Evaluating energy dissipation during expansion in a refrigeration cycle using flue pipe acoustic resonators

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008."June 2008."Includes bibliographical references (p. 27-29).This research evaluates the feasibility of using a flue pipe acoustic resonator to dissipate energy from a refrigerant stream in order to achieve greater cooling power from a cryorefrigeration cycle. Two models of the acoustic operation of flue pipe resonant systems are examined: an electrical circuit analog that represents the linear approximation of the acoustic system and a numerical model based on empirical data. The electrical analog yields a symbolic representation for the power that can potentially be dissipated from the acoustic stream. Ongoing research into these acoustic systems, however, shows that the electrical analog, which neglects nonlinear effects, is incomplete and overestimates the operation of a pipe. However, the analogy can be used to quickly find the order of magnitude of power dissipated from the acoustic resonator. A subsequent data-based model allows for a more accurate quantitative estimation of the potential efficiency of the flue pipe in extracting work and thereby dissipating energy from a refrigerant stream. The efficiency of extracting work from a refrigerant stream using the acoustic system analyzed here ranges from 10% to 60%. The range is so large because the quality factor of the experimental flue pipe is unknown. This quality factor is imperative in determining the power dissipation. Further research should optimize the quality factor. A large quality factor causes less amplitude attenuation than a small one, but a smaller one dissipates more of the stored energy. The results of the models are compared to the efficiencies of existing technology, specifically the recently invented thermo acoustic expansion valve (TEV). It is found that the efficiency of the TEV is less than the theoretical results deduced from the numerical model. At an efficiency of approximately 10%, the technology represents a gain in cooling power, but further optimization using the results of this research can increase this gain even more.by Maria N. Luckyanova.S.B

Detecting coherent phonon wave effects in superlattices using time-domain thermoreflectance

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 79-85).Superlattices (SLs), structures consisting of periodic layers of thin films of several angstroms to tens of nanometers thick, have unique electrical and thermal properties that make them well suited for applications in optoelectronics and as fundamental learning tools in the realm of thermoelectrics. One unique characteristic of SLs is their low thermal conductivity compared to a bulk material with the same molecular composition. This property has given rise to extensive theoretical and experimental investigations regarding thermal transport through SLs. The different thermal transport characteristics have been studied in the context of various transport regimes. In this thesis, an experimental investigation of thermal transport in the coherent regime through a SL is presented. The trend in thermal conductivity that can be expected if such coherent wave effects exist is derived from the Landauer-Biittiker formalism, which treats energy transport as a transmission process. The frequency-dependent transmission probability for phonons through the SL is found via an application of the transfer matrix method (TMM). The calculations show that the integral effect of the buildup of phonon stopbands in the SL is minimal. Thus, if coherent wave effects are present, the conductance of the SL is nearly constant as the number of periods is increased, and the thermal conductivity, which is the product of the conductance and the total thickness of the SL, increases linearly with number of periods. To test the predictions, five GaAs/AlAs SLs with one, three, five, seven, and nine periods of one layer of GaAs of 12 nm thickness, and one layer of AlAs of 12 nm thickness are grown using MOCVD. The thermal conductivities of the SLs are measured using a transient thermoreflectance (TTR) technique at temperatures ranging from 30K to 300K. The results are the first-ever experimental evidence for the presence of coherent wave effects in heat transport through SLs.by Maria N. Luckyanova.S.M

Observation and manipulation of the wave nature of phonon thermal transport through superlattices

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 101-130).As the scale of electronic, photonic, and energy harvesting devices has shrunk, the importance of understanding nanoscale thermal transport has grown. In this thesis, we investigate thermal transport through superlattices (SLs), periodic layers of thin films, to better understand thermal conduction at these small scales. The classical picture of nanoscale thermal transport invokes a picture of diffusive scattering of phonons, or lattice vibrations, at the interfaces and boundaries in structures. This picture has been used to explain experimental thermal transport results for a wide variety of nanostructures. Despite the omnipresence of this particle-transport picture of phonon heat conduction, the community has continuously sought an experimental demonstration of the wave regime of thermal transport in nanostructures. In this thesis, we report the first experimental observations of the regimes of coherent phonon transport and phonon localization in thermal conduction through nanostructures. First, in order to better understand thermal transport through SLs, we present measurements of anisotropic thermal conductivity in the same GaAs/AlAs SLs using two different optical techniques, time-domain thermoreflectance (TDTR) for cross-plane measurements, and transient thermal grating (TTG) for in-plane measurements. The results of this study lend insight into the role of interface scattering, previously understood to be the dominant scattering mechanism in these structures, in SLs. The experimentally measured thermal conductivities are compared to results from first principles simulations, and the agreement between the two helps to validate atomistic simulation techniques of transport through SLs. The role of coherent phonon transport is explored by using the TDTR technique to measure the thermal conductivities of SLs with the same period thicknesses but varying numbers of periods. This experimental approach is a departure from traditional studies of SLs where period thicknesses are varied while the SL is grown to be thermally thick. This shift in the experimental paradigm allows us to explore previously elusive phenomena in nanoscale thermal transport. Combined with first principles and Green's functions simulations, the results of these experiments are the first experimental observation of coherent phonon transport through SLs. Finally, experiments on GaAs/AlAs SLs with varying concentrations of ErAs nanodots at the interfaces show the ability to destroy this phonon coherence. The thermal conductivities of such SLs with constant period thicknesses and varying numbers of periods show an overall reduction in thermal conductivity with increasing ErAs concentration. In addition, at low temperatures samples with ErAs at the interfaces show a maximum in thermal conductivity with shorter sample length and then a drop-off for longer samples. These results are signatures of phonon localization, a previously unobserved thermal transport phenomenon.by Maria N. Luckyanova.Ph. D