A study of the effects of flashing inception and the formulation of a non-equilibrium model for maximum and minimum critical two-phase flow

A combined experimental and theoretical investigation of critical two-phase flow in horizontal tubes has been performed. The focus of the experimental study was to study the cause and the effects of flashing inception on critical two-phase flow. The results of this study led to the development of a non-equilibrium model for predicting critical flow rates over a wide range of conditions. Additionally, a dimensional analysis led to the development of a simple correlation for critical two-phase flow rates as well as a criteria for predicting the onset of flow instabilities.;It has long been recognized that a knowledge of both the location and the parameters which affect the onset of flashing inception during critical flow is crucial to the determination of the critical flow rate. At the time of this study, the conditions which influence the onset of flashing were not well understood. In the present experimental investigation a new method of accurately controlling the location of flashing inception during critical flow was developed. The method involved the use of a cavitating ring that could be easily positioned axially along the test section length. As a result, a systematic study of the effects of the flashing process on the critical flow of saturated and subcooled water through long tubes was undertaken. The results of varying the location of flashing inception for different stagnation conditions and test section lengths are presented. These results show that a range in the critical mass flux exists for each stagnation condition. This range was more pronounced at lower subcooled stagnation states and can be attributed to the variation in the location of flashing inception. Maximum critical flow rates occurred with flashing inception located near the pipe exit; while minimum critical flow rates occurred with the flashing front located further upstream. For a fixed location of flashing inception, the critical mass flux was found to increase with a decrease in the superheat at flashing inception. A decrease in this superheat also occurred as the location of flashing inception was moved upstream. Visualization studies reveal that the source of stable flashing inception nuclei was found to depend on the presence of available cavitation sites rather than pre-existing free steam heterogeneous and/or wall nucleation sites. In the absence of cavitation/nucleation sites without the cavitating ring), two-phase flow instabilities were observed over a wide range of stagnation conditions.;The results of the experimental investigation led to the development of two mathematical/computer models for predicting both the maximum and minimum critical mass flux.;In addition to the mathematical models, a simple correlation is proposed based on a dimensional analysis of the governing equations. The correlation also predicts the available data on critical two-phase flow well. (Abstract shortened by UMI.)...