Heat-transfer coefficients variation along heating surface

The heat transfer surface is the area under the stepped curve, which is a = 0.454 m2. A solution that takes into account the substantial variation of the heat transfer coefficients along the condenser gives the result A = 0.385 m2 (Webb and McNaught, in Chisholm, 1980, p. 98). 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

D. Variation of Local Heat Transfer Coefficient along the Surface... 

Nonuniform Surface Temperature. The previous section was devoted to uniform-temperature plates. In practice, however, this ideal condition seldom occurs, and it is necessary to account for the effects of surface temperature variations along the plate on the local and average convective heat transfer rates. TTiis is required especially in the regions directly downstream of surface temperature discontinuities, e.g., at seams between dissimilar structural elements in poor thermal contact. These effects cannot be accounted for by merely utilizing heat transfer coefficients corresponding to a uniform surface temperature coupled with the local enthalpy or temperature potentials. Such an approach not only leads to serious errors in magnitude of the local heat flux, but can yield the wrong direction, i.e., whether the heat flow is into or out of the surface. 

FIGURE 18.25 Variation of spraying density and heat-transfer coefficient along central axis of field of action of water jet on metal surface being cooled at water pressure before Sprayer of 1.5 MPa (after Urbanovich et al. [141]). 

Figure 17. Variation of heat transfer coefficient along the length of a membrane wall heat transfer in the pilot plant CFB combustor section at two different temperatures. Z is the distance from the top of the membrane surface, suspension density is approximately constant over the length of the wall, (Wu et al, 1987).

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