Turbulent Jet Flames Research Articles

Flame and flow characteristics of lean premixed turbulent NH3/H2/N2 - air flames with increasing Karlovitz numbers

Premixed flames of partially cracked ammonia (NH3) hold significant promise for the decarbonization of internal combustion engines and gas turbines, since they can burn at a similar laminar flame speed to methane but have notably high blow-out resistance. Understanding turbulent premixed flames with partially cracked NH3 is highly relevant from both academic and application perspectives. This study aims to enhance our understanding of such premixed NH3/H2/N2-air flames subjected to increasing turbulence. For this purpose, a specific fuel mixture, consisting of 40vol% NH3, 45vol% H2, and 15vol% N2, is selected to match the laminar flame characteristics of methane at the same equivalence ratio. Turbulent jet flames are stabilized in a piloted burner with increasing bulk velocities from 30 to 180 m/s and Karlovitz numbers from approximately 75 to 2,140. One-dimensional (1D) simulations of freely propagating flames and strained counter-flow flames are performed, emphasizing temperature and species axial profiles and flame response to strain rate. Further, turbulent flow and flame structures are characterized using simultaneous particle image velocimetry (PIV) and laser-induced fluorescence of OH radicals (OH-LIF) measurements. Flame surface density and curvature distributions are evaluated, revealing the dominant role of turbulence over differential diffusion in shaping the flame surface topology. It is also found that the OH intensity gradient serves as a marker for local reactivity and thermo-diffusive instabilities, being higher at positive curvatures than at negative ones. Flat flames dominate the surface topology but show significant discrepancies in as they appear both upstream and downstream of leading edges. The thickness of the OH layer is not broadened by turbulence, even at Ka = 2,140, suggesting that eddies cannot penetrate into the main reaction zone marked by OH radicals, which are formed at higher temperatures than the preheat layer.

Local strain rates on OH layers at stabilization region for hydrogen and syngas turbulent non-premixed jet flames near blowoff

Local strain rates on OH layers at stabilization region for hydrogen and syngas turbulent non-premixed jet flames near blowoff

Extended Fourier Neural Operators to learn stiff chemical kinetics under unseen conditions

Extended Fourier Neural Operators to learn stiff chemical kinetics under unseen conditions

Reinforcement Learning for Submodel Assignment in Adaptive Modeling of Turbulent Flames

Reinforcement learning (RL), an unsupervised machine learning approach, is innovatively introduced to turbulent combustion modeling and demonstrated through the automated construction of submodel assignment criteria within the framework of zone-adaptive combustion modeling (AdaCM). In AdaCM, the appropriate combustion submodel—whether the cost-effective species transport model or the advanced transported probability density function (TPDF) method—is adaptively assigned to different regions based on a criterion crucial for performance. The use of RL avoids the extensive manual optimization that involves repetitive calculations and struggles to account for multiple factors. Specifically, RL agents observe local variables as the state and determine the appropriate submodel through a policy. The policy is refined to maximize a reward measuring both accuracy and efficiency through the interaction between RL agents and the AdaCM solver. The methodology is demonstrated for a turbulent non-premixed jet flame, and a sophisticated RL criterion exhibiting a nonlinear and nonmonotonic dependency on the two-dimensional state of mixture fraction and Damköhler number is learned. The AdaCM with the trained criterion provides predictions that are nearly indistinguishable from those obtained using the TPDF method for the whole computational domain, while substantially reducing the computational cost with the speedup of 3.4 and only 22% of cells for TPDF.

Experimental study on the radiation characteristic of downward turbulent diffusion jet flame

Experimental study on the radiation characteristic of downward turbulent diffusion jet flame

The study of flame length for the horizontal jet flame of carbon dioxide and propane mixed gas

The study of flame length for the horizontal jet flame of carbon dioxide and propane mixed gas

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Assessing turbulence–flame interaction of thermo-diffusive lean premixed H2/air flames towards distributed burning regime

Assessing turbulence–flame interaction of thermo-diffusive lean premixed H2/air flames towards distributed burning regime

Comparative study on the influence of chemical reaction mechanisms on turbulent jet flame

Abstract This paper focus on the influence of chemical reaction mechanisms on the simulation of turbulent jet flame of 11 different chemical reaction mechanisms. These mechanisms are mainly selected from GRI series and their reduced mechanisms, and their reaction kinetics were analyzed. This paper uses LES-TPDF method to simulate Sandia Flame D turbulent jet flame and then analyze the difference of time-averaged temperature, heat release rate and species concentration results in different mechanisms. The results show that the kinetic performance of different chemical mechanisms is very different. The ignition delay time of JL4, SMOOKE and z42 mechanisms are either overestimated or underestimated due to the omission of multi-carbon species and other important species. The differences of the LES results in different mechanisms increase along the axis. The differences near the fuel inlet are mainly due to the difference of kinetic performance, while the difference away from the fuel inlet is more due to the different species and elementary reactions involved in different chemical reaction mechanisms. For all chemical mechanisms, there is a tendency that the more species used in the chemical reaction, the more concentrated the region of high heat release rate, the shorter the flame length, and the higher the maximum temperature throughout the field.

Learning-Based Super-Resolution Imaging of Turbulent Flames in Both Time and 3D Space Using Double GAN Architectures

This article presents a spatiotemporal super-resolution (SR) reconstruction model for two common flame types, a swirling and then a jet flame, using double generative adversarial network (GAN) architectures. The approach develops two sets of generator and discriminator networks to learn topographic and temporal features and infer high spatiotemporal resolution turbulent flame structure from supplied low-resolution counterparts at two time points. In this work, numerically simulated 3D turbulent swirling and jet flame structures were used as training data to update the model parameters of the GAN networks. The effectiveness of our model was then thoroughly evaluated in comparison to other traditional interpolation methods. An upscaling factor of 2 in space, which corresponded to an 8-fold increase in the total voxel number and a double time frame acceleration, was used to verify the model’s ability on a swirling flame. The results demonstrate that the assessment metrics, peak signal-to-noise ratio (PSNR), overall error (ER), and structural similarity index (SSIM), with average values of 35.27 dB, 1.7%, and 0.985, respectively, in the spatiotemporal SR results, can reach acceptable accuracy. As a second verification to highlight the present model’s potential universal applicability to flame data of diverse types and shapes, we applied the model to a turbulent jet flame and had equal success. This work provides a different method for acquiring high-resolution 3D structure and further boosting repeat rate, demonstrating the potential of deep learning technology for combustion diagnosis.

Implementation of the P1 Radiation Model in a Velocity-Turbulent Frequency-Composition Joint PDF Method

ABSTRACT This research employs a more detailed radiation treatment to improve the accuracy of the velocity-turbulent frequency-composition joint probability density function (PDF) methods in numerical solution of the turbulent non-premixed flames. The radiation heat transfer in the traditional velocity-frequency-composition joint PDF methods has been disregarded or treated simply using an emission-only radiation model, even though radiation is important in the combustion systems. In the present PDF method, radiation is modeled using the P1-approximation and the radiative properties are calculated by the weighted-sum-of-gray-gases (WSGG) model. The results of present method are first validated by solving a laboratory turbulent piloted jet flame and comparing them with the measured data. Then, two constructed large-scale flames are modeled and simulated. Results reveal that reabsorption of emitted radiation increases from 2.98% in a laboratory-scale flame to 56.8% in the scaled-up flame and has a strong effect on the temperature distribution in the scaled-up flame. It is shown that reabsorption of emitted radiation increases the peak temperature on the flame axis by 119 degrees. Furthermore, radiation becomes more significant with the increase of the flame scales, so that the radiation fraction increases from 4.15% in the small-scale flame to 16.6% in the large-scale flame.

Lifting of Transversely Forced Turbulent Nonpremixed Coaxial Jet Flames

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior.

Direct numerical simulations of pure and partially cracked ammonia/air turbulent premixed jet flames

Ammonia has been identified as a promising fuel to diminish greenhouse gas emission. However, ammonia combustion presents certain challenges including low reactivity and high NO emission. In the present study, three-dimensional direct numerical simulations (DNS) of ammonia/air premixed slot jet flames with varying Karlovitz numbers (Ka) and cracking ratios were performed. Three cases were considered, including two pure ammonia/air flames with different turbulence intensities and one partially cracked ammonia/air flame with high turbulence intensity. The effects of turbulence intensity and partial ammonia cracking on turbulence–flame interactions and NO emission characteristics of the flames were investigated. It was shown that the turbulent flame speed is higher for the flames with high turbulence intensity. In general, the flame displacement speed is negatively correlated with curvature in negative curvature regions, while the correlation is weak in the positive curvature regions for highly turbulent flames. Most flame area is consumed in negatively curved regions and produced in positively curved regions. It was found that the NO mass fraction is higher in the flame with partial ammonia cracking compared to the pure ammonia/air flames. The NO pathway analysis shows that the NH → NO pathway is enhanced, while the NO consumption pathway is suppressed in the partially cracked ammonia/air flame. The NO mass fraction is higher in regions of negative curvature than positive curvature. Interestingly, the NO mass fraction is found to be negatively correlated with the local equivalence ratio, which is consistent in both the DNS and the corresponding laminar premixed flames.

Flow characteristic investigation of localized arc filament plasma assisted turbulent jet and Bunsen flame

Localized arc filament plasma actuators (LAFPAs) have shown the capability to alter the entrainment of freestream air into a jet. This paper presents an investigation into the effects of LAFPAs on the air jet and methane/air premixed Bunsen flames at different Reynolds numbers. The flow disturbance covering the laminar to turbulent transient conditions was generated by a high voltage plasma discharge system of LAFPAs. The high speed Z-type Schlieren technique was applied to visualize the instantaneous flow and flame structures, and an optical flow algorithm was used to estimate the velocity distribution and further analyze the turbulent effect induced by the plasma. The results illustrate that, in the presence of LAFPA operation, the turbulence jet was accelerated and the jet mixing enhanced. Meanwhile, with the help of LAFPAs, the global consumption burning velocity increased significantly by the area enhancement via plasma–flame interaction. The flame front response to the flow disturbance consists of more surface stretch and flame separation. Furthermore, turbulence spectra analysis of the images reveals a considerable increase in turbulent fluctuations in both cold flow and reacting flow.

Morphological Characteristics of Propane–Hydrogen Diffusion Flame: Experiments and Correlations

ABSTRACT This study investigated the flame height and liftoff height of turbulent diffusion jet flames of the Propane–Hydrogen mixed gas. A series of experiments have been conducted to examine their flame height and liftoff height with different concentrations of propane and hydrogen. The experimental results show that the flame height decreases with the increase in hydrogen concentration. The accuracy of the Froude number for the prediction of flame height has been verified. The liftoff height is proportional to the propane flow rate, while it was inversely proportional to the hydrogen concentration due to the increased laminar flame speed. A theoretical model incorporating laminar flame speed, heat release rate, and fuel exit velocity has been obtained. These investigations are crucial for enhancing safety in hydrogen-enriched pipeline systems, which is helpful for the use of hydrogen in energy systems and global sustainability efforts.

Large eddy simulation of turbulent non-premixed oxy-fuel jet flames with different Reynolds numbers

Large eddy simulation of turbulent non-premixed oxy-fuel jet flames with different Reynolds numbers

A deep-learning super-resolution reconstruction model of turbulent reacting flow

A deep-learning super-resolution reconstruction model of turbulent reacting flow

The Study of Hydrogen Volume Fraction Effects on the Flame Temperature of Turbulence Diffusion Propane Jet Flames

This paper studies the influence of hydrogen volume fraction effects on the temperature distribution of diffusion turbulent propane jet flames. Numbers of experimental scenarios have been carried out to investigate the evolution of temperature distribution under different hydrogen volume fractions. In the continuous region, these experimental results show that the temperature distribution and the maximum temperature of diffusion of turbulent jet flames are proportional to the hydrogen volume fraction under the same heat release rate of propane. Considering the model of virtual point source and the three-stage model, the theoretical model between the hydrogen volume fraction and flame temperature has been analyzed. The relationship among the temperature distribution, hydrogen volume fraction, and heat release rate has been modified. It can provide some important references for the fire risk assessment of turbulent diffusion jet flames.

Data-driven propagation prediction of subsonic and supersonic turbulent jets by combining self-similarity analysis model and artificial neural network

Data-driven propagation prediction of subsonic and supersonic turbulent jets by combining self-similarity analysis model and artificial neural network

Influence of LES Inflow Conditions on Simulations of a Piloted Diffusion Flame

This study investigates the influence of different turbulent inflow conditions on the Large Eddy Simulation (LES) of turbulent combustion problems. Four turbulent inflow conditions are studied, including two Synthetic turbulence methods (White Noise and Random Spot), and two Precursor simulation methods (Library and Library-Rescale). The simulation results are compared with experimental measurements. While the White Noise method yields a longer flame with poor agreement against experiments, the Random Spot method has good predictions initially but overpredicts turbulent fluctuations downstream. The results obtained by the Library and Library-Rescale methods are similar, and the discrepancies mainly lie in the region around the centerline. By rescaling the turbulent inlet library to match the experimental profiles, the Library Rescale method improves the prediction of turbulence fluctuations near the nozzle. The present study shows the pronounced sensitivity of LES accuracy in turbulent jet flames to the choice of turbulent inflow conditions.