MadSci Network: Chemistry

Re: How does a torch work?

Date: Tue Oct 28 13:41:21 2003
Posted By: Joseph Weeks, Engineer
Area of science: Chemistry
ID: 1066595934.Ch

Combustion of liquid fuels is a complex topic because of the number of processes involved. These processes include mass transfer, heat transfer, chemical kinetics, and a wide range of high temperature chemical reactions. The wick of a torch also introduces surface chemistry into the mix.

I referred to my "Chemical Engineering Handbook" by Perry and Chilton for guidance on the combustion process of liquid fuels. It states, under the Heat Generation section "In vaporizing burners, heat from the flame continually converts liquid fuel into a vapor in the combustion air, thus sustaining the flame. This principle is used in blow torches, in pot- type home heating furnaces, and all wick burners such as kerosene lamps, kerosene stoves, and cigarette lighters." So, your torch is one of these wick burners. In the combustion process, heat from the flame causes liquid fuel within the torch to vaporize, allowing oxygen in the air to mix and react with the vaporized fuel.

You can learn a lot about combustion by studying a candle flame. If you look closely at a candle flame, you may notice that at the very base of the flame, the flame doesn't actually start at the wick, but instead starts a little way away from the wick. It takes a little time for the vaporizing wax to heat and mix with air so that it can support combustion.

When a solid surface is wetted by a liquid, the liquid tends to spread over that surface. When you put a small diameter tube in a liquid that wets the walls of the tube, the liquid will crawl up the walls of the tube, in opposition to the forces of gravity. The smaller the diameter of the tube, the higher the liquid will climb up the tube. We typically call this effect "capillary action." We think about capillary action taking place against gravity, but it actually takes place in all directions; capillary action takes place in weightlessness. If you orient a capillary tube in the horizontal direction, liquid will run along the entire length of the tube. If the liquid in the tube has a high surface tension, it may just form a small droplet at the end of the tube; if it has a low surface tension, it may just run out the end of the tube. (If you manufactured ink jet cartridges, you might use this effect to make ink run into your print head where small droplets can be squirted on paper while not running out of the cartridge altogether.)

The wick on your torch simply attempts to distribute liquid fuel throughout the wick according to capillary forces. If liquid at the outer edges of the wick is being removed by vaporization and combustion, liquid within the center of the torch will tend to move toward the outside of the torch. A simple way to visualize this effect is to put a drop of water on cloth or a paper towel. Regardless of the direction that the cloth is oriented in, the drop expands essentially in a circular direction away from where the droplet was placed. Try swinging the cloth over your head and see if you can generate enough force to influence the direction of the wetting process. A little experimentation may show that your intuition about the relative strength of capillary versus centrifugal forces will need a bit of revision.

Thanks for the question.

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2003. All rights reserved.