3D particle-scale modeling of gas–solids flow and heat transfer in fluidized beds with an immersed tube Hadi Wahyudi a,c , Kaiwei Chu a,b,⇑ , Aibing Yu b a School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia b Laboratory for Simulation and Modelling of Particulate Systems, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia c Department of Mechanical Engineering, Sultan Ageng Tirtayasa University, Jl. Jend. Sudirman KM 3, Cilegon, Banten, Indonesia article info Article history: Received 15 October 2015 Received in revised form 5 February 2016 Accepted 14 February 2016 Keywords: Gas–solids flow Heat transfer Fluidized bed Computational fluid dynamics Discrete element method An immersed tube abstract In this work, a fully three-dimensional (3D) model of combined computational fluid dynamics and discrete element method (CFD–DEM) is for the first time developed to study the gas–solids flow and heat transfer in fluidized beds with an immersed tube. A critical bed thickness is first determined at which the bed can be regarded as fully 3D. Then the validity of the model using the critical bed thickness is tested both qualitatively and quantitatively. It is shown that the model can successfully reproduce the typical relationship between pressure drop and gas velocity, and flow and heat transfer characteristics such as the four distinct stages of bubble transit through the tube and the peak of heat transfer coefficient between tube and the bed for certain gas velocity (which are however not well-predicted by previous 2D CFD–DEM and 2D CFD–3D DEM models). Finally the results are analyzed to improve the fundamental understanding of the system. It is demonstrated that both the gas and solids phases have 3D flow char- acteristics including the unique feature of 3D orientations of gas velocity vector field around the bubble. It is predicted that the maximum heat transfer coefficient is a result of the competition between surface– particle conduction and surface–fluid convection. The obtained results should be useful to the develop- ment of the fundamental understanding of the flow and heat transfer characteristics in a fluidized bed with immersed tubes. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The bubbling fluidized beds have been widely used in chemical and industrial processes for many years due to their performances as characterized by good solid mixing, high efficiency of heat trans- fer, and fast chemical reaction. Moreover, their application as heat treating furnaces has become increasingly popular in recent years as an alternative to other environmentally hazardous processes using oils, molten salts, and molten leads. Despite its potential applications, the technology improvement on the commercial scale units is slow due to the lack of accurate information on hydrody- namics and heat transfer behaviors on this scale. In many applications such as power plant and heat treatment industries, the bodies are immersed in a fluidized bed to allow the heat to be transferred or removed from/to the bed. Therefore, the heat transfer between an immersed tube surface and a fluidized bed, which can be represented by heat transfer coeffi- cient, has become an interesting subject for many years. To date, many experimental studies have been conducted on this subject in order to investigate the effects of operating conditions, geome- tries, surface temperatures, and particle properties on heat transfer coefficient (HTC) between the bed and the surface, see for exam- ple: Kim et al. [1], Denloye and Botterill [2], Ozkaynak and Chen [3], Doherty et al. [4], Friedman et al. [5], Masoumifard et al. [6]. One of the most important parameters that have been attracting many investigators to study is the superficial gas velocity. In such studies, low temperature system of fluidized beds (tube tempera- ture 6100 °C and initial bed and gas are at room temperatures) are frequently used. As summarized by Grewal and Saxena [7], Tsukada and Horio [8], and Masoumifard et al. [6], an important and general finding have been derived from such experimental studies, i.e., the HTC increases with the increase of gas velocity, reaches a maximum value, and then decreases gradually with the further increase of gas velocity. Another important finding obtained from previous experimental studies is that the HTC between the bed and the tube surface decreases as the particle diameter increases [6,9]. While the data or correlations generated http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.02.038 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Laboratory for Simulation and Modelling of Particu- late Systems, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia. Tel.: +61 3 99050848. E-mail address: kevin.chu@monash.edu (K. Chu). International Journal of Heat and Mass Transfer 97 (2016) 521–537 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt