Numerical investigation on the impact of circumferentially varying temperature on the buoyancy and heat transfer characteristics of supercritical CO2 natural circulation loop

Abstract

The non-uniform heating in the supercritical natural circulation loop (SCNCL) and its associated heat transfer behaviour and buoyancy variation are not well understood. Since buoyancy is the primary force that drives fluid motion in the SCNCL, any factor affecting buoyancy will have an impact on overall performance. The present study investigated the circumferentially varying temperature (CVT) at the source, compared it to constant uniform temperature conditions, and examined the associated variation in buoyancy, and heat transfer characteristics. This study establishes a numerical model for a three-dimensional circular cross-section rectangular SCNCL. The SCNCL was examined at different pressures ranging from 8 MPa to 12 MPa, with 1 MPa pressure increments, to cover a wide range of applications. The results indicated that at 12 MPa pressure, the heat transfer rate and heat transfer coefficient decreased by a maximum of 14 % and 50 %, respectively, compared to that of constant uniform temperature. In contrast, the mass flow rate shows minimal variation between the CVT and a constant temperature condition. The fluid density increases significantly at higher pressure, resulting in decreased buoyancy generation in the loop. Therefore, for all cases, velocity increases up to 10 MPa and then begins to decrease. Thus, the peak velocity was observed at 10 MPa pressure. The CVT negatively impacts both horizontal and vertical heater configurations, leading to a reduction in overall performance. Additionally, non-uniform heating has a greater impact on the horizontal heater configuration compared to the vertical heater configuration.