Chemical Vapor Plume Detection Using the Schlieren Optical Method

Open Access
Bigger, Rory
Graduate Program:
Mechanical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
Committee Members:
  • Gary Stuart Settles, Thesis Advisor
  • vapor detection
  • standard method
  • quantitative schlieren
  • homeland security
  • schlieren
  • plume buoyancy
Every year, millions of people, vehicles, and packages pass through security checkpoints in the United States, and it is impossible to physically inspect them all for contraband. One way to improve this situation is to sample the air at checkpoints for traces of the vapors of contraband substances. This research investigates the use of the schlieren optical method for this purpose. A list of compounds of interest was compiled, and several of these (cyclohexanone, decane, sytrene, toluene, 1-propanol, 2-methoxyethanol, $alpha$-pinene, nitromethane, and ethanol) produced vapors that were qualitatively detectable with an ordinary, inexpensive schlieren system in initial experiments. It is cumbersome, however, to test all possible vapor sources for detectability so a quantitative method for predicting detectability was required. A weak lens standard method was tested using a known refractive field (free convective boundary layer over a flat plate) and then used to develop methods for predicting detectability accurate to within 15\% error. Thermal effects that affect vapor sampling detection by changing plume buoyancy were experimentally investigated and found to predictable using simple theory. Finally, realistic detection of vapors from compounds inside containers was tested and found to be possible with some limitations.