Numerical assessment of scale-up criteria in a wall-cooled reactor for hydrogen combustion in supercritical water

A laboratory-scale wall-cooled reactor for hydrogen hydrothermal combustion was scaled up by factors of 10 and 100 using four scale-up criteria: constant velocity (CV), constant residence time (CRT), constant volume to jet momentum ratio (CM), and constant volume to jet kinetic energy ratio (CK). Computational fluid dynamics (CFD) simulations were then conducted to evaluate the performance of the scaled reactors. The results show that the overall flow field is predominantly controlled by the momentum ratio of the multiple jets, rather than the nozzle's internal flow regime, specifically the Reynolds number. Reactor scale-up weakens radial jet diffusion, slightly reducing local fuel–oxidizer mixing and suppressing the radial transport of high-temperature combustion products. The global residence time follows the order CV > CK > CM > CRT, consistent with reactor volume changes induced by scale-up. The peak temperature along the centerline exhibits a slight decrease and shifts downstream with increasing reactor size, due to reduced entrainment and weakened radial momentum exchange. Wall temperatures decrease under the CV criterion but increase under CRT, CM, and CK, reflecting the combined effects of heat transfer area per unit heat load and local fuel–oxidizer mixing intensity. Species concentrations in the reactor core remain nearly constant, with scale-up effects only marginally observed in the cooling water layer near the wall. The CV criterion is recommended for scaling up hydrothermal combustion, as it reduces peak and wall temperatures, thereby lowering material performance requirements for large-scale reactors.