57 4.8 Scatterplots of temperature (first column), heat release rate (second column) and mass fraction of n-heptane (third column) as a function of mixture fraction at several times for case 3. 89 5.12 Scatterplots of temperature (first column), release rate of heat (second column) and mass fraction of n-heptane (third column) as a function of mixture fraction at several instants for case 3.
The fundamental of HCCI combustion
HCCI combustion with thermal stratification
For a third method, it is less effective in reducing the peak HRR for fuels that exhibit typical single-stage ignition characteristics such as iso-octane, gasoline, and ethanol. For fuels that exhibit typical two-stage ignition characteristics, such as n-heptane, diesel, and dimethyl ether (DME), their ignition characteristics are quite complicated and unusual due to the interactions between the low-temperature (e.g., cool) flame chemistry and negative temperature coefficient (NTC ) regime) and turbulence intensity.
HCCI combustion with fuel stratification
The effectiveness of fuel stratification in reducing the peak HRR and bubble intensity from a successive ignition event is highly dependent on the φ-sensitivity of the fuel. In fact, a certain level of fuel stratification can extend the operating limits of the HCCI engine at low loads.
HCCI combustion with reactivity stratification
At high loads, the high-reactivity fuel only plays a role as an ignition source to initiate combustion, while most of the heat is released from the low-reactivity fuel. Another solution is to stratify both fuels by injecting low reactivity fuel and high reactivity fuel directly into the engine cylinder.
Scope and objective
Based on these things, a better understanding of the ignition characteristics of HCCI combustion can help in developing HCCI engines. Therefore, the low-reactivity fuel/air charge is homogeneously distributed throughout the engine cylinder, while the high-reactivity fuel generates inhomogeneities in the reactivity, equivalence ratio and temperature of the homogeneous mixture.
Results and discussion
For T in the high temperature regime (T > 1000 K), however, τig0 is relatively less sensitive to the variation of PRF number and φ, but becomes very sensitive to that of T. Therefore, the overall RCCI/SCCI combustion in the high temperature regime it is more likely to occur similarly.
Combustion mode analysis
In addition, there is a widely distributed PDF of |∇τig0| in the RCCI cases this may indicate a high probability of the combustion deflagration mode (see figures). Third, for T0 in the high-temperature regime, the spontaneous ignition mode is predominant over the deflagration mode for both RCCI and SCCI combustion.
Effect of turbulence
As such, the development of nascent ignition nuclei into deflagration is delayed, thus slowing down the overall combustion process. 45] found that turbulence can increase overall combustion by increasing the turbulent flame region when the shortest ignition delay of the initial mixture is significantly less than τt. In the present RCCI/SCCI cases, the shortest initial mixture ignition delays (see Figures 2.6a–c) are quite comparable to τ, and as such no increase in overall combustion can be expected.
Conclusions
Note that CEMA has been applied to several DNS studies to elucidate the chemical aspects of turbulent combustion and is now considered as one of the useful computational flame diagnostic tools for the systematic detection of important species and reactions in premixed flames and flame ignition/extinction . Because the Jacobian of the chemical source term, Jω (≡∂ω/∂y), preserves the chemical information of the local mixture, the chemical characteristic of the mixture can be determined from the Jacobian. To capture the chemical feature in CEMA, a chemical mode is defined as an eigenmode of Jω, which is associated with an eigenvalue, λe, and a corresponding pair of the left and right eigenvectors, ae and be.
Results and discussion
Overall RCCI/SCCI combustion
The results revealed that during the first phase ignition and the start of the main combustion of the RCCI case, is a significant amount of heat. However, these local mixtures have a lower n-heptane concentration (eg ~PRF50 with φ= 0.74) and as such less heat is released during first stage ignition, resulting in delayed second stage ignition. It is important to understand the difference between chemical processes occurring in these local mixtures in RCCI and SCCI combustion.
CEMA: 0-D ignition
For the SCCI case, the onset of primary combustion also comes from large Z mixtures. Second, near first-stage ignition (~t/τig0 = 0.27), temperature, n-heptane, and CH2O are identified as most important. variables for CEM, which is characterized by the peak PIs of the low-temperature chain branching reactions, O2QOOH → KOOH + OH and KOOH →OH + KO (see Fig. 3.2c). As such, temperature, CO and OH are identified as the main CEM variables in the second stage ignition.
CEMA: 2-D DNS of RCCI/SCCI combustion
As such, temperature, n-heptane and CH2O are the most important variables for the CEM. In contrast to the early stage of combustion, most heat is released from high-temperature region (T > 1500 K) where HTC represented by CO oxidation (R24) and hydrogen chemistry (R1, R9 and R13) is dominant for both RCCI and SCCI combustion. As such, temperature, CO and OH are identified as the most important variables for the CEM and HTHR determines temperature increase at this stage.
Conclusions
The mixing layer effect between the n-heptane jet and the isooctane/air charge is explained by varying the relative jet velocity, U0. For more details on the physical models and relevant engine conditions, readers are referred to [50]. To account for evaporative cooling of the direct injection fuel, we also assumed a negatively correlated (NC) T–φ distribution in the n-heptane jet.
Overall ignition characteristics
More specifically, during the early phase of the first-stage ignition (see first column), some low-temperature ignition nuclei develop near the mixing layer in the n-heptane stream in which the scalar dispersion rate, χ, is very large. low (not shown here), similar to previous studies [77, 80]. After the consumption of the n-heptane stream is almost complete, they spread in liquid mixtures from the fuel to the iso-octane/air stream (see column five). Therefore, the entire n heptane stream is sequentially consumed by the mixture layer in the center of the jet during the combustion process.
Combustion mode analysis
For cases 1 and 2, however, the heat release takes place in the self-ignition mode during third-stage ignition in the form of the end-gas self-ignition. In the case of third-stage ignition (see Fig. 4.10c), however, the reaction term is significantly larger than the diffusion term upstream of the reaction. However, the characteristics of the combustion waves are quite similar to those of case 1: the diffusion terms are comparable to the diffusion terms, especially in first- and second-stage ignition, implying that they are deflagration waves in the diffusion limit.
Chemical explosive mode analysis (CEMA)
EI and PI indicate the normalized contribution of each variable and reaction to the CEM, respectively, and thus control species and reactions for the ignition of dan-heptane jet in the iso-octane/air stream can be elucidated by evaluating their values. These results indicate that the first-stage ignition of Case 1 is determined by the first-stage ignition of the n-heptane jet and the evolution of cool flames from the first-stage ignition kernels. Figures 4.14 and 4.15 show the conditional means of HRR, EI of important variables, and PI of critical reactions given temperature and mixture fraction for Case 3 at the first-stage ignition,t/τig0 = 0.635, and the second-stage ignition,t / τig0 = 0.841, respectively.
Conclusions
Direct numerical simulation of time-evolving twin-n-heptane jet ignition in an iso-octane/air flow under RCCI. Thus, the effect of the mixing layer between the double n-heptane jet and iso-octane/air charge is elucidated by varying the jet distance, djet, and relative jet velocity, U0. The chemical aspects of the ignition under these conditions are also elucidated by applying CEMA to DNS.
Numerical methods and initial conditions
It has been reported from Wang and Rutland [78] that at the end of the evaporation process of n-heptane fuel atomization in turbulent flow, the local temperature is inversely proportional to the local equivalence ratio, φ, due to the evaporative cooling effect of the fuel spray and therefore the maximum temperature difference is in the engine cylinder about 100 K, which is consistent with recent experimental results. Based on previous experimental results, the average temperatures of the iso-octane/air stream and the n-heptane jet are 1000 and 900 K, respectively. To investigate the effect of the jet spacing between two n-heptane jets, four different cases were simulated by djet vary: from case 2a to 2d, djet and 1.9 mm, respectively, and also to investigate the relative velocity between the two fuel/air charges. four different cases were simulated by varying U0: from Cases 1 to 4, U and 20 m/s, respectively.
Overall ignition characteristics
Effect of jet distance
It is easily observed that the first-stage HRR (see the first column) and most of the second-stage HRR (see the second column) occur in the n-heptane jet in all cases. Meanwhile, iso-octane/air mixtures outside the n-heptane jet begin to participate in heat generation after the n-heptane jet is almost used up. More specifically, during the first-stage ignition (see the first column), low-temperature ignition occurs in entire regions of a -heptane jet.
Effect of jet velocity
Second, the second stage ignition in the n-heptane jet starts much later and the total heat release decreases with increasing U0 than those in the jet. During first stage ignition (t/τig) a relatively small amount of heat is released from the n-heptane jet with ξ > 0.8 for both cases. For (a) 2nd stage ignition timing and maximum HRR and (b) combustion duration and n-heptane jet area .
Chemical explosive mode analysis (CEMA)
To assess the contribution of the low-, medium-, and high-temperature chemistries (LTC, ITC, and HTC) of the oxidation of the current heptane and isooctane to the overall combustion, we evaluate the conditional mean values of EI and PI for important variables and reactions. These results indicate that the first-stage ignition of Case 1 is determined by the first-stage ignition of the heptane jet. In this region, temperature and CO are also identified as important variables for CEM.
Discussion
On the other hand, for previous study (not shown here), the n-heptane jet mixture condition is located in a low-temperature regime. This is consistent with recent experimental results [99, 100] that investigated the effect of injection pressure on the overall combustion under RCCI engine operation. However, the effect of injection pressure on the combustion process with different initial temperature regime is not well understood.
Conclusions
The combustion process of n-heptane in iso-octane/air charge is investigated. Relative n-heptane stream velocity to iso-octane/air charge effect on overall ignition characteristics. Distance and relative velocity of twin heptane jet in iso-octane/air charge, effect on overall ignition characteristics.
Future work
Yoo, “Direct numerical simulations of the ignition of a lean biodiesel/air mixture with temperature and composition inhomogeneities at high pressure and intermediate temperature,” Combust. Yoo, “Direct numerical simulations of ignition of reference lean fuel/air mixtures with temperature inhomogeneities,” Combust. Bai, “Direct numerical simulation of weak hydrogen/air autoignition in a constant volume enclosure,” Combust.
