Intense light igniting a mixture of fuel and air

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Discussion Overview

The discussion revolves around the use of focused light to ignite a mixture of fuel and air, specifically focusing on the energy required for combustion and the relationship between light intensity and temperature needed for ignition. The context includes aspects of combustion chemistry and practical applications in a project setting.

Discussion Character

  • Homework-related
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant seeks to understand the energy required to start combustion at atmospheric pressure for vaporized hydrocarbons and how to calculate the necessary light focus for ignition.
  • Another participant questions the initial terminology, suggesting a distinction between carbohydrates and hydrocarbons, noting that hydrocarbons are typically more volatile and easier to vaporize.
  • A later reply clarifies that the focus is indeed on hydrocarbons, specifically alcohols like ethanol.
  • One participant introduces the concept of 'autoignition temperature' as a relevant factor for ignition.
  • Another participant challenges the assumption that power alone determines temperature, emphasizing the need for additional factors such as absorption of light and heat capacity to estimate temperature increases.
  • It is suggested that using a black object at the lens focus could enhance heat absorption for ignition purposes.
  • A participant mentions that ethanol has significant absorption in the infrared range and proposes the use of a CO2 laser, highlighting specific wavelengths that align with ethanol's absorption spectrum.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and concepts related to combustion, particularly regarding the types of fuels being discussed. There is no consensus on the relationship between light intensity and temperature for ignition, with multiple perspectives on the factors involved in the combustion process.

Contextual Notes

Participants note limitations in their understanding of combustion principles and the complexities involved in calculating ignition conditions, including assumptions about absorption and heat transfer that may not be fully addressed.

tarakan
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Homework Statement


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.

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Homework Equations

The Attempt at a Solution

 
Last edited:
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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.
 
SteamKing said:
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.

Sorry. I am asking about hydrocarbons. Alcohol, most likely.
 
tarakan said:
Sorry. I am asking about hydrocarbons. Alcohol, most likely.

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

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

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.
 
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:
IRspectrum_water21.gif
 

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