Experimental study of heat transfer and flow instabilities in heated channels with supercritical CO2

Heating and cooling may lead to flow instabilities in systems with single-phase natural circulation loops, two-phase boiling channels and various kinds of channels containing fluids at supercritical pressure. Previous experience and theories developed for two-phase flow instabilities can be used to study the flow excursions and oscillations of supercritical fluids due to the similar basic features, which is more interesting in the supercritical water-cooled reactors. Many numerical models have been developed to simulate heated channels with increased heating power to capture the threshold heating power and the excursion or oscillation effects on the mass flow rates and temperatures. However, very few experimental studies have been found for flow instabilities of supercritical fluids. This paper described a closed experimental system with CO2 as the working fluid. The system stability with supercritical CO2 was tested using the constant pressure output mode and the constant mass flow rate output mode of a supercritical pump, with different external characteristics. By programming the input voltage and current on the experimental channel, CO2 was heated by a linearly or stepwise increasing heating power. The internal characteristic of the heated channel, i.e., the relationship between the pressure drop of the channel and the mass flow rate, was obtained. Flow excursion of a sudden decrease in the mass flow rate, which is called the Ledinegg instability was found in the experiment. The convection heat transfer characteristics and the influence of the buoyancy force and the flow acceleration on the heat transfer were analyzed for condition of the Ledinegg instability