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Homework Help: Intense light igniting a mixture of fuel and air

  1. May 13, 2014 #1
    1. The problem statement, all variables and given/known data
    We are working on a project for a class.
    I was given a part of the project where focused light is used to burn off carbohydrates.

    I want to know names of the following values so I can look them up and their relationships.

    Energy required to start combustion at atmospheric pressure, with a specific concentration of vaporized carbohydrates dissolved in the air.

    Knowing the amount of light (in Watts) coming out of the source, how much does this light need to be focused to obtain a specific temperature for combustion to occur?

    I don't know how to write a formula because I have very little knowledge about combustion, mostly from a chemistry class I took 4 years ago. This is business school. I forgot everything I was taught before it.

    I did take some Calculus and Physics classes before I went to business school.
    I just don't know what I am looking for.

    I wish I could take a different role in this project, but I am the only one who may be able to solve problems like this.

    I can get away with ignoring convection, refraction index of the tube and other variables that will make the calculation very difficult.

    Thank you.


    2. Relevant equations

    3. The attempt at a solution
    Last edited: May 13, 2014
  2. jcsd
  3. May 13, 2014 #2


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    Are you sure you are asking about igniting 'carbohydrates' (sugars, starches, et al.) or 'hydrocarbons' (gasoline, diesel fuel, jet fuel, methane, propane, etc.)? The latter are usually relatively volatile and easy to vaporize, while the former, not so much.
  4. May 13, 2014 #3
    Sorry. I am asking about hydrocarbons. Alcohol, most likely.
  5. May 13, 2014 #4


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  6. May 13, 2014 #5


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    Not to be nitpicky, but ethanol is an alcohol, and that's neither carbohydrate nor hydrocarbon :wink:

    It doesn't work this way. There is no relationship between just power and temperature (lens being the least of your problems here). You need much more than that. Light has to be absorbed (and it won't get absorbed all), this will give amount of heat, and that can be in turn used to estimate by how much temperature goes up (you will need a heat capacity).

    Perhaps the best approach is to put something black in the lens focus, to absorb as much heat as possible.
  7. May 14, 2014 #6
    Ethanol absorbs quite a bit in the infrared range. Perhaps you would want to use a CO2 laser. The 9.4um band lands in a great spot on the ethanol absorption spectrum. The 10.6um band would be less useful.
    The bottom chart is for ethanol:
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