Finally, the temporal evolutions of Sd∗ and the fraction of the heat release rate from defla- gration are shown in Figs. 3-14 and 3-15 to identify the combustion mode. It is readily observed from Fig. 3-14 that regardless of the magnitude ofu0,Sd∗ is nearly identical for cases with large T0(Cases 3, 6, 9, and 12–15) except in the final stage of ignition. This is because the deflagration mode of combustion is predominant at the reaction fronts, verifying that the deflagration speed is not affected by turbulence. The retardation of PRF50 ignition under the highu0 condition is also identified from the figure. However, for smallT0cases with largeu0(Cases 10–12)S∗dcannot be evaluated numerically because combustion occurs mostly as spontaneous ignition similar to 0-D auto-ignition and, hence, the reaction fronts do not exhibit deflagrative characteristics.
FractionofHRR
-1.0 -0.5 0.0 0.5 1.0
t/τ0ig
MeanHRR(J/mm3 s)
0.0 0.2 0.4 0.6 0.8 1.0
0 50 T′= 15 K 100
u′= 0.5 m/s 60 K
2.5 m/s 0.5 m/s
2.5 m/s PRF100
PRF80 PRF50
Figure 3-15: Temporal evolution of the fraction of the heat release rate from the deflagration mode and the mean heat release rate for Cases 1, 4, 7, 10–12 (T0 = 15 K) and 3, 6, 9, 13–15 (T0= 60 K).
These combustion characteristics are also found in the fraction of ˙q from the deflagration mode as shown in Fig. 3-15. Note that the total heat releases from the deflagration mode for Cases 12–15 are approximately 34, 32, and 28 %, respectively, which are relatively smaller than those of Cases 3, 6, 9 with smallu0, implying that high turbulence intensity can effectively homogenize the mixture such that the portion of combustion by deflagration is decreased. More- over, as mentioned above, the retardation of PRF50 combustion results in more homogenization such that the total heat release from the deflagration mode of PRF50 is the smallest among the three cases.
3.6 Conclusions
The effects of PRF composition, thermal stratification, and turbulence intensity on the auto- ignition of lean homogeneous PRF/air mixtures at constant volume and elevated pressure are investigated by direct numerical simulations with a 116-species reduced mechanism. In the
first parametric study, nine cases of HCCI combustion were studied with different degrees of temperature fluctuations for three different PRF/air mixtures. The chemical explosive mode (CEM), displacement speed, and Damk¨ohler number analyses verify that, in general, larger T0 induces greater temporal spreading of the mean heat release rate regardless of PRF/air mixtures because the deflagration mode is predominant at the reaction fronts for large T0. On the contrary, spontaneous ignition prevails for small T0 and, hence, simultaneous auto-ignition occurs throughout the whole domain, resulting in an excessive rate of pressure rise. It was also found that the effect of fuel composition on the ignition of PRF/air mixtures vanishes for cases with largeT0 because the deflagration mode prevails at the reaction fronts and the propagation characteristics of deflagrations are nearly identical.
In the second parametric study, the effect of turbulence intensity on the ignition charac- teristics of PRF/air mixtures was elucidated. It was found that turbulence with large u0 and short τt can effectively homogenize the mixtures such that the overall ignition is more apt to occur by spontaneous ignition in all the cases. It was, however, found that turbulence with large u0 retards the overall combustion of PRF50 more significantly because nascent ignition kernels of a PRF50/air mixture are more likely to be dissipated or extinguished by turbulence than are those of the other PRF/air mixtures. Therefore, these propagation and extinction characteristics result in the retardation of the overall combustion of PRF50/air mixture with largeu0.
These results suggest that large thermal stratification provides smooth operation of HCCI engines regardless of the PRF composition. In addition to the ignition characteristics, the propagation and extinction characteristics of deflagrations of different PRF/air mixtures should be considered for the design and operation of HCCI engines.
Chapter 4
Ignition of lean biodiesel/air
mixtures under HCCI/SCCI
conditions
Chapter 4 is devoted to investigating the effects of the stratifications of temperature,T, and equivalence ratio,φ, on the ignition characteristics of a lean homogeneous biodiesel/air mixture at high pressure and intermediate temperature by using a new reduced kinetic mechanism for biodiesel. A 73-species reduced mechanism for biodiesel was developed that consists of 25%
methyl decanoate (MD), 25% methyl 9-decanoate (MD9D), and 50%n-heptane by volume.
4.1 Introduction
A strategy of controlling mixture inhomogeneities has been proposed as a promising means of controlling the ignition timing and PRR. Thermal stratification can be introduced to a fuel/air mixture by high levels of exhaust gas recirculation (EGR), intake charge heating, and wall heat transfer controls. It has been demonstrated experimentally and numerically that thermal stratification can tailor PRR by prolonging combustion duration, thereby enabling HCCI combustion under conditions of higher load [44–51, 53, 54, 90, 91].
However, the manipulation and exploitation of the thermal stratification of in-cylinder charge is not easy and remains a challenge [1, 92, 93]. For this reason, stratified-charge compres- sion ignition (SCCI) combustion has been investigated as an alternative solution for enlarging the operating range of HCCI combustion [48, 92, 94]. With the help of fuel stratification, a sequential ignition event can be achieved as a locally-richer mixture tends to ignite initially and then ignition propagates towards a nearby leaner mixture. As a result, SCCI combustion enables a smooth combustion sequence, preventing any rapid release of energy and reducing the peak rate of pressure rise. In practice, fuel stratification can be achieved by multiple high- pressure injectors with flexible injection timing [16, 17, 48]. In two-stage injection, for instance, a major fraction of the fuel is initially supplied by port fuel injection to generate a relatively- homogeneous mixture [48]. The remainder of the fuel (up to 20% of total fuel volume) is then directly injected during the late compression stroke or close to the top dead center (TDC) to introduce a certain amount of equivalence ratio (φ) fluctuations [17].
SCCI combustion under HCCI condition has been studied extensively [1,48,52,55,62,92–107].
Recently, Bansal and Im [52] investigated the ignition characteristics of a hydrogen/air mixture with both temperature and composition inhomogeneities using two-dimensional (2-D) DNSs. It was found that composition inhomogeneities together with temperature fluctuations spread out the heat release rate (HRR) more than temperature fluctuations alone. However, it is not clear that the results will be directly applicable to hydrocarbon/air mixtures exhibiting two-stage ignition.
The objective of the present study is, therefore, to provide a fundamental understanding of the ignition characteristics of a hydrocarbon/air mixture with temperature and composition