Ignition delay studies on hydrocarbon fuel with and without additives M. Nagaboopathy 1, Gopalkrishna Hegde 1, K.P.J. Reddy 1, C. Vijayanand 2, Mukesh Agarwal 2, D.S.S. Hembram 2, D. Bilehal 2, and E. Arunan 2 1 Department of Aerospace Engineering, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India 2 Inorganic and Physical Chemistry Department, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India Summary. Single pulse shock tube facility has been developed in the High Temperature Chemical Kinetics Lab, Aerospace Engineering Department, to carry out ignition delay studies and spectroscopic investigations of hydrocarbon fuels. Our main emphasis is on measuring ignition delay through pressure rise and by monitoring CH emission for various jet fuels and finding suitable additives for reducing the delay. Initially the shock tube was tested and calibrated by measuring the ignition delay of C 2H 6 O 2 mixture. The results are in good agreement with earlier published works. Ignition times of exo-tetrahdyrodicyclopentadiene (C 10H 16), which is a leading candidate fuel for scramjet propulsion has been studied in the reflected shock region in the temperature range 1250-1750 K with and without adding Triethylamine (TEA). Addition of TEA results in substantial reduction of ignition delay of C 10H 16. 1 Introduction Our laboratory has been doing thermal decomposition studies using single pulse shock tube for molecules of interest to atmospheric chemistry [1, 2]. Recently the shock tube has been modified to study ignition delay of various hydrocarbon fuels and to find out suitable additives for enhancing their ignition. There are several reports available on various hydrocarbon fuel ignition delay studies. Generally ignition delay of hydrocarbons increases with increasing number of carbon atom in the structure. In alkanes ethane has the lowest ignition delay and methane has the longest delay, branched hydrocarbons have longer ignition delay time than their linear chain counterparts [3, 4, 5]. Ignition delay of ringed hydrocarbons does not follow any order as found in chained hydrocarbons, but it mainly depends on the structure of the molecule [6]. Exo-tetrahydrodicyclopentadiene is a large ringed structure hydrocarbon molecule, produced by hydrogenation of dicyclopentadiene and having molecular formula C 10 H 16. It has high volumetric energy density and relative stability than other cyclic compounds and this is proposed as a leading candidate fuel for supersonic combustion ramjets (scramjet) propulsion. Knowledge about the ignition behaviour of C 10 H 16 at a given condition will help to predict the design of combustion system for aerodynamic vehicle. Shock tube is an effective tool for understanding the combustion process of C 10 H 16, but low vapour pressure of C 10 H 16 at room temperature makes it difficult for shock tube study in gaseous state. Some investigations on ignition delay times have already been done using shock tube [7, 8, 9, 10, 11]. The present study addresses the ignition delay measurements of C 10 H 16 with and without adding Triethylamine (TEA), through pressure rise in the reflected shock condition and recording CH radical emission due to the ignition. Initially the shock tube was calibrated with C 2 H 6 O 2 mixture, ignition studies then continued with C 10 H 16 mixtures.
2 Nagaboopathy et. al. 2 Experimental Method 2.1 Experimental setup Experiments were performed in high purity helium driven stainless steel shock tube CST2, which is of 39mm diameter, having driver section of 1.97 m length and driven section of 4.2 m, separated by an aluminum diaphragm. A ball valve is introduced at 1.5 m from the end flange of the driven section for uniform heating of the test sample. Two optical view ports have been provided near to the end flange of driven section to study real time absorption and emission spectra during combustion. One of the view ports is connected with a vacuum monochromator coupled with photomultiplier by an optical fiber bundle to carry out emission spectroscopic studies. The schematic diagram of the CST2 assembly is given in the Fig. 1. For every experiment, the shock tube was pumped down to a pressure less than 10 4 torr. Initial pressure (P 1 ) of the sample section was measured using an IRA pressure transducer, and shock velocities were measured through three PCB pressure transducers (PT), which are connected over the last 1.5 m of the driven section. Reflected shock wave parameters are calculated using standard normalshock relations. Fig. 1. Schematic sketch of CST2 2.2 Ignition studies Initial study of ignition delay was carried out with C 2 H 6 O 2 mixture diluted in argon and these results have been used to calibrate the shock tube. Experiments were performed with ethane mixture of equivalent ratio (φ) 1. The measured ignition delay times from S-curve pressure rise were in good agreement with the published results. The efforts of studying ignition delay by optical diagnostics method have been developed successfully and tested with CH radical emission with the monochromator - PMT centred at 431.5
Ignition delay studies on hydrocarbon fuel with and without additives 3 nm. Here, the ignition delay time is defined as the time between the arrival of reflected shock and the onset of CH emission. Investigation of ignition times of C 10 H 16 O 2 without adding TEA are done in the reflected shock region in the temperature range of 1250-1750 K. A mixture of 0.2% concentration of C 10 H 16 added with oxygen in the equivalent ratio 1 and diluted in argon has been taken for the study. The test sample has been mixed uniformly in a separate stainless steel chamber for a period of one hour using a circulation pump. Further investigations on addition of TEA with C 10 H 16 are carried out to explore the ignition behaviour. In this study C 10 H 16 and TEA have been taken in the proportion of 0.9% and 0.1%, mixed with oxygen in the equivalent ratio of 1 and diluted with argon. The mixture is circulated for one hour to yield proper mixing of the compounds. Experiments are performed with the mixture under the same conditions as those done without addition of TEA and a substantial reduction in the ignition delay time is observed. 3 Results & Discussion 3.1 Ignition Time Data Ignition times are obtained in several experiments (some of the results are shown in the Table-1) for C 10 H 16 with and without adding TEA.Typical pressure rise and CH emission due to ignition of C 10 H 16 at 1397 K is shown in the Fig. 2. The ignition delay time reffered here is the measure of the time delay between the pressure rise due to the arrival of the reflected shock and that due to the onset of ignition. In this case ignition delay was observed as 340 µs. Fig. 3 shows the pressure and CH emission signal for C 10 H 16 with TEA mixture at 1281 K. In all the cases the ignition time depends on reflected shock temperature - pressure, equivalent ratio of the mixture and the concentration of the sample loaded. It is observed that an increase in the value of any of these parameters leads to a decrease in the ignition delay time. The calibration efforts of shock tube with C 2 H 6 O 2 mixture at various temperatures showed ignition delay between 92 µs - 1.44 ms. Experiments carried out on C 10 H 16 without and with addition of TEA results in ignition delay of 50-900 µs and 70-690 µs respectively. The log τ vs 1/T plot for these experiments with and without addition of TEA are shown in Fig. 4 and 5. It is clearly evident from the two plots that the ignition delay time of C 10 H 16 reduced with addition Fig. 2. Pressure rise and CH emission signal due to ignition of C 10H 16
4 Nagaboopathy et. al. Fig. 3. Pressure rise and CH emission signal due to ignition of C 10H 16 + TEA of TEA as compared to the ignition delay of pure C 10 H 16. These plots have been used to determine the Arrhenius parameters of the reaction. Table-2 has a comparison of our ignition delay measurements with earlier reported results. It shows that the activation energy is significantly reduced in the presence of TEA. Table 1. Some experimental results on ignition delay of C 10H 16 with and without TEA addition Ignition delay of C 10H 16 Ignition delay of C 10H 16 with TEA T 5 (K) P 5 (atm) Delay (µs) T 5 (K) P 5 (atm) Delay (µs) 1433 16.43 900 1375 14.55 530 1391 15.30 780 1384 14.71 690 1385 15.56 800 1403 15.22 130 1583 18.84 160 1422 15.36 390 1516 18.86 310 1460 15.99 560 1517 18.80 240 1474 16.66 450 1438 15.44 280 1476 17.06 400 1439 16.93 520 1486 17.48 330 1465 16.32 190 1491 16.54 250 1471 16.56 120 1506 16.79 350 1540 17.61 50 1512 18.60 340 1575 20.31 70 1546 18.46 80 1675 19.02 110 1563 16.90 310 1438 15.44 280 1574 17.99 110 1439 16.93 520 1575 18.91 110 1661 20.40 70 1581 16.97 220 1516 18.22 280 1593 18.31 150 1516 17.83 300 1627 17.97 70 1458 16.49 420 1690 19.27 110 1496 17.76 550 1721 19.54 70
Ignition delay studies on hydrocarbon fuel with and without additives 5 Fig. 4. Arrhenius plot of results from ignition delay experiments on C 10H 16 Fig. 5. Arrhenius plot of results from ignition delay experiments on C 10H 16 + TEA Table 2. Arrhenius parameters for the ignition delay data on C 10H 16 with and without TEA Reference T5 (K) Pressure (atm) Log A E a (Kcal/mole) [7] 1350 1550 1.2-53.7 [8] 1150 1500 3 8-43.1 [9] 1149 1688 1.7 9.3-34.8 C 10H 16 (this work) 1267 1686 13.5 20.4-4.00 43.2 (± 4.1) C 10H 16 + TEA (this work) 1335 1721 13.9 19.5-2.15 30.7 (± 4.3)
6 Nagaboopathy et. al. 4 Conclusion The chemical shock tube facility developed at the high temperature chemical kinetics laboratory has been modified for ignition delay measurements. A database of ignition delay measurement through S curve pressure rise and CH emission are compiled for a large ringed structure hydrocarbon fuel C 10 H 16. With the addition of TEA, an appreciable reduction in the ignition delay times of C 10 H 16 is noticed. TEA addition also reduces the activation energy of the fuel by more than 20%. More experiments are to be carried out in the near future with different additives. Fourier Transform Infrared spectroscopic investigations and Gas Chromatograph analysis of the initial and final products are to be done for the mixtures. This will help in understanding the kinetics and the combustion mechanism of C 10 H 16. Acknowledgement. The chemical shock tube facility has been established at the High Enthalpy Aerodynamics Laboratory by the active collaboration between IPC and AE departments. This effort has been supported by funds from ISRO, DRDL, DST-FIST and IISc. References 1. B. Rajkumar, K.P.J. Reddy, E. Arunan: J. Phys. Chem A 106, 8366 (2002) 2. B. Rajkumar, K.P.J.Reddy, E. Arunan: J. Phys. Chem A 107, 9782 (2003) 3. N. Lamoureux, C.-E. Paillard, V. Vaslier: Shock Waves 11, 309 (2002) 4. J.M. Simmie: Prog. Energy and Comb. Science 29, 599 (2003) 5. A. Toland, J.M. Simmie: Combustion and Flame 132, 556 (2003) 6. V.G. Slutsky, O.D. Kazakov, E.S. Severin, E.V. Bespalov, S.A. Tsyganov: Combustion and Flame 94, 108 (1993) 7. D.F. Davidson, D.C. Horning, R.K. Hanson: AIAA 99, 2216 (1999) 8. M.B. Colket, L.J. Spaddaccini: J.Propulsion and Power 17, 315 (2001) 9. E. Olchansky, A. Burcat: ISSW24 5962 10. D.W. Mikolaitis, C. Segalm, A. Chandy: J.Propulsion and Power 19, (2003) 11. S.C. Li, B. Varatharajan, F.A. Williams: AIAA 39, (2001)