ABSTRACT

Critical heat flux (CHF) is one of the key design factors for boiling heat transfer equipment. Thus, CHF, especially under upward flow condition, has been widely investigated, but the characteristics of CHF for downward flow are quite different from for upward flow. In downward flow, the complex flow structure is formed due to the counter force between the buoyancy and inertia. In view of thermal-hydraulic characteristics, downward flow should be avoided in boiling equipment. However, in a research reactor, downward flow is applied as a simplified cooling method of the reactor core. The objective of this investigation is to grasp the influence of the tube diameter on CHF in downward flow, because CHF in downward flow is closely related to the flow structure, which is influenced by the diameter and the flow velocity. The CHF experiment was carried out with a forced convective boiling system by using various inner diameter tubes in upward and downward flows. On the basis of the obtained pressure drop of the test section, behavior of inlet fluid temperature, and the location of CHF, the experimental CHF could be classified into four modes, i.e., complete dryout of falling liquid film, CHF due to flooding, CHF caused by hydraulic instability, and liquid film dryout in annular flow. These CHF are discussed by using the CHF model based on the regime-based modelling.