Low-grade heat utilization by supercritical carbon dioxide Rankine cycle: Analysis on the performance of gas heater subjected to heat flux and convective boundary conditions

Abstract

The design and optimization of gas heater in supercritical carbon dioxide Rankine cycles faces some challenges, among which an urgent one is the effect of thermal boundary condition on the performance of gas heater. This work focused on performance comparison and effects of major operating parameters under two most common thermal boundary conditions regarding to low-grade heat sources, by employing a modified Shear-Stress Transport model where a variable turbulent Prandtl formulation was incorporated. Results show that in the pseudo-critical region thermal boundary condition obviously affected the performance of supercritical carbon dioxide gas heater. Compared with the uniform heat flux condition, at convective boundary condition impairment occurred in both the local enhancement at high mass flux and local deterioration at low mass flux, due to the self-regulation in local heat input. A nearly uniform thermal field under convective boundary condition was achieved by increasing the mass flux of heat source fluid, while increasing the inlet temperature of source fluid was ineffective to that end. Opposite to constant-property fluid heater, flow arrangement dramatically affected axial profiles of local heat transfer coefficient while had a much weaker effect on local heat flux in supercritical gas heater. Temperature distribution of supercritical carbon dioxide along the heater was insensitive to the flow arrangement. Further studies reveal that thermal boundary effect was closely related to the buoyancy effect. Thermal boundary condition has a minimal effect on heat transfer of supercritical carbon dioxide when buoyancy effect is negligible. Under heavy influence of buoyancy, thermal boundary effect was obvious in the form of much weaker local deterioration under convective boundary. Finally, the Jackson Nusselt correlation was found applicable to the prediction of overall heat transfer rate under convective boundary condition, with relative deviations within ±15%.