Experimental Investigation of Finite‐Amplitude Acoustic Oscillations in a Closed Tube

The paper reports an experimental investigation of heat transfer in the closed-tube aerosyphon (aerated-thermosyphon) for a range of conditions representative of northern field applications. In particular, attention is focused on the effect of using tubes with heated lengths not only greater than the cooled lengths, but very much greater than the tube diameter. Using three heated sections and one cooled section, the geometry of the device has been varied systematically with 10 < LH/d < 50 and 1 < LH/LC < 20. For any given geometry, the effect of air bubbling rate has been studied in the range of 0 < V˙ < 5 × 10−5 m3/S. Using these ranges it has been possible to make comparisons with other thermosyphon and aerosyphon data. The results indicate that heat transfer coefficients are reduced by increasing either length-diameter ratio or heated-cooled length ratio. They also reveal that, in general terms, the aerosyphon is almost an order-of-magnitude more effective than the single-phase thermosyphon. Some obervations on the flow regimes are offered, and an empirical correlation is presented.

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Analysis of degenerate mechanisms triggering finite-amplitude thermo-acoustic oscillations in annular combustors

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.720 ◽

2019 ◽

Vol 881 ◽

pp. 384-419 ◽

Cited By ~ 1

Author(s):

Sandeep R. Murthy ◽

Taraneh Sayadi ◽

Vincent Le Chenadec ◽

Peter J. Schmid ◽

Daniel J. Bodony

Keyword(s):

Heat Source ◽

Modal Analysis ◽

Large Scale ◽

Pressure Fluctuations ◽

Harmonic Excitation ◽

Finite Amplitude ◽

Dimensional System ◽

Acoustic Oscillations ◽

Strong Response ◽

One Dimensional

A simplified model is introduced to study finite-amplitude thermo-acoustic oscillations in $N$-periodic annular combustion devices. Such oscillations yield undesirable effects and can be triggered by a positive feedback between heat-release and pressure fluctuations. The proposed model, comprising the governing equations linearized in the acoustic limit, and with each burner modelled as a one-dimensional system with acoustic damping and a compact heat source, is used to study the instability caused by cross-sector coupling. The coupling between the sectors is included by solving the one-dimensional acoustic jump conditions at the locations where the burners are coupled to the annular chambers of the combustion device. The analysis takes advantage of the block-circulant structure of the underlying stability equations to develop an efficient methodology to describe the onset of azimuthally synchronized motion. A modal analysis reveals the dominance of global instabilities (encompassing the large-scale dynamics of the entire system), while a non-modal analysis reveals a strong response to harmonic excitation at forcing frequencies far from the eigenfrequencies, when the overall system is linearly stable. In all presented cases, large-scale, azimuthally synchronized (coupled) motion is observed. The relevance of the non-modal response is further emphasized by demonstrating the subcritical nature of the system’s Hopf point via an asymptotic expansion of a nonlinear model representing the compact heat source within each burner.

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