Dissertation/Thesis Abstract

On dissipative flow structures in supersonic pressure exchange - An experimental investigation
by Bulusu, Kartik Venkat, D.Sc., The George Washington University, 2010, 478; 3397599
Abstract (Summary)

It has been envisioned that crypto-steady pressure-exchange can be obtained by a supersonic jet impinging on a freely spinning rotor (axial configuration). Relative to the laboratory, a supersonic primary fluid forms a helical pattern whereby a secondary fluid gets entrained in the interstices of the helices and, by the use of an appropriate shroud, is forced into a duct or channel. Thus, work is done by the expanding primary fluid on the compressible secondary fluid by pressure forces acting across the helical boundary between the two fluids. The benefits of pressure exchange technology are elucidated in this dissertation, with potential applications in automotive refrigeration and sea-water desalination, and with the implementation of a practical device such as an ejector of higher efficiency and environmental benefits within its grasp.

The closest counterpart of pressure exchange ejector in the spectrum of momentum transferring devices is a steady flow ejector. The device is least complex, having no moving parts, but relies on highly dissipative turbulent entrainment for operation. Sensor based wall pressure measurements indicate the presence of a normal shock and associated maximum static pressure rise at a location 31 diameters length from exit plane of the supersonic nozzle in the diffuser region of an experimental “mixing” ejector. Mixing between the two fluid streams was not directly measured, but inferred to be an ongoing process in the typical “mixing” ejector, from the location of the normal shock. The length scale of mixing zone in the steady flow ejector was therefore established to be approximately 31 diameters length from exit plane of the supersonic nozzle. The overall maximum efficiencies (turbomachinery analog model) of two laboratory scale ejectors, viz., compact and mixing ejectors of different mixing region lengths were determined to be as low as 18% and 8.5% respectively. The limitations of steady flow ejectors from low overall efficiencies as consequences of turbulent mixing and normal shocks (entropy generators), and large lengths of mixing regions, conclusively outweighed the advantage of low complexity.

Concurrently, pressure exchange technology seeks to build on those aforementioned limitations by circumventing mixing processes, adding the complexity of one non-steady element (rotor), in the laboratory frame of reference. Cone-vane type of rotors (Ramp Vane and Double Cone type) with three vanes in a primary supersonic flow field produce “pseudoblades” that are highly compliant, natural, fluidic vanes of greater density than the entrained secondary fluid. Pseudoblades mimic the action of solid impellers in conventional turbomachinery but are constrained by spatio-temporal deterioration characteristics and shear layer growth.

Shear layer growth was qualitatively observed using a non-invasive technique called Schlieren Photography and quantitatively analyzed using another non-invasive experimental technique called Laser Doppler Velocimetry (LDV). Shear layer turbulence intensity measurements (TI) from the LDV technique present the most important contribution of this dissertation research, i.e., the determination of the “effective persistence length of stationary pseudoblades”, in the presence of a rotor, the nose of which is axisymmetrically aligned on the exit plane of a supersonic nozzle. This length is determined to be approximately equal to the diameter of the supersonic nozzle.

Steady, supersonic, viscous, k − &epsis; CFD (FLUENT) results suggest that a rotor design termed as Double Cone Rotor (25 deg. semi-cone vertex angle) is the most conducive for pseudoblade formation. But prior research using commercial CFD solvers show varying levels of shear layer growth and pseudoblade deterioration under the choice of solvers. The dissertation experimentally confirms the presence of pseudoblades in a rotor-supersonic flow field by computing the Meyers correlation coeffient (C or instantaneous velocity - data rate correlation coefficient) using the LDV technique, and solves the dilemma of commercial CFD solvers such as FLUENT.

Schlieren images highlighted the presence of entropy generating oblique shocks that are consistent with theory and remain salient in rotor-supersonic pressure exchange flow fields. From a non-dimensional entropy rise parameter [special characters omitted] computed from schlieren images, it was inferred that Double Cone Rotor produces 3 orders of magnitude higher entropy than the Ramp Vane Rotor accounted by oblique shocks, inspite of better pseudoblade structure. A lesser than 25 degree semi-cone vertex angle for the Double Cone Rotor is suggested that will commensurate with lower entropy rise and greatly enhance the efficiency of pressure exchange devices.

Indexing (document details)
Advisor: Garris, Charles A., Jr., Plesniak, Michael W.
Commitee: Cutler, Andrew D., Haque, Muhammad I., Keidar, Michael
School: The George Washington University
Department: Mechanical and Aerospace Engineering
School Location: United States -- District of Columbia
Source: DAI-B 71/04, Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Aerospace engineering, Mechanical engineering, Plasma physics
Keywords: Compressible flows, Dissipative flow, Ejectors, Experimental fluid dynamics, Pressure exchange processes, Supersonic pressure exchange
Publication Number: 3397599
ISBN: 9781109696271
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