An overview on hypersonic flow research with infrared thermography

Abstract

The technological development achieved in instruments and methodology concerning both flights and ground hypersonic experiment (employed in space plane planning) goes towards an updating and a standardization of the heat flux technical measurements. In fact, the possibility to simulate high enthalpy flow relative to reentry condition by hypersonic arc-jet facility needs devoted methods to measure heat fluxes. Aim of this work is to demonstrate that InfraRed (IR) thermographic measurements with new heat flux sensor (IR-HFS) can be used as powerful tool in hypersonic high enthalpy flow research. 1. Introduction In the modern hypersonic aero-thermodynamic design all the components of the spatial vehicle (wings, propulsion, fuselage, Thermal Protection System) are strongly coupled with each other and the acquisition of the technologies necessary for the development of a space plane requires both flight and ground experiments. In particular, the design and the optimization of the Thermal Protection System, TPS are critical aspects either to ensure the structural integrity and habitability of the hypersonic vehicle or to improve its aerodynamic performances. This goal needs aero-thermodynamic heating measurement and catalytic evaluation of the TPS materials. The possibility to simulate the hypersonic heat flux loads in order to test material and to solve any criticality regarding shapes, local heating and aerothermodynamic details, is one of the main goal of the actual re-entry simulator facilities. Usually, measuring convective heat fluxes requires both a sensor (with its corresponding thermal model) and some temperature measurements. In the ordinary techniques, where temperature is measured by thermocouples, resistance temperature detectors or pyrometers, each transducer yields the heat flux at a single point, or in the space-averaged region of the same one; hence, in terms of spatial resolution, the sensor itself can be considered as zero-dimensional. This constraint makes experimental measurements particularly troublesome whenever temperature, and/or heat flux, fields exhibit high spatial gradients. The Infrared Scanning Radiometer (IRSR) constitutes a true two-dimensional temperature transducer since it allows the performance of accurate measurement of surface temperature maps even in the presence of relatively high spatial temperature gradients. Correspondingly, the heat flux sensor may become two-dimensional. In particular, infrared thermography can be fruitfully employed to measure convective heat fluxes, in both steady and transient techniques [1]. The thermal map obtained by means of currently available computerized thermographic systems is formed through a large amount of pixels (20k to 300k and more) so that IRSR can be practically regarded as a two-dimensional array of thin films. However, unlike standard thin films, which have a response time of the order of microseconds, the typical response time of IRSR is of the order of 10 -1 ÷10 -2 s.