In flow boiling, the nucleate and convective components are superimposed by a very complex mechanism, which so far is not well understood. Two models exist in present literature, one by Chen f3] (1963), using addition of the two components with a suppression factor; and one by Shah fB] (1976), using the "greater of" the two components with a Bo-number simplified correlation. Neither model presents a satisfactory solution, as attested by the numerous methods published since then, mostly based only on regression analysis-derived correction factors. In this article a new model, based on asymptotic addition of the two boiling components, is introduced. It follows the established principles offlow boiling and converges correctly to the extremes of all parameters. Tested on the University of Karlsruhe data bank containing over 13,000 data points in vertical flow boiling, results superior to previous correlations are demonstrated.
In order to select the appropriate correlations for prediction of horizontal tubeside condensation heat transfer coefficients, it is necessary to estimate what types of flow patterns exist at various points along the tube. The main criteria required are shown to be the ratio of shear to gravity forces on the condensate film and the ratio of vapor volume to liquid volume. A recently proposed prediction method by Taitel and Dukler is compared with observed flow regimes for condensation in horizontal tubes. The theoretically obtained parameters are shown to characterize the flow regimes well. Based on these parameters, a simplified procedure for prediction of local heat transfer coefficients for pure component condensation in horizontal tubes is proposed.
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