Olin Jarrett

Multispecies coherent anti‐Stokes Raman scattering instrument for turbulent combustion

A nonintrusive diagnostic system is described which permits simultaneous measurement of temperature, nitrogen number density, and oxygen number density in hostile combustion environments. This system is pumped by a 10‐Hz neodymium YAG laser, and signals are detected with an intensified photodiode array and dual photomultipliers. The system utilizes a folded BOXCARS beam arrangement, two probe dye lasers, dynamic range expansion, polarization rejection of nonresonant background, and multiplexing of the signal beams.

Coherent anti-Stokes Raman spectroscopy temperature measurements in a hydrogen-fueled supersonic combustor

Coherent anti-Stokes Raman spectroscopy (CARS) thermometry has been used to obtain static temperature cross sections in a three-dimensional supersonic combustor flowfield. Data were obtained in three span wise planes downstream of a single normal fuel injector which was located downstream of a rearward-facing step. The freestream flow was nominally Mach 2 and was combustion heated to a total temperature of 1440 K (yielding a static temperature of about 800 K in the freestream) to simulate the inflow to a combustor operating at a flight Mach number of about 5.4. Since a broadband probe laser was used an instantaneous temperature sample was obtained with each laser shot at a repetition rate of 10 Hz. Thus root-mean-square (rms) temperatures and temperature probability density functions (pdfs) were obtained in addition to mean temperatures.

Intensified silicon photodiode array detector linearity: application to coherent anti-Stokes Raman spectroscopy.

The linearity of an intensified silicon photodiode array multichannel detector is studied with coherent anti-Stokes Raman and other similar signals. Studies with diffuse and focused (spherically and cylindrically) signals resolved apparent saturation problems which limit the dynamic range of the detector. In addition, it has been shown that there is no short-range wavelength (473–532-nm) dependence on this saturation. Theoretical explanations for these phenomena are also included.

Comparison of CARS combustion temperatures with standard techniques

This paper describes a comparative study of temperatures measured with Coherent Anti-Stokes Raman Spectroscopy (CARS) and those using standard measurement techniques. CARS temperatures from nitrogen vibrational-rotational spectra have been obtained in a premixed hydrogen-air laboratory flame supported on a flat flame burner over a temperature range from about 1000 K to 2100 K. The techniques used for comparison were sodium line reversal, thin wire thermocouples and heat balance calculations. These studies show a good agreement over the entire temperature range studied.

CARS measurements of temperature and species number density in supersonic combusting flow

A supersonic combustion burner has been probed by coherent anti-Stokes Raman Spectroscopy and the results compared with computational fluid dynamics. Simultaneous measurement of temperature, nitrogen number density and oxygen number density have been acquired throughout the external combustion region. Preliminary calculations have been made on this system to estimate the burner performance. Comparisons of these techniques are included.

Simple technique for sequential q-switching of molecular lasers.

A simple technique for sequentially Q-switching molecular lasers (e.g., CO(2), CO, HF) is discussed in which an optical scanner is used as an optical folding element in a laser cavity consisting of a stationary diffraction grating and partially reflecting mirror. Sequential Q-swtiching of a conventional CO(2) laser is demonstrated in which over sixty-two transitions between 9.2 micro and 10.8 micro are observed. Rapid repetition rates (200 Hz) and narrow laser pulses (<5 microsec) allow conventional signal processing techniques to be used with this multi-wavelength laser source which is a versatile tool for laser propagation studies, absorption spectroscopy, and gain measurements. Results of a preliminary experiment demonstrating the utility of measuring selective absorption of CO(2) laser wavelengths by C(2)H(4) is shown, but selective absorption by atmospheric CO(2), NH(3), and O(3) can also be studied. With appropriate optical sca ners and diffraction gratings, sequential Q-switching of CO and HF lasers is possible.

Laser diagnostics on a hypersonic combustor

NASA-Langley has implemented a laser-based multipoint/multiparameter diagnostics system at its hypersonic direct-connect combustor, in order to measure both temperature and majority species densities in two dimensions, using spatially-scanned CARS; in addition, line-imaged measurements of radical densities are simultaneously generated by LIF at any of several planes downstream of the fuel injector. Initial experimental trials have demonstrated successful detection of one-dimensional images of OH density, as well as CARS N2-temperature measurements, in the turbulent reaction zone of the hypersonic combustor.

Simultaneous sampling technique for two spectral sources

A technique is described that uses a bundle of fiber optics to simultaneously sample a dye laser and a spectral lamp. By the use of a real-time display with this technique, the two signals can be superimposed, and the effect of any spectral adjustments can be immediately accessed. In the NASAs CARS system used for combustion diagnostics, the dye laser mixes with a simultaneously pulsed Nd:YAG laser at 532 nm to probe the vibrational levels of nitrogen. An illustration of the oscilloscopic display of the system is presented.

Coherent Anti-Stokes Raman scattering with reflective optics.

Attention is given to the implementation of a novel reflective optics configuration in which the lens normally employed for focusing and crossing the input laser beams in Coherent Anti-Stokes Raman spectroscopy (CARS) is replaced by a spherical mirror, allowing large crossing angles to be obtained with a single focusing device and permitting the laser generation and collection equipment to be located on one side of the sample region. The experimental results obtained demonstrate the feasibility of mirror-based CARS in remote combustion diagnostics.