Engineering
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Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron
Chapters
Summary June 9th, 2016
Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.
Transcript
The overall goal of this experimental procedure is to characterize the material response and gas surface interaction phenomena of ablative thermal protection materials and high NTP flows to provide data for numerical model development and validation. This method can help answer key questions in the field of thermal protection material ground testing such as how the material decomposes and how the reactive bond layer is affected. The main advantage of this technique is that we only apply optical methods, which give a wide spectrum of information and which can be relatively easily standardized for material analysis.
Generally people new to the ground testing in plasma ven-tun-ele will struggle because of the complexity of the measurement techniques. With a visual demonstration, it will be a help. The implications of this technique are meant to be extended to many heat shield materials such as ceramic composites, and especially pyrolyzing carbon phenolics.
First align the optical system using a vertical and horizontal line laser by bringing all components to the same height as the test sample and aligning the lens perpendicular to the sample stagnation line. Focus the optical path by placing the lens at the calculated distance from the test sample and the optical fiber ends at the calculated distance from the lens. Illuminate the sample stagnation point with a pencil style Mercury calibration lamp and position the fiber ends at the location of the best focused image.
Once the lens fiber system is aligned, send a laser point through the spectrometer sided fiber ends and observe the focused laser on the sample side with a white paper sheet to confirm correct position and focusing in front of the test sample. Prevent any emission except that from the focal point from entering the optical fiber ends by enclosing the optical path for instance with black cardboard. Send a laser beam through the optical fibers to check that no light emitted by the fiber end is able to reach the lens directly.
Following this, observe the test sample with a high speed camera or HSC perpendicular to the sample surface. Use the sample lens system access for horizontal and vertical alignment of the camera optics making sure the center of field of view of the HSC coincides with the center of the lens and the sample stagnation point. Synchronize the HSC and emission spectrometers with a digital delay generator or DDG.
Trigger the HSC recording with a single voltage peak from the DDG and trigger each spectrum recording with the desired frequency. For radiometry, use a two color pyrometer for observation of the surface temperature in combination with a quartz window at the test chamber. To set up the HSC software, set the high exposure time to 90 milliseconds to align and focus the HSC prior to the experiment with the test sample in place and take a pre-test image.
Change the exposure time for the experiment. Set the post-trigger to maximum and set the correct recording rate to cover the full experiment. After setting the initial F-number to 16, set the DDG to the desired repetition rate at which the spectra shall be recorded by the spectrometers.
Next, set up the spectrometer acquisition software. After confirming that the optical system is positioned correctly, take a background image with each instrument and save it. Following this start the plasma facility and bring it to the desired test condition.
Then start recording of the HD camera. Then start recording of the pyrometers. Take a free stream spectrum with all spectrometers for calibration comparison.
When finished, lower the integration time from 200 milliseconds to 50 milliseconds to prevent saturation. Trigger the HSC and spectrometers via the DDG by pressing trig and setting the mode from external to internal. Then inject the test sample into the plasma flow.
Once the testing is complete, stop the DDG. After saving the HSC images, stop the pyrometer acquisition. Following this, send a laser point through the spectrometer sided optical fiber end's spectrometer side.
Observe the laser focus with HSC and save this image to mark the position of the spectrometer. After repeating the previous step with each spectrometer, place a chessboard at the position of the test sample and record the image with the HSC for calibration. Once the test sample has been removed, record its weight.
After taking photographs of the sample, store it in sample storage to protect the brittle charred layer composed of oxidized fibers. At this point, perform an intensity calibration of each optical system by placing a tungsten ribbon lamp in the focus of each collection optic inside the test chamber. Record the calibration lamp's spectrum.
Next observe the sample injection and ejection times on the HSC video file for correct test time estimation. Observe the pixel location of the test sample stagnation point at injection from the HSC video file. Export the images previously taken and find the pixels of the spectrometer probing locations as bright spots on the image indicating the X and Y positions.
Following this, open the file containing the wavelength vector of the calibrated spectra and identify and identify the row indices corresponding to the relevant wavelengths. Plot the spectrally integrated emission of each of spectrometer as a function of the spectrometer distances from the surface. For better interpretation of the results, perform a polynomial fit of the data and plot the results.
For SEM analysis, select one single well observable fiber with the SEM system. Estimate the virgin carbon fiber thickness and fiber length with the tools provided by the SEM system software according to the manufacturer's instructions. Cut the brittle material using a scalpel.
Then estimate the depth in which the fibers are thinned by comparing the thickness of ablated fibers to the virgin fiber thickness. The results show that caliper roll recession measurement generally resulted in larger values than those performed by HSC imaging. The HSC determined recession rates in air plasma did not differ much probably due to a diffusion controlled ablation regime.
Integrated CN admission intensities plotted over distance from the ablating surface show good agreement with respect to each other. CN violet experimental spectra at low and high pressure were compared to simulated spectra in order to obtain gas temperatures. The estimated temperatures yielded a high deviation from thermal equilibrium at low pressure.
The retrieved temperatures at low pressure were 8200 Kelvin for the translation of rotational temperature and 21, 000 kelvin for the vibrational electronic temperature close to the wall with the latter decreasing through the boundary layer. This is in contrast to the equilibrium condition throughout the boundary layer at higher pressure. Micro graphs demonstrated that carbon oxidation in air plasma led to an icicle shape of the ablated fibers with an oxidation depth of around 0.2 millimeters.
Bright sparking was observed during some ablation tests which might be caused by hot fiber clusters detaching from the surface. Ablation in nitrogen plasma led to highly degraded fibers along their surface which led to a slow recession of the material by nitridation. While attending is positive, it is important to remember that experimental on numerical calculation of the plasma jets are both necessary for the understanding of the acquired data.
This method can easily be performed also on pyrolyzing materials in order to answer additional questions like other dren-sen pyrolyze the outgassing varies over time and how its time scale is different from surface ablation processes. After its development, this technique paved the way for researchers in aerospace engineering to develop models for ablation of composite materials. Use experimental data on the material response in the gas phase.
Don't forget that working with lasers and carbon fiber materials can be hazardous, and precautions such as lab coats, gloves, glasses need to be taken when performing this procedure.
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