Keith D. Rein

Application of time-division-multiplexed lasers for measurements of gas temperature and CH4 and H2O concentrations at 30 kHz in a high-pressure combustor.

Two time-division-multiplexed (TDM) sources based on fiber Bragg gratings were applied to monitor gas temperature, H(2)O mole fraction, and CH(4) mole fraction using line-of-sight absorption spectroscopy in a practical high-pressure gas turbine combustor test article. Collectively, the two sources cycle through 14 wavelengths in the 1329-1667 nm range every 33 μs. Although it is based on absorption spectroscopy, this sensing technology is fundamentally different from typical diode-laser-based absorption sensors and has many advantages. Specifically, the TDM lasers allow efficient, flexible acquisition of discrete-wavelength information over a wide spectral range at very high speeds (typically 30 kHz) and thereby provide a multiplicity of precise data at high speeds. For the present gas turbine application, the TDM source wavelengths were chosen using simulated temperature-difference spectra. This approach is used to select TDM wavelengths that are near the optimum values for precise temperature and species-concentration measurements. The application of TDM lasers for other measurements in high-pressure, turbulent reacting flows and for two-dimensional tomographic reconstruction of the temperature and species-concentration fields is also forecast.

Fourier-transform absorption spectroscopy in reciprocating engines

We have adapted our in-cylinder Fourier-transform spectroscopy technique to measure absorption spectra in a reciprocating engine. Previously, we had used the technique for emission spectroscopy; the upgrade to absorption spectroscopy mode is important because it allows for more quantitative analysis of gas properties than is possible with emission spectroscopy. Here, we discuss fuel, H(2)O, and CO(2) spectra measured in an engine using a spark-plug-based probe for optical access and use the water portion of the spectra to determine in-cylinder gas temperature. The temperature results show that heat transfer effects can significantly bias thermometry when fiber-coupled engine probes are used.

In-cylinder Fourier-transform infrared spectroscopy

We have adapted a standard Fourier-transform infrared (FTIR) spectrometer (Thermo-Scientific Nexus 670) for in-cylinder measurements of gas spectra. During engine operation, the engine shaft encoder signal is logged continuously along with the FTIRs He–Ne laser signal and infrared interferogram signal. The engine piston and FTIR mirror move in an uncorrelated fashion, so that after many minutes of engine operation, a complete interferogram is populated at each piston position. Afterward, the data are compiled into a series of spectra versus crank-angle degree. Here, we present a near-infrared H2O thermal emission spectrum measured through a fiber-optic spark plug connected to an engine (Briggs 128603-OHV).

Multi-spectral, multi-path absorption spectroscopy at 30 kHz using time-division multiplexed lasers

We report on the development and application of novel laser sources in absorption spectroscopy. Our aim is to apply near-infrared absorption spectroscopy to measure gas properties at ~ 30 kHz in practical combustion test articles using multiple lines-of-sight. Numerous multiwavelength laser sources have been considered for this task, including multiplexed fixed-wavelength distributed feedback (DFB) lasers, frequency-comb sources, swept-wavelength or discrete wavelength time-division-multiplexed (TDM) sources, and even simple broadband superluminescent sources (analyzed by a grating spectrometer). There are many tradeoffs to be considered when selecting the light source. TDM sources are often very attractive candidates, because of the ability to affordably access numerous discrete (10-100) wavelengths or to continuously scan over key spectral ranges of interest. Therefore, we have adopted TDM sources in much of our work in this area. Here we briefly review selected TDM sources and present results from experiments in aeropropulsion test articles.