Performance assessment of heat transfer and friction characteristics of a packed bed heat storage system embedded with internal grooved cylinders

The packed storage system is an important part of an energy harvesting system. The heat storing elements such as pebbles, rocks are commonly used for low to high-temperature heat storing applications. In the present work, an extensive experimental study is conducted to investigate the effects of system and operating parameters on the heat transfer and friction characteristics of packed bed storage system integrated with solid cylinders embarked with internal grooves. The investigation is done for the four different types of heat storage elements in which one is a solid cylinder and the rest three are cylinders with six internal grooves. The diameters of all the four types of elements are kept at 100 mm, while the lengths used are 100 mm for the solid cylinder and 100 mm, 150 nun, and 200 mm for the other three cylinders, respectively. The heat transfer and pressure characteristics in terms of Nusselt number (Nu) and friction factor (f) are assessed experimentally by using aspect ratio (phi = 0.66-1.33) and void fraction (epsilon = 0.47-0.60) as variable parameters. The range of Reynolds number (Re) in the present investigation is from 200 to 2050. The Nusselt number and friction factor both increases with the increase of aspect ratio from 0.66 to 1 and attains a maximum at an aspect ratio of 1. The increase of void fraction decreases the heat transfer and friction penalty. The statistical correlations are developed for the Nusselt number and friction factor and the predicted values are within +/- 10% and +/- 15% for the Nusselt number and friction factor, respectively.