CRITICAL HEAT FLUX IN AXIALLY NON-UNIFORM-HEATED CHANNELS

Experiments on critical heat flux (CHF) have been performed with directly heated tubes, annuli, and rod-bundles having uniform or non-uniform axial power profiles. These test sections were cooled with either Freon or water flow at high pressure and high-flow conditions. CHF (dryout) occurrence was detected using either fixed thermocouples, installed along the heated tubes and annuli, or moveable thermocouples inside rod-bundle geometries. The test-section power corresponding to the initial CHF occurrence has been referred to as the critical power or dryout power. Simultaneous CHF occurrences at several locations were observed at some flow conditions and axial power profiles. Local CHF values were calculated using the measured critical power and dryout locations. Two approaches in correlating CHF values were examined for various non-uniform axial power profiles. Applying the local CHF approach tends to result in significant data scatter leading to increased prediction uncertainty. The boiling-length approach appears to collapse the CHF data in consistent and systematic trends, and hence results in an improved prediction accuracy of the critical power.