In this work four different commercial and research CFD codes have been compared for the simulation of two combustion test cases. The aim was to get an overview of the capabilities of these different tools to simulate combustion flows in premixed regimes. Codes tested were Fluent,

CFX, StarCD and Tanit. Three combustion models have been applied, namely the Eddy Break Up, the Eddy Dissipation Model and the Turbulent Flame Closure, the turbulence model used being the standard k-epsilon. Numerical results have been found to fairly fit experiments and helped to show some drawbacks of combustion models. In its theoretically correct range of applicability the TFC model has been found to give the better agreement with experiments

Barbato, Maurizio

Talice, Marco

Zimont, Vladimir L.

P-arch

Numerical Simulation of Premixed Combustion Flows:a Comparative StudyM. Barbato, M. Talice, V.L. ZimontFluid Dynamics and Combustion Area - CRS4 Research CentreSesta Strada Ovest – Z.I. Macchiareddu Uta (CA) I-09010 ItalyAbstractIn this work four different commercial and research CFD codes have been compared for thesimulation of two combustion test cases. The aim was to get an overview of the capabilities ofthese different tools to simulate combustion flows in premixed regimes. Codes tested were Fluent,CFX, StarCD and  Tanit. Three combustion models have been applied, namely the Eddy BreakUp, the Eddy Dissipation Model and the Turbulent Flame Closure, the turbulence model usedbeing the standard k-epsilon. Numerical results have been found to fairly fit experiments andhelped to show some drawbacks of combustion models. In its theoretically correct range ofapplicability the TFC model has been found to give the better agreement with  experiments.IntroductionPremixed combustion is becoming more and more relevant in the industrial combustiontechnologies being one of the approaches that can make possible to achieve very low pollutantsemissions. Design and optimisation of modern burners does rely more and more on the use of CFDtools.  Commercial and research CFD codes have reached a good level of sophistication and todaythey include a number of physical models which are available to describe different combustionregimes offering the user a variety of approaches to best fit his needs.The most important requirement for a model which has to be used in industrial application is itscapability of correctly describing chemistry. This is the challenge of turbulent combustion [1]. Forexample, at given turbulence conditions, the Eddy Dissipation Model gives a dependence onchemistry which is quasi-laminar overestimating in this way the dependence on temperature [2].On the other hand, the Turbulent Flame Closure is a model that is able to give more correctdependence on chemistry, as it has been shown in [3].Assumed that correct dependency on chemistry is very important in evaluating the real capabilityof a combustion model to represent industrial combustion processes, the model itself has to be ableto reproduce experimental data in a very accurate way. But even this is not enough.  In fact, thesame importance has the ability of a model to give correct answer for trends, that is to giveprediction on what the behaviour of the system will be in different conditions. In this work thethree most diffused commercial CFD packages have been tested, namely: CFX, Fluent, andStarCD. Besides the research solver Tanit [4] has been also considered.Being turbulent premixed combustion the physical phenomenon of interest, only some of themodels which can be applied to this regime have been taken into account. In particular only theEddy Break Up (EBU) [5], the Eddy Dissipation Model (EDM) [6], and the Turbulent FlameClosure (TFC) [7] have been investigated.The chosen test cases are two turbulent premixed combustion flows in experimental combustors ofsimple geometry for which a wide spectrum of experimental data exists [8, 9].Combustion models.The Eddy Break Model [5] is based on two main ideas: the mixture can be thought as made ofcompletely reacted and completely unreacted gas; the flame region draws inside the fresh mixture,like a jet-flow entrains the fluid from its surroundings. Then the trapped mixture burns with a ratewhich depends on the local level of turbulence. This dependence is expressed in the source termfor the consumption/production of chemical species by the presence of the macroscopic turbulenttime scale.The Eddy Dissipation Model [6] relates the combustion reaction velocity with the dissipation rateof turbulent eddies.  The source term for chemical reactions is then expressed accounting for thelevel of turbulent kinetic energy, the rate of dissipation of turbulent kinetic energy and the meanreactants concentration.The Turbulent Speed Closure [7] is based on the concept of a turbulent premixed flame brushformed by thickened wrinkled flamelets. The source term for chemical reactions depends on theturbulent flame speed which is defined as a function of the properties of both the laminar flameand the local turbulence.Test Cases DescriptionThe combustor hereinafter indicated as “Moreau” [8] is shown in Fig. 1. The combustor is fed witha methane-air mixture with equivalence ratio = 0.87. The flame is ignited and stabilised using aburned gas flow entering the lower part of the combustion chamber. The inlet conditions used forthe CFD calculation are those summarised in Table 1.The second test case carried out is the combustor named ENSMA [9]. As shown in Fig. 4 a flameholder is used in order to stabilise the flame. The propane-air mixture composition and fluiddynamics properties of the flow at inlet section are shown in Table 2 and Fig. 5. Moreau Combustor ResultsFigure 2 shows the average x-velocity profiles in a section of the combustor located at 650 mmfrom inlets. The exam of Fig. 2 indicates a wide scattering of the results obtained using the EBU-class models with exception of the StarCD one which is near to what can be achieved by usingTFC model. Accordingly, the same trend has been found for the methane concentration, as shownin Fig. 3.Moreau Combustor DiscussionSimulations based on the EBU model do fail in reproducing the velocity  because they give a toolow burning rate. In fact, the low combustion intensity  determines an insufficient acceleration ofproducts (see Fig. 2). A tuning of this combustion model in order to better fit the experimental dataled to unphysical results i.e., produced a flame lip attached to the upper wall. This effect, due tothe turbulent time scale included in the expression of the EBU source term [5], leads toexcessively large source term values near solid walls.  This problem has been overcome by StarCDwhich allows the user to make use of a turbulent time scale averaged on the whole flow-field. Theresults obtained using the TFC model show light differences depending upon the solver which hasbeen used. However, all of them are in good agreement with experiments.  Furthermore the TFC isalso capable to fairly reproduce the flame brush width as shown in Fig. 3 which presents methaneconcentration profiles. In contrast from the same figure is quite evident that the EBU basedcalculations cannot follow properly the slope of the experimental data, leading to an unrealistic toothin flame brush.ENSMA Combustor ResultsProfiles of average x-velocity at 10 mm behind the flame holder are plotted in Fig. 6. In this casethe recirculation zone behind the obstacle is fairly reproduced by the EBU based calculations, notby the TFC based ones. The analysis of  Fig. 7 shows a general overestimation of peaktemperature, while the profile’s slope is fairly reproduced. Moving further downstream, both peaktemperature and profile’s shape are not well captured by all the models, as shown in Fig. 8.ENSMA Combustor DiscussionThe present test case has been found particularly difficult to solve. Any attempt of using EBUmodel did fail in achieving physically consistent results. Simulations performed using EDM andTFC models raised a series of questions which led the authors to make a deeper analysis of thistest case. From fluid-dynamics point of view, calculations have shown a too high level of turbulentkinetic energy behind the flame holder. This behaviour could have at least two possible reasons: a)the turbulence intensity level imposed at inlet was too high; b) the used standard model may fail indescribing properly the complex recirculating flow. Both these hypothesis have been investigatedby the authors. At first only the inlet turbulence intensity has been reduced in order to better fit themeasured turbulence level behind the flame holder. The obtained turbulent kinetic energy profile isthe one labelled “TFC-LowK” in Fig. 9, 10 and 11. The exam of Fig. 10, in particular, shows theachieved improvement in the velocity profile due only to the modified turbulence level imposed atinlet. Then both RNG and RSM turbulence models have been used to try and get furtheramelioration in the solution quality. The same figures show the obtained results. Summarising: thelowering of value at inlet has improved the quality of results even for the simple standard model,but the best solution can be obtained only by using more sophisticated turbulence models.A second issue is whether the combustion model applied is really effective for simulating thisflame. An analysis had then to be performed in order to classify the ENSMA flame. Figure 12shows the Borghi diagram [10] for premixed combustion regimes, together with the representativepoints of the two test cases carried out in this work. The exam of this figure shows that theENSMA flame falls in the thick flames regime, whereas the Moreau flame lays in the premixedthickened wrinkled flamelets regime. Being the TFC model soundly based on the thickenedwrinkled flamelets physical background [7], the model itself is to be expected to fail inreproducing the ENSMA flame.  Nevertheless it has been found able to give some qualitativeinsights even in this case.ConclusionsIn this work the capabilities of four different premixed combustion model of giving qualitativelyand quantitatively results on two test cases have been examined.  On the only basis of whatpresented here some conclusions can be achieved. In particular:In the Moreau test case, which fully fit the wrinkled thickened flamelets regime, TFC model givesthe best results, almost independently from which solver has been used. This is not surprisingbecause TFC model is based on this flamelets combustion mechanism [7]. The fact that  thisregime is quite common in industrial gas turbine could suggest that TFC model is actually able togive the best results also in real applications.The definition of the flame regime is a preliminary and necessary condition for the choice of theright combustion model to use for each specific problem. As doesn’t exist an universal turbulencemodel, so doesn’t exist an universal combustion model either.The authors do not recommend the ENSMA test case to validate turbulent combustion modelswhich have to be used to simulate industrial combustors. In fact the analysis of the Borghi’sdiagram [10] (Fig. 12) shows that the combustion regime is almost laminar, being very close to theReynolds number unit line. This is a situation which is quite impossible to achieve in realapplications. Nevertheless this test case remains useful to study fundamental phenomena like, forexample, counter gradient transport [9].The best results for a specific test case depend on the connection of the right turbulence andcombustion models. It has been shown how a better resolution of the turbulent flow field leads to asignificantly improvement in the quality of the achieved results, even if in situations which arelimits for the combustion model.AcknowledgementThis work has been carried out in the frame of the project named: “Accordo di Programma ENEA-MURST nel Settore Calcolo- Riunione Sottoprogetto 1”.This work has been financially supportedby the Sardinian Regional Government.Fig. 1: Moreau combustor geometry.Burnt gas inlet Unburned gas inletU [m/s] 108 65V [m/s] 0 0turb. intensity 21% 12%turb. lenght scale [m] 0.0014 0.0056Equivalence ratio 0.87 0.87Temperature [K] 2000.0 (2200.0) 600.0Tab. 1: Inlet conditions for Moreau combustor simulation.Fig.2: Average x-velocity profiles at 650 [mm] from inlet section.Fig. 3: Average methane concentration at 522 [mm] from inlet section.Fig. 4: ENSMA combustor geometry.Unburned gasY(C3H8) 0.04515Equiv. Ratio 0.65T [K] 293.0Tab. 2:   Inlet conditions for ENSMAcombustor simulation.Fig. 5:    Inlet profiles for x-velocity, y-velocityand turbulent kinetic energy.Fig. 6: Average x-velocity at 10 [mm] from the flame holder.Fig. 7: Average temperature profiles at 10 [mm] from the flame holder.Fig. 8: Average temperature profiles at 60 [mm] from the flame holder.Fig. 9: Turbulent kinetic energy profiles at 10 [mm] from the flame holder.Fig. 10: Average x-velocity at 10 [mm] from the flame holder.Fig. 11: Average temperature profiles at 60 [mm] from the flame holder.Fig. 12: Borghi’s diagram: (160$ZRUNLQJSRLQW 0RUHDXZRUNLQJSRLQWFRPEXVWLRQregimes: DC = distributed; WFC = wrinkled flamelets; WTFC = wrinkled-thickened flamelets);u’: turb. intensity; UL: lam. flame speed; lt: turb. integral length scale; dL: lam. flame thicknessReferences1. Bray, K.N.C., “The Challenge of Turbulent Combustion”, in Twenty-Sixth Symposium (Int.)on Combustion , pp. 1-26. The Combustion Institute, 1996.2. Zimont V., Barbato M., Murgia F., “On the Limit of Industrial Premixed CombustionSimulation Models”, in Proceeding of the Mediterranean Combustion Symposium – 99, pp.372-3833. Karpov V., Lipatnikov A., Zimont V., “A Test of an Engineering Model of TurbulentPremixed Combustion” in Twenty-Sixth Symposium (Int.) on Combustion , pp. 249-257. TheCombustion Institute, 1996.4. EU ESPRIT Contract N. 8835, “Development of  a Turbulent Combustion Code for IndustrialApplications”, 1997.5. Spalding , D.B., “Development of the Eddy Break Up Model of Turbulent Combustion”,  inSixteenth Symposium (Int.) on Combustion , pp. 1657-1663. The Combustion Institute, 1976.6. Magnussen, B.F. and Hjertager, B.H., “On the Mathematical Modeling of TurbulentCombustion with Special Emphasis on Soot Formation and Combustion.”, in SixteenthSymposium (Int.) on Combustion , pp. 719-729. The Combustion Institute, 1976.7. Zimont, V.L., “Theory of Turbulent Combustion of a Homogeneous Fuel Mixture at HighReynolds Number”, Combustion, Explosion, and Shock Waves, V. 15, N.3, pp.305-311,1979.8. Moreau, P., and Boutier, A. “Laser Velocimetry Measurements in a Turbulent Flame”, inSixteenth Symposium (Int.) on Combustion , pp. 1747-1756. The Combustion Institute, 1976.9. Bailly, P., Garreton, D., Simonin, O., Bruel, P., Champion, M., Deshaies, S., Duplantier, S.,and Sanquer, S., “Experimental and Numerical Study of a Premixed Flame Stabilized by aRectangular Section Cylinder”, in Twenty-Sixth Symposium (Int.) on Combustion , pp. 923-929. The Combustion Institute, 1996.10. Borghi, R. and Destriau, M., “La Combustion et Les Flammes”, Editions Technip, 1995.

Numerical simulation of premixed combustion flows: a comparative study

http://www.p-arch.it/bitstream/handle/11050/1047/1003.pdf?sequence=1&isAllowed=y

Numerical simulation of premixed combustion flows: a comparative study

Abstract

Similar works

Full text

Available Versions

P-arch