03. EXTERNAL AND INTERNAL PLASMA-ASSISTED COMBUSTION

Transcription

1 03. EXTERNAL AND INTERNAL PLASMA-ASSISTED COMBUSTION A.I.Klimov, V.A.Bityurin, A.S.Kuznetsov, B.Tolkunov, N.N Sukovatkin, N.B.Vystavkin Institute for High Temperature RAS, Izhorskaya 13/19, , Moscow, Russia Abstract. Plasma-assisted combustion (PAC) in subsonic airflow and supersonic airflow was studied in our previous works [1,4-9,12-15]. Present work is devoted to continuation of plasma- assisted combustion study. Four main tasks are considered in this work, namely: o Advanced plasma-assisted fuel-airflow mixing, o External and internal plasma- assisted combustion in cold airflow and hot airflow, o o Electric discharge and plasma parameters in PAC region, Fuel combustion completeness in airflow. Experimental researches of internal PAC are carried out in quartz duct WT-1. Supersonic airflow (M<2, P st <1 Bar, T st <1000K) is created in the test section of this experimental set up. Powerful streamer HF discharge (mean power up to 10 kw) is used for airflow pre-heating and fuel- airflow radical generation. Modulated HF streamer discharge is used to accelerate fuel-airflow mixing and plasma- assisted combustion in some experiments. Optical and IR spectroscopy, chemical analysis are used to study plasma and radical generation in airflow. Gas flow parameters (T st, P st, P 0 and others) are measured before combustion region and after it by pressure sensors and thermocouples. Experimental results on power balance (calorimetric measurements) and fuel combustion completeness are considered in this work. External PAC is studied in wind tunnel WT-2. Airflow parameters are the followings: o Mach number M< 2 o Static pressure P st <1 Bar o Stagnation temperature T 0 ~300K Stable local external PAC region is created near aerodynamic model. Surface pressure distribution on the model s surface is measured in plasma-aerodynamic experiment. Model s drag is measured in this experiment also. Supersonic flow around model is studied by shadow optical method. Considerable drag decrease up to 30-40% and surface pressure decrease up to 50% were measured at local external plasma-assisted combustion generation. Part 1. Internal PAC Experimental Set up Experimental set up WT-1 was considered in [1] in detail. Modification of this set up used in present experiments is shown in Fig.1. Pulse repetitive HF discharge is created in test section of this set up. Experimental conditions were the followings: o Airflow mass flux < 50 g/s o Propane mass flux < 1 g/s o Airflow parameters M< 1,2, P st < 1 Bar o Mean PG (HF) power N d < 10 kw o HF frequency F= 0,5-13,6 MHz o Operation regime continuous and pulse repetitive o Pulse repetitive frequency F i < 10 khz o Pulse duration T i > 100 mcs Electrical parameters (U d, I d ) are measured in HF discharge. In the result the electrical power input N d in plasma is estimated in these experiments also. Gas temperature behind combustion region is measured by thermocouples, optical pyrometer and optical spectroscopy [1]. Airflow parameters are measured by flow rate sensors and pressure sensors. As a result the power balance measurements (calorimetric measurements) are obtained in PAC experiment. Final PAC s species (gas composition) are studied by IR absorption spectroscopy. Gas samples are evacuated behind combustion region by chemical probes, Fig.1. This probe is consisted of the small container (~100 cm 3 ) and thin quartz tube connected with this container. Namely these tubes are placed behind PAC region (see Fig.1). These containers are previously vacuumed by the pump. Then the gas sample is tested by IR spectrometer Specord M80. Carl Zeiss Jena. Wave range of this IR spectrometer is about cm -1. Note that the optical lines of the PAC s species (CO, CO 2, H 2 O, NO x and others) are located in this IR wave range namely. Minimal wave range resolution is about 0,4 cm -1 in this spectrometer. The well-knowing Optical Tables [2,3] are used in IR spectrum processing. 33

2 Fig.1. Scheme of experimental set up WT-1: 1- vortex chamber, 2- nozzle, 3-quartz duct, 4-Tesla s coil, 6- MW generator, 7- PMA, 8- pyrometer, 9- pressure sensor, 10- thermocouple and chemical probe, 11- mass gas flow sensor. Experimental results. Internal PAC Pulsed repetitive HF streamer discharge and combined discharge (DC+ pulsed repetitive high voltage discharge) are used in internal PAC experiment. These pulsed discharges help us to minimize electric power input in plasma namely. Note that subsonic airflow (M<0,4) is used in power balance experiment namely. The accurate measurements of plasma and gas flow parameters by different probes (chemical probes, thermocouples and others) are possible in subsonic airflow only (no bow shock wave generation before these probes, no combustion species freezing inside chemical probes and so on). The following main results were obtained in PAC experiment: 1. Stable HF discharge is created in airflow (M<0.4; P st ~1Bar). Volt- Ampere characteristics (VACh) of this discharge are measured by resistive divider, current shunt - and oscilloscope Tetronix TDS3012. Note that measured mean power input in plasma is about N d = W in our experiments. This value is much smaller than chemical fuel power N ch = m p = 42kW/g 0,4g/s = 16.8kW. The minimal value N d depends on Mach number and airflow mass rate. Remember that maximal gas temperature T R inside HF streamer is about K [1]. This temperature was measured by optical spectroscopy in HF plasma (2 nd positive band of excited N 2 ) [1,4]. Gas temperature of airflow behind discharge (or combustion) zone is measured indirectly by thermocouples and optical pyrometer arranged 84 cm from electrodes. 2. Stable propane combustion without plasma assistance is obtained in airflow at velocity V af <2m/s only (quartz tube was pre-heated by electric discharge previously). Gas temperature behind combustion region measured by thermocouple is about T g1 = C in this regime. Gas temperature behind combustion region measured by optical pyrometer is about Fig.2. Plasma assisted combustion. HF streamer modulated discharge in airflow. Mass airflow rate 13 G/s; propane mass flow rate 1G/s; F i = 100 Hz; T i =5 ms. 34

3 T g2 = C in this regime. Note that T g1 ~T g2. So, gas temperature is measured behind PAC region correctly. 3. Stable PAC was created in subsonic airflow (M 0.4, P st ~1 Bar) by pulsed repetitive HF discharge, Fig.2. It is needed to note that airflow velocity range corresponding to stable PAC (V af <140m/s) is much higher than the one corresponded to propane combustion without plasma assistance (V af 2m/s). Measured gas temperature in PAC region is about T g ~ C. This gas temperature is depended on airflow velocity. 4. Small characteristic propane ignition time (T i ~ mcs) and its combustion time (T c ~ mcs) were measured in cold supersonic airflow (M<1,2; P st < 1Bar; T 0 ~300K ) at plasma assistance [1,4-9,13-15]. 5. Selective radical generation (CH, C 2, O, O 3, CN, OH and others) was obtained in combined electric discharge [1,4-9,13-15]. 6. Bad mixing of fuel and plasma formation was revealed in PAC experiment. This result is connected with two plasmoid s properties in supersonic airflow: Electric double layer creation on plasmoid s surface. Charged fuel molecules are deflected and reflected by this electric double layer from the plasmoid surface. Shock wave generation before hot plasmoid in supersonic airflow. This bow shock wave curves the streamlines and deflects fuel jet. 7. It is need to generate electric discharge with optimal parameters (E/N, N d, T g.) in airflow to create stable PAC. It was revealed that stable PAC in airflow was realized at high electric current in discharge region only (see experimental results, part 1.2). Gas temperature is T g ~1500K and higher in discharge region. Discharge parameter E/N is about Td and higher. Typical value of electric power input in plasma is about several percentages of total chemical fuel power (and higher). 8. Fuel excitation and dissociation without combustion were obtained in some PAC experiments. For example, PAC is not realized at small electric power input in plasma (small electric current in discharge). Stable external PAC is realized in supersonic airflow in the experimental set up WT-2 at I d > 1 Amp only (see experimental results). 9. It was revealed that there is non-homogeneous and non-stationary PAC in airflow as a rule. Different instability is generated in PAC zone in airflow. Charged excited carbon clusters stimulate these instabilities generation. The regular self-organized structures (cells, vortexes, tubes and others) are created inside PAC zone in some experiment. 10. Longitudinal plasma formations in high speed airflow (M>1). It is well- known that there are characteristic ignition times and combustion times in the PAC in airflow. These values are depended on plasma and gas parameters. The typical value of ignition time is about ~ mcs and higher in our experiments. So, the typical gas dynamic residence time of fuel in plasma has to be the same one or higher. So, typical discharge plasma length has to be 50mm and higher in PAC experiment. Note that the creation of longitudinal plasma formation in cold supersonic airflow is difficult technical task due to fast plasma cooling by airflow and discharge plasma instabilities. 11. Fuel completeness is main problem in PAC experiment. This problem is not good studied now. Power balance and chemical analysis have to be studied in future PAC experiment. Another side of this problem is toxic impurity generation in PAC region. It was revealed that high concentration of CN is created in our experiment namely. So, it is possible HCN creation in PAC region. So, ecology problems are arising with PAC realization in airflow. 12. Preliminary results on propane combustion completeness were obtained in airflow (M af < 0,4) by power balance method (calorimetric method). Where = N AIR / P CHEM, N AIR = C p M AIR T g, P CHEM =M p, M p propane mass flow rate, - specific propane combustion heat, M AIR mass airflow rate. These preliminary experimental results are shown in Tabl.1. It was revealed that is about 30% at plasma off. This value is increased up to 90% at plasma on and lean propane-airflow mix. So, propane combustion completeness is increased at plasma-assistance considerably. 13. IR spectroscopy. Preliminary results. IR absorption spectroscopy was used to study the final PAC species composition. Preliminary experimental results were obtained and analyzed. It was revealed that plasma-chemical kinetics of lean propane-air mixture in airflow at HF plasma assistance is non-standard one. It is differed from the standard combustion kinetics without plasma absolutely. Conclusions to Part 1 1. Stable internal PAC was created and studied in subsonic airflow (M < 0.4; P st 1 Bar). It is needed to note that airflow velocity range (M 0,4) corresponding to stable PAC is much 35

4 higher than the one at propane combustion without plasma assistance (V~ 1-2 m/s). 2. Propane combustion completeness in airflow (M af < 0,4) is studied in our experiment. It was revealed that its value is about 30% at plasma off. This value is increased up to 90% at lean propane-airflow mix and HF plasma assistance. So, propane combustion completeness at plasma-assistance is higher than the one at plasma off. 3. IR spectroscopy is used in PAC experiment. Preliminary experimental results were obtained and analysed. It was revealed that plasmachemical kinetics of lean propane-air mix combustion in airflow at HF plasma assistance was absolutely different one comparing with the standard combustion kinetics without plasma. Part 2. External local PAC near model F in supersonic airflow Study of external PAC near model F in supersonic airflow (M~2) with external electrode was carried out in our previous work [1]. Present work is continuation of the previous one. Main goal of this work is measurement of the model s drag force in supersonic airflow at local external PAC generation. Scheme of the experimental set up is shown in Fig.3,4 [1]. External needle electrode is located before model F. Distance between model s electrode and external one is about 10 mm. Fig.3. Scheme of the modified experimental set up WT-2 Fig.4. Scheme of experimental set up for external PAC experiment. 1- nozzle; 2- external hot needle electrode; 3- aerodynamic model; 4- fuel injection tube-grounder electrode; 5- surface pressure measurment tubes. 36

5 Experimental conditions: o Airflow Mach number M<2 o Static pressure P st < 100 torr o Diameter cylindrical mm model o Propane mass flow < 0,4 G/s o DC current <1 Amp o Power supply voltage <5 kv Force sensor Honeywell FGN-15 is used for model s drag force measurement. Stable external PAC was obtained in this experiment. Measured model s drag is shown in Fig.6. One can see that drag decrease is about 20% at DC discharge and about 40% at external PAC. Experimental results. External PAC Typical picture of the external PAC near model F is shown in Fig.5. Fig.6. Time evolution of the model s drag at local external PAC generation. Model s diameter 18 mm. F=70 G/div; T=0,4 s/div, red line- initial drag s level. Fig.5. External PAC near model F 1 (diameter 12 mm) in supersonic airflow (M~2, P st ~100 Torr), down. Propane mass flow rate about 0.4/s. One can see electromagnetic noise in drag s signal (short pulses). These pulses are generated by non-stationary electrical discharge in supersonic airflow. So, real electric discharge is not DC discharge but pulse repetitive discharge. Mean power input in electric discharge is about 1 kw. It was revealed that it is needed to use high current Fig.7. Optical spectra obtained near model F. Fibber location L=5 mm from its head part 37

6 electric discharge I DC >0,5 Amp to realize stable external PAC. Reduced electric field measured in electric discharge in airflow is about E/N ~ 30 Td. This value is obtained in DC discharge without accounting of the gas heating. This gas heating increases reduced electric field value by factor 6 (measured gas temperature is about 2000K). This temperature was measured by optic spectroscopy and optical pyrometer. Typical optical spectra obtained in this experiment are shown in Fig.7. One can see typical exited nitrogen (2 nd positive system) and CN optical lines in optical spectra. So, there are optimal conditions for radical generation and molecular dissociation in this discharge. Note that combustion completeness in this experiment is not high. Remember that chemical power of the propane combustion is about N = M p ~ 17 kw (where M p - propane mass flow rate, ~42MW/kGspecific heat of propane combustion). So, chemical power is higher than the electric power (~ 1kW) and mechanic power of airflow (~ 4 kw). However the measured drag decrease is about 40% only. Conclusions to Part 2 1. External PAC near model F in supersonic airflow ( < 2, st <100 Torr) was studied in wind tunnel WT-2. Electric combined discharge (DC + pulsed repetitive discharge) was used in this experiment. 2. Stable external PAC is created near model F in supersonic airflow at high electric current I DC > 0,5 only. Acknowledgments Financial support of Russian Academy of Science, the European Office of Research and Development (EOARD) under Contract Work 2127P and the John Hopkins University, APL under Contract Work and are gratefully acknowledged. Authors thank Dr. Kolesnichenko for the help of the optic spectrum processing and discussion of experimental results and Dr. Nikitin and his colleagues for the help in our work. References 1. Klimov A., Bityurin V., Brovkin V., Vystavkin N., Kuznetsov A., Van Wie D., Plasma Assisted Combustion, 4 th Workshop on MPA, 9-11 April 2002, Moscow, IVTAN, P Shimanouchi T., Tables of Molecular Vibrational Frequencies, Consolidated Volume I, NBS, 1972, NIST/EPA Gas-Phase Infrared Database 4. Klimov A., Bityurin V., Brovkin V., Vinogradov V., Van Wie D., Plasma Assisted Combustion, Paper AIAA , 39 th AIAA Aerospace Sciences Meeting & Exhibit, 8-11 January 2001/ Reno, NV, P.9, 5. Bityurin V.A., Klimov A.I., Lebedev P.D. et.al. Study of Ignition and Burning of Fuel-Air Mixture in Supersonic Flow Stimulated by HF Streamer Discharge. Report IVTAN, 1998, P Klimov A., Brovkin V., Bityurin V., et. al., Plasma Assisted Combustion, 3 rd Workshop on MPA, April 2001/ Moscow, IVTAN, P Klimov A., Brovkin V., Bityurin V., et. al., Plasma Assisted Combustion, Paper AIAA , 40 th AIAA Aerospace Meeting& Exhibit, January 2002/ Reno, NV, P Klimov A., Bityurin V., Brovkin V., Vystavkin N., Kuznetsov A., Van Wie D., Optimization of Plasma Generators for Plasma Assisted Combustion, Paper AIAA , 32 nd AIAA Plasma dynamics and Lasers Conference and 4 th WIG Workshop, June 2001/ Anaheim, CA, P Klimov A., Bityurin V., Brovkin V., Leonov S., Plasma Generators for Combustion, Workshop on Thermo-chemical Processes in Plasma Aerodynamics, Saint Petersburg, May 30- June 3, 2000, P Morris R.A., Arnold S.T., Viggano A.A., Maurice L.Q., Carter C., Sutton E.A. Investigation of the Effects of Ionization on Hydrocarbon-Air Combustion Chemistry. 2 nd Weakly Ionized Gases Workshop, Norfolk, Vinogradov V., Goltsev V., et.al., Influence of Active Radical Concentration on Self-Ignition Delay of Hydrocarbon Fuel- Air Mixture, Appl.Phys., 2000, P Ball Lightning in Laboratory, Editors: Avramentko R., Bychkov V., Klimov A., Sinkevich O., Moscow, Chimiya publishes, (in Russian), 1994, P Klimov A., Beaulieu W., Bityurin V., et.al. Jet Noise Reduction by Plasma Formations, Proc. of the 2 nd Workshop on Magneto- Plasma- Aerodynamics in aerospace applications, Moscow, 5-7 April, 2000, IVTAN, P Klimov A., Bityurin V., Brovkin V., Vystavkin N., Kuznetsov A., Van Wie D., Optimization of Assisted Combustion, Paper AIAA , 33 rd AIAA Plasma dynamics and Lasers Conference, May 2002/ Maui, Hawaii P Klimov A., Biturin V., Vystavkin N., Vasiliev M., Supersonic Airflow Around Model E with Plasmoid Created by Combined Discharge, Proc.5 th Workshop on Magneto-Plasma Aerodynamics for Aerospace Applications, Moscow, 7-10 April 2003, IVT RAN. 38