Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt Natural convective heat transfer of heated packed beds Dong-Young Lee, Myeong-Seon Chae, Bum-Jin Chung ⁎ Department of Nuclear Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea ARTICLE INFO Keywords: Natural convection Packed beds Heated sphere Packed bed height Analogy Heat transfer ABSTRACT Natural convection heat transfer of heated packed bed was investigated. Experiments were performed for a single heated sphere buried in unheated packed beds varying its locations and for packed beds with all heated spheres varying the heights of packed beds from 0.02 m to 0.26 m. Mass transfer experiments using a copper electroplating system were performed based upon the analogy between heat and mass transfer. The diameter of sphere was 0.006 m, which corresponds to Ra d of 1.8 × 10 7 . For the single heated sphere cases, the measured results agreed well with the existing natural convection heat transfer correlations for packed beds and even with those for a single sphere in an open channel. For all heated sphere cases, the average heat transfers decrease with increasing packed bed heights. 1. Introduction The natural convection heat transfer in the fluid-saturated high temperature packed beds has been widely studied due to its various applications ranging from solar collectors to high temperature gas cooled reactor. The phenomenon has been gaining interest, as the coupled mechanism of conduction, natural convection and thermal radiation in packed beds improves the cooling performance of many applications [1]. The natural convection heat transfer was known to be affected by Ra d , Pr [2] and especially dominated by the temperature difference between solid spheres and fluid [1]. The measurements of temperature and velocity in packed beds are difficult due to the randomly packed structure of packed beds. Also, the uniformly heated condition for all spheres in packed beds is very dif- ficult to realize in experiments. The natural convection heat transfer in packed beds has been studied as the porous media. This, in general, is the case encountered in packed beds, which are made up of roughly uniform particles [3]. The existing studies have been performed for two largely different heat source conditions: First, the packed beds are not heat source and the heat sources are located inside or outside of the packed beds [4–16]. Second, parts of the packed beds worked as the heat source [1–2,5,17–22]. In the former cases, the existing studies focused on the fluid behavior by the heat source in so-called porous media. Most studies for the former case were performed for side wall or below heating conditions [4–13]. Other studies were aimed at the measuring the temperature or flow field around a concentrated heat source embedded in the porous media [14–15]. In the latter cases, the existing studies focused on the heat transfer between the heat source and fluid by heating the parts of the packed beds. In this case, the natural convection heat transfer was mainly carried out for the single heated sphere condition varying the heat flux of sphere in unheated packed beds [2,17–22]. Other studies investigated the temperature distribution in packed beds for all heated spheres condition [1,5,16]. Relatively less studies were performed for all heated spheres in packed bed. In this study, experiments were performed in two parts: first, the single heated sphere buried in unheated packed beds and second, packed beds with all heated spheres. First, the experiments were per- formed the single heated sphere buried in unheated packed beds varying the axial and radial locations of the heated sphere. Second, the experiments were carried out varying the heights of packed beds for all heated spheres. Based on the analogy concept, mass transfer experiment was per- formed instead of heat transfer experiments by adopting an electro- plating system. In the copper sulfate‑sulfuric acid (CuSO 4 –H 2 SO 4 ) electroplating system, the cathode acts as the heated surface as the reduction of cupric ions results in the decrease of fluid density and hence induces buoyancy. Thus the cathode copper sphere simulates the heated sphere. 2. Theoretical background The porous media modeling was adopted to predict the fluid flow or heat transfer due to the complicated flow through a packed bed [23]. The porous media modeling simulate the packed beds in two aspects: first, in the fluid flow aspect, the fluid flow in packed bed is simulated http://dx.doi.org/10.1016/j.icheatmasstransfer.2017.08.013 ⁎ Corresponding author. E-mail address: bjchung@khu.ac.kr (B.-J. Chung). International Communications in Heat and Mass Transfer 88 (2017) 54–62 0735-1933/ © 2017 Published by Elsevier Ltd. MARK