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Papers by Stig Grafsrønningen
This thesis is a collection of articles with an introduction which motivates the present work and... more This thesis is a collection of articles with an introduction which motivates the present work and relates the articles to each other. The main body of this thesis consist of six journal papers, which at the time of writing,
International Journal of Thermal Sciences, Feb 1, 2017
Large eddy simulations of a buoyant plume forming above a heated horizontal cylinder with a Rayle... more Large eddy simulations of a buoyant plume forming above a heated horizontal cylinder with a Rayleigh number of 9.4E7 is carried out and compared with experimental data. Natural convection heat transfer from a horizontal cylinder at this intermediate Rayleigh number involve a laminar to turbulent transition downstream the cylinder. A laminar to turbulent transition will alter the flow characteristic downstream cylinder considerably, thus it is important that the transition is captured in the simulations. Subgrid stresses are accounted for using the dynamic Smagorinsky model which allow for both laminar and turbulent flow through the dynamic procedure. The results show a considerable difference between the numerical and experimental results. Plume center vertical velocity is highly overpredicted compared to the experimental data. The computed half-width about 1.5 y/D downstream the cylinder is comparable to the experimental data, however, 3.5 y/D downstream cylinder, the half-width is only about half that of the experimental data. The half-width growth rate measured in the experiments remain higher than the computed growth rate throughout the domain of interest.
Oil and Gas Facilities, Dec 1, 2016
In a subsea facility, the pipelines will be surrounded by sea water at 4 • C, which will actively... more In a subsea facility, the pipelines will be surrounded by sea water at 4 • C, which will actively cool the production fluid inside. If temperatures drop below a critical value, water and gas will form unwanted solids called hydrates. Dead legs are inactive parts of production pipelines occupied by stagnant hydrocarbons. These areas pose a major hydrate formation risk, and needs to be insulated based on a prior heat transfer analysis. If a dead leg contains access points for e.g remotely operated vehicles, these areas need to be kept uninsulated, and will act as cold spots. Due to internal natural convection, these cold spots will potentially influence the temperatures throughout the system, and it is therefore crucial to predict the degree of influence. In this master thesis, experimental and numerical heat transfer analysis was conducted on a T-shaped plexiglass pipe, representing a production header with a vertical dead-leg. The header was insulated, while the dead-leg was uninsulated and carried a cold spot on top. In the first of two experimental phases, water was circulated through the header at constant flow rate, mimicking steady state production. In the second phase, the flow was enclosed and the water was cooled down over a period of 3 hours. During both phases, internal and external temperatures were measured with RTD's and thermocouples respectively, while velocities in the dead leg were measured using PIV. It was shown how the mean velocity field rotated periodically in a clockwise and counterclockwise manner during both phases. A numerical model was created in Workbench, and simulations were carried out using RANS with a k − ω SST formulation in CFX. Temperatures were correctly predicted for 3 hours of cool down, by modelling the cold spot as an isothermal wall, even though simulations failed to recreate the periodic mean velocity field observed in the experiment. First of all, I would like to thank my main supervisor Atle Jensen for experimental guidance and fruitful discussions. Many thanks are also directed towards my second supervisor, Mikael Mortensen, for helping me with the numerical analysis. I would also like to thank my external supervisor Stig Grafsrønningen for an introduction on performing cool down experiments, and help with creating a numerical model. As most master students, I had limited lab experience, and setting up an experiment with so many components involved would not have been possible without the lab engineer at the "Hydrodynamic Wave lab" at UiO, Olav Gundersen. He has provided technical guidance, as well as help with setting up the experiment and designing the aluminium cold spot. I would also like to thank Trine Jelstad Olsen for help with fixing my numerical model when I could not figure out what made the meshing process fail over and over again, Jostein Kolaas for setting up the laser, and Bjørn Frostmann at Armacell for providing me with complimentary Armaflex insulation. Being surrounded by motivating master and Ph.D. students has also been very beneficial, and I would especially like to thank Anis Awal Ayati for his useful advice and feedback. Last but not least, I would like to thank my family and my girlfriend Isabell B. Johansen for supporting and motivating me.
International Journal of Heat and Mass Transfer, Feb 28, 2013
Results from an experimental investigation of natural convection heat transfer from a vertical ar... more Results from an experimental investigation of natural convection heat transfer from a vertical array of three evenly heated horizontal cylinders, with dissimilar cylinder spacings, is presented. Nusselt numbers are presented for Rayleigh numbers 1.96E7 and 5.35E7 with separation distances 2D, 3D, 4D, and 5D. The results show that the Nusselt number on the second cylinder, in the three-cylinder array, increases compared to the lowermost cylinder for both Rayleigh numbers investigated and all four separation distances. The Nusselt number for the upper cylinder in the array increases compared to the lowermost/unconfined cylinder, but is comparable to the middle cylinder for the separation distances investigated. Enhanced heat transfer due to turbulent mixing on the upper cylinders in the array is discussed.
In many future subsea projects, there will be a requirement to cool various fluid streams, either... more In many future subsea projects, there will be a requirement to cool various fluid streams, either multi-phase or single phase. To meet this need, FMC Kongsberg Subsea AS (FMC) has undertaken a project to develop a practical and robust subsea cooler. The cooler is passive in that heat is transferred to the surrounding sea water by natural convection only. Because of the subsea application, the cooler must have a special geometry to meet requirements for modularization and easy installation/removal. The passive nature of the cooler means that the flow rate of the seawater coolant is not an independent variable, but is directly linked to the cooler geometry. Developing a design method for such coolers requires detailed knowledge of the important heat transfer parameters, to an accuracy far in excess of that normally required for industrial cooler design. This problem has been approached on several levels, including an extensive literature search, theoretical studies, and model testing. One of the first observations was that little research had been done previously on this type of cooler. Much information is available for various pieces of the problem, but it became clear that designing the cooler would require significant development work. Based on the knowledge gained during the initial theoretical studies, a theory for calculating cooler performance presuming one dimensional external coolant flow has been developed. While it is clear that the actual external flow is three-dimensional, the simplified theory gives important insights into how the various design parameters affect cooler performance. To fill in the gaps in theoretical knowledge, a series of model tests designed to quantify internal and external heat transfer coefficients for the special geometry is being proposed. The testing program covers several technical areas and has required the utilization of a number of advanced measurement techniques. For the next phase of the testing program, a complete new test facility has been constructed capable of testing coolers with cross-flows typical of ocean bottom currents. The cooler development program has provided new technology which will be used to construct robust and compact subsea coolers. Because of the emphasis on basic research, fundamental knowledge and insight of the heat transfer mechanisms governing the performance of this type of cooler are acquired. This knowledge gives FMC the capability to design and manufacture subsea coolers which are custom-made to match the exact requirements of a given application.
This paper presents an experimental investigation conducted on two evenly heated vertically arran... more This paper presents an experimental investigation conducted on two evenly heated vertically arranged horizontal cylinders with a diameter of 54 mm in water. The change in Nusselt number on the upper cylinder compared to a single cylinder is presented for seven Rayleigh numbers ranging from 1.82E7 to 2.55E8 for vertical separation distances S = 1.5D, 2D, 3D, 4D, and 5D. The results show that the average Nusselt number on the upper cylinder increases for 32 out of the 35 investigated cases. A statistical description of the ensemble averaged velocity field above the upper cylinder is also presented for Rayleigh numbers 1.82E7 and 5.18E7 for S = 2D and 4D. A comparison of the buoyant plume above the upper cylinder with the plume above a single cylinder showed that the flow characteristics change considerably. The plume is significantly wider and the velocity fluctuations are about two times larger than for a single cylinder. The total kinetic energy level above the second cylinder is larger than above a single cylinder. Far downstream the plume above the second cylinder approaches a Gaussian profile associated with turbulent planar plumes.
Results from an experimental investigation of the simultaneous temperature and velocity fields ab... more Results from an experimental investigation of the simultaneous temperature and velocity fields above an evenly heated horizontal cylinder with a Rayleigh number of 9.4E7 is presented. Ensemble averaged twodimensional velocity and temperature fields, velocity fluctuations, temperature variance, and velocitytemperature correlations are computed from 2700 instantaneous data sets from simultaneous laserinduced fluorescence (LIF) and particle image velocimetry (PIV). The vertical velocity and temperature field in the plume are compared with similarity solutions from turbulent planar plumes. The production of turbulent kinetic energy (TKE) due to mean shear and buoyancy is evaluated and shows that the production of TKE is dominated by velocity shear.
status: …, 2011
ABSTRACT: During a storm in October 2002, wind induced ovalling oscillations were observed on sev... more ABSTRACT: During a storm in October 2002, wind induced ovalling oscillations were observed on several empty silos of a closely spaced group of 8 by 5 silos in the port of Antwerp (Belgium). Present day standards describe only basic wind load cases, unable to ...
This thesis is a collection of articles with an introduction which motivates the present work and... more This thesis is a collection of articles with an introduction which motivates the present work and relates the articles to each other. The main body of this thesis consist of six journal papers, which at the time of writing,
This thesis is a collection of articles with an introduction which motivates the present work and... more This thesis is a collection of articles with an introduction which motivates the present work and relates the articles to each other. The main body of this thesis consist of six journal papers, which at the time of writing,
International Journal of Thermal Sciences, Feb 1, 2017
Large eddy simulations of a buoyant plume forming above a heated horizontal cylinder with a Rayle... more Large eddy simulations of a buoyant plume forming above a heated horizontal cylinder with a Rayleigh number of 9.4E7 is carried out and compared with experimental data. Natural convection heat transfer from a horizontal cylinder at this intermediate Rayleigh number involve a laminar to turbulent transition downstream the cylinder. A laminar to turbulent transition will alter the flow characteristic downstream cylinder considerably, thus it is important that the transition is captured in the simulations. Subgrid stresses are accounted for using the dynamic Smagorinsky model which allow for both laminar and turbulent flow through the dynamic procedure. The results show a considerable difference between the numerical and experimental results. Plume center vertical velocity is highly overpredicted compared to the experimental data. The computed half-width about 1.5 y/D downstream the cylinder is comparable to the experimental data, however, 3.5 y/D downstream cylinder, the half-width is only about half that of the experimental data. The half-width growth rate measured in the experiments remain higher than the computed growth rate throughout the domain of interest.
Oil and Gas Facilities, Dec 1, 2016
In a subsea facility, the pipelines will be surrounded by sea water at 4 • C, which will actively... more In a subsea facility, the pipelines will be surrounded by sea water at 4 • C, which will actively cool the production fluid inside. If temperatures drop below a critical value, water and gas will form unwanted solids called hydrates. Dead legs are inactive parts of production pipelines occupied by stagnant hydrocarbons. These areas pose a major hydrate formation risk, and needs to be insulated based on a prior heat transfer analysis. If a dead leg contains access points for e.g remotely operated vehicles, these areas need to be kept uninsulated, and will act as cold spots. Due to internal natural convection, these cold spots will potentially influence the temperatures throughout the system, and it is therefore crucial to predict the degree of influence. In this master thesis, experimental and numerical heat transfer analysis was conducted on a T-shaped plexiglass pipe, representing a production header with a vertical dead-leg. The header was insulated, while the dead-leg was uninsulated and carried a cold spot on top. In the first of two experimental phases, water was circulated through the header at constant flow rate, mimicking steady state production. In the second phase, the flow was enclosed and the water was cooled down over a period of 3 hours. During both phases, internal and external temperatures were measured with RTD's and thermocouples respectively, while velocities in the dead leg were measured using PIV. It was shown how the mean velocity field rotated periodically in a clockwise and counterclockwise manner during both phases. A numerical model was created in Workbench, and simulations were carried out using RANS with a k − ω SST formulation in CFX. Temperatures were correctly predicted for 3 hours of cool down, by modelling the cold spot as an isothermal wall, even though simulations failed to recreate the periodic mean velocity field observed in the experiment. First of all, I would like to thank my main supervisor Atle Jensen for experimental guidance and fruitful discussions. Many thanks are also directed towards my second supervisor, Mikael Mortensen, for helping me with the numerical analysis. I would also like to thank my external supervisor Stig Grafsrønningen for an introduction on performing cool down experiments, and help with creating a numerical model. As most master students, I had limited lab experience, and setting up an experiment with so many components involved would not have been possible without the lab engineer at the "Hydrodynamic Wave lab" at UiO, Olav Gundersen. He has provided technical guidance, as well as help with setting up the experiment and designing the aluminium cold spot. I would also like to thank Trine Jelstad Olsen for help with fixing my numerical model when I could not figure out what made the meshing process fail over and over again, Jostein Kolaas for setting up the laser, and Bjørn Frostmann at Armacell for providing me with complimentary Armaflex insulation. Being surrounded by motivating master and Ph.D. students has also been very beneficial, and I would especially like to thank Anis Awal Ayati for his useful advice and feedback. Last but not least, I would like to thank my family and my girlfriend Isabell B. Johansen for supporting and motivating me.
International Journal of Heat and Mass Transfer, Feb 28, 2013
Results from an experimental investigation of natural convection heat transfer from a vertical ar... more Results from an experimental investigation of natural convection heat transfer from a vertical array of three evenly heated horizontal cylinders, with dissimilar cylinder spacings, is presented. Nusselt numbers are presented for Rayleigh numbers 1.96E7 and 5.35E7 with separation distances 2D, 3D, 4D, and 5D. The results show that the Nusselt number on the second cylinder, in the three-cylinder array, increases compared to the lowermost cylinder for both Rayleigh numbers investigated and all four separation distances. The Nusselt number for the upper cylinder in the array increases compared to the lowermost/unconfined cylinder, but is comparable to the middle cylinder for the separation distances investigated. Enhanced heat transfer due to turbulent mixing on the upper cylinders in the array is discussed.
In many future subsea projects, there will be a requirement to cool various fluid streams, either... more In many future subsea projects, there will be a requirement to cool various fluid streams, either multi-phase or single phase. To meet this need, FMC Kongsberg Subsea AS (FMC) has undertaken a project to develop a practical and robust subsea cooler. The cooler is passive in that heat is transferred to the surrounding sea water by natural convection only. Because of the subsea application, the cooler must have a special geometry to meet requirements for modularization and easy installation/removal. The passive nature of the cooler means that the flow rate of the seawater coolant is not an independent variable, but is directly linked to the cooler geometry. Developing a design method for such coolers requires detailed knowledge of the important heat transfer parameters, to an accuracy far in excess of that normally required for industrial cooler design. This problem has been approached on several levels, including an extensive literature search, theoretical studies, and model testing. One of the first observations was that little research had been done previously on this type of cooler. Much information is available for various pieces of the problem, but it became clear that designing the cooler would require significant development work. Based on the knowledge gained during the initial theoretical studies, a theory for calculating cooler performance presuming one dimensional external coolant flow has been developed. While it is clear that the actual external flow is three-dimensional, the simplified theory gives important insights into how the various design parameters affect cooler performance. To fill in the gaps in theoretical knowledge, a series of model tests designed to quantify internal and external heat transfer coefficients for the special geometry is being proposed. The testing program covers several technical areas and has required the utilization of a number of advanced measurement techniques. For the next phase of the testing program, a complete new test facility has been constructed capable of testing coolers with cross-flows typical of ocean bottom currents. The cooler development program has provided new technology which will be used to construct robust and compact subsea coolers. Because of the emphasis on basic research, fundamental knowledge and insight of the heat transfer mechanisms governing the performance of this type of cooler are acquired. This knowledge gives FMC the capability to design and manufacture subsea coolers which are custom-made to match the exact requirements of a given application.
This paper presents an experimental investigation conducted on two evenly heated vertically arran... more This paper presents an experimental investigation conducted on two evenly heated vertically arranged horizontal cylinders with a diameter of 54 mm in water. The change in Nusselt number on the upper cylinder compared to a single cylinder is presented for seven Rayleigh numbers ranging from 1.82E7 to 2.55E8 for vertical separation distances S = 1.5D, 2D, 3D, 4D, and 5D. The results show that the average Nusselt number on the upper cylinder increases for 32 out of the 35 investigated cases. A statistical description of the ensemble averaged velocity field above the upper cylinder is also presented for Rayleigh numbers 1.82E7 and 5.18E7 for S = 2D and 4D. A comparison of the buoyant plume above the upper cylinder with the plume above a single cylinder showed that the flow characteristics change considerably. The plume is significantly wider and the velocity fluctuations are about two times larger than for a single cylinder. The total kinetic energy level above the second cylinder is larger than above a single cylinder. Far downstream the plume above the second cylinder approaches a Gaussian profile associated with turbulent planar plumes.
Results from an experimental investigation of the simultaneous temperature and velocity fields ab... more Results from an experimental investigation of the simultaneous temperature and velocity fields above an evenly heated horizontal cylinder with a Rayleigh number of 9.4E7 is presented. Ensemble averaged twodimensional velocity and temperature fields, velocity fluctuations, temperature variance, and velocitytemperature correlations are computed from 2700 instantaneous data sets from simultaneous laserinduced fluorescence (LIF) and particle image velocimetry (PIV). The vertical velocity and temperature field in the plume are compared with similarity solutions from turbulent planar plumes. The production of turbulent kinetic energy (TKE) due to mean shear and buoyancy is evaluated and shows that the production of TKE is dominated by velocity shear.
status: …, 2011
ABSTRACT: During a storm in October 2002, wind induced ovalling oscillations were observed on sev... more ABSTRACT: During a storm in October 2002, wind induced ovalling oscillations were observed on several empty silos of a closely spaced group of 8 by 5 silos in the port of Antwerp (Belgium). Present day standards describe only basic wind load cases, unable to ...
This thesis is a collection of articles with an introduction which motivates the present work and... more This thesis is a collection of articles with an introduction which motivates the present work and relates the articles to each other. The main body of this thesis consist of six journal papers, which at the time of writing,