Abstract
Research and/or Engineering Questions/Objective
Using the simplest alcohol – methanol – as a fuel for spark ignition (SI) engines offers an increase of thermal efficiency. Additionally, with the enrichment of hydrogen rich gas from methanol reforming (syngas) using exhaust heat, the efficiency can be further improved. The complexity of optimizing such an arrangement asks for numerical support. However, there is no research that publishes the effect of unburned mixture temperature and equivalence ratio on the laminar burning velocity of methanol-syngas blends, which is needed for developing an engine cycle code to simulate methanol fueled SI engines with syngas addition from exhaust gas fuel reforming.
Methodology
A one-dimensional chemical kinetics code (CHEM1D) was used in this study to predict the laminar burning velocities of methanol/syngas/air mixtures under different unburned gas temperatures and equivalence ratios. The models were simulated for a wide range of equivalence ratios (0.5–2) and unburned gas temperatures (300–900 K) at atmospheric pressure with Li’s mechanism and validated with experimental data from previous studies. The influence of initial temperature on laminar burning velocities of methanol-syngas flames were investigated using the correlation uL = uL0.(Tu/Tu0)α. The variation of temperature power exponent α with equivalence ratio was calculated, and then compared to α values from well-known correlations used in engine simulation tools. The normalized sensitivity coefficients are also calculated to know which reaction influences the combustion most.
Results
The calculation of laminar burning velocities based on the energy fraction mixing rule gave the best fit with simulated data at lean conditions. With current studied syngas composition (50% H2 – 50% CO by volume), the main contribution of H radical producing is reaction (R3) H2 + OH = H2O + H. The peak production rate of H radial in that reaction of methanol-air and syngas-air combustion is 0.826E-2 mol/cm3.s and 2.1E-2 mol/cm3.s, respectively. The comparison of α values showed that the existing correlations by Gulder, Metghalchi, Saeed and Liao are not able to give a correct result of laminar burning velocity for the simulation of SI engine fueled with methanol-syngas blends. Further development of a laminar burning velocity correlation of methanol-syngas mixtures is required for engine cycle codes.
Limitation of this study
The simulation has not been validated with measured laminar burning velocity values. Additionally, there are only two components in syngas and these composition was fixed; however, the number of reformed products and these concentration varies for different engine conditions.
What does the paper offer that is new in the field including in comparison to other work by the authors?
The first database of laminar burning velocity and temperature power exponent for methanol-syngas-air premixed flames with different mixing rules.
Conclusions
Syngas addition has a significant influence on the laminar burning velocity of methanol-air mixtures; however, there is no existing correlation can predict well these values with different unburned gas temperatures. Further validation of the correlation will be done through experimental studies using a constant volume combustion chamber.
KEYWORDS : Laminar burning velocity; Temperature power exponent; Methanol; Syngas; Chemical kinetics