Experimental investigation of heat transfer and second law analysis in a pebble bed channel with internal heat generation

This paper studies experimentally the forced convection heat transfer of turbulent flow in a cylindrical pebble bed channel with internal heat generation. Exergy and entropy generation analyses are performed to optimize energy conversion in the system identify the destruction of exergy in the pebble bed channel. Stainless steel spheres are used in stacked pebble bed channel. Internal heating is generated uniformly by electromagnetic induction heating method in metallic spheres. Dry air is used as the working fluid in the process of cooling of the heated spheres. The experiment is performed for turbulent flow regimes with Reynolds (Re-d) number (based on the diameter of the spheres) in the range of 920-2570, which is equal to Reynolds (Re) number, based on channel diameter, in the range of 4500-10,000. The effects of different parameters, including spheres diameter (d = 5.5, 6.5 and 7.5 mm), inlet volumetric flow rate (over dot(V)) and internal heat generation (Q) on the forced convection heat transfer, exergy transfer and entropy generation are studied. For second law and exergy analyses, mean exergy transfer Nusselt number (Nu(e)) and entropy generation number (N-s) are investigated. Results show that for a fixed d and Q the mean exergy transfer Nusselt number (Nu(e)) decreases with the increase of Re-d number until it becomes zero for a critical Re-d number. This critical Re-d number found to be about 1450, 1800 and 2300 for d = 5.5, 6.5 and 7.5 mm, respectively. Further increase in the Re-d number, decreases Nu(e) to negative values. It is found for spheres with diameter of d = 5.5 mm and for a fixed Q, as Re-d increases, the entropy generation number N-s increases monotonically. While, ford > 5.5 mm and fixed Q, the entropy generation number (N-s) decreases with the increase of Re-d number up to a critical Re-d value that makes N-s to be, minimum. Further increase in Red number, increases N-s. It is also found that for Re-d > 1800, among the sphere diameters studied in this work, balls with highest diameters yield the minimum entropy generation in the system. (C) 2017 The Authors. Published by Elsevier Ltd.