Papers by Mandhapati Raju
Computing Research Repository, 2009
The study deals with the parallelization of 2D and 3D finite element based Navier-Stokes codes us... more The study deals with the parallelization of 2D and 3D finite element based Navier-Stokes codes using direct solvers. Development of sparse direct solvers using multifrontal solvers has significantly reduced the computational time of direct solution methods. Although limited by its stringent memory requirements, multifrontal solvers can be computationally efficient. First the performance of MUltifrontal Massively Parallel Solver (MUMPS) is evaluated
Innovations and Advances in Computer Sciences and Engineering, 2009
Page 1. Parallelization of Finite Element Navier-Stokes codes using MUMPS Solver Mandhapati P. Ra... more Page 1. Parallelization of Finite Element Navier-Stokes codes using MUMPS Solver Mandhapati P. Raju Research Associate, Case Western Reserve University, Cleveland, OH, 44106, USA E-mail: [email protected] Abstract ...
Hydrogen refueling in a metal hydride based automotive hydrogen storage system is an exothermic r... more Hydrogen refueling in a metal hydride based automotive hydrogen storage system is an exothermic reaction and therefore an efficient heat exchanger is required to remove the heat for fast refueling. In this paper a helical coil heat exchanger embedded in a sodium alanate bed is modeled using COM SOL. Sod iu m alanate is present in the shell and the coolant flows through the helical tube. A three-dimensional COMSOL model is developed to simu late the exothermic chemical reactions and heat transfer. Due to memo ry limitations, only a few turns of the coil are included in the computational domain. Practical difficult ies encountered in modeling such three dimensional geometries as well as suitable approximat ions made to overcome such difficult ies are discussed. The distribution of temperature and hydrogen absorbed in the bed for a sample case is presented. A paramet ric study is conducted using COMSOL-Mat lab interface to determine the optimal bed diameter, helical rad ius and helical pitch fo r maximu m gravimetric capacity.
The study deals with the parallelization of finite element based Navier-Stokes codes using domain... more The study deals with the parallelization of finite element based Navier-Stokes codes using domain decomposition and state-of-art sparse direct solvers. There has been significant improvement in the performance of sparse direct solvers. Parallel sparse direct solvers are not found to exhibit good scalability. Hence, the parallelization of sparse direct solvers is done using domain decomposition techniques. A highly efficient sparse direct solver PARDISO is used in this study. The scalability of both Newton and modified Newton algorithms are tested.
Numerical Heat Transfer, Part B: Fundamentals, 2008
Navier-Stokes finite-volume formulations are usually solved using segregated methods. Development... more Navier-Stokes finite-volume formulations are usually solved using segregated methods. Development of sparse direct solvers using multifrontal solvers has significantly reduced the computational time of direct solution methods. This study demonstrates the performance of multifrontal solvers in the context of finite-volume formulations for combustion problems. Here UMFPACK (Unsymmetric Multi-Frontal PACKage) has been used to solve the fully coupled linear system. The use of direct solvers can significantly reduce the computational time (subject to its memory limitations). The efficiency of multifrontal solvers is first demonstrated for a differential cavity benchmark problem and then extended to an axisymmetric candle flame. The feasibility of using multifrontal solvers for three-dimensional problems is also discussed.
Journal of Porous Media, 2008
Two-phase flow inside an externally heated axisymmetric porous wick is studied using a numerical ... more Two-phase flow inside an externally heated axisymmetric porous wick is studied using a numerical model. The motivation for this work is to. simulate the heat and mass transport taking place inside a candle wick. An axisymmetric wick is dipped inside a liquid candle wax reservoir. The wick is externally heated both from the top and the cylindrical surface. The heat and mass transfer equations in the porous media with phase change are solved to investigate the steady state two-phase flow structure in the porous wick. Only the funicular regime is studied. In this regime, there are two regions in the wick: a purely liquid region near the base of the wick and a vapor-liquid two-phase region above. The physics behind the two-phase flow driven by capillarity and evaporation has been studied in detail. The effect of external heat flux, permeability of the porous wick, and gravity is analyzed.
Journal of Porous Media, 2007
International Journal of Hydrogen Energy, 2012
Optimal heat exchanger design Helical coil heat exchanger a b s t r a c t Design of the heat exch... more Optimal heat exchanger design Helical coil heat exchanger a b s t r a c t Design of the heat exchanger in a metal hydride based hydrogen storage system influences the storage capacity, gravimetric hydrogen storage density, and refueling time for automotive on-board hydrogen storage systems. The choice of a storage bed design incorporating the heat exchanger and the corresponding geometrical design parameters is not obvious. A systematic study is presented to optimize the heat exchanger design using computational fluid dynamics (CFD) modeling. Three different shell and tube heat exchanger designs are chosen. In the first design, metal hydride is present in the shell and heat transfer fluid flows through straight parallel cooling tubes placed inside the bed. The cooling tubes are interconnected by conducting fins. In the second design, heat transfer fluid flows through helical tubes in the bed. The helical tube design permits use of a specific maximum distance between the metal hydride and the coolant for removing heat during refueling. In the third design, the metal hydride is present in the tubes and the fluid flows through the shell. An automated tool is generated using COMSOL-MATLAB integration to arrive at the optimal geometric parameters for each design type. Using sodium alanate as the reference storage material, the relative merits of each design are analyzed and a comparison of the gravimetric and volumetric hydrogen storage densities for the three designs is presented.
International Journal of Hydrogen Energy, 2012
System simulation models for automotive on-board hydrogen storage systems provide a measure of th... more System simulation models for automotive on-board hydrogen storage systems provide a measure of the ability of an engineered system and storage media to meet system performance targets. Thoughtful engineering design for a particular storage media can help the system achieve desired performance goals. This paper presents system simulation models for two different advanced hydrogen storage technologies e a cryo-adsorption system and a metal hydride system. AX-21 superactivated carbon and sodium alanate are employed as representative storage media for the cryo-adsorbent system and the metal hydride system respectively. Lumped parameter models incorporating guidance from detailed transport models are employed in building the system simulation models.
International Journal of Hydrogen Energy, 2011
Drive cycle a b s t r a c t
Combustion and Flame, 2010
A novel implementation for the skeletal reduction of large detailed reaction mechanisms using the... more A novel implementation for the skeletal reduction of large detailed reaction mechanisms using the directed relation graph with error propagation and sensitivity analysis (DRGEPSA) is developed and presented with examples for three hydrocarbon components, n-heptane, iso-octane, and n-decane, relevant to surrogate fuel development. DRGEPSA integrates two previously developed methods, directed relation graph-aided sensitivity analysis (DRGASA) and directed relation graph with error propagation (DRGEP), by first applying DRGEP to efficiently remove many unimportant species prior to sensitivity analysis to further remove unimportant species, producing an optimally small skeletal mechanism for a given error limit. It is illustrated that the combination of the DRGEP and DRGASA methods allows the DRGEPSA approach to overcome the weaknesses of each, specifically that DRGEP cannot identify all unimportant species and that DRGASA shields unimportant species from removal. Skeletal mechanisms for n-heptane and iso-octane generated using the DRGEP, DRGASA, and DRGEPSA methods are presented and compared to illustrate the improvement of DRGEPSA. From a detailed reaction mechanism for n-alkanes covering noctane to n-hexadecane with 2115 species and 8157 reactions, two skeletal mechanisms for n-decane generated using DRGEPSA, one covering a comprehensive range of temperature, pressure, and equivalence ratio conditions for autoignition and the other limited to high temperatures, are presented and validated. The comprehensive skeletal mechanism consists of 202 species and 846 reactions and the hightemperature skeletal mechanism consists of 51 species and 256 reactions. Both mechanisms are further demonstrated to well reproduce the results of the detailed mechanism in perfectly-stirred reactor and laminar flame simulations over a wide range of conditions. The comprehensive and high-temperature n-decane skeletal mechanisms are included as supplementary material with this article.
Volume 2: Instrumentation, Controls, and Hybrids; Numerical Simulation; Engine Design and Mechanical Development; Keynote Papers, 2014
ABSTRACT Representative Interactive Flamelet (RIF) and Detailed Chemistry based combustion models... more ABSTRACT Representative Interactive Flamelet (RIF) and Detailed Chemistry based combustion models are two commonly used combustion models for non-premixed diesel engine simulations. RIF performs transient chemistry calculations on a one-dimensional grid based on the mixture fraction coordinate. Hence, the chemistry calculations are essentially decoupled from the computational fluid dynamics (CFD) grid. The detailed chemistry model, on the other hand, solves transient chemistry in the 3D CFD domain. An efficient parallelization strategy is used for the computation of the multiple flamelets RIF model. The multiple flamelets RIF and detailed chemistry combustion models are applied for modeling a constant volume spray combustion case and a diesel engine case, with a view to compare the differences between the two models. Results for ignition delay, flame lift-off length, cylinder pressure, and emissions are compared with experimental data. The effect of number of flamelets is evaluated. Finally, the effect of spray cooling is investigated based on the results from the RIF model and the detailed chemistry based combustion model.
SAE Technical Paper Series, 2013
ABSTRACT A set of reduced chemical mechanisms was developed for use in multi-dimensional engine s... more ABSTRACT A set of reduced chemical mechanisms was developed for use in multi-dimensional engine simulations of premixed gasoline combustion. The detailed Primary Reference Fuel (PRF) mechanism (1034 species, 4236 reactions) from Lawrence Livermore National Laboratory (LLNL) was employed as the starting mechanism. The detailed mechanism, referred to here as LLNL-PRF, was reduced using a technique known as Parallel Direct Relation Graph with Error Propagation and Sensitivity Analysis. This technique allows for efficient mechanism reduction by parallelizing the ignition delay calculations used in the reduction process. The reduction was performed for a temperature range of 800 to 1500 K and equivalence ratios of 0.5 to 1.5. The pressure range of interest was 0.75 bar to 40 bar, as dictated by the wide range in spark timing cylinder pressures for the various cases. In order to keep the mechanisms relatively small, two reductions were performed. The first mechanism, referred to here as HIGHP (123 reactions, 502 reactions), was reduced under a pressure range of 20-50 bar. The second mechanism, referred to here as LOWP (110 species, 488 reactions), was reduced for a pressure range of 2-10 bar. The reduced mechanisms were coupled with Adaptive Mesh Refinement (AMR), a multi-zone chemistry solver, and a RANS turbulence model to predict premixed gasoline combustion under a wide range of engine conditions. First, a Turbo-charged Direct Injection (TCDI) engine was simulated for a variety of engine speeds, engine loads and displacements. Next a Port-Fuel Injected (PFI) engine with a Charge Motion Control Valve (CMCV) was simulated under a range of valve lift profiles, spark timings, and control valve geometries. Reasonable agreement with the available experimental data was achieved for both the DI and PFI cases
Volume 2: Instrumentation, Controls, and Hybrids; Numerical Simulation; Engine Design and Mechanical Development; Keynote Papers, 2014
ABSTRACT Representative Interactive Flamelet (RIF) and Detailed Chemistry based combustion models... more ABSTRACT Representative Interactive Flamelet (RIF) and Detailed Chemistry based combustion models are two commonly used combustion models for non-premixed diesel engine simulations. RIF performs transient chemistry calculations on a one-dimensional grid based on the mixture fraction coordinate. Hence, the chemistry calculations are essentially decoupled from the computational fluid dynamics (CFD) grid. The detailed chemistry model, on the other hand, solves transient chemistry in the 3D CFD domain. An efficient parallelization strategy is used for the computation of the multiple flamelets RIF model. The multiple flamelets RIF and detailed chemistry combustion models are applied for modeling a constant volume spray combustion case and a diesel engine case, with a view to compare the differences between the two models. Results for ignition delay, flame lift-off length, cylinder pressure, and emissions are compared with experimental data. The effect of number of flamelets is evaluated. Finally, the effect of spray cooling is investigated based on the results from the RIF model and the detailed chemistry based combustion model.
International Journal of Hydrogen Energy, 2015
ABSTRACT In a hybrid wind power system, the excess wind energy is stored in the form of compresse... more ABSTRACT In a hybrid wind power system, the excess wind energy is stored in the form of compressed hydrogen. Wind turbine generates electricity from the wind. The excess power (after meeting the load requirements) is used to generate hydrogen using an electrolyzer. The generated hydrogen is compressed using a compressor and sent to storage in a high pressure storage tank. The compressor should be operated at near isothermal conditions to reduce the power consumed by the compressor, thereby increasing the efficiency of the system. This paper deals with CFD modeling of a novel water spray cooled reciprocating hydrogen compressor which provides efficient cooling of the system during compression. Water is sprayed directly into the compressor cylinder during the compression stage. The water spray breaks into droplets, which provides large surface area to absorb the heat of compression thereby reducing the temperature. The heat capacity of water being order of magnitudes higher than that of hydrogen provides efficient cooling of the compressed gas with small water to hydrogen volumetric ratio. The concept of water spray cooling during compression is demonstrated through a three dimensional computational fluid dynamics (CFD) simulation.
SAE Technical Paper Series, 2012
ASME 2012 Internal Combustion Engine Division Fall Technical Conference, 2012
SAE Technical Paper Series, 2013
ABSTRACT A set of reduced chemical mechanisms was developed for use in multi-dimensional engine s... more ABSTRACT A set of reduced chemical mechanisms was developed for use in multi-dimensional engine simulations of premixed gasoline combustion. The detailed Primary Reference Fuel (PRF) mechanism (1034 species, 4236 reactions) from Lawrence Livermore National Laboratory (LLNL) was employed as the starting mechanism. The detailed mechanism, referred to here as LLNL-PRF, was reduced using a technique known as Parallel Direct Relation Graph with Error Propagation and Sensitivity Analysis. This technique allows for efficient mechanism reduction by parallelizing the ignition delay calculations used in the reduction process. The reduction was performed for a temperature range of 800 to 1500 K and equivalence ratios of 0.5 to 1.5. The pressure range of interest was 0.75 bar to 40 bar, as dictated by the wide range in spark timing cylinder pressures for the various cases. In order to keep the mechanisms relatively small, two reductions were performed. The first mechanism, referred to here as HIGHP (123 reactions, 502 reactions), was reduced under a pressure range of 20-50 bar. The second mechanism, referred to here as LOWP (110 species, 488 reactions), was reduced for a pressure range of 2-10 bar. The reduced mechanisms were coupled with Adaptive Mesh Refinement (AMR), a multi-zone chemistry solver, and a RANS turbulence model to predict premixed gasoline combustion under a wide range of engine conditions. First, a Turbo-charged Direct Injection (TCDI) engine was simulated for a variety of engine speeds, engine loads and displacements. Next a Port-Fuel Injected (PFI) engine with a Charge Motion Control Valve (CMCV) was simulated under a range of valve lift profiles, spark timings, and control valve geometries. Reasonable agreement with the available experimental data was achieved for both the DI and PFI cases
Energy Systems, 2010
Energy Syst DOI 10.1007/s12667-010-0013-6 ORIGINAL PAPER Numerical methods for on-line power syst... more Energy Syst DOI 10.1007/s12667-010-0013-6 ORIGINAL PAPER Numerical methods for on-line power system load flow analysis Siddhartha Kumar Khaitan · James D. McCalley · Mandhapati Raju Received: 4 February 2010 / Accepted: 20 April 2010 © Springer-Verlag 2010 ...
International Journal of Hydrogen Energy, 2012
Hydrogen storage Discharge dynamics Fuel cell Thermal management Wind energy a b s t r a c t
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Papers by Mandhapati Raju