Development of Adaptive Mesh Refinement Scheme and Conjugate Heat Transfer Model for Engine Simulations

Spray modeling is a critical component to engine combustion and emissions simulations. Accurate spray modeling often requires a fine computational mesh for better numerical resolutions. This, in turn, will require extensive computer time. A major concern for the successful application of computational methods in an industrial environment is its capability to handle complex configurations with acceptable accuracy at reasonable human and computational costs. To assure the accuracy and reliability of the solution, grid modification and grid refinement studies are often necessary within an iterative process. Adaptive algorithms are a promising approach to realize discretizations that are able to automatically resolve the physically relevant phenomena at reduced costs. The first goal of the dissertation work is to developed a methodology that uses a locally dynamically refined mesh in the spray region for engine spray simulations. An h-refinement adaptive scheme is developed and implemented into an existing computer code. It is a dynamic process that adapts an initial mesh by employing local cell division and recovery. Adaptation of the cells is composed of isotropic division of one hexahedron into eight sub-cells in three dimensions. The concept of polyhedral elements is implemented to treat any possible hanging node configuration that occurs at the interface between the divided and undivided zones in a natural way. This flexibility of this method was demonstrated to handle successive grid adaptation and efficient data management when it is extended to multi-level refinement process. A special data structure based on octree has been developed for high storage efficiency. The solid-cone and hollow-cone sprays under direct-injection gasoline engine conditions were simulated. Predicted spray characteristics using different mesh densities with various refinement levels were compared. Results show that the present mesh refinement scheme can accurately predict spray structures with reduced computer times. A significant computational speed-up was achieved by using a relatively coarse mesh with multi-level refinement while maintaining a good level of accuracy. On the other hand, accurate modeling of the wall heat transfer characteristics within an engine is important for engine design because the amount of heat transfer through the piston, head, and liner surfaces can influence engine efficiency and performance, exhaust emission levels, and engine durability. The surface temperature is a key element for heat transfer, thus an accurate chamber wall surface temperature prediction is crucial for engine heat transfer modeling. The second part of this study developed a conjugate heat transfer model to predict the combustion chamber surface temperature of an engine. First, the code was modified to account for a non-uniform temperature distribution and was run with the uniform temperature profile specified in the input file. The results were compared with the experimental data. The conduction heat transfer modeling capability was added to the code to predict the heat diffusion inside the solid wall by solving a simplified energy equation with the same numerical method used in fluid region. A fully coupled numerical procedure, which conserved the continuous temperature and heat flux condition, was developed to simultaneously solve the heat transfer in fluid flows and heat conduction in solid. Model validation showed the predicted results agreed well with the analytical solutions. The method was applied to simulate a transient diesel engine with fuel spray. The non-uniform spatial temperature distribution on the piston surface caused by fuel spray was predicted by the conjugate heat transfer model. The present model can be used to predict the temperature of the engine combustion chamber under combustion conditions in future studies

Development Of Conjugate Heat Transfer Capability To An Unstructured Flow Solver - U2NCLE

A precise prediction of the heat loads in metal materials in contact with the hot gas is an increasingly demanding problem in the design phase of the complex cooling schemes in the modern turbine engines. The coupled calculation of the fluid flow and the heat transfer is a promising approach as heat transfer coefficients are not necessary in the calculation and the heat transfer itself is part of the calculation and can be derived from local heat fluxes. Therefore, it is useful to incorporate an appropriate scheme for directly coupled heat transfer computations (conjugate heat transfer), capable of handling complex geometries into the existing Computational fluid dynamics (CFD) codes. The intent of the present work is to add the conjugate heat transfer solving capability to an existing flow solver. The coupled approach is achieved by maintaining a continuous local heat flux and a common temperature at the points along the fluid-solid interface. At every iteration, the temperature which is directly calculated via the equality of the local heat fluxes passing the fluid-solid contacting cell faces serves as the thermal boundary condition on the interfaces, instead of traditional isothermal/adiabatic thermal boundary conditions. In the solid domain, simplified energy equation is solved using the discretization and computational methods which have been used in the flow by introducing an effective equation of state. The connectivity is built for the points at the fluid-solid interfaces in order to communicate the thermal conditions with each other. Validation of the developed conjugate capability has been investigated. Computed results have been compared with theoretical or experimental results for laminar flat plate, high pressure guide vane, cooled plate, and effusion-cooled plate. All results obtained thus far compare rather favorably with theoretical or experimental results

Development of Adaptive Mesh Refinement Scheme and Conjugate Heat Transfer Model for Engine Simulations

Spray modeling is a critical component to engine combustion and emissions simulations. Accurate spray modeling often requires a fine computational mesh for better numerical resolutions. This, in turn, will require extensive computer time. A major concern for the successful application of computational methods in an industrial environment is its capability to handle complex configurations with acceptable accuracy at reasonable human and computational costs. To assure the accuracy and reliability of the solution, grid modification and grid refinement studies are often necessary within an iterative process. Adaptive algorithms are a promising approach to realize discretizations that are able to automatically resolve the physically relevant phenomena at reduced costs. The first goal of the dissertation work is to developed a methodology that uses a locally dynamically refined mesh in the spray region for engine spray simulations. An h-refinement adaptive scheme is developed and implemented into an existing computer code. It is a dynamic process that adapts an initial mesh by employing local cell division and recovery. Adaptation of the cells is composed of isotropic division of one hexahedron into eight sub-cells in three dimensions. The concept of polyhedral elements is implemented to treat any possible hanging node configuration that occurs at the interface between the divided and undivided zones in a natural way. This flexibility of this method was demonstrated to handle successive grid adaptation and efficient data management when it is extended to multi-level refinement process. A special data structure based on octree has been developed for high storage efficiency. The solid-cone and hollow-cone sprays under direct-injection gasoline engine conditions were simulated. Predicted spray characteristics using different mesh densities with various refinement levels were compared. Results show that the present mesh refinement scheme can accurately predict spray structures with reduced computer times. A significant computational speed-up was achieved by using a relatively coarse mesh with multi-level refinement while maintaining a good level of accuracy. On the other hand, accurate modeling of the wall heat transfer characteristics within an engine is important for engine design because the amount of heat transfer through the piston, head, and liner surfaces can influence engine efficiency and performance, exhaust emission levels, and engine durability. The surface temperature is a key element for heat transfer, thus an accurate chamber wall surface temperature prediction is crucial for engine heat transfer modeling. The second part of this study developed a conjugate heat transfer model to predict the combustion chamber surface temperature of an engine. First, the code was modified to account for a non-uniform temperature distribution and was run with the uniform temperature profile specified in the input file. The results were compared with the experimental data. The conduction heat transfer modeling capability was added to the code to predict the heat diffusion inside the solid wall by solving a simplified energy equation with the same numerical method used in fluid region. A fully coupled numerical procedure, which conserved the continuous temperature and heat flux condition, was developed to simultaneously solve the heat transfer in fluid flows and heat conduction in solid. Model validation showed the predicted results agreed well with the analytical solutions. The method was applied to simulate a transient diesel engine with fuel spray. The non-uniform spatial temperature distribution on the piston surface caused by fuel spray was predicted by the conjugate heat transfer model. The present model can be used to predict the temperature of the engine combustion chamber under combustion conditions in future studies.</p

Large-Eddy Simulation (LES) of Spray Transients: Start and End of Injection Phenomena

This work reports investigations on Diesel spray transients, accounting for internal nozzle flow and needle motion, and demonstrates how seamless calculations of internal flow and external jet can be accomplished in a Large-Eddy Simulation (LES) framework using an Eulerian mixture model. Sub-grid stresses are modeled with the Dynamic Structure (DS) model, a non-viscosity based one-equation LES model. Two problems are studied with high level of spatial and temporal resolution. The first one concerns an End-Of-Injection (EOI) case where gas ingestion, cavitation, and dribble formation are resolved. The second case is a Start-Of-Injection (SOI) simulation that aims at analyzing the effect of residual gas trapped inside the injector sac on spray penetration and rate of fuel injection. Simulation results are compared against experiments carried out at Argonne National Laboratory (ANL) using synchrotron X-ray. A mesh sensitivity analysis is conducted to assess the quality of the LES approach by evaluating the resolved turbulent kinetic energy budget and comparing the outcomes with a length-scale resolution index. LES of both EOI and SOI processes have been carried out on a single hole Diesel injector, providing insights in to the physics of the processes, with internal and external flow details, and linking the phenomena at the end of an injection event to those at the start of a new injection. Concerning the EOI, the model predicts ligament formation and gas ingestion, as observed experimentally, and the amount of residual gas in the nozzle sac matches with the available data. The fast dynamics of the process is described in detail. The simulation provides unique insights into the physics at the EOI. Similarly, the SOI simulation shows how gas is ejected first, and liquid fuel starts being injected with a delay. The simulation starts from a very low needle lift and is able to predict the actual Rate-Of-Injection (ROI) and jet penetration, based only on the prescribed needle motion. Finally, guidelines and future improvements of the model are discussed concerning the simulation of the transient injection phases

Computational Modeling of Biomass Thermochemical Conversion in Fluidized Beds: Particle Density Variation and Size Distribution

The design and scale-up of fluidized-bed reactors is an important step to commercialize viable conversion pathways (such as fast pyrolysis) for biomass into hydrocarbon intermediates and fuels that lead to “drop-in” replacements for jet fuel, diesel, gasoline, and other petroleum-based products. Detailed information about the particle size distribution (PSD) and particle density evolution throughout the fluidized-bed reactor can play a critical role in determining in situ catalyst selectivity, intermediate components, and reactor performance. This work presents an Euler–Euler computational fluid dynamics (CFD) model applied to biomass thermochemical conversion for use in fluidized-bed reactor simulations. The complex chemical and physical processes of particle devolatilization and their interaction with the reacting gas environment are described within a multifluid framework based on the kinetic theory of granular flows. The direct quadrature method of moments is used to describe the biomass PSD. Continuously varying particle density due to mass evolving to the gas flow was applied to describe the evolution of particles’ physical properties. The global kinetic model is based on superimposed hemicellulose, cellulose, and lignin reactants. The calculations of the stiff chemical source terms and convection are decoupled using a time-splitting method. The CFD model is applied to simulate the fast pyrolysis of red oak in a laboratory-scale fluidized-bed reactor and validated against experimental data. The simulated product yields at the reactor outlet are presented and compared with monodisperse results and the experimental measurements. It is demonstrated that our current CFD model is able to predict in detail the dynamic particle processes, mixing and segregation, char particle elutriation, and produced gas composition at the reactor outlet needed to optimize the reactor operating conditions

Large-Eddy Simulation (LES) of Spray Transients: Start and End of Injection Phenomena

This work reports investigations on Diesel spray transients, accounting for internal nozzle flow and needle motion, and demonstrates how seamless calculations of internal flow and external jet can be accomplished in a Large-Eddy Simulation (LES) framework using an Eulerian mixture model. Sub-grid stresses are modeled with the Dynamic Structure (DS) model, a non-viscosity based one-equation LES model. Two problems are studied with high level of spatial and temporal resolution. The first one concerns an End-Of-Injection (EOI) case where gas ingestion, cavitation, and dribble formation are resolved. The second case is a Start-Of-Injection (SOI) simulation that aims at analyzing the effect of residual gas trapped inside the injector sac on spray penetration and rate of fuel injection. Simulation results are compared against experiments carried out at Argonne National Laboratory (ANL) using synchrotron X-ray. A mesh sensitivity analysis is conducted to assess the quality of the LES approach by evaluating the resolved turbulent kinetic energy budget and comparing the outcomes with a length-scale resolution index. LES of both EOI and SOI processes have been carried out on a single hole Diesel injector, providing insights in to the physics of the processes, with internal and external flow details, and linking the phenomena at the end of an injection event to those at the start of a new injection. Concerning the EOI, the model predicts ligament formation and gas ingestion, as observed experimentally, and the amount of residual gas in the nozzle sac matches with the available data. The fast dynamics of the process is described in detail. The simulation provides unique insights into the physics at the EOI. Similarly, the SOI simulation shows how gas is ejected first, and liquid fuel starts being injected with a delay. The simulation starts from a very low needle lift and is able to predict the actual Rate-Of-Injection (ROI) and jet penetration, based only on the prescribed needle motion. Finally, guidelines and future improvements of the model are discussed concerning the simulation of the transient injection phases