Docstoc

Autoignitions of Diesel FuelAir Mixtures Behind Reflected Shock Waves

Document Sample
Autoignitions of Diesel FuelAir Mixtures Behind Reflected Shock Waves Powered By Docstoc
					    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
                             2
                              General Electric Global Research Center, Niskayuna, NY, USA

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 <). For stoichiometries φ = 0.5 – 1; 0.5 - 2, overall empirical approximations
for ignition delays have been derived from the experimental data. Critical post-shock temperatures required for
strong and transient auto-ignitions of Diesel fuel were established from measurements.

Introduction
    Most aviation fuels are mixtures of a large number        studies in Diesel fuel/Air mixtures. Pressure variations
of hydrocarbons and the chemical kinetic processes            at different cross-sections of the tube were measured by
occurring in combustion of such fuels at high                 three high-frequency PCB pressure sensors Model
temperatures have not been sufficiently validated. There      113A24 with a rise time less than 1.5 μs and with a 1.5-
is scarcity of data on auto-ignition of heavy fuels at high   mm spatial resolution. The end wall PCB pressure
temperatures and pressure. Only limited data have been        transducer measured the reflection time and pressure
reported for ignition of Diesel fuels in jet-stirred          history behind reflected shock waves in the vicinity of
reactors [1] and in shock tubes [2]. Thus, there is an        the reflecting surface. For controlling ignition and
obvious lack of experimental data for high-temperature        reaction times the additional ion current sensor was
combustion and auto-ignition of Diesel fuel and its           mounted into reflecting surface of the shock tube. To
surrogates in air. The objectives of this study are as        detect arrival times of reaction front two additional ion
follows:                                                      current sensors were installed along the shock tube
-         to investigate systematically combustion and        channel at different distances from the reflecting wall.
ignition properties of Diesel No.2 at 4.7 – 10.4 atm,         This setup ensured obtaining information on
temperatures 1065 - 1838 K, and stoichiometries φ = 0.5       propagations of shock and reaction fronts over the
– 2;                                                          length of 340 mm upstream the end wall. Pressure and
-         to compare auto-ignition of premixed Diesel         ion current signals were recorded and processed by an
fuel/Air mixtures with a Jet-A/air mixture at equivalent      automatic 10-bit data acquisition system connected with
post-shock conditions;                                        a central computer.  
-         to obtain reference data on ignition of Diesel          To provide spectroscopic observations two quartz
fuel/Air mixtures at high temperatures itself.                rods of 8 mm in diameter has been mounted into the
                                                              plane end wall. The first rod ensured emission
Experimental setup                                            observations from the gas column of 5 mm in diameter
Stainless steel heated shock tube of 76 mm in diameter        along the centerline of the tube, the second – from the
was used in the experiments. The wall thickness was 9         similar gas column along the tube wall in the boundary
mm. The length of the tube was 6.0 m. The tube channel        layer. To fix the instant at which the luminosity
was calibrated with accuracy of 0.005 mm. Four                commences, both gas volumes (∅ 5 mm) has been
independent current circuits provided an independent          imaged onto a photomultiplier detectors installed in a
heating of the four shock tube sections to ensure the         focus of the collecting quartz lens f= 400 mm. The 1-
uniform temperature distributions along the tube length       mm apertures mounted in front of the photomultipliers
with accuracy ± 50 C. For shock tube tests presented in       ensured the angle selection of transmitted radiation and
this report the initial temperature of the tube was kept      passed only light beams propagating along the shock
between 100 - 1100 C.                                         tube axis. The beam-splitters divided output spectrum
    The commercial No. 2 Diesel fuel was used for these       into three optical paths to provide the simultaneous co-
studies. The density of used Diesel sample was                axial emission observations at several spectral bands.
(0.8237±0.01) g/ml at 26 0 C. The runs were performed         The luminosities of OH radicals (transitions A2Σ – X2Π)
in stoichiometric φ = 1, lean φ = 0.5, and rich φ = 2         at wavelength of 308.9 nm and CH radicals (transitions
                                                              2
Diesel fuel /Air mixtures within the range of post-shock        Δ –2Π) at wavelength of 431.5 nm were implemented
pressures of 4.7 – 10.4 atm and temperatures of 1065 –        to measure auto-ignition of the mixture along the
1838 K (Table 1).                                             centerline of the shock tube. The luminosity of C2
    Figure 1 presents the drawing of the test section for     radicals (transitions 2Π – 2Π) at wavelength of 516.5 nm


1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009                                                                    
                     Table 1. The experimental conditions for No. 2 Diesel fuel /Air mixtures

 Diesel - Air          Equivalence              Post-shock                  Post-shock               Post-shock
                         ratio, φ              Pressure [atm]             Temperature [K]          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 shock
tube: 1 – high-frequency PCB pressure transducers; 2 – ion current sensors; 3 – thermocouple; 4 –reflecting wall
with 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 the            calculated by processing shock-arrival times at pressure
boundary layer. Ignition times were controlled also by        sensors along the tube in laboratory frame of reference,
pressure and ion current measurements at the reflecting       u is flow velocity behind incident shock wave.
wall.                                                             For stoichiometric Diesel fuel / Air mixture, Figure
    The ignition or induction time of the mixture was         3 illustrates the dependencies of reflected shock-wave
defined as the time difference between shock arrival at
the end wall and the onset of emission within measuring
gas columns (Fig.2). The applied optical setup was
sensitive to the onset of auto-ignition at selected gas
volumes and generated induction times of studied
mixtures from the beginning of normal reflection of the
incident shock wave. To obtain a correct temperature
dependence of ignition time on activation energy of the
mixture and fuel/oxygen concentrations all comparative
shock-tube series were performed at approximately
constant post-shock density. It means that fuel, oxygen,
and nitrogen concentrations were kept nearly constant
within a studied temperature range behind reflected
shock waves.
    Absolute velocities of reflected shock wave (RSW)
in a frame of reference attached to gas flow moving              Figure 2. Ignition time definition criteria and their
behind incident shock wave and pressures at different         positions along the history of reflecting wall pressure
locations identified the auto-ignition modes of the           and gas emission in stoichiometric Diesel fuel / Air
mixture (strong, transient and weak) [3-6]. The               mixture: A – OH and CH emissions along the centerline
absolute RSW velocity in the end part of the tube was         of the shock tuber. Stoichiometry φ = 1. Post shock
defined as V = V5 + u, where V5 is RSW velocity               temperature is T = 1188 K.


1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009                                                                   
Velocity on post-shock temperatures at different             transitions 2Δ –2Π), along the tube wall in boundary
locations along the tube. On the basis of pressure and       layer at λ = 516.5 nm (C2, transitions 2Π – 2Π ), pressure
emission observations the inflection point of velocity       and ion current measurements at the reflecting wall. It is
curve at low temperatures for distances of 140 mm was        apparent that the data obtained by different methods and
used for determining positions of the strong explosion       at different initial temperatures correlate well in a
limit. For stoichiometric mixture, this critical             studied range of post-shock temperatures. The induction
temperature was 1251 K. It should be mentioned that          times of aviation kerosene Jet-A [6] obtained at nearly
explosion behavior of Diesel fuel was observed also at       the same post-shock conditions are drawn on the same
temperatures higher than 1140 K due to the mechanism         graph. For stoichiometric mixture, No. 2 Diesel fuel
of deflagration to detonation transition in a shock          demonstrates 2.5 –2.6 times longer induction periods in
compressed gas volume. As is seen in the Figure 3 this       comparison with Jet-A. Figure 4 shows that at high
transient ignition mode was realized within the              temperatures ( > 1210 K ) No. 2 Diesel exhibit almost
temperature range of 1140 – 1251 K.                          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
    Figure 3. Velocities of reflected shock wave at 140      temperatures 16437 K (32.6 kcal/mole), which is very
and 340 mm from reflecting wall vs. post-shock               close to our results for Jet-A and No. 2 Diesel.
temperature in stoichiometric Diesel fuel / Air mixture.         In accordance with chosen criterion for strong
Positions of the strong and transient ignition limits are    ignition limits experiments demonstrate that the critical
indicated on the Graphs.                                     post-shock temperature required for strong initiations
                                                             was equal to T = 1251 K. The transient ignition mode
    The main measurement uncertainties were                  were detected within the temperature range of T = 1140
associated with several factors. The first one is a 0.5%     - 1251 K and M = 2.6 - 2.8, respectively. The
uncertainty     in   incident    shock-wave       velocity   measurements of the steady-state CJ detonation
measurements, which produces ≈ 0.75%, 1.5%, 0.7%             velocities in stoichiometric Diesel fuel /Air mixtures
experimental errors in temperature (T), pressure (P), and    give VCJ ≈ 1580 ± 15 m/s. The appropriate value in
density (ρ) of the mixture behind reflected shock waves.     stoichiometric Jet-A mixture is equal to VCJ ≈ 1670 m/s.
The second one is an uncertainty connected with the
definition of specific heat and enthalpy of the Diesel
fuel. Usually, this can result in a 1-1.5 % error in
determination of post-shock parameters of the gas. The
last one is an uncertainty connected with the definition
of ignition-delay time. Usually, this can result in a 2-15
% error in ignition-delay time for studied range of
parameters

Results
For post-shock density of 1.94 ± 0.29 kg/m3 in
stoichiometric Diesel fuel/Air mixtures, the temperature
dependence of induction times is plotted in the Figure 4.
The experiments were performed within the temperature
range of 1078 – 1655 K. Experimental points for No.2
Diesel fuel correspond to ignition times measured by
using co-axial emission observations along the                   Figure 4. Mean activation energies for
centerline of the shock tube at λ = 308.9 nm (OH,            stoichiometric Diesel fuel/ and Jet-A/Air (φ = 1.0)
transitions A2Σ – X2Π) and at λ = 431.5 nm (CH,              mixtures at equivalent post-shock conditions.


1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009                                                                    
    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           Figure 6. Ignition delay time vs. reciprocal
and φ = 1.0 exhibit the same activation energy. The lean    temperature for rich Diesel fuel/ and Jet-A/Air mixtures
mixture demonstrates approximately ≈ 1.5 times longer       (φ = 2.0) at equivalent post-shock conditions.
induction periods. The similar trend has been observed
for Jet-A fuel in our previous studies [6].                 Arrhenius law within the studied temperature range of
    Experiments demonstrate that the critical post-shock    916 – 1838 K. In comparison with Jet-A Diesel fuel
temperature required for strong initiations is equal to T   demonstrates ≈ 3 - 6.4 times longer induction periods at
= 1294 K. The transient ignition modes were detected        equivalent post-shock conditions.
within the temperature range of T = 1170 – 1294,                For rich and stoichiometric Diesel fuel /Air
respectively. For lean mixtures, measurements of the        mixtures, Figure 7 shows the comparison of induction
steady-state CJ detonation velocity give VCJ ≈ 1450 ±       times. As is seen form the graphs, linear approximations
20 m/s. The appropriate value for lean Jet-A mixture is     of the experimental data for φ = 2.0 and φ = 1.0 exhibit
≈ 1480 m/s.                                                 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             Figure 7. Mean activation energies for
scatter of experimental data the temperature dependence     stoichiometric and rich Diesel fuel/ Air mixtures at
of induction period for rich mixture follow the             equivalent post-shock conditions.

1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009                                                                   
    For rich Diesel fuel /Air mixtures, the critical post-                    coincidence with experimental observations for φ = 2.
shock temperature required for strong auto-ignitions is                       This correlation results in 27 % standard deviation from
equal to T = 1260 K. The transient ignition modes were                        the fitted induction times.
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):

                               ⎛ 15473 ⎞
                                       ⎟ × [O2 ]        × [Diesel ]
                                                                   −0.54218
τ ( μs ) = 8.0663 × 10 −6 × exp⎜                0.28653

                               ⎝ T ⎠


where, τ is the ignition time in (μsec) , T is the post-
shock 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
                                                                                 Figure 9. Ignition delays of Diesel fue/Air mixtures
is 30.7 ± 0.26 kcal/mole. Equation (1) gives the
                                                                              correlated using equations (2) vs. reciprocal post-shock
excellent agreement with experimental data points for
                                                                              temperature. Units: τ ( μs ); [Diesel], [O2] ( mole/cm3 );
stoichiometries φ = 0.5 –1. This correlation results in 13
                                                                              T (K).
% standard deviation from the fitted induction times.

                                                                              Conclusions
                                                                              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
                                                                              <).
                                                                                  Critical post-shock temperatures T = 1251 K (φ = 1),
                                                                              T = 1294 K (φ = 0.5), and T = 1260 K (φ = 2.0) required
   Figure 8. Ignition delays of Diesel fuel/Air mixtures                      for strong auto-ignitions of Diesel fuel/Air mixtures
correlated using equations (1) vs. reciprocal post-shock                      were established from experimental measurements.
temperature. Units: τ ( μs ); [Diesel], [O2] ( mole/cm3 );                        Within the temperature ranges of T = 1140 - 1251 K
T (K).                                                                        (φ = 1), T = 1170 - 1294 K (φ = 0.5), and T = 1145 -
                                                                              1260 K (φ = 2.0) transient auto-ignitions of Diesel
                                                                              fuel/Air mixtures were observed in experiments.
        For a wider stoichiometry range of φ = 0.5 - 2,                           CJ detonation velocities in Diesel fuel / Air mixtures
overall empirical approximation for ignition delays is                        are VCJ = 1735 ± 20 m/s (for φ = 2); 1580 ± 15 m/s (for
(2) (Fig.9):                                                                  φ = 1); and 1450 ± 20 m/s (for φ = 0.5).

                                                                              Acknowledgments
                              ⎛ 13789 ⎞
τ ( μs ) = 1.5563 × 10 −5 × exp⎜      ⎟ × [O2 ]
                                               −0.43055
                                                          [
                                                        × Diesel    ]
                                                                    −0.0404
                                                                              This work was sponsored by General Electric Global
                              ⎝ T ⎠
                                                                              Research.
with the global activation energy of Diesel fuel obtained
                                                                              References
from regression analysis is 27.3 ± 0.42 kcal/mole.
Approximation (2) results in the satisfactory


1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009                                                                                     
1. Spadaccini LJ, TeVelde JA (1982) Autoignition
   Characteristics 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 the
   measurement 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, Varatharajan
   B. Proc. Combust. Inst. 31 (2007) 2481-2488.
7. N.B. Colket, L.J. Spadaccini, 14th Int. Symp. on
   Airbreathing Engines, Florence, Italy, September 5-
   10, 1999.
8. Davidson DF, Horning DC, Herbon JT, Hanson RK,
   Proc. Combust. Inst. 28 (2000) 1687-1692.




1* Corresponding author: penyaz@dnp.itmo.by 
Proceedings of the European Combustion Meeting 2009        

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:10
posted:3/9/2010
language:
pages:6
Description: Autoignitions of Diesel FuelAir Mixtures Behind Reflected Shock Waves