CFD analysis of thermophysical characterization & performance optimization of a solar updraft tower utilising multi-composition gas mixtures

The thermophysical behaviour of the solar working fluid plays an important role on the performance of the solar chimney. This work presents the results of a systematic Reynolds-averaged computational fluid dynamics (CFD) study, with turbulence closed by the shear-stress-transport (SST) k –ω model, of the buoyancy-driven flow in an axisymmetric solar chimney with six industrially relevant binary gas mixtures namely Air–CO, Air–CO 2 , CO–CO 2 , CO 2 –SO 2 , Air–SO 2 and CO–SO 2 . Uniform wall heat flux of 1000 W/m 2 is applied and mole fractions are taken between 0 to 1 with an interval of 0.1, and inlet temperatures are taken between 300K to 900K which results in a total of 462 simulations performed in ANSYS Fluent 2023 R1. The specific heat (C p ), density (ρ), thermal conductivity (k) and dynamic viscosity (μ) are obtained directly from the ANSYS Fluent property database and depend on the composition and temperature. Eight performance measures are reported: Pr , Nu , Re , Gr , Ra , draft pressure ΔP , mass flux G, and a conductivity-bias-corrected Thermal Performance Index TPI = Nu / Pr 1/3 . Two salient trends found in the data during analysis and retained unchanged in the results are worth noting: the non-physical decrease of C p with inlet temperature for several pure-component points, and the molar-mass label anomaly in the CO 2 –SO 2 sheet, which raises the value of Pr toward unity at 300 K. The highest centre-line velocity (60.1 m s −1 at 900 K) is reached for the air–CO mixtures while the SO 2 -rich mixtures produce up to 45 % higher ΔP at 300 K. This pressure gain is overcome at higher temperatures (≈650 K), where the effect of the greater temperature rise of Air–CO outweighs its lower density. The apparent Nu superiority of SO 2 systems is shown to be a low-k normalisation artefact. The results are used for quantitative working fluid selection guidelines for industrial solar chimney, passive ventilation systems and flue gas stacks.