On dissipative flow structures in supersonic pressure exchange - An experimental investigation
Open AccessIt 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 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 Schlieren Photography and quantitatively analyzed using another non-invasive experimental technique called Laser Doppler Velocimetry (LDV). Shear layer turbulence intensity measurements (T I) 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-e model, 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. The dissertation experimentally confirms the presence of pseudoblades in a rotor-supersonic flow field by computing the Meyers correlation coefficient (C or instantaneous velocity - data rate correlation coefficient) using the LDV technique, and solves the dilemma of varying levels of shear layer growth of commercial CFD solvers such as FLUENT. From a non-dimensional entropy rise parameter (dS/R ) 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.
- All rights reserved
Notice to Authors
If you are the author of this work and you have any questions about the information on this page, please use the Contact form to get in touch with us.