Transient Thermal Measurements Using Thermographic Phosphors for Temperature Rate Estimates

Abstract

This paper addresses the potential for predicting heat flux from thermographic phosphor measurements. Temperature can be measured using thermographic phosphors by extracting the intensity decay of the phosphor, which is temperature dependent. This measured temperature can then be used to estimate boundary heat fluxes, which is often called the inverse heat conduction problem. Heating rate can also be estimated with the use of thermographic phosphors, from which heat flux can also be determined. In this case, the solution to the inverse problem appears more stable. The purpose of this work is to demonstrate the feasibility of measuring change in decay rates and the ability to determine the first derivative of temperature from these measurements. Preliminary analysis shows that by determining dT/dt instead of temperature, a better estimate of heat flux can be made. The experiment uses a millisecond phosphor, excited by an LED pulsed at 100 Hz. The phosphor is painted on a tungsten filament, which can be heated to hundreds of degrees in under a second. The temperature change during a single pulse is significant enough to affect the decay rate, which is necessary to achieve reasonable heating rate measurement. The measurements of heating rate are used to determine the volumetric generation rate (Joule heating) and the heat transfer loss from the system by convection and radiation. Early data show that estimates from heating rate data, as opposed to temperature data, result more accurate predictions with less error.