Preliminary experiments on entrained fluid mixing for enhancing ejector performance

Transcription

1 39th AIAA Fluid Dynamics Conference June 29, San Antonio, Texas AIAA Preliminary experiments on entrained fluid mixing for enhancing ejector performance Kartik V. Bulusu and Charles A. Garris Jr. The George Washington University, Washington DC, 252, USA. Bruno Peyrou École Supérieure d Ingénieurs de Poitiers, Poitiers, France A novel ejector based on the concept of supersonic crypto-steady pressure exchange rather than the more energy dissipative turbulent entrainment phenomenon is being developed. Such an ejector would have higher efficiency and environmental benefits. The process of pressure exchange occurs where flows exchange mechanical energy through work of mutually exerted pressure forces at their interfaces. The premise of our research is that if fluid mixing can be spatially or temporally delayed and the interface between primary and secondary fluid be held in a rotating frame of reference, the pressure exchange phenomenon can be fully utilized. An attempt was made to experimentally determine a longitudinal length in the mixing of supersonic or transonic primary (motive) and entrained subsonic secondary fluid under varying upstream pressures of primary flow. This length would be a critical design parameter for the pressure exchange ejector. Both schlieren imagery and laser velocimetry techniques were utilized in the experiments. This paper is being presented mainly to report the progress we made in experimentally understanding the phenomena of entrainment and turbulent mixing in the context of steady flow ejectors. The goal is to provide criteria that would eventually help in designing a fully functional ejector based on pressure exchange. Nomenclature L Distance between the beams after beam-splitting (mm) P Pressure (psi) X Axial distance from the exit plane of the nozzle (inches) d Diameter of nozzle (inches) f l Focal length of the transmitting optics (mm) f d Doppler shift frequency (Hz) n Refractive index of a transparent medium v Speed of fluid (ms 1 ) Subscripts e o p t Nozzle exit plane Stagnation quantity Primary fluid Nozzle throat Graduate Research Assistant, Dept. of Mechanical & Aerospace Engineering, The George Washington University. Professor of Engineering, Dept. of Mechanical & Aerospace Engineering, The George Washington University. Research Intern, Dept. of Mechanical & Aerospace Engineering, The George Washington University. 1 of 15 Copyright 29 by the American Institute of Aeronautics and American Astronautics, Institute Inc. All of rights Aeronautics reserved. and Astronautics

2 Symbols x λ θ Fringe spacing (m) Wavelength of the laser (m) Angle at laser beam intersection (degrees) I. Introduction While steady-flow ejectors operate on entrainment and turbulent mixing between the primary (driving) flow, and the secondary (driven) flow, the turbulent entrainment mechanism itself, is inherently dissipative of energy and little can be done to improve it. An ejector based on a working principle that the interaction between a high energy ejector primary fluid and a low energy secondary fluid can be based on the interface pressure forces acting between them has been called a pressure exchange ejector. The physical principal upon which the steady-flow ejector functions is that of entrainment of a secondary flow by a relatively higher energy primary flow, eliminating any mechanically moving parts. Through tangential shear stresses acting at the interface between the two contacting streams and by virtue of the work of turbulent shear stresses, mixing occurs between primary and secondary streams. When the primary and the secondary fluids which come in contact have a difference in kinetic energy, the viscous interface between them leads to a dragging phenomenon which is known as entrainment. This is followed by turbulent mixing of the two fluids, a process which is considered to be adiabatic. While this mechanism is quite effective, and has been widely adopted in many applications, an inherent characteristic of mixing processes is to dissipate valuable mechanical energy. The process of mixing, which is irreversible is not accounted for explicitly in any existing definitions of ejector efficiency. There is also an exchange of momentum between the fluid streams. In a conventional steady flow ejector the momentum exchange is sustained for a relatively short time span and is localized to the large scale structure of eddies generated at the fluid interface before the onset of a turbulent mixing process. The process of momentum exchange between fluids streams in direct contact if sustained, and the onset of complete turbulent mixing delayed, outcome of pressure exchange is produced. The difference between the processes of turbulent mixing and momentum exchange lies in the production of entropy. The former is inherently an entropy generator and dissipator of useful mechanical energy. The later process has slower rates of entropy generation while maintaining similar thermodynamic end results as the former. Well known experimental techniques of schlieren imagery and laser velocimetry were employed in this study. Several nozzle configurations were considered and free jet mean velocity measurements were made using a single component Laser Doppler Velocimeter (LDV) system along the core of the primary jet. Laser velocimetry experiments were performed on free jets of unseeded primary and seeded secondary fluid which is entrained by the primary fluid. Schlieren imagery technique was used to visualize the entrainment by a supersonic jet. These experiments are expected to serve as precursor to more careful experiments being planned and are therefore being called preliminary experiments. II. Pressure exchange phenomenon and devices In devices such as conventional turbomachines, a solid vane exerts forces against a fluid and thereby transfers momentum and energy from the solid surface to the fluid. This is very efficient because the solid vane is pushing the low energy fluid and it is the pressure forces that do the work. However, this process requires a non-steady flow in the laboratory frame of reference since pressure forces exerted against a stationary object can do no work. 1 In direct flow induction devices, a primary fluid, rather than a solid vane, exerts forces on a secondary fluid and similarly transfers momentum and energy from the energetic primary fluid to the low energy secondary fluid. A pressure exchange direct contact flow induction device seeks to achieve the same benefits as those of indirect devices by establishing a moving fluid vane structure which exerts pressure forces on the secondary fluid. These fluid vanes themselves are called as pseudo-blades. 2 of 15

3 An attempt was made to experimentally determine a longitudinal length scale indicating the onset of mixing of supersonic or transonic primary (motive) and entrained subsonic secondary fluid under varying upstream pressures of primary flow. This length scale would be a critical design parameter for the pressure exchange ejector, since the necessary condition for pressure exchange to occur is the existence of fluid-fluid interface between a supersonic or transonic primary fluid and an entrained subsonic secondary fluid. The fluid structure generated due to the interaction between a supersonic or transonic primary fluid and an entrained subsonic secondary fluid would provide insights into onset of mixing so that suitable design strategies can be employed to Figure 1. CAD Model of a Pressure Exchange delay mixing fluid dynamically. One such strategy is based on Ejector 2 a rotor-vane configuration as shown in figure 1 where the interface between the primary fluid psuedoblades and secondary fluid is maintained in a rotating frame of reference long enough spatially and temporally. A number of practical applications have been conceived and patented which include fuel cell pressurization and refrigeration. 3 5 A simple analytical model describing the concept of pressure exchange can found in recent computational study conducted by Zhang and Garris. 6 The effect of pressure exchange application on a thermal vapor compression type desalination system was conducted analytically and under ideal conditions by Chabukswar and Garris 7. III. Brief description of experimental set-up The experimental set-up was based on making non-invasive fluid dynamic measurements of velocity and optical visualization of flow structure. Schlieren flow visualization technique (Z-type arrangement) was used to study entrainment of a subsonic jet by a supersonic jet. Laser velocimetry is a technique used to measure mean flow velocities and turbulence intensities. Due to subsonic seeded secondary jet entrained by a supersonic or transonic unseeded primary jet, the expected variation of doppler burst count with upstream total pressure of primary flow and axial distance from the nozzle exit plane is shown in figure 2. The axial distance (x) from the nozzle can be varied while holding the (P op ) constant and the (P op ) can be varied holding (x) constant. The graphs drawn in figure 2 are hypothetical and should be considered mutually exclusive of each other although, for the sake of understanding the variations in the doppler burst count it has been drawn on the same scale. Figure shows the schematic of the plenum and a nozzle for primary flow used in the LDV experiments. The nozzles were fabricated such that they can be screwed into the fixture and thereby providing a plenum for the secondary flow. The data reduction of measurements was performed using a simple signal conditioning and data analysis algorithm executed in real time along with a digital oscilloscope using a MATLAB TM program. A. Schlieren Photography In the experimental arrangement shown in figure 4, parabolic mirrors are aligned in the Z-type configuration. Care was taken to place a knife edge at the focal length of the parabolic mirror before an optical screen. The cutting off of light using a knife edge was done repeatedly and iteratively until the best possible image could be visualized. The digital camcorder was placed at the knife edge location and the lens of the camcorder was used to project the schlieren images on the CCD arrays inside the camcorder instead of a stand-alone optical screen. Schlieren images were recorded as MPEG movies to be later analyzed using a post processing sofware. Schlieren illuminance level responds to first order spatial derivative ( n x ) of refractive index (n) that arise due to density changes in the velocity field. This relationship is described that by the Gladstone-Dale 3 of 15

4 Free shear layer initiation, growth and breakdown mixing region δ shear Seeded subsonic secondary fluid (s) Z Unseeded supersonic / transonic primary fluid (p) X Seeded subsonic secondary fluid (s) Doppler burst count Upstream total pressure of primary flow (P op ) held constant Location of measurement (x) held constant Upstream total pressure of primary flow (P op ) Axial distance from primary nozzle ( x ) Symbol indicating location of LDV measurement volume Symbol indicating particle laden secondary flow Figure 2. Expected variations of doppler burst counts with axial location and upstream total pressure of primary flow relation (eqn. 1) shown below, ρ = K(n 1) (1) where K is the Gladstone-Dale constant, which is dependent on the chemical composition of the fluid. The failure to effectively cut-off a portion light at the focal point would result in visualization of second order spatial derivatives ( 2 n x ) of refractive index (n) and another technique of optical flow visualization known 2 as shadowgraphy. The authors suggest further reading on this technique with reference to a book titled, Schlieren and Shadowgraph techniques by G.S. Settles. 8 B. Laser Doppler Velocimetry The details of the experimental setup of the single component LDV is shown in figures 6(a), 6(b), 6(c) and 6(d). The single component LDV system (TSI system) is arranged in forward-scatter mode. Forward scatter mode was chosen since the intensity of mie-scattering is expected higher in this mode than in back-scatter mode. The Laser source in the arrangement is a multiline laser (Omnichrome 532-A) that produces eight wave lengths of light with an inclusive range between 457nm and 514nm. The beam with a wavelength 514nm was chosen and isolated using an optical band-pass filter with a narrow bandwidth, carefully aligned in the transmitting optics in front of the laser head. Since the power associated with 514nm was the highest, the intensity of mie-scattering would also be proportionally higher compared to the other lower power beams. The following equation is used to calculated flow field velocities using the LDV technique: v = f d ( x) (2) 4 of 15

5 Figure 3. Schematic of nozzle for primary flow and inlet for entrained secondary flow Figure 4. Experimental arrangement for schlieren photography The focal length of the transmitting optics and the geometry of the beam splitter is shown in figure 5. The fringe spacing was calculated using equation 3. x = λ 2sin( θ2 ) (3) From figure 5 and using simple trigonometry one can conclude that θ L = tan 1 ( ) 2 2fl (4) Substiuting equation 4 in equation 3 we can calculate the fringe spacing. The doppler shift frequency (fd ) in equation 2 is measured using a photomultiplier tube (RCA 4526) housed in the receiving optics assembly. The laser head, transmitting optics and the receiving optics are carefully aligned in forward-scatter mode to gather data from the measurement volume that generate mie-scattered light at a doppler shift frequency, when a particle laden flow is sent through it. The direction of the fringes is set to perpendicular to the dominant direction of the flow field. The doppler signals generated by mie-scattering are sent to a digital oscilloscope (TDS 224B) set in a triggering mode. The triggering threshold voltage and time per division is 5 of 15

6 Beam splitter L Fringe pattern θ f L Figure 5. Geometry of transmitting optics set up arbitrarily by the user after a few test runs. The voltage-time domain signals generated by the photomultiplier are on real time, visualized on the screen of the oscilloscope and are acquired by the MATLAB TM program with a hardware generated dead-time or the time between successive acquisitions. The dead-time between velocity samples was observed to be roughly 3-4 seconds and consequently much greater the twice the integral timescale of flow. Hence, only the mean velocities could be processed using the oscilloscope method. Figures 6(a), 6(b) and 6(c) shows the hardware installed to generate the primary and secondary flows. The seeded subsonic secondary jet is generated using a six-jet atomizer (TSI Model 9396A) with olive oil used as seeding fluid (Figure 6(a)). Table 1. Table of experiments Nozzle throat Nozzle exit Location of measurement volume diameter diameter from nozzle exit plane (x, inches) (d t, inches) * (d e, inches) 2 inches 4 inches Figure 8(a) Figure 8(b) Figure 9(a) Figure 9(b) Figure 1(a) Figure 1(b) Figure 11(a) Figure 11(b) Figure 12(a) Figure 12(b) Entrainment visualization using schlieren imagery Figures 7(a) to 7(e) IV. Results and discussion A summary of the experiments conducted is being presented in table 1 indicating the pertinent dimensions of the nozzles used and corresponding figures for results. Entrainment of fluid by the direct action of a supersonic jet can be visualized by schlieren photography and in the interest of demonstrating the concept, a subsonic diffusive flow field of a smoke plume was set up in the vicinit of a supersonic jet. In both conventional and the novel pressure exchange ejectors the secondary fluid is drawn into the device by the dragging effect of the primary jet. Figures 7(a), 7(b), 7(c), 7(d), 7(e), show the various stages in the entrainment of a diffusive flow field of smoke created by lighting an incense stick, and acted upon by a supersonic jet of air. Stage 1 shows the diffusion of smoke plume after lighting the incense stick. At this stage there is no schlieren possible since there is no effective first order spatial derivative of refractive index. The smoke plume is visible because of the lighting of the room in which schlieren imagery was performed. Stage 2 shows the onset of entrainment 6 of 15

7 (a) Laser, optics and other hardware in experimental set-up (b) Another view of experimental set-up (c) Hot-film included at 6 inches from nozzle exit plane (d) Concealed measurement volume due to unseeded primary flow and seeded secondary flow Figure 6. Experimental arrangement for entrained fluid mixing studies with the bending of the smoke plume towards the nozzle. At this stage the nozzle was producing a subsonic flow field since the designed upstream total pressure was not supplied for generating supersonic flow. The subsonic flow without any convective heat transfer cannot produce density changes hence remains inviscible in schlieren. Stages 3, 4 and 5 show the development of the supersonic flow structure and entrainment of smoke plume. The nozzle was designed to produce a Mach 2 flow at P op = 12 psig. In all the experiments the measurements volume was fixed at 2 inches and 4 inches from the exit plane and along the axis of the nozzle. The upstream total pressure (P op ) was then varied from 12psig at particle arrival time t = seconds to P op = psig. In other words, eulerian measurements were made at both 2 inches and 4 inches axial distance (x) from the exit plane of the nozzle. A hand-held hot film probe (TSI V elocicalc c ) was located at 6 inches from the nozzle exit plane in all experiments. The velocities ranging between 55ms 1 were recorded using the TSI V elocicalc c and served in calibrating the experiment in lower upstream total pressures. Figures 8, 9, 1, 11, 12, show the measurements of mean velocities made using a digital oscilloscope (TDS 224B) in real time with a MATLAB TM program developed for instrument control and data reduction. The spread of the particle velocities each experiment seem to be greater at the initial stages than towards the ending stages of the experiments. One reason for this spread of data could be attributed to the varying size of seeding particles passing through the measurement volume. But the six-jet atomizer (TSI Model 936A) is designed to produce (monodisperse) aerosol particles of almost the same size at certain operating 7 of 15

8 (a) Stage 1: Diffusive flow field of smoke generated from incense (b) Stage 2: On set of entrainment when the flow was subsonic stick (c) Stage 3: Supersonic jet and signs of smoke entrainment (d) Stage 4: Supersonic jet and signs of smoke entrainment (e) Stage 5: Supersonic jet and clear indication of smoke entrainment Figure 7. Schlieren images depicting stages of entrainment of smoke from an incense stick due to flow from a supersonic nozzle 8 of 15

9 pressure, and hence this reason can be discounted. The second reason could be the rate of entrainment of the particles from the secondary flow. The secondary flow particles are being constantly sheared out of their flow regime by the primary flow by the action of eddies and may be coalescing together. The smaller the size of the particle the greater the probability that it will follow the velocity of the flow field more accuately. The particle concentration however is at a minimum, at both locations (x = 2, 4 ) of the measurement volume since at the initial stages of the experiments mixing of secondary flow particles with the primary flow occurs at a location further downstream of the flow field. Consequently, it can be concluded that higher the upstream pressure total pressure of the primary flow, higher the velocity of the primary flow and greater is the axial mixing length. The range of the velocity is getting smaller towards at the end of the experiments. This phenomenon is comprehensible, since the velocity of the primary jet is decreasing with a decrease in upstream total pressure and proportionally, the rate of entrainment of secondary flow is decreasing. But there is a sharp increase in the particle concentration of the secondary flow since the axial mixing length is decreasing and moving towards the location of the measurement volume. This experimental approach taken by the LDV technique provides a qualitative argument for axial mixing length. The exact magnitude of the mixing length cannot be determined with this approach but proportionality of the axial length at which mixing between primary and an entrained secondary flow can be established on the basis of the experimental results and hypothesis of doppler burst count in figure 2. Further experimentation is being carried out in the supersonic shear layer regime and the results will be reported with a rigorous analysis of all the experimental data in due course. V. Conclusions So far the results generated from LDV experiments were indicative of mixing between primary and secondary fluids simply because the secondary flow was seeded and the primary was unseeded. Schlieren images on the other hand confirm the phenomenon of entrainment of a subsonic flow field by a supersonic jet. LDV experiments involving turbulence intensity calculations will enable the characterization of a length scale for mixing. Such experiments are planned with varying the location of measurement volume while holding the upstream total pressure constant so that the nozzle produces a velocity field with a design mach number. Furthermore, the measurement volume is moved to various locations in straight line along the edge of the nozzle exit plane, where the interface between the supersonic primary and the entrainment subsonic secondary exists as shear layer. A velocity-data rate correlation coefficient will computed by repeating the experiments with consecutive seeding of primary and secondary flows at every measurement location. This approach is expected to yield better quantitative results and current study helped in refining and conceiving this new approach. This study was undertaken to provide basic insights into self-entrained, unconfined coaxial jet mixing under varying upstream total pressures while measuring mean velocity using LDV and analyzing the flow structure using schlieren technique. The ultimate goal is to provide design criteria for the pressure exchange ejector based on the experimental characterization supersonic turbulent mixing of primary jet with entrainment subsonic secondary jet. The lessons learnt from these preliminary experiments provided the necessary methodology for more careful future experiments. Acknowledgments The authors would like to thank Dr. Azim Eskandarian of the Civil and Environmental Engineering Department, GWU for the extending his support as a Co-PI through the Energy in Transportation grant awarded by the DoE. We would also like to acknowledge of the valuable contribution of machinists Willard Morten and William Rutkowski, graduate student David Gould. A special thanks to Prof. A. D. Cutler of MAE Department, GWU for his demonstrations of implementing schlieren photography using a handheld camcorder and to distinguished research scientist James F. Meyers of NASA Langley for his invaluable guidance on the LDV technique. 9 of 15

10 References 1 R.C. Dean, J., On the necessity of Nonsteady Flow in Fluid Machines, Trans. ASME., Vol. 1, 1959, pp Bulusu, K. V. and C.A. Garris, J., Characteristics of flow around cone-vane configurations for a novel crypto-steady pressure exchange ejector system, Proceedings of the Third International Conference on Energy Sustainability, ASME, San Francisco, CA, C.A. Garris, J., A Pressure Exchanging Ejector and Refrigeration Apparatus, U. S. Patent 5,647,221, C.A. Garris, J., Pressure Exchanging Ejector and Methods of Use, U. S. Patent 6,138,456, 2. 5 C.A. Garris, J., Pressure Exchanging Compressor-Expander and Methods of Use, U. S. Patent 6,434,943, Zhang, H. and Jr., C. G., Crypto-steady supersonic pressure exchange: A simple analytical model, Applied Energy, Vol. 85, 28, pp Chabukswar, K. A. and C.A. Garris, J., Analysis of application of pressure exchange device in thermal vapor compression desalination system, Proceedings of the Third International Conference on Energy Sustainability, ASME, San Francisco, CA, Settles, G. S., Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media, Springer, of 15

11 4 3 Velocity trace at 2 inches from the nozzle exit plane Velocity from hot film at 6" from nozzle exit plane (m/s) (a) d t =.263, x = Velocity trace at 4 inches from the nozzle exit plane Velocity from hot film at 6" from nozzle exit plane (m/s) (b) d t =.263, x = 4 Figure 8. Mean velocity measurements at x = 2, 4 from exit plane of nozzle(d t =.263 ) 11 of 15

12 2 Velocity trace at 2 inches from the nozzle exit plane Velocity from hot film 6" from nozzle exit plane (m/s) (a) d t =.377, x = 2 2 Velocity trace at 4 inches from the nozzle exit plane Measurement Count Velocity from hot film 6" from nozzle exit plane (m/s) (b) d t =.377, x = 4 Figure 9. Mean velocity measurements at x = 2, 4 from exit plane of nozzle(d t =.377 ) 12 of 15

13 14 12 Velocity trace at 2 inches from the nozzle exit plane Velocity from hot film 6" from nozzle exit plane (m/s) (a) d t =.453, x = 2 12 Velocity trace at 4 inches from the nozzle exit plane Velocity from hot film 6" from nozzle exit plane (m/s) (b) d t =.453, x = 4 Figure 1. Mean velocity measurements at x = 2, 4 from exit plane of nozzle(d t =.453 ) 13 of 15

14 Velocity trace at 2 inches from the nozzle exit plane Time of arrival of particles (sec) Velocity from hot film 6" from the nozzle exit plane (m/s) (a) d t =.694, x = Velocity trace at 2 inches from the nozzle exit plane Time of arrival of particles (sec) Velocity from hot film 6" from the nozzle exit plane (m/s) (b) d t =.694, x = 4 Figure 11. Mean velocity measurements at x = 2, 4 from exit plane of nozzle(d t =.694 ) 14 of 15

15 14 12 Velocity trace at 2 inches from the nozzle exit plane Time of arrival of particles (sec) Velocity from hot film 6" from nozzle exit plane (m/s) (a) d t =.7656, x = 2 12 Velocity trace at 4 inches from the nozzle exit plane Time of arrival of particles (sec) Velocity from hot film 6" from nozzle exit plane (m/s) (b) d t =.7656, x = 4 Figure 12. Mean velocity measurements at x = 2, 4 from exit plane of nozzle(d t =.7656 ) 15 of 15