Timothy P. Koehler

Direct Simulation Monte Carlo: The Quest for Speed.

In the 50 years since its invention, the acceptance and applicability of the DSMC method have increased significantly. Extensive verification and validation efforts have led to its greater acceptance, whereas the increase in computer speed has been the main factor behind its greater applicability. As the performance of a single processor reaches its limit, massively parallel computing is expected to play an even stronger role in its future development.

Measuring the Thermal Conductivity of Porous, Transparent SiO2 Films With Time Domain Thermoreflectance

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO 2 -based nanostructured films.

Direct simulation Monte Carlo investigation of the Richtmyer-Meshkov instability

The Richtmyer-Meshkov instability (RMI) is investigated using the Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics. Due to the inherent statistical noise and the significant computational requirements, DSMC is hardly ever applied to hydrodynamic flows. Here, DSMC RMI simulations are performed to quantify the shock-driven growth of a single-mode perturbation on the interface between two atmospheric-pressure monatomic gases prior to re-shocking as a function of the Atwood and Mach numbers. The DSMC results qualitatively reproduce all features of the RMI and are in reasonable quantitative agreement with existing theoretical and empirical models. Consistent with previous work in this field, the DSMC simulations indicate that RMI growth follows a universal behavior.

Thermal Contact Conductance of Radiation-Aged Thermal Interface Materials for Space Applications

Thermal interface materials (TIMs) serve a critical role in thermal management by enhancing heat transfer across contact interfaces. Specifically, they are most commonly used in electronics to enhance the flow of heat from source to sink by decreasing the overall thermal resistance of the system. In space, these materials are exposed to high doses of Gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years, total radiation exposure can be significant and can adversely affect the thermal contact resistance (TCR) of the TIM. In this manuscript, we report the effect of radiation-aging on the TCC of several commercially available electrically insulating, thermally conductive interface materials that are commonly used in satellite systems. Although radiation dose levels can vary significantly during the course of a space mission, a dosing of 10 Mrad per year for TIMs is a reasonable estimate. The TIMs were aged in a Gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad to simulate mission lengths of 5 and 10 years, respectively. The TCR of each radiation-aged sample, as well as un-aged samples, were measured under vacuum (less than 3 × 10−4 Pa). Radiation-aging of the TIMs led to a significant increase in the TCR of the tested samples. For example, the pressure-dependent TCR was shown to increase 20–150% for Cho-Therm 1671 and 50–250% for ThermaCool R10404 samples subjected to 50 Mrad of gamma-ray irradiation. These results show that radiation-aging of TIMs cannot be ignored in the design and simulation of space systems.Copyright

Effect of Gamma-Ray Irradiation on the Thermal Contact Conductance of Carbon Nanotube Thermal Interface Materials

Thermal interface materials (TIMs) serve a critical role in the thermal management of electronic systems by enhancing the flow of heat from source to sink. Nanostructured materials, such as arrays of carbon nanotubes (CNTs) have been shown to outperform many commercially available TIMs due to their low intrinsic resistance and large compliance that enables them to conform to rough surfaces. These characteristics, combined with their low density and ability to withstand vacuum environments and extreme temperatures, make CNT-based TIMs very suitable for space applications. In space, materials are exposed to high doses of gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years or even more, total radiation exposure can be significant and can affect the structure and performance of the TIM. In this work, the potentially adverse effects on the thermal performance of CNT TIMs of gamma-ray irradiation is reported. CNT TIMs were irradiated in a gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad. The thermal interface resistance was measured before and after gamma-ray irradiation using a transient photoacoustic (PA) method at room temperature and a contact pressure of 134 kPa and indicated no adverse effects of gamma-ray exposure on thermal performance.Copyright

Direct simulation Monte Carlo investigation of hydrodynamic instabilities in gases

The Rayleigh-Taylor instability (RTI) is investigated using the Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics. Here, two-dimensional and three-dimensional DSMC RTI simulations are performed to quantify the growth of flat and single-mode-perturbed interfaces between two atmospheric-pressure monatomic gases. The DSMC simulations reproduce all qualitative features of the RTI and are in reasonable quantitative agreement with existing theoretical and empirical models in the linear, nonlinear, and self-similar regimes. At late times, the instability is seen to exhibit a self-similar behavior, in agreement with experimental observations. For the conditions simulated, diffusion can influence the initial instability growth significantly.

Characterization of Gamma-irradiated Carbon Nanotube and Metallic Foil Thermal Interface Materials for Space Systems

Removal of waste heat generated via Joule heating during the operation of electronic devices is critical to overall system performance and reliability. A significant fraction of the overall thermal budget is consumed by heat transfer across the interface of contacting materials. To enhance the flow of heat from source to sink, thermal interface materials (TIMs) are used to reduce thermal contact resistance (TCR) by increasing real contact area at the interface. In space systems, TIMs are exposed to high doses of gamma radiation not encountered in typical terrestrial applications. With typical design lifetimes of 5 years or more, total radiation exposure can be significant and can affect the structure and performance of the TIM. Here, we report measurements of the pressure-dependent TCR of metallic foils and carbon nanotube TIMs (CNT-TIMs) in both vacuum and ambient air environments. The TIMs were irradiated in a gamma cell at a rate of 200 rad/s to a total dose of 50 Mrad. TCR was measured before and afte...

Optical Measurements of the Thermal Conductivity of Porous SiO2 Films

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. Recently, a novel pump-probe geometry was used in Time Domain Thermoreflectance (TDTR) to determine the thermal conductivity of liquids. In this work, we develop a thermal algorithm to solve for the temperature change and heat transfer in this TDTR geometry in which a thin film which is subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our thermal algorithm with TDTR measurements of the thermal conductivity and on a series of porous SiO2 -based nanostructured films.Copyright

Nano-Engineering by Optically Directed Self-Assembly

Lack of robust manufacturing capabilities have limited our ability to make tailored materials with useful optical and thermal properties. For example, traditional methods such as spontaneous self-assembly of spheres cannot generate the complex structures required to produce a full bandgap photonic crystals. The goal of this work was to develop and demonstrate novel methods of directed self-assembly of nanomaterials using optical and electric fields. To achieve this aim, our work employed laser tweezers, a technology that enables non-invasive optical manipulation of particles, from glass microspheres to gold nanoparticles. Laser tweezers were used to create ordered materials with either complex crystal structures or using aspherical building blocks.