Surface-to-bed heat transfer in fluidised beds: Effect of surface shape Francesco Di Natale ⁎ , Amedeo Lancia, Roberto Nigro Dipartimento di Ingegneria Chimica, Università di Napoli “Federico II”, P.le Tecchio, 80, 80125 Napoli, Italy Received 9 December 2005; received in revised form 3 January 2007; accepted 9 January 2007 Available online 20 January 2007 Abstract In recent times, the possible application of fluidisation technologies to the surface treatments of engineering materials becomes a subject of growing interest both for manufacturing and chemical industries. Heat and mass transfer rates between the surface and the fluidised bed strongly influence the performance of the surface treatment. Experimental results of heat transfer between a submerged surface and a fluidised bed are presented in this article. This work is focused on the influence of bed material properties and surface geometry on heat transfer coefficient. Experimental tests show that the heat transfer coefficient is notably affected by the shape of the immersed surface resulting higher for surfaces with better aerodynamic shape. An interpretative model, based on the dimensional analysis, has been used for the description of the experimental results. © 2007 Elsevier B.V. All rights reserved. Keywords: Heat transfer; Bubbling fluidised bed; Bed material properties; Exchange surface shape 1. Introduction Surface treatments are used to improve the functions and service lives of engineering materials controlling friction and wear, increasing corrosion resistance, changing physical properties (e.g., conductivity, resistivity, and reflection), dimensions and appearance (e.g., colour and roughness) also obtaining a costs reduction. Surface treatments can be roughly schematized as a series of elemental steps involving heat and mass transfer processes. The overall efficiency of a superficial treatment is related both to the improvement of the surface properties and to the degree of homogeneity of the treated surface. This last is a crucial para- meter since a non-uniform quality of the treatment may lead to the creation of weak points on the surface. This problem is more relevant when the process is carried out by immersion of the surface in a flowing fluid. In this case, differences in the fluid dynamic field around the surface alter the local heat and mass transfer rates and reduce the uniformity of the superficial treat- ment. Surface geometry directly influences the treatment efficiency. Fluidised beds are characterised by high values of heat and mass transfer rates and solid mixing [1] which may suggest the use of these reactors for surface treatments. Classical applications of fluidisation technologies to surface coating and decoating are available in literature [1–5] and, in recent times, examples of more complex surface treatments have been reported [6–9]. For example, fluidised beds have been used for the Thermal Reactive Deposition [6], a hard coating technology used to enhance the tool life, while fluidised bed plasma reactors have been used to modify the surface of fine alumina powders [7] and to alter the superficial properties of polymers, such as wettability and adhesion, without changing their bulk properties [8]. Finally, fluidised bed reactors have been used for surface pre-treatments required by film and solid covering depositions [9]. Accurate estimations of the heat and mass transfer coefficient between the fluidised bed and the immersed surface are crucial to an efficient reactor design and operation. However, at this moment, the prediction of heat and mass transfer rates in different working conditions is still not addressed. Only few experiments exist on the mass transfer rate which is mainly related to gas phase dynamics near the immersed surface and may be qualitatively described by classical models [1,10]. On the contrary, heat transfer coefficient has been widely studied [11–22] and its features have been mainly related to the so- Powder Technology 174 (2007) 75 – 81 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +39 81 7682246; fax: +39 81 5936936. E-mail address: fdinatal@unina.it (F. Di Natale). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.01.010