O.G. Penyazkov et al- Autoignitions of Diesel Fuel/Air Mixtures Behind Reflected Shock Waves

8/3/2019 O.G. Penyazkov et al- Autoignitions of Diesel Fuel/Air Mixtures Behind Reflected Shock Waves

1/6

1* Corresponding author:[email protected] of the European Combustion Meeting 2009

Autoignitions of Diesel Fuel/Air Mixtures Behind Reflected Shock Waves

O.G. Penyazkov1*

, K.L. Sevrouk1, V. Tangirala

2, N. Joshi

2

1 Heat and Mass Transfer Institute, Minsk, Belarus2

General Electric Global Research Center, Niskayuna, NY, USA

AbstractThe ignition times and auto-ignition modes of Diesel fuel/Air mixtures behind reflected shock waves were measured

at pressures 4.7 10.4 atm, temperatures 1065 - 1838 K, and stoichiometries = 0.5 - 2. It was shown that forstudied range of post-shock conditions the reaction rate of Diesel fuel oxidation exhibit a nonlinear Arrheniusdependence with global activation energies ranged from 32.6 kcal/mole at high temperatures (> 1200 K) to 20.4

kcal/mole at low temperatures (1200 K


2/6


Table 1. The experimental conditions for No. 2 Diesel fuel /Air mixtures

Diesel - Air Equivalence

ratio, Post-shock

Pressure [atm]Post-shock

Temperature [K]Post-shock

Density [kg/m3]

Mixture 1 1 4.68 10.4 1078 -1665 1.49 2.71

Mixture 2 0.5 5.6 9.8 1117 1903 1.3 2.47

Mixture 3 2.0 5.7 10.0 916 1838 1.4 3.17

Figure 1. Schematic of the test section for auto-ignition studies in Diesel fuel/Air mixtures in a 76-mm shocktube: 1 high-frequency PCB pressure transducers; 2 ion current sensors; 3 thermocouple; 4 reflecting wallwith inserted quartz rods; 5 lens f = 40 cm; 6 beam splitter; 7 aperture diaphragms; 8 doubled

monochromatic filters; 9- photomultipliers.

were detected to identify the auto-ignition in theboundary layer. Ignition times were controlled also by

pressure and ion current measurements at the reflectingwall.

The ignition or induction time of the mixture wasdefined as the time difference between shock arrival atthe end wall and the onset of emission within measuringgas columns (Fig.2). The applied optical setup wassensitive to the onset of auto-ignition at selected gas

volumes and generated induction times of studiedmixtures from the beginning of normal reflection of the

incident shock wave. To obtain a correct temperaturedependence of ignition time on activation energy of themixture and fuel/oxygen concentrations all comparativeshock-tube series were performed at approximatelyconstant post-shock density. It means that fuel, oxygen,and nitrogen concentrations were kept nearly constant

within a studied temperature range behind reflectedshock waves.

Absolute velocities of reflected shock wave (RSW)in a frame of reference attached to gas flow moving behind incident shock wave and pressures at differentlocations identified the auto-ignition modes of themixture (strong, transient and weak) [3-6]. Theabsolute RSW velocity in the end part of the tube wasdefined as V = V5 + u, where V5 is RSW velocity

calculated by processing shock-arrival times at pressuresensors along the tube in laboratory frame of reference,

u is flow velocity behind incident shock wave.For stoichiometric Diesel fuel / Air mixture, Figure

3 illustrates the dependencies of reflected shock-wave

Figure 2. Ignition time definition criteria and their positions along the history of reflecting wall pressure

and gas emission in stoichiometric Diesel fuel / Airmixture: A OH and CH emissions along the centerline

of the shock tuber. Stoichiometry = 1. Post shocktemperature is T = 1188 K.


3/6


Velocity on post-shock temperatures at differentlocations along the tube. On the basis of pressure andemission observations the inflection point of velocitycurve at low temperatures for distances of 140 mm wasused for determining positions of the strong explosionlimit. For stoichiometric mixture, this critical

temperature was 1251 K. It should be mentioned thatexplosion behavior of Diesel fuel was observed also at

temperatures higher than 1140 K due to the mechanismof deflagration to detonation transition in a shock

compressed gas volume. As is seen in the Figure 3 thistransient ignition mode was realized within thetemperature range of 1140 1251 K.

Figure 3. Velocities of reflected shock wave at 140and 340 mm from reflecting wall vs. post-shock

temperature in stoichiometric Diesel fuel / Air mixture.

Positions of the strong and transient ignition limits areindicated on the Graphs.

The main measurement uncertainties wereassociated with several factors. The first one is a 0.5%uncertainty in incident shock-wave velocity

measurements, which produces 0.75%, 1.5%, 0.7%experimental errors in temperature (T), pressure (P), and

density () of the mixture behind reflected shock waves.The second one is an uncertainty connected with thedefinition of specific heat and enthalpy of the Diesel

fuel. Usually, this can result in a 1-1.5 % error indetermination of post-shock parameters of the gas. The

last one is an uncertainty connected with the definitionof ignition-delay time. Usually, this can result in a 2-15% error in ignition-delay time for studied range ofparameters

Results

For post-shock density of 1.94 0.29 kg/m3 instoichiometric Diesel fuel/Air mixtures, the temperaturedependence of induction times is plotted in the Figure 4.

The experiments were performed within the temperaturerange of 1078 1655 K. Experimental points for No.2Diesel fuel correspond to ignition times measured byusing co-axial emission observations along the

centerline of the shock tube at = 308.9 nm (OH,transitions A

2 X

2) and at = 431.5 nm (CH,

transitions2 2), along the tube wall in boundary

layer at = 516.5 nm (C2, transitions2

2 ), pressure

and ion current measurements at the reflecting wall. It isapparent that the data obtained by different methods andat different initial temperatures correlate well in a

studied range of post-shock temperatures. The induction

times of aviation kerosene Jet-A [6] obtained at nearlythe same post-shock conditions are drawn on the samegraph. For stoichiometric mixture, No. 2 Diesel fueldemonstrates 2.5 2.6 times longer induction periods incomparison with Jet-A. Figure 4 shows that at hightemperatures ( > 1210 K ) No. 2 Diesel exhibit almost

the same activation energy 16449 K (32.6 0.2kcal/mole) as aviation kerosene Jet-A. This value is lessthan activation energies for n-heptane (40.2 kcal/mole)

and JP-10 (44.6 kcal/mole) reported in [7, 8]. Although,generally, the ignition delay times of Diesel fuel followthe Arrhenius law, the significant decreasing of

activation energy up to 10292 K (20.4 0.2 kcal/mole)has been observed in our experiments at lowtemperatures T < 1210 K. For stoichiometric No. 2Diesel fuel /Air mixture, the comparison of currentresults with existing literature data of Spadaccini &TeVelde [1] measured in continuous flow reactor atinlet air temperatures of 650 1000 K, pressures 10

30 atm, and stoichiometries 0.3 1 exhibit significantdeviations with our observations. At the same time,these experiments [1] show a noticeable decreasing ofmean activation energy of Diesel fuel at hightemperatures 16437 K (32.6 kcal/mole), which is veryclose to our results for Jet-A and No. 2 Diesel.

In accordance with chosen criterion for strong

ignition limits experiments demonstrate that the critical post-shock temperature required for strong initiations

was equal to T = 1251 K. The transient ignition modewere detected within the temperature range of T = 1140- 1251 K and M = 2.6 - 2.8, respectively. Themeasurements of the steady-state CJ detonationvelocities in stoichiometric Diesel fuel /Air mixtures

give VCJ 1580 15 m/s. The appropriate value instoichiometric Jet-A mixture is equal to VCJ 1670 m/s.

Figure 4. Mean activation energies for

stoichiometric Diesel fuel/ and Jet-A/Air ( = 1.0)mixtures at equivalent post-shock conditions.


4/6


For the post-shock density of 1.97 0.28 kg/m3 inlean Diesel fuel/Air ( = 0.5) mixtures, the temperaturedependence of induction times is plotted in the Figures5. Experiments were performed within the temperaturerange of 1117 1903 K and pressures 5.6 9.8 atm. For

lean Jet-A/ Air mixture ( = 0.5), induction times at

similar post-shock conditions are drawn on the samegraph. Likewise in the stoichiometric mixtures, No. 2Diesel fuel demonstrates 2.3 2.9 times longer induction

periods and slightly lower activation energy 15483 K

(30.7 0.2 kcal/mole) in comparison with Jet-A. Forlean mixture, the temperature dependence of ignitiondelays follows the Arrhenius law within the studiedtemperature range of 1117 1903 K. Within the scatter

of the experimental data both stoichiometries = 0.5and = 1.0 exhibit the same activation energy. The leanmixture demonstrates approximately 1.5 times longerinduction periods. The similar trend has been observed

for Jet-A fuel in our previous studies [6].

Experiments demonstrate that the critical post-shocktemperature required for strong initiations is equal to T= 1294 K. The transient ignition modes were detectedwithin the temperature range of T = 1170 1294,respectively. For lean mixtures, measurements of the

steady-state CJ detonation velocity give VCJ 1450 20 m/s. The appropriate value for lean Jet-A mixture is

1480 m/s.

Figure 5. Ignition delay time vs. reciprocaltemperature for lean Diesel fuel/ and Jet-A/Air mixtures

( = 0.5) at equivalent post-shock conditions.

For the post-shock density of 2.2 0.4 kg/m3 of richDiesel fuel/Air ( = 2.0) mixture, the temperaturedependence of induction times is plotted in the Figure 6.Experiments were performed within the temperaturerange of 916 1838 K, and pressures 5.7 10 atm.

Induction times for rich Jet-A/ Air mixture ( = 2.0) aredrawn on the same graph. In contrast to lean andstoichiometric Diesel fuel blends, the rich Dieselmixture demonstrates much longer ignition times and

significantly lower activation energy 12330 K (24.45 0.2 kcal/mole) in comparison with Jet-A. Within thescatter of experimental data the temperature dependenceof induction period for rich mixture follow the

Figure 6. Ignition delay time vs. reciprocal

temperature for rich Diesel fuel/ and Jet-A/Air mixtures

( = 2.0) at equivalent post-shock conditions.

Arrhenius law within the studied temperature range of

916 1838 K. In comparison with Jet-A Diesel fueldemonstrates 3 - 6.4 times longer induction periods atequivalent post-shock conditions.

For rich and stoichiometric Diesel fuel /Airmixtures, Figure 7 shows the comparison of inductiontimes. As is seen form the graphs, linear approximations

of the experimental data for = 2.0 and = 1.0 exhibitsubstantially different activation energies. Fortemperatures higher than 1200 K, the rich mixturedemonstrates longer ignition times. At low temperatures

< 1200 K, induction times are approximately equal in both cases within the scatter of the experimental data.

Simultaneously, at low temperatures < 1200 K the

stoichiometric and rich Diesel fuel/Air blends exhibitvery close activation energies equal to 10292 K (20.4 0.2 kcal/mole) and 12330 K (24.45 0.2 kcal/mole)(Fig. 7), respectively. For = 2.0, in contrast to Dieselfuel the Jet-A demonstrates absolutely different behavior [6]. Auto-ignitions of rich Jet-A/Air mixtureresults in shorter induction times within the temperaturerange of 1000 1520 K with the same activation energy

as for = 0.5 and = 1.0.

Figure 7. Mean activation energies forstoichiometric and rich Diesel fuel/ Air mixtures atequivalent post-shock conditions.


5/6


For rich Diesel fuel /Air mixtures, the critical post-shock temperature required for strong auto-ignitions isequal to T = 1260 K. The transient ignition modes weredetected within the temperature range of T = 1145 1260 and M = 2.83 - 3.05, respectively. Measurements

give the steady-state CJ detonation velocity VCJ 1735

20 m/s. For rich Jet-A mixture, the correspondingvalue is 1780 m/s.

For stoichiometries = 0.5 - 1, the overallempirical approximation for ignition delays have beenderived from the experimental data (1) (Fig.8):

[ ] [ ] 54218.028653.026 15473exp100663.8)(

= DieselO

Ts

where, is the ignition time in (sec) , T is the post-shock temperature in (K), [Diesel] is the Diesel fuelconcentration in (mole/cm3), and [O2] is the Oxygen

concentration in (mole/cm3). The global activationenergy of Diesel fuel obtained from regression analysis

is 30.7 0.26 kcal/mole. Equation (1) gives theexcellent agreement with experimental data points for

stoichiometries = 0.5 1. This correlation results in 13% standard deviation from the fitted induction times.

Figure 8. Ignition delays of Diesel fuel/Air mixtures

correlated using equations (1) vs. reciprocal post-shocktemperature. Units: ( s ); [Diesel], [O2] ( mole/cm

3);

T (K).

For a wider stoichiometry range of = 0.5 - 2,overall empirical approximation for ignition delays is(2) (Fig.9):

[ ] [ ] 0404.043055.02513789

exp105563.1)(

= DieselO

Ts

with the global activation energy of Diesel fuel obtained

from regression analysis is 27.3 0.42 kcal/mole.Approximation (2) results in the satisfactory

coincidence with experimental observations for = 2.This correlation results in 27 % standard deviation fromthe fitted induction times.

Figure 9. Ignition delays of Diesel fue/Air mixtures

correlated using equations (2) vs. reciprocal post-shock

temperature. Units: ( s ); [Diesel], [O2] ( mole/cm3 );

T (K).

Conclusions

The ignition delay times and auto-ignition modes of

Diesel fuel/Air mixtures behind reflected shock waveswere measured at pressures 4.7 10.4 atm, temperatures

1065 - 1838 K, and stoichiometries = 0.5 - 2.For stoichiometries = 0.5 1 0.5 -2 the overall

empirical approximations for ignition delays have beenderived from the experimental data. It was shown thatfor studied range of post-shock conditions the reactionrate of Diesel fuel oxidation exhibit a nonlinearArrhenius dependence with global activation energiesranged from 32.6 kcal/mole at high temperatures (>

1200 K) to 20.4 kcal/mole at low temperatures (1200 K


6/6


1. Spadaccini LJ, TeVelde JA (1982) AutoignitionCharacteristics of Aircraft-Type Fuels. Combust.Flame 46: 283.

2. Haylett DR, Lappas PP, Davidson DF, Hanson RK(2009) Application of an aerosol shock tube to themeasurement of diesel ignition delay times. Proc. of

the Comb. Inst. 32 (1): 477.3. Voevodsky VV, Soloukhin RI, Proc. Combust. Inst.

10 (1965) 279-283.4. Meyer JW, Oppenheim AK, Proc. Combust. Inst. 13

(1971) 1153-1164.5. Penyazkov OG, Ragotner KA, Dean AJ,

Varatharajan B, Proc. Combust. Inst. 30 (2005)1941-1947.

6. Dean AJ, Penyazkov OG, Sevruk KL, VaratharajanB. Proc. Combust. Inst. 31 (2007) 2481-2488.

7. N.B. Colket, L.J. Spadaccini, 14th Int. Symp. onAirbreathing Engines, Florence, Italy, September 5-10, 1999.

8. Davidson DF, Horning DC, Herbon JT, Hanson RK,Proc. Combust. Inst. 28 (2000) 1687-1692.

Documents

O.G. Penyazkov et al- Autoignitions of Diesel Fuel/Air Mixtures Behind Reflected Shock Waves