Temperature and number density measurement in non-uniform supersonic flowfields undergoing mixing using toluene PLIF thermometry

Abstract

visualization and quantification of many flow properties, including scalar concentration and temperature measurements, and it has been long used in both low-and highspeed flows. PLIF imaging based on a variety of tracers has been considered through the literature [1]. Historically, acetone and 3-pentanone [2, 3] have been widely used for scalar and temperature imaging, and more recently, naphthalene [4,5], krypton [6], 1-methylnaphthalene [7], and anisole [8] have been utilized in a variety of applications.Toluene has been identified as a versatile candidate for PLIF studies owing to its large fluorescence quantum yield and strong temperature sensitivity [9][10][11][12][13]. Toluene PLIF has found many applications for the study of mixing, especially in internal combustion engine studies [10,12,[14][15][16][17] where the strong oxygen quenching [18] has been used to obtain a direct measurement of air/fuel ratio [1,12,19]. Some of the challenges (particularly related to pressure effects) on the use of toluene for fuel/air ratio measurements have also been identified [12,15,20,21]. The properties of oxygen quenching on fluorescence have also been exploited for a direct measure of oxygen concentration in both single-tracer [22] and dual-tracer [23] configurations. Furthermore, because of the relatively large fluorescence quantum yield, toluene PLIF has also been successfully used for high-framing-rate imaging configurations where laser pulse energies are typically limited [14,20,24].Toluene PLIF has proven to be a particularly robust approach for temperature measurements. Both singleand dual-band-detection schemes have been proposed [10, 13] and applied [16,[25][26][27][28][29]. Its use as a thermometry technique stems from the strong temperature dependence of the integrated fluorescence quantum yield and the temperature-induced red shift of the spectral fluorescence Abstract Single-excitation, dual-band-collection toluene planar laser-induced fluorescence (PLIF) is used to measure temperature and number density (or partial pressure) fields in non-uniform supersonic complex flows in the presence of mixing and compressibility. The study provides a quantitative evaluation of the technique in transverse jets in supersonic crossflow (JISCF). It is found that toluene PLIF is highly effective in visualizing the structure of supersonic flows and that temperature can be accurately inferred with acceptable signal-to-noise ratios (of order 30) even when mixing occurs. The technique was applied to several JISCFs that differ by jet fluid properties with resulting different structures. In the presence of compressibility and mixing, it is found that the PLIF signal is non-unique, a feature that is used to identify the mixing region of the transverse jet. Measurement errors due to camera registration errors have also been quantified. Because of the complexity of the flowfield, it is found that minute misalignment (<0.1 pixels) between the two PLIF images can introduce measurable errors on the order of tens of Kelvins and significa...

The reacting transverse jet in supersonic crossflow: physics and properties (invited)

“…PLIF images). 57 In the PLIF images of figure 8, the mixing portion of the shear layer (E, H and K) is rendered as the high PLIF signal portion between the core of the jet and the post-shock region (both low PLIF signal regions). In some sense, this feature corresponds to the OH reacting layers presented in figure 4.…”

“…An account of the design and operation of this facility is given by Heltsley et al; 58 recent modeling efforts of this facility have been attempted by Örley et al 59,60 A more detailed accounting of the operation and calibration of the facility to produce the desired conditions used in this studies are also presented elsewhere 27,56,57,61 and are thus not repeated here in detail. The operational principle and a theoretical analysis of the performance of an expansion tube to generate high-enthalpy supersonic and hypersonic flows can be found in the seminal work of Trimpi.…”

“…Temperature is measured using the single-wavelength exci- 56 and extended to non-uniform supersonic flows by Gamba et al 57 A detailed description of the method can be found in the cited references. For the experiments, the excitation laser light was provided by a frequency quadrupled injection seeded Nd:YAG laser.…”

“…56 The information from each camera was then combined to infer the local temperature and tracer number density. 57 Because the tracer (toluene) was seeded only in the crossflow, the measured number density refers to the local number density of fluid originated from the crossflow.…”

“…Toluene PLIF temperature imaging 56 was used to measure the temperature and number density field in the JISCF system as a marker of mixing between (hot) crossflow and (cold) jet fluids. 57 In the following sections a brief description of the flow facility, experimental configurations and flow diagnostics are given, followed by a description of the main observations on the flowfield, flame structure and mixing properties. The importance of the main flow features that control ignition and flame stabilization processes will be emphasized.…”