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

TL;DR

This study uses direct numerical simulations to analyze heat transfer enhancement due to recombination reactions in turbulent reacting boundary layers at high pressures, revealing significant effects on heat loads and species composition.

Contribution

It provides new insights into the role of recombination reactions and turbulence in heat transfer and chemical composition in high-pressure boundary layers.

Findings

Recombination reactions increase wall heat loads by up to 20%.

Gas composition deviates significantly from equilibrium, with quenching observed.

Secondary reaction zones involve radical production, affecting energy release.

Abstract

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 Damkoehler 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.