Reacting Flow Calculations Research Articles

Design and performance analysis of hydrogen-fueled micro combustion chamber with focus on back pressure minimization

The combustion chamber is a crucial component in power generation within a micro gas turbine. This paper prioritizes practical over theoretical considerations in designing an efficient, small-scale combustion chamber for micro gas turbine applications. The investigation covers the temperature profile within the combustion chamber, employing 19 reversible reactions and considering 9 different chemical species in reactive flow calculations. Preliminary experiments demonstrate hydrogen as a feasible fuel in a micro combustion chamber, generating approximately 1 kW of thermal power. Turbulence physics are assessed using the accurate k-Ɛ model. Findings indicate a reactant inlet temperature of 300 K and a primary zone temperature of 1750 K. This research suggests that minimizing the back pressure effect in a steady-state micro combustion chamber can improve turbine performance.

Computing continuum-level explosive shock and detonation response over a wide pressure range from microstructural details

This paper builds upon our previous work where we defined distinct physically-based parameters of the Scaled Uniform Reactive Flow (SURF) model of Shaw and Menikoff for heterogeneous high explosives (HE) reactive flow calculations. We call this Physically-Informed version of the model PiSURF, or πSURF. Here, we re-derived the model in a manner that resulted in a dependency on the fraction of the overall specific surface area (associated with the void space in a consolidated sample) that is ‘activated’ by a shock wave. This perspective is appealing and consistent with the mechanism of combustion in this class of materials at lower pressures. We apply the model to three case studies having slight perturbations in the nominal microstructure of the explosive PBX 9502, characterized by a void volume distribution in the 0.1 nm–10 µm void size range, over a wide pressure range (0.01–0.6 Mbar/1–60 GPa). We concluded that the model provided meaningful predictions that agreed with our intuitive expectations, qualitatively supported by experimental evidence, for the entire relevant pressure range in a manner consistent with a more generalized picture of combustion in heterogeneous materials. This approach takes a significant step forward towards the broader goal of a priori calculation of the hydrodynamic behavior of HE from measurable microstructural details.

Intelligent Assessment of Active and Reactive Power Flow with Satisfying Accuracy for N - k1 - k2 Cascading Outages

Addressed to the $N-k_{1}-k_{2}$ cascading outages, it is computationally burdensome for the reliable calculation of active and reactive power flows. This paper builds a comprehensive framework with three algorithms, including the distribution factor (DF), the Newton-Raphson (NR), and the first iteration of NR algorithm (termed as 1J). Classifiers are designed to determine whether the NR algorithm should be employed for accuracy. Classifier features are extracted upon the analytical error of 1J. As reactive power is partially considered in the 1J but neglected in the DF algorithm, the deviation between the solutions is taken as one crucial feature. The support vector machine (SVM) is then utilized for classifier training. As the deep integration of the causal inference and the statistical paradigm, this framework calculates active and reactive power flows rapidly, reliably, and robustly. The effectiveness and robustness are fully validated in three typical IEEE systems.

Computational study of the effect of cavity geometry on the supersonic mixing and combustion of ethylene

In this numerical study, the supersonic combustion of ethylene in three model combustor configurations namely, baseline (no cavity), square cavity and inclined cavity are investigated. To this end, 3D, compressible, turbulent, non-reacting (with fuel injection) and reacting flow calculations using single step and 10-step chemical kinetics in conjunction with a one equation turbulence model have been carried out. In the mixing study, predictions of the flow features with fuel injection, fuel trajectories, contours of total pressure loss along the combustor are compared between the combustor models. In the combustion study, contours of heat release are compared across the combustor models. Overall performance metrics such as mixing efficiency, total pressure loss and combustion efficiency are also compared between the combustor models. The comparison of combustor top wall static pressure and exit total pressure predictions with experimental data reported in the literature are also presented and discussed. The results clearly show that the model combustor with a shallow and inclined aft wall cavity has the highest residence time and maximum heat release. In addition, the role of a smaller L/D ratio cavity is shown to be minimal on the predictions of residence time and the heat release.

Numerical investigation of the local and global supersonic combustion characteristics of ethylene fuel

In supersonic combustors, cavities play an active role in the process of fuel injection and flameholding. This study numerically investigates the influence of chemical kinetics on the local and global heat release predictions of supersonic combustion of ethylene. To this end, three-dimensional compressible, turbulent, Favre-averaged Navier-Strokes reacting-flow calculations have been carried out with a single-step chemistry model and a multi-step chemistry model. The reacting flow calculations have been performed with and without the presence of an inclined cavity in a model supersonic combustor. The heat release patterns obtained using the single-step and multi-step chemistry are compared. Furthermore, the heat release patterns are correlated with the variation of reaction rates involved in different chemistry models. In addition, the overall performance metrics such as mixing efficiency and combustion efficiency are used to draw inferences on the nature (whether mixing- or kinetic-controlled) and the completeness of the combustion process. The predicted values of the dimensionless wall static pressure are compared with experimental data reported in the literature. The calculations show that not only the multi-step chemistry model predicts the heat release better than single step chemistry, but also, the intricate details of the combustion process inside the combustor configurations.

Reduced Kinetic Schemes for Complex Reacting Flow Computations of Propane–Air Combustion

This paper describes the development of two simplified reduced kinetic models for high-temperature oxidation of propane, which can be incorporated into complex turbulent flame simulations. Equilibrium, 0D or 1D propagating premixed flames and 2D co-flowing laminar jet flames, with or without preheating, attached or lifted, are computed during the iterative optimization process. Accompanying computations with the USC-II mechanism, as well as available experimental data are exploited for validation. Comparisons demonstrate that these reduced kinetic models ensure satisfactory agreement with data over the investigated parameter space.

Reduced Kinetic Schemes for Complex Reacting Flow Computations of Propane-Air Combustion

Reduced Kinetic Schemes for Complex Reacting Flow Computations of Propane-Air Combustion

Simulation Investigation on Combustion Characteristics in a Four-Point Lean Direct Injection Combustor with Hydrogen/Air

To investigate the combustion characteristics in multi-point lean direct injection (LDI) combustors with hydrogen/air, two swirl–venturi 2 × 2 array four-point LDI combustors were designed. The four-point LDI combustor consists of injector assembly, swirl–venturi array and combustion chamber. The injector, swirler and venturi together govern the rapid mixing of hydrogen and air to form the mixture for combustion. Using clockwise swirlers and anticlockwise swirlers, the co-swirling and count-swirling swirler arrays LDI combustors were achieved. Using Reynolds-Averaged Navier–Stokes (RANS) code for steady-state reacting flow computations, the four-point LDI combustors with hydrogen/air were simulated with an 11 species and 23 lumped reaction steps H2/Air reaction mechanism. The axial velocity, turbulence kinetic energy, total pressure drop coefficient, outlet temperature, mass fraction of OH and emission of pollutant NO of four-point LDI combustors, with different equivalence ratios, are here presented and discussed. As the equivalence ratios increased, the total pressure drop coefficient became higher because of increasing heat loss. Increasing equivalence ratios also corresponded with the rise in outlet temperature of the four-point LDI combustors, as well as an increase in the emission index of NO EINO in the four-point LDI combustors. Along the axial distance, the EINO always increased and was at maximum at the exit of the dump. Along the chamber, the EINO gradually increased, maximizing at the exit of chamber. The total temperature of four-point LDI combustors with different equivalence ratios was identical to the theoretical equilibrium temperature. The EINO was an exponential function of the equivalence ratio.

3D numerical study of the performance of different burner concepts for the high-pressure non-catalytic natural gas reforming based on the Freiberg semi-industrial test facility HP POX

In this work, new test burner concepts for the non-catalytic partial oxidation of natural gas are evaluated using numerical modeling. The 3D CFD model for reactive flow calculations incorporates a detailed reaction mechanism containing 28 species and 112 reactions, and is extensively validated against data obtained in the semi-industrial test facility HP POX in Freiberg. The validation cases operate at 61bar(a) and outlet temperatures of approximately 1688K. The CFD models demonstrate that the optimized burner concepts result in a significantly higher natural gas conversion rate, which allows for a more compact reactor design or, alternatively, for a higher throughput in existing industrial facilities. In addition, the final burner concepts are tested experimentally. Optical measurements demonstrate that the CFD model is capable of reproducing the complex 3D flame structures and reliably predicting the overall performance of the reactor.

ANALYSIS OF OPTIMUM OPERATING MODES OF POWER TRANSFORMERS UNDER OPERATING CONDITIONS

Purpose. The study of parallel operation optimal modes of transformer equipment for a variety of operating conditions: same or different types of transformers, with or without reactive power flows. Methodology. Losses of energy in transformers make 30 % of all losses. Therefore the choice of the economically justified parallel operation of transformers is effective action to reduce losses. Typically, in the calculations of reactive power flows in the transformers are not taken into account. It is interesting to analyze the optimal operating conditions of transformers with and without reactive power flows. Results. Calculations for transformers in distribution networks showed that the inclusion of reactive power flows in transformers significant impact on the calculated optimum regimes of transformers.

Modeling Auto-Ignition Transients in Reacting Diesel Jets

The objective of the present work is to establish a framework to design simple Arrhenius mechanisms for simulation of diesel engine combustion. The goal is to predict auto-ignition over a selected range of temperature and equivalence ratio, at a significantly reduced computational cost, and to quantify the accuracy of the optimized mechanisms for a selected set of characteristics. The methodology is demonstrated for n-dodecane oxidation by fitting the auto-ignition delay time predicted by a detailed reference mechanism to a two-step model mechanism. The pre-exponential factor and activation energy of the first reaction are modeled as functions of equivalence ratio and temperature and calibrated using Bayesian inference. This provides both the optimal parameter values and the related uncertainties over a defined envelope of temperatures, pressures, and equivalence ratios. Nonintrusive spectral projection (NISP) is then used to propagate the uncertainty through homogeneous auto-ignitions. A benefit of the method is that parametric uncertainties can be propagated in the same way through coupled reacting flow calculations using techniques such as large eddy simulation (LES) to quantify the impact of the chemical parameter uncertainty on simulation results.

Investigation on supersonic combustion of hydrogen with variation of combustor inlet conditions

The present work numerically investigated the effect of variation in the inlet Mach number and stagnation temperature on the mixing of fuel with the oxidizer and the subsequent stabilization of a flame in a combustor at supersonic conditions. Dimensions of the studied combustor were taken from literature. It had a 10° wedge located at the top wall of the combustor. The combustor was modeled and analyzed using ANSYS FLUENT software. Three-dimensional, compressible, reacting flow calculations with a detailed chemistry model were performed. Turbulence was modeled using SST k-ω model. Necessary grid refinement was done to capture the incident oblique shock formed at the 10° wedge. Hydrogen was injected through the fuel inlet port. The computations were performed for Mach numbers of 2.0, 2.5 and 3.0 at the combustor inlet for a combustion inlet stagnation temperature of 1500 K. Later, the combustor inlet Mach number was kept constant at 2.5 and the combustor inlet stagnation temperature was varied as follows: 1500 K, 1700 K, and 1900 K. The results indicated that as the combustor inlet Mach number increased, the location of incidence of the oblique shock shifted to the downstream of the fuel inlet and it resulted in the better mixing of the fuel with cross flow stream of air and led to better degree of combustion of hydrogen. The contours of mole fraction of OH radical and hydrogen also corroborated the improvement in the mixing of fuel with the cross flow air and the subsequent flame stabilization at higher Mach numbers. The flow pattern, mixing of fuel with air and flame stabilization was not affected significantly till 1700 K whereas for 1900 K, combustion of hydrogen was more uniform.

Hydrogen non-premixed combustion in enclosure with one vent and sustained release: Numerical experiments

Numerical experiments are performed to understand different regimes of hydrogen non-premixed combustion in an enclosure with passive ventilation through one horizontal or vertical vent located at the top of a wall. The Reynolds averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) model with a reduced chemical reaction mechanism is described in detail. The model is based on the renormalization group (RNG) k-e turbulence model, the eddy dissipation concept (EDC) model for simulation of combustion coupled with the 18-step reduced chemical mechanism (8 species), and the in-situ adaptive tabulation (ISAT) algorithm that accelerates the reacting flow calculations by two to three orders of magnitude. The analysis of temperature and species (hydroxyl, hydrogen, oxygen, water) concentrations in time, as well as the velocity through the vent, shed a light on regimes and dynamics of indoor hydrogen fires. A well-ventilated fire is simulated in the enclosure at a lower release flow rate and complete combustion of hydrogen within the enclosure. Fire becomes under-ventilated at higher release flow rates with two different modes observed. The first mode is the external flame stabilised at the enclosure vent at moderate release rates, and the second mode is the self-extinction of combustion inside and outside the enclosure at higher hydrogen release rates. The simulations demonstrated a complex reacting flow dynamics in the enclosure that leads to formation of the external flame or the self-extinction. The air intake into the enclosure at later stages of the process through the whole vent area is a characteristic feature of the self-extinction regime. This air intake is due to faster cooling of hot combustion products by sustained colder hydrogen leak compared to the generation of hot products by the ceasing chemical reactions inside the enclosure and hydrogen supply. In general, an increase of hydrogen sustained release flow rate will change fire regime from the well-ventilated combustion within the enclosure, through the external flame stabilised at the vent, and finally to the self-extinction of combustion throughout the domain.

Numerical Investigation of Combustion Instability in a V-Gutter Stabilized Combustor

The stability of the combustion process in a V-gutter stabilized combustor is numerically investigated. To this end, 3D compressible turbulent and unsteady reacting flow calculations have been carried out using LES. The time history of the pressure at several locations is used to determine the frequency and amplitude of the oscillations along with the mode shapes. A shift in the dominant mode of the frequency spectra from the acoustic mode to the hydrodynamic mode is observed. A POD analysis of pressure time histories on the symmetry plane also corroborates this trend. The computational domain is divided into several subvolumes in the wake region of the V-gutter and the time histories of pressure, temperature, and heat release are collected in the individual volumes. It is seen that the fluctuation of pressure and heat release tend to oscillate from being in phase to out of phase over a time period. Unstable regions predicted by the Rayleigh index across a plane are shown to be different from those predicted in a volume owing to the three dimensionality of the flame. Quite interestingly, the calculated values of the indices show the combustor to be most unstable for an equivalence ratio of 0.1665 which is not the leanest one considered here. The global Rayleigh index is shown to correlate well with the amplitude of the dominant mode.

The Instability of Flame Fronts in Premixed Combustion of Low-Temperature Gases (Effects of Unburned-Gas Temperature)

We performed two-dimensional unsteady calculations of reactive flows, based on the compressible N-S equation, to study the effects of unburned-gas temperature on the instability of flame fronts. As the unburned-gas temperature became lower, the growth rate decreased and the unstable range narrowed. This was due mainly to the reduction of the burning velocity of a planar flame. The normalized growth rate increased with a decrease of the unburned-gas temperature, and the normalized unstable range widened at Lewis numbers smaller than unity. This was because that the thermal-expansion effects became stronger owing to the increase of the temperature ratio of burned and unburned gases, and that the diffusive-thermal effects strengthened at Lewis numbers smaller than unity owing to the increase of the Zeldovich number. Moreover, the cellular shape of flame fronts formed, which was due to intrinsic instability. The normalized burning velocity of a cellular flame increased as the unburned-gas temperature became lower, especially in premixed flames with small Lewis numbers.

The Effects of Unburned-Gas Temperature and Heat Loss on the Diffusive-Thermal Instability of Premixed Flames

The effects of unburned-gas temperature and heat loss on the diffusive-thermal instability of premixed flames were studied by two-dimensional unsteady calculations of reactive flows, based on the diffusive-thermal model equation, under the conditions of constant temperature jump through flame fronts. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened at Lewis numbers smaller than unity, which was due mainly to the increase of the burning velocity of a planar flame. As for the growth rate and unstable range normalized by the burning velocity of a planar flame, the former decreased and the latter narrowed. This was due to the reduction of Zeldovich numbers. In addition, the normalized growth rate increased and the normalized unstable range widened when the heat loss was taken into account. This indicated that the heat loss had a pronounced influence on the diffusive-thermal instability of premixed flames with high unburned-gas temperature. Furthermore, the cellular shape of flame fronts formed owing to diffusive-thermal instability. The normalized burning velocity of a cellular flame decreased as the unburned-gas temperature became higher, and increased when the heat loss was taken into account. Compared with high-temperature premixed flames where the adiabatic flame temperature was constant, the normalized level of instability intensity was low. This was because of small Zeldovich numbers.

Premixed flame response to equivalence ratio fluctuations: Comparison between reduced order modeling and detailed computations

Combustion instability events in lean premixed combustion systems can cause spatio-temporal variations in unburnt mixture fuel/air ratio. This provides a driving mechanism for heat-release oscillations when they interact with the flame. Several Reduced Order Modelling (ROM) approaches to predict the characteristics of these oscillations have been developed in the past. The present paper compares results for flame describing function characteristics determined from a ROM approach based on the level-set method, with corresponding results from detailed, fully compressible reacting flow computations for the same two dimensional slot flame configuration. The comparison between these results is seen to be sensitive to small geometric differences in the shape of the nominally steady flame used in the two computations. When the results are corrected to account for these differences, describing function magnitudes are well predicted for frequencies lesser than and greater than a lower and upper cutoff respectively due to amplification of flame surface wrinkling by the convective Darrieus-Landau (DL) instability. However, good agreement in describing function phase predictions is seen as the ROM captures the transit time of wrinkles through the flame correctly. Also, good agreement is seen for both magnitude and phase of the flame response, for large forcing amplitudes, at frequencies where the DL instability has a minimal influence. Thus, the present ROM can predict flame response as long as the DL instability, caused by gas expansion at the flame front, does not significantly alter flame front perturbation amplitudes as they traverse the flame.

Hydrogen Fuelled Micro Gas Turbine Combustion Chamber

An introductory overview of the micro and small-scale devices for the combustion in micro gas turbines is presented at the beginning of this paper. The work, thereafter presents the development of a micro combustor design, where the combustion process was simulated by CFD and tested experimentally. 3-dimensional model for combustion simulations was used. The fuel is hydrogen and primary zone equivalence ratio of 0.9 was simulated. Reactive flow calculations were carried out with 19 reversible reactions and nine species. The analysis of the simulated results was based on the obtained velocity profiles, concentration and temperature distributions within the liner. A micro combustor was manufactured and tests demonstrated its successful operation. Measurements of temperature, pressure and velocities along the length of combustion chamber were obtained. The experimental and simulation results are compared and a good qualitative and quantitative agreement was found between the experiments and the predicted val...

Effects of the Unburned-Gas Temperature and Lewis Number on the Intrinsic Instability of High-Temperature Premixed Flames

The effects of the unburned-gas temperature and Lewis number on the intrinsic instability of high-temperature premixed flames under the constant-enthalpy conditions were investigated by two-dimensional unsteady calculations of reactive flows. A sinusoidal disturbance with sufficiently small amplitude was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened. This was due to the increase of the burning velocity of a planar flame. In addition, the obtained numerical results were consistent with the theoretical solutions in small wave-number region. As the Lewis number became smaller ( larger ), the growth rate increased ( decreased ) and the unstable range widened ( narrowed ), which was due to diffusive-thermal effects. The dispersion relation yielded the linearly most unstable wave number, i.e. the critical wave number. The critical wave number increased as the unburned-gas temperature became higher. Thus, the critical wavelength shrank, and then the cell size shrank. To clarify the characteristics of cellular flames induced by intrinsic instability, a finite disturbance with the critical wavelength was superimposed. The superimposed disturbance evolved, and a cellular-shaped front formed. In all Lewis numbers, the behavior of cellular flames became milder as the unburned-gas temperature became higher, even though the growth rate increased. The normalized burning velocities of cellular flames decreased monotonously. This was because that the thermal-expansion effects became weaker owing to the decrease of the difference in temperature between the burned and unburned gases, which was generated by the conditions of constant enthalpy, i.e. constant burned-gas temperature.

Adaptation and application of the In Situ Adaptive Tabulation (ISAT) procedure to reacting flow calculations with complex surface chemistry

In this article, the In Situ Adaptive Tabulation (ISAT) procedure, originally developed for the efficient computation of homogeneous reactions in chemically reacting flows, is adapted and demonstrated for reacting flow computations with complex heterogeneous (or surface) reactions. The treatment of heterogeneous reactions within a reacting flow calculation requires solution of a set of nonlinear differential algebraic equations at boundary faces/nodes, as opposed to the solution of an initial value problem for which the original ISAT procedure was developed. The modified ISAT algorithm, referred to as ISAT-S, is coupled to a three-dimensional unstructured reacting flow solver, and strategies for maximizing efficiency without hampering accuracy and convergence are developed. These include use of multiple binary tables, use of dynamic tolerance values to control errors, and periodic deletion and/or re-creation of the binary tables. The new procedure is demonstrated for steady-state catalytic combustion of a methane–air mixture on platinum using a 24-step reaction mechanism with 19 species, and for steady-state three-way catalytic conversion using a 61-step mechanism with 34 species. Both reaction mechanisms are first tested in simple 3D channel geometry with reacting walls, and the impact of various ISAT parameters is investigated. It is found that the temperature of the reacting wall dictates the retrieval rate from the ISAT table. As a final step, the catalytic combustion mechanism is demonstrated in an laboratory-scale monolithic catalytic converter geometry with 57 channels discretized using 354,300 control volumes (4.6 million unknowns) after employing quarter symmetry. For this particular case, the use of ISAT-S resulted in reduction of the overall CPU time from 19.3 to 13.6 h. For all of the cases considered, the reduction in the time taken to perform surface chemistry calculations alone was found to be a factor of 5–11.