Hydrodynamics of a bubbling fluidized bed are mainly affected by bubble properties (size, rising velocity, frequency, fraction and size distribution). In a fluidized bed heat exchanger (FBHE), bubble flow pattern in the tube banks plays an important role in the solid mixing in the bubble wakes as well as the passage in tube banks. Therefore, bubble characteristics in FBHE are important to understand the heat transfer properties.
The effects of gas velocity and internal tube geometry on bubble properties (chord length, rising velocity, frequency and fraction) have been determined by using an optical fiber probe in the FBHE (0.34×0.50×0.64m). Sand particles were used as the bed material. The mean diameter and apparent density of the sand were 240 m and 2582 kg/㎥, respectively.
The average bubble chord length and bubble rising velocity increase with the excess gas velocity and the bed height. In the FBHE, the average bubble size growth rate and rising velocity are depressed by the internals. The obtained bubble rising velocities in the freely bubbling bed and in the FBHE have been correlated with the excess gas velocity and bubble chord length as follows;
Freely bubbling bed : E[l]=1.43 $d_e^{0.58}$
FBHE : E[l]=1.29 $d_e^{0.66}$
FBHE : E[l]=1.29 $d_e^{0.66}$
Bubble sizes in FHBE with tube arrangement of square and triangular pitch are similar, but bubble rising velocities are more faster in FBHE of triangular pitch.
Bubble frequency and bubble fraction increase with the excess gas velocity and decrease with the bed height.
Bubble fraction has been correlated with Froude number of gas velocity considering actual gas velocity in the tube bank as follows;
$f_b=B[\frac{d_pg}{u^2_{mf}(\frac{u}{u_{mf}}-A)^2}]^c$, A=0.735, B=0.134, C=-0.324
Bubble diameter has been correlated with the operating variables considering the tube pitch length as follows;
1) under tube bank : $d_e=1.4\rho_sd_p(\frac{U_o}{U_{mf}})H_d+d_{e0}$
2) in tube bank :
(1) under tube ; $d_e < S_h \to d_{e,u} = d_{e,under}$
$d_e > S_h \to d_{e,u} = S_h$
(2) above tube ;
$d_e=1.4\rho_sd_p(\frac{U_oA}{U_{mf}})H_{d,oi}+d_{eu}, H_{d,oi}=H_d - H_t$
