Global uncertainty and sensitivity analysis of a reduced chemical kinetic mechanism of a gasoline, n-butanol blend in a high pressure rapid compression machine.

A detailed evaluation of a recently developed combined n-butanol/toluene reference fuel (TRF) reduced chemical kinetic mechanism describing the low temperature oxidation of n-butanol, gasoline and a gasoline/n-butanol blend was performed using both global uncertainty and sensitivity methods with ignition delays as the predicted output for the temperature range 678 - 858 K, and an equivalence ratio of 1 at 20 bar. The results obtained when incorporating the effects of uncertainties in forward rate constants in the simulations, showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Global sensitivity analysis of the mechanism based on predicted ignition delays of stoichiometric TRF mixtures, showed the toluene + OH route = phenol + CH3 to be among the most dominant pathways in terms of the predicted output uncertainties but an update on the mechanism based on data from a recent study led to the toluene + OH hydrogen abstraction reaction becoming the most dominant reaction as expected. For the TRF/n-butanol blend, hydrogen abstraction reactions by OH from n-butanol appear to be key in predicting the effect of blending. Uncertainties in the temperature dependence of relative abstraction rates from the α and γ sites may still be present within current mechanisms, and in particular may affect the ability of the mechanisms to capture the low temperature delay times for n-butanol. Further studies of the product channels for n-butanol + OH for temperatures of relevance to combustion applications could help to improve current mechanisms. At higher temperatures, the reactions of HO2 and that of formaldehyde with OH also became critical and attempts to reduce uncertainties in the temperature dependent rates of these reactions would be useful

Low temperature ignition properties of n-butanol: key uncertainties and constraints

A recent kinetic mechanism (Sarathy et al., 2012) describing the low temperature oxidation of n-butanol was investigated using both local and global sensitivity/uncertainty analysis methods with ignition delays as predictive targets over temperature ranges of 678-898 K and equivalence ratios ranging from 0.5-2.0 at 15 bar. The study incorporates the effects of uncertainties in forward rate constants on the predicted outputs, providing information on the robustness of the mechanism over a range of operating conditions. A global sampling technique was employed for the determination of predictive error bars, and a high dimensional model representation (HDMR) method was further utilised for the calculation of global sensitivity indices following the application of a linear screening method. Predicted ignition delay distributions spanning up to an order of magnitude indicate the need for better quantification of the most dominant reaction rate parameters. The calculated first-order sensitivities from the HDMR study show the main fuel hydrogen abstraction pathways via OH as the major contributors to the predicted uncertainties. Sensitivities indicate that no individual rate constant dominates uncertainties under any of the conditions studied, but that strong constraints on the branching ratio for H abstraction by OH at the α and γ sites are provided by the measurements

Global Uncertainty and Sensitivity Analysis of a Reduced Chemical Kinetic Mechanism of a Gasoline, N-Butanol Blend in a High Pressure Rapid Compression Machine

A detailed evaluation of a recently developed combined n-butanol/toluene reference fuel (TRF) reduced chemical kinetic mechanism (Agbro, 2017) describing the low temperature oxidation of n-butanol, gasoline and a gasoline/n-butanol blend was performed using both global uncertainty and sensitivity methods with ignition delays as the predicted output for the temperature range 678 - 858 K, and an equivalence ratio of 1 at 20 bar. A global sampling technique was applied in the simulations in order to quantify the uncertainties of the predicted ignition delays when incorporating the effects of uncertainties in forward rate constants in the simulations. In addition, a variance-based global sensitivity analysis using a high dimensional model representation (HDMR) method was carried out to understand and rank the parameters responsible for the predicted uncertainties. The results showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Global sensitivity analysis of the mechanism based on predicted ignition delays of stoichiometric TRF mixtures, showed the toluene + OH route = phenol + CH3 to be among the most dominant pathways in terms of the predicted output uncertainties but an update on the mechanism based on recent data from the study of Seta led to the toluene + OH hydrogen abstraction reaction becoming the most dominant reaction as expected. For the TRF/n-butanol blend, hydrogen abstraction reactions by OH from n-butanol appear to be key in predicting the effect of blending. Uncertainties in the temperature dependence of relative abstraction rates from the α and γ sites may still be present within current mechanisms, and in particular may affect the ability of the mechanisms to capture the low temperature delay times for n-butanol. Further studies of the product channels for n-butanol + OH for temperatures of relevance to combustion applications could help to improve current mechanisms. At higher temperatures, the reactions of HO2 and that of formaldehyde with OH also became critical and attempts to reduce uncertainties in the temperature dependent rates of these reactions would be useful

Experimental and Chemical Kinetic Modelling Study on the Combustion of Alternative Fuels in Fundamental Systems and Practical Engines

In this work, experimental data of ignition delay times of n-butanol, gasoline, toluene reference fuel (TRF), a gasoline/n-butanol blend and a TRF/n-butanol blend were obtained using the Leeds University Rapid Compression Machine (RCM) while autoignition (knock) onsets and knock intensities of gasoline, TRF, gasoline/n-butanol and TRF/n-butanol blends were measured using the Leeds University Optical Engine (LUPOE). The work showed that within the RCM, the 3-component TRF surrogate captures the trend of gasoline data well across the temperature range. However, based on results obtained in the engine, it appears that the chosen TRF may not be an excellent representation of gasoline under engine conditions as the knock boundary of TRF as well as the measured knock onsets are significantly lower than those of gasoline. The ignition delay times measured in the RCM for the blend, lay between those of gasoline and n-butanol under stoichiometric conditions across the temperature range studied and at lower temperatures, n-butanol acts as an octane enhancer over and above what might be expected from a simple linear blending law. In the engine, the measured knock onsets for the blend were higher than those of gasoline at the more retarded spark timing of 6 CA bTDC but the effect disappears at higher spark advances. Future studies exploring the blending effect of n-butanol across a range of blending ratios is required since it is difficult to conclude on the overall effect of n-butanol blending on gasoline based on the single blend that has been considered in this study. The chemical kinetic modelling of the fuels investigated has also been evaluated by comparing results from simulations employing the relevant reaction mechanisms with the experimental data sourced from either the open literature or measured in-house. Local as well as global uncertainty/sensitivity methods accounting for the impact of uncertainties in the input parameters, were also employed within the framework of ignition delay time modelling in an RCM and species concentration prediction in a JSR, for analysis of the chemical kinetic modelling of DME, n-butanol, TRF and TRF/n-butanol oxidation in order to advance the understanding of the key reactions rates that are crucial for the accurate prediction of the combustion of alternative fuels in internal combustion engines. The results showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Further studies of the key reaction channels identified in this work at the P-T conditions of relevance to combustion applications could help to improve current mechanisms. Moreover, the chemical kinetic modelling of the autoignition and species concentration of TRF, TRF/n-butanol and n-butanol fuels was carried out using the adopted TRF/n-butanol mechanism as input in the engine simulations of a recently developed commercial engine software known as LOGEengine. Similar to the results obtained in the RCM modelling work, the knock onsets predicted for TRF and TRF/n-butanol blend under engine conditions were consistently higher than the measured data. Overall, the work demonstrated that accurate representation of the low temperature chemistry in current chemical kinetic models of alternative fuels is very crucial for the accurate description of the chemical processes and autoignition of the end gas in the engine

Experimental Investigation in the Autoignition Behaviour of Aviation Jet Fuel in a Rapid Compression Machine

Abstract: Jet A-1 fuel’s autoignition behaviour was examined experimentally in a newly developed rapid compression machine (RCM) at the University of Sheffield. The experiment was conducted at a compressed temperature at compressed temperatures of 697 K ⩽ Tc ⩽ 796 K, compressed pressure of 6 and 10 bar, and under lean and stoichiometric conditions (φ = 0.75 and 1.0). The initial chamber wall temperature was varied from 115°C - 135°C. The effect of the compressed temperature, compressed pressure and equivalence ratio was seen to have influence on the ignition delay time (IDTs) of Jet A-1. In the experimental condition studied, Jet A-1 exhibited an Arrhenius behaviour that is increasing the compressed temperature, compressed pressure and equivalence ratio decreases monotonically with IDTs. Evidence of two-stage ignition delay and absence of Negative Temperature Coefficient (NTC) behaviour was observed within the condition studied

Experimental Investigation in the Autoignition Behaviour of Aviation Jet Fuel in a Rapid Compression Machine

Abstract: Jet A-1 fuel’s autoignition behaviour was examined experimentally in a newly developed rapid compression machine (RCM) at the University of Sheffield. The experiment was conducted at a compressed temperature at compressed temperatures of 697 K ⩽ Tc ⩽ 796 K, compressed pressure of 6 and 10 bar, and under lean and stoichiometric conditions (φ = 0.75 and 1.0). The initial chamber wall temperature was varied from 115°C - 135°C. The effect of the compressed temperature, compressed pressure and equivalence ratio was seen to have influence on the ignition delay time (IDTs) of Jet A-1. In the experimental condition studied, Jet A-1 exhibited an Arrhenius behaviour that is increasing the compressed temperature, compressed pressure and equivalence ratio decreases monotonically with IDTs. Evidence of two-stage ignition delay and absence of Negative Temperature Coefficient (NTC) behaviour was observed within the condition studied

Sample-tip interaction of piezoresponse force microscopy in ferroelectric nanostructures

We report on qualitative and quantitative implications of the sample-tip interaction in piezoresponse force microscopy. Our finite-element analysis of adsorbate effects, sample heterogeneities, and tip asymmetries is in agreement with experimental observation of ferroelectric nanostructures. Qualitative discrepancies arise from locally asymmetric tip-sample interaction. Any quantitative determination of field-related material parameters as required for the verification of semiempirical models of the ferroelectric limit typically relies on an overestimated field across the sample. Our findings indicate that adsorbates reduce the actual field across the nanograin by roughly one order of magnitude

Low Temperature Oxidation of n-Butanol: Key Uncertainties and Constraints in Kinetics

A recent chemical kinetic mechanism (Sarathy et al., 2012) describing the low temperature oxidation of n-butanol was investigated using both local and global uncertainty and sensitivity methods within the context of predicting ignition delay times in a rapid compression machine (T = 678–898 K, ϕ = 0.5–2.0, P = 15 bar) and species profiles in a jet stirred reactor (T = 800–1150 K, ϕ = 0.5–2.0, P = 10 atm) in order to determine the most important reactions driving the predictive uncertainty, and the constraints provided by the experimental measurements. A global sampling technique was employed for the determination of predictive uncertainties, and a high dimensional model representation (HDMR) method was further utilised for the calculation of global sensitivity indices following the application of a linear screening method. The calculated global sensitivity indices were used to identify and rank the rate parameters driving the predicted uncertainties across the conditions studied. Predicted ignition delay distributions spanning up to an order of magnitude indicate the need for better quantification of the most dominant reaction rate parameters. The calculated first-order sensitivities from the HDMR study show the main fuel hydrogen abstraction pathways via OH as the major contributors to the predicted uncertainties. Sensitivities indicate that no individual rate constant dominates uncertainties under any of the conditions studied, and that the target outputs are largely insensitive to the total rate of OH with n-C₄H₉OH. However, strong constraints on the branching ratio for H abstraction by OH at the α and γ sites are provided by the RCM measurements. In the JSR simulations, predicted n-C₄H₉OH and CH₂O concentration profiles at T = 800 K, were particularly sensitive to H abstraction reaction by HO₂ from the α site. Although abstraction by OH from the α site plays an important role for predicted n-C₄H₉OH profiles at higher temperatures, in general, better constraint is provided on the n-C₄H₉OH + HO₂ abstraction rate by the measured concentration profiles of n-C₄H₉OH and CH₂O at lower temperatures than for abstraction by OH