Convection transition region

In the bnlk, to the contrary, concentration gradients are leveled only as a result of convection, and diffnsion has practically no effect. In the transition region we find both diffnsional and convective transport. The concentration gradient gradnally falls to zero with increasing distance from the surface. 

Because the diffusion coefficient depends upon y over a much larger region than the interaction forces, a transition region must also be considered. Neglecting convection in this region, one may write... 

The tropical tropopause, essentially defined by the 380 K potential temperature surface (see Chapter 21), is located at about 16-17 km. Tropical convection occurs up to an altitude of about 11-12 km. Between these two levels lies a transition region between the tropopause and the stratosphere, call the tropica tropopause layer (TTL). Together with the lowermost stratosphere, the region is referred to as the upper troposphere/lower... 

Certainly, the most important convective heat-transfer process industrially is that of cooling or heating a fluid flowing inside a closed circular conduit or pipe. Diflferent types of correlations for the convective coefficient are needed for laminar flow below 2100), for fully turbulent flow above 6000), and for the transition region (/Vr between 2100 and 6000). 

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

The problem of burn-out prediction is a difficult one, and one on which a great deal of experimental work is being carried out, particularly in connection with nuclear-reactor development. Much of the earlier literature is rather confused, due to the fact that the mechanics of the burn-out were not carefully defined. Silvestri (S8) has discussed the definitions applicable to burn-out heat flux. It appears possible to define two distinctly different kinds of burn-out, one due to a transition from nucleate to film boiling, and one occurring at the liquid deficient point of the forced-convection region. The present discussion treats only the latter type of burn-out fluxes. The burn-out point in this instance is usually determined by the sudden rise in wall temperature and the corresponding drop in heat flux and heat-transfer coefficient which occur at high qualities. 

Clearly, the solution of this equation at forced-convection electrodes will depend on whether the fluid flow is laminar, in the transition regime, or turbulent. Since virtually all kinetic investigations have been performed in the laminar flow region, no further mention will be made of turbulent flow. The reader interested in mass transport under turbulent flow is recommended to consult refs. 14 and 15. 

Purely forced, purely free, and mixed convective regions in assisting flow over a vertical plate. (Based on results obtained by Patel K., Armaly B.F., and Chen T.S., Transition from Turbulent Natural to Turbulent Forced Convection Adjacent to an Isothermal Vertical Plate , ASME HTD, Vol. 324, pp. 51-56, 1996. With permission.)... 

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