Bromberg, L.

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PSFC/JA-05-14CFD modeling ofPlasmatron Methane ReformerL. BrombergAugust 25, 2005Massachusetts Institute of TechnologyPlasma Science and Fusion CenterSupported by Chevron Texaco, ArvinMeritor and Department of Energy, Office ofFreedomCar and Vehicle TechnologiesAbstractPlasmatron fuel converters have been investigated for the conversion of a wide range offuels.  This is a paper of a series of papers describing an effort to investigateexperimentally and computationally plasmatron methane reformers.  This paper describesthe fluid dynamics modeling of the plasmatron methane reformer, with the hope ofunderstanding air/methane mixture and mixing rates in the region where the plasma islocated.  The modeling includes fluid dynamic effects, without chemistry.1. IntroductionUnderstanding the flows in plasmatron reformers is important in order to provide thebasis for optimization of the device and minimizing transients. It is useful to have ahandle of the mixing conditions (stratified, homogeneous) of the fuel/air prior and pastthe discharge, as well as determining the mixing rates.Although there are codes available that can model the chemistry with the complexhydrodynamic flows, we have opted for performing the chemistry and the fluid dynamicsseparately.  The individual problems are complicated enough that solution of a combineddynamic-chemistry model may not provide hints needed an understanding of theprocesses.  However, because of the lack of chemistry in the model, the results obtainedwith the fluid dynamics code are good only upstream of the discharge region.In this paper we concentrate in modeling the flows upstream from the discharge, and seehow the air/fuel mixing can be modified by changing flow patterns.  The goal is toproduce an understanding of the level of stratification and the mixing time that can thenbe used with chemistry codes, the Perfectly Stirred reactor (PSR) [1] and Partially StirredReactor (PaSR) [2] chemical reaction codes. The results from the chemistry codes canthen be compared with steady state experiments [3] and transient experiments [4].The background, the applications for small reformers and different plasma reformers arereviewed in the accompanying paper that describes the steady state performance of theplasmatron methane converter [3] and will not be repeated here. The fluid dynamic modelis described in section 2. Results for two methods of methane injection are described insections 3 and 4.  Conclusions are presented in section 5.2. CFD modelThe geometry of the plasmatron used in the experiments is shown in Figure 1, and hasbeen described in an accompanying paper [3]. Two independent airs are used: the wall airand the swirl air.  The wall air is injected in the axial direction at the same axial locationwhere the methane nozzle is located. The swirl air is injected downstream from the wallair and the methane, with a large amount of vorticity.  The swirl gas moves the dischargeinto the central region of the plasmatron, and provides the rotation motion that moves thearc roots on the electrodes to minimize electrode erosion. Two locations for introductionof the methane have been investigated: the methane can be introduced through the axialnozzle, upstream from the plasma, without any air (described in section 3 of this paper);or the methane can be premixed with the swirl air and injected as the plasma gas(described in section 4).Downstream from the plasmatron head there is a 5 in long section of the reactionextension cylinder which is also included in the model.The axisymmetric, steady state solution of the fluid dynamics in the plasmatron methanereformer has been calculated using a commercially available CFD code, FLUENT. Themodel includes compressibility of the air, using ideal gas. Wall temperatures are at thesame temperature as the gas (300K). Inlet conditions in all three ports (swirl air, wall airand methane) are as mass flow rate. The outlet condition is atmospheric pressure.The effect of changing the grid structure has been investigated to determine theconvergence of the problem, by using a grid that has four times the node density, and ithas been determined that the mesh is appropriate for the problems investigated in thispaper.The model uses a turbulent model based upon the k-epsilon RNG model, with standardwall functions. The RNG mode is better suited for analysis of flows with substantialswirl, as is the case with the plasmatron methane reformer. The model solved the steadystate conditions, using an implicit formulation and segregated solver. The convergencecriteria for momentum, swirl, axial and radial velocities, k, epsilon and speciesconcentrations was 10-4. The energy was monitored and controlled, with a convergencecriteria of 10-8.Figure 1. Schematic diagram of the plasmatron used in the methane reformingexperiments and modeling.Axial gas Central nozzleElectrode gapSwirl gasSince no chemistry is included in the model, the model cannot be used to describe theflows downstream from the plasma. The object of these calculations is to determine theconditions at and immediately following the plasma zone. In particular, the presence ofrecirculating flows that constitute exhaust gas recirculation, are likely to have substantialeffect on the performance of the plasmatron methane reformer.3. Axial injection of methaneIn this section, the results for the case when the methane is injected introduced throughthe axial nozzle are presented.  Although this is not the best configuration for methane, itis the preferred configuration for liquid fuels, which require air-assist atomization.  Axialinjection minimizes wall wetting.(a)(b)Figure 2.    Contours of constant (a) velocity magnitude; (b) swirl velocityReaction extension cylinderAxis of symmetryAxial injection of methaneWall air  Swirl gasUpstream electrodeDownstream electrodeThe swirl air and wall air flows in the model are1.29 and 2.59 g/s respectively, while theinput of methane is 0.45 g/s, corresponding to an O/C ratio of about 1.8. The high O/Ccorresponds to that shown experimentally to produce good conversion of the methane andmoderate hydrogen yield in a non-catalytic reactor. The wall air is injected without swirl,but the plasma gas has a swirl ratio of 12/40 (radial to tangential speed).Figure 2 shows the velocity profiles in the plasmatron head and the entrance to thereaction extension cylinder. The left region in Figure 2 corresponds to the top of theplasmatron in Figure 1 (as the plasmatron is axially symmetric only half of theplasmatron in Figure 1 is shown, rotated clockwise 90 degrees,). The flows are labeled inFigure 2 (a). Only a small section of the reaction extensions cylinder is shown, as themodel is not applicable to this region. The methane is injected through the axial nozzleand at the extreme left of the figure.(a)(b)Figure 3. Contours of constant (a) k, turbulent kinetic energy; (b) ε, dissipation rateThe injection velocity of the methane is high (on the order of 120 m/s).  The velocity hasto be high in order to provide the narrow radial distribution of the methane that results instratification of the methane, with high concentration on axis. The methane velocitydecreases to the bulk velocity downstream, but upstream from the electrode gap region.The swirl gas (in this case, air) has swirl velocities on the order of 20 m/s in theplasmatron, with velocities decreasing to about 10 m/s downstream from the plasmaregion. It should be noted that the swirl gas flows upstream from the injection point,resulting in longer residence time in the plasmatron.  As the swirl flows modify theplasma, it part of the plasma, and the upstream root of the plasma, should move upstreamfrom the electrode gap. This has been observed experimentally.The turbulence parameters k (turbulent kinetic energy) and ε (turbulent kinetic energydissipation) are shown in the Figure 3. These parameters are important for the chemicalmodeling because they drive the turbulent mixing. The mixing time, which is the time formixing of the flows, is determined by the ratio of k to ε, τmixing ~ k/ε. k is in the rangefrom 1-5 m2/s2 in the plasma region, while ε varies from 2 104 – 6 104 m2/s3. The mixingtime, for these conditions, vary from 0.1 –  0.5 msFigure 4. Contours of constant methane molar concentrationThe molar concentration of methane is shown in Figure 4.  The methane is stratified inthe plasmatron head region, with highest concentration on the axis, decaying radially. It isthought that reforming in plasmatron fuel converters proceeds through a two-stageprocess, as described in the experimental and chemical model papers [1,2]. Reforming ismaximized when there are substantial regions with stoichiometric methane/air mixture.The partial mixing is expected to result in ignition of some regions with goodflammability characteristics, with mixing controlled reaction proceeding through the lowflammability region. Both the Perfectly Stirred Reactor model [1] and the PartiallyStirred reactor PaSR model [2] are used in the accompanying papers to investigate partialmixingOnly in the region of the reaction extension cylinder the methane and air are well mixed.It should be noted that there is no methane present within the strong swirl next to theplasma electrode. It is thought that the lack of fuel in this plasma region is responsible forpoor experimentally determined reforming performance in the case of axial methaneinjection, and has suggested introduction of at least some of the fuel through the swirlgas.  This condition will be discussed in the next section.Figure 5. Radial molar concentration of methane along a radius at planes coinciding withthe upstream and the downstream electrodes, shown in Figure 2(a).The radial concentration of methane, measured along a radius upstream and downstreamfrom the electrode gap (shown in Figure 2(a)), is shown in Figure 5. As expected,because the swirl flow does not contain any methane, the upstream and downstreamconcentrations are very similar.Stoichiometric combustion of methane occurs for a methane molar concentration of 9%(at an O/C = 4). As the methane concentration gradient is large in the zone where themethane concentration is 9%, the zone with approximately stoichiometric methane/airmixtures is small.4 Methane injection with swirl gasIn this section, results will be shown for the case when the methane is premixed with theswirl air, and injected through the gap between the two electrodes, with strong vorticity.In this case, there is no input through the axial nozzle, and wall air and swirl air flows atthe same levels as in section 3.(****** I do not think so…********(a)(b)Figure 6. Same as Figure 2 but for case of methane through swirl port.Figure 6 shows contours of constant magnitude of the velocity magnitude and the swirlvelocity for the same air flows as Figure 2, but for methane through mixed with the swirlair. The swirl velocities are larger, due to the increased flow of swirl gas (which nowcontains also the methane, about 100 lpm STP in Figure 6, vs 60 lpm in Figure 2). Theswirl motion also moves further upstream in Figure 6 than in Figure 2.(a)(b)Figure 7. Same as Figure 3 but for methane injected through the swirl port.The turbulence parameters, k and ε, do not change substantially in the region of theplasma when the location of the methane injection is varied, as can be seen by comparingFigure 3 and 7.  Thus, the mixing time is not sensitive of the place of injection of themethane.However, the methane concentration profile changes substantially in the two cases.Contours of constant methane concentration are shown in Figure 8 for methane throughthe swirl port. The gradients are in the opposite direction from that of figure 4, withhighest concentration near the wall and smallest near the axis.Figure 8. Molar concentration of methane for the case of methane premixed with swirlair.Figure 9 shows the molar concentration of methane at the location of the upstream anddownstream electrodes. As opposed to the case shown in Figure 5, the gradients in themethane concentration are small near the region with stoichiometric methane/air mixture(at around a radius of 0.055 m), which results in a larger fraction of the methane nearcombustion conditions, and should be better suited to two-stage reforming. The locationof the stoichiometric conditions is not that different from the case shown in Figure 5, butit is substantially larger because the gradients are smaller.Figure 9.  Radial concentration of methane at the location of the upstream anddownstream electrodes.In this case, there is a difference between the upstream and downstream planes, as themethane is part of the swirl flow, and some of it moves upstream while the rest movesdownstream.Thus, the conditions generated in the case of methane premixed with the swirl air aremore conducive to two-stage reforming than that for axial injection of methane. Resultsnot shown in this paper indicate that premixing the methane with the wall air has similarmethane concentration profiles than the case with axial injection, and thus should havecomparable reforming characteristics.  It has been experimentally shown [3,4] that thecase of methane through the swirl port results in improved reforming (defined as methaneconversion at a given overall O/C and at a given power).ConclusionsComputational fluid dynamics tools have been used to investigate the effects ofalternative injection points for the fuel, in this case, methane. The purpose was todetermine the characteristics of the air/fuel mixture at the location of the plasma, as wellas to determine the mixing rates. It was determined that changes in injection pointchanges significantly the distribution of the methane in the region of the plasma, but doesnot significantly change the mixing parameters at this location.This information will be used in accompanying papers to investigate the characteristics ofmethane reforming in plasmatron fuel converters.AcknowledgementsThis work was performed under auspices of ArvinMeritor, ChevronTexaco andDepartment of Energy, Office of FreedomCar and Vehicle Technologies.  The supportand interest from Dr. S. Diamond from DoE, R. Smaling and N. Khadya, fromArvinMeritor, and T. Rufael, from ChevronTexaco, are appreciated.References[1] L. Bromberg and N. Alexeev, Plasma assisted reforming of methane: Two stagePerfectly Stirred Reactor (PSR) simulation, Plasma Science and Fusion Center ReportJA-05-12[2] L. Bromberg, Modeling of plasma assisted reforming of methane II: Partially StirredReactor (PASR) simulation, Plasma Science and Fusion Center Report JA-05-13 [3] L. Bromberg, K. Hadidi and D.R. Cohn, Experimental Investigation of PlasmaAssisted Reforming of Methane I: Steady State Operation, Plasma Science and FusionCenter Report JA-05-10[4] L. Bromberg, K. Hadidi and D.R. Cohn, Experimental Investigation of PlasmaAssisted Reforming of Methane II: Start-up, Plasma Science and Fusion Center ReportJA-05-11

CFD modeling of Plasmatron Methane Reformer

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CFD modeling of Plasmatron Methane Reformer

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