Wind-aided

For turbulent wall fires, from the work of Ahmad and Faeth [18], it can be shown that 

The radiation dependence has been simplified using only Xr, but in general it is quite complex and significant for tall wall flames ( 1 m). 

FIRE SPREAD ON SURFACES AND THROUGH SOLID MEDIA 

Example 8.1 It is found that flame length on a vertical wall can be approximated as 

Take Q = 425 kW/m2 for the foam burning in air at 25 °C. The adjacent surface immediately outside the spreading flame is heated to 225 °C. 

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In addition to George s acclaimed work with locomotives, he and his son Robert designed numerous contraptions such as a scarecrow with wind-aided arms, a sundial, an oil light that burned underwater, and an automatic cradle rocker. While George... 

Since accidental fire spread mostly occurs under natural convection conditions within buildings and enclosures, some examples of configurations leading to opposed or wind-aided types of spread are illustrated in Figure 8.3. Flame spread calculations are difficult... 

This follows by a steady state energy balance of the surface heated by qe, outside the flame-heated region S. It appears that a critical temperature exists for flame spread in both wind-aided and opposed flow modes for thin and thick materials. Tstmn has not been shown to be a unique material property, but it appears to be constant for a given spread mode at least. Transient and chemical effects appear to be the cause of this flame spread limit exhibited by 7 smln. For example, at a slow enough speed, vp, the time for the pyrolysis may be slower than the effective burning time ... 

Thus, for opposed flow spread, the steady state thermal flame spread model appears valid. In wind-aided flame spread, it seems appropriate to modify our governing equation for the thermally thin case as... 

From Figure 8.8, Vf > vp, so cbcf/dxp > 1. Hence, transient effects will cause a lower speed for wind-aided spread. Again, since the gas phase response time is much faster than the solid, we can use steady gas phase results for qf and <5f in these formulas for flame spread on surfaces. 

Consider the control volume in Figure 8.9 where the thermally thick case is drawn for a wind-aided mode, but results will apply in general for the opposed case. The control... 

Loh and Femandez-Pello [8] have shown that the form of Equation (8.30) appears to hold for laminar wind-aided spread over a flat plate. Data are shown plotted in Figure 8.17 and are consistent with... 

Thus, we can replace u00 in Equation (8.36) and apply it to both opposed and wind-aided cases. For upward or wind-aided spread the speed increases as cos (f> increases to the vertical orientation. For downward or opposed flow spread, the speed is not significantly affected by changes in until the horizontal inclination is approached for the bottom orientation (—90 < stagnation plane flow results from the bottom. Figure 8.19 gives sketches of the... 

Thomas [21,22] has analyzed and correlated data for porous, woody material representative of forest debris, grass fields, cribs and even blocks of wooden houses. He finds that vp Moo/pb for the wind-aided case, and vp p[[1 for natural convection, where pb is the bulk density of the fuel burned in the array. Figure 8.20 shows the wind-aided data and Figure 8.21 is the no-wind case. From Thomas [21,22], an empirical formula can be shown to approximate much of the data ... 

Figure 8.20 Wind-aided fire spread through porous arrays as a function of wind speed [21]...

Flames can spread over a solid surface in two modes. In the wind-aided flame spread mode, flames spread in the same direction as the surrounding airflow. The second mode is referred to as opposed-flow flame spread, which occurs when flames spread in the opposite direction of the surrounding airflow. These two modes are illustrated for flame spread over a flat surface in Figure 14.2. 

By far the most common and most practical approach to measure the rate of flame spread over a flat surface involves recording the location of the flame tip (wind-aided spread) or flame front (opposed-flow spread) as a function of time based on visual observations. However, in the case of wind-aided flame spread, it is very difficult to track propagation of the pyrolysis front (boundary between the pyrolyzing and nonpyrolyzing fuel) as it is hidden by the flame. This problem can be solved by attaching fine thermocouples to the surface at specified locations as ignition results in an abrupt rise of the surface temperature. This approach is very tedious and not suitable for routine use. An infrared video camera has been used to look through the flame and monitor the upward advancement of the pyrolysis front in a corner fire.62... 

As mentioned earlier, the fire hazard of interior finish materials is primarily due to the potential for rapid wind-aided flame spread over the surface. It is therefore not a surprise that reaction-to-fire requirements for interior finish materials in U.S. building codes are primarily based on performance in a wind-aided flame spread test. The apparatus of this test is often referred to as the Steiner tunnel. The Steiner tunnel test is described in ASTM E 84. Although the test does not measure any material properties that can be used in a model-based hazard assessment, a discussion of the test is included here due to its practical importance for the passive fire protection of buildings in the United States. 

Flame spread (opposed flow and wind-aided flame spread, fire growth rate, etc.)... 

The Steiner Tunnel Test. The primary intent of the steiner tunnel test is to quantify the wind-aided flame spread propensity of the material tested. It is... 

R Macedonio, L. Katzirc, N. Geismac, S. Simone, E. Drioli, and J. Gibon, Wind-Aided Intensified evaporation (WATV) and Membrane Crystallizer (MCr) integrated brackish water desalination process Advantages and drawbacks. Desalination 273 (2011) 127-135. 

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