New seeding methodology for gas concentration measurements.

This paper presents the first demonstration of the pulsed laser ablation technique to seed a laminar non-reacting gaseous jet at atmospheric pressure. The focused, second harmonic from a pulsed Nd : YAG laser is used to ablate a neutral indium rod at atmospheric pressure and temperature. The ablation products generated with the new seeding method are used to seed the jet, as a marker of the scalar field. The neutral indium atoms so generated are found to be stable and survive a convection time of the order of tens of seconds before entering the interrogation region. The measurements of planar laser-induced fluorescence (PLIF) with indium and laser nephelometry measurements with the ablation products are both reported. The resulting average and root mean square (RMS) of the measurements are found to agree reasonably well although some differences are found. The results show that the pulsed laser ablation method has potential to provide scalar measurement for mixing studies.

Beam displacement as a function of temperature and turbulence length scale at two different laser radiation wavelengths.

Narrow laser beams directed from aircraft may at times pass through the exhaust plume of the engines and potentially degrade some of the laser beam characteristics. This paper reports on controlled studies of laser beam deviation arising from propagation through turbulent hot gases, in a well-characterized laboratory burner, with conditions of relevance to aircraft engine exhaust plumes. The impact of the temperature, laser wavelength, and turbulence length scale on the beam deviation has been investigated. It was found that the laser beam displacement increases with the turbulent integral length scale. The effect of temperature on the laser beam angular deviation, σ, using two different laser wavelengths, namely 4.67 µm and 632.8 nm, was recorded. It was found that the beam deviation for both wavelengths may be semiempirically modeled using a single function of the form, σ=a(b+(1/T)(2))(-1), with two parameters only, a and b, where σ is in microradians and T is the temperature in °C.

Solvent effects on two-line atomic fluorescence of indium.

We aim to investigate the potential of four different organic solvents, namely, acetone, ethanol, methanol, and isopropanol, and the organic-solvent-water mixtures as a seeding medium for the two-line atomic fluorescence technique. Water is used as the reference case. Indium, which has been previously shown to have suitable spectroscopic attributes, is chosen as the thermometry species in the present study. Acetone and methanol are shown to enhance the fluorescence signal intensity the most (approximately threefold to fivefold at stoichiometric conditions) when used. Acetone and methanol are shown to improve the fluorescence emission over the entire stoichiometric envelope of flame, most significantly in the rich combustion region, as well as a twofold enhancement in the signal-to-noise ratio.

Instantaneous temperature imaging of diffusion flames using two-line atomic fluorescence.

This work investigates the first demonstration of nonlinear regime two-line atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is approximately 6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.

Development of temperature imaging using two-line atomic fluorescence.

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.