Turbulent Combustion Characteristics Research Articles

The decoupled effect of oxidizer mass flux and combustion pressure on turbulent combustion characteristics of finite-length solid fuel

Combustion pressure and oxidizer mass flux are key factors influencing the performance of hybrid rocket motors. In the present work, the decoupled effects of oxidizer mass flux and combustion pressure on turbulent combustion characteristics were investigated. First, a finite-length combustion theory of fuel grains was developed to identify the main sensitive parameters, including flame structure and the recirculation zone, which affect the combustion of solid fuel. Then, a two-dimensional transient numerical model, based on the coupling characteristics of the heat transfer process in solid fuel grains and dynamic combustion flow, was developed and validated through fire experiments under a wide range of inflow conditions. A quantitative analysis was conducted to assess the impact of flame structure and the recirculation zone on the dynamic combustion characteristics of finite-length solid fuel. The results showed that the fuel characteristic temperatures of the front and central parts are negatively correlated with flame height, while the characteristic temperature of the rear part is positively correlated with the recirculation zone temperature. Both the mass flow rate and combustion pressure are negatively correlated with flame height and positively correlated with the temperature of the recirculation zone. Compared to combustion pressure, the influence of mass flow rate is more significant. The transient processes of the solid fuel regression rate under different inflow conditions exhibit a consistent tendency: an increase in mass flow flux and combustion pressure results in faster attainment of peak regression rate and stabilization.

Effect of the Smagorinsky Constant on Turbulent Combustion Characteristics for GH2/LOx Shear Coaxial Injectors at Supercritical Pressure

Effect of the Smagorinsky Constant on Turbulent Combustion Characteristics for GH2/LOx Shear Coaxial Injectors at Supercritical Pressure

Experimental observation of the combustion modes based on a novel multistage pre-chamber turbulent jet ignition (TJI) system

Transient jet flame propagation under newly proposed multistage pre-chambers is studied in a constant-volume combustion chamber with a high-speed schlieren photography system. Various combustion behaviors, including the flame tip velocity, jet emergence timing, projected flame area, pressure, and heat release rate, are investigated under different pre-chamber structures. The present work will provide constructive insight into the design, manufacture, and application of turbulent jet ignition engines. It is shown that the pre-chamber structure determines the main chamber flame development by influencing the flame development inside the pre-chamber. As the flame is accelerated by an obstacle in the pre-chamber, faster exit velocity of hot jet and intense turbulence are observed in the main chamber. In addition, the overall development of the jet flame in the main chamber can be separated into two stages, the former of which is dominated by jet flows, while the latter stage is controlled by the chemical reaction under different excess air coefficients, presenting turbulent combustion characteristics. In this work, six ignition modes under ultra-lean conditions are observed, including (1) jet ignition occurrence on the entire jet surface due to the sufficiently high reactivity; (2) local ignition in the middle of the hot jet; (3) local multipoint ignition and ignition at the jet tip; (4) ignition induced by delayed burning at the jet root; (5) jet tip ignition with backward flame propagation; and (6) global extinction. For the effect of initial pressure, it is found that under stoichiometric conditions, the initial pressure has a minor influence on flame tip propagation, while it significantly influences pressure evolution and heat release rate. However, the increase in initial pressure can improve flame propagation and pressure evolution under lean conditions. Under near-extinction conditions, the ignition mode could be switched from unstable ignition to stable ignition. A numerical simulation is also conducted to reveal the flame development inside the pre-chamber under different pre-chamber structures.

Measurement and scaling of turbulent burning velocity of ammonia/methane/air propagating spherical flames at elevated pressure

This study reports the accurate laminar burning velocity, turbulent burning velocity and its correlations of ammonia/methane/air propagating spherical flames. The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber with ammonia molar content varying from 20% to 60% and the initial pressure up to 3 bar. The turbulent burning velocity decreases with the ammonia content due to the weakening effect of the laminar burning velocity under all turbulence intensities and pressures studied. Since the weakening of flame chemistry is dominated by the enhancement of turbulence eddies, the normalized turbulent burning velocity increases with the ammonia content. The turbulent expanding flame of ammonia/methane/air is self-similar under different ammonia content. This self-similar propagation follows the one-half power-law correlation between the normalized turbulent burning velocity, ST/SL, and the turbulent flame Reynolds number, which is quantitatively consistent with that of unity Lewis number methane/air flames (Chaudhuri 2012). The pressure dependence of turbulent burning velocity can be represented roughly as a 0.4 power law of ST/SL and (u′/SL)(P/P0). However, there is a quantitative gap between the pre-exponential factor of the present experimental data and the literature data based on this correlation, which could attribute to the difference in the turbulence eddy scales of different experimental apparatus. The integral length scale characterized the largest turbulence eddies is introduced to consider the turbulent length scale effect. A modified general correlation ST/SL∼(LI/L0)0.5[(u′/SL)(P/P0)]0.41 with the consideration of the integral length scale effect is obtained, which is able to predict a variety of spherical flame data regardless of temperatures, pressures, and fuel types. In addition, it is verified that turbulent burning velocity of ammonia flame could be expressed by the correlation of Karlovitz and Damköhler numbers: ST/SL∼Ka·Da=ReT,flow0.5. It can be seen that ammonia has similar turbulent combustion characteristics as hydrocarbon fuel. These findings indicate that it is feasible to simulate and optimize ammonia combustors utilizing previous turbulent burning velocity correlations based on hydrocarbon fuel.

Effects of ammonia addition on combustion characteristics in partially-premixed swirling ammonia/methane/air flame

Ammonia combustion has received intense research interest recently for its potential to reduce CO2 emission. This study aims to investigate the turbulent combustion characteristics in a bluff-body burner for CH4/NH3 mixtures with different ammonia blending ratios (15%, 30%, and 45% by mole fraction) through large eddy simulation and experiments. The simulations are conducted using openFOAM with a low Mach number solver and the partially stirred reactor combustion model with a detailed reaction mechanism. The flow field of one typical case is measured using the particle image velocimetry technique to verify the accuracy of the numerical results.The combustion characteristics are discussed. As the ammonia blending ratio increases, the flame height shortens, the flame color gradually changes from blue to orange, and the intermittent local quenching zone moves upstream, indicating that the combustion is becoming unstable. Meanwhile, the flow fields exhibit similar characteristics though the ammonia concentration varies greatly. The CO and NO emissions are also discussed. The CO emission decreases and the NO emission increases as the ammonia blending ratio increases

Experimental study on characteristics of CH4/H2 oxy-fuel turbulent premixed flames

In this work, a systematic investigation was conducted on CH4/H2 oxyfuel turbulent premixed flames. The effects of hydrogen addition and CO2 concentration on turbulent combustion characteristics of methane oxy-fuel were studied. Based on planar laser induced fluorescence (PLIF) measurement of OH radical, the turbulent flame speed and flame front structure were derived. The probability density function (PDF) of curvature radius of hydrogen enriched turbulent premixed flames was found to be similar to that of cellular flames on flat burner, which identified the role of thermal-diffusive instability. Proper orthogonal decomposition (POD) was further applied to the multi-scale analysis of flame front structure. With increase in hydrogen fraction, the effect of small-scale wrinkles was enhanced by thermal-diffusive effect, which promoted the flame propagation and made the flame prone to instability.

Investigation of Low-Temperature Lean Combustion Characteristics in Power Plants with External Heating of Components

An integral part of any open-type gas turbine plant is a low-emission combustion chamber, which is usually two-zone and cooled. One of the ways to reduce emission of harmful substanc-es is organizing low-emission low-temperature lean combustion with external heating of compo-nents. This paper investigates the effect of external heating of air and fuel gas on expansion of the lower combustion limit and stable flame position in a single-zone uncooled combustion chamber of a microgas turbine power plant. Stable position of the flame front in combustion chambers of this type mainly depends on the ratio between the average flow rate of the combus-tible-air mixture and the rate of turbulent combustion. This ratio depends on thermal, gas-dynamic, thermochemical and geometric factors. The purpose of this work is to substantiate the possibility of using the relative flow rate as a generalized characteristic. This goal was achieved in processing a large amount of published experimental data and numerical modeling of low-temperature combustion of lean mixtures. The most significant research result is determination of the range of relative flow rate (gk = 0.3…3.5·10-4 kg⁄s∙N), at which it is possible to ensure sta-ble flame position in a single-zone combustion chamber. Significance of the obtained results lies in the fact that using the relative flow rate makes it possible to quickly determine and analyze the geometric and gas-dynamic parameters and characteristics of turbulent combustion in com-bustion chambers of micro-gas turbine power plants.

A Numerical Investigation on the Rapid Turbulent Filtration Combustion under High Background Pressure

ABSTRACT A numerical investigation of the rapid turbulent filtration combustion within a packed pebble bed is conducted at a fixed superficial velocity of 0.5 m/s, equivalence ratio of 1.0 (methane/air) under two background pressures of 1.0 and 0.5 MPa. Turbulence is modeled with the detached eddy simulation (DES) method. The interaction between turbulence and chemical reaction is considered through the fast reaction method, i.e., burning velocity model (BVM). To validate the DES method, the relative turbulence intensities in a 3D staggered cylinders are verified previously which are in excellent agreement with the experimental data (no reaction). With the confidence in the DES method, the turbulent combustion characteristics in a 3D packed pebble bed are studied in detail, including pressure fluctuation, spatial inhomogeneities of velocity and temperature fields, and evolution of the flame surface. Numerical results show that a strong pressure fluctuation arises after ignition due to thermal expansion contributing significantly to the velocity and temperature fluctuations for the case of 1.0 MPa, whereas it is relatively moderate for the case of 0.5 MPa. Though the evolution of the flame surface both can be divided into two stages for the two cases, i.e., initial-rapid spreading stage, and stable propagating stage, there is a slight difference between them. Specifically, it closely resembles to an atomic blast for the former, while it is similar to a volcanic eruption for the later. Undergoing this stage, about 20 ms, the wrinkle and fragmented flames or flamelets creep over the pebbles through the thin layers close to the walls. Statistical stability is identified in the process of flame propagating upstream. Turbulent flame speeds obtained in this study agree well with the experimental data, especially for the case of 1.0 MPa.

ГАЗОДИНАМИКА УСТОЙЧИВОГО ГОРЕНИЯ В КАМЕРЕ СГОРАНИЯ ГАЗОТУРБИННОЙ УСТАНОВКИ С ВНЕШНИМ ПОДОГРЕВОМ КОМПОНЕНТОВ

The relevance of the study is due to the need to organize stable low-emission low-temperature combustion of a lean mixture in a single-zone uncooled combustion chamber, which is an integral part of a micro-gas turbine power plant. The stable position of the flame front in combustion chambers of this type mainly depends on the ratio between the average flow rate of the combustible-air mixture and the rate of turbulent combustion. This ratio depends on many factors, the main of which are the pressure and temperature of the components supply, the excess air ratio, the pulsating speed of the gas flow and autoturbulization of the flame, the consumption of the combustible-air mixture. In this work, we investigate the effect of external heating of the components on the expansion of the lower combustion limit and on the stable position of the flame front. The aim of the study is to obtain areas of breakthrough, stable position and flame blowout when organizing low-temperature lean combustion with large values of the excess air ratio; determination of the ranges of relative flow rate, at which a stable position of the flame front is observed, using experimental data and the results of numerical modeling; development of recommendations for determining the geometric appearance of a single-zone uncooled chamber of a micro-gas turbine power plant in the presence of air and fuel gas recuperators. As a result, the dependences of the normal combustion rate on the excess air ratio were obtained taking into account the lower combustion limit, the relationship between the average flow rate and the turbulent combustion rate in various combus-tion modes, and ranges for the relative flow rate in the region of low-temperature stable combustion. Based on the developed gas-dynamic model, the geometric and gas-dynamic parameters and characteristics of turbulent combustion in the flow in the combustion chambers of micro-gas turbine power plants with a capacity of 100 and 300 kW with external heating of air and fuel gas using a turbocompressor with a compression ratio of 3.0 are determined and analyzed.

A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methane–air flames

Ammonia has been identified as a promising energy carrier that produces zero carbon dioxide emissions when used as a fuel in gas turbines. Although the combustion properties of pure ammonia are poorly suited for firing of gas turbine combustors, blends of ammonia, hydrogen, and nitrogen can be optimized to exhibit premixed, unstretched laminar flame properties very similar to those of methane. There is limited data available on the turbulent combustion characteristics of such blends and important uncertainties exist related to their blow-out behavior. The present work reports experimental measurements of the blow-out limits in an axisymmetric unconfined bluff-body stabilized burner geometry of NH3/H2/N2-air flame, comprised of 40% NH3, 45% H2, and 15% N2 by volume in the “fuel” blend. Blow-out limits for the NH3/H2/N2-air flames are compared to those of methane–air flames. OH PLIF and OH chemiluminescence images of the flames just prior to blow-out are presented. Furthermore, two large-scale Direct Numerical Simulations (DNS) of temporally evolving turbulent premixed jet flames are performed to investigate differences in the turbulence-chemistry interaction and extinction behavior between the NH3/H2/N2-air and methane–air mixtures. The experiments reveal that the blow-out velocity of NH3/H2/N2-air flames is an order of magnitude higher than that of methane–air flames characterized by nearly identical unstretched laminar flame speed, thermal thickness and adiabatic flame temperature. Results from the DNS support the experimental observation and clearly illustrate that a methane–air mixture exhibits a stronger tendency towards extinction compared to the NH3/H2/N2-air blend for identical strain rates. Furthermore, the DNS results reveal that, even in the presence of intense sheared turbulence, fast hydrogen diffusion into the spatially distributed preheat layers of the fragmented and highly turbulent flame front plays a crucial role in the enhancement of the local heat release rate and, ultimately, in preventing the occurrence of extinction.

Large Eddy Simulation on the Effects of Coal Particles Size on Turbulent Combustion Characteristics and NOx Formation Inside a Corner-Fired Furnace

Abstract The effects of pulverized coal particles’ sizes on the coal combustion characteristics are numerically studied in a laboratory-scale tangentially fired furnace. The turbulent gas flow and the coal particle motion are solved by employing the large eddy simulation (LES) and the discrete phase model (DPM). The mixture fraction probability density function (MF-PDF) is coupled to simulate the non-premixed pulverized coal combustion. It is found that the coal combustion efficiency is positively affected by the dispersion of coal powders. The particle dispersion and the coal combustion are augmented by the intensive impingement caused by the corner-injected flow. Large coal particles, with their greater inertia, enhance particle agglomerations, which limit the combustion of volatile and char. Accordingly, the average flame temperature decreases with the growing particle sizes. Also, the O2 concentration increases slightly because of the incomplete coal combustion, and the CO2 concentration decreases gradually. In contrast, the CO concentration increases markedly in the furnace center due to the presence of a reducing atmosphere. The NO concentration exhibits an exponential decline with the increased particle size. A relatively stable combustion and a relatively low NOx formation are acquired inside such a corner-fired furnace when the particle Stokes number is a little greater than 1.

Experimental study on premixed turbulent flame of methane/air in a spherical closed chamber

A turbulent combustion test platform is designed. It is demonstrated that the turbulent environment in a constant volume combustion chamber is isotropic and controllable. Based on the platform, the turbulent combustion characteristics of methane/air are studied. The collected flame image and combustion pressure data are analysed. The results show that when the initial temperature is 300 K, the initial pressure is 1 bar, and the equivalence ratio is ϕ=1, the flame shape maintains self-similarity in the propagation process under different turbulence intensities. With the increase in turbulence intensity, there are more cracks on the flame surface and more wrinkles on the flame front; with the increase in turbulence intensity, the combustion propagation rate increases, the flame propagation rate increases at the same flame radius, and the maximum combustion pressure and the rate of pressure rise increase. When the turbulence intensity is constant, the test results of equivalence ratios of 0.8, 1.0 and 1.2 are compared. At an equivalent ratio of 1, the flame propagation rate and the stretch rate of the flame are maximized, and the maximum burning pressure and pressure rise rate are maximized.

Turbulent burning velocity of stoichiometric syngas flames with different hydrogen volumetric fractions upon constant-volume method with multi-zone model

Syngas has been widely concerned and tested in various thermo-power devices as one promising alternative fuel. However, little is known about the turbulent combustion characteristics, especially on outwardly propagating turbulent syngas/air premixed flames. In this paper, the outwardly propagating turbulent syngas/air premixed flames were experimentally investigated in a constant-volume fan-stirred vessel. Tests were conducted on stoichiometric syngas with different hydrogen volumetric fractions (XH2, 10%–90%) in the ambience with different initial turbulence intensity (u'rms, 0.100 m/s~1.309 m/s). Turbulent burning velocity was taken as the major topic to be studied upon the multi-zone model in constant-volume propagating flame method. The influences of initial turbulent intensity and hydrogen volumetric fraction on the turbulent flame speed were analysed and discussed. An explicit correlation of turbulent flame speed was obtained from the experimental results.

Effect of carbon dioxide content in biogas on turbulent combustion in the combustor of micro gas turbine

China has huge potential on biogas resources and the distributed energy system (DES) based on micro gas turbine provides an effective way for biogas utilization. In this work, turbulent combustion characteristics in the combustor of micro gas turbine for DES were numerically and experimentally studied, and the effect of carbon dioxide content in biogas on the turbulent flame was mainly analyzed. Turbulent flames structures of natural gas and three kinds of biogas under various equivalence ratios were measured by three-dimensional (3D) combustion diagnostic technique based on computed tomography of chemiluminescence (CTC). The results show that the carbon dioxide content in the fuel has a great influence on flame structure of turbulent combustion. The increase of carbon dioxide content in the fuel not only reduces turbulent combustion rate, but also makes the combustion stability worse. With the decrease of equivalence ratio, the effect becomes greater. With the increase of carbon dioxide volume fraction, combustion flame is gradually away from the outlet of the nozzle. When volume fraction of carbon dioxide is 50%, the distance between the combustion flame and the nozzle outlet increases the 100 mm. Furthermore, carbon dioxide weakens the effect of excess air on the CH* distribution and turbulent flame.

Experimental study on the turbulent premixed combustion characteristics of 70%H2/30%CO/air mixtures

Abstract To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H2/30% CO/air mixtures at various equivalence ratios and turbulence intensities were conducted in a turbulent combustion bomb at atmospheric temperature and pressure. The turbulent burning velocity and flame curvature were used to study turbulent combustion characteristics. The results show that the turbulent burning velocity grew nonlinearly as the equivalence ratio increased, while the normalized turbulent burning velocity tended to decrease. When the equivalence ratio was relatively low, the turbulence intensity was a greater determinant of the burning velocity. The normalized turbulent burning velocity increased as the turbulence intensity increased. Re and Da were found to be directly and inversely proportional to u’/uL, respectively. A linear relationship was observed between uT/uL and ln Re. As the turbulence intensity increased or equivalence ratio decreased, the wrinkle degree of the flame front increased, and the maximum and minimum values of flame front curvature increased and decreased, respectively. Meanwhile, the range of the flame front curvature increased gradually. The proportion of components with smaller absolute value of flame front curvature gradually decreases.

Characteristics of hydrogen jet combustion in a high-enthalpy supersonic crossflow

The facilitation of a stable combustion process is of utmost importance for the realizability and performance of hypersonic propulsion systems. To elucidate the turbulent combustion characteristics, wall-modeled large eddy simulations of a transverse jet injection into a heated supersonic flow are conducted employing a detailed reaction mechanism. The computation framework utilizes an adaptive central-upwind weighted essentially nonoscillatory (WENO-CU) scheme to achieve the sixth-order accuracy in smooth flowfields, while keeping a good shock-capturing ability. The reacting zones agree well with experimental measurements in terms of the instantaneous distribution of OH radicals. And the flame penetration height has been predicted with an error of less than 17%. It is found that the turbulent reacting flow is dominated by nonpremixed combustion mainly taking place in the near-wall region and jet windward shear-layer. Moreover, the autoignition process, which plays a critical role in stabilizing supersonic combustion, shows to favor a fuel-lean or not very fuel-rich environment of a high enthalpy. Local scalar dissipation induced by turbulence gives rise to a rapid fuel mixing with the surrounding air. However, this effect may also lead to the decrease in local temperature.

Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure

Ammonia is one of the most promising alternative fuels. In particular, ammonia combustion for gas turbine combustors for power generation is expected. To shift the fuel for a gas turbine combustor to ammonia step-by-step, the partial replacement of natural gas by ammonia is considered. To reveal the turbulent combustion characteristics, CH4/NH3/air turbulent premixed flame at 0.5 MPa was experimentally investigated. The ammonia ratio based on the mole fraction and lower heating value was varied from 0 to 0.2. The results showed that the ratio of the turbulent burning velocity and unstretched laminar burning velocity decreased with an increase in the ammonia ratio. The reason for this variation is that the flame area decreased with an increase in the ammonia ratio as the flame surface density decreased and the fractal inner cutoff increased. The volume fractions in the turbulent flame region were almost the same with ammonia addition, indicating that combustion oscillation can be handled in a manner similar to that for the case of natural gas for CH4/NH3/air flames.

Verification and Validation of a Low-Mach-Number Large-Eddy Simulation Code against Manufactured Solutions and Experimental Results

To investigate turbulent reacting flows, a low-Mach number large-eddy simulation (LES) code called ‘LESsCoal’ has been developed in our group. This code employs the Germano dynamic sub-grid scale (SGS) model and the steady flamelet/progress variable approach (SFPVA) on a stagger-structured grid, in both time and space. The method of manufactured solutions (MMS) is used to investigate the convergence and the order of accuracy of the code when no model is used. Finally, a Sandia non-reacting propane jet and Sandia Flame D are simulated to inspect the performance of the code under experimental setups. The results show that MMS is a promising tool for code verification and that the low-Mach-number LES code can accurately predict the non-reacting and reacting turbulent flows. The validated LES code can be used in numerical investigations on the turbulent combustion characteristics of new fuel gases in the future.

Large eddy simulation of cavity-stabilized hydrogen combustion in a diverging supersonic combustor

Turbulent combustion characteristics in a one-sided diverging scramjet combustor are investigated by large eddy simulation (LES), especially for the flame regimes and turbulence/chemistry interactions. The model combustor designed by National University of Defense Technology consists of a wall-mounted cavity and an injector that allows a sonic hydrogen jet injecting into the high-enthalpy supersonic crossflow. In the simulation, an adaptive central-upwind 6th-order weighted essentially nonoscillatory (WENO-CU6) scheme along with multi-threaded and multi-process MPI/OpenMP parallel is adopted to improve the accuracy and parallel efficiency of solver. Supersonic turbulent boundary layer is generated by a multi-wall recycling/rescaling method at the inflow, in which turbulent fluctuations support a more rapid jet mixing. In addition, the chemical source term is closed by an assumed probability density function (PDF) approach that accounts for the fluctuations of temperature and composition. Validation is performed for the distribution of wall pressure and time-averaged structures, and the predicted results show in good agreement with experimental data. Through the bow shock, supersonic fluid with high enthalpy is stagnated in front of the jet plume, leading to the occurrence of auto-ignition. However, the local scalar dissipation rate here is so large that strong chemical reaction is inhibited. It is found that the combustion is anchored at the trailing edge of cavity accompanied by violent heat release, which is governed by a cavity-stabilized diffusion flame. The jet-cavity interactions extend the residence time of fluid and entrain plenty of hot intermediate products into the cavity, which is beneficial to the ignition and flame stabilization.

Hydrogen–air turbulent flame propagation in the presence of microdroplets

A technique has been developed for evaluating turbulent combustion characteristics in the presence of microdroplets obtained by vapor condensation in the course of rapid expansion. Experiments have been conducted to visualize spark-initiated flame propagation through hydrogen-air mixtures at various turbulent intensities, water-vapor volume fractions, and microdroplet concentrations.The influence of microdroplet suspensions on iginition and flame propagation is investigated.