Nanofluid Convection in Horizontal Annuli With Decoupled Thermal and Concentration Boundaries

ABSTRACT This study examines fully developed laminar nanofluid convection in a horizontal concentric annulus using the Buongiorno two‐component model, accounting for Brownian diffusion and thermophoresis. A new boundary configuration is introduced in which nanoparticle concentration is prescribed at the isothermal outer wall, while the inner wall is adiabatic, thereby decoupling concentration from the heat source. The nonlinear governing equations are solved as a boundary‐value problem with concentration‐dependent thermophysical properties and a reciprocal iteration scheme to enforce the target bulk concentration. Validation against the benchmark studies demonstrates good agreement for velocity, temperature, and concentration fields. Results show that, under the decoupled boundary condition, the average Nusselt number decreases monotonically with increasing Brownian‐to‐thermophoretic diffusivity ratio ( N BT ). Alumina nanofluids exhibit higher heat‐transfer performance than titania, while ternary nanofluids display nearly N BT ‐independent behavior. The pressure‐drop parameter depends strongly on nanoparticle concentration and axial forcing, approaching saturation at high N BT due to Brownian homogenization. These findings highlight the critical role of boundary placement in governing nanoparticle migration and thermal performance in annular nanofluid systems.