Natural Convection Effects on Freezing in Vertical Cylinders

Freezing is an important thermal process with solar applications. The Phase Change Material (PCM) undergoes freezing process and the rate of freezing is influenced by natural convection in the melt if it is above the melting point. Experimental study was conducted on two PCMs namely Paraffin wax (m.p 63–65°C) and n-octadecane (m.p 27.7 °C). Steel and copper containers with insulation at both bottom and top by thermocol (1 cm thick) and acrylic plates (1 cm thick) are used to allow only radial heat flux. The PCMs were separately heated in a container to a known superheat and placed in an isothermal bath (± 1°C) containing the Heat Transfer Fluid (HTF). Water is used as HTF in this case. The temperature distributions in the PCM were measured using PT100 sensors with digital indicator to cover 10–16 points in radial and axial locations. In another separate set of experiments, the PCM under identical initial conditions of superheat is placed in the cylindrical metal container without any temperature sensors. Separating the frozen mass and unfrozen liquid at different time intervals and weighing, generate the frozen mass vs. time data following Sparrow et al (International Journal of Heat and Mass Transfer, 24, 273–284,1981). The range of parameters studied are given below: • Container L/D ratio: 0.6–5.4; • Initial Superheat of PCM: 5–35 °C; • Range of water temperature: 5–25 °C; • Volume of PCMs: 35–500 ml. From the experimental data of temperature distributions, frozen mass vs. time data were generated following El Dessouky et al (Journal of Solar Energy Engineering, Transactions of ASME, 121, 98–109, 1999), whose data pertains to melting outside tubes. The data of melt separation and weighing of n-octadecane and paraffin wax are compared with temperature-based data. A tentative correlation of entire data is presented.