Bubble heat transfer coefficients

These are the average laminar and transition to turbulent Nusselt numbers over the respective single-phase lengths L. In Gnielinski s correlation the friction factor is taken as / = (1.821ogjQ Re -1.64) . The bubble heat transfer coefficient, a, was determined by applying simple conduction theory through the liquid film ... 

Internal Regenerator Bed Colls. Internal cods generate high overall heat-transfer coefficients [550 W / (m -K)] and typically produce saturated steam up to 4.6 MPa (667 psi). Lower heat fluxes are attained when producing superheated steam. The tube banks are normally arranged horizontally in rows of three or four, but because of their location in a continuously active bubbling or turbulent bed, they offer limited duty flexibdity with no shutdown or start-up potential. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Bubble action provides agitation about equivalent to that of mechanical stirrers, and thus about the same heat-transfer coefficients. 

In experiments with bubble-columns containing suspended sand particles with average diameter 0.12 mm, an increase in heat-transfer coefficient was observed with increasing sand concentration, maximum values of 6000 kcal/m2-hr-°C being measured for suspensions containing 50% sand (based on the liquid volume). 

Experiments with bubble-columns containing suspended solid particles of varying diameter have demonstrated an increase of heat-transfer coefficient with increasing particle size. 

At higher values of AT the bubbles form more rapidly and form more centres of nucleation. Under these conditions the bubbles exert an appreciable agitation on the liquid and the heat transfer coefficient rises rapidly. This is the most important region for boiling in industrial equipment. 

To provide the pr equisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrod3aiamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve... 

The heat transfer coefficient (/i) has been determined by measuring the temperature difference between the immersed heater and the bed. The h value increases with increasing Ug (Fig. 1(a)), but exhibits a maximum value with increasing (Fig. 2(a)). The effects of Ug on h is dominant, since the bubbling phenomena become more vigorous due to the... 

For liquid metals the superiority of nucleate boiling heat transfer coefficients over those for forced-convection liquid-phase heat transfer is not as great as for ordinary liquids, primarily because the liquid-phase coefficients for liquid metals are already high, and the bubble growth period for liquid metals is a relatively short fraction of the total ebullition cycle compared with that for ordinary fluids. In the case of liquid metals, the initial shape of the bubbles is hemispheric, and it becomes spherical before leaving the heating surface. This is because of very rapid... 

The heat transfer coefficient h from the bubble layer to the bulk flow is constant before DNB. 

Substantial decrease of heat transfer coefficient Bubbly flow has less void but higher Ap than annular flow condensation rate depends on flow regime Transformation wave propagates along the system... 

The heat transfer coefficient (h) for a vapor bubble rising through a boiling liquid is given by... 

Figure 45. Comparison of heat transfer coefficient measured in 20 MW bubbling bed combustor vs prediction from MIT cold test. (From Glicksman et al, 1987)...

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

Figure 1. Average heat transfer coefficients at surface of horizontal tube in bubbling fluidized bed. (From Chandran, Chen and Staub, 1980.)...

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