Effect of fluctuations on time-averaged multi-line NO-LIF thermometry measurements of the gas-phase temperature

Abstract

chemistry and therefore, e.g., combustion efficiency and pollutant formation. Multi-line nitric oxide (NO)-laserinduced fluorescence (LIF) thermometry imaging enables accurate measurements of gas-phase temperature distributions in a spatial plane without the necessity of calibration [1]. The technique relies on the temperature dependence of NO LIF excitation spectra [1][2][3]. In thermal equilibrium, the temperature dependence of the population of the rotational ground state energy levels is described by the Boltzmann distribution. A laser is tuned across the vibronic transitions of the electronic A-X absorption band and the population fractions of individual rotational quantum states are probed by recording the respective LIF signal intensity with an intensified CCD camera. Excitation spectra are then extracted for each pixel from the stack of images taken for each laser wavelength position. Simulated spectra are then fitted to the experimental spectra with absolute temperature as the adjustable parameter. Spectral line broadening and background signals (e.g., scattered laser light or broad-band fluorescence) are included in the fitting routine.Multi-line techniques have demonstrated their robustness in stable low-pressure [4,5], atmospheric pressure [6], and high-pressure flames [7,8] as well as in systems with a large scattering background such as sooting flames [1] and spray systems [9,10]. In contrast to conventional two-color temperature imaging techniques (e.g., [11]), the multi-line approach has the advantage of being calibrationfree and robust against background signal as long as the shape of the excitation spectra can still be distinguished on top of the background signal and the background does not strongly vary with excitation wavelength within the investigated spectral range. Therefore, especially in highpressure flames, NO-LIF transitions must be chosen that do not interfere with LIF from hot O 2 [12]. The method, however, lacks single-shot capability because the spectra Abstract Multi-line NO laser-induced fluorescence (LIF) thermometry enables accurate gas-phase temperature imaging in combustion systems through least-squares fitting of excitation spectra. The required excitation wavelength scan takes several minutes which systematic biases the results in case of temperature fluctuations. In this work, the effect of various types (linear, Gaussian and bimodal) and amplitudes of temperature fluctuations is quantified based on simulated NO-LIF excitation spectra. Temperature fluctuations of less than ±5 % result in a negligible error of less than ±1 % in temperature for all cases. Bimodal temperature distributions have the largest effect on the determined temperature. Symmetric temperature fluctuations around 900 K have a negligible effect. At lower mean temperatures, fluctuations cause a positive bias leading to over-predicted mean temperatures, while at higher temperatures the bias is negative. The results of the theoretical analysis were applied as a guide for interpreting experimental multi-line ...

The accuracy and precision of multi-line NO-LIF thermometry in a wide range of pressures and temperatures

“…Moreover, the calculations using Eqs. ( 12), ( 13) and (14) show that the total line broadening at 293 K is 0.28 cm −1 , which is slightly higher than the values obtained at 1599 and 1556 K, 0.26 cm −1 .…”

“…The limitation of the technique is that it requires a data acquisition period longer than the timescales of turbulence and chemical reaction. Nonetheless, the technique has been applied in mildly turbulent flames to obtain temporally averaged temperatures [14,15]. However, the temperature bias observed in the flame contour region, where the ambient air and burnt gases mix vigorously, could be as high as ±25%.…”