Ray-tracing images for melting PCM bridges in microgravity

Abstract

A ray-tracing algorithm is developed to obtain synthetic images for tracking the melting of the phase change material (PCM) n-octadecane in a liquid bridge configuration. The translucent nature of the solid n-octadecane is modelled through a sigmoid function which provides a distance-dependent illumination intensity throughout the solid. The code is validated against two types of experiments: front shapes made of 3D-printed PLA and cast front shapes of solid n-octadecane. Comparison with these experiments allows the numerical constants involved in the model to be tuned to appropriate values. For application to the MarPCM microgravity experiment (Porter et al., 2023), the current study provides the expected images of the solid/liquid front obtained by the optical diagnosis module for three different values of the dimensionless volume (defined with respect to a perfect cylinder), V=0.9, 1, and 1.1, and two applied temperature differences, ΔT=5 and 30K. The solid/liquid front shapes are obtained numerically from a two-dimensional axisymmetric code as in Varas et al. (2021). The results reveal a strong dependence of image quality on the liquid fraction L. Visual inspection leads to the conclusion that, below L≈60%, an accurate determination of the front location may be difficult, particularly for the relatively shallow layer of liquid near the thermocapillary interface.