Burnout in highly subcooled water flow boiling in small diameter tubes

Abstract

The present work deals with the critical heat flux (CHF) in subcooled flow boiling in short tubes. The field of application is in the very high heat flux region (up to 60 MW m −2) of interest to fusion technology (heat removal from divertors), which calls for a knowledge of the heat transfer under very high heat loading conditions. The experimental work was carried out with water at pressures ranging from 0.1 to 2.5 MPa and water velocities from 10 to 40 m s −1, employing stainless steel 2.5 mm i.d. tubes. The heated length was 0.1 m ( L/D = 40) and the wall thickness was 0.25 mm. The effects due to variation of thermal hydraulic parameters (velocity, subcooling, pressure) on the heat transfer are presented together with a comparison of the experimental data with existing correlations and theoretical models. The main result achieved in the experiment is the possibility of reaching such high values of the CHF using water in subcooled flow boiling inside smooth tubes. The parameters that seem to be determinant are the level of subcooling of the coolant and its velocity. Considering that other parameters, such as the tube diameter not investigated here, may have an influence on the CHF, it would also seem possible to come to the right compromise—with an optimized choice of parameters—between high values of the CHF and pressure loss involved (high with high velocity and small tube diameter) using this simple cooling technique.