International Journal of Chemical Engineering and Applications, Vol. 2, No. 2, April 2011 84 Abstract—The solid particles distribution in circulating fluidized beds (CFB) has been extensively studied because of its importance in the design of CFB boilers and reactors. The previous studies report that in the circulating fluidized beds, the solids near the wall move down ward as a film with axial variable thickness. Therefore, it is very important to predict solids film thickness near the wall to estimate the heat transfer between a solid suspension in the bed and wall surface. In the present paper, an expression for the local solids film thickness has been suggested from the data of local solids flux measurements by using U-tube probe. The data measured in two laboratory scale CFB units which consist of 0.0508m diameter & 3 m height and 0.1016m diameter & 6m height. Sand (182 µm and 2560 kg//m3) and FCC particles (99 µm and 1600 kg/ m3) are used as bed material. The operating gas velocity and solid circulation rates are in the range of 1-6m/s and 5-125kg/m2-s respectively for the material considered. The R2 values for the correlation developed are 0.577 for literature data, 0.3904 for small column (0.0508m ID & 4m Height) and 0.886 for large column (0.1016m ID & 6m height). Index Terms—Circulating Fluidized Bed, Dilute Core, Dense Annulus, Down Flowing Solids Clusters, Flow Structure, Film Thickness, Cross-sectional Average Voidage, Gas velocity & Solids Circulating Rate. I. INTRODUCTION Circulating fluidized beds (CFBs) have been employed commercially in numerous gas–solid contacting processes such as combustors, coal gasifiers and catalytic reactors. Therefore, the gas-solid flow structure in the CFBs has got important role in understanding and design of these processes. The flow structure strongly depends on operating conditions such as gas velocity & solid circulation rate and bed geometry. Also it is possible to control the solid circulation rate in the circulating fluidized bed, therefore high concentration within the bed may result by promoting turbulent regime into the fast-fluidized bed regime. The solid in the fast-fluidized bed may typically occupy up to 20% of the bed volume and is in a state of extreme turbulence marked by refluxing of dense Manuscript received January 17, 2011. The authors gratefully acknowledge the financial support of the Technical Education Quality Improvement Program (TEQIP) of Indian Government given to University College of Technology, Osmania University, Hyderabad, India in doing this work. They also acknowledge the University of New Brunswick, Fredericton, New Brunswick, Canada for providing a part of fund in preparation of this work. Prof. V.V.Basava Rao is with the University College if Technology, Osmania University, Hyderabad-500 007, India (phone: +91-040-27098901; fax: +91-04027098472; e-mail: profbasavarao_1964@ yahoo.com). Dr. T. Balanarasaiah., is with the Centre for Chemical Sciences & Technology, IST, JNTU Hyderabad, HYDERABAD, INDIA -500 085. (email: balutumma@yahoo.com) Dr. B.V.Reddy is with the Department of Mechanical Engineering, University of New Brunswick, FREDERICTON, NB, CANADA - E3B 5A3 (email: bv_reddy@hotmail.com) clusters. The flow structure for fast fluidization is summarized in Figure 1. Figure 1: Flow structure of CFB (Core – Annulus) Under S-shaped axial voidage profile conditions typical of fast fluidization, the core-annulus flow persists over the whole length of the riser with a solids film near the wall in which solids are continuously flow downward [1]. In the film, near the wall, the solids concentration may be quite high so that the instantaneous voidage can be equal to voidage at the minimum fluidization condition of the bed [2] and the flow structure in the film is not uniform in radial direction [3]. Such solid down flow has been observed by measuring the radial distribution [4], the radial distribution of solids flux [5] and the radial solids momentum distribution [6]. Further, the solid flow downward near the wall in the form clusters and flow up ward in the core as dilute suspension. Also, Berruti & Kalogerakis [7], Senior & Brereton [8], Harris & Davidson [9] assumed in their models a Core/annulus two-region structure in which solids flow in the riser consists of a dilute up-flowing suspension in the central core region, surrounded by dense wall where solids fall downward as clusters. The solids film thickness near the wall depends again on operating conditions and bed geometry of the CFB. This solid film plays a significant effect on heat and mass transfer predictions. In this article, an attempt has been made to characterize the flow structure in the film and model for estimating the film thickness. In initial stages of the CFB investigations, the mean bed void fraction was described as a function of the gas velocity and solid rate in the fluidization regime diagram. This mean bed void fraction was obtained by averaging over the entire volume of the CFB riser column, assuming no variation along the bed height. This school of thought considered that the clusters were distributed uniformly over the bed volume. Prediction of Falling Solids Film Thickness near the Wall in Circulating Fluidized Bed Risers V.V.Basava Rao*, T.Bala Narsaiaha and B.V. Reddyb