Numerical study of mixing and heat transfer of SRF particles in a bubbling fluidized bed
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Numerical study of mixing and heat transfer of SRF particles in a bubbling fluidized bed Mohamed Sobhi Alagha1 · Botond Szucs1 · Pal Szentannai1 Received: 9 July 2019 / Accepted: 29 November 2019 © The Author(s) 2019
Abstract In this article, numerical investigations on mixing and heat transfer of solid refused fuel (SRF) particles in a bubbling fluidized bed are carried out. The numerical model is based on the Eulerian–Eulerian approach with empirical submodels representing gas–solid and solid–solid interactions. The model is verified by experimental data from the literature. The experimental data include SRF vertical distribution in SRF–sand mixtures of different sand particle sizes ( dpm = 654, 810 and 1110 μ m) at different fluidization velocities ( u∕umf = 1.2–2.0). We proposed magnification of drag force exerted by the gas on SRF particles based on Haider and Levenspiel (Powder Technol 58(1):63–70, 1989) drag coefficient. The proposed model shows good agreement with the experimental data at high fluidization velocities ( u∕umf = 1.5–2.0) and poor predictions at low fluidization velocities ( u∕umf = 1.2–1.5). Heat transfer results showed that the present model is valid and gives good agreement with the experimental data of wall–bed heat transfer coefficient. Keywords Mixing · Heat transfer · SRF · Fluidized bed · Fluidization velocity List of symbols A, B, C, D Coefficients (–) CD Drag coefficient (–) Cp Particle specific heat ( J kg−1 K−1) dp Particle diameter (μm) 𝜀 Void fraction (–) g Gravity acceleration (ms−2 ) h Heat transfer coefficient (Wm−2 K−1 ) Kgs Momentum exchange coefficient (kgm−3 s−1 ) 𝜆 Thermal conductivity (Wm−1 K−1 ) 𝜇 Viscosity (Pas) hd Nu Nusselt number = 𝜆p (–) 𝜇Cp Pr Prandtl number = 𝜆 (–) 𝜌 Density (kgm−3 ) 𝜌 d |u −u | Re Reynolds number = g p 𝜇 g p (–) T Temperature (◦ C)
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* Botond Szucs [email protected] Mohamed Sobhi Alagha [email protected] Pal Szentannai [email protected] 1
Department of Energy Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics (BME), Budapest, Hungary
u Velocity (ms−1 ) 𝜓 Particle shape factor (–) Subscripts and superscripts D Drag g Gas m Mean mf Minimum fluidization p Particle s Solid
Introduction Fluidized beds offer excellent mixing and heat transfer characteristics which make them efficient in thermal conversion applications, i.e., combustion/gasification of low-grade coal, biomass, and solid refused fuel (SRF) [38]. Practically, fuel particles are of irregular shapes and they enter the fluidized bed at a size range of 5–10 mm [27]. The overall fuel concentration within the fluidized bed combustor is very low compared to the bed material (2–5 mass%) [27]; however, the big differences in the density and size ratios can significantly affect the homogeneity of the bed (segregation phenomenon) [32]. In general, the segregation occurs during fluidization of binary beds containing two or more solid components of different sizes and/or densities. The heavier
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