Mini-Channel Supercritical CO2 Heat Transfer Measurements for Brayton Cycle Regenerators

Abstract

Recently, it has become increasingly important to improve efficiency and reduce capital costs in nuclear power plants. This prompted significant work in studying advanced Brayton cycles for high temperature energy conversion. A particular improvement in the operation of an advanced carbon dioxide cycle, is the use of compact, highly efficient, diffusion bonded heat exchangers for the recuperators. These heat exchangers operate near the pseudo-critical point of liquid carbon dioxide, making use of the drastic variation of the thermo-physical properties. This paper focuses on the experimental measurements of heat transfer and pressure drop characteristics within mini-channels. Two test section channel geometries are studied: a straight channel and a zig-zag channel. Both configurations are 0.5m in length and constructed out of 316 stainless steel with a series of nine parallel 1.9mm semi-circular channels. The zig-zag configuration has an angle of 115 degrees with an effective length of ∼0.58m. Heat transfer measurements are conducted for varying ranges of inlet temperatures, pressures, and mass flow rates. Local and average heat transfer coefficients near the critical point are determined from measured wall temperatures and calculated local bulk temperatures.