Abstract
Combustion in a CI engine is initiated by self-ignition of fuel–air mixture caused by high pressure and high temperature; a process known as autoignition. Autoignition is a challenging problem to simulate as the temperature increases from initial temperature to the adiabatic flame temperature in a very short duration. Numerical study of a turbulent flow using RANS/LES encounters a closure problem. Accuracy of closure models can be improved through experimental results, theoretical reasoning and direct numerical simulation (DNS) data. A review of DNS of autoignition in a turbulent non-premixed medium is presented in this chapter. As observed from DNS study, autoignition sites in a turbulent non-premixed medium are not randomly distributed but follow a pattern in the mixture fraction-scalar dissipation rate space. Turbulent flow is always three-dimensional in nature. 2D DNS of autoignition shows that ignition delay time increases with increase in initial turbulence intensity, which contradicts with the experimental observation. 3D DNS of autoignition resolves this conflict. The conflict is mainly due to the absence of vortex-stretching phenomenon in 2D DNS. Homogeneous charge compression ignition (HCCI) is being considered as one of the strategies toward improving performance of conventional CI engines. However, HCCI engines suffer from drawbacks like lack of control over combustion and limited operating regime. One of the modifications suggested in the HCCI technology to overcome these drawbacks is the use of stratification in the fuel–air mixture. Therefore, a few DNS studies on autoignition in the stratified medium have been discussed here. Further, discussion on the conditional moment closure (CMC) model and its validation using DNS data has been presented. Ignition delay time predicted by CMC was found to be in good agreement with DNS predictions.
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Abbreviations
- CFD:
-
Computational Fluid Dynamics
- CI:
-
Compression Ignition
- CMC:
-
Conditional Moment Closure
- DNS:
-
Direct Numerical Simulation
- HCCI:
-
Homogeneous Charge Compression Ignition
- HRR:
-
Heat Release Rate
- LES:
-
Large Eddy Simulation
- MR:
-
Most Reactive
- RANS:
-
Reynolds-Averaged Navier–Stokes
- SI:
-
Spark Ignition
- TDC:
-
Top Dead Center
- 2D/3D:
-
Two Dimensional/Three Dimensional
- \( c_{P} \) :
-
Specific heat at constant pressure
- D :
-
Fickian diffusion coefficient
- Da :
-
Damköhler Number
- Ea :
-
Activation energy of a reaction
- h r :
-
Heat of reaction
- Δh α :
-
Heat of formation of a species
- H F :
-
Heat of combustion of fuel
- N S :
-
Number of species
- \( Q(\eta ;x,t) \) :
-
Conditional average of any scalar Y on ξ = η
- R :
-
Universal gas constant
- T :
-
Temperature
- T a :
-
Activation temperature of a reaction (E a /R)
- T in :
-
Initial Temperature
- u rms :
-
Root mean square value of velocity fluctuations
- Y i :
-
Mass fraction of ith species
- α, β :
-
Indices of species
- η :
-
Sample space of ξ
- \( \theta \) :
-
Excess temperature given by \( (T - T_{in} )c_{P} /h_{r} \)
- λ:
-
Thermal Diffusivity
- ξ:
-
Mixture fraction (Eq. 2 )
- ρ :
-
Density
- σ 2 :
-
Conditional variance of excess temperature θ
- τ ign :
-
Ignition delay
- τ t :
-
Integral timescale of turbulence
- τ 0 :
-
Ignition delay in homogeneous mixture (or other reference timescale)
- φ αβ :
-
Factor defined by Eq. 11
- χ:
-
Scalar dissipation rate (Eq. 1)
- \( \dot{\omega }_{i} \) :
-
Chemical source term of ith species
- \( {\bigwedge } \) :
-
Integral length scale of turbulence
- \( {\bigwedge }_{k} \) :
-
Kolmogorov scale of turbulence
- \( \wedge_{\upxi} \) :
-
Integral scale of initial scalar distribution
- \( \mho \) :
-
Vortex locating index (Sect. 10.2.1)
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Acknowledgements
Most of these works were carried out by one of the authors, during his PhD, under the supervision of Prof. K. N. Lakshmisha, IISc, Bangalore.
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Bhide, K.G., Sreedhara, S. (2018). Direct Numerical Simulation of Autoignition in Turbulent Non-premixed Combustion. In: De, S., Agarwal, A., Chaudhuri, S., Sen, S. (eds) Modeling and Simulation of Turbulent Combustion. Energy, Environment, and Sustainability. Springer, Singapore. https://doi.org/10.1007/978-981-10-7410-3_10
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