Author:
Publisher:
Published: 2015
Total Pages: 113
ISBN-13:
DOWNLOAD EBOOK →Fluids operating in the supercritical state have promising characteristics for future high efficiency power cycles. In order to develop power cycles using supercritical fluids, it is necessary to understand the flow characteristics of fluids under both supercritical and two-phase conditions. In this study, a Computational Fluid Dynamic (CFD) methodology was developed for supercritical fluids flowing through complex geometries. A real fluid property module was implemented to provide properties for different supercritical fluids. However, in each simulation case, there is only one species of fluid. As a result, the fluid property module provides properties for either supercritical CO2 (S-CO2) or supercritical water (SCW). The Homogeneous Equilibrium Model (HEM) was employed to model the two-phase flow. HEM assumes two phases have same velocity, pressure, and temperature, making it only applicable for the dilute dispersed two-phase flow situation. Three example geometries, including orifices, labyrinth seals, and valves, were used to validate this methodology with experimental data. For the first geometry, S-CO2 and SCW flowing through orifices were simulated and compared with experimental data. The maximum difference between the mass flow rate predictions and experimental measurements is less than 5%. This is a significant improvement as previous works can only guarantee 10% error. In this research, several efforts were made to help this improvement. First, an accurate real fluid module was used to provide properties. Second, the upstream condition was determined by pressure and density, which determines supercritical states more precise than using pressure and temperature. For the second geometry, the flow through labyrinth seals was studied. After a successful validation, parametric studies were performed to study geometric effects on the leakage rate. Based on these parametric studies, an optimum design strategy for the see-through labyrinth seals was proposed. A stepped labyrinth seal, which mimics the behavior of the labyrinth seal used in the Sandia National Laboratory (SNL) S-CO2 Brayton cycle, was also tested in the experiment along with simulations performed. The rest of this study demonstrates the difference of valves' behavior under supercritical fluid and normal fluid conditions. A small-scale valve was tested in the experiment facility using S-CO2. Different percentages of opening valves were tested, and the measured mass flow rate agreed with simulation predictions. Two transients from a real S-CO2 Brayton cycle design provided the data for valve selection. The selected valve was studied using numerical simulation, as experimental data is not available.
Author:
Publisher:
Published: 2014
Total Pages: 241
ISBN-13:
DOWNLOAD EBOOK →This report is divided into four parts. First part of the report describes the methods used to measure and model the flow of supercritical carbon dioxide (S-CO2) through annuli and straight- through labyrinth seals. The effects of shaft eccentricity in small diameter annuli were observed for length-to-hydraulic diameter (L/D) ratios of 6, 12, 143, and 235. Flow rates through tooth- cavity labyrinth seals were measured for inlet pressures of 7.7, 10, and 11 MPa with corresponding inlet densities of 325, 475, and 630 kg/m3. Various leakage models were compared to this result to describe their applicability in supercritical carbon dioxide applications. Flow rate measurements were made varying tooth number for labyrinth seals of same total length. Flow rate measurements were also made for a stepped labyrinth seal similar to shaft seal found in Sandia National Laboratories research facility. The effect of eccentricity on flow through small diameter annuli was found to be minimal for the lengths typically found in labyrinth type shaft seals. There is an expected flow increase when moving a shaft from a concentric position to an eccentric position which is driven by a change in the fluid friction. This flow increase was found to be small for short annular orifices. An observed increase in flow rate of 3% was observed for short length annular orifices and was increased to 8.5% when the orifice length was increased beyond a distance equal to the developing entrance length and frictional effects were manifested. Flow rate measurements for a straight through labyrinth seal with three teeth were made at inlet pressures of 7.7, 10, and 11 MPa with corresponding inlet densities of 325, 475, and 630 kg/m3. Various labyrinth seal leakage models were applied to the data calculated to compare applicability. Applying the Homogeneous Equilibrium Model (HEM) with an experimentally determined discharge coefficient to predict the mass flow rate gave results with less than 5% error for the higher pressure cases of 10 and 11 MPa and less that 14% error for the lower pressure case of 7.7 MPa. The HEM model works well when the inlet condition chokes prior to entering the two phase region and begins to deviate when two-phase effects become more prevalent. Other models were unable to predict property changes along with poor response to changes in geometry due to their lack of complexity. A Stepped labyrinth seal was designed to mimic the geometry used in the supercritical flow research loop at Sandia National Laboratories. This provided a more complex geometry to further test the capabilities of the facility and validate models. The results showed that the data could be used to scale to larger diameters and apply to more practical geometries. Three-tooth and four-tooth cases were tested an inlet pressure of 10 MPa with a corresponding inlet density of 325 kg/m3. It was found that increasing the tooth number decreased the flow by 5% from the three-tooth case to the four-tooth case. Second part of the report describes the computational study performed to understand the leakage through the labyrinth seals using Open source CFD package OpenFOAM. Fluid Property Interpolation Tables (FIT) program was implemented in OpenFOAM to accurately model the properties of CO2 required to solve the governing equations. To predict the flow behavior in the two phase dome Homogeneous Equilibrium Model (HEM) is assumed to be valid. Experimental results for plain orifice (L/D ~ 5) were used to show the capabilities of the FIT model implemented in OpenFOAM. Error analysis indicated that OpenFOAM is capable of predicting experimental data within "10% error with the majority of data close to "5% error. Following the validation of computational model, effects of geometrical parameters and operating conditions are isolated from each other and a parametric study was performed in two parts to understand their effects on leakage flow.
Author: David Yang (S.M.)
Publisher:
Published: 2014
Total Pages: 121
ISBN-13:
DOWNLOAD EBOOK →This thesis presents an experimental assessment of the internal flow behavior of supercritical carbon dioxide. The investigation focused mainly on assessing condensation onset during rapid expansion of CO2 into the two-phase region. An experimental blowdown test-rig with a modular test section was developed with the capability of operating a converging-diverging nozzle with a wide range of charge conditions. The test-rig demonstrated repeatable results with relative errors of less than 1 percent. An innovative method to measure the speed of sound through the use of Helmholtz resonators was developed and investigated. Shock tube experiments and static high pressure air tests in the blowdown facility were conducted to determine the viscous damping in the resonators. The results indicate that a Helmholtz resonator neck Reynolds number of 106 is required for underdamped response. The speed of sound measurement technique was demonstrated at relevent Reynolds numbers and at static conditions in air, showing promise for blowdown testing in S-CO2 Blowdown experiments were conducted in supercritical CO2 from charge conditions both away and near the critical point to sonic conditions in the two-phase region. To determine whether there was condensation onset, static pressure measurements were compared to theoretical and numerical models. Numerical models utilized the Span-Wagner equation of state extrapolated into the two-phase region to characterize the metastable state. Away from the critical point, for operating conditions typically encountered in S-CO2 compressor stages, condensation was not observed. Near the critical point, results were inconclusive and future work is proposed to provide more conclusive assessment of condensation near the critical region.
Author:
Publisher:
Published: 2015
Total Pages: 0
ISBN-13:
DOWNLOAD EBOOK →Fluids operating in the supercritical state have promising characteristics for future high efficiency power cycles. In order to develop supercritical fluid power cycles, it is necessary to understand the flow characteristics of fluids under both supercritical and two-phase conditions. In this study, a Computational Fluid Dynamic (CFD) methodology was developed for supercritical fluids flowing through complex geometries. A real fluid property module was implemented to provide properties. In each simulation case, there is only one species of fluid. As a result, the fluid property module provides properties for either supercritical CO2 (S-CO2) or supercritical water (SCW). The homogeneous equilibrium model (HEM) was employed to model the two-phase flow. HEM assumes two phases have the same velocity, pressure, and temperature, making it only applicable for the dilute dispersed two-phase flow situation. Three example geometries, including orifices, labyrinth seals, and valves, were used to validate this methodology with experimental data. For the first geometry, S-CO2 and SCW flowing through orifices were simulated and compared with experimental data. The maximum difference between the mass flow rate predictions and experimental measurements is less than 5%. This is a significant improvement as previous works can only guarantee 10% error. In this research, several efforts were made to help this improvement. First, an accurate real fluid module was used to provide properties. Second, the upstream condition was determined by pressure and density, which determines supercritical states more precisely than using pressure and temperature. For the second geometry, the flow through labyrinth seals was studied. Based on the successful validation of the proposed methodology, parametric studies were performed to study geometric effects on the leakage rate. Based on these parametric studies, an optimum design strategy for the see-through labyrinth seals was proposed. A stepped labyrinth seal, which mimics the behavior of the labyrinth seal used in the Sandia National Laboratory (SNL) S-CO2 Brayton cycle experiment compressor, was also tested in the experiment along with simulations performed. The study demonstrates the difference of valves' behavior under supercritical fluid and normal fluid conditions. A small-scale valve was tested in the experiment facility using S-CO2. Different percentages of opening valves were tested, and the measured mass flow rate agreed with simulation predictions. Two transients from a real S-CO2 Brayton cycle design provided the data for valve selection. The selected valve was studied with the proposed numerical methodology, as experimental data is not available.
Author: Guillaume Paul H. Mignot
Publisher:
Published: 2008
Total Pages: 206
ISBN-13:
DOWNLOAD EBOOK →Author: Sandeep R. Pidaparti
Publisher:
Published: 2014
Total Pages:
ISBN-13:
DOWNLOAD EBOOK →To meet future energy needs the use of alternative fuel sources are gaining popularity. The supercritical carbon dioxide Brayton cycle has been proposed as a possible cycle for next generation nuclear and concentrated solar power generation. Large density fluctuations of carbon dioxide in the supercritical region can be exploited to maintain compressor inlet conditions close to the critical point and thereby, reducing the compressor work and the back work ratio. In order to improve the efficiency of turbomachinery equipment it is important to reduce internal leakage through seals. A computational study was performed to understand the leakage through seals subject to large pressure differential using Open source CFD software OpenFOAM. FIT (Fluid Property Interpolation Tables) program is implemented in OpenFOAM to accurately model the properties of CO2 required to solve the governing equations. To predict flow behavior in the two phase dome HEM (Homogeneous equilibrium model) is assumed to be valid. Effects of geometrical parameters and operating conditions are isolated from each other and a parametric study was performed in two parts to understand the effects of both geometrical parameters and operating conditions. Results of the geometrical parameter study indicated that the carryover coefficient of a seal is independent of pressure drop across the seal and is only a function of geometry. A model for carryover was developed as a function of c/s (clearance to pitch ratio) and w[subscript cavity]/c (cavity width to clearance). It has been identified that the major non-dimensional parameter influencing the discharge through an annular orifice is w[subscript tooth]/c (tooth width to clearance) and a model for C[subscript d] (discharge coefficient) can be developed based on the results we obtained. Flow through labyrinth seals can be considered as a series of annular orifices and cavities. Using this analogy, leakage rate equations can be written for each tooth and the mass flow rate can be modeled as a function of the discharge coefficient under each tooth and the carryover coefficient, which accounts for the turbulent dissipation of kinetic energy in a cavity. The discharge coefficient of first tooth in a labyrinth seal is similar to that of an annular orifice, whereas, the discharge coefficient of the rest of the tooth was found to be a function of the C[subscript d] of the previous tooth and the carryover coefficient. To understand the effects of operating conditions, a 1-D isentropic choking model is developed for annular orifices resulting in upper and lower limit curves on a T-s diagram which show the choking phenomenon of flow through a seal. This model was applied to simulations performed on both an annular orifice and a labyrinth seal. It has been observed that the theory is, in general, valid for any labyrinth seal, but the upper and lower limit curves on a T-s diagram depend on number of constrictions. As the number of constrictions increases these two curves move farther away from the critical point. Finally, some experimental results for a plain orifice (L/D ~ 5) were used to show the capabilities of the FIT model implemented in OpenFOAM. Error analysis indicated that OpenFOAM is capable of predicting experimental data within a 10 % error with the majority of data close to a 5 % error. This validates the FIT model and HEM assumption. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151884
Author: Intergovernmental Panel on Climate Change. Working Group III.
Publisher: Cambridge University Press
Published: 2005-12-19
Total Pages: 59
ISBN-13: 052186643X
DOWNLOAD EBOOK →IPCC Report on sources, capture, transport, and storage of CO2, for researchers, policy-makers and engineers.
Author: Vannessa Goodship
Publisher: iSmithers Rapra Publishing
Published: 2005
Total Pages: 152
ISBN-13: 9781859574942
DOWNLOAD EBOOK →SCFs are currently the subjects of intense research and commercial interest. Applications such as the RESS (rapid expansion of supercritical fluid solutions) process are part of standard industrial practice. In view of their ever-growing importance in the polymer industry there is a need to fully comprehend how supercritical fluids interrelate with polymeric materials to realise the potential that can be gained from their use. The authors review the basic principles of SCFs and their application within the polymer industry: characteristics and properties, extraction of unwanted residual products, polymerisation solvents, and polymer impregnation. Processing applications such as plasticisation, foaming and blending are also considered. There is discussion of the potential within the polymer recycling industry for use of SCFs as cleaning agents or within supercritical oxidation processes. Around 400 references with abstracts from recent global literature accompany this review, sourced from the Polymer Library, to facilitate further reading. A subject index and a company index are included.
Author: Kanokwan Kanno
Publisher:
Published: 2002
Total Pages: 324
ISBN-13: 9789741744978
DOWNLOAD EBOOK →The expansion of supercritical fluid solution through a nozzle is a key step for producing fine particles. It makes use of the density change of a fluid from its supercritical state to its gaseous state due to expansion to lower pressure environment. Simulation of rapid expansion of supercritical carbon dioxide requires a set of basic equations of fluid flow field and thermodynamic properties of carbon dioxide from suitable equation of state (EOS). From investigation, it was found that Soave-Redlich-Kwong EOS incorporated with the compressibility factor of carbon dioxide is suitable for estimating of the carbon dioxide thermodynamic properties. The explicit finite-difference and implicit finite-different methods are employed to solve two-dimensional Euler equations for fluid flow field. The calculation results shown that the inlet temperature and inlet pressure gave impact effects on the predicted temperature of flowing fluid passed through the nozzle and for inlet velocity, it could be seen that it rarely gave effect on the predicted temperature of fluid.