Analytical and numerical study of inlet velocity in radiation–convection duct systems

This research presents an analytical model for predicting the inlet air velocity in a radiation–convection coupled duct system. The proposed model integrates heat balance, mass conservation, and momentum equations, incorporating radiative heat transfer, temperature-dependent viscosity, and buoyancy effects. The derivation includes a viscosity-corrected term based on Sutherland’s law and incorporates viscous pressure losses due to high-temperature flow conditions. To validate the theoretical predictions, computational fluid dynamics simulations using a k–ε shear stress transport turbulence model with radiative–convective coupling were conducted across seven thermal reservoir temperatures (Tb = 750–1050 K). Results show that the proposed theoretical model maintains a deviation below 6.2% from the simulated inlet velocities. The model accurately captures flow regime transitions and velocity suppression due to increased viscosity, providing a robust foundation for the thermal-fluid design of solar duct systems. These findings support compact system configurations (H/D < 10) and efficiency enhancement (η > 5%) through geometric and material optimization.