frame cavities thus simplifying the original combined heat transfer problem to a merely two-dimensional conductive one. The equivalence is referred to a rectangular cavity and is not able to fully recover the same radiative heat flux involved in the original problem. In view of that, the paper is focused on the radiative heat transfer taking place in the air cavities and aims to check if different equivalence criteria could lead to improved results. Thus, numerical tests involving an accurate description of radiative heat transfer in air cavities are compared to the simplified fully-conductive one provided by the standard. Results show that different criteria lead to quite different results. The optimal criterion turns out to depend on both geometrical and surface radiative parameters. It is also shown that, in any case, a proper radiative resistance but not the one suggested by the ISO 10077 should be adopted.

Thermographic Inspection at the Interface of Dry Sliding Surfaces

During the last decades, in order to clarify the coupled thermal and frictional aspects in dry sliding contacts, researchers have been involved in theoretical models as well as in experimental testing. In order to outline both the analytical and the experimental approach, a procedure for estimating the maximum temperature increase in dry sliding contacts is introduced. The procedure is based on an analytical solution for the two-dimensional temperature field in a slab subjected to a suitably shaped moving heat source. Experiments were carried out on a specifically designed pin-on-disk device with the aim of taking into account the unavoidable uneven friction distribution under the coupled surfaces. The subsequent data reduction led to quite satisfactory agreement with analytical predictions and provided a suitable shape for the heating source distribution thus allowing for proper maximum temperature rise estimate. Since measuring the interface temperature of a friction pair is a difficult task, temperature data were collected by means of infrared thermography; this technique seems to be the most effective and valuable due to its ability of performing continuous temperature map recording with relatively high resolutions when compared to other traditional methods. It is a matter of facts that high energy rates are dissipated by friction during short periods, thus transient and localized thermal phenomena with high thermal gradients are to be expected. The latter conditions still suggest the use of IR thermography.

Dielectric constant measurements by IR thermography in microwave heating

In this paper a technique based on microwave heating and IR thermography measurement for dielectric constant estimation is proposed. The effect of the propagating EM wave on the temperature distribution inside the sample under test is studied for short observations time. A model was derived and a suitable data reduction is accomplished. Preliminary results are presented and discussed for the pure water case.