I was considering the gasoline vapor ignition question further and thought that a simple math example of the effect would shed light upon the subject. An intense amount of local heating is generated when a small region of vapor mix is ignited by some means. This released heat energy then proprogates outward from the ignition volume into the surrounding material and if the intensity is adequate the flame will proceed onward. Recall that volume of a sphere is proportional to radius cubed while surface area is proportional to radius squared. A result of these relationships is that less heat per unit area escapes into the surrounding medium as the size of the active sphere becomes smaller. This implies that a certain minimum volume of material must be ignited before the surrounding mix can see enough energy for it to ignite as well. The UV rays apparently do not activate a volume that is great enough for further activity. Lets choose an easy example to calculate which actually bears little comparison to the actual case at hand but demonstrates the process. Assume that we have ignited the gas vapor mix within a volume that releases 1 joule of heat energy. Our test region has a radius of 1 cm for the example. The area of a sphere is 4*pi*R*R or 4 * 3.14159 * 1 centimeter * 1 centimeter = 12.566 square cm. The energy density leaving the sphere is thus 1 joule / 12.566 square cm. Perhaps this is enough to ignite the material at the surface of the sphere and the flame would proprogate. Now lets only ignite a ½ cm radius sphere of vapor mix. The heat released would now be ½ * ½ * ½ or 1/8 joule. But the area would also shrink to the new value of 4 * pi * ½ * ½ = 3.1416 square cm. Thus the net energy density leaving our new smaller volume is exactly ½ the original amount which might not be enough to ignite the adjacent material. Dave
-----Original Message----- From: mixent <mix...@bigpond.com> To: VORTEX <vortex-l@eskimo.com> Sent: Sun, Apr 22, 2012 6:02 pm Subject: [Vo]:Ignition Hi, Here's a little conundrum that has troubled me for some time. ake a cup of gasoline and place it in open sunlight. It will slowly evaporate. ring a flame near it and it will suddenly ignite. Why don't the UV rays from sunlight cause ignition? Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html -----Original Message----- From: mixent <mix...@bigpond.com> To: VORTEX <vortex-l@eskimo.com> Sent: Sun, Apr 22, 2012 6:02 pm Subject: [Vo]:Ignition Hi, Here's a little conundrum that has troubled me for some time. ake a cup of gasoline and place it in open sunlight. It will slowly evaporate. ring a flame near it and it will suddenly ignite. Why don't the UV rays from sunlight cause ignition? Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
