Final Report: A Novel Pressure-Exchange Ejector Refrigeration System with Steam as the RefrigerantEPA Grant Number: R825324
Title: A Novel Pressure-Exchange Ejector Refrigeration System with Steam as the Refrigerant
Investigators: Mavriplis, Catherine , Garris Jr., Charles A.
Institution: George Washington University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999
Project Amount: $175,000
RFA: Technology for a Sustainable Environment (1996) RFA Text | Recipients Lists
Research Category: Economics and Decision Sciences , Sustainability , Pollution Prevention/Sustainable Development
Objective:The pressure-exchange ejector offers the possibility of attaining a breakthrough in the level of performance of ejectors by means of utilizing nondissipative, nonsteady flow mechanisms. Yet, the device retains much of the mechanical simplicity of conventional steady-flow ejectors. If such a substantial improvement in performance is demonstrated, its application to ejector refrigeration will be very important. Such a development would provide significant benefits for the environment in terms of both chlorofluorocarbon (CFC) usage reduction and greenhouse gas reduction.
Several new innovations in pressure-exchange ejectors have been conceived during the grant and three patents applied for by the principal investigator; two have been granted and the other is pending. The structure of this new ejector is such that the primary fluid is brought to the zone of interaction in the form of a plurality of supersonic expanding jets, which rotate about a common axis, thereby introducing nonsteadiness in the laboratory frame of reference. The mutual deflection of primary and secondary flows incur pressure forces between the flows that do the work of flow induction. After interaction, the flow is channeled into a diffuser, whereby the secondary flow is trapped in the interstices between the helically formed primary flow discharge jets.
The purpose of this research program was to study the complex nonsteady flow processes and induction mechanisms within the pressure-exchange ejector from the experimental, computational, and theoretical perspectives; how they relate to overall ejector performance; and to obtain fundamental design information, which would allow this technology to be applied to ejector refrigeration systems. If this new concept in ejector technology based on nonsteady flow processes can be shown to be viable, the impact on refrigeration and air conditioning in the commercial, automotive, and residential sectors would be enormous, which would enable environmentally benign refrigerants such as water to replace the harmful CFCs, and reduce the effluence of greenhouse gases by reducing fuel consumption through the use of waste heat and improved efficiency.
Experimental Program. The experimental program had the near-term goal of understanding the behavior of the pressure-exchange ejector, and the long-term goal of implementing it in a refrigeration system. Recent work concentrated on the former. During the course of the grant, three different configurations of the test rig were designed, fabricated, and incorporated in our testing facility. Air is provided through a blow-down air supply. The system is instrumented, and pressures, temperatures, and flow rates are measured in the primary and secondary flow circuits. All flow variables are monitored, and data is compiled via a computerized data acquisition system. During the course of the grant, flow visualization by means of smoke and tufts has been used and incorporated with high-speed photography.
It must be appreciated that this type of ejector is completely new, and that there is not any previous design experience upon which to build. Before experiments can be conducted, the ejector must satisfy unforgiving mechanical design constraints, including bearing design, structural design, and dynamic sealing. The operating parameters that must be studied include primary/secondary stagnation pressure ratio, primary/secondary stagnation temperature ratio, ejector discharge back pressure, and working fluid (air, steam, etc.). To obtain an optimum configuration, the geometrical parameters that must be studied include: area ratio of supersonic primary nozzle (Mach Number); configuration of nozzle (planar, axisymmetric, minimum-length, curved, etc.); number of nozzles; spin angle of nozzles (peripheral speed); diffuser spacing (distance between vaneless diffuser surfaces); diffuser angle; secondary to primary throat area ratio; and aerodynamic design configuration.
A major goal of this research program is to explore all of these aspects. However, to date, we have dedicated much of our effort towards satisfying the mechanical design constraints. In the process, we have encountered difficulty with bearing failures, vibration instability, structural failure, and sealing problems. As we made progress in identifying these problems and achieving an integrated design that balances each requirement, new designs evolved leading to entirely new flow induction environments. The latest invention offers hope of alleviating all of these design difficulties, while maintaining simplicity. It involves a dramatically different type of turbo-machine than any that appears in the literature. It seeks to exploit the advantages of supersonic design, particularly oblique shock waves and Prandtl-Myer expansion fans, to promote flow interaction. The new design exploits these characteristics and thereby eliminates the need for high-tolerance seals, and reduces the difficulty of axial thrust management because of the drastically different pressure field embodied in a supersonic flow. However, there are many basic issues of the fluid dynamics that remain to be resolved.
Computational Simulation Program. To model the characteristics of this novel pressure exchange concept computationally, the commercial fluid flow analysis code TASCflow was acquired. TASCflow solves the three dimensional Navier-Stokes equations by utilizing a finite element-based finite volume method over structured hexahedral grids. It was chosen over other software packages primarily for its ability to handle time periodic boundary conditions and rotating frames of reference, making it well-suited for modeling rotating machinery. Other features particularly suitable to this research include the abilities to accurately model real gas compressible flows, turbulence, heat transfer, shocks resulting from transonic and supersonic flows, and special fluids such as wet steam and refrigerants.
Our computational fluid dynamics (CFD) research has been geared to supporting our experimental program. Because three configurations of pressure exchange ejector have been developed, the CFD program has been useful in studying these configurations from the purely fluid dynamics perspective.
Another activity in parallel has been the use of the commercially available CFD program FLUENT to solve subsonic-design pressure exchange ejector problems. It is not clear at this juncture which ejector configuration is optimal from the aerodynamic point of view. Although the subsonic configuration suffers from sealing and thrust management problems, the supersonic configuration must deal with oblique shock waves. The entire goal for attaining a commercially viable technology can be reduced to the search for the configuration that provides minimal energy dissipation. The advantage of CFD is that it allows us to divorce the practical mechanical problems from the aerodynamics problems.
Summary/Accomplishments (Outputs/Outcomes):The heart of the novel pressure-exchange ejector refrigeration system of this program is the ejector. Although theoretic results suggest the possibility of major improvements in performance, integrating mechanical design requirements with aerodynamic design in a confined environment is difficult. From the aerodynamic perspective, among the most important findings is the fact that the rotor of the pressure-exchange ejector must attain nearly free-spinning speed. It was found that this is necessary to minimize the mixing that occurs and to exploit to the fullest, the reversible work of interface pressure forces?pressure-exchange. Unfortunately, the achievement of this condition is tightly linked to the mechanical aspects of the ejector, particularly radial bearings and thrust bearings, and to sealing the flow path between the high pressure primary fluid and the low pressure secondary fluid. The most important achievement in the program has been the finding that supersonic flow characteristics can be utilized to provide good aerodynamic performance and yet alleviate the sealing problems and thrust management problems. It is commonly known in turbo-machinery that oblique shock waves can be managed so as to minimize the energy losses. However, with the novel device conceived in this program, much research must be done on the nature of the complex internal supersonic flow field and the momentum exchange processes occurring therein. Our combined experimental and computational approach has been slowly providing insight.
Another finding has been concerned with the design and implementation of nearly frictionless gas bearings. Because the rotor is free spinning, the radial loading is very light; hence, gas bearings using the working fluid of the ejector are feasible. We have entered into a collaborative relationship with Mohawk Innovative Technology who has custom designed their patented compliant foil bearings for our application. This development may make it possible to finally achieve the operating conditions commensurate with our theoretical analysis.
Our fluid dynamics studies have revealed an interesting kinematical feature of the flow induction process. It was found that after the pressure exchange process, primary and secondary fluids are deflected to a common orientation in the rotating frame of reference. However, in the laboratory frame of reference, the primary fluid rotates in the direction opposite to the rotor, while the secondary fluid rotates in the same direction as the rotor. This characteristic has been utilized to design a means for separating primary and secondary fluids after pressure exchange. This opens the door for the development of an entirely new class of turbo-machines for applications such as turbo-chargers, fuel-cell compressor-expanders, and air cycle refrigerators.
The nature of the ejector is such that excellent performance requires management of a variety of loss mechanisms. Our combined experimental and computational approach has been useful in incrementally attacking each one of a multitude of design characteristics associated with the many parameters involved. Parameters include pressure ratio, rotor vane angle of attack, vane wedge angle, secondary fluid inlet shrouds, and diffuser geometry. Much insight is being obtained in achieving proper combinations.
In this grant, some findings were made in relation to the refrigeration system as a whole. A basic ejector refrigeration system was fabricated to attain familiarity with it. The utilization of this system was seen to be capable of utilizing waste heat. One characteristic that was seen to be very important was the condenser because it must reject not only the heat absorbed in the evaporator of the refrigeration system, but also it must reject the heat introduced in the primary flow. For an inefficient system, the size of this heat exchanger can be very large. A study was made to identify the relationship between the efficiency of the ejector and the size of the condenser. It was seen that the pressure-exchange ejector will provide substantial reductions in system size if we obtain the levels of efficiency that we seek.
M. Shepard, D. Hand, R. Good, and V. Salazar served as summer interns. They worked in the laboratory under the Research Experiences for Undergraduates Program.
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||Bender S, Sadik OA. Direct electrochemical immunosensor for polychlorinated biphenyls. Environmental Science & Technology 1998;32(6):788-797.||