Predicting the onset of thermoacoustic oscillations in supercritical fluids
Abstract
Sound waves in gas are capable of converting energy from one form to another. In these 'thermoacoustic' processes heat or chemical energy can be converted into acoustic power. In small channels, the acoustical power can couple with the heat transfer process to create oscillations. Pressure and flow oscillations in convective heat transfer applications have been reported in literature for many years. Both bulk mode/Helmholtz low frequency (frequency < 60) and acoustic high frequency (frequency > 60) have been reported. Herring studied these oscillations at Purdue University using JP-10. Using the same facility as Herring a similar parameter space was explored using the same small diameter (0.125 in) tubes with a different fluid. Methanol was chosen as a simulant fluid because of its similar properties to JP-10 in the supercritical phase. A test matrix was established that covered a range of parameters including tube inner diameter, tube length, fluid pressure, mass flow rate, and power. During the experiments two types of oscillations were observed. Bulk mode oscillations were observed at a relatively low frequencies of 0.05 to 10 Hz. Acoustic oscillations were also observed in pressure data between 360 and 581 Hz. Many observations and notes as the to the behavior of both types of oscillations were made based on the relationship they had with test parameters. A similar parameter space to Herring was outlined as a region of possible thermoacoustic instabilities. No method of damping the oscillations in this region was discovered. A model was developed to analyze the bulk behavior of system similar to the one tested in these experiments. The model is a starting point and successfully addresses bulk mode behavior in the subcritical temperature range. The model failed to produce bulk mode oscillations but should provide groundwork for future models.
Degree
M.S.A.A.
Advisors
Heister, Purdue University.
Subject Area
Aerospace engineering
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