Interpretation of optical emissions for sensors in liquid fueled combustors

Abstract

The optical emissions from a turbulent, nonpremixed, liquid fueled (n-heptane) combustor were investigated using two optical imaging systems: a high spectral resolution (~0.3 nm), single wavelength system; and a low resolution (~5 nm), multi-wavelength system intended to mimic the response of a practical sensor. The goal was to develop signal interpretation strategies that would allow an optical sensor system to monitor needed properties in liquid-fueled combustors and engines. The research focused on the chemiluminescent emissions from OH*, CH*, and C 2 *. Data were acquired for a range of fuel and air flow rates (equivalence ratios 0.6-1.3), and for radial slices through the nominally axisymmetric combustor over the complete combustor length. Without correction, both OH* and CH* emission were found to be good indicators of heat release rate only over a narrow band of lean equivalence ratios. In addition, OH* provides information on the postcombustion zone conditions. A nearly linear relationship was found between the overall equivalence ratio of the combustor and the ratio of the emission integrated over the combustor volume from two radical species, with the ratio C 2 */OH* exhibiting the greatest sensitivity. The correlation was also applied to estimate the radially averaged equivalence ratio of the reaction zone along the length of the combustor. With the multiwavelength sensor set to simultaneously monitor the broadband background from the flame, its background corrected output agreed well with the high spectral resolution results, suggesting that a properly designed optical sensor system would be capable of determining local combustor conditions such as reaction zone equivalence ratio and heat release rate.

Internal entrainment effects on high intensity distributed combustion using non-intrusive diagnostics

“…Consequently, one can deduce the equivalence ratio from the chemiluminescence intensities. Numerous researchers have shown that the ratios of chemiluminescence from different radicals, such as OH ⁄ /CH ⁄ and C 2 ⁄ /OH ⁄ , vary with equivalence ratio in simple flames [17][18][19][20][21][22][23][24] including liquid fueled flames [25]. In all this investigations, a similar trend was shown where the OH ⁄ /CH ⁄ ratio was shown to increase significantly for lean equivalence ratios.…”

A measuring method based on photodiodes for the diagnostic of optimal combustion conditions

“…In many works, the intensity emissions and the ratio of different radicals have been used in order to identify clearly the main flame zones, air-fuel ratio, and pollutant emissions amongst others. Thus, the ratios CH*/OH*, C 2 */OH* and C 2 */CH* show a nearly linear function of equivalence ratio [19,22]. Also, the correlation between chemiluniscence of any radicals and the flame temperature has been investigated [22].…”

“…Thus, the ratios CH*/OH*, C 2 */OH* and C 2 */CH* show a nearly linear function of equivalence ratio [19,22]. Also, the correlation between chemiluniscence of any radicals and the flame temperature has been investigated [22].…”

“…The specific installation of these devices is shown in figure 1, where the photodiode array is cooled by means of fresh gas and air flux and the distance between the photodiodes and the flame basis is 8cm. The reaction zone is observed, with a FOV of 72º, where the formation of these radicals is strong [3,22]. The experimental results were performed in a central heating boiler of 150 kW, equipped with a Riello GS20 gas burner.…”

“…In particular, the massive use of radiometric techniques [3,4,7,10,11,22] has contributed to a better understanding of the combustion processes. It is known that in the reaction zone of hydrocarbon flames, the oxygen of the air reacts with the fuel to liberate the chemical energy stored in the molecular bond.…”