FREEZING AND MELTING WITH MULTIPLE PHASE FRONTS ALONG THE OUTSIDE OF A TUBE

This paper addresses the modeling and analysis of thermal storage syst ems involving phase change with multiple phase fronts. The problem inv olves a fluid flowing inside a long tube surrounded by a phase-change material (PCM). The fluid temperature at the tube inlet cycles above a nd below the freezing temperature of the PCM, causing alternating liqu id and solid layers to form and propagate from the tube outside surfac e. The objective of this paper is to predict the dynamic performance, tempera ture distribution, and phase front distribution along the trib e. The problem is modeled as axisymmetric and two dimensional. Axial c onduction is neglected and the problem is discretized into axial segme nts. Each of these axial sections is modeled as a transient, one-dimen sional problem involving phase change with the possibility of multiple phase boundaries. The boundary element method (BEM) is used to obtain the transient solution in each axial section. Each axial segment comm unicates with downstream segments through the fluid flowing inside the tube. In order to ensure numerically stable results, a fully implicit discretization is used in both the axial and time variables. Results are presented for the time and axial evolution of the phase fronts and temperatures in response to a fluid inlet temperature that periodical ly alternates between values above and below the freezing temperature. This BEM is tested against the thermal network method (TNM) and the n egligible sensible heat approximation (NSH) by comparing the outlet te mperature and the latent state of charge. Results are found to be cons istent and accurate.