Heat Transfer to a Single Sphere Immersed in Beds of Particles Supplied by Gas at Rates above and below Minimum Fluidization

Abstract

The coefficient of heat transfer to a mobile sphere in a fluidized bed of relatively large particles has been measured. Measurements were also made for U < U mf , where the solids are stationary. One feature of these experiments is that the heat-transfer sphere was of a size comparable to that of the bed particles. Two methods of measuring heat-transfer coefficients in the bed were employed: (i) The rate of cooling of a sphere, initially hot, was measured by means of a tiny inserted thermocouple with fine lead wires connected to a temperature recorder external to the bed, and (ii) a small number of spherical CO 2 particles (dry ice) were put into an air-fluidized bed of inert particles. The subsequent concentration of CO 2 in the off-gas from the bed provided a measure of the evaporation rate of the dry ice particles and, hence, of the heat-transfer rate to each dry ice particle. Good agreement between values of h from the two methods implies that the thermocouple lead wires of method (i) did not restrain the free motion of the heat-transfer sphere. The meaning of the heat-transfer coefficient is discussed with regard to its applicability to a transient experiment, and a criterion is suggested for determining when it is valid to derive a heat-transfer coefficientsa steady-state parametersfrom a transient experiment. At high Reynolds numbers (100 < Re mfs < 830) and with small spheres in a bed of large particles (0.2 < d s /d p < 2.75), the dominant mechanism of heat transfer is found to be to the flowing gas. In this case, Nu ) 2 + 1.0Re mfs 0.6 (d s /d p ) 0.26 .

The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

“…That Eq. (7) holds for U/U mf 1, as well as for U > U mf , but then using Re = U mf d s / , is confirmed by subsequent work by Scott et al (2004), who found a very similar correlation for spheres of the much lighter material graphite as the heat transfer particle. Given all the above, it might be expected that for the conditions of this study, there is the mass transfer analogue of Eq.…”

Particle Scale Simulation of Heat Transfer in Fluid Bed Reactors

“…The cooling of copper spheres at different initial locations in a gas fluidzied bed was examined by the model (Zhou et al, 2009). In physical experiments, the temperature of hot spheres is measured using thermocouples connected to the spheres (Collier et al, 2004;Scott et al, 2004). But the cooling process of such hot spheres can be easily traced and recorded in the DPS-CFD simulations, as shown in Fig.…”

“…In physical experiments, Collier et al (2004) and Scott et al (2004) used different materials to examine the HTCs of hot spheres, and found that there is a general tendencyfor the HTC of hot sphere increasing first with gas superficial velocity in the fixed bed (U<U mf ), and then remaining constant, independent on the gas superficial velocities for fluidized beds (U>U mf ). The DPS-CFD simulation results also exhibit such a feature (Fig.…”

“…Physical experiments usually experience difficulty in quantifying the heat transfer mechanisms at a particle level identified many years ago (for example, Yagi & Kunii, 1957). To overcome this difficulty, in recent years, heat transfer behaviour in a fluidized bed at a microscopic, particle scale has been examined experimentally (Prins et al, 1985;Baskakov et al, 1987;Agarwal, 1991;Parmar & Hayhurst, 2002;Collier et al, 2004;Scott et al, 2004). In such a study, the transient temperature of a hot sphere immersed in the bed is measured using an attached thermocouple, and then its HTC can be determined.…”

The analysis of hot spots in large scale fluidized bed reactors

“…20,21 But in uidized beds, although researchers have paid much attention to this problem, their results are insufficient to reach a general conclusion. Behjat 22 and Yiannoulakis et al 23 found that the higher supercial velocity had a positive effect on the heat transfer coefficient, while Scott, 24 Zhou 25 and Li et al 13 found that increasing the gas velocity did not lead to an increase of the heat transfer coefficient. And for the effect of bed diameter, Matsen 26 found that larger units gave higher mixing rates, but there is no practicable conclusion for industrial uidized beds, since most of the experiments are in laboratory scale.…”