Stray light is the main noise source for planar imaging measurement technique, which can affect the accuracy of results directly. A method of Structured Laser Illumination Planar Imaging (SLIPI) was used to solve this issue. The key of SLIPI is periodic modulation of laser spatial intensity and implementation of post filtering algorithm. In this paper, cylindrical micro lens array was used to modulate the spatial intensity of laser periodically, which was compared with Ronchi ruling. The post filtering algorithm adopts phase-locked detection method. The signal results can be separated from the noisy image using only one measurement image by this method. SLIPI method has been used in Temperature Sensitive Paints measurement experiments. A diagnostic optical path combining cylindrical micro lens array and cylindrical mirror was designed for the need of surface light source irradiation. The results show that the method of SLIPI can be applied to most planar imaging measurement techniques, and the accuracy of two-dimensional parameter measurement can be further improved.
In order to measure the 2-D velocimetry distribution without seed injection in the high-speed flow field of the engine, the 2-D HTV (hydroxyl tagging velocimetry) technique in the complex combustion field has been developed. The image processing method combining difference method with cross correlation method is presented. The background noise is suppressed by the difference method. The variable-template is used to perform cross correlation operation with experimental images, and the obtained correlation images is fitted with two-dimensional polynomial. Not only can the complex background interference be suppressed, but also can realize accurate extraction position of the tagging cross grid center.So the extraction accuracy is better than 0.25 pixels, when the image SNR is higher than 2. At the same time, based on Matlab software, the data processing program is written. The velocimetry distribution is gained by processing the experimental data in the flow field of the scramjet model. The speed calculation error is less than 5%, which meets the requirement of measurement accuracy of the system.
The key to improve Hydroxyl Tagging Velocimetry (HTV) Measurement precision in the laser supersonic combustion diagnosis research is to improve the effect of image processing. In terms of strong OH background and low signal-to-noise ratio (SNR) issues, the approach for HTV signal extraction in supersonic combustion filed is proposed. Firstly, the method of compensation-and-correction-window-filter and the progressive approach characteristic filtering window approach are adopted to remove background for image preprocessing. Then the algorithm combined image segmentation and the skeleton extraction is employed for signal extraction, that improves the signal identification ability in the interferences of fierce combustion zone of a mass of hydroxyl background, solves the insufficient precision problem of extracting hydroxyl effective signal, what's more, achieves the effective information of velocity distribution in combustion flow.
Nonlinear regime Two-line Atomic Fluorescence (NTLAF) is a promising technique for two-dimensional thermometry. A key challenge is seeding of indium atoms into flame. This work aims at investigating the mechanism of Indium LIF signal generation in a fuel-rich InCl<sub>3</sub>-ethanol premixed flame. Several types of images including natural emission of the flame itself, natural emission of CH, natural emission of OH, natural emission at 410 nm/451 nm of indium atom, and laser induced fluorescence at 410 nm/451 nm were obtained. The indium atom was generated in the flame front, and could survive in the post-flame zone for a while which is benefit for making NTLAF measurements. Further detail mechanism of fluorescence signals generation in InCl<sub>3</sub>-ethanol solution burning was investigated. The conclusion which probable to be drew is that to gain high NTLAF signals, the size of liquid droplets should be well controlled, neither to be too large nor to be gasified.
For the purpose of measuring the flow velocity in a scramjet test model, an special designed measurement system was established, including the strong vibration suppression, optical transport consideration, the movable device etc. The interference of the strong vibration to the velocity measurements was avoided by two ICCD cameras capturing the reference tag lines image and moved tag lines image together during an experiment. According to the tag lines image feature, data processing including correlation algorithm, data fitting by a Gauss function were used respectively to extract the positions of the reference tag lines and the moved tag lines. The velocity measurements were carried out at the isolation section and the cavity section. The results showed that the well SNR could be achieved in the H<sub>2</sub>/air combustion heating flow, but in the kerosene fuel combustion flow, the measurements images might be interfered by the strong OH background from the chemical reaction, and the signal intensity could be reduced due to the tag laser attenuation through the absorption by kerosene vapor. But when the combustor model was run at a low chemical equivalent, the interference could be suppressed to an accepted level.
In order to obtain the velocity with high dependability at extreme combustion condition, a 2-D interferometric Rayleigh scattering (IRS) velocimetry based on detecting the Doppler frequency shift of molecular scattering with Fabry–Perot etalon was developed. The 2-D IRS measurement system was set up with a multi-beam probe laser, aspherical lens collection optics, and a solid Fabry-Perot etalon. A multi-beam probe laser with 0.5mm intervals was formed by collimating a laser sheet passing through a cylindrical microlens arrays. The aspherical lens was used to enhance the intensity of Rayleigh scattering signal. The 2-D velocity results of a Mach 1.5 air flow were obtained. The velocity in the flow center is about 450 m/s. The reconstructed results are in accordance with the characteristic of flow, which indicates the validity of this technique.