Thermal Stability and Heat Transfer Characteristics of RP-2 | Heat Transfer | Temperature

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  REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.   1. REPORT DATE (DD-MM-YYYY)   30-06-2008 2. REPORT TYPE   Technical Paper 3. DATES COVERED (From - To)   4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Thermal Stability and Heat Transfer Characteristics of RP-2 (Preprint) 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Matthew Billingsley (AFRL/RZSA) 5d. PROJECT NUMBER 5e. TASK NUMBER 48470244 5f. WORK UNIT NUMBER   7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER Air Force Research Laboratory (AFMC) AFRL/RZSA 10 E. Saturn Blvd. Edwards AFB CA 93524-7680 AFRL-RZ-ED-TP-2008-259   9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)   10. SPONSOR/MONITOR’S ACRONYM(S)   Air Force Research Laboratory (AFMC) AFRL/RZS 11. SPONSOR/MONITOR’S 5 Pollux Drive NUMBER(S)   Edwards AFB CA 93524-7048 AFRL-RZ-ED-TP-2008-259   12. DISTRIBUTION / AVAILABILITY STATEMENT Approved for public release; distribution unlimited (PA #08259A). 13. SUPPLEMENTARY NOTES   For presentation at the 44 th  AIAA Joint Propulsion Conference, Hartford, CT, 20-23 July 2008. 14. ABSTRACT In an effort to enable reusable, high-performing liquid rocket engines, a comprehensive experimental and numerical investigation of the thermal  performance (thermal stability and heat transfer characteristics) of RP-2 is underway at the Air Force Research Laboratory (AFRL), Edwards AFB, CA. In the current work, the High Heat Flux Facility (HHFF) was used to provide initial RP-2 thermal performance information under conditions simulative of those encountered in the cooling channels of a real engine. RP-2 was thermally stressed while flowing through circular copper tube test sections. Short-duration thermal stressing tests provided heat transfer information which closely followed existing empirical correlations for RP-1. Effects of wall temperature, bulk temperature, and flow rate on heat transfer were observed and were consistent with expected behavior. Longer-duration tests at elevated wall temperatures provided the first steps in elucidating the conditions under which solid carbon deposits form. The test sections were analyzed post-test with optical and scanning electron microscope and carbon deposition burn-off for signs of coke formation. The results from these analyses indicate the presence of solid carbon deposition for high-wall temperature tests exceeding 30 min. in duration, although further testing is required to make more conclusive comparisons. 15. SUBJECT TERMS   16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON Matthew Billingsley a. REPORT Unclassified b. ABSTRACT   Unclassified c. THIS PAGE   Unclassified SAR 11 19b. TELEPHONE NUMBER (include area code)    N/A Standard Form 298 (Rev. 8-98)   Prescribed by ANSI Std. 239.18     American Institute of Aeronautics and Astronautics Distribution A: Approved for Public Release. Distribution is Unlimited. 1 Thermal Stability and Heat Transfer Characteristics of RP-2 Matthew C. Billingsley  Air Force Research Laboratory, Edwards AFB, CA, USA In an effort to enable reusable, high-performing liquid rocket engines, a comprehensive experimental and numerical investigation of the thermal performance (thermal stability and heat transfer characteristics) of RP-2 is underway at the Air Force Research Laboratory (AFRL), Edwards AFB, CA. In the current work, the High Heat Flux Facility (HHFF) was used to provide initial RP-2 thermal performance information under conditions simulative of those encountered in the cooling channels of a real engine. RP-2 was thermally stressed while flowing through circular copper tube test sections. Short-duration thermal stressing tests provided heat transfer information which closely followed existing empirical correlations for RP-1. Effects of wall temperature, bulk temperature, and flow rate on heat transfer were observed and were consistent with expected behavior. Longer-duration tests at elevated wall temperatures provided the first steps in elucidating the conditions under which solid carbon deposits form. The test sections were analyzed post-test with optical and scanning electron microscope and carbon deposition burn-off for signs of coke formation. The results from these analyses indicate the presence of solid carbon deposition for high-wall temperature tests exceeding 30 min. in duration, although further testing is required to make more conclusive comparisons. I. Introduction MPROVED understanding and characterization of fuel thermal stability is required for the design and development of long-life, reusable liquid rocket engines. Regeneratively-cooled hydrocarbon engines maintain wall conditions below failure limits by actively cooling the thrust chamber assembly with fuel prior to fuel injection. During this process, the fuel absorbs a tremendous amount of heat and may undergo molecular decomposition, eventually resulting in insoluble products depositing on the cooling channel walls. This deposition tends to insulate the wall material, causing localized hot spots which can ultimately lead to structural failure. Thoroughly characterizing the chemical and physical processes of fuel decomposition and deposition and the conditions under which they occur will enable engine designers in the process of developing reusable, highly operable engines. Improving a fuel’s ability to absorb heat without coking is also desirable from a performance standpoint. The  present work is an experimental investigation of the thermal stability of the kerosene-based rocket fuel RP-2 in a heated tube under realistic fluid and thermal conditions. Thermal stability was gauged by several factors, including deposit formation and heat transfer. Coking temperature is often referred to as the temperature above which solid deposition readily occurs, and wetted wall temperatures exceeding this limit are intentionally avoided in the design of thrust chamber assemblies. However, the chemical process of thermal decomposition and the physical process of deposit formation are influenced by several factors: fuel composition, wall roughness and material, residence time, bulk fluid conditions, and numerous temperature-dependent physical properties. The variety of contributing factors leads to a wide variability in reported coking temperatures for rocket kerosene. For RP-1 (MIL-DTL-25576E, 2006), a narrow-range kerosene fraction developed in the 1950’s, reported coking wall temperature limits range from 550-850°F (561-727K). To properly quantify a fuel’s thermal stability in terms of coking temperature, understanding the effects of the aforementioned influences is important. With this in mind, AFRL’s High Heat Flux Facility (HHFF), Edwards AFB, CA, was used to thermally stress RP-2 in a high-temperature, high-pressure environment. The effort discussed in this paper is part of a comprehensive  program intended to develop and transition improved hydrocarbon fuels for use in liquid rocket engines. This includes full characterization of the fuel’s thermal performance (thermal stability and heat transfer characteristics). A specific goal for the current work was to provide an initial measure of the thermal performance of RP-2. This was attempted by flowing fuel at relatively low velocity and high wall temperature channels and observing signs of solid deposition formation. I  print) (Preprint)( Preprint)   American Institute of Aeronautics and Astronautics Distribution A: Approved for Public Release. Distribution is Unlimited. 2  Figure 1. Experimental test section configuration.    Left view shows cradle assembly and a portion of test section tube. Right view is a cutaway showing thermocouple and heater block contact with tube.Test section tube is OFE copper, 1/8-in. O.D., 0.032-in. wall thickness (nominal). Thermocouple bead  sare silver-soldered to test section tube. II. Experimental Setup and Procedures The HHFF is a relatively new rig capable of simulating a realistic cooling channel environment and providing fuel thermal stability data over a wide variety of operating conditions. It allows for flexibility in several regards, including cooling channel geometry and material, wetted wall temperature, flow velocity, fuel composition, and channel surface features. The ability to preheat the fuel allows examination of the effect of bulk inlet temperature and comparison of core flow/boundary layer chemistry effects. A thorough discussion of the facility design is  provided in Reference 1. The current testing took advantage of flexibility in test section geometry by utilizing a simplified channel configuration. The simplified geometry was used for several reasons. First, it provided a useful comparison with existing heated tube data such as the Heated Tube Facility at NASA’s Glenn Research Center  2  (RP-2 thermal stability testing is currently being conducted in that facility at different conditions, with comparisons forthcoming). Second, in fuel characterization work, an extensive experimental effort is necessary, and a simplified geometry is an efficient way of examining the general thermal performance, specifically fuel decomposition and its effects. When testing conditions such as wetted wall temperature and residence time are established which produce measurable levels of coke, realistic test section geometries and flow conditions are planned. Third, the test sections are relatively inexpensive and readily available. The experimental setup for the current work is shown in Figure 1. Heat transfer from the copper heater block to the test section occurred asymmetrically across a semi-circular contact surface area formed by a groove in the heater  block in which the test section fit snugly. The test section tube rested in a cradle of low-thermal conductivity ceramic, minimizing conduction to the assembly and simulating asymmetric heat transfer. In turn, the test section and its cradle rested in a higher strength ceramic cradle which was supported by an aluminum fixture suspended in a vacuum chamber. Conducting experiments under high vacuum minimized convective losses to the surroundings and oxidation on the copper surfaces. Five K-type right-angle ribbon thermocouples were brazed along the bottom centerline of each test section with 0.4-in. (10-mm) spacing. Thermocouple leads passed through holes in both ceramic cradles. One K-type spring-loaded thermocouple measured the temperature near the bottom of the heater block during the test. The heater block temperature was maintained with twenty-five 800W custom Watlow Firerod cartridge heaters with embedded K-type thermocouples with mineral insulated leads, controlled at the console through a Watlow MLS 332 controller. Transducer data was collected at a sample rate of 100 Hz with Pacific Instruments 6013 8-channel amplifier-digitizer cards, and previewed/recorded using Pacific Instruments PI660 software. High-pressure bladder tanks were used to pressurize the fuel, with pressure regulated by a Tescom ER3000 electronic pressure controller using a PID algorithm. A Coriolis mass flow meter measured the fuel flow rate with a stated uncertainty of ±0.1% at the flow rates tested. A preheater upstream of the test section was used to raise the  bulk fluid temperature when desired. Cavitating venturis of varying throat area maintained constant mass flow rate despite fluctuations in downstream pressure. Most of the venturis used in the testing operate effectively at up to 75-80% pressure recovery. A backpressure control valve was used to maintain test section pressure greater than 1000  psi (48 kPa) to minimize boiling and two-phase phenomena and sharp transport property gradients near the critical  point. Downstream of the test section, the fuel was collected and either reloaded or discarded, depending on the test objectives. A typical test procedure involved increasing the fuel preheater and heater block to their specified temperatures, increasing system pressure, and adjusting the backpressure valve and pressure-regulating ER3000 to obtain the desired test section velocity and pressure. When flow conditions were reached, data recording began and the heater  block was lowered onto the test section tube. An increase in the tube thermocouples occurred instantly. Flow Heater blockBrazed TC Test section tube Assembly cradle Test section cradle   American Institute of Aeronautics and Astronautics Distribution A: Approved for Public Release. Distribution is Unlimited. 3 Figure 2. Measured temperature histories for a representative short-duration heat transfer test.  Data used for heat transfer calculations is bounded by thick dashed lines. Thermocouple axial location is indicated inarentheses. Figure 3. Temperature contours for axisymmetric heat transfer totubular test section (scale in K). Conjugate heat transfer calculationsreveal large temperature gradientsaround inner (wetted) wall circumference. conditions were maintained and the heater block remained in contact with the test section tube for the duration of the test, which ranged from 2 – 40 min. for the results presented. After this time, the heater block was raised, fuel flow was stopped, and the test section was purged with low-pressure nitrogen to remove any residual fuel. The block and tube assembly then cooled under vacuum, and finally the test section was removed and prepared for analysis. III. Results and Discussion A. Heat Transfer Results Several shorter-duration tests were conducted to examine the relationship between fuel bulk temperature, wetted wall temperature, and heat transfer. Minimal coke formation was expected with little influence on the measured heat transfer rates. For this reason, test sections were reused for several tests, which helped improve repeatability of thermo-fluid conditions. Surface and fluid temperature measurements were averaged over the steady-state  portion of each run, usually between 2-3 min. During this time, surface temperatures varied little, but in some cases experienced an approximate 3% decrease as heat transfer occurred along the tube. Typical variation in measured surface temperature between axial locations was within 3% of the average measured temperature. This can be seen in Figure 2, showing measured surface temperatures for a representative run. The thick dashed lines indicate the user-selected portion of the test for which data were included in heat transfer calculations, and the parenthesized numbers in the legend are the axial distance of the temperature measurement, with conduction heat transfer from the  block beginning at 0 in. Temperatures at all tube locations decrease initially, remain relatively constant for most of the test, and increase slightly near the end. The variation in measured temperature over time was due to corresponding temperature fluctuations in the heater block itself. The tube temperature is greatest at the location of T2, and decreases in the upstream and downstream directions. The decreasing measured temperature in the downstream direction may have been caused by slight misalignment  between the heater block and the tube, resulting in relatively high thermal contact resistance and lower heat transfer rates at certain locations compared with others. The experimental setup in the current work improves on previous efforts 3  by making a surface temperature measurement closer to the wetted wall temperature. However, a small temperature difference is still expected, due to exact location and size of the brazed ribbon thermocouple junction, minor conduction losses through thermocouple leads, and temperature gradient across the 0.032-in. tube wall. The first two of these were considered negligible. A one-dimensional conduction model was used to estimate the wetted wall temperature based on the average enthalpy increase of the fuel and the thermal conductivity of the tube. Assuming the energy increase in the fuel was due to conduction around the circumference of the tube, a less than 3% decrease in temperature from the outer surface to the inner surface was calculated. A more significant complication in temperature reporting arises from the circumferential variation in temperature around the inner surface of the tube. The asymmetric heat transfer results in large differences between, for example,
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