Details

Autor(en) / Beteiligte

• Perakis, Nikolaos
• Haidn, Oskar J.
• Ihme, Matthias

Titel

Heat transfer augmentation by recombination reactions in turbulent reacting boundary layers at elevated pressures

Ist Teil von

• International journal of heat and mass transfer, 2021-10, Vol.178, p.121628, Article 121628

Ort / Verlag

Elsevier Ltd

Erscheinungsjahr

2021

Link zum Volltext

Quelle

Elsevier ScienceDirect Journals Complete

Beschreibungen/Notizen

• •A reacting boundary layer at elevated pressures has been studied using a set of DNS.•Exothermic chemical reactions are induced by the low enthalpy in the boundary layer.•The recombination reactions lead to wall heat loads 20% higher than in the inert flow.•The gas composition deviates strongly from the chemical equilibrium conditions.•A quenching of the composition is found in regions where the Da number drops below 1. A study of a reacting boundary layer flow with heat transfer at conditions typical for configurations at elevated pressures has been performed using a set of direct numerical simulations. Effects of wall temperatures are investigated, representative for cooled walls of gas turbines and sub-scale rocket engines operating with hydrocarbon as fuels. The results show that exothermic chemical reactions induced by the low-enthalpy in the boundary layer take place predominantly in the logarthimic sub-layer. The majority of the heat release is attributed to the exothermic recombination of OH and CO to produce CO2 and H2O. The recombination reactions result in an increase of the wall heat loads by up to 20% compared to the inert flow. The gas composition experiences strong deviations from the chemical equilibrium conditions. In fact, a quenching of the major species is observed within the viscous sub-layer and the transition region. Analysis of chemical time-scales shows that the location of quenched composition coincides with the region where the Damköhler number decreases below unity. Within the viscous sub-layer, a secondary reaction zone is detected, involving the production of formyl and formaldehyde radicals that provide an additional source of energy release. The analysis of the reaction paths showed that reactions with zero activation energy are responsible for this change in gas composition, which also account for the initial branching of hydrocarbon fuels decomposition according to previous auto-ignition studies. The effect of the secondary recombination reactions is more prominent for the lower wall temperature case. Finally, the role of turbulent fluctuations on the species net chemical production rates is evaluated, showing a strong correlation between species and temperature fluctuations. This leads to a pronounced deviation of the mean reaction rates ω˙(Yk,T)¯ from the reaction rates obtained under the assumption of laminar finite rate.