Volume 229, Issue 6 - Special Issue: Combustion, Part 3 Dedicated to Professor Henning Bockhorn on the occasion of his 70th birthday / Eva Gutheil, Hans-Jürgen Troe

Experimental and kinetic modeling studies are carried out to characterize nonpremixed and premixed combustion of producer gas and mixtures of producer gas and methane. The producer gas, employed in the study, is made up of 55.00% carbon monoxide, 2.34% hydrogen, 12.72% methane, 5.24% ethene, and 24.7% carbon dioxide by mass. The primary focus is on characterizing the chemical influence of addition of producer gas on combustion of methane. The kinetic modeling studies are carried out employing a detailed chemical-kinetic mechanism, called the San Diego Mechanism, a skeletal mechanism, and a reduced mechanism made up of five global steps. Experiments on nonpremixed combustion are carried out employing the counterflow configuration. Critical conditions of extinction are measured for producer gas, methane, and mixtures of producer gas and methane. They are compared with predictions obtained using the detailed, skeletal, and reduced mechanism. Critical conditions of autoignition are measured for producer gas and compared with the predictions obtained employing the detailed mechanism. Experimental data and predictions show that with increasing amounts of producer gas in the mixture, the flame is more difficult to extinguish. Flame structures show that at a fixed value of the strain rate, leakage of oxygen from the reaction zone decreases with increasing amounts of producer gas in the combustible mixture. This is attributed to enhanced consumption of oxygen in an overall chain branching step that consumes hydrogen. Thus, the increase in the overall reactivity of the combustible mixture is attributed to presence of hydrogen in producer gas. Computations are performed, using the detailed mechanism and the skeletal mechanism, to investigate aspects of premixed combustion of stoichiometric mixtures of producer gas, methane, oxygen, and nitrogen at fixed values of adiabatic temperature. Burning velocities are calculated. They are found to be less sensitive to the amount of producer gas in the mixture. This is qualitatively different from that observed for nonpremixed combustion. It is attributed to complete consumption of all reactants including oxygen and combined influences of H 2 and CO in producer gas on overall combustion of methane. A third set of computations was performed on strained premixed flames stabilized in counterflow between a stream of a stoichiometric mixture of producer gas, methane, oxygen, and nitrogen, and a stream of nitrogen at fixed adiabatic temperature. Critical conditions of extinction were obtained. The strain rate at extinction increased with increasing amounts of producer gas. The increase was comparable to that observed for nonpremixed combustion of these fuels.

Nowadays the mathematical description of chemically reacting flows uses very often reaction mechanisms with far above hundred or even thousand chemical species (and, therefore, a large number of partial differential equations must be solved), which possibly react within more than a thousand of elementary reactions. These chemical kinetic processes cover time scales from nanoseconds to seconds. An analogous scaling problem arises for the length scales. Due to these scaling problems the detailed simulation of three-dimensional turbulent flows in practical systems is beyond the capacity of even today's super-computers. Using simplified sub-models is a way out of this problem. The question arising in mathematical modeling of reacting flows is then: How detailed, or down to which scale has each process to be resolved (chemical reaction, chemistry-turbulence-interaction, molecular transport processes) in order to allow a reliable description of the entire process. Both the chemical source term and the transport term have one important property, namely, they cause the existence of low-dimensional attractors in composition space. When these manifolds can be constructed (described) and parametrized by a small number of variables, it can be used to reformulate and reduce the mathematical description for modeling reacting flows. In this work the hierarchical nature of these low-dimensional manifolds of slow motions is discussed. It is demonstrated how this important feature of reacting flows is accounted for by the standard model reduction methods (like e.g. PEA and QSSA methods) as well as by recently developed concepts of model reduction. The use of the hierarchical nature for identification of the low-dimensional manifolds to devise hierarchical modeling concepts (e.g. for turbulent reacting flows) is additionally discussed.

In the present numerical study laminar oxygen-enhanced and oxy-fuel CH 4 /N 2 –O 2 /N 2 non-premixed flames were systematically investigated in counter-flow configuration. The compositions of the fuel and the oxidizer stream were chosen in a way, which allows a study of the flame structure at different flame temperatures in a range of approximately 1800–3000 K at constant stoichiometric mixture fraction of Z st  = 0.2. The calculations were performed with the GRI 3.0 as a base kinetic mechanism, while selected cases were also investigated with the Appel et al. mechanism and the GDFkin ® 3.0. The flame structure, extinction and flame broadening effects were investigated for various strain rates by analysing the heat release rate and temperature profiles. Four different heat release rate zones were identified, namely the radical chain branching, the main water and carbon dioxide formation, the fuel oxidation and the fuel pyrolysis zone. While those zones overlap in case of air-combustion, a separation accompanied by a flame broadening is found in oxygen-enhanced flames as a result of the higher flame temperatures. The four heat release zones get affected dissimilarly by increased oxygen content and flame temperature, while the flame broadening is dominated by the shift of the radical branching zone towards the oxidizer side. A comparison between air combustion and pure oxygen combustion shows that the flame is broadened by a factor of approximately 10 as long as the flame conditions are not close to the extinction limits. With increasing strain rate flame extinction starts from the oxidizer side. An increased radical pool and temperature level in the chain branching zone extents the extinction limits towards higher strain rates in oxygen-enhanced flames. On the fuel side, the increased oxygen-content with the associated higher temperature level leads to a strong endothermic zone and an intense activation of the C 2 -pathway.

Alternatives to traditional petroleum derived transportation fuels, particularly alcohols, have been investigated increasingly over the last 5 years. Isopropanol has received little attention despite bridging the gap between smaller alcohols (methanol and ethanol) and the next generation alcohols (butyl alcohols) to be used in transportation fuels. Previous studies have shown that decomposition reactions that dehydrate are important in the high-temperature oxidation of alcohols. Here we report new data on the dehydration reaction for isopropanol ( i C 3 H 7 OH → C 3 H 6  + H 2 O) in a Variable Pressure Flow Reactor at 12.5 atm pressure and temperatures from 976–1000 K. Pyrolysis experiments are performed in the presence of a radical trapper (1,3,5 trimethyl benzene or toluene) to inhibit secondary reactions of radicals with the fuel and product species. The recommended rate constant for the dehydration reaction is determined using an indirect method along with Latin Hypercube sampling to estimate uncertainties. Comparison of the rate constant data to previous works show that the reaction is considerably more rapid than the high level theoretical predictions of Bui et al. (Bui et al., J. Chem. Phys., 2002). The dehydration reaction rate for isopropanol is well described by k  = 8.52 × 10 6 T 2.12  exp (− 30, 667/ T ) with an estimated uncertainty of σ   ln   A 2  = 0.0195. The C–C bond fission reaction is also investigated, but the insensitivity of the decomposition data to this reaction results in an uncertainty in the determined rate constants to approximately 2 orders of magnitude. Theoretical estimates lie within these experimental uncertainties.

The present work is focused on the airblast atomization in a gas turbine model combustor. Swirl flows in such combustion chambers form an inner recirculation zone that can develop a precessing vortex core. This instability affects the velocity field at the exit of the nozzle, where the fuel is atomized. The influence of the precessing vortex core on the airblast atomization is examined in this work. The flow field in the combustion chamber is examined under reacting and non-reacting conditions. The velocity amplitudes under reacting conditions are more than twice as high compared to non-reacting conditions. The influence of the precessing vortex core on the airblast atomization process is examined in detail at an atomization test rig with a perspex nozzle. High speed shadowgraphy and POD analysis have been performed of the liquid film on the prefilmer as well as on the ligaments formed at the atomizing edge. It was observed that although the PVC does affect the liquid film flow of the prefilming surface, it does not affect the breakup into ligaments, i. e. primary atomization.

A space-time adaptive method is presented for the reactive Euler equations describing chemically reacting gas flow where a two species model is used for the chemistry. The governing equations are discretized with a finite volume method and dynamic space adaptivity is introduced using multiresolution analysis. A time splitting method of Strang is applied to be able to consider stiff problems while keeping the method explicit. For time adaptivity an improved Runge–Kutta–Fehlberg scheme is used. Applications deal with detonation problems in one and two space dimensions. A comparison of the adaptive scheme with reference computations on a regular grid allows to assess the accuracy and the computational efficiency, in terms of CPU time and memory requirements.

Partial homogeneous oxidation of methane (CH 4 ) within stationary engines may be one concept for conversion of available energy to alternatively mechanical energy, heat, and additional useful chemicals like syngas (CO/H 2 ), formaldehyde (CH 2 O), methanol (CH 3 OH) or hydrocarbons (e.g. C 2 H 4 ). The present study investigates the formation reactions of chemicals experimentally and theoretically. Methane oxidation is studied under fuel-rich conditions ( ϕ  = 17.50–22.25) at high pressures (6 bar) and high temperatures ( T   max   = 1030 K) for long residence times in a tubular reactor. The gas composition is determined experimentally by time-of-flight mass spectrometry for different reactor temperatures. Through variation of reactor temperature an overview of the maximum mole fractions of target chemicals, the temperature of observed reaction onset, and the optimal temperature to increase target yields can be determined. The experimental results are compared to kinetic simulations of the methane conversion using literature mechanisms to assess how well the data are reproduced for these uncommon reaction conditions. The potential of activating the conversion reactions with ethane (C 2 H 6 ) and propene (C 3 H 6 ) as additives is investigated. Methanol is chosen as one target compound. Its yield is increased by both additives. In addition, propene as additive reduces the temperature of reaction onset in the experiments and in the simulation. CH 2 O and C 2 H 4 can be identified as other useful chemicals produced in the experiments and the influence of the additives on the yields is discussed.

The high temperature sensitivity of the luminescence emission of thermographic phosphor (TP) particles favours its utilization for the measurement of wall or gas temperatures in combustion devices. The present work focuses on the luminescence behavior and suitability of two lanthanide ion types (Tm and Dy) doped in a YAG host crystal for high temperature applications. The Dy:YAG is a well-known thermographic phosphor and is used here as a reference for the comparison with Tm:YAG TP. Additionally its dopant concentration is varied to improve its signal intensity for high temperature applications. For the studied thermographic phosphors phosphorescence spectra, intensity ratios and luminescence decay time are characterized for a wide temperature range up to 1573 K.

The addition of OH radical to benzene and other aromatic species to form a cyclohexadienyl radical ( C•HD -OH ), is the first step for conversion (oxidation) of aromatic species in atmospheric chemistry. Reaction of this C•HD -OH intermediate, with ground-state molecular oxygen forms a number of hydroxyl-cyclohexadienyl peroxy ( CHD-OH -OO•) isomers, which further react to form aerosols. Ab initio and density functional calculations were used to study the structures and thermochemical properties ( Δ f H o 298 , S o ( T ) and C p ( T ) of the hydroxyl-cyclohexadienyl peroxy isomers ( CHD-OH -OO•) ((i) para- cis -(Z) and para- trans -(E) (OO• relative to the OH group); (ii) ortho- cis -(Z) and ortho- trans -(E) at pseudo-equatorial (e') and pseudo-axial (a') positions). The thermochemical properties are important for modeling studies on the kinetics of the aerosol formation. Molecular structures and vibration frequencies were determined at the B3LYP/6-31G(d,p) level. Energy calculations were performed at the CBS-lq, CBS-4, G3(MP2)//B3LYP/6-31G(d,p), and BLYP/6-311++G(2d,p)//BLYP/6-31G(d,p) levels. Enthalpies of formation ( Δ f H o 298 ) were calculated at each level using group balance isodesmic reactions. An analysis is performed for entropy from inclusion of internal rotor analysis versus use of torsion frequencies and conformer statistics. Standard entropy, S o ( T ), and heat capacity, C p ( T ), from vibrational, translational, and external rotation contributions were calculated using statistical mechanics based on the vibration frequencies and structures. Hindered rotational contributions to S o ( T ) and C p ( T ) were calculated from the energy levels, where the internal rotation potential was calculated at the B3LYP/6-31G(d) level. This analysis of the hindered internal rotor contributions yields higher entropy than the values calculated when the rigid-rotor-harmonic-oscillator approximation (RRHO) applied for the calculated torsion frequencies. Differences between the uses of RRHO for torsions with correction for statistical mix of conformations, versus hindered internal rotor analysis are as high as 3.5 cal mol – 1  K – 1 in entropy and 3.6 cal mol – 1  K – 1 in heat capacity at 50 ≤  T /K ≤ 5000. A number of intramolecular interactions are found to stabilize the CHD-OH -OO• isomers: (i) intramolecular hydrogen bonding (between the OH and the peroxy OO• groups, OO–H–O), (ii) a negative hyperconjugation where the lone pair on oxygen atom in OH group is hyperconjugated with the antibonding σ * orbital of adjacent C=C bond, (iii) an anomeric interaction in which antibonding σ * orbital of C–O bond in a pseudo-axial position with a neighboring double bond π C–C . The eclipsing steric repulsion between OH and OO• groups counteract the stabilizing hydrogen bonding interaction. The importance of these effects are evaluated in determining the thermodynamic stability of the CHD-OH -OO• isomers. Evaluations of data from the isodesmic reaction at G3(MP2)//B3LYP/6-31G(d,p) level, results in the enthalpy of formation of the most stable isomers of CHD-OH -OO• in a range of − 0.47 to −3.62 kcal mol – 1 at 298 K (for (Z)-o- II , −0.47 kcal mol – 1 and (E)-o- II , −3.62 kcal mol – 1 ). The reaction energies ( Δ r H o 298 ) of hydroxycyclohexadienyl radical ( C•HD -OH ) + O 2  ↔  CHD-OH -OO• isomers were determined to be − 10.63 ∼ − 13.67 (− 10.56 ∼ − 13.34) kcal mol – 1 . Entropy ( S o 298 ) of CHD-OH -OO• was in range of 93.17 ∼ 96.18 (90.82 ∼ 95.55) cal mol – 1  K – 1 . Data in parentheses are based on RRHO/torsion frequency approximation with correction for the mix of conformers. The literature values of reaction energy ( Δ r H o 298   K ) and entropy ( Δ r S o 298   K ) were reported −10.5 ± 1.3 kcal mol – 1 and −33.9 ± 1.4 cal mol – 1  K – 1 , respectively, based on second-law analysis at temperature between 265 and 345 K.