Krithika Narayanaswamy

An alternative way to formulate transportation fuel surrogates using model predictions of gas-phase combustion targets is explored and compared to conventional approaches. Given a selection of individual fuel components, a multi-component chemical mechanism describing their oxidation kinetics, and a database of experimental measurements for key combustion quantities such as ignition delay times and laminar burning velocities, the optimal fractional amount of each fuel is determined as the one yielding the smallest error between experiments and model predictions. Using a previously studied three-component jet fuel surrogate containing n-dodecane, methyl-cyclohexane, and m-xylene as a case study, this article investigates in a systematic manner how the surrogate composition affects model predictions for ignition delay time and laminar burning velocities over a wide range of temperature, pressure and stoichiometry conditions, and compares the results to existing surrogate formulation techniques, providing new insights on how to define surrogates for simulation purposes. Finally, an optimisation algorithm is described to accelerate the identification of optimal surrogate compositions in this context.

Surrogate fuels are often used in place of real fuels in computational combustion studies. However, many different choices of hydrocarbons to make up surrogate mixtures have been reported in the literature, particularly for jet fuels. To identify the best choice of surrogate components, the capabilities of different surrogate mixtures in emulating the combustion kinetic behavior of the real fuel must be examined. To allow extensive assessment of the combustion behavior of these surrogate mixtures against detailed experimental measurements for real fuels, accurate and compact kinetic models are most essential. To realize this goal, a flexible and evolutive component library framework is proposed here, which allows mixing and matching between surrogate components to obtain short chemical mechanisms with only the necessary kinetics for the desired surrogate mixtures. The idea is demonstrated using an extensively validated multi-component reaction mechanism developed in stages (Blanquart et al., 2009; Narayanaswamy et al., 2010, 2014, 2015), thanks to its compact size and modular assembly. To display the applicability of the component library framework, (i) a jet fuel surrogate consisting of n-dodecane, methylcyclohexane, and m-xylene, whose kinetics are described in the multi-component chemical mechanism is defined, (ii) a chemical model for this surrogate mixture is derived from the multi-component chemical mechanism using the component library framework, and (iii) the predictive capabilities of this jet fuel surrogate and the associated chemical model are assessed extensively from low to high temperatures in well studied experimental configurations, such as shock tubes, premixed flames, and flow reactors.

Experimental and computational investigation is carried out to elucidate the influence of stoichiometric mixture fraction, ζst, on the structure and critical conditions of extinction of nonpremixed dimethyl ether (DME) flames. The stoichiometric mixture fraction represents the location of a thin reaction zone in terms of a conserved scalar quantity. The counterflow configuration is employed, wherein two reactant streams flow towards a stagnation plane. One stream is made up of DME and nitrogen (N2) and the other stream is oxygen and N2. Previous studies have shown that critical conditions of extinction depend on (Formula presented.) and the adiabatic temperature (Formula presented.). Therefore, the present investigation is carried out with the composition of the reactants in the counterflowing streams so chosen that the adiabatic temperature is the same for different values of (Formula presented.). The strain rate at extinction, (Formula presented.), is measured for values of (Formula presented.) up to 0.8. Computations are performed using detailed kinetic mechanisms and critical conditions of extinction and flame structures are predicted. The measurements and predictions show that, with increasingζst, the strain rate at extinction first decreases and then increases. The predictions agree with measurements for = ζst < 0,4, but significant deviations between measurements and predictions are observed at higher values of (Formula presented.). The scalar dissipation rate at extinction, χst,q is calculated using measured and predicted values of aq. With increasing (Formula presented.), the measured and predicted values of χst,q first increase and then decrease. It is noteworthy that changes in values of (Formula presented.) with (Formula presented.) for dimethyl ether flames are similar to those for methane flames, while the changes in values of (Formula presented.) with (Formula presented.) are remarkably different. Flame structures are predicted and they are found to be qualitatively similar to those for hydrocarbon fuels.

Biodiesel is being considered as a renewable fuel candidate to completely or partially replace fossil diesel. Understanding its combustion is key to assess its applicability in practical compression ignition engines. Significant progress has been made in understanding biodiesel combustion through experimental studies, development of reaction kinetics to describe its oxidation, and simulations in typical engine environments. The use of surrogates in place of the real biodiesels plays a crucial role in this endeavour. This chapter reviews the existing studies revolving around surrogate fuels for biodiesels. Thereafter, the challenges ahead in this context to further enhance our knowledge of biodiesel combustion are presented, and possible options to address these are discussed where appropriate.

Addition of oxygenates to diesel fuel has been found to reduce soot emissions, nonetheless, the resulting changes in the extinction and auto-ignition characteristics of the fuel mixture have not yet been fully understood. The present work investigates this aspect using laminar non-premixed flames. Dimethyl ether and methanol are two oxygenates considered in this study owing to their importance as potential alternative fuels/additives to gasoline and diesel. The fuel stream diluted with nitrogen is injected into a mixing layer from one duct of the counterflow burner, while air is injected from the other duct placed coaxially. The strain rate at extinction is calculated as a function of the mass fraction of fuel in the fuel stream. Further, the temperature of air at auto-ignition is determined as a function of strain rate for a fixed value of the mass fraction of fuel in the fuel stream. These data are obtained for the neat diesel, as well as for the blends of diesel and oxygenates. A flux analysis is used to explain the differences in extinction characteristics upon addition of oxygenates to diesel. A surrogate mixture of n-dodecane and m-xylene is used to represent the diesel fuel.

Biodiesel is considered as a potential candidate to replace petrodiesel. The complex reaction scheme of actual biodiesel fuel oxidation could be simplified using surrogate formulations with suitable representative fuels. A chemical kinetic mechanism is developed considering n-dodecane (n-C12H26), methyl butanoate (CH3CH2CH2C(=O)OCH3) and methyl crotonate (CH3CH=CHC(=O)OCH3) as components of biodiesel surrogate. Starting with a detailed kinetic model for methyl butanoate and n-dodecane, revisions are introduced to the C0-C4 chemistry based on the recent AramcoMech, and the resulting mechanism is combined with a short model for methyl crotonate, derived from a suitable reference mechanism. The results for high temperature ignition of n-dodecane and methyl butanoate in the combined mechanism show good agreement with the experimental datasets. The combined mechanism is then validated for ignition delays of methyl crotonate at high temperatures in a shock tube for a wide range of equivalence ratios. Laminar flame speeds of methyl crotonate are compared with the experimental data. The present work will progress towards developing low temperature chemistry for methyl crotonate and defining a surrogate for actual biodiesel.

Biofuels, including biodiesel have the potential to partially replace the conventional diesel fuels for low-temperature combustion engine applications to reduce the CO2 emission. Due to the long chain lengths and high molecular weights of the biodiesel components, it is quite challenging to study the biodiesel combustion experimentally and computationally. Methyl crotonate, a short unsaturated fatty acid methyl ester was chosen for the chemical kinetic study as a model biodiesel fuel. Auto-ignition experiments were carried out in a rapid compression machine at pressures of 20 and 40 bar under diluted conditions over a temperature range between 900 K and 1074 K, and at different equivalence ratios (φ = 0.25, 0.5 and 1.0). The updated mechanism showed satisfactory agreement with the ex- perimental data with significant improvements in low-temperature ignition behavior. The key reactions at various combustion conditions and the improved reactivity of the modified mechanism were analyzed by performing sensitivity and path flux analysis. The importance of low-temperature pathways in predicting the ignition behavior of methyl crotonate at intermediate and low temperatures was demonstrated.

Biodiesel is a promising candidate to partially or completely replace diesel. A surrogate approach to simplify the chemical kinetic representation of its long chain methyl ester components is adopted in this work to circumvent the difficulty in integrating its large reaction schemes with engine combustion simulations. Firstly, a compact reaction scheme for potentially important candidates for biodiesel surrogates is derived by combining a detailed mechanism for methyl butanoate (Dooley et al., Combust. Flame, 2008) and n-dodecane (Narayanaswamy et al., Combust. Flame, 2014) and is comprehensively assessed for its component kinetic description. Thereafter, a constrained optimization approach is used to propose a surrogate for biodiesel fuels. The surrogate and its kinetics are evaluated for these real fuels. Furthermore, the importance of representing the unsaturated content in the biodiesel is investigated by using 1-butene as a surrogate component. This work serves as our first step towards the development of a compact reaction scheme for a biodiesel surrogate which will be coupled with an engine CFD code to study the application of biodiesels and its blends with diesel in compression ignition engines.

Ignition delay time (IDT) is an important global combustion property that affects the thermal efficiency of the engine and emissions (particularly NO (Formula presented.)). IDT is generally measured by performing experiments using Shock-tube and Rapid Compression Machine (RCM). The numerical calculation of IDT is a computationally expensive and time-consuming process. Arrhenius type empirical correlations offer an inexpensive alternative to obtain IDT. However, such correlations have limitations as these typically involve ad-hoc parameters and are valid only for a specific fuel and particular range of temperature/pressure conditions. This study aims to formulate a data-driven scientific way to obtain IDT for new fuels without performing detailed numerical calculations or relying on ad-hoc empirical correlations. We propose a rigorous, well-validated data-driven study to obtain IDT for new fuels using a regression-based clustering algorithm. In our model, we bring in the fuel structure through the overall activation energy ((Formula presented.)) by expressing it in terms of the different bonds present in the molecule. Gaussian Mixture Model (GMM) is used for the formation of clusters, and regression is applied over each cluster to generate models. The proposed algorithm is used on the ignition delay dataset of straight-chain alkanes (C (Formula presented.) H (Formula presented.) for n = 4 to 16). The high level of accuracy achieved demonstrates the applicability of the proposed algorithm over IDT data. The algorithm and framework discussed in this article are implemented in python and made available at https://doi.org/10.5281/zenodo.5774617.

An experimental and computational investigation is carried out to characterize the influence of reactants on critical conditions for extinction and for autoignition of propane and n-heptane in nonpremixed counterflow configurations. Propane or vaporized n-heptane mixed with nitrogen is transported in one stream while the other stream is made up of air mixed with nitrogen. Measurements of the oxidizer stream temperature needed for autoignition are made at fixed values of the strain rate, either with the fuel mass fraction varied at a fixed oxygen mass fraction or with the oxygen mass fraction varied at a fixed fuel mass fraction. Extinction strain rates for propane are measured as a function of the oxygen mass fraction with room-temperature feed streams and the fuel mass fraction fixed and for n-heptane as a function of the fuel mass fraction with the oxygen mass fraction and feed-stream temperatures fixed. Predictions of critical conditions for extinction and autoignition are made employing detailed kinetic mechanisms. Predictions of critical conditions for extinction are in reasonable agreement with measurements, but there are significant discrepancies for autoignition. Measurements show that increasing the mass fraction of either fuel or oxygen increases the overall reactivity thereby reducing the autoignition temperature. The kinetic models predict the increase in reactivity of the mixing layer with increasing mass fraction of fuel but predict very little change in reactivity of the mixing layer with increasing mass fraction of oxygen, thus failing to predict the influence of oxygen on autoignition. It is concluded that there may exist kinetic pathways responsible for this disagreement that are yet to be discovered, and paths that fail to explain the results are identified.