In this research, to study the hypersonic boundary-layer transition, experiments were conducted on a large-scale flat plate with a length of 3.2 m at a zero angle of attack in the hypersonic shock tunnel duplicating flight conditions. Surface-mounted piezoelectric pressure sensors and coaxial thermocouples were, respectively, used to measure the pressure fluctuations and wall heat transfer. The spatial distribution of heat transfer was used to distinguish the transition. Under the test conditions of Ma = 7.0, T0 = 2120 K, and Re∞ = 6.08 × 105 m−1, no transition occurred, and under the test conditions of Ma = 7.0, T0 = 2220 K, and Re∞ = 1.23 × 106 m−1, the transition position was s = 2.06 m. The repeatability of the experiment was found to be good. Furthermore, focus was placed on the spectral and spatial/temporal evolution characteristics of pressure fluctuations in the laminar boundary layer. The experiment captured the three frequency distributions of mode waves in the laminar flow zone. Among the mode waves distributed in the three frequency bands, the low-/high-frequency bands were dominant, and the mid-frequency band exhibited a staged contribution. The amplitude energy percentages of the high- and low-frequency mode waves exhibited opposite trends in both time and space, which means that the disturbance energy will be distributed among the various harmonics in the laminar stage. 

Experimental study on the evolution of mode waves in laminar boundary layer on a large-scale flat plate

Measuring the heat flux density is one of the main tasks in the research field of hypersonic experimental aerodynamics. Considering the fact that refined wind tunnel tests require small-sized sensors, and the sensitivity of atomic layer thermopile (ALTP) heat-flux sensor is dependent on the effective length of the sensitive element, the sensitive elements are connected in series by conductive films. New-generation ALTP sensors with multiply enlarged sensitivities is developed in this study. The general features of new ALTP sensors, including the sensitivity, linearity and amplitude-frequency curves, are obtained by static and dynamic calibration tests. Then a discussion is presented on the tradeoff between high sensitivity and fast dynamic response. Finally, the practice performance of new ALTP sensors is confirmed by experiments conducted in the hypersonic low-density wind tunnel, since the frequently used thin-skin calorimeters and infrared thermographic techniques have the problems of short effective time and low signal-to-noise ratio. 

High-sensitivity atomic layer thermopile heat-flux sensor and its application in hypersonic low-density wind tunnel tests

Coaxial thermocouples have the advantages of fast response and good durability. They are widely used for heat transfer measurements in transient facilities, and researchers have also considered their use for long-duration heat transfer measurements. However, the model thickness, transverse heat transfer, and changes in the physical parameters of the materials with increasing temperature influence the accuracy of heat transfer measurements. A numerical analysis of coaxial thermocouples is conducted to determine the above influences on the measurement deviation. The minimum deviation is obtained if the thermal effusivity of chromel that changes with the surface temperature is used to derive the heat flux from the surface temperature. The deviation of the heat flux is less than 5.5% when the Fourier number is smaller than 0.255 and 10% when the Fourier number is smaller than 0.520. The results provide guidance for the design of test models and coaxial thermocouples in long-duration heat transfer measurements. The numerical calculation results are verified by a laser radiation heating experiment, and heat transfer measurements using coaxial thermocouples in an arc tunnel with a test time of several seconds are performed.

https://www.mdpi.com/1424-8220/20/18/5254/pdf

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Influence of test model material on the accuracy of transient heat transfer measurements in impulse facilities