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Space Power Systems *W?!J AGARDograph 123 t» u M O Q < < Space Power Systems PART II o NORTH ATLANTIC TREATY ORGANIZATION H (# DISTRIBUTION OF THIS DOCUMENT IS UNLIMrUES INITIAL DISTRIBUTION IS LIMITED FOR ADDITIONAL COPIES SEE BACK COVER DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. AGARDograph 123 NORTH ATLANTIC TREATY ORGANIZATION ADVISORY GROUP FOR AEROSPACE RESEARCH AND DEVELOPMENT (ORGANISATION DU TRAITE DE L'ATLANTIQUE NORD) SPACE POWER SYSTEMS Published in Two Parts PART II DISTBIBUTION OF THIS DOCUMENT IS UNLIM^ This Lecture Series was sponsored by the Propulsion and Energetics Panel and the Consultant and Exchange Programme of the Advisory Group for Aerospace Research and Development. It was held at the Universite' Libre de Bruxelles, Belgium from 2 to 6 October 1967. 629.78:539. 1 Published November 1969 ^ Printed by Technical Editing and Reproduction Ltd Harford House, 7-9 Charlotte St, London, WiP iHD 11 CONTENTS PART II Page IV B. TURBOMACHINERY FOR SPACE POWER by Eugene B.Zwick 349 I. Basic Turbine Concepts and Terminology 351 II. Turbomachlnery Performance Estimation Procedures 356 III. Example of N^ - Dg Diagram Use for Space Power Plant 358 IV C. ALTERNATORS FOR SPACE POWER APPLICATIONS by Eugene B.Zwlck 371 Review of Basic Alternator Concepts and Terminology 373 Solid Rotor Brushless Alternators 376 Alternators in Space Power Systems 378 Characteristics of a Typical Space Power Machine 378 Minimum Rotor Diameter 379 Rotor Drag 379 10 Kw Design Study 380 V A. TECHNOLOGY OF THERMOELECTRIC AND THERMIONIC ENERGY CONVERSION by Ned S.Rasor 397 Introduction 399 Thermoelectric Energy Conversion 400 Thermionic Energy Conversion 406 Comparison of Thermoelectric and Reversible Thermionic Conversion 407 V B. ENGINEERING ASPECTS OF THERMIONIC ENERGY CONVERSION by Ned S.Rasor 415 Introduction 417 Synopsis of Converter Technology 418 Nuclear Reactor Application 424 Radioisotope Generator 429 Solar Generator 430 Flame-Heated Generators 430 VI. ELECTROCHEMICAL SPACE POWER SOURCES by Ernst M.Cohn 443 Introduction 445 Electrochemical Background 448 Primary Batteries for Space 452 Primary Fuel Cells for Space 458 Secondary Batteries for Space 465 Design of Electrochemical Power (Sub) Systems 470 Outlook for Electrochemical Power 472 ill Page VII. PHOTOVOLTAIC DEVICES AND SYSTEMS by M.Rodot and H.Daspet 503 1. Outline of Nature of Solar Radiation 505 2. Solar Photocells 507 3. Photovoltaic Systems 522 Appendix I. OPTIMIZATION OF ENERGY STORAGE FOR SOLAR SPACE POWER by George C.Szego and B.Paiewonsky 603 Appendix II. PANEL DISCUSSION ON SPACE POWER SOURCES. 619 • IV 349 IVB. TURBOMACHINERY FOR SPACE POWER by Eugene B. Zwick 8901 Zelzah, Northridge, California, (213) 345-6078 SUMMARY Recent advances in turbomachine technology for missile and space power applications have far reaching implications for all power system designs. Low specific machine performance has been greatly improved. In addition, new optimized turbomachlnery design data is being derived. Machine per­ formance and design data can now be found on N„ - D„ diagrams. These data are in a form which is immediately usable by systems analysts. No specia­ lized turbomachlnery background knowledge is required. The preliminary design of a 20 kW Biphenyl Rankine cycle power plant was used to illustrate the application of the Ng - Dg concepts and charts. 351 IVB. TURBOMACHINERY FOR SPACE POWER Eugene B.Zwick INTRODUCTION Dynamic heat engine cycles derive their work output from the difference between the expansion work of a high temperature working fluid and the compression work required by the same fluid at low temperatures. Both reciprocating and turbomachlnery can be used in dynamic space power systems to perform the expansion and compression processes required. Turbo- machinery is used in most of the systems which are currently under development. Turbo- machinery technology has generally been paced by the demands of power generating systems. The early development of efficient turbines was stimulated by the installation of large steam power plants for the generation of electricity. Development in this field reached a plateau which was well described by Stodola in his work on steam and gas turbines. During the second world war and afterwards the development of turboprop and turbojet engines further stimulated turbomachlnery research. Impulse turbine technology was improved and considerable work was done during this period of time on axial flow and radial flow compressors. These machines have high specific speeds. They have relatively small work per stage with large power output and large volume flow of fluid. The development of missile power systems in the early 1950' s led to requirements for efficient turbines in the low specific speed regime. These machines were characterized by low power output with energy being extracted from a gaseous stream with very high specific energy content. The developments which took place under the stimulus of missile power requirements led to two extremely beneficial results. First there was a great increase in the efficiency of low specific speed turbines. Secondly, a generalized approach to optimization and selection of turbines, compressors, and pumps was developed. This generalized approach has had a great influence in the development of space power systems, and I am confident that it will have a considerable Impact in many future applications of turbomachlnery. In the present lecture we will examine this generalized approach and see how it is applicable to the design of a typical space power system. In order to make these results significant one must first have a background in the basic concepts of turbomachlnery. A preliminary discussion of these concepts is therefore presented below. I. BASIC TURBINE CONCEPTS AND TERMINOLOGY Figure 1 shows the typical configuration of an axial flow turbine. The turbine rotor is preceded by a nozzle which draws a supply of gas from an upstream supply line. The gaseous working fluid flows through the nozzles where it is accelerated to high velocity and directed towards the turbine blades. The gas flows through the blade passages, and after emerging from the downstream side of the wheel it passes onto the next stage of the machine or into an exhaust duct. The mechanism by which the turbine provides power to the shaft is the change in momentum of the gas stream as it flows through the turbine blades. In an Impulse turbine, see Figure 2, there is no pressure drop across the blading and ideally the gas velocity is constant. In this case the change in momentum of the gas stream is accomplished only by changing the gas flow direction. In a reaction turbine. Figure 3, there is a pressure drop across the blades. This results in acceleration of the gas as it passes through the wheel. The change in momentum which occurs in a reaction machine arises from both the change in flow direction and the acceleration of the flow. 352 A. Impulse Turbines To understand the impulse turbine better it is necessary to examine in detail the velocity change which occurs through the wheel. Figure 5 shows the ideal velocity diagram for an impulse machine. The fluid leaves the nozzle and flows towards the blades in a tangential direction with a velocity C imparted by the expansion through the nozzle. The flow enters the turbine blades with a relative velocity W with respect to the turbine blades. This velocity is reduced compared to C by the magnitude of the tangential velocity of the blade system. The relative velocity is thus given by Wj = C - U . In this ideal case the fluid in the blade passages is turned through an angle of 180° and leaves the wheel flowing tangentially with the same velocity relative to the blade surfaces with which it entered (W^ = W^). As noted in diagram 5 the absolute velocity of the flow with respect to the stationery nozzle structures is now Cg = C - 2U . The analysis of this case can be approached from either of two directions. We can examine the fluid velocities before entering and after leaving the wheel, and establish the change in energy which has taken place. This must be reflected in power which has been extracted by the turbine. Alternatively we can examine the forces acting on the turbine blades, caused by the change in flow direction. Since this force acts on a moving surface we can immediately establish the rate of power extraction. The equations for the external velocity approach are presented below. The inlet energy flow rate, E^ , is given by E^ = imC^ . The specific energy flow at the exit is Ej = iiiiC^ = i m (C - 2U) ^ . The decrease in energy of the flow gives the power extracted by the wheel P = El - Ej = 2mU(C - U) . The corresponding development for the force on the blades is the following. The change in momentum of the flow gives the force on the blades F = mCw^ - Wi) = 2m(C - U) . The rate of doing work is then P = F.U = 2mU(C - U) . 353 We may now form an expression for the efficiency with which the turbine has converted the energy which was available in the nozzle stream into useful work.
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