Micro-Ct Investigation of Urea Crystal Structures for Frost Analysis

Abstract

To model the process of frost formation, a detailed understanding of the evolving three-dimensional microstructures formed under varying environmental conditions is essential. This study introduces a novel approach employing micro-computed tomography (Micro-CT) for scanning the sample combined with an evaluation process based on the solution of the 3D heat equation (Laplace equation) by utilizing the finite-element method. Using urea as a non-melting substitute for frost, we performed a detailed, quantitative analysis of the thermophysical properties (e.g. thermal conductivity and mass diffusivity of the heterogeneous sample) related to the micro-crystalline structure. This study distinguishes between 'simple' and 'complex' microstructures that may or may not contribute to the overall diffusive pathway. These naturally formed microstructures significantly influence the samples' thermal conductivity. A significant variability in these properties along the sample height underscores the critical role of considering the internal structural configurations in the modelling process.Notably, the study revealed a substantial deviation between observed properties and those predicted by current empirical and heterogeneous material models which at best showed a 40.1% error in predicting the sample bulk conductivity, highlighting their limitations when applied to some complex, frost-like structures. The combination of Micro-CT scanning with a detailed analysis based on solving the conservation equations on the domain of the heterogeneous material holds significant potential to enhance the accuracy of frost modelling. This approach promises to improve the design and efficiency of cooling technologies sensitive to frosting conditions.

Keywords: Computed tomography, Experiment, Frost formation, Porosity, Effective thermal conductivity, Effective mass diffusivity, Urea, Laplace equation, Heterogeneous material