Heat Flux Measurement Method And Instrument For Hypersonic Flight Test

In hypersonic flight, the accurate measurement of the aerodynamic heating load is the basis for the design of the aircraft thermal protection system(TPS), and it is also crucial for the improvement of the structural efficiency and the optimization of the flight performance. The heat flux generated from aerodynamic heating is generally calculated by some engineering methods and simulations based on some assumptions and experimental identifications or the high speed wind tunnel tests, which diverge from the real flight situation to some extent. The hypersonic flight test is the only technique to validate and improve the aero-heating algorithms, the high speed wind tunnel tests and the correlation between the ground and flight tests. Since the 1950 s, a lot of surface heat flux measurement techniques have been conducted in many countries based on the hypersonic flight tests via embedding and building-in measurement devices, accumulating precious flight test data. But many of these techniques are limited to the situations of high heat flux in the re-entry stage or the low heat flux with long time period. In the recent two decades, more and more investigations contributes to the development of the novel hypersonic vehicles, and the thermal protection techniques are highly promoted. The coupling between the thermal protection materials and the service environment is also very important and received more attention. Additionally, it is in a great demand to accurately acquire the aerodynamic heating load in harsh environment.Based on the cold wall heat flux measurement technique using embedding devices, a measurement program with wider scope of application and an inverse heat conduction algorithm are carried out in this dissertation to meet the developing demands for the development of hypersonic aircraft and the acquirement of aerodynamic heating load of real flight. In conjunction with the typical flight conditions, the heat flux measurement devices for flight tests are designed, and some experiments are carried out to determine the contact parameters. The phase change heat sink mechanism is introduced to avoid overloading. In addition, two calibration test platforms are built based on the error propagation system. Furthermore, the effectiveness of heat flux identification and structur al reliability of the measurement devices are verified through the calibration experiments and the gas-jet test platform. The details are listed as follows.By taking a cylinder with variable cross-section as the basic shape of the sensor, which comprises a one-dimensional area and a heat sink area, the transient heat flux is predicted using the Levenberg-Marquardt method. The temperature signals from two different measurement points are used in this method, one of which is treated as the variable of the objective function and the other as the boundary condition. The effectiveness of the Levenberg-Marquardt method for one-dimensional problems is also verified via the simulation experiment under various heat flux conditions, with various thermal physical properties of metals as the input parameters. Based on it, the oxygen free copper is chosen as the sensitive material. A multilayer transition deaden program consisting of wedge retainers is implemented to avoid the thermal mismatch induced heat transfer within the structures and alleviate the error of the one-dimensional model. Based on the predicted results of the one-dimensional heat transfer model, the optimization of heat flux is conducted using the DownhillSimplex method, in which the objective function is derived based on the measured temperature. Hence, the interference caused by the lateral heat transfer within structures is corrected. The improved plans by adding measurement points or the thermal barrier layers are also proposed, which are verified by simulation experiments.Based on the former experimental and analytical methods, a general design method is proposed considering the trajectory, the geometric profile, and the thermal protecting material and structure. The effects of wall temperature on t he heat transfer process and the inverse algorithm are analyzed under the condition of convection. Based on the trajectory and profile of Apollo, the heat flux is predicted by the reference enthalpy method, and the “wall temperature-convection heat flux” coupling method, which is incorporated with the finite element method, is introduced into the thermal analysis of sensor and the identification of heat flux. Besides, The thermal cutting-out of thermal protection structure caused by the sensor is also analyzed.To address the decrease of reliability caused by the thermal cutting-out of sensors, a metal with low melting point is used in the design of heat flux sensor, and the latent heat of phase change is utilized to keep the temperature below its melting point and thus control the energy flow of the whole structure. The copper foam is used to enhance the thermal conductivity and achieve the modification of phase change material. The equivalent thermal physical properties of the former structure are obtained based on simulation and verified by tests. It is shown that the method of using phase change material is more efficient than the method of increasing the mass of heat sink.The investigation on the experiment of heat flux calibration: the calibration experiment is used to confirm the input-output relationship of the calibrated instrument and evaluate measurement efficiency. Based on the error propagation system and the absolutely calibrated Gordon gages, a radiation calibration facility and a convection calibration facility are designed, with a heat flux of 100 k W/m2 and 270 k W/m2, respectively. The core part of the radiation calibration facility is a tungsten halogen lamp with an ellipsoidal structure, and its homogeneity and stability is evaluated by the ray tracing analysis and the Gordon gage measurement. The core part of the convection calibration facility is an air heater which consists of a grading heater, a controllable wind source and a nozzle; the subsonic flow around a flat model and the heat transfer process are simulated by the CFD analysis. The flow quality, the homogeneity and the stability of the model are also verified by experiments. Additionally, the uncertainty analysis is conducted, in which the extending factor equals to 2. The relative uncertainties of the radiation facility in three different states are 7.98%, 6.85%, and 6.44%, respectively, and that of the convection facility are 11.46%, 8.09%, and 6.95%, respectively.Experimental investigation on heat flux sensor: a heat flux sensor spe cimen is prepared, and the contact parameters is estimated using a specimen with a similar profile. Calibration experiments of radiation and convection are conducted by the former facilities, obtaining the correction factors of these two different heating mechanisms. Comprehensive tests of typical thermal protection structure containing sensor are conducted using the modified jet facility with rectangular nozzle, and thus validates the effectiveness of the prediction of heat flux. Furthermore, a split deaden sensor specimen containing phase change material is fabricated, and the thermal response of the sensor is characterized and analyzed using the radiation calibration facility.