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 there
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4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AIR1326A AEROSPACEINFORMATION REPORTAIR1326 REV. AIssued 1974-01 Revised 1997-12
5、Reaffirmed 2014-07 Superseding AIR1326 Aircraft Fuel System Vapor-Liquid Ratio Parameter RATIONALE AIR1326A has been reaffirmed to comply with the SAE five-year review policy. FOREWORDChanges in this revision are format/editorial only.1. SCOPE:The AIR is limited to a presentation of the historical b
6、ackground, the technical rationale which generated the V/L fuel condition interface requirement in specifications between the aircraft fuel delivery system and the aircraft engine fuel system, and limitations in the usage of the V/L concept.1.1 Purpose:Since the introduction, in July 1951, of the va
7、por-liquid (V/L) ratio fuel condition parameter as an airframe/engine interface design requirement for military applications, many questions have arisen concerning interpretation of the requirement (1, 2).1The more important questions concern; (a) applicability of the concept to transient as well as
8、 steady state endurance demonstrations of engine fuel pump capability and, (b) limitations of the formula employed to calculate V/L ratio with respect to accuracy and range of conditions covered.Therefore, the primary purpose of this Aerospace Information Report (AIR) is to present the background wh
9、ich led to the introduction of the vapor-liquid (V/L) ratio parameter defining the condition of the fuel at the airframe/engine interface and to interpret its application to specification requirements. A secondary purpose is to promote a better understanding of the subject among airframe and engine
10、fuel system designers and users by providing a bibliography of the many, but not all, of the documents and papers published on the subject.2. REFERENCES:See Appendix A.1. Numbers in parentheses refer to the Bibliography in Appendix A.3. HISTORICAL BACKGROUND:3.1 Early History:Early reciprocating eng
11、ine aircraft, prior to WW II, which used wide boiling range high vapor pressure hydrocarbon fuels (aviation gasoline) were found to be altitude limited due to deficiencies in the airframe/engine fuel supply system. This situation was caused by the lack of definitive design requirements in early airc
12、raft specifications. The resolution of this problem was largely accomplished by the actual physical testing of the design changes installed in the aircraft itself.The advent of the turbojet engine brought about the need for new fuel requirements (3). The first fuel developed, AN-F-32, JP1, a low vap
13、or pressure fuel, did result in a short-lived period of successful operation, but still incomplete requirements for airframe/engine fuel system design prevailed. In 1947, the requirement for a wide boiling range jet fuel, AN-F-58, JP3, was issued. The requirement for this fuel was largely dictated b
14、y the desire to establish a military fuel of maximum availability anticipating that a national emergency might produce fuel shortages. This fuel permitted the conversion of a greater percentage of the crude oil to aviation fuel. Approximately only 10% of a barrel of crude oil is utilized to produce
15、kerosene, whereas approximately 50% of the barrel can be utilized for JP-3 fuel. Considerable opposition, however, developed against the high volatility of JP-3 fuel (RVP of 5-7 psia). After engine performance studies and testing, it was determined that a fuel having a Reid vapor pressure of 2.0 - 3
16、.0 psia would be satisfactory and would represent a compromise between engine performance and fuel availability. The new fuel became known asJP-4 and was included in MIL-F-5624A issued in May, 1951. JP-4 is now accepted as the prime military fuel.The development of the turbojet engine during the 195
17、0 decade, when operation at higher altitudes, higher mach numbers, higher tank fuel temperatures and increased fuel flows were required, created the need for more definitive design parameters at the airframe/engine interface. The evolution of these requirements are described further in this report._
18、 SAE INTERNATIONAL AIR1326A Page 2 of 183.2 Design Parameter Evolution:During the initial years of WW II, it had been recognized that Net Positive Suction Head (NPSH), commonly used in the commercial pumping industry for single boiling point fluids, was not adequate to define the possible two-phase
19、condition that could be generated in aircraft fuel systems using wide boiling range hydrocarbon fuels. Hence, the Coordinating Research Council (CRC) was asked to advise on this matter and subsequently provided a section to the CRC Handbook on Vapor Lock (January 1946 edition) which presented the me
20、ans for predicting Vapor-Liquid Ratios in dynamic fuel systems using hydrocarbon fuels (4).The military then published requirements for turbojet powered aircraft that limited aircraft fuel delivery systems to 3 and then subsequently 4 inches of mercury line drop (tank to engine inlet) at a specified
21、 flight altitude, usually 6000 feet, and at the specified engine power setting identified in the engine model specification (5, 6, 7, 8). The objective of this requirement was to create a worst case situation, i.e., “No Assistance From Airplane Boost Pump”, at a nominal to high power setting. Attemp
22、ts to apply this aircraft system “worst-case” requirement to the engine fuel system by simulated test techniques were not entirely satisfactory due mainly to variables possible in the test set-ups which produced inconsistent results. These factors then led the military to conclude that the Vapor-Liq
23、uid Ratio (V/L) parameter should be used as the design criterion for the condition of fuel at the aircraft/engine interface. This design and test requirement was then subsequently incorporated into engine military specifications (1, 2, 9, 10). Some years later new Military Aircraft Fuel System Speci
24、fications also incorporated V/L as the design parameter.3.3 Development of V/L Measurement Instrument:Fuel Pump Panels in the Aircraft Industries Association (AIA) and the Society of Automotive Engineers (SAE Panel A-1), circa 1952, began the joint task of exploring the feasibility of and initiating
25、 the design of a measuring device or meter to physically measure the Vapor-Liquid ratio of the flowing fluid at the inlet of a fuel pump during test (11). Several companies subsequently became interested in the design and development of a V/L meter. Three meters; one based on a light beam and photo-
26、electric sensitive plate, one based on a capacitance bridge comparison between a flowing and a non-flowing test section and a later design, currently in use, based on sensing the average dielectric constant of the flowing fluid, were designed and tested (12). The use of the V/L meter is covered late
27、r in paragraph 9._ SAE INTERNATIONAL AIR1326A Page 3 of 183.4 V/L Calculation Method (SAE ARP492A):The CRC method of calculating the V/L from known or set test conditions was initially used (4). Since the V/L calculation method depends on air solubility and vapor equilibrium conditions in the initia
28、l and final states, which is not easily attained or determined, and since the test set-up and test procedure has a marked effect on the release of dissolved air from fuel, SAE Committee A-16 and later AE-5 took up the task of standardizing the test set-up and procedure.SAE ARP492A, “Aircraft Fuel Pu
29、mp Cavitation Endurance Test”, was then prepared and issued after a long period of coordination with the Military and Industry (13). It was issued as an Industry Recommended Practice in 1957. The ARP defines the procedure for testing an aircraft engine fuel pump for the sole purpose of determining i
30、ts resistance to deterioration during endurance testing under Military Specification (emergency) V/L conditions at the pump inlet. The test was structured for the use of MIL-F-5624, grade JP-4 fuel, as it is the prime military fuel.WADC Technical Report 55-422, “Physical Properties of JP-4 Fuels and
31、 Development of Equations for Predicting Fuel System Performance Under Two-Phase Flow Conditions”, and the work of other authored documents and papers referenced in the Technical Report were used extensively in preparing ARP492A (14).3.5 Establishment of V/L Value:The emergency “No Assistance From A
32、ircraft Boost Pump” interface fuel condition in military specification MIL-E-5007A was established at 0.45 V/L. This value was established on the basis of the calculated V/L for a single engine fighter using high vapor pressure Aviation Gasoline (AN-F-48) at 6000 feet fuel tank altitude, 110 F fuel
33、temperature and 4 inches of Hg line loss (tank to engine inlet), at a specified engine power level (fuel flow rate) plus a safety margin (10). Turbo-propeller powered aircraft specifications, usually cargo type, specified a V/L requirement of 0.30 because of the difference in performance needs.In la
34、ter years the use of lower vapor pressure fuels (primarily JP-4) did ease the need for high V/L capabilities at the airframe/engine fuel system interface; however, retention of the 0.45 and 0.30 V/L values by the military is considered justifiable for safety and return flight margins after battle da
35、mage with aircraft power and support sub-systems inoperative.3.6 Design Intent:The design intent of current airframe and engine specifications related to the V/L requirement at the engine fuel inlet is to insure the continued operation of the engine within a defined flight envelope upon the unexpect
36、ed loss of tank booster pumps. It is an emergency capability and pumps to date have been accordingly designed for limited life when operated at the maximum V/L condition. The life test in the specifications is also of limited duration compared to the normal operational test._ SAE INTERNATIONAL AIR13
37、26A Page 4 of 184. AIRCRAFT FUEL SUPPLY CONDITIONS:4.1 Two-Phase Fuel Flow In Aircraft Fuel Supply Systems (15, 16):Two-phase fuel flow in relation to an aircraft feed system is defined as a condition where liquid fuel in combination with a gaseous product, evolved from the fuel, are flowing togethe
38、r in a line from the fuel tank to the engine inlet.This occurs as follows: When hydrocarbon fuel is placed in a vented container, it releases or dissolves air until the sum of the partial pressures of vapor and air within the container equals the ambient atmospheric pressure. Accordingly, for a give
39、n fuel there is an ultimate decrease in the amount of dissolved air with each increase in fuel vapor pressure or with a decrease in ambient air pressure. Since a reduction in air pressure above the fuel results in a decrease in the amount of dissolved air within the fuel, it follows that air is evol
40、ved from the fuel throughout all parts of the system, wherein a drop in absolute pressures occurs during climb of an airplane to altitude. At system absolute pressure levels which are high relative to the vapor pressure of fuel, the gaseous product is mainly air which had been previously dissolved i
41、n the fuel. As the pressure level is reduced, the vapor space contains an increasing fraction of fuel vapor until at absolute pressure levels which approach the vapor pressure of the fuel, the gaseous mixture is mainly the volatile components of the fuel. It is for the latter reason that the CRC met
42、hod of calculating the two-phase flow (V/L ratio) as used in SAE ARP492A has questionable accuracy at high fuel temperatures and high V/L ratios. (See paragraph 8.)Some of the more important factors in the formation of two-phase fuel flow in a feed line are:a. The type or batch of fuelb. Fuel temper
43、ature and changes in temperaturec. Initial absolute pressure of the fuel subsequent to delivery in the feed lined. The line pressure loss and the degree to which equilibrium conditions are again established in the fuel in the feed line4.2 Aircraft Operation-”With Assistance From Tank Boost Pump”:Ope
44、ration of the aircraft fuel supply system “With Assistance From Tank Boost Pump” is generally not a problem because the tank mounted fuel booster pumps can operate with some liquid boiling or cavitation taking place within the impeller - which must then separate the vapor or depend upon reabsorbtion
45、 by the fuel. The minimum pressure specified at the engine fuel pump inlet or connection is usually well above the true vapor pressure of the fuel (5 psi in MIL-E-5007) and at a condition of zero V/L._ SAE INTERNATIONAL AIR1326A Page 5 of 184.3 Aircraft Operation-”Without Assistance From Tank Boost
46、Pump”:Operation of the aircraft fuel supply system “Without Assistance From Tank Boost Pump” is the condition where two-phase flow is encountered. The specification generally requires the aircraft system to supply and the engine pump(s) to accept, at the specific fuel inlet temperature and V/L ratio
47、, the fuel required to meet engine performance guarantees within a defined flight envelope. This condition is an emergency requirement and unless otherwise spelled out in the aircraft and engine specification applies to steady state operation only.5. DEFINITION OF VAPOR-LIQUID RATIO (V/L):Definition
48、: V/L = the equilibrium ratio of vapor volume (actually air and fuel vapor) to volume of liquid, both at the same temperature.Relationship: In terms of one volume of Vapor and Liquid, often referred to as Percent Quality:V + L = 1V = 1 - LV = Vol. of vaporL = Vol. of liquid both in same unitsExample: Using 0.45 V/L as an example and substituting:Solving for L,
