Regions forced-convection, boiling

For the saturated boiling region, the power-law interpolation method has been used by, for instance, Steiner and Taborek [259] and Wattelet [262]. Steiner and Taborek deal with forced convective boiling in the quality region and hence use Eq. 15.226 as a basis. Their expression is as follows ... 

C4. Clerici, G. C., Garriba, S., Sala, R., and Tozzi, A., A catalogue of the commonest burn-out correlations for forced convection in the quality region, paper presented at Symp. Boiling and Two-Phase Flow, EURATOM, ISPRA, June, 1966. 

Single-phase flow region at the inlet the liquid is below its boiling point (sub-cooled) and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat-transfer coefficient in this region. 

Sub-cooled boiling in this region the liquid next to the wall has reached boiling point, but not the bulk of the liquid. Local boiling takes place at the wall, which increases the rate of heat transfer over that given by forced convection alone. 

The values of the constants that have been determined by various researchers are given in Table III. In all cases Eq. (76) is based on data from both the nucleate boiling and the two-phase forced-convection regions. 

Referring to Fig. 12, it is clear that not all the three regions need necessarily be present when a liquid is boiled in a tube. For instance, the exit velocity of the two-phase mixture may never exceed that required for two-phase forced convection to take place, or the heat flux may be such that with an inlet velocity F, nucleate boiling cannot occur and only forced convective heat transfer takes place. 

For the forced convective region, only limited data are available on the effects of the different variables involved, since the existence of this region has only been recognized recently. As previously mentioned, the velocity required for the suppression of nucleate boiling increases with pressure further, an increase in pressure reduced the specific volume of the vapour and hence the linear velocity of the two phase mixture at a given quality will be reduced. Thus higher velocities and steam qualities would be required for the forced convective region to be entered at the same heat flux. The effect of diameter is, as far as can be seen from the work of previous experiments and from these experiments, that to be expected with convective heat transfer, namely, that the coefficient is proportional to the diameter or the equivalent diameter to the power —02. 

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. 

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