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Partially Homogenous HCCI Combustion Chemical Kinetics Model Initial Conditions Development by CFD Modeling

Babazadeh, Amin | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48116 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Hosseini, Vahid
  7. Abstract:
  8. Homogeneous Charge Compression Ignition (HCCI) engines are the new generation of internal combustion engines which have diesel-like thermal efficiency, while they have low nitrogen oxide and soot emission. One of the major problems of HCCI engines is combustion timing control due to its dependence on reactivity of the charge and chemical kinetics. Moreover, limiting operation range between knock and misfiring is the other drawback of using these kind of engines. In the present work, partially homogeneous HCCI combustion in a single cylinder diesel engine is modeled using Computational Fluid Dynamics (CFD) coupled with chemical kinetics. Simulation includes the engine cycle from exhaust valve closure of intake stroke to exhaust valve opening of exhaust stroke. Chemical kinetic mechanism of the blend of n-heptane and iso-octane including 41 species and 130 reactions is used for simulating the combustion of gasoline fuel. The results of two different cases (with and without Exhaust Gas Recirculation) show that combustion parameters like combustion timing and the maximum in-cylinder pressure have good agreement with experimental results. Although these engines are named HCCI, the results of present work show that temperature and chemical composition of the charge at the beginning of the compression stroke and at the start of auto-ignition reactions is not completely homogeneous and is actually partially homogeneous. By using ignition delay as a measure for reactivity and investigating its stratification in the combustion chamber at the start of auto-ignition reactions, we found that thermal stratification has much more effect on the reactivity stratification than compositional stratification. Relying on the validity of the model, effects of variation of fuel composition, Exhaust Gas Recirculation (EGR) fraction and intake temperature as methods of combustion timing control are studied. The results show that by increasing the intake temperature and fraction of n-heptane in the fuel and reducing the EGR fraction, combustion starts earlier, combustion duration decreases and the maximum in-cylinder pressure rises due to rapid combustion. By increasing intake temperature, chemical reactions that are exponentially dependent on temperature are initiating earlier. By raising fraction of n-heptane in the fuel, overall fuel resistance against auto-ignition decreases. By increasing EGR fraction, its species absorb heat of pre-ignition reactions because of their higher thermal capacity, as a result, it delays the ignition timing
  9. Keywords:
  10. Ignition Delay ; Chemical Kinetic Simulation ; Computational Fluid Dynamics (CFD) ; Homogeneous Charge Compression Ignition (HCCI) ; Chemical Kinetic ; Combustion Timing

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