M.Sc Thesis


M.Sc StudentBrustin Tom
SubjectMass Transfer and Working Temperature of a Wet
Thermoacoustic Engine
DepartmentDepartment of Energy
Supervisor ASSOCIATE PROF. Guy Ramon


Abstract


Low-grade heat, namely heat from low temperature sources such as industrial waste streams, solar-thermal and geothermal, represents a significant source of energy that is currently not being fully utilized. Low-grade heat harvesting technologies include electric power generating devices and thermal distillation plants as well as other solutions. In order to be cost-effective, these technologies must be simple and efficient, as sources are dispersed and not necessarily power-dense. Moreover, these technologies have to present a low working temperature that will enable them to use heat streams in a wide range of temperatures.

Wet Thermoacoustic Engines (WTE) are simple, robust heat engines. WTEs employ a working fluid mixture of gas and an evaporable liquid to convert heat to kinetic power in the form of a sound wave. While previous studies focused on the potential of WTE technology as a low grade heat to power solution, this work focuses on WTE's unique mass transport and low working temperature properties, which make it a potential candidate in thermo-chemical separation process, specifically water desalination.

In a set of experiments, the working behavior of a water-based, standing-wave, WTE was studied. A protocol was designed in order to achieve an extended time interval of engine in steady state operation, characterized by a stable working temperature profile and pressure amplitude. Temperature, mean pressure, pressure amplitude and resonance frequency were recorded throughout the experiment duration. In addition, mass measurements were made in order to assess the rate of water evaporation. The link between gas molecule physical characteristics, its transport properties and the implications to engine behavior is discussed based on the kinetic theory of gases. As engine performance is strongly linked to the diffusive properties of the working gas, the experiments were conducted with a variation of the working gas between three candidates: Argon, carbon dioxide (CO2) and sulfur hexafluoride (SF6). The gas range was designed to include candidates in a variety of molecular geometries and masses, in order to obtain a wide range of diffusive properties of the working fluid.

The results suggest that this type of devices have several remarkable traits: The WTE is capable of producing a substantial flux of water vapor while exhibiting a maximum working temperature that is significantly lower than the boiling point. This trait, referred to below as 'Thermoacoustic Maximum Temperature' is, presumably, a trait unique to thermoacoustic engines that is reported here for the first time. The possible underlying physics giving rise to the phenomenon in question are briefly discussed. In addition, the results confirm that the characteristics of the working gas have an important role in setting the performance parameters of the device. Specifically, it was shown that in the tested range, a heavier, more complex-structured gas has significantly enhanced the steady-state acoustic intensity and mass evaporation rate of the device, while exhibiting a yet lower maximum temperature. Additionally, comparisons of temperature and mass flux measurements to previously designed numerical model predictions are presented. In conclusion, potential applications of the device and future research avenues are discussed.