Heat transfer wall to bed

Weekman and Myers (W3) measured wall-to-bed heat-transfer coefficients for downward cocurrent flow of air and water in the column used in the experiments referred to in Section V,A,4. The transition from homogeneous to pulsing flow corresponds to an increase of several hundred percent of the radial heat-transfer rate. The heat-transfer coefficients are much higher than those observed for single-phase liquid flow. Correlations were developed on the basis of a radial-transport model, and the penetration theory could be applied for the pulsing-flow pattern. 

Ostergaard (02) measured the wall-to-bed heat-transfer coefficient in a bed of 3-in. diameter. The media were air, water, and glass ballotini of0.5-mm diameter. It was observed that the heat-transfer coefficient for a liquid fluidized bed near the point of incipient fluidization could be approximately... 

Wall-to-bed heat-transfer coefficients were also measured by Viswanathan et al. (V6). The bed diameter was 2 in. and the media used were air, water, and quartz particles of 0.649- and 0.928-mm mean diameter. All experiments were carried out with constant bed height, whereas the amount of solid particles as well as the gas and liquid flow rates were varied. The results are presented in that paper as plots of heat-transfer coefficient versus the ratio between mass flow rate of gas and mass flow rate of liquid. The heat-transfer coefficient increased sharply to a maximum value, which was reached for relatively low gas-liquid ratios, and further increase of the ratio led to a reduction of the heat-transfer coefficient. It was also observed that the maximum value of the heat-transfer coefficient depends on the amount of solid particles in the column. Thus, for 0.928-mm particles, the maximum value of the heat-transfer coefficient obtained in experiments with 750-gm solids was approximately 40% higher than those obtained in experiments with 250- and 1250-gm solids. 

Gabor, J. O., Wall-to-Bed Heat Transfer in Fluidized and Packed beds, AIChE Symp. Series, 66(105) 76-86 (1970)... 

Flamant, G., Fatah, N. and Flitris, Y. (1992). Wall-to-Bed Heat Transfer in Gas-Solid Fluidized Beds Prediction of Heat Transfer Regimes. Powder Tech., 69,223. 

Two-fluid simulations have also been performed to predict void profiles (Kuipers et al, 1992b) and local wall-to-bed heat transfer coefficients in gas fluidized beds (Kuipers et al., 1992c). In Fig. 18 a comparison is shown between experimental (a) and theoretical (b) time-averaged porosity distributions obtained for a 2D air fluidized bed with a central jet (air injection velocity through the orifice 10.0 m/s which corresponds to 40u ). The experimental porosity distributions were obtained with the aid of a nonintrusive light transmission technique where the principles of liquid-solid fluidization and vibrofluidization were employed to perform the necessary calibration. The principal differences between theory and experiment can be attributed to the simplified solids rheology assumed in the hydrodynamic model and to asymmetries present in the experiment. 

Figure 19 shows, as an example, the evolution and propagation of bubbles in a 2D gas-fluidized bed with a heated wall. The bubbles originate from an orifice near the heated right wall (air injection velocity through the orifice s 5.25 m/s, which corresponds to 2 Uj. The instantaneous axial profile of the wall-to-bed heat transfer coefficient is included in Fig. 19. From this figure the role of the developing bubble wake and the associated bed material refreshment along the heated wall, and its consequences for the local instantaneous heat transfer coefficient, can be clearly seen. In this study it became clear that CFD based models can be used as a tool (i.e., a learning model) to gain insight into complex system behavior.

Kuipers, J. A. M., Prins, W., and van Swaaij, W. P. M., Numerical calculation of wall-to-bed heat transfer coefficients in gas-fluidized beds. AIChE. J. 38(7), 1079 (1992c). 

Syamlal, M., and Gidaspow, D., Hydrodynamics of fluidization Prediction of wall-to-bed heat transfer coefficients. AlChE. J. 31(1), 127 (1985). 

K Wall-to-bed heat-transfer Le Equivalent height of emul-... 

Ocone R, Sundaresan S, Jackson R (1993) Gas-Particle Flow in a Duct of Arbitrary Inclination with Particle-Particle Interactions. AIChE J 39(8) 1261-1271 Patil DJ, Smit J, van Sint Annaland M, Kuipers JAM (2006) Wall-to bed heat transfer in gas-sohd bubbling fluidized beds. AIChE J 52(l) 58-74 Peirano E, Leckner B (1998) Fundamentals of Turbulent Gas-Solid Flows Applied to Circulating Fluidized Bed Combustion. Proc Energy Combust Sci 24 259-296... 

Investigator Type of correlation Phases involved System Li and Finlayson [37] Wall-to-bed heat transfer coefficient in packed bed Fluid-solid Spherical particle-air system... 

The wall-to-bed heat transfer coefficient has been investigated by many researchers. Examining the published data, Li and Finlayson [37] found that many of the data on hw had entrance effect. Considering the data that were free from entrance effect, they proposed correlation Eq. 13.2.12 for spherical particle-air systems. They also correlated the data for cylindrical particle-air systems as given by correlation Eq. 13.2.13. 

Recently Nasr et al. [38] studied the augmentation of heat transfer by embedding the heat transfer surface in a packed bed. They found that in the presence of particles, the wall-to-bed heat transfer coefficient was up to 7 times greater than that for the case where heat transfer surface was placed in a cross flow. It was shown that the heat transfer coefficient increases with decreasing particle diameter and increasing thermal conductivity of the packing mate-... 

Wall-to-Bed Heat Transfer. The wall-to-bed heat transfer coefficient increases with an increase in liquid flow rate, or equivalently, bed voidage. This behavior is due to the reduction in the limiting boundary layer thickness that controls the heat transport as the liquid velocity increases. Patel and Simpson [94] studied the dependence of heat transfer coefficient on particle size and bed voidage for particulate and aggregative fluidized beds. They found that the heat transfer increased with increasing particle size, confirming that particle convection was relatively unimportant and eddy convection was the principal mechanism of heat transfer. They observed characteristic maxima in heat transfer coefficients at voidages near 0.7 for both the systems. 

Packed-bed heat transfer can be conveniently expressed by the concept of effective thermal conductivity, which is based on the assumption that on a macroscale the bed can be described by a continuum. In general, the effective thermal conductivity increases with increasing operating pressure. The wall-to-bed heat transfer coefficient increases with decreasing particle diameter. 

K. Muroyama, M. Fukuma, and A. Yasunishi, Wall-to-Bed Heat Transfer in Liquid-Solid and Gas-Liquid-Solid Fluidized Beds. Part I Liquid-Solid Fluidized Beds, Can. J. Chem. Eng. (64) 399, 1986. 

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