Jeffrey Santner

Effect of Water Content on Syngas Combustion at Elevated Pressure

This work experimentally and numerically investigates the effect of water addition up to a mole fraction of 0.15 on the burning rates of H2 and H2/CO mixtures from 1 atm 10 atm at a flame temperature near 1600K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with the newly updated hydrogen kinetic model for H2 flames, and H2/CO flames. The results show a monotonic decrease in mass burning rate with water addition at pressures above 1 atm, as well a decrease in the pressure at which the maximum burning rate occurs. Path analyses reveal the influence of water addition on the radical pool through H2O+O=2OH.

Development of Reduced Kinetic Models for Petroleum-Derived and Alternative Jet Fuels

The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fuels derived from coal and natural gas, and also to the pressure and equivalence ratio dependences of the combustion responses of conventional Jet–A fuel. The formulation of surrogate fuels for Syntroleum S-8, Shell SPK and Sasol IPK, is described. Assuming these compositions, a detailed chemical kinetic model construction previously elaborated upon is extended and tested against reference data sets of shock tube ignition delay and laminar burning velocity. Calculations with the detailed kinetic model, containing 3147 species correctly represent the experimentally measured reactivity of the target fuels for shock tube ignition delay. The model also captures trends in the ignition delay for a reference Jet-A as a function of pressure and equivalence ratio. The earlier reported detailed model is expanded to encompass a range of n-alkane carbon numbers up to C16 and iso-cetane. The expanded model is validated against available shock tube ignition delay in detailed form and against laminar burning velocity datasets using a series of numerically reduced models of decreasing dimension for n-hexadecane, iso-cetane, and their mixtures. Though the detailed model reproduces the general kinetic behavior for the ignition delays of each jet fuel, the predicted values are generally longer than experimental results. A series of reduced models of the order of 100 species in size, are produced for simulation of flame environments. Calculations for laminar premixed flames for each jet fuel are similar with burning velocities for IPK flames marginally lower than those for the conventional Jet-A which in turn are marginally lower than those for S-8. The requirement for severely reduced, but high fidelity chemical kinetic numerical schemes that retain predictive capacities for the combustion behaviors of real liquid transportation fuels is addressed through the introduction of a strategy to produce “compact” models of the order of 35 species. The strategy utilizes calculations of the detailed model construct as a fundamental and scientific standard, to which engineering approximations achieved through adjusting reaction rates and omitting or diverting the fate of select reaction pathways at high carbon numbers are applied. The strategy is tested for the exemplar real fuel test case of the S-8 ignition delay and laminar

Comparative Evaluation of Global Combustion Properties of Alternative Jet Fuels

Global high temperature and low temperature combustion properties of one conventional jet fuel (JP-8) and four alternative jet fuels are examined. The global combustion characteristics of these five fuels were hypothesized a priori based on a set of four property targets used in the formulation of surrogate jet fuels in recent studies by the authors. The high temperature combustion properties were evaluated by measuring two near-limit flame behaviors: the radical index derived from extinction limits of diffusion flames, and the critical flame initiation radius in outwardly propagating premixed flames. Two fundamental flame measurements reveal that Shell SPK exhibits the strongest high temperature reactivity, whereas Sasol IPK exhibits the lowest reactivity among the tested fuels. The relative low to intermediate temperature reactivities of these five jet fuels were compared by performing oxidation reactivity experiments in a high pressure flow reactor. The oxidation reactivity profiles demonstrated that Sasol IPK has no low temperature reactivity. The remaining jet fuels demonstrated extensive reactivity between 500 to 750 K. Of interest, despite sharing similar derived cetane numbers, Shell SPK shows a more pronounced low temperature reactivity compared to HRJ Camelina. Consequently, the results in this study indicate that the proposed four property targets can be used to predict the global combustion properties of petroleum derived jet fuels and perhaps for blends with alternative jet fuels up to 50%, but further refinements appear to be needed to precisely predict the temperature dependencies of global combustion properties of alternative jet fuels containing no aromatics.

Characterization of Global Combustion Properties with Simple Fuel Property Measurements for Alternative Jet Fuels

Global combustion characteristics of one conventional jet fuel (JP-8) and four non-petroleum alternative jet fuels (Shell Synthetic Paraffinic Kerosene (SPK), Sasol Iso-Paraffinic Kerosene (IPK), Hydrotreated Renewable Jet (HRJ Camelina and HRJ Tallow)) are experimentally examined. The ranking of the fully pre-vaporized global combustion characteristics of these five fuels has been hypothesized

Measurements and Modeling of the Laminar Flame Speeds of n-Propyl and 1,3,5-TriMethyl Benzenes at Moderate Pressures

Aromatics are major components of real and surrogate jet fuels and can produce rich stabilized radicals to inhibit the mixture reativity. The laminar flame speeds of the two aromatic C 9H12 isomers, n-Propylbenzene and 1,3,5-TriMethyl benzene are measured at atmospheric pressure. A kinetic model have been developed, and tested against these new experimental data as well as against previously published data. Both experiments and computations show that n-Propylbenzene exhibits higher flame speeds than the other isomer, due to the weaker bonds in the propyl side chain. The increase of pressure results in an increased inhibitive effect on the flame velocities for both fuels because of the competition between the two reactions H+O 2=O+OH and H+O 2(+M)=HO 2(+M). The oxidation pathway of the 1,3,5-triMethyl benzene isomer has been delineated and is computed to be little sensitive to the pressure. However, results show that the degradation scheme of n-Propyl benzene is observed to be more strongly pressure dependent.

HP-Mech: A High Pressure Kinetic Mechanism for C2 Flames with Exhaust Gas Dilution

This work represents continued development of a high pressure combustion chemistry mechanism (HP-Mech) for C2 mixtures with exhaust gas recirculation (EGR) at Princeton University. New burning velocity measurements were performed for ethane flames using the expanding spherical flame method at pressures from 1 to 20 atm with carbon dioxide and water vapor dilution. By holding the adiabatic flame temperature constant with dilution, the chemical effects of these diluents are investigated. These new measurements, as well as previous work with acetylene and ethylene, are used to improve and validate HP-Mech for C2 fuels.

Experimental Assessment of Transport and Chemical Kinetic Impacts on Critical Flame Initiation Radius in Outwardly Propagating Premixed Flames

This work experimentally and numerically investigates the effects of fuel specific chemistry and transport properties on the critical flame initiation radius and associated critical strain rate. To isolate the effects of chemistry from those of transport, two sets of fuel comparisons are performed. Transport effects are varied without altering chemistry by comparing n-decane with n-heptane, chemical kinetic effects by comparing the C9 alkylated benzene isomers, 1,3,5-trimethylbenzene vs. n-propylbenzene. Measurements and simulations are performed using outwardly propagating spherical flames in air at 1 atm and 400 K preheat. A considerable kinetic effect is identified as a lower critical strain rate and larger critical radius for 1,3,5-trimethylbenzene as compared to n-propylbenzene. Transport effects in n-alkanes appear only for the critical radius, but were minimal for critical strain rates. A linear relationship is found between the critical strain rate and the maximum OH radical in the stretched flame, confirming that fuel specific chemical kinetics and its subsequent impact on radical pool population drive the behavior of the critical strain rate at the conditions investigated. A radical index for premixed flames has been further derived based on the rate of OH production and incorporated with transport-weighted enthalpy to interpret the behaviors of the critical radius for all tested fuels. It is found that that the critical radius is a fundamental property of unsteady premixed flame propagation specific to the chemical kinetics of fuel oxidation.

The Critical Radius of Flame Initiation and Propagation for n-Decane/Air Premixed Flames

The initiation outwardly propagating spherical flames has been studied experimentally and numerically using n-Decane/air mixtures at atmospheric pressure. The flame trajectories, laminar flame speeds, and the critical radius have been measured and simulated at various equivalence ratios. A kinetic model has been used to study the flame structure of transient flame initiation to propagation phenomena. Both experiments and computations show that the Lewis number (preferential diffusion of heat and mass affects) both spherical flame initiation and flame kernel evolution after ignition. It is found that the generation of the radical pool plays important role in initiation of the flame kernel and sequential transition from ignition, unstable branch, and to the normal stable flame regimes. Paticualarly, the critical radius for the successful flame initation is investigated and the results indicate that the outwardly propagating spherical flames in a stable branch can be achieved by overcoming the critical stretch rates at the critical radius.

High Pressure Burning Rates and Model Comparisons for High-Hydrogen CO, CH 4 , C 2 H 4 , and C 2 H 6 Mixtures

Experimental measurements of burning rates and analyses of key reaction pathways and sensitive rate constants were performed for lean hydrogen flames with the addition of small concentrations of C1-C2 hydrocarbons. Pressures from 1-25 atm and (diluted) low flame temperatures were studied. The species added individually to hydrogen were CO, CH4, C2H4, and C2H6, each at a molar ratio of H2 to additive of nine to one. Flame speeds were measured using outwardly propagating flames. While results show good agreement of all model predictions with experimental data at 1 atm, large discrepancies among model predictions and with experimental data are observed at higher pressures. Kinetic analyses were performed to identify controlling kinetic pathways and major sources of disagreement. Differences in the H2 submodel were demonstrated to be largely responsible for differences among various model predictions, though predictions of the various models using a common H2 submodel still yield burning rate predictions that vary substantially. Sensitivity analysis indicates that reactions of CH3 with major radical species, CH3+H(+M)=CH4(+M) and CH3+HO2=CH3O+OH, and HCO consumption pathways, HCO+M=H+CO+M and HCO+O2=CO+HO2, become increasingly important with hydrocarbon addition to H2/additive mixtures in flames at high pressure and low flame temperature.