Additives to Increase Fuel Heat Sink Capacity

Transcription

1 Additives to Increase Fuel Heat Sink Capacity 41 st AIAA/ASME/SAE/ASEE Joint Propulsion Conference James Nabity Dr. David T. Wickham, P.I. Bradley D. Hitch Jeffrey R. Engel Sean Rooney July 11, 2005 Research Inc. Wheat Ridge, CO

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3 Overview Application of endothermic fuels. Initiated thermal cracking reactions. Results of laboratory experiments to measure initiated heat sink capacity. Results obtained with pilot scale fuel/air heat exchanger.

4 NASA Application for Endothermic Fuels Improve commercial access to space. Current cost is about $10,000 per pound. Goal is to reduce cost to $100 per pound. Cost reductions will require: Single stage to orbit (SSTO) vehicles. Rocket-based combined cycle (RBCC) engines using hydrocarbon fuel. At speeds between Mach 5 and 10, heat loads exceed cooling available from sensible heating of the fuel. Thermal cracking reactions may provide the additional heat sink capacity.

5 Applications of Endothermic Fuel Cooling Cool solid panels Heat Load Fuel Cool air out Fuel Reduce air temperature Hot Air In

6 Thermal Cracking Reactions Heptane cracking. C 7 H 16 CH 4 + C 2 H 4 + C 2 H 6 + C 3 H 6 + C 3 H 8 + ΔH Btu/lb Proceeds by a free radical mechanism. Initiation - slow step C 7 H 16 C 7 H 15 + H Ethylene formation by β scission - fast C 7 H 15 C 2 H 4 + C 5 H 11 Chain propagation - fast C 7 H +CH 16 3 C 7 H 15 + CH 4 The overall rate is limited by the initiation step, which is slow at working temperatures.

7 Addition of Chemical Initiator Increases the rate of radical generation because the R-R bond is weaker than the C-H bond. R-R 1 R + R 1 R and R 1 then react with the fuel. R +C 7 H 16 C 7 H 15 + R-H The rest of the process is identical to the mechanism without the initiator. The chemical initiator only starts the reaction - it has no effect on reaction stoichiometry. Low concentrations required (less than 3 wt%).

8 Characteristics of the Initiator Consists of carbon, hydrogen, and oxygen. Is soluble in normal paraffin fuels. Is stable in its concentrated form at ambient temperatures. It is not a highly toxic chemical.

9 Previous Results with n-heptane 1.20 Reaction Rate / mole heptane/cc h Initiator Concentration/ wt% Initiator Research Proprietary Temperature / C

10 Objective of Current Project Measure heat sink capacity of real fuels such as JP-7 with and without initiator. Use kinetic data to design and construct a pilot scale heat exchanger and demonstrate initiator under realistic heat flux.

11 Laboratory Apparatus INITIATORS 1000 ml VENT VENT TO WASTE COLLECTOR AO PT DRAIN VALVE VACUUM CHAMBER HEPTANE O 2 SENSOR HPLC PUMP TEST SECTION PT EXIT PRESSURE TRANSDUCER N 2 SPARGER 5µ FILTER AO PT PCV To Vent HPLC PUMP PREHEATER N 2 QUENCH GC 5 GAL WASTE COLLECTOR N2 Sparge AO AIR INLET Mass Flow Controller A HIGH PRESSURE N 2 INLET AO Mass Flow Controller A

12 Test Section Used Annular Fuel Flow Path

13 Test Section Installed in a Vacuum Chamber to Reduce Convective Losses

14 Power Measurement V1 V2 V3 Heater High Precision Resistor Power Power = V * I V = V1 - V2 I = (V2 - V3) / r Measurements were made at 1000 Hz with a digital oscilloscope

15 Significant Power Increase with Initiator Addition n-heptane P = 550 psi 1.1 Degrees C Inject 500 C 133 watts Reactor out Reactor In 162 watts Wall-3 Wall-2 Wall-1 Inject 550 C 192 watts 243 watts Fraction Full Power to Heater Discontinue 100 Initiator Run Time (hrs)

16 1000 The Initiator Improves the Heat Sink Capacity of JP-7 Heat Sink Capacity from 25 C (Btu/lb) Sensible Heating Only wt% Initiator 4wt% Initiator JP-7 LHSV = 1000 h psi Fluid Out Temperature (C)

17 Substantial Increases in n-decane Heat Sink Capacity 1100 Endotherm from 25C (Btu/lb) Sensible Heating Measured - 0% Initiator Measured - 4% Initiator n-decane LHSV = 1000 h psi Fluid Out Temperature (C)

18 The Initiator is Very Effective with a 1150 Mixture of Normal Paraffins Endotherm from 25C (Btu/lb) Sensible Heating Measured 0% Initiator Measured 4% Initiator Norpar 13 Pressure = 550 psi LHSV = 1000 h Fluid Out Temperature (C) endotherm_plots_2

19 Cyclohexane is Thermally Stable without Initiator 900 Sensible Heating 0% Init Endotherm from 25C (Btu/lb) % Init Cyclohexane LHSV = 1000 h -1 Pressure = 550 psi endotherm_plots_2 Fluid Out Temperature (C)

20 The Initiator Reduces the Activation Energy of the Cracking Reaction 1100 Heat Sink from 25 C / Btu/lb Sensible Heating Data without Initiator Data with 4% Initiator Model without initiator Model with initiator n-decane P = 550 psi LHSV = 1000 h -1 V = 3.15E4 moles/s cm 3 P fuel Ea = 39 Kcal/mole V = 2.26E10 moles/s cm 3 P fuel Ea = 64 Kcal/mole cracking_models Temp / C

21 Kinetic Data for JP Heat Sink from 25 C / Btu/lb Sensible Heating Data without Initiator Data with 4% Initiator Model without initiator Model with initiator V = 1.4E4 moles/s cm 3 P fuel Ea = 38.5 Kcal/mole JP-7 P = 550 psi LHSV = 1000 h V = 1.45E11 moles/s cm 3 P fuel Ea = 67 Kcal/mole cracking_models Temp / C

22 Design and Construct Pilot Scale Air/Fuel Heat Exchanger Demonstrate heat sink capacity under realistic conditions. Heat flux of approximately 100,000 Btu/ft 2 h. T air in = 780 C, T air out = 350 C T fuel in = 65 C, T fuel out = 450 C

23 Schematic of Ethylene Burner and the Heat Exchanger Fuel in/out Cross section of the coiled fuel flow path Hot exhaust gas Air flow out Air Ethylene Ethylene Burner

24 Finned Inconel Tubing for Fuel 32 feet total finned tubing length 25.5 in overall unit length 3 in coil diameter 41 total wraps ~9.4 in length per wrap

25 Kinetic Model Used to Predict Cracking Level JP-7 - without initiator LHSV = 630 h -1 Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h Temperature / C % of the fuel reacted Percent Fuel Cracked 100 fuel temperature Reactor Length / ft

26 Addition of Initiator Increases the Fuel Cracking Reaction JP-7 with 2% Initiator LHSV = 630 h -1 Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h Temperature / C fuel temperature 23% cracking Percent Fuel Cracked Reactor Length / ft

27 Installed in Test Rig Main air exit Heat exchanger housing Ethylene burner

28 We Measured Non Condensable Flow 5ft length 14 Round Duct AO Flow sensor 14 Axial Fan - rated for 1100cfm Non-Condensable Flow - 12 scfm maximum Diluting Air Flow 1/2 Pipe 1/2 Tubing HX Effluent VENT 25 psig Mass flow meter Waste Container 200 psig pressure rating

29 Pilot Scale Rig in Operation

30 Initiator Causes an Increase in the Fuel Heat Sink Capacity JP-7 Pressure = 550 psi Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h LHSV = 630 h -1 Air Inlet temperature Temperature (C) Fuel out temperature 2% Initiator 2% Initiator 2% Initiator 450 Air out temperature _26_JP7_Data Run Time (min)

31 Initiator Produces Significant Increase In Non Condensable Flow Non Condensable Flow (slpm) JP-7 Pressure = 550 psi Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h LHSV = 630 h -1 Fuel Temperature = 564 C Initiator on 10% Fuel Temperature = 578 C Initiator on 14% Initiator off Fuel Temperature = 600 C Initiator on 11% 20% % Initiator off 3.8% _26_JP7_Data Run Time (min)

32 Model for JP-7 Fits the Data Well 1000 Heat Sink from 25 C ( Btu/lb) JP-7 Pressure = 550 psi Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h LHSV = 630 h -1 Revised Model - without Initiator Model - with Initiator Sensible Heating Measured - with Initiator Measured - without Initiator Model_Measured_charts Fuel Temperature (C)

33 Initiated Cracking Adds Substantial 1000 Heat Sink Capacity for n-decane Heat Sink from 25 C ( Btu/lb) n-decane Measured - Pressure = 550 psi with Initiator Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h LHSV = 630 h -1 Measured - Model - without Initiator Model - with Initiator Sensible Heating without Initiator 650 Maximum heat flux = 91,000 Btu/h ft Model_Measured_charts Fuel Temperature (C)

34 The Initiator Can Reduce the HX Temperature JP-7 Pressure = 550 psi Air Side Flow = 1121 lb/h Fuel Flow = 117 lb/h LHSV = 630 h -1 Air In Temperature / C wt% Initiator 0 wt% Initiator Model_run_summaries Fuel Out Temperature

35 Coke Deposition Rates are a Strong Function of Temperature 240 Total Coke Deposition / mg Pressure = 550 psi LHSV = 12 h -1 Run Duration = 4 h n-dodecane n-decane n-heptane Temperature / C From Coke_compare

36 Summary The initiator produces significant increases in the fuel heat sink capacities of JP-7 and model fuel compounds. The initiator reduces the activation energy for the thermal cracking reaction. We demonstraed the effectivness of the initiator in a fuel/air heat exchanger that operated at realistic heat flux. The initiator reduces the HX temperature, which could substantially reduce coke deposition.

37 Acknowledgements Funding provided by NASA SBIR Program, contract number NAS Diane Linne, Contract Monitor. Air Force Research Laboratory for providing JP-7 fuel.