The development of a light-collecting probe with high spatial resolution applicable to randomly fluctuating combustion fields

Abstract

A high-luminosity-light-collection system for highly spatial detection of chemiluminescence of radical species in flames has been developed. The system, multi-colour integrated Cassegrain receiving optics (MICRO) is based upon a Cassegrain-type configuration, which implies that it employs only reflective components (in combination with an optical fibre for light collection). It provides therefore spherical- and chromatic-aberration-free detection, which is of importance for high-spatial-resolution measurements and for the simultaneous monitoring of signals in different wavelength regions from a given spatial volume. The effective light-collection volume has been estimated to be only 1.6 mm × 0.2 mm × 0.2 mm by ray-tracing techniques, which is more than three orders of magnitude smaller than that provided by a corresponding simple single-lens system and comparable to that of laser-based techniques, e.g. Doppler anemometry. The system is also easily aligned since the active probe volume can be visualized by sending in visible light through the system in the reverse direction. In order to demonstrate the performance of the system, OH-radical chemiluminescence in a Bunsen flame was monitored using MICRO and compared with the ion-current signal from a Langmuir probe with a minute sensor tip. A good correlation between the fluctuations in the two signals could be obtained, proving the high spatial and temporal resolution of the MICRO system.

Numerical evaluation of equivalence ratio measurement using OH∗ and CH∗ chemiluminescence in premixed and non-premixed methane–air flames

“…It has been used for identification of the reaction zone [2], to compute Rayleigh index maps [3], as a marker of heat release rate [4][5][6], and to measure flame equivalence ratios [7][8][9][10][11][12]. By use of a Cassegrain optics-based Chemiluminescence Sensor (CS), similar to that used by Akamatsu et al [13] & Kojima et al [2], Hardalupas and Orain [7] demonstrated in their chemiluminescence study that the intensity ratio of the two excited radicals OH* and CH* is independent of strain rate in premixed counterflow flames at atmospheric pressure, in contrast to that of OH* and C 2 *, which is a function of strain rate.…”

Experimental and Numerical Study of Chemiluminescence Characteristics in Premixed Counterflow Flames of Methane based Fuel blends

“…The spatially and temporally resolved detection system is a MICRO (Multi-colour Integrated Cassegrain Receiving Optics) system 33,34 , modified in previous studies 5,6 . The Cassegrain-based optical Sensor (CS) with the working distance of 300 mm had a primary and a secondary mirror of 150 and 50 mm diameters respectively, see Figure. 2.…”

“…This ensured a probe volume of the sensor with nominal diameter of 200 µm and length of 1.6 mm 6 . There are two main advantages of the CS over the line of sight: one is avoiding chromatic aberrations for different wavelengths 33 , another is reducing the contribution of flame emissions from outside the probe volume 6,33 . The collected light from the CS was focused onto a pinhole placed in front of a UV-grade optical fibre (Thorlabs lnc, FT1500UMT), whose core diameter is 1500 µm and Numerical Aperture (NA) is 0.39.…”

Optical measurement and numerical simulation of spray combustion

“…The single-lens optics has poor spatial resolution due to the spherical aberration and suffers from chromatic aberration in multi-wavelength-light measurements. In order to resolve these problems involved in single-lens optics, we developed a light-collection probe of Cassegrain type of a finite conjugate ratio named the Multi-color Integrated Cassegrain Receiving Optics (MICRO) (Akamatsu et al, 1999). The MICRO consists of an optimized pair of concave and convex mirrors and a light-detection unit connected by an optical fiber cable.…”