Ignition delay studies on hydrocarbon fuel with and without additives

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Summary. Single pulse shock tube facility has been developed in the High Temperature Chem- ical Kinetics Lab, Aerospace Engineering Department, to carry ...
Ignition delay studies on hydrocarbon fuel with and without additives M. Nagaboopathy1 , Gopalkrishna Hegde1 , K.P.J. Reddy1 , C. Vijayanand2 , Mukesh Agarwal2 , D.S.S. Hembram2 , D. Bilehal2 , and E. Arunan2 1

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Department of Aerospace Engineering, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India 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 C2 H6 − O2 mixture. The results are in good agreement with earlier published works. Ignition times of exo-tetrahdyrodicyclopentadiene (C10 H16 ), 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 C10 H16 .

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 C10 H16 . 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 C10 H16 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 C10 H16 , but low vapour pressure of C10 H16 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 C10 H16 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 C2 H6 − O2 mixture, ignition studies then continued with C10 H16 mixtures.

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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 (P1 ) 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 C2 H6 − O2 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

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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 C10 H16 − O2 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 C10 H16 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 C10 H16 are carried out to explore the ignition behaviour. In this study C10 H16 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 C10 H16 with and without adding TEA.Typical pressure rise and CH emission due to ignition of C10 H16 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 C10 H16 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 C2 H6 − O2 mixture at various temperatures showed ignition delay between 92 µs - 1.44 ms. Experiments carried out on C10 H16 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 C10 H16 reduced with addition

Fig. 2. Pressure rise and CH emission signal due to ignition of C10 H16

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Fig. 3. Pressure rise and CH emission signal due to ignition of C10 H16 + TEA

of TEA as compared to the ignition delay of pure C10 H16 . 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 C10 H16 with and without TEA addition Ignition delay of C10 H16

Ignition delay of C10 H16 with TEA

T5 (K) P5 (atm) Delay (µs) T5 (K) P5 (atm) Delay (µs) 1433 1391 1385 1583 1516 1517 1438 1439 1465 1471 1540 1575 1675 1438 1439 1661 1516 1516 1458 1496

16.43 15.30 15.56 18.84 18.86 18.80 15.44 16.93 16.32 16.56 17.61 20.31 19.02 15.44 16.93 20.40 18.22 17.83 16.49 17.76

900 780 800 160 310 240 280 520 190 120 50 70 110 280 520 70 280 300 420 550

1375 1384 1403 1422 1460 1474 1476 1486 1491 1506 1512 1546 1563 1574 1575 1581 1593 1627 1690 1721

14.55 14.71 15.22 15.36 15.99 16.66 17.06 17.48 16.54 16.79 18.60 18.46 16.90 17.99 18.91 16.97 18.31 17.97 19.27 19.54

530 690 130 390 560 450 400 330 250 350 340 80 310 110 110 220 150 70 110 70

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Fig. 4. Arrhenius plot of results from ignition delay experiments on C10 H16

Fig. 5. Arrhenius plot of results from ignition delay experiments on C10 H16 + TEA Table 2. Arrhenius parameters for the ignition delay data on C10 H16 with and without TEA Reference [7] [8] [9] C10 H16 (this work) C10 H16 + TEA (this work)

T5 (K) 1350 1150 1149 1267 1335

– – – – –

1550 1500 1688 1686 1721

Pressure (atm) Log A Ea (Kcal/mole) 1.2 3–8 1.7 – 9.3 13.5 – 20.4 13.9 – 19.5

-4.00 -2.15

53.7 43.1 34.8 43.2 (± 4.1) 30.7 (± 4.3)

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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 C10 H16 . With the addition of TEA, an appreciable reduction in the ignition delay times of C10 H16 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 C10 H16 . 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. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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