Research
Sponsors: |
 |
 |
 |
 |
 |
 |
 |
| 
|
| NSF | AFRL | TVA | IMS | DOE | Darpa | Vanderbilt | Sandia | DTRA | LM/ATL |
Ultrasonic pyrometry
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.
Sponsor: AFRL
Thermographic phosphors
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
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.
Sponsor: DARPA
Quantum transport in energy conversion devices
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.
Sponsor: NSF