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Combustion
Experimental and numerical investigations of structure and stability of premixed swirl-stabilized CH4/ O2/CO2 flames in a model gas-turbine combustor Medhat Ahmed Nemitallah, Ahmed A. Abdelhafez, and Mohamed A. M. Habib Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04438 • Publication Date (Web): 16 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019
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Experimental and numerical investigations of structure and stability of premixed swirl-stabilized CH4/O2/CO2 flames in a model gasturbine combustor Medhat A. Nemitallah*, Ahmed Abdelhafez, and Mohamed A. Habib
TIC on CCS and Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia
Abstract Premixed CH4/O2/CO2 flames were studied experimentally and numerically in a swirl-stabilized combustor. The bulk throat velocity of the incoming combustible mixture was maintained constant at 5.2 m/s. The oxygen fraction of O2/CO2 oxidizer was kept constant at 60% vol., while the effect of equivalence ratio was examined from the blowout limit to the flashback one. The LES computational model was successfully validated through comparisons with experimental data in terms of axial and radial temperature profiles, as well as predicted OH* concentration maps vs. visual flame appearance. Three different flame structures were observed while varying the equivalence ratio, namely (I) double conical flame, (II) corner-stabilized flame, and (III) swirl-stabilized (V-shaped) flame. The double conical flame is observed at low equivalence ratios near the blowout limit. This flame stabilizes within both the inner and outer shear layers and has two distinct reaction zones with two inner recirculation zones (IRZ) separated by a colder, reaction-free corner recirculation zone (CRZ). The corner-stabilized flame is observed at higher equivalence ratios. The downstream reaction zone diminishes and merges with the upstream one to form a flame that stabilizes within a reacting CRZ. Increasing the equivalence ratio further induces a transition to the typical V-shape of swirl-stabilized flames with significantly stronger IRZ and weaker CRZ. This flame is thus stabilized by its IRZ. The Vshape prevails until flashback occurs, when the hot reaction zone moves closest to the burner throat. Keywords: Premixed flames; Oxy-methane combustion; CO2-diluted oxy-flames; Swirl-stabilized flames; Flame structure; Stabilization mechanism. * Corresponding author: M.A. Nemitallah; E-Mail:
[email protected]; Tel: +96613-860 4959; Fax: 966-13-860-4959.
Nomenclature AFT CH4 CO2 CRZ IRZ LES O2 O.F. PVC
Adiabatic flame temperature Methane Carbon dioxide Corner recirculation zone Inner recirculation zone Large eddy simulation Oxygen Oxygen fraction, volumetric percentage of oxygen in the O2/CO2 oxidizer mixture Processing vortex core
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SLPM Standard liters per minute Z Axial location inside the combustor Greek symbols φ Equivalence ratio ρ Density of reactant mixture μ Dynamic viscosity of reactant mixture
1. Introduction Heavy-duty gas turbines are widely used in power plants in a very excessive way. There are two dilemma associated with operating such heavy duty gas turbines for long time, these two dilemmas lie on the abundance of fossil fuels for power generation (natural gas and coal) and increasing the environmental restrictions and constraints regarding the flue gas emissions (CO2, CO and NOx). To satisfy such restrictions, researchers have developed different combustion techniques including hydrogen-rich fuel combustion,1,2 and oxy-fuel combustion with carbon capture.3,4 Gas turbines combustors for power generation normally adapt non-premixed flame type thanks for its reasonable combustor performance and higher flame stability characteristics. Nowadays, this kind of combustors is obsolete as non-premixed flames are the main source of NOx emissions. The development of more restricted environmental regulations forced the researchers to invent novel combustors capable of meeting such strict regulations. In the last two decades, new technologies developed for gas turbine power production industry including, lean premixed combustion (LPM) and catalytic combustion (CC). Catalytic combustion is highly expensive with low safety and durability.1 While in LPM, fuel and oxidizer mixed upstream before introduced to the combustor burner. In such LPM system, fuel burned under lean combustion condition to prevent the creation of elevated-temperature spots inside the flame core. Consequently, NOx emissions are significantly reduced.2,3 However, such kind of LPM flames suffers from lower ranges of stable flame operation as compared to non-premixed flames, in addition to that fact that NOx emissions are not totally eliminated. The LPM flames are more prone to static (flashback and blow-out limit) and dynamic (thermo-acoustic) instabilities.4-7 Oxy-fuel combustion is such a promising method for controlling CO2 emissions from power plants. This technique if merged with carbon capture and sequestration (CCS) technologies will facilitate the capturing process due to the high concentration of CO2 in the exhaust stream.8,9 The main challenge in this case is to retrofit the current air-fuel combustors with oxy-fuel burners at minimum required modifications and performance deficiency. In oxy-fuel power plants, oxygen separation units were commonly used to separate oxygen from air; however, these separation units consume great portion of the output power for the separation of oxygen in the oxycombustion power plants.10 Therefore, oxy-fuel combustion systems are preferred to be operated near stoichiometry (equivalence ratio near unity). Such combustion systems are also designed to accommodate flue gas recirculation in order to control the combustion temperature.11,12 Combining premixed combustion with oxy-combustion may result in complete control of NOx emissions with the benefit of ease of CO2 capture under oxy-combustion condition. However, the use of CO2 as a diluent in oxy-combustion, instead of N2 in normal air combustion, introduces additional challenges on flame stabilization owing to the dissimilar physical properties of CO2 and N2, which necessitates the modification of the burner design to meet such
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differences. In addition, CO2 is radiative in the infrared zone and becomes active and participates in the chemical reactions under high-temperature operation, which may reduce the rates of reaction kinetics within the flame and add more restrictions on flame stability.13,14 The aim of the present work is to develop a computational tool able to calculate the combustion characteristics of premixed oxy-methane flames. This area of research, where the fully premixed methane flames are burnt in O2/CO2 mixtures, have been limitedly covered in the literature.14-21 In our recent publications,22,23 experimental investigations of premixed oxy-fuel combustion are carried out with different aspects. These studies investigate the flame stability limits and typical premixed flame characteristics, including laminar flame speed and the adiabatic flame temperatures, of a premixed gas turbine model combustor. The results at fixed inlet velocity showed that the flames flashback and blowout at fixed adiabatic flame temperature (AFT). Marsh et al.15 investigated the methane combustion in N2 and CO2 diluents based on the measurements of the stability limits, flame stabilization position and emissions at burner capacity of 37.5 KW. They investigated different oxygen fraction varied from 0.21 to 0.70. They reported that the CO2 dilution has a remarkable impact than N2 on flame position and amount of heat release. This was attributed to the differences in the transport properties of CO2 and N2. The dilution of CO2 results in high concentration of CO carried out with the exhaust gases due to lower flame temperature associated with the thermal dissociation of CO2. On the other hand, increasing N2 dilution results in more NOx emissions. Jourdain et al.16 performed an experimental study to compare between the premixed CO2 diluted and N2-diluted flames to investigate their stabilization mechanisms. The study revealed that, for a similar swirl number, CO2 and N2 diluted flames feature similar shapes while maintaining the AFT, the equivalence ratio, and the incoming jet velocity the same. This study revealed that CO2-diluted flames could be stabilized with the same flame configuration as N2-diluted flames without hardware modifications. Watanabe et al.17 studied experimentally premixed oxy-flames and air-flames in a swirl-stabilized combustor. They wanted to compare between oxygen and air flames by maintain the same equivalence ratio, AFT, and Reynolds number as well. They observed at high equivalence ratio that the flame stabilizes within the shear layers of the flame. An important feature of swirling flow is the so-called Processing Vortex Core (PVC), which is identified as a source of incoherent frequency leading to uncomfortable noise.20 When the recirculation zone is not stable, the PVC take place and start spinning around the axis of symmetry. Furthermore, a recent experimental and numerical investigation of swirl flames in a gas turbine model combustor is carried out by Weigand et al.24,25 They report their analysis in terms of two major aspects: (a) combustion flow field, flame macrostructure, temperature and species concentration; and (b) turbulence chemistry interactions. They reported the flame macrostructure using the planar laser induced fluorescence (PLIF) showing OH and CH radicals. They highlighting that the most representative way to report flame macrostructure is the OH radical molar concentration within the combustor domain. Nemitallah et al.26 studied lean premixed propane-air flame stability in a backward-step facing combustor using the large eddy simulation (LES) turbulent scheme. They reported that the flame stabilizes strongly on the back-step side of the combustor at higher equivalence ratio and, at lower equivalence ratio, the flame lifts-off the step point and the lift-off distance increases while reducing the equivalence ratio. The aim of this study is to investigate flow-field, combustion and stability characteristics of CO2-diluted CH4/O2 flames in a swirl-stabilized premixed combustor. Specifically, to show how
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the operability limits, visual flame appearance (flame macrostructure) and the combustion characteristics are changing as function of the operating equivalence ratio at fixed O.F. of 60%. To perform such analysis, a computational code was developed based on LES model in the full 3-D domain of the combustor. Experimental measurements were also carried out to characterize the flame in terms of temperature distributions and flame shape. The developed computational model was successfully validated using the measured data of the same combustor.
2. Experimental setup 2.1 Test conditions This study aims at examining the effect of equivalence ratio on the macrostructure and combustion characteristics of atmospheric, non-preheated, swirl-stabilized, premixed CH4/O2/CO2 flames at fixed oxygen fraction (O.F.) of 60% and a common inlet bulk velocity of 5.2 m/s. The O.F. is defined as (1) Where and are the volume flow rates (in SLPM) of O2 and CO2, respectively. The reader is referred to previous studies by the authors22,23 for a detailed explanation of how the flow rates of CH4, O2, CO2 were adjusted for each data point to maintain the same inlet bulk velocity for any given combination of equivalence ratio and oxygen fraction. Figures 1 and 2 show schematics of the test setup and combustor used to examine premixed swirl-stabilized CH4/O2/CO2 flames. Detailed descriptions of the combustor design, dimensions, and supporting instrumentation are available in past studies.22,27,28 It is important to emphasize here, though, that the combustor was designed to ensure an almost perfectly premixed reactant mixture (CH4+O2+CO2). This is to avoid having variations in local equivalence ratio at burner throat, which will significantly affect flame stability and macrostructure. Perfect premixing also allows for a direct comparison of the reported experimental and numerical results, especially that all the numerical simulations conducted here have considered perfectly premixed inlet mixtures without any variations in local equivalence ratio. Visual flame shape was quantified experimentally by capturing images of visual flame appearance using a 10-Mega-pixel camera with 1/60-s shutter speed, 5.6 f-stop, and 1600 ISO. Local temperature was also measured at selected locations within the combustor using an R-type (PtRh13%-Pt) thermocouple of 1-mm junction diameter. The readings were corrected for radiation error to the surroundings using the model of Brohez et al.29 It is also worth mentioning here that the combustor power density ranged from ~3.0 to ~5.5 MW/m3/bar, which is comparable to most industrial gas turbines, as these are typically operated in the range 3.5-20 MW/m3/bar.30
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Figure 1: Schematic diagram of the experimental test setup.
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Figure 2: Schematic diagram of the premixing combustor.
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2.2 Experimental procedure The CH4, CO2 and O2 mass flow rates are varied depending on each other for maintaining the bulk flow velocity constant at the throat of the burner. In this way, all three-flow rates are connected for any combination of ϕ and OF. The flow rates adjustment is required for every movement from one data point to the other, even for the case when both data points share the same ϕ or OF. For plotting flame stability maps of this reactant mixture, oxygen fraction was scanned one value at a time in steps of 5%, from 15% to 70%. While the equivalence ratio spanned from 0.1 to 1.0. Following approach is used for finding out the flashback and blowout limits of flame, for each OF. For safety, and avoiding flashback, fuel line is equipped with an emergency shut-off valve, which instantly stops the fuel flow. For further information regarding the equipment, previously published work can be referred.22
3. Numerical modelling of the premixed flames This section presents the numerical geometry, grid independency study, governing equations, and models used to simulate the examined swirling oxy-methane flames. The numerical model used is a combination of the presumed probability density function (PDF) and the large eddy simulation (LES) viscous model, while the premixed combustion model was applied for simulating oxy-fuel flames stabilized over a 55o swirler. 3.1 Geometric model A 3-D cylindrical-based geometry of the gas turbine model combustor was established using Gambit to be used in the full 3-D numerical simulations performed using Ansys-Fluent 19.2. The swirl-stabilized premixed gas turbine combustor model is represented in Figure 3 with all dimensions and the swirler location. The length of combustion chamber is 30 cm. D= 5 cm
Exhaust Section D= 2 cm
D= 7.5 cm
Swirler
Combustion Chamber
Figure 3: Dimensions of the combustor geometry. 3.2 Grid independency study Quadrant meshing was developed for the 3-D geometry to reduce the computational time, wherein 28k tetrahedral cells were implemented. A mesh independency study was conducted to assure that the results are independent of mesh size. Figure 4 compares three different mesh
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sizes, namely 28k, 33k, and 36k. As shown in the Figure, increasing the total number of cells beyond 28k yields no significant differences; therefore, 28k cells were used in this study to save the computational cost and time.
Figure 4: Grid independency study for different total number of cells. 3.3 LES combustion modeling Solving the conversation equation is challenging, particularly in the case of turbulent reactive flows that involve a wide spectrum of length and time scales. The multi-scale and multi-physics attributes of turbulent combustion necessitate the use of modeling approaches in order to simplify the pathetical description of the complex physical phenomena. Three different approaches may be identified: (i) Direct Numerical Simulation (DNS), (ii) Reynolds-Averaged Navier-Stokes (RANS), and (iii) Large-Eddy Simulation (LES); a detailed description of each approach was reviewed by Denis and Vervisch.31 In case of LES, rather than averaging the effect of turbulence, the equations are filtered: The larger turbulent eddies are explicitly resolved and computed, as they have a more significant influence on the flow and are more dependent on geometry, whereas the effect of smallest eddies is modeled using sub-filter-scale models. Thus, in contrast to RANS, there is partial resolution of the turbulent fluctuations in LES, which decreases the uncertainty associated with modeling but increases the computational cost. Turbulence transfers energy from large eddies to the small eddies where energy is dissipated according to the following spectrum:32
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(1) Where, is the Kolmogorov constant. In LES, the various flow quantities, Φ, are filtered in the spectral space (by suppressing the components greater than a given cut-off length), or in the physical space (weighted averaging in a given volume). The filtered quantity Φ is expressed as follows:
(2) Where, g is a filter function. For variable density flows, like in the present study, Favre, or density-weighted, filtered quantity, Φ, can be expressed as follows: (3) Often, the grid is used as the spatial filter. The governing equations for LES are obtained by Favre filtering operation to each term in the conservation equations, including, continuing equation,1 momentum equation,2 energy equation,3 species transport equation.4 These equations are presented below as following: Continuity Equation:26,33 (1) Where
is the filtered velocity vector.
Conservation of momentum:26,33 (2) Where and refer to the filtered viscous stress tensor and the corresponding sub-grid scale (SGS) term, respectively. Species Transport Equation:26,33 (3) Where D is the molecular diffusivity, and Yk and reaction rate, respectively.
refer to the specie mass fraction and
Conservation of Energy:26,33 (4) Where refers to the filtered total specific energy, and is the filtered heat flux. More details on modeling the unresolved terms can be found in.34,35 All of the aforementioned transport equations were discretized by a second-order upwind scheme using the finite volume and underrelaxation method provided in Ansys fluent.
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During turbulent combustion, chemical reactions are confined to thin reacting layers at small scales that cannot be resolved on typical LES grids. Therefore, most of the turbulence-chemistry interactions need to be modeled. This modeling of the reaction rates presents a major challenge in turbulent premixed combustion, because reaction rates are highly nonlinear functions of temperature and species mass fractions. An integral component of LES is thus the turbulent combustion sub-grid model, which is necessary to incorporate the effect of turbulence-chemistry interactions at the under-resolved scales on the reaction rate. Researchers to model the filtered reaction rates corresponding to specific flame regime have adopted various approaches. The probability density function (PDF) model is used in this study. In this presumed-PDF framework, the reaction rate and the thermo-chemical properties are averaged over the presumed probability density function, which does not require solving a transport equation. Furthermore, the reaction chemistry can be taken into account using the flamelet generated manifold (FGM) approach. This technique focuses on including detailed chemistry effects within the combustion model by reducing the chemical subspace using a set of reference laminar flamelet computations and storing the chemical flame structure inside a lookup table. Hence, this approach can reduce the computational cost of preforming reacting flow with extensive chemical kinetic mechanisms. In this work the focus is on the presumed-PDF approach coupled with the FGM technique and a description of the method can be found in Ref.33 LES is very attractive for inherently unsteady simulations, especially those dominated by largescale turbulent structure, including combustion instability and fire spread. LES is compelling for reacting flows because of several reasons. One reason is that the large turbulent eddies are anisotropic, long lasting, and depend on the geometry. The small eddies, on the other hand, are isotropic, more easily modeled, and can be determined by the large eddies as well. In addition to that, the LES is a more accurate turbulence model because LES has few or no adjustable constants as compared to the RANS model constants, which require adjustment to free jets, jets in cross flow, swirl, buoyancy, etc. In contrast to RANS, the LES modeling error decreases as grid resolution is increased. Furthermore, combustion is often controlled by the rate the large eddies mix reactants with products, and these large eddies are directly resolved in LES. Unlike other turbulence models, the LES model treats effectively the flow near walls. In the current study, large-eddy simulations were conducted using the presumed probability density function (PDF) coupled with the flamelet generated manifold (FGM). The effect of equivalence ratio (φ) is examined. All simulations were performed at a common oxygen fraction (O.F.) of 60% and inlet bulk velocity of 5.2 m/s, similar to the experimental tests. Table 1 summarizes the considered operating conditions.
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Table 1: Summery of the operating conditions and configuration parameters. Oxy-fuel combustion (CH4/O2/CO2) with oxygen fraction (O.F.) = 60% Equivalence ratio, φ 0.35 – 0.70 Combustor power (based on equivalence ratio) 2.25 – 4.25 kW Inlet mixture bulk velocity 5.2 m/s Operating pressure 1 bar Inlet mixture temperature 300 K Turbulence intensity Viscosity ratio
5% 10%
To consider convection and radiation heat transfer, a mixed thermal boundary condition was chosen for the combustor wall. A coupled scheme was selected for the pressure-velocity coupling, while the first-order upwind scheme was selected for the spatial discretization, such as momentum, turbulent kinetic energy, etc. The convergence criterion was 10-3 for the continuity, momentum, and species equations and 10-6 for the energy equation.
4. Results and discussion 4.1 Comparisons of measured and predicted temperature and flame shape For the sake of validating the numerical work, Figure 5 presents a comparison between the predicted centerline temperature distribution and the corresponding experimental data at φ = 0.5 and O.F. = 60% with a theoretical adiabatic flame temperature (AFT) of ~2500 K. It has to be mentioned here that the theoretical adiabatic flame temperature does not account for CO2 dissociation, which was proven in past studies to reduce that theoretical value by 100–200 K only.15 Good agreement can be observed between the numerical and experimental results with only 5% deviation. A peak temperature of about 1900 K is predicted at an axial location Z ≈ 2.5 cm, which indicates the presence of an inner recirculation zone (IRZ) where the local temperature is below AFT. A radial profile would thus reveal that the temperature does not peak at the centerline.
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Figure 5: Predicted centerline temperature distribution compared to the corresponding experimental data. Figure 6 presents another comparison between predicted and experimental data in form of radial temperature profiles at Z = 5 cm, φ = 0.5, and O.F. = 60% and 50% with respect to the normalized combustor radius. The corresponding values of AFT are ~2500 and 2170 K, respectively. Good agreement can again be observed with
