Experimental Investigation of Critical Heat Flux and Post-Critical Heat Flux in a Carbon Dioxide Cooled Tube at High Subcritical Pressures.

Abstract

Supercritical fluids such as water or CO₂ are an attractive choice in various engineering applications (e.g. in energy conversion systems or nuclear power systems such as SCWR or SMART reactor concept). The exact knowledge of the heat transfer in a trans-critical pressure is important for the design of those systems. In subcritical pressure especially knowing how to avoid the reduction of heat removal when exceeding the critical heat flux (CHF).

A series of experiments are carried out with the SCARLETT (Supercritical CARbon dioxide Loop at IKE StuTTgart) test facility at the Institute of Nuclear Technology and Energy Systems of the University of Stuttgart, focusing on investigations of CHF and post-CHF heat transfer with CO₂ as a working fluid. Four test sections have been used – each with a different internal diameter. In the test section with a 4 mm internal diameter and a heated length of 2000 mm and the test section with 8 mm internal diameter and heated length of 1640 mm, the wall temperature was monitored every 40 mm along the tube with Pt100 sensors. The other two test sections with 10 mm and 6 mm internal diameters and heated lengths of 2000 mm were equipped with glass fiber in a stainless-steel capillary for semi-continuous wall temperature measurement and thermocouples for the reference temperature measurement. All test sections were instrumented accordingly for pressure, differential pressure, mass flow, inlet, and outlet temperature measurements. The paper shows the experimental results in vertical upward flow with reduced pressures of Π>0.7, mass fluxes ranging from 250 to 3000 kg/m²s, and heat fluxes from 40 to 200 kW/m².