Uncertainty in velocity measurement based on diode-laser absorption in nonuniform flows

Abstract

This work investigates the error caused by nonuniformities along the line-of-sight in velocity measurement using tunable diode-laser absorption spectroscopy (TDLAS). Past work has demonstrated TDLAS as an attractive diagnostic technique for measuring velocity, which is inferred from the Doppler shift of two absorption features using two crossing laser beams. However, because TDLAS is line-of-sight in nature, the obtained velocity is a spatially averaged value along the probing laser beams. As a result, nonuniformities in the flow can cause uncertainty in the velocity measurement. Therefore, it is the goal of this work to quantify the uncertainty caused by various nonuniformities typically encountered in practice, including boundary layer effects, the divergence/convergence of the flow, and the methods used to fit the Doppler shift. Systematic analyses are performed to quantify the uncertainty under various conditions, and case studies are reported to illustrate the usefulness of such analysis in interpreting experimental data obtained from a scramjet facility. We expect this work to be valuable for the design and optimization of TDLAS-based velocimetry, and also for the quantitative interpretation of the measurements.

Infrared laser-absorption sensing for combustion gases

“…Despite this complication several researchers have 1325 developed strategies to provide useful and quantitative measurements of gas properties in non-uniform gases [172,173,32,174,175,176,177,178,179,180,181,182,171]. This section will focus on discussing solutions applicable to single-LOS measurements, however, a discussion of some of the most widely used and emerging tomographic LAS techniques will conclude this section.…”

“…Several researchers have developed strategies to identify temperature non-uniformities [174,178] and quantify the measurement uncertainty they induce [182]. Seitzman and Scully [174] developed a broadband sensor for monitoring temperature non-uniformity and H 2 O mole fraction in a gas-turbine combustor.…”

Two-color absorption spectroscopy strategy for measuring the column density and path average temperature of the absorbing species in nonuniform gases

“…However, there are many important applications in which absorption spectroscopy measurements are needed to characterize highly nonuniform environments that commonly result from heat transfer, flow mixing, combustion, and phase change. For example, absorption measurements have been used to study flames [6], quantify combustion progress in engines [7], and measure bulk flow speed and temperature in various facilities [8,9]. As a result, some researchers have developed absorption spectroscopy strategies for nonuniform flows [6][7][8][9][10][11][12][13][14][15][16][17].…”

“…For example, absorption measurements have been used to study flames [6], quantify combustion progress in engines [7], and measure bulk flow speed and temperature in various facilities [8,9]. As a result, some researchers have developed absorption spectroscopy strategies for nonuniform flows [6][7][8][9][10][11][12][13][14][15][16][17]. More specifically, strategies have been designed to identify nonuniformities [10,11], reduce sensitivity to nonuniformities [7,10,12], address boundary layer effects [6,9,13], provide active control of nonuniform flows [14,15], and actively resolve LOS distributions of temperature [16,17].…”

Multiplexed absorption tomography with calibration-free wavelength modulation spectroscopy

“…Laser absorption spectroscopy has become a ubiquitous tool for combustion scientists due to its ease of implementation, species-selectivity, capability of measuring both scalar and vector flow parameters (i.e., temperature, species concentration, pressure, and velocity), 1,2 and high-sensitivity when combined with cavity enhanced/ring-down spectroscopy (CEAS/CRDS), [3][4][5][6] wavelength modulation spectroscopy (WMS), 7,8 or photoacoustic spectroscopy. 9 Over the past decades, absorption spectroscopy has been empowered by significant advances and reduced costs of lasers, detectors, and data acquisition systems and matured into a flexible and sensitive technique.…”