Dump Combustor Research Articles

Large-Eddy Simulations of Turbulent Flows in an Axisymmetric Dump Combustor

A hybrid Eulerian-Lagrangian, mathematical/computational methodology is developed and evaluated for large-eddy simulations of turbulent combustion in complex geometries. The formulation for turbulence is based on the standard subgrid-scale stress models. The formulation for subgrid-scale combustion is based on the filtered mass density function and its equivalent stochastic Lagrangian equations. An algorithm based on high-order compact differencing on generalized multiblock grids is developed for numerical solution of the coupled Eulerian-Lagrangian equations. The results obtained by large-eddy simulations/filtered mass density function show the computational method to be more efficient than existing methods for similar hybrid systems. The consistency, convergence, and accuracy of the filtered mass density function and its Lagrangian-Monte Carlo solver is established for both reacting and nonreacting flows in a dump combustor. The results show that the finite difference and the Monte Carlo numerical methods employed are both accurate and consistent The results for a reacting premixed dump combustor also agree well with available experimental data. Additionally, the results obtained for other nonreacting turbulent flows are found to be in good agreement with the experimental and high-order numerical data. Filtered mass density function simulations are performed to examine the effects of boundary conditions, subgrid-scale models, as well as physical and geometrical parameters on dump-combustor flows. The results generated for combustors with and without an inlet nozzle are found to be similar as long as appropriate boundary conditions are employed.

Effect of Cavity Width on the Unsteady Pressure in a Low-Mach-Number Cavity

T HE flow over a cavity (Fig. 1) has drawn the attention of many researchers (e.g., see the review articles by Rockwell and Naudascher [1] and Colonius [2]) because of its relevance to a range of engineering applications. These include the flow past windows and sunroofs in automobiles, airplane wheel wells, and dump combustors. The flow in these circumstances is characterized by selfsustained, high-pressure fluctuations, which can produce vibration and fatigue of the underlying structure, a high level of noise, and drastic increase in the drag force on the body containing the cavity. Hence, understanding and controlling the unsteadiness associated with cavity flows are important. Rossiter [3] identified the basic mechanism leading to strong, selfsustained oscillations in cavities. More specifically, he proposed that small disturbances in the shear layer separating at the upstream edge of the cavity are amplified, forming periodic vortex structures that travel downstream and interact with the aft edge of the cavity, generating strong pressurefluctuations. Thesefluctuations propagate (“feedback”) acoustically to the separation edge and reexcite the shear layer, hence, sustaining the cavity oscillations. A notable deviation from the Rossiter mechanism is the case where the vortex structures do not form upstream of the cavity’s aft edge. In this situation, self-sustained oscillations could still exist, but they are driven by convective waves, which cause large lateral motion (or “flapping”) of the shear layer near the downstream lip of the cavity (e.g., Sarohia [4] and Chatellier et al. [5]). Another important aspect of self-sustained oscillations of a cavity is the nature of the feedback. As M ! 0, the disturbance-edge interaction is inefficient in producing acoustic disturbances and the feedback is driven by the conservation of the fluid mass within the cavity (e.g., see Martin et al. [6] and Rockwell [7]). Although there is a large body of literature on cavity flows, very few studies have focused on three-dimensional aspects of the flow. Cell structures along the span of the cavity were found in the experiments of Maull and East [8]. Rockwell and Knisely [9] found that secondary, spanwise-periodic, streamwise vortices formed and distorted the primary spanwise vortex structures. More recently, three-dimensional instability analysis of a two-dimensional cavity flowwas carried out by Bres and Colonius [10]. It was found that the most amplified three-dimensional mode had a typical frequency that was an order of magnitude smaller than the frequency of the selfsustained cavity oscillations. This modewas linked to the centrifugal instability of the primary recirculation flow inside the cavity. The objective of this study is to examine the influence of the cavity width on the behavior of the unsteady pressure acting on the cavity floor and how this behavior depends on Reynolds number. A unique aspect of the present investigation is that an axisymmetric cavity geometry is employed to establish a “reference” condition that is free of any end-wall influences. The behavior of this cavity flow can then be compared against that of finite-width cavities byfilling portions of the axisymmetric cavity (see the next section for further details). It is significant to contrast the present work to the study ofMaull and East [8], who employed a rectangular cavity geometry that had to be terminated with end walls (even for the widest cavity). In addition, Maull and East did not report any quantitative measurements of the cavity unsteadiness. As will be seen, the present findings show that, asM ! 0, the accepted criteria, based on cavity depth and length as well as boundary layer thickness, for the occurrence of self-sustained oscillation are not sufficient to guarantee the establishment of the oscillation. The cavity width must also be accounted for; this offers a possible explanation for the discrepancy between recent (Grace et al. [11] and Ashcroft and Zhang [12]) and earlier studies at a low Mach number.

Effect of the Unmixedness of Unburned Gases on the Pressure Fluctuations in a Dump Combustor

Combustion instability is a serious obstacle for the lean premixed combustion of gas turbines and can even cause fatal damage to the combustor and the entire system. Thus, enhanced understanding of the mechanisms of combustion instability is necessary for designing and operating gas turbine combustors. In this study, in order to elucidate the instability phenomena, an experimental study was conducted in a rearward-step dump combustor with LPG and air. The fuel supply conditions and the mixing distances ( L fuel) between fuel and air are used as experimental parameters to examine the effects of fuel modulation and unmixedness. The fluctuations of pressure, heat release, and equivalence ratio were measured by a piezoelectric pressure sensor and a high speed intensified charge coupled device (ICCD) camera, respectively. The unmixedness was measured by acetone laser induced fluorescence (LIF) at nonreacting flow because of stratification of the fuel in air. Various combustion modes occurred in accordance with...

Large-Eddy Simulation of Lean Premixed Turbulent Flames of Three Different Combustion Configurations using a Novel Reaction Closure

This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.

Step Geometry and Countercurrent Effects in Dump Combustor, Part 1: Cold Flow

Nonreacting flow in a backward-facing step combustor is studied while employing a novel countercurrent shear (or counterflow) concept. Counterflow is used to manipulate the turbulent shear layer created by the step to increase turbulent burning velocities, and thereby, reduce ignition delay time. Unfortunately, this concept also leads to a smaller residence time because of a shorter recirculation vortex. These competing challenges of achieving higher burning velocities and longer residence time demand modification of the step geometry. Changes in the step geometry will alter the size and characteristics of the recirculation vortex and the shear layer dynamics within the combustor. These issues are addressed in this paper via a numerical study. For the simulations, Reynolds-averaged Navier–Stokes equations are solved in the framework of the realizable k–� turbulence model. A two-layer approach is used for the near-wall modeling. The paper includes a detailed account of the benefits of countercurrent shear technology, a parametric study based on step-geometry modifications, and an aerodynamic performance evaluation.

Controlling Volumetric Heat Release Rates in a Dump Combustor Using Countercurrent Shear

Research carried out in a backward-facing step combustor burning prevaporized JP10-air mixtures has examined the implementation of counterflow as a means to enhance turbulent combustion and increase volumetric heat release rates. The combustor is characterized by a nominally two-dimensional primary flow of lean reactants entering a rectangular channel before encountering a 2:1 single-sided step expansion. A secondary flow is created via suction at the dump plane as a fluidic control mechanism to enhance the naturally occurring countercurrent shear layer caused by separation. Counterflow is shown to elevate turbulence levels and volumetric heat release rates downstream of the step while concomitantly reducing the scale of the recirculation zone. Modifications to the rearward-facing step geometry are investigated using particle image velocimetry under isothermal flow conditions in an effort to extend the near-field interaction between the recirculation zone and the incoming primary flow, further exploiting the benefits of counterflow. Relative heat release rates quantified using chemiluminescence are shown to increase by 70% with a counterflow level of 6% of the primary mass flow rate in the step geometry, and by 150% when 3.5% counterflow is introduced into a modified step geometry.

Computational studies of turbulent premixed flame based dump combustor

Abstract The present work reports the computational studies of turbulent premixed flame based dump combustor. The effects of various flow parameters such as inlet Reynolds number, inlet turbulence intensity and expansion ratio on important flow quantities like axial velocity, turbulent kinetic energy and turbulent dissipation rate have been studied extensively. It was found out that the axial velocities and the radial velocities within the flowfield are many times higher as compared to the cold flow case due to the heat release and volumetric expansion. The reattachment length for the reacting flow case is found to be lower than that of the cold flow cases due to higher recirculation velocities. A maximum reduction of 64.6% compared to the cold flow reattachment length was observed in the case of turbulence intensity, I = 10%, equivalence ratio, ϕ = 1.00 with an expansion ratio of 2. In contrast, minimum of 41.51% reduction in recirculation length was observed in the case of I = 5%, ϕ = 0.5 for the same expansion ratio. This reduction was quite significant at higher inlet turbulence intensities and at higher equivalence ratios.

연소실 압력 변동이 버너내부의 당량비 변조에 미치는 영향

In order to understand the phenomena of combustion instability, an experimental study was conducted at the moderate pressure and ambient temperature conditions. The flame behavior and the pressure fluctuations were measured in a dump combustor. Various types of combustion modes occurred in accordance with the equivalence ratio and the fuel supplying conditions. The fluctuation of pressure, heat release and equivalence ratio were measured by piezoelectric pressure sensor, high speed Intensified Charge Coupled Device (HICCD) camera and gas chromatography respectively. Two representative modes were self-excited pressure oscillations at the resonance of combustion chamber (200㎐) and instabilities related to the modulated fuel flow rate through the fuel holes (10㎐). It is found that, especially in an unchoked fuel flow condition, the modulation of the fuel flow rate affects the characteristics of flame behavior and pressure fluctuations in a lean premixed flame.

Experimental and Numerical Investigation of a Swirl Stabilized Premixed Combustor Under Cold-Flow Conditions

Planar velocity measurements under cold-flow conditions in a swirl-stabilized dump combustor typical of land-based gas turbine combustors were carried out using two-dimensional particle image velocimetry (PIV). Axial, radial, and tangential velocity components were measured sequentially using two experimental configurations. Mean and root-mean-squared velocity components are presented along with instantaneous realizations of the flowfield. A numerical study of the flowfield using large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) techniques was conducted in an effort to help understand the complex hydrodynamics observed in the experiments. The agreement between the experimental data and LES simulation was good with both showing evidence of a precessing vortex core. The results of the RANS simulation were not as encouraging. The results provide a fundamental understanding of the complex flowfield associated with the relatively simple geometry and also serve as a baseline validation dataset for further numerical simulations of the current geometry. Validation of LES models in a highly swirled, nonreacting flowfield such as the work presented here is an essential step towards more accurate prediction in a reacting environment.

덤프 연소기에서의 연소불안정에 대한 실험적 연구

The combustion instability in a model dump combustor with an exhaust nozzle and the possibility of combustion control using a loudspeaker to these instabilities were studied. By changing inlet velocity, combustor length and equivalence ratio, dynamic pressure signals and flame structures were simultaneously taken. Because inlet velocity and combustor length affect the life time of vortex in the dump combustor, the results showed that as the combustor length increased and the inlet velocity decreased, the instability frequency decreased and the maximum power spectral density of the dynamic pressure generally decreased. Also, instability frequency and maximum power spectral density of the dynamic pressure increased with the increment of equivalence ratio. From the data of close-loop control, the optimum time-delay control using a loudspeaker was confirmed to be able to reduce the vortex shedding induced from the mixed acoustic-convective mode combustion instability.

Active control of lean blowout in a swirl-stabilized combustor using a tunable diode laser

A novel tunable diode laser temperature sensor is used to prevent lean blowout (LBO) in a swirl-stabilized, partially premixed dump combustor which serves as a model of gas turbine combustors. The fast-response temperature sensor is based on rapid, repetitive (2 kHz) wavelength-scanned laser absorption of two vibrational overtone transitions of water vapor near 1.4 μm using a single fiber-coupled diode laser. Although the gas composition and temperature are not uniform along the line-of-sight, low-frequency temperature fluctuations are identified in the FFT power spectrum of the real-time temperature measurements at an optimized location. We found that the fraction of the FFT power at low-frequencies (0–50 Hz) increases exponentially as the flame approaches LBO. The intensity of these low-frequency temperature fluctuations is used to detect the proximity to LBO and as a control variable for the active LBO control system. Successful prevention of LBO during power reduction and transient fuel variation is demonstrated even without knowledge of the LBO fuel/air ratio limit for the specific operating conditions of the combustor. The active control system also can maintain the flame at very lean conditions (0.03 above the LBO equivalence ratio), and thus reduce the LBO margin. To our knowledge, this is the first application of laser-based sensors in active LBO control of swirl-stabilized flames. The results demonstrate the potential of tunable diode laser sensors for closed-loop combustion control applications.

AERODYNAMICS OF FILM COOLING JET INTERACTION IN A DUMP COMBUSTOR

A numerical study of the interaction between a wall jet and the swirling turbulent flow has been conducted in an axisymmetric dump combustor. The streams are considered to be isothermal. The necessary conditions for the formation of a wall film from the injected jet are analyzed. It is observed that the wall film can be formed more readily (i.e., with less injected air mass) when the slot is located at the corner and on the peripheral wall rather than on the wall of the inlet plane. A higher rate of injection is required to establish the film over the complete wall when the inlet swirl number is higher. Tangential velocity in the wall jet is found to reduce the ability to form a continuous wall film. The injected jet reduces the size of the central recirculation zone, which is even more prominent when the wall film is completely formed by thwarting the corner recirculation in the dump combustor.

재순환 및 선회 유동에 대한 대와동모사(LES)의 성능검토

The objective of this study is to evaluate the efficiency and the prediction accuracy of developed large eddy simulation (LES) program for complex turbulent flows, such as recirculating and swirling flows. To save the computational cost, a Beowulf cluster system consisting 16 processors was constructed. The flows in backward-facing step and dump combustor were examined as representative recirculating and swirling flows. Firstly, a direct numerical simulation (DNS) for laminar backward-facing step flows was previously conducted to validate the overall performance of program. Then LES was carried out for turbulent backward-facing step flows. The results of laminar flow showed a qualitative and quantitative agreement between simulations and experiments. The simulations of the turbulent flow also showed reasonable results. Secondly, LES results for non-swirling and swirling flows in a dump combustor were compared with the results of Reynolds-averaged Navier-Stokes (RANS) using standard κ-ε model. The results show that LES has a better performance in predicting the mean axial and azimuthal velocities, corner recirculation zone (CRZ) and center toroidal recirculation zone (CTRZ) than those of RANS. Finally, it was examined the capability of LES for the description of unsteady phenomena.

Active Control of Lean Blowout for Turbine Engine Combustors

A complete, active control system has been developed to permit turbine engine combustors to operate safely closer to the lean-blowout (LBO) limit, even in the presence of disturbances. The system uses OH chemiluminescence and a threshold-based identification strategy to detect LBO precursor events. These nonperiodic events occur more frequently as the LBO limit is approached. When LBO precursors are detected, fuel entering the combustor is redistributed between a main flow and a small pilot, so as to increase the equivalence ratio near the stabilization region of the combustor. This moves the effective LBO limit to leaner mixtures, thus increasing the safety margin. The event-based control system was demonstrated in an atmospheric pressure, methane-air, swirl-stabilized, dump combustor. The NOx emissions from the piloted combustor were found to be lower than those from the unpiloted combustor operating at the same safety margin and same nominal velocity field. The controller minimizes the NOx at constant total power by keeping the pilot fuel fraction at the lowest value needed to limit the number of precursor events to an acceptable level.

혼합기 공급방식에 따른 덤프연소기의 연소 불안정성에 관한 실험적 연구

The combustion instability of turbulent flames is the most important problem of the gas turbine combustor. Thus improved understanding of mechanisms of combustion instability is necessary for the design and operation of gas turbine combustors. In this study, the cause of the combustion instability in a rearward-step dump combustor was investigated with respect to the fuel flow modulation; choked fuel flow, unchoked fuel flow and fully premixed mixture flow. We observed various types of combustion instabilities with respect to the change of equivalence ratio, fuel flow conditions and fuel injection location. Particularly in the unchoked fuel flow condition, it was found that the oscillation time of combustion instability is strongly related to the convection time of the fuel and that the pressure fluctuation in a lab-scale combustor is highly related to the vortex and the equivalence ratio fluctuations due to fuel flow modulation and unmixedness of the fuel and air.

Experimental study on flame structure and temperature characteristics in a lean premixed model gas turbine combustor

Experimental study was carried out in an atmospheric pressure, laboratory-scale dump combustor showing features of combustion instabilities. Flame structure and heat release rates were obtained from OH emission spectroscopy. Qualitative comparisons were made between line —integrated OH chemiluminescence image and Abel—transformed one. Local Rayleigh index distributions were also examined. Mean temperature, normalized standard deviation and temperature fluctuations were measured by coherent anti-Stokes Raman spectroscopy (CARS). To see the periodic behavior of oscillating flames, phase—resolved measurements were performed with respect to the pressure wave in the combustor. Results on system damping and driving characteristics were provided as a function of equivalence ratio. It also could be observed that phase resolved temperatures have been changed in a well—defined manner, while its difference between maximum and minimum reached up to 280K. These results would be expected to play an important role in better understanding of driving mechanisms and thermo-acoustic interactions.

A fractal flame-wrinkling large eddy simulation model for premixed turbulent combustion

Combustion plays an important role in a wide variety of industrial applications, such as gas-turbines, furnaces, spark-ignition engines, and various air-breathing engines. The ability to predict and understand the behavior of reacting flows in practical devices is fundamental to improved combustors with higher efficiency and reduced levels of emissions. At present, large eddy simulation is considered the most promising approach for premixed combustion modeling since the large-scale energy containing flow structures are resolved on the grid. However, the typically thin reaction zone cannot be resolved. To overcome this difficulty flamelet models, in which the reaction is assumed to take place in thin layers, wrinkled by the turbulence can sometimes be used. In these models, the turbulent flame speed can be represented as the product of the laminar flame speed, S u, corrected for the effects of stretch (strain and curvature) and the flame-wrinkling, Ξ. In this study, we propose to model Ξ using fractal theory. This model requires sub-models for the fractal dimension, and the inner and outer cut-offs—the latter being set by the grid. A model is proposed for the inner cut-off, whereas an empirical parameterization is used to provide the fractal dimension. The proposed model is applied to flame kernel growth in homogeneous isotropic turbulence in a fan-stirred bomb and to a lean premixed flame in a plane symmetric dump combustor. Good qualitative and quantitative agreement with experimental data were obtained for the proposed model in both cases. Comparison with other well-known turbulent flame speed closure models shows that the proposed model behaves at least as good, or even better, than the reference models.

희박 예혼합 덤프 연소기에서 OH 자발광을 이용한 열 방출에 관한 실험적 연구

Measurements of OH chemiluminescence in an atmospheric pressure, laboratory-scale dump combustor at equivalence ratios ranging from 0.63 to 0.89 were reported The signal from the first electronically excited state of OH to ground state was detected through a band-pass filter with an ICCD The objectives of this study are two: One is to see the effects of equivalence ratio on global heat release rate and local Rayleigh index distribution To get the local Rayleigh index distribution, the line-of-sight Images were inverted by tomographic method, such as Abel de-convolution Another aim is to investigate the validity of using OH chemiluminescence acquired with an ICCD as a qualitative measure of local heat release. For constant inlet velocity and temperature, the overall intensities of OH emission acquired at different equivalence ratio showed periodic and higher value at high equivalence ratio OH intensity averaged over one period of pressure increased exponentially with equivalence ratio. Local Rayleigh index distribution clearly showed the region of amplifying or damping the combustion instability as equivalence ratio increased. It could provide an information/insights on active control such as secondary fuel injection. Finally, local heat release rate derived from reconstructed OH Images were presented for typical locations.

CARS를 이용한 희박 예혼합 가스터빈 연소기내 온도 측정 (II)-당량비가 위상별 온도에 미치는 영향-

The effect of equivalence ratio and fuel/air mixing quality on the phase-resolved gas temperatures at different phases of the oscillating pressure cycle was experimentally investigated An atmospheric pressure, optically accessible and laboratory-scale dump combustor operating on methane With heat release rate of 1 59㎾ was used Temperature measurements were made using coherent anti-Stokes Raman spectroscopy (CARS) at several spatial locations for typical unstable combustion conditions Analysis was conducted using parameters such as phase-resolved averaged temperature, normalized standard deviation and temperature probability distribution functions (PDFs). Also the probability on the occurrence of high temperature (over 1900K) was investigated to get the information on the perturbation of equivalence ratio and NOx emission characteristics It was shown that most of temperature histograms exhibit Gaussian profile which has short breadth of temperature fluctuation at equivalence ratio of 0 6, while beta profile was predominant for the cases of other equivalence ratios (φ=0.55, 0 50) It was also shown that phase-resolved averaged temperature oscillated In phase with pressure cycle, while normalized standard deviations which represent temporal turbulent intensity of temperature showed nearly constant value around 0 1 The characteristics on the occurrence of high temperature also displayed periodic wave form which was very smilar to the pressure signal And the amplitude of this profile went larger as the fuel/air mixing quality became poorer These also provided additional information on the perturbation of equivalence ratio at flame as well as NOx emission characteristics.

Large eddy simulation and experimental studies of a confined turbulent swirling flow

Laser Doppler velocimetry (LDV) measurement and large eddy simulation (LES) were used to study confined isothermal turbulent swirling flows in a model dump combustor. The aim was to gain deeper understanding of the flow and turbulence structures in dump combustors and to examine the capability of LES for prediction of turbulent swirling flows. A refractive index matching technique is used in the LDV measurement to improve the near-wall data. A high-order finite difference scheme on Cartesian grids with a scale-similarity subfilter scale model is used in the LES. Turbulent inflow boundary conditions with different energy spectra, different outflow boundary conditions, and grid resolutions are tested in the LES. Three test cases with different swirl numbers and Reynolds numbers are studied in the measurements and the simulations. The Reynolds numbers range from 10 000 to 20 000, and the swirl number is varied from 0 to 0.43. With appropriate inflow, outflow boundary conditions, and fine grid resolution, the LES results are in fairly good agreement with the LDV data. The experimental and numerical results show that turbulence in the dump combustor is highly anisotropic behind the sudden expansion and in the internal recirculation zone near the axis of the combustor. Turbulence decays rapidly along the streamwise direction downstream, and the structure of turbulence depends highly on the level of inlet swirl. At low swirl numbers, turbulence is primarily generated in the shear layer behind the sudden expansion; at high swirl numbers the near axis flow becomes very unstable and vortex breakdown occurs. The shear layer near the axis of the combustor caused by vortex breakdown generates most of the turbulent kinetic energy. Large-scale motions (coherent structures) are found in the near axis vortex breakdown region. A helical flow in the guiding pipe breaks down near the sudden expansion to form a large bubble-like recirculation zone whose center moves slowly around the axis. Downstream of the bubble the core of the rotational large scale azimuthal flow motion is off the combustor axis and rotates around the axis at a frequency about 18–25 Hz (Strouhal number about 0.17–0.4). As the swirl number increases the coherent structure becomes more evident, and the internal recirculation zone moves upstream. LES successfully simulated the vortex breakdown, the internal recirculation zones and the anisotropic turbulence structures for all the swirl numbers considered.