David Rowinski - Academia.edu (original) (raw)

Papers by David Rowinski

International Journal for Numerical Methods in Biomedical Engineering, 2021

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied usin... more A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.

Computational Fluid Dynamics (CFD) models can offer great insight into flow phenomena and complex... more Computational Fluid Dynamics (CFD) models can offer great insight into flow phenomena and complex fluidstructure interactions present in reciprocating compressors. This is often achieved, however, at large computational cost and considerable user setup time. In this study, a Cartesian cut-cell finite-volume method is applied to model a small displacement refrigeration compressor. The cut-cell method has the key feature of representing discrete cell volumes exactly without requiring the computational grid to coincide with the bounding geometry. Additionally, the grid is dynamically generated at each time step based on the instantaneous boundary positions and is automatically refined based on gradients of local flow variables. These two features make this method ideal for modeling the deformation of the valves, the motion of the piston, and the complex geometries of the suction and discharge mufflers. The model is validated against experimental data. The sources of numerical error in ...

Computational Fluid Dynamics (CFD) is a convenient and powerful tool for modeling and evaluating ... more Computational Fluid Dynamics (CFD) is a convenient and powerful tool for modeling and evaluating the performance of centrifugal compressors, fans, and pumps. Typically, the use of CFD requires significant labor in mesh generation and refinement. Moreover, for rotating transient simulations, a grid-to-grid interface is usually needed to incorporate the rotating impeller. In this work, a finite-volume-based Cartesian cut-cell method is employed to model a centrifugal fan. Both rotating transient simulations and steady steady multiple reference frame (MRF) simulations are analyzed. The cut-cell-based method automatically generates the Cartesian mesh on-the-fly based on the current location of the rotating boundaries, without requiring any grid-grid interface between the rotating part and the stationary part for the rotating transient cases. The grid is also dynamically refined based on the velocity field and y-plus values at the walls. These features greatly save the pre-processing tim...

Computational fluid dynamics has been increasingly used in the design and analysis of reciprocati... more Computational fluid dynamics has been increasingly used in the design and analysis of reciprocating compressors over the last several years. One of the major challenges in the use of such tools is the creation of the numerical grid on which the modeled equations are solved. Since these compressors typically consist of many interconnected and moving parts, manual creation of the grid can be labor-intensive. Furthermore, it is necessary that the choice of grid yields a sufficiently resolved solution, so that the numerical error is significantly less than the modeling error. In this work, a small displacement refrigeration compressor is modeled using a numerical grid created with an automatic meshing approach. The grid is then automatically adapted to the flow based on the local flow field variables at each time step. This cut-cell based grid matches the supplied fluid volume exactly and permits general motion of all bounding surfaces. An explicit two-way coupled approach is used to ac...

The recent requirements on the scroll compressors used in air conditioning systems are focused on... more The recent requirements on the scroll compressors used in air conditioning systems are focused on the reduction of the noise and the improvement of the efficiency. To achieve this, the complex fluid flow phenomenon taking place inside of the compressor must be better understood. Two modeling approaches for investigating this problem are one-dimensional multi-physics modeling and three-dimensional computational fluid dynamics (CFD) modeling. The one-dimensional multi-physics based models are available to perform these calculations with low computational cost, but the influence of detailed geometrical effects on the fluid flow behavior is not taken into account. This paper deals with the development and application of a three-dimensional CFD model for a scroll compressor. To deal with the challenge of the complicated moving geometries of the orbiting scroll and the deforming reed valves, an automated meshing strategy is employed to dynamically calculate the working chamber volumes bas...

In order to design screw compressors for optimal performance, it is crucial to understand the com... more In order to design screw compressors for optimal performance, it is crucial to understand the complex fluid flow processes within them. Computational fluid dynamics (CFD) is one approach for doing so. Considerable progress has been made over the last several years in both commercial and academic solution packages for this application; however, due to the complex moving geometries of the screw rotors and the tight clearances between the moving parts, a major challenge that remains is the generation of numerical grids that are increasingly efficient, accurate, robust, and easily created. In this study, an alternate methodology for this problem is presented. The grid is created automatically at every time step based on the instantaneous geometry using a Cartesian cut-cell based method which preserves exactly the changing control volume shapes. Automatic mesh refinement is employed to adaptively increase mesh resolution where the flow variables have large gradients in order to resolve t...

Journal of Propulsion and Power, 2020

The predictive numerical simulation of near-limit turbulent premixed combustion, in which the tur... more The predictive numerical simulation of near-limit turbulent premixed combustion, in which the turbulent intensity is high and the fuel/air mixture is near the flammability limit, remains challengin...

Combustion and Flame, 2017

upstream location but does not help improve the prediction at the downstream location where the c... more upstream location but does not help improve the prediction at the downstream location where the controlling process is chemical reaction.

Combustion Theory and Modelling, 2013

A computational study is performed on a series of four piloted, lean, premixed turbulent jet flam... more A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM1-50, PM1-100, PM1-150 and PM1-200, respectively. Calculations are performed using the RANS-PDF and LES-PDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM1-50 and PM1-100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LES-PDF calculations. The inclusion of differential diffusion in the LES-PDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H 2. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1-150 and PM1-200, and this discrepancy is also observed in the LES-PDF calculations; however, a parametric study of the LES-PDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.

Combustion Theory and Modelling, 2010

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied usin... more A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.

IOP Conference Series: Materials Science and Engineering, 2018

Understanding the details of the internal flow processes in screw compressors and expanders is ve... more Understanding the details of the internal flow processes in screw compressors and expanders is very important for their efficient and robust design. Computational fluid dynamics (CFD) provides full access to a modeled three dimensional flow field and its variation in time. However, the application of CFD to screw compressors and expanders can be difficult because of the complicated geometries involved and the need to supply the computational grid on which the modeled equations are solved over a large number of time steps. While the majority of previous research on CFD applications to screw machines features inventive techniques for generating meshes that adequately resolve the flows in the small clearances, an alternate approach is demonstrated in this work. The screw expander SE 51.2 from TU Dortmund University is analyzed here through a CFD model which generates the grid automatically based on a modified Cartesian cut-cell approach. The grid is then adaptively refined based on local gradients of velocity and temperature. At each time step, the grid is regenerated based on the geometry motion. As opposed to resolving the flow in the clearances, a model is applied so that the cells in the clearance can remain relatively large. The detailed measurements of the screw expander are used to validate the model. The operating conditions investigated include the expansion of dry air at a four-to-one pressure ratio for four different rotational speeds. The measured internal chamber pressures are compared to the results from the model, as are the average mass flow rate, indicated power, and outlet temperature. A coupled thermal-fluid approach is used to model the rotor temperatures and corresponding thermal deformation of the rotors and housing. In this approach, the fluid and solid temperatures are solved together; to deal with the problem of the disparate time scales between the fluid and solid heat transfer, the solids are periodically solved to steady state using heat transfer coefficients and near-wall temperatures computed from an energy conserving averaging over several cycles. The effects of the various leakage flow paths in the model, including the rotor-to-rotor, rotor-tip-to-housing, and bearing leakage are demonstrated and quantified. Finally, simulation and experimental results are compared in terms of different rotor-tip-to-housing clearance heights. The model considering the appropriate thermal deformation of the rotors is shown to yield the best agreement with the measurements, however there is work remaining to reduce the model calculation time, especially at low rotational speeds.

International Journal for Numerical Methods in Biomedical Engineering, 2021

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied usin... more A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.

Computational Fluid Dynamics (CFD) models can offer great insight into flow phenomena and complex... more Computational Fluid Dynamics (CFD) models can offer great insight into flow phenomena and complex fluidstructure interactions present in reciprocating compressors. This is often achieved, however, at large computational cost and considerable user setup time. In this study, a Cartesian cut-cell finite-volume method is applied to model a small displacement refrigeration compressor. The cut-cell method has the key feature of representing discrete cell volumes exactly without requiring the computational grid to coincide with the bounding geometry. Additionally, the grid is dynamically generated at each time step based on the instantaneous boundary positions and is automatically refined based on gradients of local flow variables. These two features make this method ideal for modeling the deformation of the valves, the motion of the piston, and the complex geometries of the suction and discharge mufflers. The model is validated against experimental data. The sources of numerical error in ...

Computational Fluid Dynamics (CFD) is a convenient and powerful tool for modeling and evaluating ... more Computational Fluid Dynamics (CFD) is a convenient and powerful tool for modeling and evaluating the performance of centrifugal compressors, fans, and pumps. Typically, the use of CFD requires significant labor in mesh generation and refinement. Moreover, for rotating transient simulations, a grid-to-grid interface is usually needed to incorporate the rotating impeller. In this work, a finite-volume-based Cartesian cut-cell method is employed to model a centrifugal fan. Both rotating transient simulations and steady steady multiple reference frame (MRF) simulations are analyzed. The cut-cell-based method automatically generates the Cartesian mesh on-the-fly based on the current location of the rotating boundaries, without requiring any grid-grid interface between the rotating part and the stationary part for the rotating transient cases. The grid is also dynamically refined based on the velocity field and y-plus values at the walls. These features greatly save the pre-processing tim...

Computational fluid dynamics has been increasingly used in the design and analysis of reciprocati... more Computational fluid dynamics has been increasingly used in the design and analysis of reciprocating compressors over the last several years. One of the major challenges in the use of such tools is the creation of the numerical grid on which the modeled equations are solved. Since these compressors typically consist of many interconnected and moving parts, manual creation of the grid can be labor-intensive. Furthermore, it is necessary that the choice of grid yields a sufficiently resolved solution, so that the numerical error is significantly less than the modeling error. In this work, a small displacement refrigeration compressor is modeled using a numerical grid created with an automatic meshing approach. The grid is then automatically adapted to the flow based on the local flow field variables at each time step. This cut-cell based grid matches the supplied fluid volume exactly and permits general motion of all bounding surfaces. An explicit two-way coupled approach is used to ac...

The recent requirements on the scroll compressors used in air conditioning systems are focused on... more The recent requirements on the scroll compressors used in air conditioning systems are focused on the reduction of the noise and the improvement of the efficiency. To achieve this, the complex fluid flow phenomenon taking place inside of the compressor must be better understood. Two modeling approaches for investigating this problem are one-dimensional multi-physics modeling and three-dimensional computational fluid dynamics (CFD) modeling. The one-dimensional multi-physics based models are available to perform these calculations with low computational cost, but the influence of detailed geometrical effects on the fluid flow behavior is not taken into account. This paper deals with the development and application of a three-dimensional CFD model for a scroll compressor. To deal with the challenge of the complicated moving geometries of the orbiting scroll and the deforming reed valves, an automated meshing strategy is employed to dynamically calculate the working chamber volumes bas...

In order to design screw compressors for optimal performance, it is crucial to understand the com... more In order to design screw compressors for optimal performance, it is crucial to understand the complex fluid flow processes within them. Computational fluid dynamics (CFD) is one approach for doing so. Considerable progress has been made over the last several years in both commercial and academic solution packages for this application; however, due to the complex moving geometries of the screw rotors and the tight clearances between the moving parts, a major challenge that remains is the generation of numerical grids that are increasingly efficient, accurate, robust, and easily created. In this study, an alternate methodology for this problem is presented. The grid is created automatically at every time step based on the instantaneous geometry using a Cartesian cut-cell based method which preserves exactly the changing control volume shapes. Automatic mesh refinement is employed to adaptively increase mesh resolution where the flow variables have large gradients in order to resolve t...

Journal of Propulsion and Power, 2020

The predictive numerical simulation of near-limit turbulent premixed combustion, in which the tur... more The predictive numerical simulation of near-limit turbulent premixed combustion, in which the turbulent intensity is high and the fuel/air mixture is near the flammability limit, remains challengin...

Combustion and Flame, 2017

upstream location but does not help improve the prediction at the downstream location where the c... more upstream location but does not help improve the prediction at the downstream location where the controlling process is chemical reaction.

Combustion Theory and Modelling, 2013

A computational study is performed on a series of four piloted, lean, premixed turbulent jet flam... more A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM1-50, PM1-100, PM1-150 and PM1-200, respectively. Calculations are performed using the RANS-PDF and LES-PDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM1-50 and PM1-100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LES-PDF calculations. The inclusion of differential diffusion in the LES-PDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H 2. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1-150 and PM1-200, and this discrepancy is also observed in the LES-PDF calculations; however, a parametric study of the LES-PDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.

Combustion Theory and Modelling, 2010

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied usin... more A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.

IOP Conference Series: Materials Science and Engineering, 2018

Understanding the details of the internal flow processes in screw compressors and expanders is ve... more Understanding the details of the internal flow processes in screw compressors and expanders is very important for their efficient and robust design. Computational fluid dynamics (CFD) provides full access to a modeled three dimensional flow field and its variation in time. However, the application of CFD to screw compressors and expanders can be difficult because of the complicated geometries involved and the need to supply the computational grid on which the modeled equations are solved over a large number of time steps. While the majority of previous research on CFD applications to screw machines features inventive techniques for generating meshes that adequately resolve the flows in the small clearances, an alternate approach is demonstrated in this work. The screw expander SE 51.2 from TU Dortmund University is analyzed here through a CFD model which generates the grid automatically based on a modified Cartesian cut-cell approach. The grid is then adaptively refined based on local gradients of velocity and temperature. At each time step, the grid is regenerated based on the geometry motion. As opposed to resolving the flow in the clearances, a model is applied so that the cells in the clearance can remain relatively large. The detailed measurements of the screw expander are used to validate the model. The operating conditions investigated include the expansion of dry air at a four-to-one pressure ratio for four different rotational speeds. The measured internal chamber pressures are compared to the results from the model, as are the average mass flow rate, indicated power, and outlet temperature. A coupled thermal-fluid approach is used to model the rotor temperatures and corresponding thermal deformation of the rotors and housing. In this approach, the fluid and solid temperatures are solved together; to deal with the problem of the disparate time scales between the fluid and solid heat transfer, the solids are periodically solved to steady state using heat transfer coefficients and near-wall temperatures computed from an energy conserving averaging over several cycles. The effects of the various leakage flow paths in the model, including the rotor-to-rotor, rotor-tip-to-housing, and bearing leakage are demonstrated and quantified. Finally, simulation and experimental results are compared in terms of different rotor-tip-to-housing clearance heights. The model considering the appropriate thermal deformation of the rotors is shown to yield the best agreement with the measurements, however there is work remaining to reduce the model calculation time, especially at low rotational speeds.