Ultrasonic time of flight measurements have been used to estimate the interior temperature of propulsion systems remotely. All that is needed is acoustic access to the boundary in question and a suitable model for the heat transfer along the path of the pulse train. The interior temperature is then deduced from a change in the time of flight and the temperature dependent velocity factor, which is obtained for various materials as a calibration step. Because the acoustic pulse samples the entire temperature distribution, inverse data reduction routines have been shown to provide stable and accurate estimates of the unknown temperature boundary. However, this technique is even more interesting when applied to unknown heat flux boundaries. Normally, the estimation of heat fluxes is even more susceptible to uncertainty in the measurement compared to temperature estimates. However, ultrasonic sensors can be treated as extremely high-speed calorimeters where the heat flux is directly proportional to the measured signal. This work will show that heat flux is a more natural and stable quantity to estimate from ultrasonic time of flight. We have also introduced an approach for data reduction that makes use of a composite velocity factor, which is easier to measure.
Applications include thermal load non-destructive evaluation of hypersonic vehicle exterior surfaces, engine combustion chambers, and gun barrels. Current work is sponsored by AFRL. Significant contributors to this effort include Industrial Measurement Systems, Inc., University of North Carolina Charlotte, Purdue University, and Virginia Tech. The student lead is Mike Myers.
Fluorescent decay times of rare-earth doped ceramic oxide phosphors are known to be temperature dependent, which renders them useful for non-contact remote sensing thermometry applications. These materials, referred to as thermographic phosphors (TGP), are advantageous in comparison to other temperature-sensing materials because they are chemically stable under photoexcitation, can withstand high temperature or corrosive environments, and are immune to the presence of significant blackbody radiation. Nanophosphors exhibit remarkably different luminescent properties as compared to bulk phosphors. We are interested in the fabrication, testing, and design of nanophosphor materials for temperature measurement in industrial systems such as combustion engines, pressure-sensitive paints, and even biological systems. Recently, we have been working with cerium-doped yttrium aluminum garnet (YAG:Ce) and have found that substituting atoms in the host matrix significantly lowers the temperature range where the material can be used as a TGP. These results indicate that the luminescent properties of certain phosphors can be tuned to application specific requirements. Likewise, we are developing a theoretical model which can be used to predict thermo-luminescent properties of phosphors based on particle size, rare-earth dopant, and host matrix.
Sponsors: ORNL, NSF, Industrial partners
Thermal rectification is a phenomenon where transport is preferred in one direction over the opposite. Similar to the electrical diode, thermal rectification could have widespread applications including electronics cooling and thermoelectrics with the improved ability to control thermal transport. In the current work we are investigating two possible rectification producing systems. First we are using Monte Carlo simulations to model a device with asymmetric boundaries (sensitive to phonon direction and frequency) which results in self-biasing/thermally rectifying behavior. Second we are using molecular dynamics to investigate rectification effects in a system of two dissimilar materials divided by a single interface where we have shown that the difference in allowed phonon frequencies in the two materials gives different transport properties when a temperature gradient is reversed.
As devices are being built on smaller scales, a critical point is reached at which quantum effects become a governing factor in the behavior of devices. The ability to understand these effects will allow continued scaling and improved performance.
Superlattices are nanometer-scale layers of alternating materials (Si/Ge, for example). They have been proposed as thermoelectric devices because of their ability to limit thermal conductivity. Because the performance is inversely proportional to conductivity, devices can presumably be mode more efficient. Unfortunately, nanometer-thick layers confine electrons as well as reduce thermal conductivity, and performance is a direct function of electrical conductivity.
Using quantum simulations, the Thermal Engineering Lab is developing tools to estimate the performance of thermoelectric materials including superlattices, nanocrystalline composites and other quantum structures. We have already shown good agreement with measurements compared to superlattice structures and have identified opportunities for optimization through careful design of the next generation of structures.