An Improved Data Reduction Technique for Heat Transfer Measurements in Hypersonic Flows

The re-entry phase is one of the most critical phases in the lifetime of most space missions. When the spacecraft comes into contact with the atmosphere, it is exposed to intense stresses, both mechanical and thermal. The experimental analysis of this phase is therefore of primary importance for the design of spacecrafts and for the dimensioning of thermal protection systems. High enthalpy wind tunnels are designed to simulate the extreme conditions and the complex flow phenomena which develop during re-entry. It is therefore understood how the measurement of heat fluxes is one of the crucial goals of the tests in high enthalpy wind tunnel. The measurement of heat fluxes generally is not made directly but starting from transient temperature measurements on the surface of the model; as such, this is a typical inverse problem and usually implies great sensitivity to measurement errors. The purpose of this work has been to develop a new heat flux sensor to improve the accuracy and stability of the sensors currently used, and it is based on the minimization of the functional of the sum of the mean square errors between temperature transients measured experimentally and temperature transients generated by the numerical solution of the heat conduction equation inside the model, through an optimization process in which the physical parameters h and Taw, on which the temperature rise depends, are estimated. Experimental temperature measurements are performed by means of infrared thermography. To increase the accuracy of thermographic measurements, a new method of optical/geometrical reconstruction has been developed. This method allows to identify with precision the point on the three-dimensional surface of the model on the IR image, and it also permits to take directional emissivity into account to improve the accuracy of the thermographic measurements. In chapter one a general introduction on infrared thermography is given, and the reason why a geometrical reconstruction technique is desirable is made clear. The resection technique, based on the well established pinhole camera model, is subsequently explained in chapter two. The new heat flux sensor is illustrated in chapter three; the limits of the classical thin film, the most widely used heat flux sensor, are highlighted and the mathematical model on which the new sensor is based is explained. A full numerical validation of the sensor and of its limits of applicability has been performed showing that the sensor, in its one-parameter estimation form, can be used in a wide variety of test conditions. The experimental implementation, both of the new geometrical reconstruction technique and of the new heat flux sensor, is illustrated in chapter 4 and 5. The experiments in chapter 4 have been performed in Alta’s HEAT wind tunnel on a double cone model. This is a classical test for which both previous experimental results and numerical simulations are available. It has infact been designed ad hoc for the purpose of code validation within the CAST project. The test illustrated in chapter five has been performed in CIRA’s Scirocco wind tunnel. The purpose of the test was to investigate the shock wave boundary layer interaction on a full-scale model of EXPERT 20deg flap. The materials used in the test are those intended to be used on the capsule itself. The two test cases illustrated in this work have provided a good initial experimental validation for the new heat flux sensor and have shown that it can be successfully used, coupled with the implemented geometrical reconstruction technique, both in test cases designed ad hoc and in simulations of real life models, which do not require any preliminary preparation for the sensor to be used.