Estudio de la formación de escarcha durante el almacenamiento a granel de vegetales congelados

Abstract

A model of heat and mass transfer is proposed in order to predict frost formation into a closed container filled with frozen vegetables. The physical problem is modeled as a macroporous media composed by the product itself and the surrounding air. Natural convection air flow is assumed into the container, who promotes water mass transport. As a first validation, the model is simulated for several exterior air temperatures, under environmental fluctuations (boundary conditions). Results of four temperature cycles were compared, varying average air temperature, amplitude and frequency of oscillation, one by one. As a general result, it is observed that the product temperature behavior is as expected, and it is directly associated with frost formation into the container. Frost formation increases with large amplitude of oscillation, but decreases with higher frequencies and higher mean temperatures. Model parameters were obtained for two assembling: frozen slices of carrots and air, and frozen extra thin green beans and air. Parameter definition and evaluation combines literature review, measurements and numerical simulation. In general, parameters which characterize these porous media were similar for both products, even though they display different geometries. The experimental validation is performed for carrot slices with two temperature cycles. The numerical model is able to predict air velocity field, air and product temperatures, and local frost formation. Results are validated in respect to a set of independent experimental results that shown a good agreement. Air flow circulation is as expected due to natural convection. Product temperature simulated behavior agrees with measurements, and temperature values differ by less than 12%. Respect to frost formation predictions, the model predicts correctly the most susceptible regions to frost formation. However, the quantity of frost formed predicted by the model (1.56 g/week) is lower than the experimental one (4.67 g/week), despite being of the same order of magnitude. The effect of each parameter in the model is study in order to detect how to improve the model. The most important parameters affecting total frost formation are effective mass diffusivity and convective heat coefficient into the storage container. Adjusting these parameters to twice, better results in terms of frost formation could be obtained (3.09 g/week).