Thermal storage analysis of a foam-filled PCM heat exchanger subjected to fluctuating flow conditions

Highlights: • Charging process in a PCM heat exchanger subjected to fluctuating flow is studied. • Phase change rate is expedited by filling both PCM and HTF sides with metal foam. • Fluctuation in HTF temperature greatly affects the heat exchange and energy storage. • Oscillating change is found in PCM temperature, liquid fraction and heat storage. • Sensitivity to HTF temperature fluctuation is obvious at a low foam porosity. A dynamic heat transfer model is developed to investigate the transient thermal storage characteristics of a heat exchanger with foam-filled phase change material (PCM) under the fluctuating flow conditions. The thermal energy storage (TES) configuration of double pipe is considered, while using the highly porous metal foam to intensify the heat transfer in both the PCM and heat transfer fluid (HTF) regions. The paraffin RT50 is used as the PCM and water is selected as the HTF. The energy transport between the inner and outer pipes is thoroughly taken into account. The Darcy-Forchheimer equation is adopted to model the fluid flow through metal foam, and the enthalpy-porosity method is employed to simulate the phase change process within the PCM/foam composite under the local thermal non-equilibrium (LTNE) condition. The performance of heat exchanger during the charging process is firstly predicted under steady HTF flow, and then the effects of fluctuating inlet temperature and velocity are examined respectively. In the case of steady flow, compared with the pure PCM case, inserting metal foam in both the PCM and HTF regions shortens the melting time by 93.6% and augments the heat storage rate by 9 times. Fluctuation in the inlet temperature leads to a visibly oscillating variation in the PCM temperature and heat storage, which are 15 K, 65 kJ at a fluctuation amplitude of 30 K and 17 K, 105 kJ at a periodicity of 400 s. However, fluctuation in the velocity has no considerable influence. The heat exchanger with lower porosity foam presents more sensitive to the fluctuating inlet temperature.