Three-dimensional Vorticity Effects on Extinction Behavior of Laminar Flamelets

TL;DR

This paper introduces a three-dimensional rotational flamelet model that accounts for vorticity effects, significantly extending flammability limits and improving understanding of flame behavior under shear and rotational influences.

Contribution

The study develops a novel 3D rotational flamelet model incorporating vorticity effects, enhancing predictive capabilities over traditional 2D models by capturing vital physics.

Findings

Vorticity extends flammability limits by up to 30%.

Vorticity reduces local strain rate in non-premixed flames.

Model reveals physics missing in 2D flamelet models.

Abstract

A recent rotational flamelet model is developed and tested with an improved framework of detailed chemistry and transport. The rotational flamelet model incorporates the effects of shear strain and vorticity on local flame behavior and is three-dimensional by nature. A similarity solution reduces the three-dimensional governing equations to ODEs involving a transformation to a non-Newtonian reference frame. A 9-species chemical kinetics model is used for H2-O2 combustion with non-reacting N2. Multiple flamelet cases including non-premixed, premixed, and partially-premixed flames are performed. Across all cases, vorticity extends flammability limits by up to 30% in terms of the ambient extinction strain rate and modifies both local flame structure and mixture composition. For non-premixed flames, where the location of minimum density coincides with the location of peak temperature, the centrifugal force induced by vorticity reduces the mass flow rate through the flame, effectively lowering the local strain rate. This increases residence time, thus extending flammability limits and reducing burning rates. This analysis is done also for premixed and partially-premixed flames. If minimum density lies between the flame zone and the fuel inlet boundary, centrifugal forces do not significantly modify flame behavior. Stable and unstable branches of S-curves for non-premixed and partially-premixed flames and stable branches for premixed flames show extended flammability limits due to vorticity. The capabilities of the rotational flamelet model reveal that vital physics are currently missing from two-dimensional, irrotational, constant-density, flamelet models. Improvements of detailed chemical kinetics, transport formulation, and thermo-physical properties bring the new flamelet model to par in these areas with existing models, while adding new features in terms of physical emulation.