Recently, I girded my loins and headed over the border to Jacobs University in Bremen to attend the European conference on nonlinear optical spectroscopy. In addition to discovering the joy involved in having our group dominate one of the sessions, and that Jacobs needs about 10 more years of student life before it will start to feel like a university, I learned about the existence of a whole new field. Spectroscopy for combustion physics.

Okay, that is a bit exaggerated. I knew these guys existed, I just didn't think they did anything interesting. But I have exited the valley of ignorance and have become an official fan of coherent anti-Stokes Raman spectroscopy (CARS) for combustion physics. Essentially, CARS provides the answer to the question "What the heck is going on inside a combustion chamber, anyway?". What follows below is based on talks given by Paul Danehy at NASA and Robert Lucht at Purdue. Along with some new developments, they provide an outline of how you use CARS in combustion physics and the sort of information you get out of it.

The people I listened to and spoke with are mainly associated with the aerospace industry, so they are interested in mixing and combustion in jet engines and supersonic combustion ramjets (scramjets). The aim is to measure the fuel-air mixing before combustion, the development of combustion—where is fuel burnt first, where is the oxygen, and is the combustion complete—and the temperature.

For scramjets, the idea is that a supersonic airflow will have something like hydrogen fuel injected into it. These two flows will mix as they travel downstream until they hit the throat of the scramjet, where the increase in pressure raises the temperature and initiates combustion. This further increases the pressure, expelling hot gases out the back to provide propulsion.

To make this work properly, you want a nice even burn. How can you know that you have achieved this, when the temperatures are sufficient to melt thermometers and things are moving so fast that you have to monitor the engine from the next county?

Understanding CARS

This is where optical techniques play a key role, and CARS seems to be a favorite of the community. How does CARS work? We start with a laser that is set to any old wavelength and illuminate the molecules we are interested in. But, because we didn't think too hard about what wavelength to use, the molecule ignores the light field and we see nothing... well almost nothing.

The light field does cause electrons to move in sympathy with the light—light is basically an oscillating electric field, which the electrons respond to—and, in doing so, they can set the molecule vibrating. Since the energy used to set the molecule vibrating comes from the light field, to balance the books, a molecule effectively absorbs a photon from the light field and instantaneously emits a photon with a slightly lower energy.

This process is called Raman scattering, and measuring the spectrum of Raman scattering from molecules is a standard technique in chemistry labs all over the world. But, in actual fact, Raman scattering kind of sucks because the amount of light emitted is tiny, and it goes in every direction, so you only get a small fraction of it.

Riding to the rescue is CARS. In this case, we use two or even three lasers. One is the pump laser that would normally be Raman scattered. The second laser is tuned so that it has the same wavelength as the Raman scattered light. This sets up a resonance between the molecule and the two light fields, and the molecules respond strongly to this by creating their own fields that oscillate at the same frequency as the molecular vibration.

So we have this oscillating field, but it can't radiate. This is because the vibrations of molecules have momentum associated with them, and to radiate the molecule has to give up the wrong amount of momentum. If left to its own devices, it would just vibrate crazily until it ran into something and gave up the energy in the collision.

Instead, we hit it with another laser (or the original pump laser again). This provides an additional unit of momentum, which the molecular oscillations couple to and relax. In doing so, a photon is absorbed from the light field and a photon with additional energy is emitted. And, because of the way the process was put together, every photon goes in the same direction and has the same phase.

What does that mean? It means you collect a lot more light, so most of the Raman spectrum is stronger. And you can use all sorts of cool techniques that depend on the phase of the field to extract weaker signals. It also means, that, if you want to, you can set up a CARS experiment to look at just one or two molecular vibrational modes—just be careful choosing your laser wavelengths.

CARS meets jets

What does this have to do with combustion physics? Everything. Let's take temperature sensing. Temperature is a measure of the kinetic energy of the molecules in a gas. All else being equal, the kinetic energy is spread across translation motion, rotational modes, and vibrational modes. However, it takes much more energy to excite some modes of motion compared to others. What we observe is that intermolecular collisions set molecules rotating, and these rotational modes simply fill up from the lowest energy to the highest energy.

Now imagine we have a gas that is at a high temperature, so lots of rotational modes are excited. When we perform our CARS measurement on a vibrational mode, we don't see a single peak. Rather, the single peak gets split up into multiple closely spaced peaks, each of which corresponds to the vibrational mode plus a rotational mode. One can now measure the temperature by measuring the relative intensity of each peak, which corresponds to the relative population in each rotational mode. Typically this is accurate to within a few degrees.

Those of you with your thinking caps on might think that this is a bit dodgy. After all, if you have combustion and fuel-air mixing going on, how do you distinguish changes in temperature from changes in concentration? The answer is to use a species that doesn't react: nitrogen. The good thing about nitrogen is that it is a very simple molecule for which accurate calculations can be made. So, the presence of nitrogen in the mix turns out to be necessary to extract an accurate temperature from the data.

The other thing you want is the concentration of gas species. This sounds pretty easy, right? The presence of a CARS signal indicates a vibrational mode that is associated with a particular molecule, so getting the concentration should just be a matter of comparing the relative strengths of the CARS signals. Unfortunately, life is never that easy. Because CARS is a nonlinear process that depends on the strength with which a molecule responds to the light, and the strength of the light beam. Add a turbulent medium to that, and you have a nightmare in the making.

So, yeah, you can get concentration and temperature out of these measurements, but it turns out to be pretty tricky and relies on a lot of modeling. This brings us to the work of Robert Lucht at Purdue, who has eliminated much of the modeling from temperature measurements.

Lucht has focused on using some of the recent—about 10 years old—developments in laser technology, which has enabled combustion temperature monitoring in a single laser shot at very high sample rates. To do this, he uses a laser that emits pulses of light that are just 100 femtoseconds in duration. These light pulses are then split up so that different parts of the light pulse play different roles in the CARS process.

As a result, he samples a large number of the rotational modes of nitrogen, providing him with more data from which to obtain an accurate temperature. More important, however, is the speed with which everything occurs. In 100 femtoseconds, molecules don't have a lot of time to do anything. As a result, each nitrogen molecule responds as though it was in isolation, so Lucht's measurements are not distorted by the effects of molecular collisions that would otherwise occur during the measurement. The result is that he doesn't need to estimate collision rates and the effects of collisions to fit his data and obtain a temperature.

Paul Danehy, on the other hand, seems to think that there is plenty of life left in the older horses, provided you are a bit clever. His experimental setup uses three big lasers, each emitting pulses that are 10 nanoseconds long, but with a whole lot more energy in them. He cleverly chooses his three laser wavelengths so that when combined one way, he sees the nitrogen spectrum, giving him the temperature. When combined another way, he sees the hydrogen fuel, oxygen, and other hydrocarbons, so he can measure the distribution of the fuel and oxygen in the flow. In fact, he also sees s small fraction of the rotational spectrum of hydrogen, which extends the range of his thermometry down to room temperature.

Furthermore, he goes a step further and provides flow velocity information. This is actually a pretty simple thing to do, but requires a lot of light—which he has. The idea is very simple: light that is simply scattered by the moving molecules gets Doppler shifted. If you analyze the wavelengths of the light scattered in two different directions from the flow, you can calculate the radial and longitudinal flow in the jet. That is a pretty cool addition to an already nice piece of kit.

So, what do these guys do? Well, in the case of Danehy, he provides combustion information to the designers, who use that information to improve their designs. However, it goes further than that: one of the major obstacles to improving scramjet designs is the validation of computational models. The CARS set up by Danehy provides valuable detailed information that is being used to help perform this validation. Lucht, on the other hand, seems to be much more focused on the fundamentals associated with instrumentation development. He is developing the tools that researchers like Danehy will use in the future.

Listing image by NASA