טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBracha Yehiel
SubjectInvestigation of Parameters Governing the Onset of the
Two-Phase Thermoacoustic Engine
DepartmentDepartment of Energy
Supervisors Dr. Dan Liberzon
Professor Yehuda Agnon
Full Thesis textFull thesis text - English Version


Abstract

In many industrial processes an excessive amount of low grade heat is emitted to the environment. It is defined as “low grade” emission due to the low temperature difference with that of the environment. To harvest the heat emitted using a heat engine for conversion into useful work, an engine will need a critical temperature difference to produce work. In the most common internal combustion engines there is a high temperature of about 1500 degrees Celsius and an environmental temperature at about 25 degrees Celsius. Conventional heat engines require a high temperature difference to produce work. In a typical dry Thermoacoustic engine the required temperature difference is about 300 degrees Celsius. In the following experimental work, a novel Wet Thermoacoustic Engine that produce work at very low temperature difference of about 10 degrees Celsius is presented. A Thermoacoustic heat engine in general and a Wet Thermoacoustic engine in particular, convert heat, into acoustic vibrations which appear as a monotonic sound wave. But unlike dry Thermoacoustic engines, wet engines are based on condense-able material and an inert material mixture, such as water and air mixture in our case, humid air. Therefore, the driving force of the engine is not only a temperature gradient but probably also concentration gradient. Previous work done in the area predicted a relationship between the concentration gradient and the onset, the change of state from a not working engine to a working engine also known as stability threshold, where a sustained sound wave was “fed” by heat and vapor.


From our experimental results we observe a connection between two parameters and the moment of onset; the temperature difference on the stack and the heating power supplied to the water reservoir. The relationship is not quantified, yet but qualitatively, a substantial amount of water vapor and a temperature gradient of about 10 degrees Celsius per cm are required in order to sustain acoustic oscillations. It is evident that a wet system, with water presence inside the stack, starts spontaneous oscillations more “easily”, requiring a lower temperature gradient to “ignite”. In the following work a frequency response method, to predict the onset of self-oscillations, is used in order to examine its compatibility to wet Thermoacoustic engines. This method was used for dry Thermoacoustic engines in past works.