Prediction models of critical heat flux for vertical downward flow

Abstract

Critical heat flux is one of the most important key parameters for various boiling systems, such as nuclear reactors and conventional boilers. Thus, many investigations have been reported, but most of these are concerned with the upward flow condition, and the phenomena under downward flow condition have not been fully understood. In this series of studies, the critical heat flux under the downward flow condition was classified into four mechanisms, i.e. dryout of falling liquid film, critical heat flux caused by the flooding, critical heat flux caused by the hydraulic instability and the liquid film dryout in annular flow regime. In addition, evaluation models of CHF were presented for each mechanism. Although these explanations showed the features of CHF under downward flow condition adequately, they were not suitable to explain the characteristics of CHF caused by the hydraulic instability. In this investigation, CHF of this region was formulated by two different models based on the comparison of the data in the experimental results obtained by using the tube with heated length of 200 mm and that of 400 mm. As a result, the model based on the Kelvin-Helmholtz instability was found to be suitable for relatively low mass flux condition. Under relatively high mass flux condition, the modified model of the departure from nucleate boiling (DNB) is proposed. These models show good agreement with the experimental results.