Experimental and Numerical Investigations of Inclination Angle Effect on PCM melting Behaviour in Shell and Tube Thermal Energy Storages with and without Metal Foam

Phase change materials are employed to store and release the heat energy at nearly constant temperatures due to their high energy capacity. However, the low thermal conductivity of PCMs makes them less competent to use them as thermal energy storage materials. Researchers are deploying various novel approaches to enhance the heat transfer within PCMs. In this study, heat transfer enhancement in PCM type RT-42 is experimentally and numerically evaluated with the effect of system inclination. The experimental study focused on two major aspects: the impact of inclination and the incorporation of metal foam. The first experimental study focused on the effect of system inclination on the melting behavior of PCM in square shell-and-tube thermal energy storage (TES). PCM inside the TES is heated using a high temperature fluid (HTF) flowing inside aluminum tubes at 55 oC with a constant flow rate of 2.70 l/min. The experiments are carried out by varying the inclination angle of the TES for 0o, 15o, 30o, 45o, 60o and 90o. Temperature variation at the key locations inside the TES has been recorded by thermocouples using a data acquisition system. Various characteristics of PCM, such as liquid fraction, heat transfer rate by the HTF, and energy storage, have been evaluated. It is concluded that increasing the inclination angle of the TES results in a decrease in the charging time. Vertical orientation (90o) provides the optimum thermal performance among the analyzed cases for effective charging in TES, while horizontal one (0o) is the least effective. In the second study, the effect of the melting behaviour for pure PCM TES was compared to PCM embedded with metal foam TES at 55 oC and 65 oC. The study reveals that the presence of metal foam enhanced the melting behaviour of PCM up to 3-4 times due to enhanced thermal conductivity under the same boundary conditions. While, the melting time reduced with the increase in HTF temperature. In the third study, the thermal performance of pure PCM TES and PCM embedded with metal foam was compared for two inclination angles of the TES channel. The study concluded that higher inclination angles lead to enhanced melting behaviour of the PCM, while this effect remains minimal for PCM-embedded metal foam TES. A two-dimensional model of a rectangular TES cavity containing five heated tubes was also developed in ANSYS Fluent to study the melting fraction, convective heat transfer coefficient, energy storage, and temperaturevariation inside the channel. A detailed analysis has been conducted to study and compare the melting behaviour of the PCM for different positions at the inclination angles of 0o, 15o, 30o, 45o, 60o and 90o. The research concluded that the increase in inclination angle of the TES cavity enhances the heat transfer inside the channel due to the interplay between conduction and convection modes of heat transfer and the presence of buoyancy force associated with the inclination angle. The buoyancy force promotes the thermal mixing of the PCM, which results in faster melting; however, there exists a temperature gradient at various positions inside the channel, and this temperature gradient increases with the increase in inclination. The heat transfer at the upper half of the channel is through convection, while in the lower half, it is mainly due to diffusion. As a result, some PCM remain unutilized throughout the heating cycle, thus requiring some alternate approach for the effective PCM utilization at the bottom of the channel. The system with a high inclination angle performs better as compared to the horizontal orientation, and the 90 ° inclination angle performs better in comparison to others. The numerical results are in agreement with experimental results. This research study concludes that, besides introducing porous structures, nanoparticles, or finned surfaces, the system orientation can also positively influence the heat transfer in thermal energy storage.