Internal Temperature Measurement of Ablative Bodies in High-Speed Flow

Hypersonic ablative heat transfer (HAHT) is a complex multi-physics process relevant to a number of fields, particularly thermal protection systems for high-speed aircraft and reentry spacecraft. While there are a large number of computational tools that are used to model HAHT, many of these tools have not yet been validated via experimental measurements. To obtain better models for HAHT, experimental measurements of the internal temperature field of ablating bodies in high speed flows must be collected in a spatially and temporally resolved manner. However, temperature fields within ablating bodies are difficult to measure experimentally due to energy requirements for ablation and limitations of current experimental techniques, which are usually restricted to either point or surface temperature measurements. However, through a multi-disciplinary study, a novel luminescent sensor based on pyranine was developed. The novel sensor can be embedded within optically transparent ice and is highly temperature sensitive. The sensor can thus be used to measure the spatially and temporally resolved temperature field within ice, a material commonly used in HAHT studies. Additionally, the sensor is a self-referencing, meaning that it is perfectly suited to measure temperature fields in the dynamic environments associated with ablation. As a preliminary test, the novel sensor was used to measure the temperature field inside an ablating half-cylinder in Mach 2 flow and showed agreement with analytical predictions for convective heating of a blunt body in supersonic flow. The luminescent sensor was then employed to measure the heat transfer into ablating sphere models in Mach 7 flow. The luminescent temperature sensor was used to measure the three main categories of heat transfer to the spherical test articles: 1. The heat conducted into the core of the sphere, 2. The heat reflected in a temperature rise at the surface of the sphere, and 3. The heat reflected in the melting and removing of mass from the sphere. From these measurements, three key observations were made. First, the heat conducted toward the center of an ablating test article is non-negligible compared with heat rejected through ablation. Secondly, the dynamics of heat conduction into an ablating bodies is sensitive to the stagnation temperature of the flow. Thirdly, the rate of ablation for a particular location on a sphere undergoing HAHT is constant after ablation has begun. In addition to these key observations, a model for the heat transfer over the surface of the sphere was developed using a novel analysis technique inspired by the Sparse Identification of Nonlinear Dynamics (SINDy) algorithm. The model proposed in this work describes the heat transfer rate over a blunt body of revolution in hypersonic flow as a function of flow stagnation temperature, location on the body relative to the stagnation point, and freestream flow velocity. The results of this work include spatially and temporally resolved measurements of the heat transfer rate into an ablating body, which had not been directly measured experimentally, as well as a proposed model that quantitatively describes the heat transfer rate for ablating bodies in high-speed flows.