Experimental Study of Falling Water Limitation under a Counter-Current Flow in a Vertical Rectangular Channel : 1st Report, Effect of Flow Channel Configuration and Introduction of CCFL Correlation

Abstract

Counter-current-flow limitation (CCFL) experiments were carried out for both vertical rectangular channels and vertical circular tubes varying in size and in configuration of their cross sections to clarify CCFL characteristics in the vertical rectangular channels. Quantitative understanding of critical heat flux (CHF) in a narrow vertical rectangular channel is required for the thermohydraulic design and safety analysis of research nuclear reactors in which flat-plate-type fuel is employed. Critical heat flux under downward low velocity is closely related to falling water limitation under a counter-current flow. Experimental results showed that the equivalent hydraulic diameter de, i.e., the width, and gap of the channel play an important role in determining the CCFL characteristics of a rectangular channel. However, a significant influence of channel length on CCFL characteristics was not observed in the ranges investigated. Using new dimensionless parameters, the authors propose a correlation for predicting the relationship between upward air velocity and downward water velocity based on the present experimental results.