The Impacts of Three Flamelet Burning Regimes in Nonlinear Combustion Dynamics

TL;DR

This study uses advanced simulations to analyze how different flamelet burning regimes affect nonlinear combustion dynamics and pressure stability in liquid rocket engines.

Contribution

It introduces a comprehensive simulation approach combining DDES turbulence and CFPV combustion models to explore three distinct pressure instability regimes.

Findings

Pressure oscillation amplitude decreases with fewer local extinctions

Full S-curve combustion modeling improves agreement with experimental data

Coupling between heat release rate and pressure influences instability behavior

Abstract

Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-simulation (DDES) turbulence model with the Compressible Flamelet Progress Variable (CFPV) combustion model. Three different pressure instability domains are simulated: completely unstable, semi-stable, and fully stable. The different instability domains are found by varying the combustion chamber and oxidizer post length. Laminar flamelet solutions with a detailed chemical mechanism are examined. The $\beta$ Probability Density Function (PDF) for the mixture fraction and Dirac $\delta$ PDF for both the pressure and the progress variable are used. A coupling mechanism between the Heat Release Rate (HRR) and the pressure in an unstable cycle is demonstrated. Local extinction and reignition is investigated for all the instability domains using the full S-curve approach. A monotonic decrease in the amount of local extinctions and reignitions occurs when pressure oscillation amplitude becomes smaller. The flame index is used to distinguish between the premixed and non-premixed burning mode in different stability domains. An additional simulation of the unstable pressure oscillation case using only the stable flamelet burning branch of the S-curve is performed. Better agreement with experiments in terms of pressure oscillation amplitude is found when the full S-curve is used.