<p>The growth of carbon nanotube (CNT) forests by chemical vapor deposition (CVD) is governed by a complex interplay between transport phenomena, reaction kinetics, and catalyst evolution. In this study, we investigate position-dependent CNT growth within a quartz tube reactor by combining experiments with reactor-scale multiphysics modeling. Velocity, temperature, and precursor concentration fields near the substrate surface were resolved, and a temperature-dependent surface consumption flux was introduced to quantify local reaction propensity. The results show that while the flow field remains nearly uniform, both temperature and reaction-driven precursor consumption vary with position, leading to significant differences in the Damköhler number. Although the highest Damköhler number and reaction flux are obtained at the hotter upstream positions, the experimentally measured CNT height increases toward the cooler downstream edge of the hot zone. To resolve this discrepancy, catalyst morphology was analyzed using position-resolved SEM and temperature-dependent AFM. The results reveal temperature-induced catalyst coarsening and broadening of particle size distributions, which reduce the density of active growth sites at elevated temperatures. These findings demonstrate that final CNT height is not determined solely by instantaneous reaction kinetics, but by the balance between reaction intensity and catalyst stability.</p> Graphical abstract <p></p>