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.
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 H2/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.
The OH can be generated from photo-dissociation of water in the gas phase, and the generated OH has served in tagging velocimetry using the time-flight method. The hydroxyl tagging mechanism has the advantages of non-seeding, kindly flow following character, but its application in the reaction region is limited for the fluorescence interference from nascent OH. In this paper, we explored the laser induced fluorescence spectrum of OH both from burning and photodissociation. A photo-dissociation laser induced fluorescence (PD-LIF) system with optical multichannel analysis instrument (OMA) for spectrum analysis was developed. Based on multichannel mechanism, the LIF spectrum of OH from photo-dissociation and burning were acquired simultaneously. The temporal spectrum profiles of dissociation OH both in flame and air were taken by varying the pump–probe delay. The normalized emission spectrum in flame showed a process of rotational relaxation while in air the spectrum was almost not changed. The fluorescence intensity was precisely proportional to the base states population, so we can get certain states that the OH from dissociation was predominant from the fluorescence intensity ratio of OH. This result can be further utilized for hydroxyl tagging velocimetry technology (HTV) which was less affected by burning OH.
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 InCl3-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 InCl3-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.
In order to achieve the two-dimensional (2-D) velocity measurement of a flow field at extreme condition, a 2-D interferometric Rayleigh scattering (IRS) velocimetry using a multibeam probe laser was developed. The method using a multibeam probe laser can record the reference interference signal and the flow interference signal simultaneously. What is more, this method can solve the problem of signal overlap using the laser sheet detection method. The 2-D IRS measurement system was set up with a multibeam probe laser, aspherical lens collection optics, and a solid Fabry–Perot etalon. A multibeam probe laser with 0.5-mm intervals was formed by collimating a laser sheet passing through a cylindrical microlens arrays. The aspherical lens was used to enhance the intensity of the Rayleigh scattering signal. The 2-D velocity field results of a Mach 1.5 air flow were obtained. The velocity in the flow center is about 450 m/s. The reconstructed results fit well with the characteristic of flow, which indicate the validity of this technique.
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.
With the concern of environmental protection and reducing the fossil fuel consumption, combustion processes need to be more efficient and less contaminable. Therefore, the ability to obtain important thermophysical parameters is crucial to combustion research and combustor design. Traditional surveying techniques were difficult to apply in a confined space, especially the physically intrusions of detectors can alter the combustion processes. Laser-based diagnostic techniques, like CARS, SVRS, PLIF and TDLAS, allow the in situ, non-intrusive, spatially and temporally resolved measurements of combustion parameters in hostile environments. We report here a new non-intrusive optical diagnostic technique, based on laser-induced thermal grating. Thermal gratings generated in NO2/N2 binary mixtures, arise from the nonlinear interaction between the medium and the light radiation from the interference of two pulsed, frequency-doubled Nd:YAG lasers (532 nm). This leads to the formation of a dynamic grating through the resonant absorption and the subsequent collisional relaxation. By the temporally resolved detection of a continuous wave, frequency-doubled Nd:YVO4 probe laser beam (671 nm) diffracted by LITG. The temporal behavior of the signal is a function of the local temperature and other properties of gas, various parameters of the target gas can be extracted by analyzing the signal. The accurate singleshot temperature measurements were carried out at different test conditions using a stainless steel pressurized cell, data averaged on 100 laser shots were compared with simultaneously recorded thermocouple data, and the results were consistent with each other. The LITG signal is shown to grow with increasing the gas pressure and is spatially coherent, which makes the LITG thermometry technique a promising candidate in high pressure environments.
Laser induced fluorescence has the potential to get two dimension temperature and concentration map in combustion. For understanding the fluorescence technology exactly, A detailed experiment has been developed to investigate the fluorescence spectrum. The time variation of spectrum is first presented by changing the pump-probe time delay. And this is followed by demonstrate the response of fluorescence to laser excitation. As showed by the experiment result, The intensity ratio of the (1, 1) and (0, 0) fluorescence band was varied both with time and the exiting laser line, we attribute this phenomenon to the difference rotational distribution of excited OH. And the new rule, which exciting a specific rotational level in the A2Σ(υ ' =1) excited state from two difference rotational levels in the X 2Π(υ " = 0) ground state, was experimental confirmed, and this will be a foundation to the future two line laser induced fluorescence thermometry.
The velocity components of the high temperature high speed flow produced by a simulative device were diagnosed by single-line hydroxyl tagging velocimetry. The simulative device was driven by H2/air combustion gas, worked like a shock tube, held about 10-ms. The HTV tagging lines were put after the exit of device, and the experimental images at flow’s different regions were acquired through changing the tagging lines’ positions corresponding to the exit of device. And the velocity components along the device axis on tagging lines were calculated from the images. The results indicated that the velocity values of the flow in compress region were much slower than those in expansion region. Between the flow’s first expansion and compress regions, the velocity values at center were higher than those at both sides, but in the flow’s second expansion region, the velocity values at center were slower than that at both sides.
A mobile diagnostic system based on Unstable-resonator Spatially Enhanced Detection Coherent anti-Stokes Raman
Scattering (USED CARS) geometry was established, which was used for temperature measurements in a model
supersonic combustor. The temporal and spatial resolution of this mobile CARS system are 10ns and Ф0.1mm×5mm
respectively. Single pulse nitrogen CARS spectrum with high signal to noise ratio(SNR) were obtained at the exhaust of
model combustor through high stability optical design, and the temperatures were acquired by Levenberg-Marquarat
fitting algorithm. The mean temperature of the unstable combustion is 1412K, and the mean temperature of the stable
combustion is 1705K.