Oxidation of the 1‐naphthyl radical C₁₀H₇• with oxygen: Thermochemistry, kinetics, and possible reaction pathways

The reaction of the 1-naphthyl radical C10H7• (A2•) with molecular (3O2) and atomic oxygen, as part of the oxidation reactions of naphthalene, is examined using ab-initio and DFT quantum chemistry calculations. The study focuses on pathways that produce the intermediate final products CO, phenyl and C2H2, which may constitute a repetitive reaction sequence for the successive diminution of six-membered rings also in larger polycyclic aromatic hydrocarbons. The primary attack of 3O2 on the 1-naphthyl radical leads to a peroxy radical C10H7OO• (A2OO•), which undergoes further propagation and/or chain branching reactions. The thermochemistry of intermediates and transition state structures is investigated as well as the identification of all plausible reaction pathways for the A2• + O2 / A2• + O systems. Structures and enthalpies of formation for the involved species are reported along with transition state barriers and reaction pathways. Standard enthalpies of formation are calculated using ab initio (CBS-QB3) and DFT calculations (B3LYP, M06, APFD). The reaction of A2• with 3O2 opens six main consecutive reaction channels with new ones not currently considered in oxidation mechanisms. The reaction paths comprise important exothermic chain branching reactions and the formation of unsaturated oxygenated hydrocarbon intermediates. The primary attack of 3O2 at the A2• radical has a well depth of some 50 kcal mol−1 while the six consecutive channels exhibit energy barriers below the energy of the A2• radical. The kinetic parameters of each path are determined using chemical activation analysis based on the canonical transition state theory calculations. The investigated reactions could serve as part of a comprehensive mechanism for the oxidation of naphthalene. The principal result from this study is that the consecutive reactions of the A2• radical, viz. the channels conducting to a phenyl radical C6H5•, CO2, CO (which oxidized to CO2) and C2H2 are by orders of magnitude faster than the activation of naphthalene by oxygen (A2 + O2 → A2• + HO2)

3D direct pore level simulations of radiant porous burners

Inside porous burners, chemical combustion reactions coincide with complex interaction between thermo-physical transport processes that occur within solid and gaseous phase and across phase boundary. Fluid flow, heat release and resulting heat flows influence each other. The numerical model used in this work considers gaseous and solid phases, includes fluid flow, enthalpy transport, conjugate heat transfer, and radiative heat transfer between solid surfaces, as well as combustion kinetics according to a skeletal chemical reaction mechanism, fully resolved on the pore scale in three-dimensional space (Direct Pore Level Simulation, DPLS). The calculations are performed based on the finite volume method using standard applications implemented in the OpenFOAM library. The present study presents simulations of three different structures, each at four settings of specific thermal power. Results indicate that specific surface area of the porous structure is a major influencing parameter for increasing radiation efficiency, whereas no correlation of the orientation of an anisotropic structure on radiation efficiency was observed

A CFD Study of the Performance of Horizontal Dilution Tubes

Since about 20 years, horizontal dilution tubes are in use to study soot formation close to the main reaction zone, in order to characterize the properties of nascent soot nanoparticles and to obtain insight into the soot formation process. In this study, the performance of horizontal dilution tubes, both free standing and embedded, is investigated by RANS and LES. The flame gas enters the dilutionctube through a pinhole and, in experimental studies, it is claimed to mix quickly with the cold, inert gas flow in the dilution tube. Previously, the distortion of the flow and temperature profiles around the dilution tube were investigated. Here, the orifice flow as well as the dilution process inside the tube are studied. The volume flow through the orifice is shown to be proportional to the square root of the pressure drop. The discharge coefficient is the range 0.9 0.3 in the cold air (calibration) case and drops to 0.35 under hot (flame) conditions. The resulting dilution ratio is roughly a factor of 5 below typical literature data. The gas sample is found to remain in the wall boundary layer and, the mixing process is not complete at the end of the dilution tube. Turbulence decays rapidly behind the tube inlet and, in the main body of the tube, the flow is in the laminar to turbulent transition regime. Turbulence increases significantly in the outlet section which has much smaller pipe cross sections. Despite its relatively low Reynolds number, the outlet flow to the particle sizer (or to the gas analyzer) is clearly turbulent and, interactions with the wall are probable. The results are in agreement with previouscfindings from laminar jets in crossflow. Guidelines for optimization of the sampling conditions are suggested

Simultaneous Compression and Absorption for Energy‐Efficient Dissolution of Gases in Liquid

In this study, a novel approach for energy-efficient dissolution of gases in liquid is presented, which significantly reduces the compression work. The core of the one-step process is the simultaneous operation of compression and absorption. The liquid was injected into a cylinder filled with the gas, while a piston compressed the mixture during the injection time. The solubility increases with increasing system pressure, so that the compression work of the gas phase is permanently reduced on the one hand by the permanent reduction of the gas volume and on the other hand by the nearly isothermal compression process. The approach is demonstrated in this study using liquid H$_{2}$O and gaseous CO$_{2}$ compressed up to 10 bar. The theoretical energy savings of the novel process compared to the conventional two-stage process is 41.2 % for the selected fluids. A maximum energy saving of 40.8 % was demonstrated in the experiments. The results also show that the energy saving depends on the curve of the piston speed and the injection time

Super-adiabatic flame temperatures in premixed methane-oxygen flames

Oxy-fuel combustion differs significantly from conventional air combustion. The absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. In the present work, freely propagating methane-oxygen flames were numerically calculated using a 1D model from lean to rich conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations were performed for equivalence ratios of 0.5 < Φ < 3.0 with an increment of 0.1, different inlet temperatures from 300 K to 700 K and a pressure range of 0.1 MPa to 1 MPa. Additionally, selected results were investigated with different detailed chemical reaction mechanisms. The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios Φ > 0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in slightly rich conditions (1.0 2.1). A first maximum of temperature difference is observed at an equivalence ratio of Φ = 1.5. Approximately 120 K to 180 K higher temperatures than the equilibrium ones at standard inlet conditions are locally observed, depending on the applied reaction mechanism. The first maximum at Φ = 1.5 correlates with the maximum concentration of the Hradical, which plays a key role in the first SAFT regime. A minimum over-temperature of 50 K was identified at an equivalence ratio of Φ = 2.1. By significantly increasing the equivalence ratio, the maximum flame temperature exceeded the equilibrium up to almost 400 K at Φ = 3.0 in the second SAFT regime. An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT in the second regime

Hybrid eulerian-lagrangian approach for dense spray simulations

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston

Manufacturing of a Burner Plate by Diffusion Bonding to Investigate Premixed Fuel‐Rich Oxy‐Fuel Flames at Increased Pressure and Preheating

Combustion of hydrocarbons with pure oxygen as oxidizer is used, e.g., in high-temperature processes such as the partial oxidation (POX) of hydrocarbons to produce synthesis gas of high purity. Due to the prevailing temperatures, active cooling is required for many parts. For laboratory-scale experiments, the dimensions of key parts are too small for conventional manufacturing processes. One example is the manufacturing of a burner plate especially developed for POX processes. The complex geometries of several hundreds of burner nozzles and perpendicular cooling channels across the diameter of the burner plate cannot be manufactured in a conventional way. For this burner, the advantage of chemical etching of thin sheet material and stacking of multiple sheet layouts was used to assemble the layout of the burner. The burner plate was then diffusion-bonded, allowing the complex design to be realized. The partial oxidation of CH$_4$/O$_2$ flames at the laboratory scale could thus be studied under industrially relevant conditions

DNS of Near Wall Dynamics of Premixed CH4_4/Air Flames

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Ka$_s$ , Ka$_c$ and Ka$_tot$ = Ka$_s$ + Ka$_c$, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, S$_L$ decreases with decreasing normal stretch due to curvature Ka$_c$ while approaching the cold wall. However, S$_L$ increases with decreasing Ka$_c$ while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Ma$_tot$ based on Ka$_tot$ from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of S$_L$ has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (S$_L$/S$_{L,0}$ = 1− Ma · Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region