1、_SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising theref
2、rom, is the sole responsibility of the user.”SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions.Copyright 2015 SAE InternationalAll rights reserved. No part of this publi
3、cation may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE.TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada)Tel: +1 724-776-4970 (out
4、side USA)Fax: 724-776-0790Email: CustomerServicesae.orgSAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedbackon this Technical Report, please visithttp:/www.sae.org/technical/standards/AIR1811AAEROSPACEINFORMATION REPORTAIR1811REV. AIssued 1985-09Revised 1997-10Reaffirmed 2015-
5、10Superseding AIR1811Liquid Cooling SystemsRATIONALEAIR1811A has been reaffirmed to comply with the SAE five-year review policy.FOREWORDChanges in this revision are format/editorial only.TABLE OF CONTENTS1. SCOPE .41.1 Purpose.42. REFERENCES .43. BACKGROUND54. LIQUID COOLING TECHNIQUE54.1 Benefits .
6、54.2 Types of Liquid Cooling Systems64.2.1 Direct.64.2.2 Indirect 74.2.3 Expendable .95. HEAT SINKS.95.1 Ambient or Ram Air.95.2 Fuel .105.3 Expendables .105.4 Heat Storage . 115.5 Refrigeration System Transport Loop . 11TABLE OF CONTENTS (Continued)6. SYSTEM DESIGN 126.1 Liquid Loop Cooling System,
7、 Description .126.2 Review of Liquid Cooling System Design Features 126.2.1 Thermodynamic Performance.126.2.2 Package Configuration and Weight.126.2.3 Structure146.2.4 Reliability.146.2.5 Maintainability .146.2.6 Environment156.2.7 Electromagnetic Interference 156.2.8 Corrosion Protection .156.2.9 T
8、esting.157. COOLANT SELECTION .157.1 Fluid Properties.157.1.1 Heat Transfer 157.1.2 Thermal Expansion .157.1.3 Surface Tension 167.1.4 Flash Point 167.1.5 Flammability167.1.6 Pour or Freezing Point 167.1.7 Viscosity167.1.8 Vapor Pressure .167.1.9 Thermal Decomposition 167.1.10 Dielectric Strength.16
9、7.1.11 Moisture Absorption 167.1.12 Toxicity 167.1.13 Compatibility .177.1.14 Lubricity.177.1.15 Optical Clarity177.2 General Liquid Properties .177.2.1 Glycol-Water or Methanol-Water.177.2.2 Silicate Ester .177.2.3 Silicone .177.2.4 Halogenated-Hydrocarbon187.2.5 Synthetic Hydrocarbon (Polyalphaole
10、fin)187.2.6 Water.187.2.7 Ammonia.187.2.8 Cryogenic18SAE INTERNATIONAL AIR1811A 2 OF 26TABLE OF CONTENTS (Continued)8. COMPONENTS 188.1 Heat Exchanger 188.2 Pump.208.3 Reservoir and Accumulator.208.4 Flow Control Valve 218.5 Filter and Bypass Valve.218.6 Relief Valve .228.7 Heater .228.8 Disconnect
11、228.9 Fill and Drain Connection229. CORROSION PREVENTION AND CONTROL 229.1 Types of Corrosion and General Prevention.239.1.1 Atmospheric 239.1.2 Galvanic Corrosion .239.1.3 Electrolytic Corrosion 249.1.4 Chemical Corrosion 249.2 Corrosion Prevention 249.2.1 Aqueous Ethylene Glycol259.2.2 Fluorochemi
12、cal Liquid .259.2.3 Silicate Ester Fluid 259.2.4 Expendable Water.2510. SERVICING AND SYSTEM CHECKOUT.2510.1 Closed Loop System.2610.2 Expendable System 26FIGURE 1 Cooling Methods Versus Heat Transfer Coefficients6FIGURE 2 Direct Liquid Cooling 7FIGURE 3 Indirect Liquid Cooling With Integral Cold Pl
13、ate.8FIGURE 4 Indirect Liquid Cooling With Non-Integral Cold Plate .8FIGURE 5 Liquid Loop/Heat Sink Schematic 10FIGURE 6 Refrigeration System Transport Loop 11FIGURE 7 Representative Liquid Cooling System Schematic 13SAE INTERNATIONAL AIR1811A 3 OF 261. SCOPE:This publication is applicable to liquid
14、 cooling systems of the closed loop type and the expendable coolant type in which the primary function is transporting of heat from its source to a heat sink. Most liquid cooling system applications are oriented toward the cooling of electronics. Liquid cooling techniques, heat sinks, design feature
15、s, selection of coolants, corrosion control, and servicing requirements for these systems are presented. Information on vapor compression refrigeration systems, which are a type of cooling system, is found in Reference 1.1.1 Purpose:The purpose of this Aerospace Information Report (AIR) is to provid
16、e guidelines for the selection and design of airborne liquid cooling systems.2. REFERENCES:1. “General Requirements for Application of Vapor Cycle Refrigeration Systems for Aircraft.”SAE ARP731A, April 15, 1973.2. A. E. Bergles, R. C. Chu, and J. H. Seely. “Survey of Heat Transfer Techniques Applied
17、 to Electronic Equipment.” ASME-72-WA/HT-39, November, 1972.3. “Cooling of Modern Airborne Electronic Equipment.” SAE AIR1277, May, 1976.4. “Reliability/Design Thermal Applications.” MIL-HDBK-251, January 19, 1978.5. K. H. Token, E. C. Garner, R. S. Cook, and J. E. Stone. “Heat Pipe Avionic Thermal
18、Control.” AIAA-80-1511, July, 1980.6. “Environmental Test Methods.” MIL-STD-810C, Military Standard, 10 March 1975.7. A. F. Knight. “Choice of Fluids for Cooling Electronic Equipment.” Electro-Technology; Vol. 71, No. 6, Pages 57-63, June, 1963.8. D. J. Kelly. “Resistance of Materials to Hydraulic F
19、luid.” Machine Design, January 21, 1971.9. “SAE Aerospace Applied Thermodynamics Manual.” SAE ARP1168, October, 1969.10. W. Kays and A. L. London. “Compact Heat Exchangers.” 2nd Edition, McGraw-Hill, New York, 1964.SAE INTERNATIONAL AIR1811A 4 OF 263. BACKGROUND:Liquid cooling systems offer many sig
20、nificant advantages over air cooling systems.A major application of liquid cooling systems is for avionics in military aircraft. This use is the result of sophisticated, high performance electronic components in small packaging sizes, and the need for high reliability of avionic systems.Liquid cooli
21、ng can be accomplished by direct contact of the liquid with heat producing components, or indirectly via heat exchange devices.In either case, the liquid transports heat to a heat sink. In airborne systems, the heat sink can be ambient or ram air, fuel, an expendable or heat storage material, or a r
22、efrigeration system.Expendable systems are typically used in remotely piloted vehicles or drones, for avionics cooling and as a supplemental heat sink for a refrigeration system transport loop.4. LIQUID COOLING TECHNIQUE:A major reason to use liquid for cooling is that it is more efficient than cool
23、ing with a gas, such as air. It may be used to remove heat directly or indirectly from the heat source and it may absorb heat as it changes phase and is expended.4.1 Benefits:Liquid cooling systems provide superior localized cooling than either air or gas cooling. A larger heat transfer coefficient
24、is obtainable with liquid versus air or gas, resulting in smaller differences between component temperatures and coolant supply temperatures. The smaller temperature difference allows components to operate at lower temperatures. This improves component reliability and reduces maintenance costs.Relat
25、ive heat transfer coefficients for two liquid cooling methods compared to two air cooling methods are presented in Figure 1. (See Reference 2 or 3.) The higher heat transfer coefficients of liquid cooling systems permits larger power densities to be adequately cooled. Also, liquid cools low power de
26、nsity equipment more efficiently than air cooling. Heat transfer coefficients generally increase with increased fluid velocity. However, pressure losses increase with increased velocity.The heat capacitance (mass flow rate times specific heat) of a cooling fluid is a significant factor. The large he
27、at capacitance and high density of liquids results in smaller heat sink components and transport lines sizes. This generally reduces weight, therefore, lowers aircraft fuel penalties.Disadvantages of liquid cooling systems also should be analyzed for each potential application. Major areas to be con
28、sidered are maintenance, potential leaks, fluid contamination, logistics, and potential fire hazards.SAE INTERNATIONAL AIR1811A 5 OF 26FIGURE 1 - Cooling Methods Versus Heat Transfer Coefficients4.2 Types of Liquid Cooling Systems:Direct, indirect, and expendable cooling systems are addressed in thi
29、s AIR.4.2.1 Direct: Liquid may be used to remove heat directly from the heat source. This provides the minimum thermal resistance from the heat source to the liquid. An example of direct liquid cooling of avionic components on circuit boards is shown in Figure 2.A major consideration of direct liqui
30、d cooling is compatibility of all parts being cooled with the liquid. Other design considerations are thermal expansion, liquid pressure, maintenance and servicing provisions, and controls for normal operation and failure conditions.SAE INTERNATIONAL AIR1811A 6 OF 26FIGURE 2 - Direct Liquid Cooling4
31、.2.2 Indirect: In this method, heat is transferred to the liquid in a liquid cooled cold plate, via an intermediate device. Conduction is the common method for transferring heat from components and subassemblies to a liquid cooled cold plate. An advantage of indirect liquid cooling over direct cooli
32、ng is that a heat producing component can be removed for servicing without disturbing (i.e., opening) the liquid loop.The cold plate can be an integral part of the assembly being cooled, or it can be designed to readily separate from the assembly being cooled (i.e., a non-integral design). An exampl
33、e of an integral indirect cooling design is shown in Figure 3 (from Reference 4). An example of a non-integral indirect cooling design is shown in Figure 4 (from Reference 5).Indirect cooling designs require an important thermal design consideration which is not necessary for direct cooling designs.
34、 Heat must be transferred from the heat source to the liquid. This heat should be transferred with a minimal total thermal resistance. There are two types of thermal resistances in this heat transfer path.One thermal resistance is across one or more interfaces. An objective of the indirect design is
35、 to allow removal of a component subassembly (as in Figure 3) or removal of an assembled unit (as in Figure 4). It is important that mating surfaces of an interface are flat and are held together with sufficient pressure. A low thermal resistance design becomes more important as heat transfer rate i
36、ncreases per unit area of the interface. A thin, high thermal conductivity material (e.g., silver filled silicone rubber) can be used between the two surfaces to fill the small asperities, thus decreasing overall thermal resistance.SAE INTERNATIONAL AIR1811A 7 OF 26FIGURE 3 - Indirect Liquid Cooling
37、 With Integral Cold PlateFIGURE 4 - Indirect Liquid Cooling With Non-Integral Cold PlateSAE INTERNATIONAL AIR1811A 8 OF 264.2.2 (Continued):The second thermal resistance is from the heat source to an interface or between interfaces (if more than one exists). Conductive heat transfer in copper or alu
38、minum is most commonly used (see note in Figure 3). Small heat pipes can transfer heat with a smaller thermal resistance than copper, and heat pipes weigh less. A heat pipe is a passive closed circuit heat transfer device which uses evaporation, condensation, and counter-current flow of liquid and v
39、apor. Heat pipes can be used in avionic circuit boards or modules, and in assembly sidewalls (see Figure 4) to reduce overall thermal resistance in non-integral indirect liquid cooling designs.4.2.3 Expendable: Expendable cooling systems may be used with direct or indirect techniques of cooling equi
40、pment. Expendable systems may be used to handle temporary or peak loads. The quantity of heat to be removed determines the weight of expendable fluid required. Since liquid is used as heat is removed, expendable systems are time limited. These systems use the latent heat of evaporation, plus sensibl
41、e heat, to absorb the heat load.5. HEAT SINKS:Selection of the heat sink for a liquid cooling system depends on availability and temperature of heat sinks, and on other unique design factors.Ambient or ram air, fuel, refrigeration system (such as an air cycle or vapor cycle), and expendables or heat
42、 storage materials are available types of heat sinks used for liquid cooling systems. A combination of different types may be required to optimize the system for different operating conditions. The cooling capacity of fuel and air heat sinks are related primarily to the flow rate and temperatures of
43、 these fluids. For a refrigeration system, its capacity is a function of system design. Cooling capacity of an expendable is the latent and sensible heat of the liquid and the liquid quantity. The capacity of a heat storage material is the latent heat of fusion and the quantity of the material.5.1 A
44、mbient or Ram Air:Heat from a liquid cooling system may be transferred to an ambient or ram air heat sink through an air-to-liquid heat exchanger, as shown in Figure 5. Under most aircraft operating conditions, cooling load requirements can be met with ram air. For static and low speed aircraft oper
45、ation, a fan or air driven ejector pump is used to provide adequate airflow. When a fan is used, consideration must be given to prevent potential fan overspeed. As aircraft speed increases, ram air temperature and pressure increases, and should be considered in the design. Increased airflow is requi
46、red as ram air temperature increases. Other factors to be considered in the use of air as the heat sink are effectiveness of the heat exchanger in removing heat, and matching of available pressure head of the air and pressure losses of the heat exchanger design.SAE INTERNATIONAL AIR1811A 9 OF 26FIGU
47、RE 5 - Liquid Loop/Heat Sink Schematic5.2 Fuel:Fuel, as a heat sink, may be pumped through a heat exchanger, installed in the liquid cooling system as shown in Figure 5, or a natural convection heat exchanger may be installed within a fuel tank. If the amount and rate of heat added to the fuel cause
48、s fuel temperatures to rise above allowable limits, an auxiliary heat sink must be used or the fuel must be cooled. Fuel temperature limits are established by engine design and by the airframe design.The advantage of a system with fuel pumped through a heat exchanger is high heat transfer rates, res
49、ulting in smaller and lighter heat exchangers. A major advantage of locating a heat exchanger in the fuel tank is that fuel is confined to the fuel tank, reducing the fire hazard. For a free convection heat exchanger located in a fuel tank the heat transfer rates are smaller since it is free rather than forced convection, hence, the heat exchanger is larger and heavier. Other disadvantages of this approach are servicing difficulty, loss
