Convergent Science Inc.

Middleton, WI, United States

Convergent Science Inc.

Middleton, WI, United States

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Zhou H.,Tsinghua University | Li S.,Tsinghua University | Ren Z.,Tsinghua University | Rowinski D.H.,Convergent Science Inc.
Combustion and Flame | Year: 2017

The piloted premixed jet burner (PPJB) methane–air flame series have been developed to study turbulence–chemistry interactions in lean and highly turbulent premixed flames. The two flames with the largest jet velocities, denoted as PM1-150 and PM1-200, remain challenging for combustion models up to the present. In this work, the flames with the lowest and highest jet velocities, PM1-50 and PM1-200 respectively, are simulated using a hybrid RANS-PDF method to examine the composition evolution and the mixing model performance through Lagrangian particle tracking, and to reveal the controlling physio-chemical processes through particle-level sensitivity analysis. The mixing-reaction budget analysis reveals that for the PM1-50 flame, the implied combustion modes by IEM and EMST mixing models are different: the simulation with EMST is consistent with a flame propagation process, whereas the one with IEM is consistent with an autoignition process. In this regard, for the PM1-50 in the flamelet regime, EMST predicts the correct combustion process whereas IEM does not, despite the fact that the radial profiles of species concentration predicted by these two models are almost identical and are both in good agreement with the experimentally measured values. For the PM1-200 flame closest to global blow-off, EMST and IEM yield similar characteristics: both predict a balance between mixing and reaction in the reaction zone, and both are consistent with flame propagation processes. Due to strong turbulent mixing, the mixing in the preheat zone is greatly enhanced. The change of combustion modes predicted by IEM, i.e. from an autoignition process in PM1-50 to a flame propagation process in PM1-200, is further explored by examining some representative particle trajectories in progress variable space. The PDFs and the mixing-reaction budgets of the progress variable predicted by IEM and EMST demonstrate that the localness of mixing in composition space is essential for a flame in the flamelet regime (PM1-50), while it is less important for a flame in the broken reaction zone regime (PM1-200). The particle-level sensitivity approach has been augmented to further investigate the sensitivities of combustion process to mixing and reaction with an attenuation factor R for the overall reaction rate being introduced to quantify the sensitivities to chemical reaction. It is observed that the sensitivities in PM1-50 and PM1-200 are very different. For PM1-50, the sensitivities of the progress variable to the mixing model constant Cϕ and to the attenuation factor R are both positive, indicating that the reaction progress can be promoted by enhancing either mixing or reaction. In contrast, for PM1-200, the progress variable at the upstream location shows negative sensitivities to Cϕ, which indicates that enhancing mixing suppresses combustion progress due to the fact that the flame is already near the blow-off limit. At downstream locations, the sensitivities of progress variable to reaction is significantly larger than its sensitivity to mixing, indicating that the controlling process during the reignition stage is chemical reaction. These sensitivities are insightful in explaining the observed trends in previous parametric studies of Cϕ for high speed PPJB flames, in which it is found that increasing Cϕ alleviates the overpredition of reaction progress at the upstream location but does not help improve the prediction at the downstream location where the controlling process is chemical reaction. © 2017 The Combustion Institute


Mittal G.,University of Akron | Raju M.P.,Convergent Science Inc. | Sung C.-J.,University of Connecticut
Fuel | Year: 2012

The performance of a rapid compression machine (RCM) with a creviced piston is assessed over a range of operating conditions through computational fluid dynamics simulations with systematic demonstration of the effects of compressed gas pressure, temperature, stroke length, and clearance on altering vortex formation and temperature homogeneity inside the reaction chamber. Simulated results show that as compressed gas pressure is reduced, the temperature homogeneity deteriorates due to the combined effect of thicker boundary layer and increased flow velocities. A further optimization of the creviced piston geometry is then required to completely suppress the roll-up vortex. Stroke length and clearance volume are also noted to significantly affect vortex formation. A basis for quantifying the extent of the roll-up vortex is suggested and the operating regime of an RCM with a creviced piston, that is free from the roll-up vortex, is delineated. This work emphasizes the importance of assessing the performance of an RCM over the associated range of operating conditions in order to obtain reliable chemical kinetics data. © 2011 Elsevier Ltd. All rights reserved.


Raju M.,Convergent Science Inc. | Wang M.,Convergent Science Inc. | Dai M.,Convergent Science Inc. | Piggott W.,Lawrence Livermore National Laboratory | Flowers D.,Lawrence Livermore National Laboratory
SAE Technical Papers | Year: 2012

Detailed chemical kinetics, although preferred due to increased accuracy, can significantly slow down CFD combustion simulations. Chemistry solutions are typically the most computationally costly step in engine simulations. The calculation time can be significantly accelerated using a multi-zone combustion model. The multi-zone model is integrated into the CONVERGE CFD code. At each time-step, the CFD cells are grouped into zones based on the cell temperature and equivalence ratio. The chemistry solver is invoked only on each zone. The zonal temperature and mass fractions are remapped onto the CFD cells, such that the temperature and composition non-uniformities are preserved. Two remapping techniques published in the literature are compared for their relative performance. The accuracy and speed-up of the multi-zone model is improved by using variable bin sizes at different temperature and equivalence ratios. In addition, a general n-dimensional zoning strategy is developed to include other cell variables such as pressure, mass fractions of different species, etc. to improve the performance of the zoning strategy. This paper discusses the savings in computational time achieved and the accuracy of the results using the multi-zone model for a range of scenarios. Gasoline and Diesel engine simulations are performed. Test cases are run for single fuel and multi-component fuels. Exhaust gas recirculation (EGR) scenarios are also tested. Copyright © 2012 SAE International.


Raju M.,Convergent Science Inc. | Khaitan S.,Iowa State University
International Journal of Hydrogen Energy | Year: 2011

Small hybrid wind systems are capable of storing and supplying power for residential applications. In this paper, the excess wind energy is converted into hydrogen by electrolysis and is stored in a metal hydride. Metal hydride beds are known for their high volumetric capacity compared to the compressed hydrogen storage, and offers hydrogen storage at a reasonable operating temperature and pressure. A system simulation model is developed in Matlab/Simulink platform for the dynamics of the metal hydride hydrogen storage system, which is charged by the wind energy. The thermal loads of the metal hydride storage system is met by passing water at ambient temperature for cooling the bed while hydrogen is being absorbed. The effect of the transient turbulent wind velocity profile on the storage system is analyzed. The thermal management of the storage system plays an important role in the overall design, and hence it is discussed in detail. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


Khaitan S.K.,Iowa State University | Raju M.,Convergent Science Inc.
International Journal of Hydrogen Energy | Year: 2012

Typical compressed air energy storage (CAES) based gas turbine plant operates on natural gas or fuel oils as fuel for its operation. However, the use of hydro-carbon fuels will contribute to carbon emissions leading to pollution of the environment. On the other hand, the use of hydrogen as fuel for the gas turbine will eliminate the carbon emissions leading to a cleaner environment. Hydrogen can be produced using renewable energy sources like wind, solar etc. Storage of hydrogen is a bottleneck for such a system. A high capacity sodium alanate metal hydride bed is used in this study to store the hydrogen. The dynamics of the CAES based gas turbine plant operating with hydrogen fuel is presented along with discharge dynamics of the metal hydride bed. The heat required for desorbing the hydrogen from the metal hydride bed is provided partly by the hot flue gas exiting from the low pressure turbine and partly by external heating. Thus some of the heat from the flue gas is extracted. A novel multiple bed strategy is employed for efficient desorption. Each bed consists of a shell and tube, with alanate in the shell and heating fluid flowing through the helical coiled tube. Hydrogen combustor is modeled using a simplified Continuous Stirred Tank Reactor (CSTR) assumption in CANTERA. The NOx emissions in the low pressure turbine exhaust stream are presented. Copyright © 2012 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


Khaitan S.K.,Iowa State University | Raju M.,Convergent Science Inc.
International Journal of Hydrogen Energy | Year: 2012

In small hybrid wind systems, excess wind energy is stored for later use during the deficit power generation. Excess wind energy can be stored as hydrogen in a metal hydride storage bed and reused later to generate power using a fuel cell. This paper deals with the discharge dynamics of the coupled fuel cell and metal hydride storage bed during the power extraction. Thermal coupling of the fuel cell and metal hydride bed is also discussed. The waste heat generated in the fuel cell is removed using a water coolant. The exit fuel cell coolant stream is passed through the metal hydride storage bed to supply the necessary heat required for desorption of hydrogen from the bed. This will also lead to a reduction in the load on the radiator. The discharge dynamics and the thermal management of the coupled system are demonstrated through a system simulation model developed in Matlab/Simulink platform. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


Raju M.,Convergent Science Inc. | Khaitan S.K.,Iowa State University
Journal of Power Sources | Year: 2012

This paper deals with the storage of excess wind energy, in a hybrid wind power system, in the form of compressed hydrogen. A system simulation model is developed in Matlab/Simulink platform for the charging and discharging dynamics of compressed hydrogen storage system integrated with the wind turbine and the fuel cell. Wind model is used to estimate the power generation in the wind turbine. When the wind power generation exceeds the load, the excess power is diverted to the electrolyzer to produce hydrogen. As and when the pressure inside the electrolyzer builds, a compressor is operated intermittently (for higher efficiency) to divert the hydrogen into high pressure cylinders. When demand exceeds the power generation, fuel cell supplies the power to the load. A number of fuel cell stacks are provided to meet the required load. The overall efficiency of the storage system, defined as the ratio of the useful energy derived from the storage system to the energy diverted to the storage system is found to be 24.5% for the compressed hydrogen storage based system. © 2012 Elsevier B.V. All rights reserved.


Senecal P.K.,Convergent Science Inc. | Pomraning E.,Convergent Science Inc. | Richards K.J.,Convergent Science Inc. | Som S.,Argonne National Laboratory
SAE Technical Papers | Year: 2013

A state-of-the-art spray modeling methodology, recently applied to RANS simulations, is presented for LES calculations. Key features of the methodology, such as Adaptive Mesh Refinement (AMR), advanced liquid-gas momentum coupling, and improved distribution of the liquid phase, are described. The ability of this approach to use cell sizes much smaller than the nozzle diameter is demonstrated. Grid convergence of key parameters is verified for non-evaporating and evaporating spray cases using cell sizes down to 1/32 mm. It is shown that for global quantities such as spray penetration, comparing a single LES simulation to experimental data is reasonable, however for local quantities the average of many simulated injections is necessary. Grid settings are recommended that optimize the accuracy/runtime tradeoff for LES-based spray simulations. Copyright © 2013 SAE International.


Drennan S.A.,Convergent Science Inc.
Western States Section of the Combustion Institute Spring Technical Meeting 2014 | Year: 2014

Computational Fluid Dynamic (CFD) combustion simulations of gas turbine combustion have become increasingly useful in the design and development of lower emissions, higher performing engines. However, mesh generation for traditional CFD continues to represent a significant portion of the time required to conduct CFD simulations. This paper presents the application of an automatically generated Cartesian meshing approach to a liquid fueled gas turbine combustor for Large Eddy Simulation (LES) simulations. The model combustor used in this study is a single can with a generic fuel/air mixer employing radial and axial swirlers for multi-zone combustion. The spray is modeled with discrete injections of droplets to simulate an air-blast atomizer. A modified cut-cell Cartesian method is used that eliminates the need for the computational grid to be morphed with the geometry of interest while still representing the true boundary shape. This approach allows for the use of simple orthogonal grids and completely automates the mesh generation process. The meshing approach utilizes Adaptive Mesh Refinement (AMR) to resolve the domain near geometric features and in regions near the spray and flame front. AMR allows the use of a very fine computational mesh in the vicinity of the spray and high temperature reaction zones while keeping the overall cell count relatively low. Mesh sensitivity studies are presented with modifications in the global mesh size and in mesh refinement near effusion cooled surfaces. These reacting flow results use a detailed reaction mechanism to simulate Jet-A liquid fuel. Cell-size recommendations from the above studies are used to further explore grid convergence for RANS and LES-based simulations.


Pomraning E.,Convergent Science Inc. | Richards K.,Convergent Science Inc. | Senecal P.K.,Convergent Science Inc.
SAE Technical Papers | Year: 2014

Combustion is governed by only two phenomena: chemical reactions and mixing (i.e., transport of energy, species, and momentum). A Reynolds Averaged Navier-Stokes (RANS) turbulence model is commonly employed to account for the enhanced mixing due to the presence of turbulence in fluid flow. A RANS turbulence model enhances mixing by introducing a turbulent viscosity. The addition of a turbulent viscosity not only enhances mixing but it also eliminates smaller scales in the CFD simulation. Even though the turbulent viscosity eliminates smaller scales, it is common for RANS engine combustion simulations to be under-resolved. The lack of sufficient mesh resolution to resolve the remaining scales in a RANS combustion simulation may result in a significant sub-grid term that needs to be modeled. In the context of combustion simulation, it is shown that frequently this sub-grid term is significantly more important than Turbulent Chemistry Interaction terms (TCI). It is also shown that by adding sufficient mesh resolution to a RANS simulation, accurate combustion results can be obtained by using detailed chemistry directly. Copyright © 2014 SAE International.

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