Kinetic modelling of High Density PolyEthylene pyrolysis: Part 2. Reduction of existing detailed mechanism

Nowadays, a great effort in hybrid rocket technology is being made in order to develop Computational Fluid Dynamics (CFD) models to investigate the coupled fuel pyrolysis and by-products combustion phenomena. Detailed chemistry must be considered to determine with accuracy the chemical induction delays which play a major role on the stability of the system. A highly detailed High Density PolyEthylene (HDPE) pyrolysis mechanism has been selected (7541 reactions and 1014 species) to be reduced. A new mechanism is created and validated over a temperature range from 700 K to 1200 K and for pressure from 1 bar to 100 bar. Two methods have been successively applied (Detailed Reduction and Direct Relation Graph with and without Error Propagation). Several sets of species have been defined to ensure the validity and the appropriateness of this mechanism with another existing detailed one for combustion. The final mechanism (1713 reactions and 472 species) is demonstrated to be the optimal one, according to the validation criteria (accuracy of 5 mol% on HDPE consumption and on C2H4 formation). The computation cost is reduced by one order of magnitude. This kinetic scheme is successfully implemented in a two dimensions CFD code. Its accuracy is determined quantitatively on the species content under steady-state and transient conditions. The reduction work allows quantifying the importance of β-scission reactions compared to backbiting and random scission reactions while dienes, alkanes and alkenyl radicals have been mostly removed due to their limited impact.