A critical review of the mechanisma, models arui heat-tr~~sfer correlations for nucleate boiling is presented,. The importance of nucleation is stressed.. It is pointed. out that the bulk of correlations are based on the erroneous notion that the energy transferred away from the heating surface by bubbles as such, in the form of latent heat, contribute$ insignificantly to the total flux. - A critical review of the correlations for the upper limit of the nucleate boiling regime - the peak flux is also presentede The theory of nucleation during boiling is reviewed and extended in an attempt to explain the complete absence of the nucleate regime, noted in preliminary testa conducted at low pressures. The following equation charaoteriz,ing nucleation is derived rc = <J(Tw + Ts) vV A.v(Tw - T s) J where r is the mouth radius of the smallest cavit,y for vmich nucleation can pro&eeil. under the conditions reflected in the right-hand. terms of the equation; T is the {absolute) wall temperature'; T is the (absolute.) saturltion temperature corresponding to the s§stem presaure; 0'" is the surface tension; A.v is the latent heat of vaporization; vV is the specific volume of the vapour; (all physical properties evaluated. at (Tw + Ts)/2.) j and J is the mechanical equivalent of heat. ol The nucleation mechanism embod.i.e.s in this equation is the one according to which the nucleation superheat for ap!.t'ticular surfta.c.e caYity is determined by the equilibrium (expressed by the Gibbs equation) of a vapour cap formed. over a cav:1-ty mouth, with the bubble radius of curvature. equal to the cavity mouth radius. Experimental data are reported for benzene and ethanol boiling from the same surfa{}e; at various pressures. At atmospheric ancl moderate subatmospheric pressures the usual stable boiling curves are obtained. In an intermediate region a hitherto unreported, permanently unstable, nucleate boiling regime exists, which is characterized by erratic bubble behaviour and by boiling curves of unusunl. shape. At even lower pressures the nucleate regime is altogether absent, the heat-transfer changing from natural. convection straight to film boiling as the wall superheat is raised.. Application of the nucleation equation to the stable runs andl to the point of disappearance. of the last bubble on a stable boiling curv~- (flux decreasing) allows the prediction of the size range of surface cavities available for nucleation, A method is given to determine the point where this size range becomes zero, and. where the above mechanism ceases to operate. Comparison with the experimental data shows reillmzkab~y quamtitrntively that when th:i.s mechanism ceases to operate - a, situation that can occur in the middle of a. boiling curve - nucleate boiling changes from stable to unstable and the bubble behaviour becomes, erratic:. Thus a. method of predicting the threshold of unstable nucleation which becomes operative before complete cessation of nuc~eation occurs, is presented. Photomicrographs of the heating surfac_·e employed. are shown, which illustrate the existence of cavities of such sizes as are required by the theory.

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