flow reactor study of controlled combustion kinetics gcep global
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Stanford University Thermosciences
Flow Reactor Study of Controlled Combustion Kinetics
GCEP Global Climate and Energy Project
C. T. BowmanK. Walters
Mechanical EngineeringDepartment
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Outline
• Background
- Motivation
- Objectives
• Approach
• Experimental Setup
• Results
• Future work
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Why Controlled Combustion?
• In conventional combustion devices, the chemical conversion offuel and oxidizer to products occurs rapidly in an uncontrolledand highly irreversible process (flame) and prior to work extraction,leading to a loss in cycle efficiency.
10
20
30
40
50
0 5 10 15 20 25Cycle Pressure Ratio
y
Second Law Cycle Efficiency - %
Fuel Exergy Loss in Combustion - %
Simple Brayton Cycle
T04 = 1200K
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Why Controlled Combustion?
• Modification of work producing cycles to match work extractionand heat release rates can lead to increased cycle efficiencyand, hence, reduced greenhouse gas emissions.
Cycle OPR nth Gain
Simple 20 41.2%
Reheat 92.6 48.0%13.8%
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What is Controlled Combustion?
• In controlled combustion, the rateof the fuel conversion process isvaried by imposing prescribedconditions (temperature and massfractions of the oxidizer/diluents),leading to potential reductions inirreversibilities in energy conversion(improved efficiency) and reducedemissions of pollutants andgreenhouse gases.
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Controlled Combustion
The Challenge to Implementation:
• A new regime of combustion that is poorly understood at thefundamental level needed for design optimization, especiallyfor high-pressure combustion systems, such as gas turbinesand diesel engines (HCCI).
The Objective of this Study:
• To investigate the combustion mechanisms of fuels in theintermediate preheat temperature range (1000-1300K) forpressures up to 50 bar with dilution by inert and chemicallyactive species.
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Controlled Combustion – Expected Outcomes
• Understanding of the effects of inert and chemically activeadditives on combustion rates at intermediate temperaturesand high pressures.
• Detailed and reduced mechanisms for controlled flamelessoxidation of model fuels for use in modeling and designinglow-irreversibility combustion engines, including HCCI andmulti-stage turbine burners.
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Range of Conditions Investigated
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Approach
• Flow Reactor Experiments– Measure the spatial evolution of temperature and composition– Experimental parameters:
• Temperature• Pressure• Oxygen concentration• Fuel concentration• Fuel composition: C2H6, C2H6/CH4 mixtures, surrogates, and
oxygenated fuels• Bath gas composition
• Compare the experimental data to the model results using adetailed reaction mechanism
• Optimize and validate the reaction mechanism• Model reduction by principal component analysis
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High-Pressure Flow Reactor
Gas Analyzers
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High-Pressure Flow Reactor
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Modeling Approach
• Initial studies have been conducted using CH4, C2H6and CH4-C2H6 mixtures to simulate natural gas.
• The starting reaction mechanism is GRI-Mech 3.0.
• The Chemkin and Senkin computer codes are used tomodel the reaction progress and to conduct sensitivityand reaction path analysis.
• Future studies will focus on surrogate andoxygenated fuels.
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1500 ppm C2H6: 0.5% O2
P = 1 bar
Experimental and Model Results
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1500 ppm C2H6: 3.0% O2
T0 = 1170K P = 1 bar
Experimental and Model Results
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Sensitivity Analysis Results
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1050 ppm C2H6: 0.5% O2
T0 = 1250K P = 2 bar
Experimental and Model Results
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Sensitivity Analysis Results
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Sensitivity Analysis Results
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Experimental and Model Results
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Conclusions
• Combustion times can be varied from 10 ms – 100 msec by varying initial O2concentration and diluents.
• For the fuels considered, combustion times increase with increasing pressure.
• Existing detailed reaction mechanisms capture some of the featuresof combustion of methane and ethane in the controlled combustionregime.
• The model consistently under predicts the initial rate of CO formation.
• In the controlled combustion regime, RO2 chemistry becomes increasingly important as the initial O2 concentration decreases and pressure increases.
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Future Work
• Additional experiments– Vary fuel composition and concentration– Vary O2 concentration– Vary pressure
• Measurements of stable intermediates– Aldehydes (CH2O, CH3CHO)– Alcohols (CH3OH, C2H5OH, C3H3OH )
• Update the mechanism• Mechanism reduction