Feed water heaters are large shell and tube heat exchangers. They from part of the Rankine cycle used in coal fired power plants with the main purpose being the improvement of the overall cycle efficiency. Like most heat exchangers, feed water heaters suffer from fouling. Fouling is defined as “any undesirable deposit on heat exchanger surfaces that increases resistance to heat transmission”. In the design of heat exchangers, fouling is accommodated by adding additional surface area to the heat exchanger. The amount of additional area is determined by the use of fouling factors. Although this is the only wide-spread method accepted in industry, the fouling factors in use are outdated, generally considered conservative and lead to oversized heat exchangers. The purpose of this study was to design and build a test rig that can accurately measure fouling factors of feed water heater tubes that has been in service for a full life cycle. A comprehensive literature study was performed to decide on the most effective test method, as well as the required instrument type and accuracy. The best method was found to be where the overall heat transfer coefficient for a fouled tube, outside cleaned tube (half clean) and clean tube was measured. The measured values are then converted to the internal, external and overall fouling factors. Validation test were done on the test rig. These included energy balance tests, theoretical comparison tests and repeatability tests. The results of all tests were acceptable and within measurement uncertainty limits. Five sample test tubes, obtained from a 30 year old LP heater at an Eskom power station, were tested. The results indicated that the average measured fouling factors were less than 20% of the commonly used HEI fouling factors. This is significantly lower and confirms that the fouling factors in use for this specific case are conservative. The test rig proved to be accurate and effective in measuring the fouling factors. Although the tests shows promising results, the small amount of tubes tested from only one heat exchanger are not sufficient to make meaningful conclusions. The test rig is now ready for a future study where a large sample of tubes can be tested.

A feedwater heater is a typical component in power plants which increases the cycle efficiency. Over the last decade, renewable energies have significantly developed and been employed in the power grid. However, weather conditions are inconsistent and therefore produce variable power. Fossil fuel power stations are often required to supplement the variable renewable energies, which increased the rate of power cycling to an unforeseeable extent over the past decade. Power cycling results in changes in the flow rate, pressure, and temperature of a feedwater heater’s inlet flows. In a tubesheet-type feedwater heater, these transients induce cycling stress in the tubesheet and failures due to thermal fatigue occur. The header-type feedwater is currently employed in high pressure applications as it is more resistant to thermal fatigue compared to the tubesheet-type. However, the tubesheet-type is more cost effective to construct and maintain. It would be advantageous if the cyclic thermal stresses in the tubesheet can be better analysed and alleviated to support the use of the tubesheet-type. A detailed transient temperature distribution of the tubesheet is required to understand the thermal fatigue. Normally, engineers opt towards a full CFD to obtain such results. However, the size and complexity of a feedwater heater is immense and cannot be simulated practically solely using CFD spatial elements. This study developed a multiscale approach that thermally couples 1D network elements, CFD spatial elements, and macroscopic heat transfer correlations to reduce the computational expense substantially. The combination of the various selected techniques and the specific application of this methodology is unique. This approach is capable of obtaining the detailed transient temperature distribution of the tubesheet in a reasonable time, as well as include the effects of the upstream and downstream components within the network model. The methodology was implemented using Flownex and Ansys Fluent for the 1D network and CFD solvers, respectively. The internal tube flow was modelled using 1D network elements, while the steam was modelled with CFD. Thermal discretisation, mapping, and convergence were considered to create a robust methodology not limited to feedwater heaters only. Additionally, a method was developed to analyse flow maldistribution in tube-bundles using the coupled 1D-3D approach. The implementation of the methodology consists of two parts, of which one is for development purposes, and the other serves as a demonstration. The development was done on a simple TEMA-FU heat exchanger which is representative of a feedwater heater. The methodology was tested by varying the primary fluid’s flow rates, changing the fluid media, and conducting transient simulations. The temperature distributions obtained were compared against a full CFD model and corresponded very well with errors less than 4%. A reduction in computational time of more than 40% was achieved but is highly dependent on the specific problem. Improvements to be made in future studies include the accuracy of the laminar case method and the stability of the flow maldistribution algorithm. The methodology was demonstrated by applying it to an existing industrial feedwater heater. No plant data was available to use for input conditions and therefore were assumed. The steam in the DSH was modelled using 3D CFD elements and the tube flow with 1D network elements. The condensing zone’s heat transfer was approximated using an empirical correlation. A steady state case was simulated and the outlet temperatures corresponded well with the manufacturer’s data. The temperature distribution of the tubesheet and surrounding solids were obtained. Finally, assumed sinusoidal transient perturbations to the inlet conditions were imposed. It was evident that the thermal gradients of both sides of the tubesheet were misaligned which highlights the thermal lag and inertia that cause differential temperatures. The 1D-CFD methodology was developed successfully with results that proved to correspond well, for a wide range of conditions, to full CFD. The methodology was applied and can be, in future work, validated with experimental results or extended by modelling upstream and downstream components in the network solver. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2019. / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

Large coal-fired power stations are designed to be run predominantly at full load and optimum conditions. The behaviour of plants, operating at low load and varying conditions, is getting more and more attention due to the introduction of variable renewable generation on the grid. Consequently, the need for a fully transient high-fidelity system based model has grown, as this will enable one to study the behaviour of plants under such non-ideal conditions. This report details the development of a feedwater heater, deaerator and turbine component for such a high-fidelity transient system model using the Flownex Simulation Environment, a onedimensional thermohydraulic network solver. The components have been modelled all with the aim of using minimal design input data. The feedwater heater component model includes transient effects and thermodynamic relations to represent aspects such as heater performance, level control and transient inertia. In determining the heat transfer characteristics, the model makes use of plant-performance data and correlates the amount of heat transfer by using the feedwater mass flow as the load indicating parameter. This approach eliminates the need for specific geometrical details to calculate the effective heat transfer area. The level control is modelled by using a level representation built from using heat exchanger design methods. The turbine component is modelled by using Fuls’ Semi-Ellipse law or the pressure drop modelling and Ray’s semi-empirical method for the efficiency modelling. The model also contains transient effects, which include thermal inertia due to the shaft and casing, and rotational inertia due to the shaft. The deaerator component is modelled by adapting the model presented by Banda, and modifying the model to work under various conditions. This involved using curve fit methods in Flownex to use input data to model the pressure drop over the main condensate valve. Each of the mentioned components was validated and verified with plant data and finally packaged into a compound component which is a component consisting of a subnetwork in Flownex. These compound components further contain design inputs which are easily accessible by the user. The component models were integrated into larger networks in which various scenarios can be run. A short transient scenario was run on the low-pressure feedwater train of a specific power station. The scenario involved a turbine trip where the bled steam valves for the heaters were closed suddenly. The speed of the valves closing was however unknown and after closing the valves in approximately 10 seconds, results agreed relatively well with plant data. This illustrated the short transient capabilities of the feedwater heater component model. The three component models (feedwater heater, turbine and deaerator) were finally integrated into a regenerative Rankine cycle and was set up using minimal design data. The boiler, condenser and condensate pump were set as boundary conditions in the network but all extraction points for the network were connected. Steady-state results were obtained for various load cases and the main temperature, flow and pressure results were compared. Results agree well with plant data, even at low load conditions

Manufacturers of feedwater heaters (FWHs) are obliged to disclose a specification sheet to the client that describes their FWH design. However, the client is unable to verify the performance of this FWH design without comparing it to the results that are predicted by a thermal model. An additional limitation is that the manufacturer will only disclose the minimum number of design parameters. The purpose of this study was to develop a thermal model that can predict the performance of a FWH. The model requires the minimum design input data to predict the performance parameters that may be compared to values predicted by the vendor. A FWH in a regenerative water-steam Rankine cycle achieves heat transfer to the feedwater by condensing steam on the shell side. This is called a single zone FWH. The tube plate type FWH is the most common type of FWH referenced in literature but the following variations may exist: • The Eskom fleet consist of both tube plate and header type FWHs. • FWHs may be orientated vertically or horizontally. Internal shrouded regions, that define it as a 2 or 3 zone FWH, may be present in the FWH. The length of the drains cooler (DC) zone may either be identified as long or short. A general model was required to capture all these design variations. Plant visits were arranged with engineers at several power stations to obtain the minimum input data and to confirm that these FWH design variations existed within the Eskom fleet. The model was based on existing tube plate models found in literature. It was then extended to accommodate the FWH variations mentioned above. A further improvement was made by including an additional heat transfer sub-zone that removes excess superheat in the condensing (COND) zone. The vendor does not disclose the correlations used to predict the film heat transfer coefficients (h) in their design. Therefore, the user is granted the option of selecting a correlation from a list of popular correlations, specific to a heat transfer mode. Note that the uncertainty associated with this thermal model is affected by the uncertainty of each correlation selected in the model.

The once through steam generator (OTSG) is a single pass counter flow heat exchanger in which primary pressurized water from the core is circulated. Main Feedwater is injected in an annular gap on the outer periphery of the steam generator shroud such that it aspirates steam to preheat the feedwater to saturation temperature. An important component of the OTSG and enhanced once through steam generator (EOTSG) is the auxiliary feedwater system (AFW), which is used during accident/transient scenarios to remove residual heat by injecting water through jets along the outer periphery of the heat exchanger core directly on to the tubes at the top of the OTSG. The intention is for the injected water, which is subcooled, to spread into the tube nest and wet as many tubes as possible. In this project, the main objectives were to use first principles Computational Fluid Dynamics to predict the number of wetted tubes versus flow rate in the EOTSG at the AFW injection location above the top tube support plate. To perform the fluid analysis, the losses in the bypass leakage flow and broached hole leakage flow were first quantified and then used to model a 1/8th sector of the EOTSG. Using user defined functions (UDF), the loss coefficients of the leakage flows were implemented on the 1/8th sector of the EOTSG computational model to provide boundary conditions at the bypass flow and leakage flow locations With this method, the number of tubes wetted in the sector of EOTSG for various AFW flow rates was found. Results showed that the number of wetted tubes was in very close agreement to that predicted by experimental-analytical methods by the sponsor, AREVA. With the maximum flow rate of 65 l/s a total of 318 tubes were wetted and the percentage of tubes wetted with broached holes was 8.7%. The analysis on the bypass leakage flow showed that the loss coefficients was a function of the mass flow rate or the flow Reynolds number through the gap and it increased as the Reynolds number increased from 300 to 1600. The experimental and computational loss coefficients agree to within 15% of each other. In contrast, the constant loss coefficient of 1.3 used by AREVA was much higher than that obtained in this study, particularly in the low Reynolds number range. As the Reynolds number approached 3000, the loss coefficients from this study approached the value of 1.3. This value of the loss coefficient was implemented for the bypass flow leakage in the 1/8th sector of the EOTSG model. The analysis on the broached hole leakage flow was performed using a single hole, five holes, and one, two, four and eight rows of broached holes in order to characterize the loss coefficients. The one hole and five hole computational models were validated with experiments. The computational models showed the presence of voids in the leakage flow through the tube support plate (TSP), which were not observed (visually) in the experiments. The characterization of the broached hole leakage in the one, two and four rows showed that the loss coefficient of the control broached hole increased as the number of rows increased. These results indicated that for the same height of water on the TSP, the resistance to leakage flow increased as the number of tubes increased. They also indicated that leakage flow through the broached holes was not solely a function of the height of water above the TSP but also the surrounding geometrical topology and the flow characteristics. However, the analysis done for eight rows showed that the loss coefficient became constant after a certain number of rows as the loss coefficient differed by only 5% from the results of the four rows. From these results it was determined that the loss coefficient asymptotes to an estimated value of 4.0 which was implemented in the broached hole leakage flow in the 1/8th sector of the EOTSG. Computational models of the 1/8th sector of the EOTSG were implemented with the respective loss coefficients for the bypass and leakage flows. Results showed that as the AFW flow rate increased, the percentage wetted tubes increased. The data matched closely with AREVA's experimental-analytical model for flow rates of 14.5 l/s and higher. It was also deduced that complete wetting of the tubes is not possible at the maximum AFW flow rate of 65 l/s. / Master of Science

Conventional concentrating solar power (CSP) plants typically have a very high levelised cost of electricity (LCOE) compared with coal-fired power stations. To generate 1 kWh of electrical energy from a conventional linear Fresnel CSP plant without a storage application, costs the utility approximately R3,08 (Salvatore, 2014), whereas it costs R0,711 to generate the same amount of energy by means of a highly efficient supercritical coal-fired power station, taking carbon tax into consideration. This high LCOE associated with linear Fresnel CSP technology is primarily due to the massive capital investment required per kW installed to construct such a plant along with the relatively low-capacity factors, because of the uncontrollable solar irradiation. It is expected that the LCOE of a hybrid plant in which a concentrating solar thermal (CST) station is integrated with a large-scale supercritical coal-fired power station, will be higher than that of a conventional supercritical coal-fired power station, but much less than that of a conventional CSP plant. The main aim of this study is to calculate and then compare the LCOE of a conventional supercritical coal-fired power station with that of such a station integrated with a linear Fresnel CST field. When the thermal energy generated in the receiver of a CST plant is converted into electrical energy by using the highly efficient regenerative Rankine cycle of a large-scale coal-fired power station, the total capital cost of the solar side of the integrated system will be reduced significantly, compared with the two stations operating independently of one another for common steam turbines, electrical generators and transformers, and transmission lines will be utilised for the integrated plants. The results obtained from the thermodynamic models indicate that if an additional heat exchanger integration option for a 90 MW (peak thermal) fuel-saver solar-augmentation scenario, where an annual average direct normal irradiation limit of 2 141 kWh/m2 is considered, one can expect to produce approximately 4,6 GWh more electricity to the national grid annually than with a normal coal-fired station. This increase in net electricity output is mainly due to the compounded lowered auxiliary power consumption during high solar-irradiation conditions. It is also found that the total annual thermal energy input required from burning pulverised coal is reduced by 110,5 GWh, when approximately 176,5 GWh of solar energy is injected into the coal-fired power station’s regenerative Rankine cycle for the duration of a year. Of the total thermal energy supplied by the solar field, approximately 54,6 GWh is eventually converted into electrical energy. Approximately 22 kT less coal will be required, which will result in 38,7 kT less CO2 emissions and about 7,6 kT less ash production. This electricity generated from the thermal energy supplied by the solar field will produce approximately R8,188m in additional revenue annually from the trade of renewable energy certificates, while the reduced coal consumption will result in an annual fuel saving of about R6,189m. By emitting less CO2 into the atmosphere, the annual carbon tax bill will be reduced by R1,856m, and by supplying additional energy to the national grid, an additional income of approximately R3,037m will be due to the power station. The annual operating and maintenance cost increase resulting from the additional 171 000 m2 solar field, will be in the region of R9,71m. The cost of generating 1 kWh with the solar-augmented coal-fired power plant will only be 0,34 cents more expensive at R0,714/kWh than it would be to generate the same energy with a normal supercritical coal-fired power station. If one considers that a typical conventional linear Fresnel CSP plant (without storage) has an LCOE of R3,08, the conclusion can be drawn that it is much more attractive to generate electricity from thermal power supplied by a solar field, by utilising the highly efficient large-scale components of a supercritical coal-fired power station, rather than to generate electricity from a conventional linear Fresnel CSP plant. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Conventional concentrating solar power (CSP) plants typically have a very high levelised cost of electricity (LCOE) compared with coal-fired power stations. To generate 1 kWh of electrical energy from a conventional linear Fresnel CSP plant without a storage application, costs the utility approximately R3,08 (Salvatore, 2014), whereas it costs R0,711 to generate the same amount of energy by means of a highly efficient supercritical coal-fired power station, taking carbon tax into consideration. This high LCOE associated with linear Fresnel CSP technology is primarily due to the massive capital investment required per kW installed to construct such a plant along with the relatively low-capacity factors, because of the uncontrollable solar irradiation. It is expected that the LCOE of a hybrid plant in which a concentrating solar thermal (CST) station is integrated with a large-scale supercritical coal-fired power station, will be higher than that of a conventional supercritical coal-fired power station, but much less than that of a conventional CSP plant. The main aim of this study is to calculate and then compare the LCOE of a conventional supercritical coal-fired power station with that of such a station integrated with a linear Fresnel CST field. When the thermal energy generated in the receiver of a CST plant is converted into electrical energy by using the highly efficient regenerative Rankine cycle of a large-scale coal-fired power station, the total capital cost of the solar side of the integrated system will be reduced significantly, compared with the two stations operating independently of one another for common steam turbines, electrical generators and transformers, and transmission lines will be utilised for the integrated plants. The results obtained from the thermodynamic models indicate that if an additional heat exchanger integration option for a 90 MW (peak thermal) fuel-saver solar-augmentation scenario, where an annual average direct normal irradiation limit of 2 141 kWh/m2 is considered, one can expect to produce approximately 4,6 GWh more electricity to the national grid annually than with a normal coal-fired station. This increase in net electricity output is mainly due to the compounded lowered auxiliary power consumption during high solar-irradiation conditions. It is also found that the total annual thermal energy input required from burning pulverised coal is reduced by 110,5 GWh, when approximately 176,5 GWh of solar energy is injected into the coal-fired power station’s regenerative Rankine cycle for the duration of a year. Of the total thermal energy supplied by the solar field, approximately 54,6 GWh is eventually converted into electrical energy. Approximately 22 kT less coal will be required, which will result in 38,7 kT less CO2 emissions and about 7,6 kT less ash production. This electricity generated from the thermal energy supplied by the solar field will produce approximately R8,188m in additional revenue annually from the trade of renewable energy certificates, while the reduced coal consumption will result in an annual fuel saving of about R6,189m. By emitting less CO2 into the atmosphere, the annual carbon tax bill will be reduced by R1,856m, and by supplying additional energy to the national grid, an additional income of approximately R3,037m will be due to the power station. The annual operating and maintenance cost increase resulting from the additional 171 000 m2 solar field, will be in the region of R9,71m. The cost of generating 1 kWh with the solar-augmented coal-fired power plant will only be 0,34 cents more expensive at R0,714/kWh than it would be to generate the same energy with a normal supercritical coal-fired power station. If one considers that a typical conventional linear Fresnel CSP plant (without storage) has an LCOE of R3,08, the conclusion can be drawn that it is much more attractive to generate electricity from thermal power supplied by a solar field, by utilising the highly efficient large-scale components of a supercritical coal-fired power station, rather than to generate electricity from a conventional linear Fresnel CSP plant. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

<p>Analyses of nuclear reactor safety have increasingly required coupling of full three dimensional neutron kinetics (NK) core models with system transient thermal-hydraulics (TH) codes. To produce results within a reasonable computing time, the coupled codes use different spatial description of the reactor core. The TH code uses few, typically 5 to 20 TH channels, which represent the core. The NK code uses explicit node for each fuel assembly. Therefore, a spatial mapping of coarse grid TH and fine grid NK domain is necessary. However, improper mappings may result in loss of valuable information, thus causing inaccurate prediction of safety parameters.</p><p>The purpose of this thesis is to study the sensitivity of spatial coupling (channel refinement and spatial mapping) and develop recommendations for NK-TH mapping in simulation of safety transients – Control Rod Drop, Turbine Trip, Feedwater Transient combined with stability performance (minimum pump speed of recirculation pumps).</p><p>The research methodology consists of spatial coupling convergence study, as increasing number of TH channels and different mapping approach the reference case. The reference case consists of one TH channel per one fuel assembly. The comparison of results has been done under steady-state and transient conditions. Obtained results and conclusions are presented in this licentiate thesis.</p>

With the ever advancing state of computer systems, it is imperative to maintain the most up-to-date and reliable safety evaluation data for nuclear power systems. Commonplace now is the practice of updating old accident simulation results with more advanced models and codes using today's faster computer systems. Though it may be quite an undertaking, the benefits of using a more advanced model and code can be significant especially if the result of the new analysis provides increased safety margin for any plant component or system. A series of parametric and sensitivity studies for the Loss of Normal Feedwater Anticipated Transient without Scram (LONF ATWS) for Southern Company's Vogtle Electric Generating Plant (VEGP) Units 1&2 located near Waynesboro, GA was performed using the best-estimate thermal-hydraulics transient analysis code RETRAN-02w. This thesis includes comparison to the results of a generic plant study published by Westinghouse Electric Corporation in 1974 using an earlier code, LOFTRAN, as well as Vogtle-specific analysis. The comparative analysis exposes and seeks to explain differences between the two codes whereas the Vogtle analysis utilizes data from the Vogtle FSAR to generate plant-specific data. The purpose of this study is to validate and update the previous analysis and gather more information about the plant actions taken in response to a LONF ATWS. As a result, now there is a new and updated evaluation of the LONF ATWS for both a generic 4-loop Westinghouse plant and VEGP using a more advanced code. Beyond the reference case analysis, a series of sensitivity and parametric studies have been performed to show how well each type of plant is designed for handling an ATWS situation. These studies cover a wide range of operating conditions to demonstrate the dependability of the model. It was found that both the generic 4-loop Westinghouse PWR system and VEGP can successfully mitigate a LONF ATWS throughout the core's operating cycle.

Analyses of nuclear reactor safety have increasingly required coupling of full three dimensional neutron kinetics (NK) core models with system transient thermal-hydraulics (TH) codes. To produce results within a reasonable computing time, the coupled codes use different spatial description of the reactor core. The TH code uses few, typically 5 to 20 TH channels, which represent the core. The NK code uses explicit node for each fuel assembly. Therefore, a spatial mapping of coarse grid TH and fine grid NK domain is necessary. However, improper mappings may result in loss of valuable information, thus causing inaccurate prediction of safety parameters. The purpose of this thesis is to study the sensitivity of spatial coupling (channel refinement and spatial mapping) and develop recommendations for NK-TH mapping in simulation of safety transients – Control Rod Drop, Turbine Trip, Feedwater Transient combined with stability performance (minimum pump speed of recirculation pumps). The research methodology consists of spatial coupling convergence study, as increasing number of TH channels and different mapping approach the reference case. The reference case consists of one TH channel per one fuel assembly. The comparison of results has been done under steady-state and transient conditions. Obtained results and conclusions are presented in this licentiate thesis.