Chapter IV: UV Radiation of Hypervelocity Flow Over Circular Cylinders
4.4 Radiation Measurements of Oxygen/Nitrogen Mixtures
Figure 4.15: Temperature profile from repeat experiments of M5-H6-A air compared with a state-to-state computation.
method. Statistics of the shot-to-shot repeatability is shown in Table 4.7. State- to-state calculations of bulk translational temperature and vibrational temperature of NO are also plotted. For the first 1 mm, excitation temperature fits show that the gas approaches the frozen temperature. This is confirmed with the state-to-state calculation, which shows that the vibrational and rotational/translational temperature are within 500 K of one another. After 1 mm, the computation shows that the gas is in thermal equilibrium. Extracted temperatures agree with the meanTv andTr profile of the state-to-state calculation, providing evidence that the single temperature fitting procedure can be used to gauge the relaxation of the gas in cases where radiation is not optically thin.
test gas. Radiation is measured in the experiment, with lines identified as the Si I electronic transitions. Similar to the radiation of the iron hollow cathode lamp, atomic radiation is formed as discrete lines in the spectrum. In the case of silicon, strong signals are observed in the lines, with intensities approaching as high as 7000 counts on the detector. A likely source of the silicon contamination is the vacuum grease used for preparing the o-rings for a vacuum seal. The sealant used for the facility is Dow Corning high vacuum grease, which is a silicone based lubricant.
(a) Detector Image of Contaminants (b) Extracted Spectrum
Figure 4.16: Detector image of silicon atomic lines and residual NO γ radiation for a N2 test gas experiment of M7-H8-A. White dashed line indicates surface of cylinder.
In addition, some residual NOγsignal is observable on the detector. The maximum signal of theγ(0,1) is measured to be 1144 counts. This is likely due to a combination of leaking in the driven section and air remaining from the flushing procedure. Figure 4.17 plots the integrated radiance for the entire wavelength range of the experiment.
Primarily consisting of Si I transitions, the radiation profile is measured to peak approximately 2 mm downstream of the location of the shock.
Radiation of Oxygen/Nitrogen Mixtures
Figure 4.18a shows emission spectra extracted at the x = 0.75 mm streamwise location for four test gas mixtures in the M7-H8-A freestream. Normalization is performed using the γ(0,1) peak of air. Overall detector intensity is observed to increase from theXO2 =0.273 case to theXO2 =0.105 case. The change in detector intensity is due to the reduced quenching of the excited states by other species such as O2. As the oxygen mole fraction is reduced further, detector intensity drops, as seen in the XO2 =0.063 mixture. In this limiting case, there is not enough oxygen
Figure 4.17: Integrated intensity of detector image of silicon radiation. Standoff distance for N2is taken from Table 4.1.
present to produce NO molecules in both the X and Astates, effectively reducing the amount of radiation for the extremely low oxygen cases.
(a) Spectra at x= 0.75 mm (b) Integrated Signal ofγ(0,1) Figure 4.18: Post-shock radiation of M7-H8-A for various mixtures. Figure 4.18a are spectra extracted at x = 0.75 mm downstream of the shock. Figure 4.18b are integrated signal profiles for theγ(0,1) vibrational band. Normalization is performed using the peak value of theγ(0,1) vibrational band for air in both plots.
In addition to changes in radiance, the vibrational band profiles change dramatically.
Figure 4.18b contains the integrated radiance profile of theγ(0,1) vibrational band for each test gas mixture for the M7-H8-A condition. As oxygen mole fraction is reduced, the peak of vibrational band intensity pushes downstream. This is related to the reduced rate of chemical reactions occurring, as nitrogen dissociation rates are typically slower than that of oxygen. Similar to Fig. 4.18a, intensities are also
observed to increase from theXO2 =0.273 case to the XO2 =0.105 case. The mole fraction ofXO2 =0.063 experiences a decrease in radiance, confirming observations in Fig 4.18a. As integration was performed for only theγ(0,1) vibrational band of an optically thin gas, the intensity profiles represent the number density of NO(A) along the shock layer. Identical trends in overall detector intensity and vibrational band profiles were observed in the M5-H6-A data set. It is to be noted that optical depth is affected as oxygen is removed. The equilibrium number density of NO species for XO2 = 0.063 is computed to be NNO = 2.18× 1016 cm−3, an order of magnitude lower than NNO = 1.04×1017 cm−3 for XO2 = 0.210 of M5-H6-A.
Therefore, post-shock gas of M5-H6-A approaches the optically thin limit as oxygen is removed.
(a) M7-H8-A (b) M5-H6-A
Figure 4.19: Integrated intensity profile of the dominant vibrational bands forXO2= 0.063 freestream mixtures.
The integrated radiance of other vibrational bands can be plotted for the lowest oxygen mole fraction to study variations of the population of electronically excited NO. Figure 4.19 shows extracted profiles ofXO2= 0.063 for both freestream condi- tions. The most notable difference between air and XO2= 0.063 is the increase in radiance for theγ(0,0) band compared to theγ(0,1) peak intensity. For M5-H6-A, dramatic changes in profiles are observed, where peak radiance now occurs at x = 1.25 mm and γ(0,2) no longer shares the intensity levels as γ(0,1). The shifts in peak intensities between the wavelength regions are also caused by the contributions of additional electronic transitions for low oxygen cases.