Experimental investigations on heat transfer to highly buoyant sCO2 flows in a large horizontal tube

Abstract

The horizontal layout is commonly employed for heat exchangers equipped within energy and power systems. However, experimental investigations on horizontal supercritical heat transfer are scarce, with limited published studies primarily focusing on small-diameter (below 10 mm) tubes. In this work, the flow and heat transfer experiments of horizontal sCO₂ within a large tube (22.14 mm) have been conducted under mass fluxes of $100-225\mathrm{kg}·m^{-2}·s^{-1}$, heat fluxes of $5-25\mathrm{kW}·m^{-2}$, inlet temperatures of $15-{45}^{°}C$ and pressures of $7.75-9\mathrm{MPa}$. The results indicate that buoyancy effects are significant for all the operating conditions. The ratio of ${\mathrm{Nu}}_{\mathrm{top}}/{\mathrm{Nu}}_{\mathrm{bottom}}$ gradually decreases as the buoyancy level strengthens. Further analysis reveals that the buoyancy-driven natural convection significantly enhances bottom heat transfer, with improvements even up to be eightfold; whereas for the top surface, buoyancy effects generally have a negative impact on heat transfer except within initial heating section where a limited number of enhancement data may occur. The wall temperature peak over the top surface induced by heat transfer deterioration has also been identified using the buoyancy parameter. In addition, the existing heat transfer correlations exhibit considerable deviations in predicting the experimental results, and new Nusselt correlations have been developed and validated for the sCO₂ heat transfer in this large size pipe with strong buoyancy.