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Introduction

Fuel combustion is a transient chemical reaction kinetic process. Scientists use traditional photoelectric technology, combined with laser technology, spectroscopy technology, and image processing technology to analyse the temperature, composition and other parameters of the combustion field, and optimize the spray combustion of the engine. It is of great significance to improve combustion efficiency and reduce pollutants.

There are many methods for combustion diagnosis, some directly measure the whole process of combustion with high speed, some use a spectrometer and PMT to monitor some processes indicating chemical bond luminescence to analyse the fuel ratio, and some use laser technology to produce chemical reactions during combustion. Some intermediate products (such as CH, OH, etc.), these products will absorb the excitation laser and emit characteristic spectral signals. Scientists analyse these fluorescence spectra and other information, and can analyse the instantaneous structure of the flame, the concentration of components and other information.

Application:

By analysing the ignition performance of multi-component kerosene at low temperature, scientists found that adding certain activators can greatly reduce the ignition point of kerosene, so that it can fully burn at lower temperatures,

Fig.1：Low temperature ignition test experiment diagram

In the picture, scientists drop kerosene mixed with a certain proportion of activator onto the variable temperature table, which can be controlled from 100 degrees to more than 600 degrees. During combustion, the flame luminescence is collected into the spectrometer through an optical fibre, and the decay process of the luminescence band over time is monitored by the PMT. In the whole experiment, the emission band of the OH

radical of the combustion product was monitored at 306.5 nm, the spectrometer was set to the dispersion position to this band, and then PMT was used for monitoring. During the monitoring process, the high voltage of the PMT was set to 1000V, so that it was in the photon counting state, and then the data acquisition was connected to monitor and scan the OH luminescence in real time.

When lasers are applied into combustion experiments, many interesting phenomena occur. The first is Raman spectroscopy.

Fig.2：Raman Spectroscopy Analysis of Flame Combustion

At different times of combustion, due to the incomplete combustion of the fuel at the beginning, there are many intermediate products, to full combustion, and then to slowly cooling. The distribution of Raman signals of various components in the whole process is a gradual process, and we can use these signal changes to analyse and optimize the proportional concentration of combustion gases. In the experiment, Nd:YAG pulsed laser with low repetition frequency combined with ICCD is generally used for research and analysis.

If a dye laser is applied into the experiment, pumped after the Nd:YAG laser, we can choose some special wavelength, such as the 606nm laser, and then adjust it in the time domain with two 532nm lasers , under the condition of satisfying the phase matching of CARS, we can obtain the relevant CARS signal, which can be used to analyse the temperature and component concentration information of the combustion field.

Fig.3：Schematic diagram and experimental device diagram of CARS

During the combustion process, insufficient combustion will produce many intermediate products. We can also use the laser to excite these intermediate products in the flame to obtain fluorescence for analysis, that is, laser- induced fluorescence (LIF). If the laser is shaped through optical components such as cylindrical mirrors into a thin parallel plane of light, which we call planar laser-induced fluorescence (PLIF)

Fig.4：Schematic diagram of PLIF

In the experiment, we can use ICCD or high-speed camera to photograph the distribution of fluorescence of OH molecules in the combustion process.

Fig.5：flame OH PLIF images to study the turbulent structure distribution inside the flame with ICCD

Zolix provide various products in combustion diagnosis, such as opto-mechanical products required for the construction of optical path systems, such as cylindrical mirrors, time-delayed optical path systems, etc., and also provide various spectrometer configurations, such as the combination of spectrometer and PMT, the combination of spectrometer and various cameras is also available, especially the ICCD camera which is important in combustion diagnosis experiments.