Non-reactive Mixtures Research Articles

Burner and Flame Transfer Matrices of Jet-Stabilized Flames: Influence of Jet Velocity and Fuel Properties

Abstract To achieve the decarbonization of electrical power generation, gas turbines need to be upgraded to combust high-hydrogen content fuels reliably. One of the main challenges in this upgrade is the burner design. A promising burner concept for a high-hydrogen fuel mixture are jet burners, which are highly flashback resistant thanks to their high bulk velocity. Due to its nonacoustically compact extension and the presence of hydrogen in the fuel mixture, new challenges arise in assessing the (thermo)acoustic response of this burner design. A burner transfer matrix (BTM) and the flame transfer function (FTF) or transfer matrix (FTM) are typically measured with the multimicrophone method (MMM) to assess the performance of new burner types in relation to thermoacoustic stability. With the switch toward hydrogen, the fuel/air mixture is significantly altered in its properties regarding the speed of sound and density, which are of fundamental importance for acoustic waves propagation and their reconstruction via the MMM, as highlighted in recent work. In this work, we extend this discussion by studying the influence of the gas composition within the burner when measuring BTMs, and its indirect effect on the assessment of FTFs. Experimentally, we achieve this by adapting the preheating temperature during the measurement of the BTM with a nonreactive mixture in order to match the speed of sound of the hydrogen–air mixture required to flow in the burner under reactive conditions. Additionally, we present an analytical model for the jet burner transfer matrix, which is validated against the experimental data. Since the BTM is fundamental for the assessment of the FTM and FTF, the propagation of the error of changing fuel mixtures in the burner is evaluated. The influence of the variation in reactant composition of the BTM on the FTM assessment is noticeable, particularly in the gain of the FTFs. Furthermore, the influence of the total mass flow and, thus, the bulk flow velocity on the FTF is analyzed.

Interference-free laser-based temperature and CO-concentration measurements for shock-heated isooctane and isooctane/ethanol blends

Species concentration (e.g. CO) and temperature measurements in the combustion field require fast-response technique without interfering species. In the last decade, tunable diode lasers have been established as strong technique to measure species such as CO, CO2, and H2O as well as temperature with high sensitivity. The drawback is the degree of interference that might hamper the robustness of the technique. In this work simultaneous measurements of temperature and CO concentration were carried out using an interference-free mid-infrared laser-based absorption technique behind reflected shock waves. Two transition lines of CO (P(v″ = 0, J″ = 21) and P(v″ = 1, J″ = 21)) in the fundamental vibrational band near 4.87 and 4.93 μm, respectively, were selected. Absorbance interferences from CO2 and H2O at room and high temperatures were evaluated. Spectroscopic parameters for the development of the system were measured: line strengths and collisional broadening coefficients (in Ar) of both lines were obtained at 1020–1950 K by using the scanned-wavelength direct-absorption method. The technique was demonstrated for non-reactive and reactive mixtures. For the non-reactive case, temperature and CO concentration were measured at 1030–1910 K and 1.0–3.7 bar. For the reactive case, oxidation of i-C8H18/O2/Ar and i-C8H18/C2H5OH/O2/Ar mixtures were investigated at three equivalence ratios of 2.0, 1.0, and 0.5. The two newly adopted lines exhibited good performance in the detection of CO concentration and are immune to interferences from CO2 and H2O. In addition, the simulated data from the state-of-the-art isooctane/ethanol mechanisms in literature were compared with the measured data, showing overall good agreement.

Polarized-depolarized Rayleigh scattering for simultaneous composition and temperature measurements in non-isothermal gaseous mixtures.

This paper demonstrates the application of polarized-depolarized Rayleigh scattering (PDRS) as a simultaneous mixture fraction and temperature diagnostic for non-reacting gaseous mixtures. Previous implementations of this technique have been beneficial when used for combustion and reacting flow applications. This work sought to extend its applicability to non-isothermal mixing of different gases. The use of PDRS shows promise in a range of applications outside combustion, such as in aerodynamic cooling technologies and turbulent heat transfer studies. The general procedure and requirements for applying this diagnostic are elaborated using a proof-of-concept experiment involving gas jet mixing. A numerical sensitivity analysis is then presented, providing insight into the applicability of this technique using different gas combinations and the likely measurement uncertainty. This work demonstrates that appreciable signal-to-noise ratios can be obtained from this diagnostic in gaseous mixtures, yielding simultaneous temperature and mixture fraction visualization, even for an optically non-optimal selection of mixing species.

Surface topologies and self interactions in reactive and nonreactive Richtmyer–Meshkov instability

The reactive Richtmyer–Meshkov instability (RMI) exhibits strong wrinkling of a reactive flame front after an interaction with a shock wave. High levels of deformation and wrinkling can cause the flame surface to intersect with itself, leading to the events of flame self interactions (FSI). As FSI can have a significant influence on the development and topology of the flame surface, it should be considered an important factor affecting the burning characteristics of the flame. The topological structure and statistics of FSI are analyzed using data from high-fidelity simulations of a planar shock wave interacting with a statistically planar hydrogen/air flame for stoichiometric, lean and nonreactive gas mixtures. FSI events are detected by searching for critical points in the field of the reaction progress variable c and divided into the following topological categories: burned gas mixture pocket (BP), unburned gas mixture pocket (UP), tunnel formation (TF) and tunnel closure (TC). It is found that reactivity and flame thickness are decisive factors, influencing the frequency and topological distribution of the detected FSI events. While in early RMI-stages the FSI is found to be mainly dependent on the flame thickness, later stages are heavily influenced by the reactivity, as high reactivity quickly burns out emerging wrinkled structures (in the stoichiometric case) leading to massively reduced levels of FSI. The findings are further supported by the results from the nonreactive case, which at later stages of the RMI closely resembles the less reactive lean case. Analysis of the topology distribution over time and conditioned over c, reveals further differences between the lean and stoichiometric case, as the strong wrinkling and mixing encountered with the lean case facilitates the build up of many pocket-type and tunnel-type interactions throughout the wrinkled flame front. For the stoichiometric case, mainly tunnel-type and unburned pocket topologies are found in the narrow flame funnels extending into the burned gas.

Mixed convection MHD boundary layer flow, heat, and mass transfer past an exponential stretching sheet in porous medium with temperature-dependent fluid properties

The mixed convection MHD boundary layer flow, heat and mass transfer of a Newtonian fluid mixture across an exponentially stretched permeable sheet in porous medium with Soret and Dufour effects subject to temperature-dependent fluid properties have been investigated numerically. The distribution of temperature and concentration at the surface is supposed to grow exponentially. Temperature is considered to affect fluid viscosity and thermal conductivity. The guiding partial differential equations with appropriate boundary conditions are converted into coupled, nonlinear ordinary differential equations with variable coefficients via a similarity transformation. Matlab's built in solver bvp4c is used to find numerical solutions. Using a graphical approach, the effects of several emerging physical parameters on velocity, temperature and concentration distributions on a flow field of a chemically nonreacting fluid mixture are examined and discussed. Results are compared to earlier literature available and found to be in good agreement. Furthermore, this research demonstrates that flow, heat and mass transmission are greatly influenced by mixed convection parameters, magnetic field, Prandtl number, viscosity, conductivity, permeability, and Schmidt number. Skin friction grows for mixed convection parameter, magnetic, viscosity. Highest growths of wall temperature and wall concentration gradients are observed for Prandtl number and Schmidt number, respectively. HIGHLIGHTS Heat and mass transfer rates increase with higher values of mixed convection parameters and Darcy parameter. To reduce heat transfer rate magnetic field may be used. Fluid with lower thermal conductivity and with higher Prandtl number may be employed to boost the cooling rate in conducting flows. Higher Schmidt enhances mass transfer rates. Suction stabilizes boundary layer growth. Soret effect can be used to separate light and medium molecular weight components from a fluid mixture and hence can be used for control of air pollution.

A Thermodynamical Description of Third Grade Fluid Mixtures

Abstract A complete thermodynamical analysis for a non-reacting binary mixture exhibiting the features of a third grade fluid is analyzed. The constitutive functions are allowed to depend on the mass density of the mixture and the concentration of one of the constituents, together with their first and second order gradients, on the specific internal energy of the mixture with its first order gradient, and on the symmetric part of the gradient of barycentric velocity. Compatibility with the second law of thermodynamics is investigated by applying the extended Liu procedure. An explicit solution of the set of thermodynamic restrictions is obtained by postulating a suitable form of the constitutive relations for the diffusional mass flux, the heat flux, and the Cauchy stress tensor. Taking a first order expansion in the gradients of the specific entropy, the expression of the entropy flux is determined. It includes an additional contribution due to non-local effects.

Detailed investigation of the mixing field and stability of natural gas and propane in highly turbulent planar flames

In most practical combustion devices, the actual combustion process occurs within different mixture inhomogeneity levels. Investigating the mixture fraction field upstream of the reaction zones of these flames is an essential step toward understanding their structure, stability, and emission formation. In this study, the mixture fraction fields were measured for turbulent non-reacting inhomogeneous mixtures immediately downstream from the slot burner exit, using Rayleigh scattering imaging. The slot burner had two concentric slots. The inner air slot can be recessed at distances upstream from the exit of the outer fuel slot, allowing various degrees of mixture inhomogeneity. Mixture fraction field statistics and the two-dimensional gradient were utilized to characterize the impact of the air-to-fuel velocity ratio, global equivalence ratio, fuel composition, Reynolds number, and the premixing length on the mixture mixing field, and thus flame stability. These impacts were evaluated by tracking the normalized mean mixture fraction and mixture fraction fluctuation transition across the regime diagram for partially premixed flames. The results showed that the air-to-fuel velocity ratio was the critical parameter affecting the mixture fraction field for the short premixing length. Stability results showed that the level of mixture inhomogeneity mainly influenced the flame stability. High flame stability is achieved if a large portion of the inhomogeneous mixture fraction is within the fuel flammability limits.

Saturation point and phase envelope calculation for reactive systems based on the RAND formulation

The equilibrium calculation of a reactive mixture under the phase-fraction specification, including saturation point calculation and phase envelope calculation, is desirable for the relevant process analysis and simulation. We developed the saturation RAND formulation and the corresponding algorithms for these phase-fraction specification problems. Two types of phase fraction were distinguished: the component-based one and the element-based one, which led to different constraint equations in the formulation. We further introduced element-based partition coefficients to cross the critical point of a reactive mixture in the phase envelope construction. In addition, the general formulation for reactive mixtures can be reduced and used for calculating simpler non-reactive mixtures. We illustrated the robustness and efficiency of our algorithms using four examples: Pxy diagrams for CO2-NaCl brine, a solid–liquid Tx diagram for MgCl2-water, a PT phase envelope for a reactive mixture with the alkene hydration reaction, and a PT phase envelope for a non-reactive hydrocarbon mixture.

Modeling and Simulation of Distillation of a Binary Non-Reacting Mixture using Rate Based Approach in Packed Column under Total Reflux

Modeling and simulation of a distillation column are very important in many chemical industries where ever it requires separation or purification of a liquid mixture into its pure components. Particularly in the context of improving efficiency, process scale-up, intensification and control it requires a validated model. In this work, both experimental results and simulation results are obtained. A batch distillation equipment that consists of a reboiler, a packed column, and a top condenser is used to distill a mixture of benzene and toluene with various heat inputs to the reboiler. The distillation process in the column is modeled by an evaporation rate-based approach using Raoult’s law for driving force. This particular feature is different from the stage-wise equilibrium model. The evaporation mass transfer coefficients are obtained for benzene and toluene in lab-scale apparatus. The material balance equations for the two components are solved using a finite difference method. The distillate compositions at total reflux obtained from experiments and the simulations are compared for validating of model proportionally. Further insights are obtained in regard to throughput and distillation efficiency.

Ignition delay and chemical–kinetic modeling of undiluted mixtures in a high‐pressure shock tube: Nonideal effects and comparative uncertainty analysis

AbstractHigh‐pressure shock tube ignition delay data are essential for fuel characterization and for the validation and optimization of chemical–kinetic models. Therefore, it is crucial that realistic measurement conditions are considered in modeling. Furthermore, an accurate uncertainty quantification for experimental data is the basis for evaluation of the predictive reliability of chemical–kinetic models. Several measurement aspects are investigated to improve the interpretation of measurement results: (1) A new approach for integrating the nonideal pressure rise into chemical–kinetic modeling based on a correlation to measurement data is introduced, which enables the determination at each condition and fuel–air mixture individually with minimal effort. (2) A semiempirical model for available test times of reactive mixtures is introduced, which is based on measurement data of nonreactive mixtures. It allows for a priori prediction of test times and provides experimental limits to support the measurement. (3) A literature review shows that different uncertainty sources are considered in ignition delay time uncertainty analysis. A comparative analysis is conducted to investigate the significance of different uncertainty sources for test temperature and ignition delay time. The analysis of ignition delay time uncertainty indicates that for fuels with negative temperature coefficient behavior a comprehensive uncertainty analysis has to be conducted to accurately estimate measurement uncertainty in the intermediate temperature range. Additionally, ignition delay times of dimethyl ether–air mixtures are measured at pressures of 8, 12, and 35 bar and at equivalence ratios of 0.5, 1.0, and 2.0. Furthermore, the data on first‐stage ignition delay are rather scarce and have therefore been recorded additionally. The new approach of integrating the nonideal pressure rise into modeling and the comprehensive uncertainty analysis supports the interpretation of measurement data, such that the prediction capabilities of chemical–kinetic models can be evaluated thoroughly.

Numerical Simulation of Gas Mixtures Based on the Quasi-Gasdynamic Approach as Applied to the Interaction of a Shock Wave with a Gas Bubble

A new numerical algorithm for simulating transonic flows of nonreacting gas mixtures is proposed. The algorithm is based on the finite volume method as applied to regularized, or quasi-gasdynamic, equations. The equations describing multicomponent gas flows are derived phenomenologically using an existing regularized system for one-component gas and classical equations for gas mixtures. The numerical examples include the computation of the nonstationary interaction of gas flow with heavy and light gas bubbles.

Employing one-dimensional approach to calculate mixture composition evolution caused by gas-phase reactions for the gas flowing through a cylindrical channel

Two approaches to evaluate the mixture composition with a small reactive admixture for the gas flowing through a cylindrical channel are compared. Both are based on solving the Navier-Stokes equations. In the first one, the Navier-Stokes equations are solved for the reacting gas mixture. In the second one, they are solved for non-reacting mixtures and gas-phase reactions are taken into account by solving the one-dimensional (along the flow axis) chemical kinetics equations. Comparison of the obtained results for CH4+H2 mixture have shown the possibility of using of one-dimensional approach for correct description of the mixture composition evolution caused by gas-phase reactions.

Laser-based CO concentration and temperature measurements in high-pressure shock-tube studies of n-heptane partial oxidation

This paper presents a laser-based absorption technique for measuring temperature and CO concentration in high-pressure shock tubes. Two fundamental vibrations of CO (v" = 0, P8, 4.73 µm and v" = 1, R21, 4.56 µm) were selected for high-temperature sensitivity with a reduced influence from pressure broadening compared to previous work. Single-pass absorption (80 mm path length) was measured with two quantum-cascade lasers. The technique was demonstrated by measuring time-resolved temperature for non-reactive mixtures at 1100–1960 K and 1.2–9.7 bar. During partial oxidation of n-heptane, temperature and CO concentrations were measured with 4 µs time resolution at 1360–1670 K and 5.8–8.2 bar. Interference from broadband CO2 absorption was quantified and subtracted. Measured data in the burnout state are in excellent agreement with predictions from kinetics mechanisms (Mehl et al. Proc Combust Inst 33:193, 2011; Zhang et al. Combust Flame 172:116, 2016) over the entire range of operating conditions, which validates the performance of the current laser-absorption technique in reactive-mixture measurements. Additionally, time-resolved temperature and CO-concentration measurements agree well with predictions based on the Mehl et al. mechanism.

Low-Mach hybrid lattice Boltzmann-finite difference solver for combustion in complex flows

A hybrid solver for low-Mach combustion simulations has been proposed and validated through different test-cases in a previous publication [Hosseini et al., “Hybrid lattice Boltzmann-finite difference model for low Mach number combustion simulation,” Combust. Flame 209, 394–404 (2019)]. However, all the considered configurations were laminar, far from realistic applications. To check the performance of this approach for more complex physical processes, the developed solver is used here to model a variety of transitional and turbulent reacting flows. It is first used to compute an established benchmark, the Taylor–Green vortex, for (a) an iso-thermal single-component fluid, (b) a thermal non-reacting mixture, and (c) a thermal reacting mixture (hydrogen/air flame). Detailed comparisons of the results against a high-order in-house direct numerical simulation solver show that the proposed hybrid lattice Boltzmann solver correctly captures the dynamics of the flow at relatively low numerical cost. This same solver is then used to model the interaction of a methane/air flame with a vortex pair, revealing different interaction regimes of interest for turbulent combustion models. It is further employed to model the interaction of an expanding circular flame kernel with a pair of vortices and correctly captures the characteristic regimes. To showcase its ability to deal with turbulent flows, it is finally applied to a homogeneous isotropic turbulent configuration.

Fluid phase equilibria for the isoamyl acetate production by reactive distillation

A new set of revised parameters for NRTL activity coefficient model was correlated for the quaternary mixture of acetic acid + isoamyl alcohol + isoamyl acetate + water based on available scientific experimental vapor – liquid, liquid – liquid and vapor – liquid – liquid equilibrium data. The structure of residue curve maps for the non-reactive quaternary mixture presents four binary azeotropes and two ternary azeotropes. No reactive azeotropes were found for the reactive system. The new set of parameters for NRTL activity coefficient model is more consistent with experimental data than current models and their predictions are used to calculate a complete thermodynamic characterization for the reactive and nonreactive fluid phase equilibria. The results can be used in reactive distillation calculations such as process synthesis and conceptual design.

Experimental and numerical analysis of the autoignition behavior of NH3 and NH3/H2 mixtures at high pressure

Measurements of autoignition delay times of NH3 and NH3/H2 mixtures in a rapid compression machine are reported at pressures from 20–75 bar and temperatures in the range 1040–1210 K. The equivalence ratio, using O2/N2/Ar mixtures as oxidizer, varied for pure NH3 from 0.5 to 3.0; NH3/H2 mixtures with H2 fraction between 0 and 10% were examined at equivalence ratios 0.5 and 1.0. In contrast to many hydrocarbon fuels, the results indicate that, for the conditions studied, autoignition of NH3 becomes slower with increasing equivalence ratio. Hydrogen is seen to have a strong ignition-enhancing effect on NH3. The experimental data, which show similar trends to those observed previously by He et al. (2019) [28], were used to evaluate four NH3 oxidation mechanisms: a new version of the mechanism described by Glarborg et al. (2018) [29], with an updated rate constant for the formation of hydrazine, NH2 + NH2 (+M) = N2H4 (+M), and the literature mechanisms from Klippenstein et al. (2011) [30], Mathieu and Petersen (2015) [25], and Shrestha et al. (2018) [31]. In general, the mechanism from this study has the best performance, yielding satisfactory prediction of ignition delay times both of pure NH3 and NH3/H2 mixtures at high pressures (40–60 bar). Kinetic analysis based on present mechanism indicates that the ignition enhancing effect of H2 on NH3 is closely related to the formation and decomposition of H2O2; even modest hydrogen addition changes the identity of the major reactions from those involving NHx radicals to those that dominate the H2/O2 mechanism. Flux analysis shows that the oxidation path of NH3 is not influenced by H2 addition. We also indicate the methodological importance of using a non-reactive mixture having the same heat capacity as the reactive mixture for determining the non-reactive volume trace for simulation purposes, as well as that of limiting the variation in temperature after compression, by limiting the uncertainty in the experimentally determined quantities that characterize the state of the mixture.

Broadband mid-infrared optical parametric oscillator for dynamic high-temperature multi-species measurements in reacting systems

We demonstrate time-resolved simultaneous measurements of multiple hydrocarbons in high-temperature reacting and non-reacting mixtures using a broadband (instantaneous bandwidth 2.80–3.57 µm) subharmonic mid-infrared optical parametric oscillator based on orientation-patterned gallium phosphide. High-temperature absorption spectra and concentration time-histories of methane, ethane, and ethylene are measured at pressures around 2.3–2.7 atm and temperatures around 1235–1277 K in shock tube experiments.

Single-Ended Sensor for Thermometry and Speciation in Shock Tubes Using Native Surfaces

A single-ended laser absorption sensor for time-resolved measurements of temperature and H2O, CO2, and CO concentrations was designed and deployed on a cylindrical shock tube operating at combustion-relevant conditions. The sensor transmitted four monochromatic laser beams probing strong species-specific mid-infrared absorption transitions through a single optical port and measured the backscattered laser radiation from the native surface across the 13.97-cm-diameter shock tube. Despite the non-ideal concave reflective surface and the long optical path relative to the port diameter (1.37 cm), an optimal optical configuration that maximized signal-to-noise ratio and was free of physical blockage was found and implemented using a numerical ray-tracing optimization algorithm. Wavelength-modulation spectroscopy was employed to further compensate for interference sources in the shock tube environment. The four lasers were sinusoidally injection-current modulated at various frequencies near 100 kHz to yield a measurement rate of 44 kS/s. Demonstration measurements involving non-reacting mixtures of the target species and reacting mixtures of ethane and oxygen in a N2 bath were performed in a shock tube facility at temperatures between 1100 and 1800 K and pressures between 2 and 9 atm, with measured quantities demonstrating excellent agreement with the expected values. Despite experiencing strong vibrations induced by the dynamic shock tube environment, the sensor resolved sub-millisecond transients during ethane oxidation and detected H2O, CO2, and CO mole fractions as low as 0.025%, 0.012%, and 0.005%, respectively, representing highly sensitive detection limits suitable for single-port low-concentration multi-species sensing in shock tubes and other flow channels of comparable dimensions.

Numerical Study of the Mixing Inside a Jet Stirred Reactor using Large Eddy Simulations

Jet Stirred Reactors (JSR) have been extensively used in the last decades to investigate gas phase chemical kinetics. Inside the JSR efficient mixing through turbulent jets is required in order to obtain homogeneous compositions. One of the best ways to achieve the mixing of the gas phase is to use turbulent jets obtained from nozzles. In our research, Computational Fluid Dynamics (CFD) simulations were applied to predict the mixing and flow field characteristics inside a spherical reactor. Large-Eddy Simulations (LES) were used to compute the residence time distribution and the mixing inside the JSR for different flow rates. Our simulations concern a non-reacting mixture at ambient conditions. The results agree well with tracer-decay data, experimentally measured using laser absorption spectroscopy, and with a CFD analysis of the mixing rate based on the Reynolds-Averaged Navier-Stokes (RANS) approach. Our simulations enable us to provide detailed information concerning the instantaneous turbulent structures which effectuate mixing inside the JSR.

High temperature infrared absorption cross sections of methane near 3.4 µm in Ar and CO2 mixtures

The absorption cross-sections of CH4 at two wavelengths in the mid-IR region: λpeak = 3403.4 nm and λvalley = 3403.7 nm were measured. Data were taken using three different compositions of non-reactive gas mixtures comprising CH4/Ar/CO2 between 700 < T < 2000 K and 0.1 < P < 1.5 atm in a shock tube utilizing a continuous-wave distributed-feedback quantum cascade laser. Also, broadband room temperature methane cross section measurements were performed using a Fourier transform infrared spectrometer and the cascade laser to gain a better insight into the changes of the line shapes in various bath gasses (Ar, CO2, and N2). An application of the high-temperature cross-section data was demonstrated to determine the concentration of methane during oxy-methane combustion in a mixture of CO2, O2, and Ar. Current measurements will be valuable addition to the spectroscopy database for methane- an important fuel used for power generation and heating around the world.