Nonintrusive molecular velocity measurements in air and reacting flows using hydroxyl tagging velocimetry.

摘要

A new non-intrusive molecular technique, namely Hydroxyl Tagging Velocimetry (HTV), has been developed for unseeded instantaneous flow velocity measurements. Initially at some time, t₀, single-photon photodissociation of H₂O by a pulse of an ArF excimer “write” laser operating at ∼193 nm wavelength quickly leads to the formation of OH and H photoproducts by accessing the steeply repulsive A1B₁ surface of H₂O. The ArF laser is split into 14 beams by compact micro-lens optics to produce a multiline 7 x 7 optical grid of OH. After a certain variable delay time, t ₀ + Δt, the OH lines are interrogated by tunable “read” lasers at various operating wavelengths (248, 282, or 308 nm) thus accessing different vibrational levels (3 ← 0), (1 ← 0), or (0 ← 0) respectively of the A2Σ+ (v ′ = i) ← X2Π _i (v′′ = j) OH transition. When a KrF excimer laser excites the A2Σ + (v′ = 3) ← X 2Π_i (v′′ = 0) OH band, the resulting fluorescence is relatively weak due to predissociation. Thus, only high temperature chemically-reacting flows (containing significant populations of vibrationally-excited H₂O) are successfully probed with the KrF laser.; A strong OH fluorescence “read” process must be used for low temperature HTV applications where only ground vibrational state H ₂O exists. By operating the frequency-doubled read dye at ∼308 nm or at ∼282 nm, the vibrational transitions of the (0 ← 0 ) or (1 ← 0) OH bands respectively are accessed. These strongly-fluorescing OH transitions compensate for the relatively weak 193 nm photodissociation cross-section of ground vibrational state H ₂O.; A high-sensitivity intensified charge-coupled device (ICCD) camera coupled to a UV lens collects the fluorescence from the OH from its initial and final locations thus providing the means of establishing unambiguous reference points in the experimental flows. A time-of-flight software (DaVis 6.0) determines the instantaneous velocity field based on displacement.; Detailed studies of OH formation, concentration levels, and depletion mechanisms have been conducted and are presented in this work. Chemical kinetic analyses for either low temperature (nonreacting) or high temperature (reacting) flowfields reveal the wide applicability of the newly developed velocity measurement technique. A previously developed chemical kinetics mechanism for room temperature air is tuned and validated. Fluorescence signal intensity, noise, and yields have been studied and compared to experimental results for HTV tag lifetimes. The HTV method is calibrated in a well-characterized flowfield and estimates on velocity accuracy calculations are presented. Finally, the HTV method is demonstrated measuring velocities in various reacting and nonreacting flowfields.