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Description
Shock and expansion tubes are important facilities used to investigate hypersonic conditions experienced at high Mach numbers and during planetary entry. However, they are limited by short test times depending on the facility design and test conditions used. The NASA Ames EAST shock tube, for example, has test times between 3 and 20 µs whereas the Chinese JF-12 facility has test times between 100 to 130 ms.
Optical emission spectroscopy (OES) is the primary tool used to capture spectral data on hypersonic impulse testing facilities. Short test times restrict the amount and quality of spectral data collected from the emitting test flow due to limitations in spectrometers.
The CCD and ICCD cameras used in spectrometers to capture spectral data on shock and expansion tubes only capture one image due to the low frame rate and sensitivity. Meaning multiple tests are often required to properly understand time history during experiments.
There are three important axes to consider when collecting spectral data: spectral, spatial and temporal. Spectrally resolved data helps identify the chemical species radiating in the flow, with an analysis of intensity providing temperature and number densities. Spatially resolved data allows for investigation of the post-shock relaxation region and other spatially distributed quantities. Temporally resolved data provides insight into the properties of the test gas and how it is changing over the test time; this would be particularly useful with quasi-steady, non-equilibrium, recirculating, turbulent and reacting flows.
An ideal spectrometer would be able to capture spectral data that is temporally, spatially and temporally resolved, without sacrificing its sensor’s sensitivity to low intensity emissions. Current setups with CCD and ICCD’s are only capable of spectrally and spatially resolved spectral data. Other sensor types have been used to collect temporally resolved spectral data with varying success.
Photomultiplier tubes (PMT) have been used as sensors in OES systems to collect temporally resolved data since becoming widely available in the 1930s. While being extremely sensitive and capable of high temporal resolution, the limitation comes from lack of a spatial and spectral dimension that modern ICCD based systems provide, rather providing intensity as a function of time at one location and wavelength.
Just like PMT's, streak cameras are also able to collect temporally resolved data. They can have both temporal and spectral dimensions. However, they do not simultaneously have a spatial dimension, meaning the streak camera can only spectrally resolve radiation from single point.
CCD and ICCD sensors spatial dimension makes them the best candidate for an ideal spectrometer, as seen in imaging spectrometers around the world. Now that ultra-highspeed cameras and image intensifiers have improved, and become more available, an optical emission spectrometer can now be designed allowing spectral data that is spectrally, spatially and temporally resolved to be captured on hypersonic impulse test facilities.
An ultra-highspeed optical emission spectrometer utilising a Phantom v2012 ultra-highspeed camera and a HiCATT 25 image intensifier has been used to successfully capture data that is spectrally, spatially and temporally resolved on the X2 expansion tube at the University of Queensland. This facility’s longest test time has been recorded at 100 µs. The spectrometer successfully captured spatially, spectrally and temporally resolved data at a frame rate of 100 kHz (100,000 FPS) on X2, and it is expected that a frame rate of 300 kHz could achieved.
The spectrometer achieved this speed when investigating contamination on X2, determining contaminant species. Due to the temporal resolved nature of the spectrometer, when sodium was deliberately added to certain locations in X2’s shock tube, it could be seen when it arrived at the test section, and how bright it was, at different times during the test flow.
After the experimental campaign, rigorous bench testing was conducted to fully understand the capabilities and how the spectral system worked. Upon bench testing of the system, discrepancies were noticed when the spectrometer was exposed to constant bright light source. The system would capture 500 frames of data, depending on the exposure time and gain setting of the intensifier, the number of counts detected by the sensor would decrease, despite a constant light source.
It was determined that the microchannel plate (MCP) in the intensifier, used to multiply the number of electrons after converting photons into electrons, was responsible for the drop in counts. The gain setting on the intensifier sets the level of electron multiplication. If the intensifier is unable to recharge the electrons in the MCP after a multiplication event, the gain will start to ‘drop’ reducing the multiplication effect. Over extended periods of time the gain drop can affect the intensity of light outputted by the intensifier. If this is not accounted for in intensity calibration of spectral data, it will increase uncertainties.
The investigation into contamination on X2 is just one example of the usefulness of having a spectrometer that can be spectrally, spatially and temporally resolved on hypersonic impulse test facilities. These types of spectrometers will help reduce the need for repeated shots, while increasing understanding of test flows in such facilities.
Summary
The development of an ultra-highspeed optical emission spectrometer has benefits other optical spectrometers do not. Primarily being sensitive enough to capture spectrally, spatially and temporally resolved data on hypersonic impulse test facilities with test times in the tens to hundreds of microseconds. However there are difficulties that have been encountered and worked around. This system has successfully captured data at a frame rate of 100 kHz, capturing 10 frames of data during the test time on X2. It is expected this can be pushed to a frame rate of 300 kHz.
