Method feasibility of micro-ct investigation into frost formation

Air-source heat pumps save energy and reduce emissions but frost formation on the evaporator is still a major challenge. Effective frost mitigation and de-frosting require a fundamental understanding of frost formation dynamics. Previous studies model frost growth and densification within a 15 % error margin, based on bulk and surface properties. However, efficient frost mitigation requires better prediction of local and internal properties including crystal structure. So far, limited optical accessibility has hindered experimental analysis. In this study, a method is proposed to overcome this limitation, through the use of X-ray micro-computed tomography. To test the overall methodological feasibility, additional experimental complexities related to frost melting and instability were mitigated by substituting with a urea-mixture. A sample visually resembling frost was chosen for analysis. A detailed 3-D image of the air/crystal interface, with 7.3 μm spatial resolution, was captured. The porosity of the sample was determined to be 84 %. After filtering, segmenting, and cropping, the image was exported to MATLAB.
The effective thermal conductivity and effective mass diffusivity of the sample were determined by solving the respective steady-state three-dimensional diffusion equations. The effective thermal conductivity was determined as 0.0879 W/m.K, which lay between the ideal series and parallel values. The same is true for the effective mass diffusivity of 11.75 mm2/s. The general methodological feasibility was confirmed by the successful scanning, reconstruction and analysis of the frost substitute.