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Iqbal, AsimFundamentals of Knock
Doctor of Philosophy, The Ohio State University, 2012, Mechanical Engineering
In view of the declining global oil reserves and the environmental concerns associated with automotive emissions, it is imperative to improve the fuel efficiency of engines. Using higher compression ratios or boosting the specific output through turbocharging are proven strategies to accomplish this goal. However, the ability to achieve elevated peak pressures required by either mechanism to be effective is limited by knock. The lack of understanding of knock also hinders the realization of potential benefits of homogeneous charge compression ignition, a promising technology that relies on controlled autoignition. Thus, knock is one of the most serious obstacles in the development of fuel efficient engines. For this reason, the phenomenon of knock has been studied extensively, but even after more than a century of mostly experimental research, the basic mechanism governing knock remains poorly understood. In order to develop a fundamental understanding of engine knock, detailed chemical kinetic modeling of the hydrocarbon oxidation mechanism associated with the autoignition process is conducted in CHEMKIN (a chemical kinetics software). Based on the insight gained from kinetic modeling, some of the key reactions and species that are instrumental to the autoignition of hydrocarbons are identified. The sensitivity of knock to various parameters including inlet pressure, inlet temperature, compression ratio, wall temperature, fuel-air equivalence ratio, and exhaust gas recirculation (EGR) is examined through CHEMKIN simulations. Ignition delay predictions for the autoignition of a toluene reference fuel (TRF) blend with an antiknock index of 91 (TRF 91), obtained through extensive chemical kinetic modeling in CHEMKIN for a constant volume reactor, are used to develop an improved ignition delay correlation for predicting knock in spark ignition (SI) engines. In addition to NOx control, EGR is increasingly being utilized for managing combustion phasing in SI engines to mitigate knock. Therefore, along with other operating parameters, the effects of EGR on autoignition are incorporated into the correlation to address the need for predicting ignition delay in SI engines operating with EGR. The modeling approach adopted for TRF 91 is then extended to develop an ignition delay correlation for an oxygenated surrogate fuel blend of 87 octane gasoline (with 10% ethanol). In addition, a conceptually new approach based on multiple timescales is developed to predict ignition delay for the autoignition of a primary reference fuel blend. Finally, the new ignition delay correlation for TRF 91 is implemented into the engine simulation tool GT-POWER and engine dynamometer experiments with knocking combustion are conducted to validate the knock predictions from the correlation. Comparison of knock onset predictions from GT-POWER with engine experiments illustrates the accuracy of the TRF 91 ignition delay correlation. Hence, the contributions of the present study include an enhanced understanding of the underlying physics governing knock, development of improved ignition delay correlations, and better knock predictions from engine simulations through implementation of the TRF 91 correlation in GT-POWER.

Committee:

Ahmet Selamet, PhD (Advisor); Ronald Reese, II (Committee Member); Jeffrey Sutton, PhD (Committee Member); Junmin Wang, PhD (Committee Member); Sheng-Tao Yu, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Knock; Spark Ignition Engines; Combustion; Ignition Delay; Kinetics; Ignition Delay Correlation

Nagulapalli, AdityaMethylcyclohexane Ignition Delay Times Under a Wide Range of Conditions
Master of Science (M.S.), University of Dayton, 2015, Mechanical Engineering
During the last century, our dependence on oil has increased rapidly and is projected to increase for several decades. There is a critical need to improve the design of the combustion chamber for different kinds of engines to reduce fuel consumption. Chemical kinetics of the fuel plays an important role in reducing emissions and improving engine efficiency. Studying single components of a conventional fuel allows a fuller understanding of the physical and chemical behavior of the real fuel. Many studies have been conducted on all classes of hydrocarbons, with the exception of cycloalkanes. Only a few studies exist on cycloalkanes, which is an important class of hydrocarbons. Methylcyclohexane (MCH), which is widely used as a surrogate to represent the cycloalkane portion of a fuel, was chosen as the subject of this study. The shock tube is an established tool used for measuring the ignition delay, and was used as the experimental apparatus. Ignition delay was measured using the end-plate pressure rise, the OH* and CH* chemiluminescence and white light emission. In addition, experimental results were compared with kinetic modeling data using detailed MCH mechanisms developed by Pitz et al. and Orme et al. Different modeling approaches, such as constant volume and internal energy (with and without experimental pressure profiles) and constant pressure, were used to validate the models by comparing against experimental ignition delay data. It was observed that the equivalence ratio affects the ignition delay time. For the lower argon concentration (Ar = 93%) and higher pressure (P ~ 16 atm), ignition delay times were longest for rich conditions. Additionally, they were shorter at lower temperatures (T = 1250 K) for stoichiometric conditions in comparison to lean values, but the opposite trend was observed at the higher temperatures (T > 1250 K). Ignition delay times of stoichiometric mixtures were longer than lean mixtures across the studied temperature range for low pressure (P = 2 atm) and argon concentration (Ar = 93%), as well as high pressure (P ~ 16 atm) and argon concentration (Ar = 98%). The Orme et al. model using the approach of constant U,V assumption with experimental pressure profile showed a better agreement with experimental results at low temperatures than the approach without experimental pressure profile. Both models and approaches underestimate the experimental ignition delay times at high temperatures.

Committee:

Sidhu Sukh S, Ph.D (Advisor); Philip Taylor. H. , Ph.D (Committee Member); Moshan Kahandawala, Ph.D (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

Methylcyclohexane; Ignition Delay; Jet Fuels; Surrogate Fuels; Shock Tube; Hydrocarbons;

Balagurunathan, JayakishanInvestigation of Ignition Delay Times of Conventional (JP-8) and Synthetic (S-8) Jet Fuels: A Shock Tube Study
Master of Science (M.S.), University of Dayton, 2012, Mechanical Engineering
The global depletion of petroleum-based fuels has led the world to more closely examine alternate fuels. Therefore, alternate fuels produced from feedstocks such as coal, soybeans, palm oil or switch grass through methods such as coal liquefaction, biomass gasification, and Fischer-Tropsch synthesis have been tested. Among these techniques, fuels generated using Fischer-Tropsch technologies are of interest because they produce clean burning hydrocarbons similar to those found in commercial fuels. Therefore, in this study the Fischer-Tropsch derived S-8 fuel was evaluated as a drop-in replacement for the jet fuel JP-8. The jet fuel JP-8 is comprised of n-, iso- and cyclo- alkanes as well as aromatics while the S-8 fuel is primarily comprised of n- and iso- alkanes. The composition of the fuel affects its ignition characteristics chemically and physically by either advancement or delay of time to ignition. Since this study focused on the chemical effects, the fuels were completely pre-vaporized and pre-mixed. A high pressure, high temperature heated single pulse shock tube was used for this study. The shock tube is an established experimental tool used to obtain ignition delay data behind reflected shock waves under operating conditions relevant to modern engines. The experiments were conducted over a temperature range of 1000-1600 K, a pressure of 19±2 atm, equivalence ratios of 0.5, 1 and 3, within a dwell time of 7.6±0.2 ms and an argon dilution of 93% (v/v). Ignition delay times were measured using the signal from the pressure transducer on the end plate with guidance from the optical diagnostic signal. Along with JP-8 and S-8, the ignition delay of n-heptane was also studied. N-heptane was chosen to represent the n-alkanes in the fuels for this study since it was present in both fuels and also to prove the fact that the n-alkanes were rate controlling. The results indicate that both S-8 and JP-8 fuels have similar ignition delays at corresponding equivalence ratios. The fuel-rich mixtures ignited faster at lower temperatures (<1150 K) and the fuel-lean mixtures ignited faster at higher temperatures (>1150 K). In the transition period between lower to higher temperatures (~1100-1200 K), the equivalence ratio had no significant effect on the ignition delay time. The results also show that the ignition delay time measurements of S-8 and JP-8 fuels are similar to the ignition delay of n-heptane at the equivalence ratio of Φ=0.5 and thereby indicate that the n-alkanes present in these fuels controlled the ignition under these conditions. The ignition delay results of S-8 and JP-8 at Φ=3.0 from this study were also compared to prior work (Kahandawala et al., 2008) on 2-methylheptane and n-heptane/toluene (80/20 liquid vol.%), respectively and found to be indistinguishable. This data serves to extend the gas phase ignition delay database for both JP-8 and S-8 and is the first known data taken for both these fuels at higher temperatures (>1000 K) for an equivalence ratio of 3.0 with argon as the diluent gas.

Committee:

Sukh Sidhu, Dr (Committee Chair); Philip Taylor, Dr (Committee Member); Moshan Kahandawala, Dr (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Alternative Energy; Automotive Engineering; Automotive Materials; Chemical Engineering; Chemistry; Energy; Engineering; Environmental Engineering; Mechanical Engineering; Petroleum Engineering; Technology

Keywords:

Ignition delay; shock tube; S-8; JP-8; Jet fuels; Fuel characteristics; heated shock tube; Fischer-Tropsch; Alternate fuels; alkanes; synthetic fuel; fuel; iso-alkanes; jayakishan balagurunathan

Flora, GiacomoFuel Structure Effects on Surrogate Alternative Jet Fuel Emission
Doctor of Philosophy (Ph.D.), University of Dayton, 2015, Mechanical Engineering
The emergence of alternative jet fuels has opened new challenges for the selection of practical alternatives that minimize the emissions and are suitable for existing gas turbine engines. Alternative jet fuels are in the early stages of development, and little fundamental emissions data are currently available. An accurate knowledge of their combustion behavior is highly important for a proper fuel selection based on emissions. This dissertation work investigated the oxidation of different alternative fuel surrogates composed of binary mixtures in order to correlate fuel composition with emissions. The proposed surrogate mixtures included n-dodecane/n-heptane (47.5/52.5 by liq. vol.), n-dodecane/iso-octane (47.9/52.1 by liq, vol.), n-dodecane/methylcyclohexane (49/51 by liq. vol.) and n-dodecane/m-xylene (75/25 by liq. vol.) mixtures. Experiments were carried out at the UDRI heated shock tube facility, and covered a pre-ignition temperature range of 950–1550 K at a pre-ignition pressure of ~16 atm, an equivalence ratio of 3, an argon concentration of 93% (by mol), and under homogeneous gas-phase conditions. Experimental data were modeled using the 2014 SERDP mechanism for jet fuel surrogates (525 species and 3199 reactions). Similar ignition delay times were measured for the tested surrogate blends, confirming previous observations regarding the controlling role of normal alkanes during the induction period. The experimental observation was also compared with modeling results reporting reasonably good agreements. A kinetic analysis of the SERDP 2014 mechanism was also performed, highlighting the major chemical pathways relevant to the pre-ignition chemistry, especially the role of the hydroperoxyl radical at the low temperatures. A wide speciation of combustion products was also carried out under the test conditions. All the aliphatic blends reported similar emissions, whereas the presence of m-xylene produced lower emissions than the aliphatic surrogate blends at lower temperatures. For certain species (light gases) this experimental observation was also supported by the kinetic mechanism predictions. However, aromatic species formed from combustion of n-dodecane/m-xylene surrogate blend were always overestimated by the model and in poor agreement with experimental observations. The results also confirmed the role of acetylene as assisting growth of large PAHs and formation of soot.

Committee:

Sukhjinder S. Sidhu, Ph.D. (Committee Chair); Kahandawala Moshan S. P, Ph.D. (Committee Member); Dewitt Matthew J., Ph.D. (Committee Member); Stouffer Scott, Ph.D. (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

combustion; jet fuel surrogates; ignition delay time; emissions; shock tube