Frequency modulation spectroscopy can detect hydroxyl radicals (OH) with unprecedented sensitivity. Because OH is a critical component in the combustion processes that power most vehicles, the new approach could advance the development of novel types of engines and fuels that would be more efficient and environmentally friendly.
“In the US, combustion produces 60 % of our electricity and powers 90 % of ground transportation and almost all aviation”, said Shengkai Wang, a postdoctoral research fellow in mechanical engineering at Stanford University. “The ability to examine combustion processes and understand them at a more fundamental level would aid in the development of next-generation combustion strategies that can increase efficiency and reduce pollution”, he said.
In Optics Letters, Wang and Ronald K. Hanson, professor of mechanical engineering at Stanford, report a spectroscopy-based approach that detected levels of OH radicals at least four times lower than the previous best method used to analyse OH. Among hundreds of molecular entities involved in combustion reactions, OH is the most important because it determines whether and how fast the fuel will burn.
“OH is extremely difficult to measure, especially in the dynamic and noisy environments of fuel combustion, because it is highly reactive and present in very low concentrations”, said Wang. “Our approach paves the way to practical detection of OH in the parts per billion range.”
The new approach could also be useful for applications such as studying atmospheric chemistry, where OH is a key player in the formation and depletion of ozone according to Wang.
One bottleneck to commercialising new types of engines or optimised fuels is that their combustion chemistry is not fully understood due to a lack of sensitive analysis methods. To solve this problem, Wang and his colleague developed frequency-modulation spectroscopy using ultraviolet (UV) light. Rather than using one laser wavelength, frequency modulation spectroscopy examines the differences in light absorption between multiple wavelengths, allowing any noise common among the readings to be subtracted. The method also shifts the signal coming from OH absorption to a higher frequency, thereby eliminating any low-frequency drift that challenges OH measurement.
“The general idea of frequency modulation spectroscopy has existed for a while, but we are the first to demonstrate its applicability to detecting OH at this particular wavelength range”, said Wang. “One reason this hasn’t been done before is that the high-quality UV laser source necessary to measure OH absorption [only] became available very recently.”
The researchers tested their new approach by studying the combustion reaction of a representative fuel, iso-octane, in a controlled reactor. They were able to achieve a minimum detectable absorbance of 3.0 × 10–4 at a temperature of 1330 K. This is equivalent to detecting 85 ppb of OH over 15 cm optical length and is four times better than the best record previously reported.
As a next step, the researchers plan to incorporate better optical components, which they say could improve the sensitivity by another order of magnitude. They also want to make the equipment more portable so that it could be transported on a cart to various specialised testing facilities. A portable system would also allow them to use the approach to make measurements in practical engine conditions and to eventually adapt the method for making measurements in realistic engines and combustors.