Void fractions in two‐phase steam‐water flow

Abstract

AbstractThe pressure‐drop characteristics associated with one liquid and one gaseous phase flowing concurrently in a pipe or tube have yet to be understood. The operation of evaporators, boilers, and condensers has long stimulated interest in the pressure drop of steamwater mixtures, and more recently this specialized case of one‐component, two‐phase flow has received even greater attention from the applications in cooling nuclear reactors. The two‐phase–flow problems have not been amenable to through theoretical analyses, and therefore empirical and semiempirical correlations have attained unusual prominence in practical applications. The present investigation employs a new research tool for the study of two‐phase–flow structure.A variety of geometric flow patterns is possible. Bergelin, Alves, and others have classified these patterns according to visual appearance; whereas the Martinelli classifications were based upon whether the flow in each phase was termedviscousorturbulent. The distinction between viscous and turbulent flow in either phase is rather arbitrary, and if the Reynolds number for one phase, calculated on the basis of the total tube diameter, is greater than 2,000, the flow in the phase is calledturbulent. This investigation is confined to the study of annular flow, in which most of the liquid is found in an annular ring surrounding the central vapor core and the flow in each phase is turbulent.Boiling or flashing occurs when superheated water rises in an insulated vertical tube at atmospheric pressure. For a separated two‐phase flow geometry, the mean linear steam velocity may exceed that of the water. The fraction of the tube occupied by the steam (void fraction) at a given cross section cannot be obtained directly from a determination of the thermodynamic quality. Void fractions, however, must be known for the estimation of the pressure drops due to head and momentum changes.Void fractions and pressure drops for steam‐water flows were measured in an 0.872‐in. I.D. vertical tube at atmospheric pressure over a quality range of 0 to 4%. The test section was the hot leg of a natural‐circulation loop, and the inlet liquid flow rate ranged from 1 to 3 ft./sec. A new technique for measuring void fractions was used, and the method utilizes the difference between the gamma‐ray absorption coefficients of water and steam.