Turbulent heat transfer measurements of supercritical CO₂ in a horizontal tube at relatively low Reynolds numbers

Abstract

Supercritical fluids have captured global attention due to their extraordinary properties, such as high heat transfer coefficients, solubility and heat storage capacity. Due to the requirement for more effective power generation systems, supercritical Brayton cycles were proposed because of its high potential for the development of high thermal efficiency power cycles with no need for condensers. It is known that small fluid temperature and pressure variations near critical point or pseudocritical regions, result in significant changes to the thermo-physical properties of the fluid. However, the available correlations for convection heat transfer do not show sufficient agreement with the available experimental data to simplify industrial design except in very limited conditions. Therefore, further experimental investigations are still required to better understand the thermal and hydraulic behaviors of the supercritical fluids before they can be widely used in industrial applications. In the newly developed test facility in Texas A&M University at Qatar, forced convection heat transfer of supercritical CO₂ in a circular horizontal tube at relatively low Reynolds numbers (Re <105) is investigated experimentally in a straight tube with an inner diameter of 8.7 mm. Experiments are carried out for different mass flow rates, fluid inlet temperatures, system pressures, and semi-local heat transfer coefficients, which are recorded at several locations in the pipe with constant heat flux. The influence of the fluid bulk temperature and pressure on the forced convection heat transfer in the tube was then recorded and compared to widely used empirical correlations. The results indicate that the effect of buoyancy on the heat transfer coefficient cannot be ignored in the near critical or pseudocritical region of fluids for this flow geometry. This dependency is believed to be due to extreme dependence of fluid properties on temperature in this region. The results suggest that the heat transfer correlation should include the buoyancy effects especially near the pseudocritical region. Therefore, new empirical correlations in horizontal pipes is proposed based on our experimental results of the heat transfer measurements. An effort is undertaken to design an inclined pipe test section for the existing facility to further study the effect of buoyancy of this flow.