A study of vortex breakdown in supercritical fluids

Loren C Crook, Purdue University

Abstract

The objective of this thesis is to evaluate the hydrodynamic stability theory of vortex breakdown by examining its presence in a swirling supercritical fluid jet. Supercritical fluids were chosen because they are technologically important, and because their strong density gradients near the critical point provide a self-excitable flow regime where hydrodynamic instabilities should be more easily identified. Computations were developed to provide flow conditions for experimental observation of vortex breakdown and to estimate hydrodynamic flow instabilities prior to testing. The RANS computations of an axial-plus-tangential air swirler were developed and verified as grid-independent and in agreement with experimental results reported in the literature. The computations also produced a correlation of momentum swirl number as a function of mass ratio (tangential/total) for estimating swirl number during experiments. Finally, the computations were extended to simulate supercritical CO2 in an axial-plus-tangential swirler that was compatible with the supercritical injection facility. Three mass flow ratio cases were investigated extensively: no-swirl (0%), low-swirl prior to breakdown (25%), and moderate swirl with breakdown (45%). (The three corresponding momentum swirl numbers for these mass flow ratios were 0.0, 0.30 and 0.80.) These three cases also served as the basis for the experimental portion of the work. Swirling supercritical fluid jets were observed in the injection facility using schlieren imaging for three separate swirl numbers. The Sobel method was used to locate the jet edges. The jets were characterized by their radii, including mean and standard deviation, and spreading angles as functions of swirl number and density ratio (2.3, 2.6, and 5.0). The swirl number was identified as the dominant parameter in determining the spreading angle and jet radius. Vortex breakdown was identified as the jet structure changed from straight edges to curved, signifying internal pressure gradients and recirculation. A spectral analysis showed the frequency content of the jet radius fluctuations is not in agreement with the instability theory. Velocity and normal stress measurements were obtained using an LDA system in backscatter mode, with the natural CO2/air interfaces used as flow markers. Mean axial velocity profiles at Sm = 0.80 showed the characteristic double-hump profile, conclusively demonstrating the presence of breakdown. Good accuracy was seen with the predictions, except in the cases of axial extent of recirculation region and magnitude of normal stresses. The frequency content of axial velocity fluctuation was extracted via spectral analysis and found to be in agreement with the hydrodynamic instability theory. Measured dominant frequencies for Sm = 0.30 were predicted to within 25% of those predicted by the instability theory. Measured dominant frequencies for Sm = 0.80 were within -64% to +70% of the predicted values, with the mean observed frequency identical to the predictions. The hydrodynamic instability theory also predicted the supercritical jets nearest the critical point to develop breakdown at lower mass ratios, while the opposite effect was observed during experiments: the condition nearest the critical point had either no breakdown, or the event was significantly weaker than those seen for the other conditions. The hydrodynamic instability theory does not predict this near-critical effect on breakdown. This thesis closes with a course for future work in breakdown, including numerical and experimental efforts. Design guidelines for supercritical fluid injectors are also provided highlighting the relevant properties of supercritical fluids and discussing the scope and limitations of the current results for the design of hardware.

Degree

Ph.D.

Advisors

Sojka, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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