Cheng, Xinwei
(2016)
Development of reduced reaction kinetics and fuel physical properties models for in-cylinder simulation of biodiesel combustion.
PhD thesis, University of Nottingham.
Abstract
The analyses of spray, combustion and emission characteristics for two types of biodiesel fuels, namely coconut methyl ester (CME) and soybean methyl ester (SME) are reported in this thesis. In order to produce high fidelity numerical spray and combustion representation for CME and SME, accurately developed thermo-physical properties and chemical kinetics were integrated with open-source computational fluid dynamics codes. First, the thermo-physical properties of CME and SME which include liquid and vapour properties were calculated using temperature-dependent correlations that were found in the literature. These calculated thermo-physical properties were then incorporated into Open Field Operation and Manipulation (OpenFOAM) to determine the sensitivities of the fuel properties on the spray development. Based on the sensitivity analyses, 5 of 12 thermo-physical properties, including latent heat of vaporisation, liquid density, liquid heat capacity, liquid surface tension and vapour pressure, gave the largest fluctuation to the spray development. Besides, coupled effects among the thermo-physical properties were discovered. The effects of thermo-physical properties were also varied according to the addition of unsaturation levels and combustion chemistries.
Next, a generic reduced chemical kinetic mechanism, with components of methyl decanoate, methyl-9-decenoate and n-heptane was developed to represent the biodiesel fuels. The reduced mechanism with 92 species and 360 elementary reactions was validated under 72 shock tube conditions against experimental measurements in the literature and detailed mechanism predictions, for each zero-dimensional auto-ignition and extinction process using CHEMKIN-PRO. Maximum percentage errors of less than 40.0% were recorded when the ignition delay (ID) period predictions of the reduced mechanism were compared to those of detailed mechanism. Satisfactory agreement was attained when the predictions of the reduced mechanism were validated against the measured species profiles of rapeseed methyl ester oxidation in jet stirred reactor, which were obtained from the literature. Besides, the ID periods and lift-off lengths (LOL) predicted for the reacting spray at initial temperatures of 900 K and 1000 K achieved a maximum deviation of 29.8% and 43.4%, respectively, as compared to those of the experimental measurements in the literature.
CME and SME were then numerically analysed under both the conditions of constant volume bomb and diesel engine, using the validated thermo-physical properties and reduced mechanism. The ambient oxygen level of the constant volume bomb was raised from 15.0 to 21.0% to emulate the intake air composition in the diesel engine. As such, the spray development was changed from radial to forward propagation, where LOL was reduced by 24.3%. Higher levels of carbon monoxide (CO), carbon dioxide (CO2) and soot mass concentrations were also obtained. When the unsaturation level was increased from 20.0% (CME) to 80.0% (SME), retarded spray and combustion developments were found in both the constant volume bomb and diesel engine. Besides, the CO, soot and nitric oxide (NO) emissions, including the tailpipe predictions were maximally increased by 32.0%. In overall, CME performs better than SME does because of the improved air-fuel mixing and decreased tailpipe NO, CO and CO2 emissions. Based on these, it is sufficient to deduce that the phenomena predicted in the constant volume bomb are adequate to replicate those in the diesel engine.
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