Numerical investigation on reversible reactive flow inside ribbed channels with different inclined angles

Abstract

In accordance with the United Nations Sustainable Development goal #7 – affordable and clean energy, the concept of reversible reactive flow (N₂O₄/NO₂) inside ribbed channel is proposed for low-temperature waste heat recovery. Quasi direct numerical simulations are performed to reveal the relationship between flow, heat/mass transfer, and chemical characteristics with different rib inclined angles (90° and 45°). The analyses indicate that the reaction of N₂O₄ ⇌ 2NO₂ has limited influence on flow patterns inside the ribbed channel, but intensifies the heat transfer considerably. For the 90° reactive case, the enhancement of Nusselt number reaches 112.7 % when Reynolds number is 2000. Although non-equilibrium thermal-chemical phenomenon is observed by instantaneous snapshots, time-averaged results show that the forward endothermic reaction is concentrated close to the heated wall. The flow structures transport fluid pocket consisting of “overheated” gas and triggers local backward exothermic reaction, which decreases the thickness of thermal boundary layer and thereby intensifies the overall heat transfer. For the 45° inclined reactive case, a flow circulation at local equilibrium between heat release and absorption is formed by the rib-induced large-scale vortices. The comprehensive thermal performance is further improved by 24.6 % compared to the 90° reactive case, which attributes to higher Nusselt number and lower friction loss.