Heat Transfer From Rough and Finned Horizontal Tubes in a Gas Fluidized Bed

Experimental results of the heat transfer coefficient for smooth, rough, and finned tubes of o.d. 12.7 and 50.8 mm immersed in fluidized beds of silica sand particles of average diameters (d_p) 145, 167, 488, and 788 μm are reported as a function of fluidizing velocity. The surface roughness is of V-shaped fins with the values of pitch (P_f) as 5.08, 3.18, 1.58, 0.79, 0.40, and 0.24 mm. The qualitative trends of these heat transfer coefficients in regard to their dependence on various systems and operating parameters are discussed on the concept of particle mode of heat transfer, contact geometry of the heat transfer surface, particle residence time, particle size and size range, tube diameter, etc. Results are also analyzed in terms of the fin effectiveness factor (φ) and heat transfer function (β). It is shown that φ and β are only weakly dependent on tube diameter, particle size range, and fluidizing velocity and therefore have been correlated in terms of the dimensionless ratio (P_f/d_p). It is also shown that particle residence time on the heat transfer surface controls the value of the heat transfer coefficient rather sensitively.