Eventos Anais de eventos
COBEM 2023
27th International Congress of Mechanical Engineering
TRANSIENT BEHAVIOR OF DIFFUSION FLAMES: BUOYANCY INDUCING FLICKERING IN GROOVE GEOMETRY
Submission Author:
Guilherme Alvarez , ES
Co-Authors:
Guilherme Alvarez, Eduardo Reis Sampaio Filho, Fernando Filho Fachini, Leandro M. Fernandes
Presenter: Guilherme Alvarez
doi://10.26678/ABCM.COBEM2023.COB2023-0563
Abstract
This work describes the flickering phenomenon in groove diffusion flames, using a bi-dimensional flame model to analyze the influence of a large aspect ratio on the oscillation frequency. Diffusion flames present a particular transient behavior characterized by an oscillation along the flow direction with a specific range of frequencies. The flickering phenomenon occurs due to a global instability generated by the annular buoyancy flow induced by combustion heat release. The instability analyses done so far consider axisymmetric flames, e.g. jet flames and cylindrical pool fires. To study flames that present a parallel flow, the groove flame configuration was chosen. It consists of a rectangular pool fire with a large aspect ratio, a geometry that permits the occurrence of a bi-dimensional flow. The physical model considers the fuel stream depending only on the heat transport to the liquid surface. It also considers the Burke-Schumann limit. To validate the theoretical solution, the flickering frequencies are measured in grooves of different aspect ratios. The experimental setup consists of a coif, a test cell, and a suspended base, with a honeycomb in the entry of the setup in order to laminarize the incoming flow induced by the buoyant force. Image analysis is the technique applied to measure flame oscillation. This method is based on video recording of the flame captured by a digital camera, which is divided into frames to perform the analysis. N-heptane is used in the experiment due to its flame generating favorable images to the process and the data availability. The flame height in each frame is measured, which permits following its evolution and consequently determining the oscillation frequency. The measurements show the burning regime (flickering and puffing) and that the flame oscillation frequency depends on the groove width. For the groove with an aspect ratio of 2/5 the flame exhibits the flickering phenomenon and its frequency is 11.81Hz. However, between the aspect ratios 2/5 and 3/5, and the flickering disappears and puffing is observed, the frequency drops to 7.6Hz. For the lowest aspect ratio (4/5), which is almost a square container, the frequency decreases slightly (6.8Hz). Furthermore, due to hydrodynamic instability, the flame surface is disturbed with a spatial average flame height of 67 mm. A simplified model (infinitely fast chemical kinetics, constant transport and thermodynamic coefficients, and unitary Lewis number) is developed which is able to represent qualitatively a flame height of 91mm.
Keywords
Flickering, puffing, Groove flame, Diffusion Flame, aspect ratio
