Experimental set-up for the validation of phase change models in case of direct and inverse heat transfer problems

Abstract

A number of models and modelling approaches for phase transitions of phase change materials (PCMs) have been proposed in recent years. However, many of these models have not been thoroughly validated with experimental data. This is in particular the case of the models for thermal hysteresis and partial phase transitions of PCMs, where the design and execution of relevant experiments is difficult. The most widely used experimental techniques for characterization of PCMs - Differential Scanning Calorimetry (DSC) and T-history method – require minimization of the temperature gradients in the test samples and thus the obtained results do not represent very well the behavior of PCMs in thermal energy storage (TES) systems (where large temperature gradients in PCMs are commonplace). An experimental set-up for the acquisition of data suitable for validation of phase change models have been proposed and assembled. The set-up can be used for the model validation in case of both the direct and inverse heat transfer problems. The set-up is based on the monitoring of the phase change front propagation in a rectangular cavity, where the positive or negative heat flux is introduced at one of the cavity walls. Such an arrangement results in (often significant) temperature gradients in a PCM. Unlike in similar experimental set-ups, where a heat transfer fluid (HTF) is used to introduce the heat flux at the wall, the Peltier cells are used in the proposed experimental set-up for this purpose. Also, most experiments reported in the literature only addressed the melting process (heating of a PCM) with the positive heat flux introduced at the wall. The Peltier cells allow for relatively quick switching between the positive and negative heat flux (heating/cooling) and as a result the thermal processes similar to real-life operation of TES systems can be investigated. The cubical cavity with 250 mm long internal edges is made of PMMA. The wall, at which the heat flux is introduced, is made of a 15 mm thick aluminum plate with embedded RTD temperature sensors for wall temperature monitoring. A heat flux sensor is installed on the side of the aluminum plate facing the PCM (the heat flux sensor covers the entire surface of the plate). An extended heat transfer surface in the form of a finned aluminum sink is installed on the Peltier cells to improve the heat transfer between the cells and the ambient environment.