Subcooled Flow Boiling and Condensation

Abstract

<p>Fundamental problems in two different flow regimes; subcooled water-steam bubbly condensing flow and subcooled flow boiling, in vertical conduits under low pressure and mass flux conditions, were investigated.</p> <p>For subcooled water-steam condensing bubbly flow, experiments were carried out to obtain a data base for the axial distribution of area-averaged void fraction, interfacial area concentration, interfacial condensation heat transfer and bubble relative velocity. The interfacial area concentration was obtained by measuring the distributions of bubble volume and surface area, using high speed photography and digital image processing techniques, as well as the area-averaged void fraction, using a single beam gamma densitometer, at various axial locations. The interfacial heat transfer coefficient was obtained by monitoring the rate of change of the volume of individual bubbles as a function of local conditions. The data was used to develop new correlations for interfacial area concentration and interfacial condensation heat transfer coefficients. The applicability of the proposed correlations, as closure equations in existing two-fluid model based computer codes, was checked by including them in a two-fluid model to predict the axial void fraction profiles. The prediction of the current two-fluid model was compared with the measured axial void fraction profiles as well as available data from literature with good agreement.</p> <p>For subcooled flow boiling, experimental data on the axial profile of void fraction, wall superheat and liquid subcooling along the test section were generated for various levels of mass flux, heat flux and inlet subcooling. The high speed photographic results confirmed the fact that the bubble parallel, or normal, detachment from the heating surface is not the reason of the NVG phenomenon. The digital image processing technique was used to measure the bubble size distributions as function of the local conditions. A correlation for the mean bubble diameter as a function of the mass flux, heat flux and local subcooling was obtained. Physical mechanisms causing the NVG phenomenon were investigated using the high speed photographic results. A net vapour generation model was proposed. The proposed model was based on the balance between the vapour generation and condensation rates at this point. A two-fluid model for the axial void fraction profile in subcooled boiling flow was introduced. A heat flux division model was proposed. The proposed two-fluid model was reasonably capable of predicting the axial void fraction profiles in subcooled flow boiling, without the need for prior identification of the location of the NVG point.</p>