CFD simulation does an adequate job of predicting temperature globally, so it served well to solve the NOx problems of the past 20 years. However, the combustion challenges that designers need to simulate today are kinetically driven and require detailed chemical simulation. This simulation has been accomplished for over 30 years through the use of idealized chemical reactor modeling using chemistry simulation software packages such as CHEMKIN®. But these packages have always lacked the ability to directly take into account effects of the complex 3-D flow field and geometry. Building ENERGICO networks to represent the local chemical reactions in appropriate regions of the geometry is a proven method of incorporating the effects of both the flow and the kinetics in a single simulation.

1. A WHITE PAPER BY REACTION DESIGN<br />Using Automatic Reactor Networks with CFD to Provide Optimal Accuracy While Lowering Cost<br />Combustion simulation approaches must evolve in order to help engine designers address the challenges posed by the ever widening fuels landscape and new emissions regulations. In the first paper of this series, we discussed that while CFD is the dominant combustion simulation tool used by industry; its inherent limitations on chemistry accuracy prevent users from addressing today’s design problems (e.g., fuel flexibility, low load CO performance, etc.). The second paper in this series demonstrated how more accurate chemistry is available today that can capture real fuel behavior, but it requires more complex chemistry models than CFD can handle. In this final part of the series, we will describe how accurate fuel models can be used with automatically created reactor networks to achieve cost effective solutions to today’s combustion problems and provide real value to the design process.<br />Reactor networks have proven accuracy<br />CFD simulation does an adequate job of predicting temperature globally, so it served well to solve the NOx problems of the past 20 years. However, the combustion challenges that designers need to simulate today are kinetically driven and require detailed chemical simulation. This simulation has been accomplished for over 30 years through the use of idealized chemical reactor modeling using chemistry simulation software packages such as CHEMKIN®. But these packages have always lacked the ability to directly take into account effects of the complex 3-D flow field and geometry. Building ENERGICO networks to represent the local chemical reactions in appropriate regions of the geometry is a proven method of incorporating the effects of both the flow and the kinetics in a single simulation. <br />It is critical that the reactor network be a true representation of the actual combustor flow field in order for the simulation to be accurate. Once the reactor network is created through a careful devolution of the combustor flow field, the full detailed chemistry model can be used to provide an understanding of chemical behavior and performance. Recently, researchers have proven that using a CFD case as the basis for reactor network creation dramatically improves the quality of the results. Traditionally, expert personnel would be required to create the reactor network manually; a process so time consuming that it is not practical in the design process. <br />ERN automation provides the speed and accuracy required by industry<br />The ENERGICO™ simulation package has been developed to automatically create a reactor network from a reacting-flow CFD solution. It uses a series of filters that are applied to the CFD or user-defined variables to generate the ENERGICO network for accurate prediction of combustion performance, including exit emissions (see REF _Ref281966532 Figure 4). A proprietary algorithm is used to divide the combustor flow field into zones that will form the basis of the ENERGICO network. Once the ENERGICO network is created, you can apply an accurate fuel model to predict the emissions of trace species such as NOx, CO and unburned hydrocarbons (UHC). The ENERGICO network can also be employed in a parametric variation of operating conditions and fuel composition to determine how such variations would affect performance. <br />3-D CFD SolutionAutomatically create ERNMap chemistryresults ontogeometry viewImprove your CFD model with greater kinetic understandingUse algorithm to divide flow field into reactor zones<br />Figure SEQ Figure ARABIC 4: Automatic ERN generation and solution with ENERGICO allows the use of appropriate chemistry to model real fuel behavior<br />ENERGICO parameter studies offer insight that cannot be gained using CFD alone<br />Emissions are a dominant performance metric that can carry significant financial penalties for non-compliance. Predicting emissions from combustion equipment is a critical capability in order to guarantee performance. As we have discussed previously, CFD alone does not have the capability to accurately predict emissions of interest due to a lack of accuracy in the chemical model. REF _Ref219880868 Figure 5 shows a comparison of ENERGICO network emissions results with experimental measurements for a single fuel injector from a low-NOx, industrial, gas-turbine engine. The ENERGICO network is able to accurately predict the NOx emissions at the conditions in the CFD case (represented by the circle). The real value of a well constructed ENERGICO network is demonstrated when you perform parametric variations of the inputs to increase fuel/air ratio (i.e., increase combustor exit temperature) yielding NOx predictions that are in excellent agreement with the experimental results. Similarly accurate results are obtained when looking at the impact of pilot fuel split on NOx formation. The impacts of Fuel air ratio and pilot fuel split on NOx formation are common combustor experiments. The ENERGICO simulation in these cases was conducted in a couple of hours and its results can be used to replace experimental tests that cost upwards of $100,000 to perform.<br />Nominal Case based on CFD<br />Figure SEQ Figure ARABIC 5: Manipulating an ERN to determine the impact of fuel/air ratio on NOx emissions<br />Experimental Data<br />Figure SEQ Figure ARABIC 6: ERNs show excellent ability to predict emissions for pilot fuel split<br />Another key area for today’s combustion market is fuel flexibility. ENERGICO simulations have proved valuable in simulating the effects of fuel composition variation on emissions for an industrial gas turbine as is shown in REF _Ref281979605 Figure 7. Gaseous fuel compositions with widely varying amounts of CH4, CO, CO2 and H2 are input ENERGICO and their impact on emissions of NOx are shown. ENERGICO not only predicted the correct trends for NOx emissions with the fuel composition variation but comes very close to predicting the actual values as well. When you consider how expensive it is to experimentally test a multitude of fuel composition variations, the ENERGICO approach provides an attractive alternative.<br />Experimental Data<br />Figure SEQ Figure ARABIC 7: ERNs can predict real fuel impacts for fuel flexible designs with syngas and LNG applications. (Fuel composition variation between CH4, CO, H2, CO2 and N2). <br />ERN analysis can show how to improve CFD <br />How do I know if my CFD simulation is correct? This is one of the key questions any CFD engineer must address. This is an exceedingly difficult question to answer if the only combustion simulation tool you have is CFD. Typically the question of CFD accuracy is evaluated by running a large number of CFD cases with different mesh sizes, combustion models, turbulence models, etc. and comparing the results to experimental data. This time consuming process only provides results in the context of CFD modeling and does not provide an ability to get a second opinion on CFD accuracy.<br />ENERGICO networks provide an excellent method to get that second opinion. The ENERGICO network can be run using the CFD temperatures and then compared against ENERGICO results where temperatures are determined using the more accurate fuel model. An example of this can be seen in the industrial gas turbine combustor shown in REF _Ref281978896 Figure 8. Here, the top image is the CFD result showing a very small diffusion pilot and the lower image shows the ENERGICO results with a dramatically larger pilot flame. The emissions predictions using the CFD temperatures are terrible with very little NOx and far too much CO predicted. However, when ENERGICO applied, the NOx and CO emissions are accurate to within 10%. Looking at the temperature distribution between the two images can help understand why there is such a dramatic difference in the two approaches. The flow field nearest the diffusion pilot clearly gets hotter and larger when in ENERGICO owing to increased NOx and better CO oxidation. In this case, the CFD engineer can easily see where the CFD result is deficient and can focus their efforts on improving the result by refining the mesh or improving boundary conditions. It is important to note that accurate emissions predictions from the ERN were obtained even though the CFD case was not perfect.<br />Figure SEQ Figure ARABIC 8: Accurate emissions results can be obtained with poor CFD results with ENERGICO. Comparing ENERGICO simulations to CFD provides an opportunity to improve CFD results.<br />Automatic ERN analysis is cost effective<br />Typical reacting flow CFD cases take at least 3 days to converge on a solution with some cases taking up to a week. Typical ENERGICO network creation and solution times are less than a few hours. A single ENERGICO license can handle the work generated by more than 10 CFD licenses. When you consider the manpower that is required to support 10 CFD licenses, the investment in ENERGICO works out to 10% of the total investment for CFD. As we have shown, ENERGICO can provide a valuable alternative approach that can be used to improve the quality of CFD. Getting a second opinion on CFD results for 10% seems like a good investment.<br />Summary<br />Automatic ERN analysis represents a proven technique of simulating combustion, providing accurate emissions and combustion stability assessments that will reduce development costs, improve fuel flexibility and decrease development risk.<br />CFD alone cannot incorporate the fuel chemistry accuracy that’s required for today’s combustion challenges<br />Advances in fuel chemistry understanding are available that allow the prediction of real fuel effects, but CFD cannot handle the complexity<br />Automatic ENERGICO network creation from a CFD solution is an efficient way to incorporate the required fuel chemistry complexity for accurate emissions predictions <br />Important parameter variations of fuel-air ratio, fuel splits, fuel composition, etc. can be performed on the ENERGICO network with predictive accuracy for NOx, CO and UHC exit emissions, without requiring the development of a new CFD case<br />Accurate CFD results are always desired, but not necessarily required to get good results from the ENERGICO network<br />ENERGICO provides a valuable “second opinion” that can be used to improve the CFD case <br />The increased accuracy of the combined ENERGICO and CFD simulations reduces the number of expensive experimental tests required to perfect a design<br />Quick results enable your designers to explore novel combustion concepts more efficiently<br />