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The two-color ratio pyrometry technique using a digital single-lens reflex camera has been used to measure the time-averaged and path-integrated temperature distribution in the radiating shock layer in a high-enthalpy flow. A 70 mm diameter cylindrical body with a 70 mm long spike was placed in a hypersonic shock tunnel, and the region behind the shock layer was investigated. The systematic error due to contributions from line emissions was corrected by monitoring the emission spectrum from this region using a spectrometer. The relative contributions due to line emissions on R, G, and B channels of the camera were 7.4%, 2.2%, and 0.4%, respectively. The temperature contours obtained clearly distinguished regions of highest temperature. The maximum absolute temperature obtained in the experiment was ∼2920  K±55  K, which was 20% lower than the stagnation temperature. This lower value is expected due to line-of-sight integration, time averaging, and losses in the flow. Strategies to overcome these limitations are also suggested in the paper.

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Effects of controlled heat addition into the high temperature, chemically reacting shock layer of a large angle (60°) blunt cone with a spherical nose have been experimentally investigated. The exothermic oxidation of ablated chromium from the surface of the cone at hypersonic Mach numbers triggers the heat release process. A conical skirt with a base diameter of 70 mm and a semi-apex angle of 30°, culminating into a nose of radius 30 mm, has been used as the test model. An in-house hypersonic free-piston driven shock tunnel facility, HST3, was used for the experiments at a stagnation enthalpy of 6.31 MJ/kg and a freestream Mach number of 9.84. The temperature distribution in the shock layer was experimentally measured by the two-color ratio pyrometry technique, using a Digital Single Lens Reflex (DSLR) camera as a pyrometer. The temperature field was corrected for discrete radiative line emissions obtained through emission spectroscopy. Surface heat flux measurements were taken using carefully calibrated thin film platinum heat transfer gauges mounted on an insulating Macor substrate. Shock stand-off distances were measured through Schlieren imaging of the flow using a high-speed camera at 20 000 frames per second, and also by a new intensity-scan based method using the processed color image from the DSLR camera. Calculations showed a 173 K rise in the temperature of the gas layer in the stagnation region due to chromium oxidation. The net surface heat flux on the blunt cone was also found to increase by about 31 W/cm2. The shock stand-off distance, as ascertained from Schlieren images, increased from about 3.82 mm (±1.4%) to 4.45 mm (±1.5%), a 17% rise. Analytical calculations, taking chromium oxidation reaction kinetics into consideration, to relate the total exothermic heat release to its distribution into various processes demonstrated that chromium oxidation releases about 78 W/cm2 energy in the stagnation region of the shock layer. 1.9% of this energy increases the temperature of the gas layer, 40% is convected back into the airframe, 0.4% is lost from the rear of the Macor substrate by conduction, and about 8% is radiated into the model. The remaining 50% of the heat was used up in pushing the shock layer away from the body by raising its density, thereby increasing shock stand-off distance.