AUTHOR=Tang Wei , Finney Mark , McAllister Sara , Gollner Michael 

TITLE=An Experimental Study of Intermittent Heating Frequencies From Wind-Driven Flames

JOURNAL=Frontiers in Mechanical Engineering

VOLUME=5

YEAR=2019

URL=https://www.frontiersin.org/journals/mechanical-engineering/articles/10.3389/fmech.2019.00034

DOI=10.3389/fmech.2019.00034

ISSN=2297-3079

ABSTRACT=<p>An experimental study was conducted to understand the intermittent heating behavior downstream of a gaseous line burner under forced flow conditions. While previous studies have addressed time-averaged properties, here measurements of the flame location and intermittent heat flux profile help to give a time-dependent picture of downstream heating from the flame, useful for understanding wind-driven flame spread. Two frequencies are extracted from experiments, the maximum flame forward pulsation frequency in the direction of the wind, which helps describe the motion of the flame, and the local flame-fuel contact frequency in the flame region, which is useful in calculating the actual heat flux that can be received by the unburnt fuel via direct flame contact. The forward pulsation frequency is obtained through video analysis using a variable interval time average (VITA) method. Scaling analysis indicates that the flame forward pulsation frequency varies as a power-law function of the Froude number and fire heat-release rate, <inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mi>F</mml:mi></mml:mrow></mml:msub><mml:mover accent="true"><mml:mrow><mml:mtext> </mml:mtext></mml:mrow><mml:mo>~</mml:mo></mml:mover><mml:msup><mml:mrow><mml:mrow><mml:mo stretchy="true">(</mml:mo><mml:mrow><mml:mi>F</mml:mi><mml:mi>r</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mrow><mml:msup><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mo stretchy="true">)</mml:mo></mml:mrow></mml:mrow><mml:mrow><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. For the local flame-fuel contact frequency, it is found that the non-dimensional flame-fuel contact frequency <inline-formula><mml:math id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> remains approximately constant before the local Ri<sub><italic>x</italic></sub> reaches 1, e.g., attached flames. When Ri<sub><italic>x</italic></sub> &gt; 1, <inline-formula><mml:math id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> decreases with local as Ri<sub><italic>x</italic></sub> flames lift up. A piece-wise function was proposed to predict the local flame-fuel contact frequency including the two Ri<sub><italic>x</italic></sub> scenarios. Information from this study helps to shed light on the intermittent behavior of flames under wind, which may be a critical factor in explaining the mechanisms of forward flame spread in wildland and other similar wind-driven fires.</p>