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.

超音速剥蚀传热（HAHT）是一种复杂的多元物理过程，与多个领域密切相关，尤其是高速飞机和再入航天器的热防护系统。尽管有许多计算工具被用于模拟HAHT，但其中许多工具尚未通过实验测量进行验证。为了获得更精确的HAHT模型，必须以空间和时间分辨的方式收集高速流动中剥蚀体内部温度场的实验测量数据。然而，由于剥蚀所需的能量以及对当前实验技术的限制（通常仅限于点或表面温度测量），剥蚀体内的温度场难以进行实验测量。然而，通过跨学科研究，开发了一种基于聚烯烃的新型发光传感器。该传感器可嵌入到光学透明冰中，对温度变化极为敏感。因此，它可以用于测量冰内部的时空分辨率温度场，冰是HAHT研究中常用的材料。此外，该传感器具有自参照特性，非常适合测量与剥蚀相关的动态环境中的温度场。作为初步测试，该新型传感器被用于测量马赫数为2的流动中剥蚀半圆柱体内的温度场，并与钝体超音速流动中的对流加热的解析预测相符。随后，该发光传感器被用于测量马赫数为7的流动中剥蚀球模型的热量传递。使用发光温度传感器测量了球形测试样品的热量传递的三大类别：1. 传递到球体核心的热量，2. 反射在球体表面温度上升的热量，以及3. 反射在球体熔化和质量去除的热量。从这些测量中，得出了三个关键观察结果。首先，与通过剥蚀排除的热量相比，传递到剥蚀测试样品中心的热量不可忽视。其次，热量传导到剥蚀体的动力学对流动的滞止温度敏感。第三，在HAHT开始后，球体上特定位置剥蚀的速率保持恒定。除了这些关键观察结果外，还使用受稀疏识别非线性动力学（SINDy）算法启发的创新分析方法，开发了球体表面热量传递的模型。本研究中提出的方法描述了在超音速流动中，钝体旋转体的热量传递速率作为流动滞止温度、相对于滞止点的身体位置和自由流速度的函数。本工作的结果包括对剥蚀体热量传递速率的时空分辨率测量，这些测量以前尚未直接通过实验进行，以及一个提出的模型，该模型定量描述了高速流动中剥蚀体的热量传递速率。