Recent advances in room-temperature, near-IR and visible diode laser sources for tele-communication, high-speed computer networks, and optical data storage applications are enabling a new generation of gas-dynamic and combustion-flow sensors based on laser absorption spectroscopy. In addition to conventional species concentration and density measurements, spectroscopic techniques for temperature, velocity, pressure and mass flux have been demonstrated in laboratory, industrial and technical flows. Combined with fibreoptic distribution networks and ultrasensitive detection strategies, compact and portable sensors are now appearing for a variety of applications. In many cases, the superior spectroscopic quality of the new laser sources compared with earlier cryogenic, mid-IR devices is allowing increased sensitivity of trace species measurements, high-precision spectroscopy of major gas constituents, and stable, autonomous measurement systems. The purpose of this article is to review recent progress in this field and suggest likely directions for future research and development. The various laser-source technologies are briefly reviewed as they relate to sensor applications. Basic theory for laser absorption measurements of gas-dynamic properties is reviewed and special detection strategies for the weak near-IR and visible absorption spectra are described. Typical sensor configurations are described and compared for various application scenarios, ranging from laboratory research to automated field and airborne packages. Recent applications of gas-dynamic sensors for air flows and fluxes of trace atmospheric species are presented. Applications of gas-dynamic and combustion sensors to research and development of high-speed flows aeropropulsion engines, and combustion emissions monitoring are presented in detail, along with emerging flow control systems based on these new sensors. Finally, technology in nonlinear frequency conversion, UV laser materials, room-temperature mid-IR materials and broadly tunable multisection devices is reviewed to suggest new sensor possibilities.

https://iopscience.iop.org/article/10.1088/0957-0233/9/4/001/pdf

Diode laser absorption sensors for gas-dynamic and combustion flows.

Gas turbines offer very high cycle efficiency, exceeding 50% in modern 250 MW-class combined-cycle units for power generation, as well as very low NOv in the lean premixed combustion mode with natural gas fuel. They also account for virtually all commercial aeropropulsion systems, in which case kerosene-based fuel is used. To meet future NOv and CO regulations, a higher level of understanding of turbulence, chemical kinetics and their interactions is required. NOv in particular has become a pacing consideration, although other constraints are present. Selected NOv data obtained at laboratory and machine conditions with gaseous fuel are reviewed here. Although the important chemical reactions cover a wide range in effective Damkohler number, the measure of turbulence-chemistry interactions, it appears that NOv is formed in a distributed zone manner. Equilibrium and superequilibrium effects can broaden the NOv-forming zones beyond the fine scales of turbulence, even in non-premixed flames. Pressure...

A Review of NOx Formation Under Gas-Turbine Combustion Conditions

This paper explores the use of acetone as a suitable tracer in planar laser-induced fluorescence concentration measurements in gaseous flows. The photophysics and physical properties of acetone relevant to its use as a fluorescent marker are discussed and compared to those of alternative molecular tracers, particularly the biacetyl molecule. Finally, as a direct example, concentration images obtained in a turbulent air jet seeded alternatively with acetone and biacetyl are compared.

Acetone: a tracer for concentration measurements in gaseous flows by planar laser-induced fluorescence

An overview is given of recent developments in laser diagnostic methods for the quantitative measurement of trace species concentrations and, in conjunction, of temperature in combustion systems. After a short introduction illustrating some experiments from the pre-laser era, the article presents typical applications and discusses advantages and limitations of laser techniques including laser absorption, linear, saturated, predissociative and multi-photon-excited laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), electronically resonant coherent anti-Stokes Raman scattering (resonance CARS), degenerate four-wave mixing (DFWM) and amplified spontaneous emission (ASE). Recent trends including two-dimensional imaging, multi-species detection and high-pressure applications will also be discussed. Throughout the article, an attempt is made to present typical results from a large portion of the relevant technical literature. A concluding section gives a short summary of the current status and comments on the perspectives for further research.

Laser Techniques for the Quantitative Detection of Reactive Intermediates

Flamelet theories are examined in the context of turbulent nonpremixed combustion. The criterion requiring that the reaction zone be thinner than the Kolmogoroff length scale is converted to one in which the range of mixture fraction over which reaction occurs is compared with the scalar scale for the small scale fluctuations. It is found that the criterion is violated in many of the flames of interest, particularly away from the nozzle in jet flames and in recirculating flows. Laser imaging data for scalar mixing is used to illustrate the structure of quasi-equilibrium distributed reaction (QEDR) flames at high Damkohler number. The structure is found to tend toward that of the scalar dissipation. The apparent success of flamelet theories in predicting mean concentrations of CO in hydrocarbon flames is ascribed to the metastable equilibrium of rich mixtures arising from chain terminating reactions of radicals with the fuel. Differences in flame structure for piloted jet diffusion flames near extinction of methane and CO/H 2 /N 2 fuels are discussed in terms of these fluid dynamic and kinetic insights.

The structure of turbulent nonpremixed flames

Waveguide evanescent field absorption has been measured as a function of cladding refractive index and bulk absorption coefficient. A simple theory for interpreting these results has been developed. An all‐fiber species concentration sensor has been constructed and its performance characterized.

Fiber‐optic evanescent field absorption sensor

A single-pulse, laser-induced-fluorescence diagnostic for the measurement of two-dimensional temperature fields in combustion flows is described. The method uses sheet illumination from a tunable laser to excite planar laser-induced fluorescence in a stable tracer molecule, seeded at constant mole fraction into the flow field. The temporal resolution of this technique is determined by the laser pulse length. Experimental results are presented for a rod-stabilized, premixed methane–air flame, using the Q1(22) line of the nitric oxide A2 ∑+(v = 0) ← X2Π1/2(v = 0) transition (λ ≃ 225.6 nm).

Instantaneous temperature field measurements using planar laser-induced fluorescence

Quantitative visualization of combustion species in a plane

A quantitative flow visualization technique for combustion research is described. The method uses sheet illumination from a tunable laser to excite planar laser-induced fluorescence (PLIF) which is detected using 2-D detector. This technique has been used to make single-shot simultaneous multiple-point measurements of species concentration and has recently been extended to measurements of other properties such as temperature and velocity. The light sources, image intensifiers, detector arrays, computer hardware, and computer software needed to implement this scheme are described. The detectivity, dynamic range, spatial resolution, and temporal resolution of the technique are discussed and a recent application of the technique (to view the OH concentration in a rod-stabilized flame) is presented.

George Kychakoff

Papers

Fiber‐optic evanescent field absorption sensor

Instantaneous temperature field measurements using planar laser-induced fluorescence

Quantitative visualization of combustion species in a plane

Quantitative flow visualization technique for measurements in combustion gases.

Movies and 3-D images of flowfields using planar laser-induced fluorescence.