Non-premixed Jet Flames Research Articles

The Correlation for Non-premixed Hydrogen Jet flame Length in Still Air

The experimental data on the hydrogen flame length normalized by the nozzle diameter are correlated with the dimensionless product of the density ratio (hydrogen density in the nozzle exit to the density of surrounding air) and the Mach number to the power of three. The current up-to-date experimental data on hydrogen flame length are used to build the correlation that covers laminar and turbulent flows, buoyancy- and momentum-dominated releases, subsonic, sonic and highly under-expanded supersonic jets. The density and velocity of hydrogen in the nozzle are taken either directly from experiments or calculated by the under-expanded jet theory published elsewhere. The correlation is validated in the range of hydrogen storage pressures from nearly atmospheric up to 90 MPa and nozzle diameters from 0.4 to 51.7 mm. The predictive capability of this dimensionless correlation exceeds that of previously published work based on the Froude number only.

Petascale direct numerical simulation of turbulent combustion—fundamental insights towards predictive models

The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained ‘turbulence-chemistry’ interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information is leading to new physical insight, providing benchmark data for assessing model assumptions, suggesting new closure hypotheses, and providing interpretation of statistics obtained from lower-dimensional measurements. In this paper the various roles of petascale DNS are illustrated through selected examples related to lifted flame stabilization, premixed and stratified flame propagation in intense turbulence, and extinction and reignition in turbulent non-premixed jet flames. Extending the DNS envelope to higher Reynolds numbers, higher pressures, and greater chemical complexity will require exascale computing in the next decade. The future outlook of DNS in terms of challenges and opportunities in this regard are addressed.

Implementation of the eddy dissipation model of turbulent non-premixed combustion in OpenFOAM

This work discusses the implementation of eddy dissipation model in OpenFOAM CFD toolbox. The code was validated in modeling of confined non-premixed Methane jet flame. The model predictions were extensively compared against published experimental results as well as ANSYS Fluent® predictions. The differences between the implemented model in OpenFOAM and Fluent were demonstrated.

Analysis of scalar mixing dynamics in LES using high-resolution imaging of laser Rayleigh scattering in turbulent non-reacting jets and non-premixed jet flames

Imaging of turbulent jet flames has provided insights into the dynamic structure of the thermal dissipation field. These insights motivated studies of scalar mixing in non-reacting jets, with emphasis on comparing measurements to large eddy simulations (LES). We use a progressively refined set of calculations to investigate how filter size affects the LES representations of scalar mixing. Results are analyzed using three grids that are successively refined by a factor of two in space and time. The coarsest grid contains 1.3-million cells and provides resolution similar to that used in current LES. The refined grids contain 10- and 82-million cells. Imaging measurements of the instantaneous mixture fraction and dissipation fields provide insights into scalar mixing dynamics and key length scales. They show that dissipation structures reside on the resolved- and subgrid-scales of the LES, and they provide order of magnitude estimates of the relationship between the dissipation layer thickness (the smallest relevant mixing scale), the integral scale (the largest relevant mixing scale), and the filter width. For all grid distributions, the longitudinal dimension of the dissipation layers spans multiple cells, but the ratio of the layer thickness to filter width is of order 1, which is not consistent with typical modeling assumptions. Measurements of mixture fraction were filtered to isolate the effects of spatial averaging on the dissipation field from the combined effects of spatial and temporal averaging. Statistical analysis reveals that the average mixture fraction and dissipation fields for the coarsest grid deviate significantly from the measured profiles as the flow evolves. On the finest grid, however, good agreement is obtained. Trends suggest that temporal damping and dispersion errors compound as the flow evolves downstream, which is directly related to the relationship between the dissipation layer thickness, filter width, and integral scale of the local mixing field.

Prediction of the Blowout Limits of Turbulent Nonpremixed Jet Flames Using the Premixed Combustion Theory

A method is proposed for predicting the blowout limits of lifted nonpremixed jet flames in a coflowing air stream. Based on the premixed combustion theory, the velocity at the base of a lifted flame is balanced by the turbulent flame speed. From this simple concept, Kalghatgi derived a universal formula for calculating the blowout limits of free jet flames. The authors aimed to extend the same concept to lifted flames in a coflowing stream. Local flow parameters needed to estimate the turbulent burning velocity were determined using a RANS model. Results show that at blowout conditions, the local jet velocity is approximately equal to the turbulent burning velocity estimated using Bray's model at the blowout location. Furthermore, this method allows the blowout limits to be predicted through an iterative procedure for a variety of operating conditions. It was shown that there is good agreement between calculated and experimental blowout limits.

A forced ignition probability analysis method using LES and Lagrangian particle monitoring

A large eddy simulations-based methodology is developed for predicting the probability of igniting a turbulent non-premixed jet flame with a localized spark. The method couples the LES to a Lagrangian monitoring of fluid particles and explores, through a sub-grid turbulent flame speed model, the local turbulent flow properties on upstream and downstream flame propagation in the axial direction following radial kernel growth. The velocity and mixing fields are validated by using experimental measurements. The predicted region of possible upstream flame propagation, characterized by near-stoichiometric mixture and low velocity flow, compares well with the available experimental data regarding its form and upstream flame speed. The probabilistic generation of a kernel, its convection and the subsequent flame propagation are investigated and the conditional statistics of the mixture at the ignition location, and the convection and transition to upstream propagating flame are determined. The conditional probability of formation, convection, growth and stabilization of flame kernels results in a prediction of the overall flame ignition probability, which agrees well with experiment.

Mixture fraction imaging in turbulent non-premixed flames with two-photon LIF of krypton

The use of a noble gas as an inert tracer for mixing studies in combustion systems is investigated. Simultaneous two-photon laser-induced fluorescence (LIF) of krypton and Rayleigh scattering are used for imaging measurements of mixture fraction and temperature in turbulent non-premixed jet flames. The turbulent flames investigated in this study include a piloted CH4/air flame (Sandia flame D) and a CH4/H2/N2 flame (DLR-B flame). These flames are well-documented in the literature and enable an evaluation of krypton as a tracer in different fuel mixtures with varying degrees of differential diffusion. Krypton is excited from the ground state to the 5p[3/2]2 state using 215nm laser radiation, and the fluorescence decay to the metastable state, 5s[3/2]2, is detected at 760nm. Single-shot krypton LIF and Rayleigh scattering images are analyzed in an iterative routine to determine mixture fraction and temperature. Measurements of the temperature- and species-dependent quenching rates for Kr-LIF are incorporated into this routine. The resulting average radial profiles of mixture fraction and temperature for both flames agree well with previously published measurements. The use of a noble gas as a chemically inert tracer has potential applications for mixing studies in a broad range of combustion environments.

An evaluation of the one-dimensional turbulence model: Comparison with direct numerical simulations of CO/H2 jets with extinction and reignition

A variant of the one-dimensional turbulence (ODT) model formulated in an Eulerian reference frame is applied to a planar nonpremixed turbulent jet flame and results from the model prediction are compared with DNS data. The model employed herein solves the full set of conservation equations for mass, momentum, energy, and species on a one-dimensional domain corresponding to the transverse jet direction. The effects of turbulent mixing are modeled via a stochastic process, while the full range of diffusive-reactive length and time scales are resolved directly on the one-dimensional domain. A detailed chemical mechanism consisting of 11 species and 21 reactions and mixture-averaged transport is used in this study (consistent with DNS simulations). Comparisons between the model and DNS data in physical and state space are shown, including conditional statistics. Results indicate that the model accurately reproduces the DNS data set. Turbulence-chemistry interactions, including trends for extinction and reignition, are captured by the model. Differences observed between model prediction and data are the result of early excess extinction observed in the model. The reasons for the early extinction are discussed within the model context.

Experimental study of flame-hole reignition mechanisms in a turbulent non-premixed jet flame using sustained multi-kHz PIV and crossed-plane OH PLIF

The dynamics of flame-hole reignition were studied experimentally in a turbulent non-premixed CH4/H2/N2 jet flame at Red=22,800 (flame ‘DLR-B’ from the TNF workshop). Simultaneous measurements of the OH combustion radical and velocity field were performed using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) at a sustained rate of 10kHz. The dynamics of the reignition process were tracked through time and two reignition mechanisms were identified. Particular care was taken to reduce the influence of out-of-plane motion on the analyzed events by simultaneously measuring the OH distribution in crossed planes. Flame-holes reignited due to both edge-flame propagation and turbulent transport of burning flame segments. However, the edge-flame propagation mechanism was dominant and accounted for over 90% of the flame-hole reignition rate on average. Furthermore, the presence of large scale turbulent structures adjacent to a flame-hole did not necessarily result in reignition due to turbulent transport. Instead, the edge-flames propagated around the perimeter of such structures, indicating intervening regions of well mixed gas. The range of measured edge-flame propagation speeds agreed well that of highly-preheated premixed flames, with a mode of approximately 4m/s and a mean of approximately 7m/s.

Development of high-repetition rate CH PLIF imaging in turbulent nonpremixed flames

We report on the development of high-repetition rate planar laser-induced fluorescence (PLIF) imaging of the CH radical in turbulent flames. This paper presents what the authors believe to be the first multi-frame, high-speed CH PLIF image sequences captured in a turbulent nonpremixed jet-flame. The high-repetition rate CH PLIF measurements were made using a combination of a custom pulse burst laser system operating at a 10-kHz repetition rate, an in-house optical parametric oscillator (OPO) with frequency mixing for 390-nm laser pulse generation, and a high-framing rate ICCD camera for high-speed image capture. The 1064-nm output of the pulse burst laser is frequency-tripled (355nm) and used to pump the OPO, which can be operated in a narrow bandwidth (300MHz) or broadband (4cm−1) mode. The OPO system produces a burst of laser pulses near 615nm, which can be subsequently mixed with residual 1064-nm output from the pulse burst laser to generate a series of 390-nm pulses separated by less than 100μs. The tunable ultraviolet output around 390nm is then used to excite the B–X (0,0) band allowing high-speed image sequences of the CH radical to be acquired from the B–X(0,1), A–X(1,1), and A–X(0,0) bands near 430nm. In this paper, we present results using the narrow bandwidth mode of the laser. By injection seeding the OPO with a single-frequency diode laser, we can obtain spectrally-narrow output at 390-nm, which allows imaging of CH with only 0.4mJ/pulse of laser energy. Ten-kilohertz-CH PLIF image sequences from a turbulent nonpremixed flame are reported as an example of the potential of this diagnostic system. Although, the signal-to-noise ratio (SNR) is marginal, we discuss future improvements to the system to increase the SNR to levels comparable to that achieved with traditional low-repetition rate systems.

Projection method with self‐adaptive time steps for LES of ignition and extinction in non‐premixed jet flames

AbstractIn this paper, a modified projection method is combined with a self‐adaptive time step procedure to develop numerical scheme for large eddy simulation (LES) of low Ma number turbulent reactive flows. The projection method introduced by Chorin is modified in this study to satisfy the simulation requirement of low Ma number reactive flow. The time step in this computation is automatically determined according to the time scales of both chemical reaction and turbulent fluctuations. This enables the simulation to capture detailed flow structures with less computational time. Numerical simulation of methane–air jet flames is carried out as an example to validate the developed numerical scheme. The mechanism of a simplified 4‐step chemical kinetics is applied for the methane–air reaction. The dynamic model is adopted for the turbulent motion of sub‐grid scale (SGS). The dynamic similarity model is used as the SGS model for the reaction rate. The LES results satisfactorily depict the ignition process of the turbulent jet flames and illustrate lucid and detailed coherent structures of the fully developed turbulent reactive jet flow. The LES results also exhibit the mechanism and characteristics of local extinction. The method developed in this study provides an effective way to capture more flow details with less computational time. In addition the method also helps one to investigate the mechanism of ignition and local extinction in jet flames. Copyright © 2008 John Wiley & Sons, Ltd.

Correlation of Spark Ignition with the Local Instantaneous Mixture Fraction in a Turbulent Nonpremixed Methane Jet

The correlation between spark success in a turbulent nonpremixed methane jet flame and the mixture fraction at the spark location is examined by monitoring spark emission, from which the mixture fraction, integrated over the spark volume and duration, can be determined after calibration. It was found that ignition of the whole jet flame can be achieved even if nominally nonflammable mixture is sparked and that sparking flammable mixture does not always result in ignition.

Swirling and Impinging Effects in an Annular Nonpremixed Jet Flame

The effects of swirl and downstream wall confinement on an annular nonpremixed flame were investigated using direct numerical simulation (DNS). Fully three-dimensional parallel DNS was performed employing high-order numerical methods and high-fidelity boundary conditions to solve governing equations for variable-density flow and finite-rate Arrhenius chemistry. Three swirl numbers have been examined: 0 (without swirl), 0.4 and 0.8, while the effects of downstream wall confinement have been examined for swirl numbers 0 and 0.4. Results have been presented in terms of instantaneous and time-averaged flow quantities, which have also been analysed using energy spectra and proper orthogonal decomposition (POD). Effects of swirl on the fluid dynamic behaviour of the annular nonpremixed flame were found to be significant. The fluid dynamic behaviour of the flame is greatly affected by the interaction between the geometrical recirculation zone (GRZ) near the jet nozzle exit due to the annular configuration, the central recirculation zone (CRZ) associated with swirl, the unsteady vortical structures in the jet column due to the shear instability, and the downstream wall confinement. Depending on the degree of swirl, the GRZ near the burner mouth and the CRZ may co-exist or one zone may be overwhelmed by another. At a moderate swirl number, the co-existence leads to a flame with strong reaction attached to the burner mouth; while at a high swirl number, the CRZ dominates over the GRZ. The precessing vortex core was observed to exist in the swirling flow fields. The Nusselt number distribution of the annular impinging flames differs from that of round impinging jets. The POD analysis revealed that wall effects on the flow field are mainly associated with the higher mode numbers.

Grid Resolution Effects on VSFMDF/LES

The grid dependence of LES/VSFMDF is studied on a series of grids with progressively increased resolution reaching over 10 million grids for simulation of a turbulent piloted nonpremixed methane jet flame (Sandia D). In VSFMDF, the effects of the subgrid scale chemical reaction and convection appear in closed forms. The modeled transport equation for the VSFMDF is solved by a hybrid finite-difference/Monte Carlo scheme. A flamelet model is employed to relate the instantaneous composition to the mixture fraction. The simulated results are assessed via comparison with laboratory data. In addition, the dependence of predicted statistics on the grid size of the simulation is studied. The first order moments converge for the finest grid, but the higher order statistics including the PDFs are more sensitive to the grid resolution.

Simulation of Sandia Flame D Using Velocity-Scalar Filtered Density Function

The joint velocity―scalar filtered mass density function methodology is employed for large eddy simulation of Sandia National Laboratories' flame D. This is a turbulent piloted nonpremixed methane jet flame. In velocity―scalar filtered mass density function, the effects of the subgrid-scale chemical reaction and convection appear in closed forms. The modeled transport equation for the velocity―scalar filtered mass density function is solved by a hybrid finite difference/Monte Carlo scheme. For this flame, which exhibits little local extinction, a flamelet model is employed to relate the instantaneous composition to mixture fraction. The simulated results are assessed via comparison with laboratory data and show favorable agreements.

Numerical studies of vortex shedding in forced oscillatory non-premixed flames

A comparative study has been performed to nvestigate the flow nstabilities and their nteractions n a non-premixed methane jet flame using highly accurate numerical methods. The nteraction between the jet flapping modes and buoyancy nstability has been examined by comparing a full three-dimensional case and an idealised axisymmetric case, in order to identify the three-dimensional effects on the vortex dynamics. In addition, air co-flow flames have been numericaly simulated to nvestigate the interaction between the shear and buoyancy nstabilities. It was found that three-dimensional effects dominate the flame dynamics at downstream locations, where the flame exhibits transitional behaviour. The axisymmetric simulation is only able to capture the flow dynamics close to the jet nozzle exit. The air co-flow tends to stabilise the flame and to reduce the flame unsteadiness, which may also have an mpact on the flickering frequency. Three-dimensional effects can greatly affect the strength of flame oscillation energy at downstream locations, where the flapping modes enhance the flame unsteadiness. On the contrary, co-flow tends to reduce the strength of flame oscillation, especialy at upstream locations.

Demonstration of a New Laser Diagnostic Based on Photodissociation Spectroscopy for Imaging Mixture Fraction in a Non-Premixed Jet Flame

The study of turbulent combustion calls for new diagnostics that can measure multidimensional mixture fraction under a wide range of flame conditions. A laser diagnostic technique based on photodissociation spectroscopy (PDS) is proposed to address this need. This paper describes the concept of the PDS-based diagnostic, reports its experimental demonstration in a non-premixed jet flame, and assesses its performance and applicable range. This new technique is centered around the creative use of photodissociation (PD) for flow visualization. A carefully chosen PD precursor is seeded into the flow of interest to measure mixture fraction. The precursor is chosen such that (1) both the precursor itself and the products formed from the precursor (if it reacts) can be completely and rapidly photodissociated; thus, the concentration of one of the photofragments forms a conserved scalar and can be used to infer the mixture fraction, and (2) the target photofragment offers friendly spectroscopic properties (e.g., strong laser-induced fluorescence signals and/or simple signal interpretation) so multidimensional imaging can be readily obtained. Molecular iodine (I(2)) was identified as a precursor satisfying both requirements and was seeded into a carbon monoxide (CO)-air jet flame for single-shot two-dimensional imaging of mixture fraction. This demonstration illustrates the potential of the PDS-based technique to overcome the limitations of existing techniques and to provide multidimensional measurements of mixture fraction in a variety of reactive flows.

층류 비예혼합 분류화염에서 음향가진에 의한 유동특성 연구

공명 주파수의 음파로 가진된 비예혼합 분류화염에서 가진 강도에 대한 영향을 실험적으로 조사하였다. 이를 위해 Mie 산란광 단면법, PLIF, PIV 등 광학적 가시화 방법이 사용되었다. 가진된 분류화염의 부상경로는 연료유량 및 가진 강도에 따라 크게 세 가지 영역으로 구분되는데, 비교적 큰 가진 강도에서 부상될 때 화염기저가 마치 난류화염처럼 부상되는 영역I과 작은 가진 강도에서 쉽게 부상되는 영역II 그리고 이 두 영역들 사이에서 부상된 화염기저가 부분 예혼합의 분젠화염과 같은 형태로 과도기적 영역이 존재함을 알았다. 본 연구에서는 영역I과 II로 구분되는 부상경로 차이에 대한 이해를 높이고자 가진 위상에 따른 노즐과 화염기저간의 유동 혼합과 속도 분포 특성을 아세톤 PLIF와 PIV 시스템을 사용하여 조사하였다. An experimental study has been conducted to investigate the effects of forcing amplitude on the tone-excited non-premixed jet flame of the resonance frequency. Visualization techniques are employed using the laser optic systems, which are RMS tomography, PLIF and PIV system. There are three lift-off histories according to the fuel flow rates and forcing amplitudes; the regime I always has the flame base feature like turbulent flame when the flame lift-off, while the flame easily lift-off in the regime II even if a slight forcing amplitude applied. The other is a transient regime and occurs between the regime I and regime II, which has the flame base like the bunsen flame of partial premixed flame. In the regime I and II, the characteristics of the mixing and velocity profile according to the forcing phase were investigated by the acetone PLIF, PIV system. Particular understanding is focused on the distinction of lift-off history in the regime I and II.

Three-dimensional simulations of cellular non-premixed jet flames

The formation, dynamics and structure of cellular flames in circular non-premixed jets are examined with three-dimensional numerical simulations incorporating detailed descriptions of chemistry and transport. Similar to past experiments reported in the literature, CO 2-diluted hydrogen in diluted or pure oxygen co-flowing streams in the proximity of the extinction limit are considered. As in the experiments, several preferred cellular states are found to co-exist with the particular state realized depending on initial conditions as well as on the jet characteristics. The simulations provide additionally the temporal transitions to different stationary or rotating cellular flames, their detailed structure, and the dependence of the scaling of the realized number of cells with the vorticity thickness.

A Lagrangian Model of Combustion in High-Speed Flows: Application to Scramjet Conditions

The present study is devoted to the computational modeling of non-premixed flames stabilization in high-velocity reactive flows where compressibility effects, turbulent mixing, and chemical kinetics processes are competing. The characterization of the unsteady features of such turbulent reactive flows is still a difficult task, from both experimental and numerical points of view, so that the evaluation of numerical models capabilities remains essentially performed through the comparisons of steady-state solutions with the corresponding experimental data. The Reynolds averaged Navier-Stokes (RANS) strategy still provides the most suitable framework to obtain such steady-state solutions for flows at a high Reynolds number, especially for design and optimization purposes. In turbulent non-premixed flames, the competition between molecular diffusion effects—namely, micromixing or scalar dissipation—and chemical kinetics must be taken into account. In the present work, a Lagrangian framework, able to represent non-premixed combustion in supersonic turbulent reactive flows, is set forth. The main objective of the study is to assess the relevance of the corresponding model to predict turbulent combustion in high-speed flows. The conclusions are drawn from comparisons with results from a well-documented experimental configuration that consists of a supersonic lifted co-flowing hydrogen-air non-premixed jet flame retained to evaluate the ability of the modeling proposal to describe the conversion of kinetic energy into sensible enthalpy. The comparisons between computational results and experimental data are satisfactory and suggest that, despite their complexity, the main physical processes are well described with the proposed approach.