Some Theoretical Aspects of Vapor Void Formation in Heated Vertical Channels

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Abstract
Fundamental flux vectors have been obtained for the diffusion of bubbles in heated channels by considering bubble motion in a turbulent liquid as a Markoff process. These flux vectors lead to a nonlinear partial differential equation representing the void fraction, which has been linearized for the case of small void fractions and coupled to a similar partial differential equation governing heat flow into the liquid phase. The coupled differential equations are transformed into coupled integral equations which are solved to obtain axial void fraction and temperature distributions in a heated channel. The rate of vapor production at the wall and the rate constant for bubble growth have been calculated from experimental data on void fraction distributions at constant uniform flux. The model predicts the correct shape for the void fraction distribution curve as well as providing a plausible explanation of burnout phenomena in terms of the bubble slip velocity.

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