Flame Tube Research Articles

Kinetic Simulation of Combustion Process in the Combustor of the Aero-Engine

The use of detailed chemical reaction mechanisms of aviation fuels is still very limited in analyzing the combustion process concerning complex multidimensional fluid dynamics issues. In this work, a reduced chemical kinetic mechanism containing 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species was introduced and studied in the kinetic simulation of the ignition and combustion of n-decane (n-C10H22) at various combustion regimes. Moreover, it was validated with the experimental results obtained from both premixed flame of a flat-flame burner and n-C10H22 shock-tube ignition. The results show that the calculated ignition delay time of fuel n-C10H22 in the shock-tube under typical conditions viz. a pressure of 12 or 50 bar and corresponding equivalence ratio at 1.0 or 2.0, as well as the calculated mole fractions of the main reactants and products of fuel n-C10H22 in the premixed combustion process, agree very well with the experimental data. The combustion processes in the single flame tube of a tube annular combustor were simulated through coupling this reduced reaction mechanism of surrogate fuel n-C10H22 and one step reaction mechanism of surrogate fuel C12H23 into the computational fluid dynamics (CFD) software, and this combustion process was also experimentally studied. It is found that the reduced reaction mechanism exhibits clear advantages in simulating the combustion processes and certain species formation in the single flame tube over the one step reaction mechanism. In particular, the temperature radial distribution at the outlet of the single flame tube computed by using reduced reaction mechanism of surrogate fuel n-C10H22 is found to be more reasonable and agreed better with the experimental data than that computed by one step reaction mechanism of surrogate fuel C12H23.

Effect of radiation losses on very lean methane/air flames propagating upward in a vertical tube

The stationary upward propagation of a very lean methane/air flame in a long vertical tube open at the bottom and closed at the top is simulated numerically using a single overall chemical reaction to model combustion and assuming an optically thin gas and a transparent or non-reflecting tube wall to approximately account for radiation losses from CO2 and H2O. Buoyancy plays a dominant role in the propagation of these flames and causes a large region of low velocity of the burnt gas relative to the flame to appear below the flame front when the equivalence ratio is decreased. The size of this region scales with the radius of the tube, and its presence enhances the effect of radiation losses, which would be otherwise negligible for a standard flammability tube, given the small concentration of radiating species. Heat conduction is found to be important in the low velocity region and to lead to a conduction flux from the flame to the burnt gas that causes extinction at the flame tip for a value of the equivalence ratio near the flammability limit experimentally measured in the standard tube. The effect of radiation losses decreases with the radius of the tube. Numerical results and order-of-magnitude estimates show that, in the absence of radiation, a very lean flame front fails to propagate only after recirculation of the burnt gas extends to its reaction region and drastically changes its structure. This condition is not realized for the standard flammability tube, but it seems to account for the flammability limit measured in a tube of about half the radius of the standard tube.

Bioethanol Combustion in an Industrial Gas Turbine Combustor: Simulations and Experiments

Combustion tests with bioethanol and diesel as a reference have been performed in OPRA's 2 MWe class OP16 gas turbine combustor. The main purposes of this work are to investigate the combustion quality of ethanol with respect to diesel and to validate the developed CFD model for ethanol spray combustion. The experimental investigation has been conducted in a modified OP16 gas turbine combustor, which is a reverse-flow tubular combustor of the diffusion type. Bioethanol and diesel burning experiments have been performed at atmospheric pressure with a thermal input ranging from 29 to 59 kW. Exhaust gas temperature and emissions (CO, CO2, O2, NOx) were measured at various fuel flow rates while keeping the air flow rate and air temperature constant. In addition, the temperature profile of the combustor liner has been determined by applying thermochromic paint. CFD simulations have been performed with ethanol for five different operating conditions using ANSYS FLUENT. The simulations are based on a 3D RANS code. Fuel droplets representing the fuel spray are tracked throughout the domain while they interact with the gas phase. A liner temperature measurement has been used to account for heat transfer through the flame tube wall. Detailed combustion chemistry is included by using the steady laminar flamelet model. Comparison between diesel and bioethanol burning tests show similar CO emissions, but NOx concentrations are lower for bioethanol. The CFD results for CO2 and O2 are in good agreement, proving the overall integrity of the model. NOx concentrations were found to be in fair agreement, but the model failed to predict CO levels in the exhaust gas. Simulations of the fuel spray suggest that some liner wetting might have occurred. However, this finding could not be clearly confirmed by the test data.

Initiation and formation of the corrugated structure leading to the self-turbulization of downward propagating flames in a combustion tube with external laser absorption

Combustion tube (length 45cm, inner diameter 5cm) experiments with flames of premixed gas of C2H4/CO2–O2 (Le<1) were conducted. The flame fronts propagated downward to the closed bottom of an open-ended tube. An initially steadily propagating flat flame was deformed by an external laser irradiation method to investigate its evolution under the interaction with acoustic vibration. Results showed that the locally deformed flame evolved into a corrugated structure at the flame front followed by self-turbulization. The process to form this corrugated structure was investigated in detail based on the images captured using high-speed cameras. From the observations, a possible mechanism for the initiation of the corrugated structure, explained mainly by periodic acoustic acceleration, was proposed. Then, according to the mechanism an alternative definition for the inverse Froude number is proposed in this work and used as criterion for the initiation of the corrugated flame structure. To prove the validity of the criterion two mixtures having different flame speed were tested and it was confirmed that the criterion provided transition condition very well for both tested mixtures.

3d calculation of a multiport low-emission combustor cooling system

The results of calculating coupled heat transfer for four versions of a multiport flame tube are presented. The walls of the tube of an experimental combustion chamber have a double segment structure with an impact-convective cooling system. The calculations were made taking into account the combustion and the influence of radiant fluxes. The thermal state of the flame tube walls determined by the results of a numerical experiment is compared with the data of full-level tests of the combustion chamber as a part of a gas generator of an engine. Recommendations are given concerning the modernization of the flame tube structure in order to improve the thermal state of its segments.

Quasi one-dimensional model and calculation of processes in gas turbineengine combustion chambers given the heterogeneity of the combustible mixture, changes in mass, composition, temperature and stirring efficiency of the working body

The methodology of one- dimensional calculation of the combustion chamber existing at SSAU has been significantly complemented: a) the uneven distribution of fuel in the combustion zone and its combustion in the range of excess air ratio 0.55 ... 1.65 has been taken into account; b) the amount of vaporized fuel at each site is determined by considering the size of evaporating droplets; c) for each fraction of the fuel burnt the equilibrium composition and the adiabatic equilibrium flame temperature is determined; d) a scheme of mixing the combustion products with the secondary combustion air and mixing air is proposed in order to evaluate the non-uniformity of the temperature of exhaust gas at the output of the combustor; e) a mixing quality factor is introduced. The impact of the completeness of mixing, composition and temperature of the gas on the quantity of nitrogen oxides along the length of the flame tube, the NO emission index and the non-uniformity of the temperature field in the output section is assessed. The updated method of calculation is illustrated by the low- emission aviation turbine engine combustor.

The influence of engineering changes on NOx emission in a gas-turbine endine combustion chamber

The paper presents a model of calculating nitrogen oxide formation in the combustion chamber of a gas turbine engine. The model has made it possible to make the calculations and determine the characteristics of nitrogen oxide concentration from the total air ratio in combustion chambers with various extents of opening the flame tube head. The processes in the commercial and shortened gas turbine engine combustor chambers are analyzed.

The influence of swirling of a flow with axial countercurrent on the process of burning of hydrogenoxygen mix in the stream of water steam

The paper presents comparative results of the numerical research of the combustion of stoichiometric hydrogen-oxygen mixture in the medium of water steam using various ways of mixing fuel components in the flame tube of a tubular combustion chamber.The geometry of the mixing chamber that makes it possible to achieve the highest values of combustion efficiency and temperature uniformity at the exit of the combustion chamber is defined.

Influence of the turbine ignition, flame tube suspension and nozzles on the combustion chamber characteristics

The paper presents a mathematical model of the working process of a gas turbine engine combustion chamber that makes it possible to take into account the effect of the flame tube suspension, ignition and turbine nozzles on the working process of the combustion chamber and to estimate the effect.

On choice of a swirler for the impulse combustion chamber

In this paper, some problems on implementation of an air-fuel mixture combustion process in the flame tube with a GTE impulse combustion chamber flow swirler are considered. When the peripheral swirler is placed in the flame tube head of this chamber, a number of 3-D reverse flow zones are formed to effectively mix up the air-fuel mixture providing its steady-state combustion. As a result, the thrust impulse is increased and a level of atmospheric emissions of NOx, CO, and some other detrimental compounds is significantly decreased.

Characterization of Lean Burn Module Air Blast Pilot Injector With Laser Techniques

For lean burn combustor development in low emission aero-engines, the pilot stage of the fuel injector plays a key role with respect to stability, operability, NOx emissions, and smoke production. Therefore it is of considerable interest to characterize the pilot module in terms of pilot zone mixing, fuel placement, flow field, and interaction with the main stage. This contribution focuses on the investigation of soot formation during pilot-only operation. Optical test methods were applied in an optically accessible single sector rig at engine idle conditions. Using planar laser-induced incandescence (LII), the distribution of soot and its dependence on air/fuel ratio, as well as geometric injector parameters, was studied. The data shows that below a certain air/fuel ratio, an increase of soot production occurs. This is in agreement with smoke number measurements in a standard single sector flame tube rig without optical access. Reaction zones were identified using chemiluminescence of OH radicals. In addition, the injector flow field was investigated with PIV. A hypothesis regarding the mechanism of pilot smoke formation was made based on these findings. This along with further investigations will form the basis for developing strategies for smoke improvement at elevated pilot-only conditions.

Detonation propagation characteristic of H2–O2–N2 mixture in tube and effect of various initial conditions on it

As for the premixed H2–O2–N2 gas ignited and induced by flame in tube, this paper represents systemic researches on its detonative formation process and flow field changes under different initial conditions (pressure, temperature, component concentration). The conservational Euler equation set with chemical reaction is taken as the basic gas phase equation model and the reduced elementary chemical reaction and shock wave problem are considered available so as to establish a theoretical model of premixed H2–O2–N2 combustible gas detonation process. A unity coupling TVD format with second-order accuracy is adopted to solve the gas phase equation and deduce the two-dimension Riemann invariant, and the TVD format for solution of the polycomponent convection equation with elementary chemical reaction is proposed. Meanwhile, a time splitting format is adopted to perfectly treat with the rigid problem resulted from the higher time difference value between gas phase flow characteristic time and chemical reaction characteristic time. It is shown by the calculation results that the detonation waves form certain angle with relation to the tube wall surface at the initial stage of ignition, so as to incur reflections and form reflection waves; during the propagation of the detonation waves, the reflection wave structures are propagated backwards the back of waves constantly, so the whole flow field is characterized of obvious two-dimension. Besides, the excessive pressure detonation occurs at first before formation of the stable detonation propagation process, then a stable detonation propagation process forms finally. Mixed gas detonation characteristics resulted from different calculated-initially parameters are different. The higher the initial temperature and pressure of flame is, the shorter the induction time for detonation formed due to combustion acceleration of the mixed gas is, but which nearly brings no great influence on the later propagation process of the detonation waves. The initial mixed gas component can influence the detonation characteristic of the mixed gas observably, when the quantity relative ratio is close to 1 and the mixed gas with larger reaction activity, its detonation propagation speed is rapider and the pressure after detonation waves is higher. The simulation result keeps accordant with the calculated result of the typical C–J detonation theory model.

좁은 다중 동축 석영관 내부에서의 예혼합 화염의 전파 특성에 대한 기초 실험

Flame stabilization characteristics of premixed flames in narrow annular coaxial tubes (NACT) were investigated experimentally. The NACT burner was proposed as a model of a cylindrical refractory burner, and it was made of quartz tubes. Flame stabilization conditions affected by the characteristic length of the burner was investigated with the variation of the equivalence ratio and the flow rates. Flame behaviors in narrow spaces could be directly observed. Conclusively, more wide flame stabilization conditions could be obtained at the case of the decreased channel scale. A flame instability, such as combustion noise was detected concerned with the flame oscillation observed at the surface of multi channel stage. Some flame propagation characteristics had complicated tendencies that may exist in practical porous-media combustors. Therefore, this NACT burner can be a basic configuration for the development of flame stabilization model in porous media combustor, and it will enhance our understanding about the behavior of flames in meso-scale combustion spaces.

Chemical kinetic simulation of kerosene combustion in an individual flame tube

The use of detailed chemical reaction mechanisms of kerosene is still very limited in analyzing the combustion process in the combustion chamber of the aircraft engine. In this work, a new reduced chemical kinetic mechanism for fuel n-decane, which selected as a surrogate fuel for kerosene, containing 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species was developed, and the ignition and combustion characteristics of this fuel in both shock tube and flat-flame burner were kinetic simulated using this reduced reaction mechanism. Moreover, the computed results were validated by experimental data. The calculated values of ignition delay times at pressures of 12, 50bar and equivalence ratio is 1.0, 2.0, respectively, and the main reactants and main products mole fractions using this reduced reaction mechanism agree well with experimental data. The combustion processes in the individual flame tube of a heavy duty gas turbine combustor were simulated by coupling this reduced reaction mechanism of surrogate fuel n-decane and one step reaction mechanism of surrogate fuel C12H23 into the computational fluid dynamics software. It was found that this reduced reaction mechanism is shown clear advantages in simulating the ignition and combustion processes in the individual flame tube over the one step reaction mechanism.

Thermoacoustic instability of a laminar premixed flame in Rijke tube with a hydrodynamic region

In this work, a Rijke tube with a hydrodynamic region confined is considered to investigate its non-normality and the effect of the hydrodynamic region on the system stability behaviors. Experiments are first conducted on Rijke tubes with different lengths. It is found that the fundamental mode frequency is decreased and then increased, as the flame is placed at different axial positions at the bottom half of the tube. This trend agrees well with the prediction from the thermoacoustic model developed, of which the hydrodynamic region is modelled as an oscillating ‘airplug’ and the flame dynamics is captured by using classical G-equation. In addition, the flame as measured is found to respond differently to oncoming acoustic disturbances. Modal and non-modal stability analyses are then conducted to determine the eigenmode growth rate and the transient one of acoustic disturbances. The ‘safest’ and most ‘dangerous’ flame locations as defined as those corresponding to extreme eigenmode and transient growth rate are estimated, and compared with those from the model without the hydrodynamic region. In order to mitigate such detrimental oscillations, identification and mitigation algorithms are experimentally implemented on the Rijke tube. The sound pressure level is reduced by approximately 50dB. To gain insights on the thermoacoustic system, transfer function of the actuated Rijke tube system is measured by injecting a broad-band white noise. Compared with the estimation from our model, good agreement is observed. Finally, the marginal stability regions are estimated.

Experimental Study of Atomization Characteristic of Air Blast Atomizer

Experimental investigation results of the fuel injector group in a heavy-duty gas turbine are presented. Atomization characteristic has great impact about combustion, inflame, temperature field of outlet. Obtained atomization characteristic about spray particle size and spray angle using LDV/PDPA system, determined dimension of injector group. On the basis of these tests, the combustion testing of the injector group in the flame tube is made, its every targets are arrived in the design requirements. This has demonstrated: the test systems and test methods are practical, feasible and reliable. These experimental data have provided the reliable basis for the injector group design and development.

Analysis of limit cycles sustained by two modes in the flame describing function framework

The Flame Describing Function (FDF) framework, developed for the nonlinear instability analysis of combustors, has been validated more recently in a generic configuration comprising an upstream manifold, an injection unit and a flame tube. This system featuring a wide variety of dynamical phenomena, is used here to explore a new range of self-sustained flame oscillations. Depending on the geometry, the system exhibits stable or variable amplitude limit cycles. In the first case, oscillations have an essentially constant amplitude and are well retrieved with the FDF framework, whereas in the second case, limit cycles feature different types of amplitude unsteadiness and require some further consideration. The present article is concerned with one type of unstable oscillation in which a regular period modification occurs in the presence of two modes, leading to frequency heterodyning. It is found from the FDF analysis that such oscillations sustained by two modes may occur when there is an overlap between modes corresponding to super and subcritical bifurcations. An additional condition which has to be fulfilled to obtain this behavior is inferred from experiments.

Simulation of flow and mixture formation nonstationarity in combustion chambers

In this paper, attention was focused on the matter of simulating a flow, the mixture and vapor formation in the GTE combustion chamber without regard for combustion. The software developed by the authors was used for simulation. For comparison and analysis, also presented are the simulation results obtained in Fluent software. The characteristic features of a flow and the distribution of vaporous and droplet-liquid fuel throughout the flame tube model volume are revealed.

Studies on Aerodynamic Characteristics of Dump Diffusers for Modern Aircraft Engines

The geometrical optimization of dump diffusers are extremely demanding as the flow fields and stress fields are very complex and must be well understood to achieve the required design efficiencies. In this paper parametric analytical studies have been carried out for examining the aerodynamics characteristics of different dump diffusers for modern aircraft engines. Numerical studies have been carried out using SST K- ω turbulence model. This code solves SST k- ω turbulence equations using the coupled second order implicit unsteady formulation. In the numerical study, a fully implicit finite volume scheme of the compressible, Reynolds-Averaged, Navier-Stokes equations is employed. We concluded that in addition to the dump gap ratio, the aerodynamic shape of the flame tube case and the other geometric variables are also need to be optimized judiciously after considering the fluid dynamic constraints for controlling the pressure recovery and the losses.

Scaling the flame transfer function of confined premixed conical flames

The influence of confinement side walls on the response of premixed conical flames submitted to velocity disturbances is investigated experimentally and theoretically. When the flame is sufficiently confined, the burnt gases cannot fully expand at the nozzle outlet. In that case, the confinement ratio Cr=R0/R1 between the burner and flame tube radii and the burnt to unburnt gas volume expansion ratio E=ρu/ρb need to be taken into account in the description of the flame transfer function (FTF). The main effect of confinement is an acceleration of the fresh stream of reactants induced by the over-pressure of the confined burnt gases. Experiments on steady flames reveal that the flame height increases for increasing confinement ratios Cr, leading to a gain shift and a phase drop for the FTF. A theoretical analysis is conducted to model this acceleration and examine its impact on the steady flame shape and flame response to flow disturbances. The change in the FTF phase is shown to be related to a reduction of the mean time lag between heat release rate perturbations and velocity modulations induced by both the velocity acceleration in the fresh reactants and a change in the location of the steady flame front. An expression to scale the FTF of confined flames is derived based on a modification of the classical reduced frequency ω∗ used to scale the response of unconfined flames. This expression includes explicitly the confinement ratio Cr and the volume expansion ratio E. This new dimensionless number enables to plot FTF measured for different confinements with reasonable collapse. It may also be used to transpose FTF gathered on single burners to multiple injection configurations when the burnt gases cannot fully expand due to the presence of neighboring flames.