Effects of multidimensional transport and buoyancy on the cool flame dynamics in a counterflow burner

Abstract

The one-dimensional simulation of counterflow cool flames with the plug flow assumption ignores the effects of gravity, buoyancy, and the radial velocity gradient, leading to a flow field that deviates from measurements. A two-dimensional (2D) axisymmetric multi-physics model is developed within the OpenFOAM framework, considering buoyancy and multidimensional transport to simulate the opposed flow diffusion flame of dimethyl ether operating in the cool flame regime (i.e., low-temperature combustion). Flame structures and extinction limits of diffusion-cool flames are simulated and compared with measurements and 1D simulations. While the classical 1D model (i.e., without any buoyancy effect) predicts the stagnation plane forming at the mid-plane, the 2D model with buoyancy effect predicts the formation towards the upper fuel side nozzle, deviating by ∼ 23% compared to the buoyancy-free case. The extinction limit of the cool flame has been studied at atmospheric pressures of 1, 3, and 5 atm. The 2D model allows the velocity to be perturbed at the nozzle exit without imposing any radial velocity gradient, which enables the flame to sustain higher strain rates than the 1D prediction. For 1 atm cases, predictions from the 1D and 2D models deviate from the measurements at higher fuel loading conditions (i.e., > 0.48); however, the 2D model performs significantly better. For a fuel loading of = 0.525, the difference between the measurements and the 1D model is ∼40 s⁻¹ (∼30%), while the 2D model prediction is within 14% (∼19 s⁻¹) of the measured extinction strain rates. The models predict that an increase in pressure shifts the cool flame to higher strain rates, resulting in higher cool flame extinction strain rates. At elevated pressure, the deviation between the models decreases as buoyancy effects become less pronounced. The Richardson number is identified as a critical parameter for characterizing the counterflow cool flame configuration, where the competition between buoyancy and flow inertia determines the flame location, especially when the system deviates from the classical one-dimensional assumption.