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

Autoignitions of Diesel Fuel/Air Mixtures Behind Reflected Shock Waves O.G. Penyazkov1*, K.L. Sevrouk1, V. Tangirala2, N. Joshi2 1

Heat and Mass Transfer Institute, Minsk, Belarus General Electric Global Research Center, Niskayuna, NY, USA

2

Abstract The 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 φ = 0.5 - 2. It was shown that for studied range of post-shock conditions the reaction rate of Diesel fuel oxidation exhibit a nonlinear Arrhenius dependence 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 1210 K ) No. 2 Diesel exhibit almost the same activation energy 16449 K (32.6 ± 0.2 kcal/mole) as aviation kerosene Jet-A. This value is less than 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 follow the 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 low temperatures T < 1210 K. For stoichiometric No. 2 Diesel fuel /Air mixture, the comparison of current results with existing literature data of Spadaccini & TeVelde [1] measured in continuous flow reactor at inlet air temperatures of 650 – 1000 K, pressures 10 – 30 atm, and stoichiometries 0.3 – 1 exhibit significant deviations with our observations. At the same time, these experiments [1] show a noticeable decreasing of mean activation energy of Diesel fuel at high temperatures 16437 K (32.6 kcal/mole), which is very close 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 mode were detected within the temperature range of T = 1140 - 1251 K and M = 2.6 - 2.8, respectively. The measurements of the steady-state CJ detonation velocities in stoichiometric Diesel fuel /Air mixtures give VCJ ≈ 1580 ± 15 m/s. The appropriate value in stoichiometric Jet-A mixture is equal to VCJ ≈ 1670 m/s.

Figure 4. Mean activation energies for stoichiometric Diesel fuel/ and Jet-A/Air (φ = 1.0) mixtures at equivalent post-shock conditions.

 

For the post-shock density of 1.97 ± 0.28 kg/m3 in lean Diesel fuel/Air (φ = 0.5) mixtures, the temperature dependence of induction times is plotted in the Figures 5. Experiments were performed within the temperature range of 1117 – 1903 K and pressures 5.6 – 9.8 atm. For lean Jet-A/ Air mixture (φ = 0.5), induction times at similar post-shock conditions are drawn on the same graph. Likewise in the stoichiometric mixtures, No. 2 Diesel 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. For lean mixture, the temperature dependence of ignition delays follows the Arrhenius law within the studied temperature range of 1117 – 1903 K. Within the scatter of the experimental data both stoichiometries φ = 0.5 and φ = 1.0 exhibit the same activation energy. The lean mixture demonstrates approximately ≈ 1.5 times longer induction periods. The similar trend has been observed for Jet-A fuel in our previous studies [6]. Experiments demonstrate that the critical post-shock temperature required for strong initiations is equal to T = 1294 K. The transient ignition modes were detected within 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 6. Ignition delay time vs. reciprocal temperature for rich Diesel fuel/ and Jet-A/Air mixtures (φ = 2.0) at equivalent post-shock conditions. Arrhenius law within the studied temperature range of 916 – 1838 K. In comparison with Jet-A Diesel fuel demonstrates ≈ 3 - 6.4 times longer induction periods at equivalent post-shock conditions. For rich and stoichiometric Diesel fuel /Air mixtures, Figure 7 shows the comparison of induction times. As is seen form the graphs, linear approximations of the experimental data for φ = 2.0 and φ = 1.0 exhibit substantially different activation energies. For temperatures higher than 1200 K, the rich mixture demonstrates 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 exhibit very 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 φ = 2.0, in contrast to Diesel fuel the Jet-A demonstrates absolutely different behavior [6]. Auto-ignitions of rich Jet-A/Air mixture results in shorter induction times within the temperature range of 1000 – 1520 K with the same activation energy as for φ = 0.5 and φ = 1.0.

Figure 5. Ignition delay time vs. reciprocal temperature for lean Diesel fuel/ and Jet-A/Air mixtures (φ = 0.5) at equivalent post-shock conditions. For the post-shock density of 2.2 ± 0.4 kg/m3 of rich Diesel fuel/Air (φ = 2.0) mixture, the temperature dependence of induction times is plotted in the Figure 6. Experiments were performed within the temperature range of 916 – 1838 K, and pressures 5.7 – 10 atm. Induction times for rich Jet-A/ Air mixture (φ = 2.0) are drawn on the same graph. In contrast to lean and stoichiometric Diesel fuel blends, the rich Diesel mixture demonstrates much longer ignition times and significantly lower activation energy 12330 K (24.45 ± 0.2 kcal/mole) in comparison with Jet-A. Within the scatter of experimental data the temperature dependence of induction period for rich mixture follow the 1* Corresponding author: [email protected]  Proceedings of the European Combustion Meeting 2009   

Figure 7. Mean activation energies for stoichiometric and rich Diesel fuel/ Air mixtures at equivalent post-shock conditions.

 

For rich Diesel fuel /Air mixtures, the critical postshock temperature required for strong auto-ignitions is equal to T = 1260 K. The transient ignition modes were detected 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 corresponding value is ≈ 1780 m/s. For stoichiometries φ = 0.5 - 1, the overall empirical approximation for ignition delays have been derived from the experimental data (1) (Fig.8):

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

⎛ 15473 ⎞ 0.28653 −0.54218 × [Diesel ] ⎟ × [O2 ] ⎝ T ⎠

τ ( μs ) = 8.0663 × 10 −6 × exp⎜

where, τ is the ignition time in (μsec) , T is the postshock temperature in (K), [Diesel] is the Diesel fuel concentration in (mole/cm3), and [O2] is the Oxygen concentration in (mole/cm3). The global activation energy of Diesel fuel obtained from regression analysis is 30.7 ± 0.26 kcal/mole. Equation (1) gives the excellent agreement with experimental data points for stoichiometries φ = 0.5 –1. This correlation results in 13 % standard deviation from the fitted induction times.

Figure 9. Ignition delays of Diesel fue/Air mixtures correlated using equations (2) vs. reciprocal post-shock temperature. Units: τ ( μs ); [Diesel], [O2] ( mole/cm3 ); T (K). Conclusions

Figure 8. Ignition delays of Diesel fuel/Air mixtures correlated using equations (1) vs. reciprocal post-shock temperature. Units: τ ( μs ); [Diesel], [O2] ( mole/cm3 ); T (K). For a wider stoichiometry range of φ = 0.5 - 2, overall empirical approximation for ignition delays is (2) (Fig.9):

[

⎛ 13789 ⎞ −0.43055 × Diesel ⎟ × [O2 ] ⎝ T ⎠

τ ( μs ) = 1.5563 × 10 −5 × exp⎜

]

−0.0404

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

The ignition delay 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 φ = 0.5 - 2. For stoichiometries φ = 0.5 – 1 0.5 -2 the overall empirical approximations for ignition delays have been derived from the experimental data. It was shown that for studied range of post-shock conditions the reaction rate of Diesel fuel oxidation exhibit a nonlinear Arrhenius dependence 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