Combustion involves very complicated physical and chemical reactions of fuels. Chemical reactions transform the fuel energy into thermal energy, accompanied by high temperature as well as combustion products at high pressure. The thermal energy then drives mechanical devices for mechanical movements and greatly promotes the industrial development. However, the combustion efficiency and the working temperature range of the fuel determine the performance and service life of the combustion equipment. Also, the combustion process inevitably generates carbon oxides, nitrogen oxides and other pollutants, which can seriously damage human health and the global environment. It is essential to explore the reactions of complicated combustion fields and reveal their states in real time for combustion optimization and intrinsic exploitations.
The distributions of temperature field and gas component concentration inside the combustion reveal the combustion performance more intuitively. The transient changes of the flame temperature directly reflect the stability of the combustion process, and are closely related to the combustion efficiency, gas pollutant emission and unburned carbon loss. The gas component concentration distribution is also an important indicator of the fuel combustion efficiency and combustion cleanliness. For the combustion reaction mechanism and combustion performance improvement, the online monitoring of temperature and gas component concentration is the prerequisite. However, these reactions often occur in harsh environments with high temperatures and pressures, and the confined layout of the measurement space poses a serious challenge to these measurements.
With the development and innovation of lasers, laser spectroscopy has been widely used in combustion monitoring and turns to be one of the important tools for combustion diagnosis. The continuous vibration in the combustor, the radiation from the violent fluctuation of the flame, and the high-speed turbulence of the flow all bring great distortions into the detection of optical intensities. Meanwhile, the actual combustion process changes very drastically, and the flame parameters, such as temperature, gas fraction concentration, and flow rate, are non-uniformly distributed in the confined space. If only the projections along a single laser path are measured, the spatial resolution along the path is missing and it fails to reveal the distribution along the path. For multi-dimensional imaging of gas parameters in the combustion field, absorption data from multiple laser paths across the region of interest are used to reconstruct the distributions inside by tomographic techniques.
In recent decades, laser absorption spectroscopy (LAS) has been widely used in combustion diagnosis, benefiting from the development of low-cost and easy-to-use distributed feedback laser diodes. As a non-contact method with high sensitivity and rapid response, LAS has been combined with computed tomography (CT) methods for cross sectional imaging by using spectral data from multiple laser paths at multiple angles. In this way, real time visualizations of flame temperature and gas component concentration distributions are realized for postprocessing of the combustion reaction mechanism. LAS is also a preferable technology in complicated combustion diagnosis due to its advantages of simple structure and good environment adaptability.
Laser absorption spectroscopy tomography and its application in monitoring of dynamic and complex combustion field are reviewed. Firstly, the measurement principles of common LAS methods, including direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS) and amplitude modulation spectroscopy (AMS), are briefed. The application of these measurement techniques to the intrinsic parameter monitoring in combustion field along a single laser path is also described. Secondly, the state-of-the-art of optical sensing module and circuit module is illustrated for LAS tomography instruments. For combustion field of interest in different cases, tomogrpahic sensors in terms of moving scanning sensors and fixed angle sensors are compared for specific applications. Also, data acquisition systems for the tomographic images are included, such as systems of high frame rate raw signals at high speed for a short period of time and systems at a low frame rate for a long period of monitoring time. Then the principle and development of LAS imaging technology are introduced. Image reconstruction methods, e.g., analytical method, iterative method and nonlinear method are presented to monitor the intrinsic parameters of complex combustion field. These methods have unique advantages in certain applications, such as fast solution speed or high solving accuracy. Finally, the specific applications of LAS tomography in laboratory flames and harsh field experiments are briefed.
Laser absorption spectroscopy has made great strides in spectral acquisition methods, data acquisition systems, image reconstruction algorithms and other key techniques, and has got progress in the application of combustion field parameters monitoring in both laboratory and industrial sites. However, there still exist urgent needs for further developments and thorough investigations, including but not limited to the development of wide spectrum laser sources, spectral data acquisition in extreme environments, image reconstruction models in cases of very few angular projections, new image reconstruction methods incorporating combustion models, and sensor systems suitable for ultra-high dynamics, etc. Further in-depth studies are expected to meet the increasing demand for onsite applications over wide temperature ranges, high velocity dynamics and multi-component distributions.
