Off-design Operating Conditions Research Articles

Effects of propeller cavitation on ship propulsion performance in off-design operating conditions

The propeller cavitation is closely related with its hydrodynamic performance. When the propeller works in the cavitating flow, the propeller hydrodynamic performance, in terms of thrust and torque coefficients, is reduced due to the propeller cavitation. When the high-speed and high-power ship sails in adverse sea, the varying loads on propeller will cause severer cavitation on propeller. The reduction in propulsion performance caused by cavitation will further affect the propulsion system, impacting the ship sailing performance and safety. The VP1304 propeller cavitation CFD simulation are conducted to validate the CFD method. Furthermore, the influence of propeller advance ratio and cavitation numbers on the performance of low-speed high-power series propeller in cavitating flow are studied and illustrated. This influence of cavitation has also been transferred to the propulsion system and this process are calculated by the ship propulsion system numerical model. Additionally, the sailing performance of ship propulsion system under off-design conditions, where the cavitation is more likely to occur, has been investigated in this article. Under severe heavy load conditions, it can even result in a complete loss of propulsion capability, posing significant risks. Therefore, it is necessary to further optimize control strategies for the propulsion system to mitigate the hazards caused by cavitation.

Impact of extracorporeal blood pump gap sizes on the performance and hemocompatibility under off-design operation.

Hemocompatibility remains the dominant challenge in rotary blood pumps, and more information on the relationship between individual pump design features, hemodynamics, and blood trauma in various operation conditions is necessary. The study evaluated the variation of gap sizes in extracorporeal blood pumps concerning their influence on blood compatibility, particularly during off-design conditions. We developed a parametric generic blood pump framework for in-silico and in-vitro design feature analysis. Thirty-six designs with varying axial and radial gap sizes between 0.5 mm and 3 mm were generated. CFD was applied to calculate and compare device hemodynamics and evaluate the performance and hemocompatibility during off-design and target operation conditions. The following quantities were analyzed: pressure difference, hemolysis potential, residence times, hydraulic efficiency, and recirculation ratio. The in-vitro prototype showed excellent agreement with in-silico predictions regarding hydraulic performance (R2 = 0.996 with a RMSE = 2.07). Our results show a modest impact of gap size variations ±10% on key metrics. Domain-resolved analyses revealed a significant contribution of the gap regions to the device's overall hemolytic performance, with an increasing contribution for off-design flow rates. Overall elevated hemolysis levels were identified if at least one gap size was held minimal. We introduced and showed the feasibility of a parametric rotary blood pump framework to systematically investigate design feature impact. Results suggest, larger and uniformly sized gaps being overall beneficial regarding hemocompatibility.

Model-based optimisation of solar-assisted ORC-based power unit for domestic micro-cogeneration

Integrating flat solar thermal collectors and organic Rankine cycle (ORC)-based power units in micro-cogeneration systems ensures a reduction in CO2 emissions in domestic applications. The key component of these systems is the expander, which must withstand frequent off-design operating conditions owing to the intermittent nature of the solar source. Despite being in the first stage of technological development, scroll expanders are widely adopted in small-scale applications owing to their operating flexibility and robustness. In this study, a domestic micro-cogeneration unit equipped with a scroll expander is characterised experimentally. The experimental data are used to calibrate the expander and ORC unit models. The models are used to evaluate the operating limits of the units as functions of the main operating parameters. The maximum power and efficiency are obtained as 300 W and 3 %, respectively, for hot water temperatures between 90 °C and 100 °C, close to the rated performances. Finally, the effect of adopting a dual-intake port on the expander and unit performance is assessed. The technology facilitates the widening of the operating range (20–140 g/s mass flow rate of WF) and a peak power production of 1.2 kW.

Thermodynamic analysis of two very-high-temperature gas-cooled reactor-driven hydrogen and electricity cogeneration systems under off-design operating conditions

This study conducts a thermodynamic analysis for two very-high-temperature gas-cooled reactor-driven hydrogen and electricity cogeneration systems integrating an iodine-sulfur cycle and a helium turbine direct cycle under off-design operating conditions. Control strategies are proposed based on inventory control. The calculation model is established for the main components of the system under off-design conditions. Two operation modes are proposed, one is to maintain the split ratio at the reactor outlet at the rated value, and the other is to maintain the hydrogen generation rate at the rated value. The off-design performance of the two systems under partial load in two modes is analyzed. For Systems 1 and 2, the suggested lower limits of reactor thermal power are 68.80 % and 69.00 % of the rated value in mode 1, and 72.04 % and 64.98 % in mode 2. As the reactor thermal power decreases from the rated value to the suggested lower limit, in modes 1 and 2, the hydrogen-electricity efficiency of System 1 decreases from 37.46 % to 36.20 % and 31.81 %, and that of System 2 decreases from 42.40 % to 41.45 % and 35.21 %. The decrease in overall efficiency is more significant in mode 2.

A case study of thermal performance of gas-steam combined cycle with gas turbine inlet air cooling and condenser deep cooling

To enhance power generation during high summer temperatures and address the power supply and demand imbalance in gas-steam combined cycle, this study explored the exhaust gas waste heat refrigeration scheme, along with the optimal scheduling of cooling capacity between the precooling of inlet air for the gas turbines and deep cooling for condenser in steam cycle. Using the Ebsilon software, we investigate the thermal performance of the gas cycle, steam cycle, and the overall gas-steam combined cycle under various off-design operating conditions, including varying cooling capacity distributions, ambient temperatures, and generation loads. Based on our calculations, it is advisable to prioritize cooling capacity for precooling the ambient air for the gas cycle, balancing power generation and thermal efficiency. After the transformation, the power generation of the gas-steam combined cycle remains relatively stable across varying ambient temperatures, achieving a significant 10.9 % increase compared to pre-transformation levels under a 39 °C ambient temperature and 100 % generation load. Furthermore, the thermal efficiency of the gas-steam combined cycle improves by 1.11 % under a 40 % generation load condition. As ambient temperatures rise, the benefits of the inlet air cooling scheme become more apparent, validating its effectiveness in addressing the inherent summer power supply and demand challenges.

Assessment of Fiber Bragg Grating sensors for monitoring induced strains on the draft tube cone of hydraulic turbines

Abstract In hydraulic turbines, several flow instabilities can take place inside the draft tube cone during off-design and transient operating conditions such as the rotating vortex rope which can severely damage the structure if sustained in time. In the frame of the AFC4Hydro H2020 research project, an extensive measurement campaign has been carried out to monitor and predict this rotating vortex rope phenomenon in a reduced scale Kaplan turbine model at the Vattenfall Research and Development facility in Älvkarleby, Sweden. The hydraulic turbine model has been operated in propeller mode with a fixed blade angle corresponding to its best efficiency point. Several sensors have been placed along the test stand to monitor vibrations, strains and pressures. Concretely, the present paper assesses the performance of using Fiber Bragg Grating sensors to measure the strains induced on the draft tube cone walls with high-spatial resolution in three different zones of influence: the upper and lower flanges and the vertical cone wall between the runner outlet and the elbow. To do so, a total of 3 arrays embedding a total of 48 Fiber Bragg Grating sensors were glued inside three grooves previously machined on these particular areas of the draft tube cone. Analysing the frequency response of the different Fiber Bragg Grating sensors, the strain patterns induced by the rotating and plunging components of the rotating vortex rope have been precisely determined. Moreover, their impacts at the different part load conditions tested have also been quantified.

Design and Performance Assessment of District Heating Systems in Latvian Region

The energy consumed by buildings for air conditioning accounts for a large percentage of global energy consumption. To promote efficiency and sustainability, the scientific community is making great efforts to develop renewable technologies. A well-known but unfortunately underestimated solution is the development of centralized heating and cooling systems that consistently reduce energy consumption.The work proposes a comparison between three heating configurations covering the demand of a settlement in the Latvian region: 1) centralized district heating (DH) system; 2) 5th generation district heating & cooling (5GDHC) system and 3) individual home heating (HH) systems. Thermal and electrical loads are evaluated by transient simulations of a residential area with 80 buildings for the Riga climate and compared with the same settlement in a Mediterranean region (Milan, IT). The energy plants are based on different technologies: combined heat and power (CHP) plants, gas-fired boilers, and domestic heat pumps.The analysis includes the option of power exchange with the national grid. A transient numerical model has been developed for each solution. Every component is modelled according to performance maps provided by the manufacturers, allowing an accurate simulation in both design and off-design operating conditions. The study includes energy, economic and environmental aspects.The result of the simulation highlights the large difference between the two locations, not only in terms of annual load, but also in terms of load distribution. On an annual basis, the Latvian residential complex requires almost twice as much energy as the Italian one. The thermal losses in the district systems are 4.21% in Milan solution and 5.65% in Riga. The district heating system coupled with heat pump represents the best layout in terms of primary energy consumption in both locations, with energy savings of 50% compared to other solutions. The use of 5GDHC is a good compromise that could increase the use of renewable energy. The adoption of cogeneration plant is a good choice in case of centralized district system that allows the installation of high efficiency genset. On the contrary, for small application as residential, the installation of cogeneration system results expensive and the conversion efficiency does not justify the installation.

Performance investigation of solar ejector cooling system: Case study area-Addis Ababa, Ethiopia

This study investigates the performance of a solar ejector cooling system under both on-design and off-design operating conditions, focusing on the case study area of Addis Ababa, Ethiopia. Data crucial to the analysis were gathered from the Ethiopian National Meteorology Agency (ENMA) over six consecutive years (2013-2018). The system's performance was evaluated through hourly and monthly analyses under on-design conditions, yielding promising results. Under on-design conditions, the system demonstrated favorable performance metrics. Specifically, at noontime in March, when the design points were set at Tg = 90°C, Tc = 30°C, and Te= 12°C, the system achieved a total hourly maximum coefficient of performance (COP) of 0.2049 and a cooling capacity of 186.9 W/m². Furthermore, the study determined that a collector area of 22.3 m² per ton of cooling capacity is required during noontime for all cooling seasons in Addis Ababa, Ethiopia. Additionally, an off-design operating condition map was developed for the system. The findings indicate that the solar ejector cooling system can effectively provide cooling during office hours (8:00-15:00) in Addis Ababa, Ethiopia.

Investigation of Transonic Flow Features in an Axial Flow Compressor Rotor

Numerical investigation of an axial flow rotor is carried out for its performance characterization and aerodynamic behavior during the design and off-design operating conditions. The study focuses on capturing the transonic flow features from the choke point (CP) to the near stall (NS) point in the rotor. This includes the analysis of passage shock structure, its movement in the blade passage with varying back-pressure, shock boundary layer interaction, tip leakage flow structure, and resulting losses. The study is carried out from 60 % to 100 % of the design speed using steady and unsteady RANS simulations. Three turbulence models, namely; SST, k-ε, and Reynolds stress models, are employed. The SST model predicted the closest approximation to the experimental data. The rotor aerodynamic performance is predicted in terms of total pressure ratio, efficiency, and flow contours. Unsteady analysis revealed that the primary and secondary tip leakage vortices, combined with the suction side tip corner separation, are the major instabilities near the stall region.

Numerical analysis of flow instability with vortex cavitation in venturi tube

The swirling flow with cavitation can cause pressure pulsations in a draft tube of a hydro turbine at off-design operating conditions. Especially at the full load condition, the cavitation volume may fluctuate greatly, causing pressure pulsations with flow rate fluctuations known as cavitation surge throughout the hydraulic system. The cases that predicted the phenomenon qualitatively using the CFD analysis have been reported previously, whereas the methods to predict the cavitation volume and the fluctuation frequency in the cavitation surge condition quantitatively have not yet been established. Therefore, in this study, unsteady CFD analysis was conducted for the experimental apparatus that simulates the flow in the draft tube downstream of a runner using the stationary guide vane and the venturi tube. In CFD analysis, the influence of the turbulence models by using LES and SST and the influence of the number of meshes in LES simulation were investigated. It was founded that LES simulation captured the vortex generation finely and improved the prediction accuracy of the cavitation volume than SST simulation. On the other hand, it was founded that too fine mesh has a small effect on the prediction accuracy of the cavitation volume, although the resolution of the vortex is improved.

Formation and evolution of vortex breakdown consequent to post design flow increase in a Francis turbine

Draft tube flow instability encountered under off-design operating conditions in hydraulic turbines significantly limits their operational flexibility. The instability arises consequent to a higher than threshold swirl content in the runner outflow and leads to vortex breakdown phenomenon in the draft tube cone. At high load condition, the phenomenon presents as an enlarged vortex core counter-rotating with respect to the runner. The flow situation is known to compromise the turbine efficiency besides the generation of unwanted effects such as power swings and large-scale pressure fluctuations. The present paper is the first to encapsulate a thorough numerical investigation on the formation and evolution of the enlarged vortex core alongside the consequent effects. A transient operating sequence between best efficiency and high load operating points in a model Francis turbine is simulated. Turbulence closure has been attained using the shear stress transport-scale adaptive simulations turbulence model. Dynamic meshing based on a Laplacian smoothing scheme has been used to account for mesh deformation arising from guide vane motion during load change. The pressure and velocity fields have been determined and analyzed to elucidate the physics of vortex breakdown, the phenomenon underlying the formation of the enlarged vortex core. Furthermore, pressure fluctuations at salient points in the domain have been analyzed using Fourier and short-time Fourier transforms. Finally, the enlarged vortex core formed in the draft tube has been visualized through the λ2 criterion. The core takes the shape of a cork-screw like compactly wound spiral structure extending up to the draft tube elbow.

Design and Performance Comparison of District Heating Systems in Milan and Riga

Abstract The work proposes a comparison between three heating configurations covering the demand of a new settlement: 1) centralized district heating system (DHC); 2) 5th generation district heating & cooling system (5GDHC) and 3) individual home heating and cooling (HHC) systems. Thermal and electrical loads are evaluated by transient simulations of a residential area with 80 buildings. The energy plants are based on different technologies: combined heat and power plants, gas-fired boilers, and domestic heat pumps. A transient numerical model has been developed for each solution. Every component is modelled according to performance maps provided by the manufacturers, allowing an accurate simulation in both design and off-design operating conditions. On an annual basis, the Latvian residential complex requires almost twice as much energy as the Milan one. The thermal losses in the district systems are 4.21 % in the Milan solution and 5.65 % in Riga. The district heating system coupled with a heat pump represents the best layout in terms of primary energy consumption in both locations, with energy savings of 50 % compared to other solutions. The use of 5GDHC is a good compromise that could increase the use of renewable energy. The adoption of a cogeneration plant is a good choice in the case of a centralized district system that allows the installation of high-efficiency genset. On the contrary, for small applications such as residential, the cogeneration results are expensive, and the conversion efficiency does not justify the installation.

Techno-economic analysis of a new thermal storage operation strategy for a solar aided liquid air energy storage system

The SA-LAES system is an economical and efficient energy storage solution that combines solar energy with LAES. However, the inherent instability of solar energy can result in uncertainty regarding heat storage, thereby compromising the stability of the energy storage system. The development of more economical, efficient, and stable strategies for the SA-LAES system represents a challenging task. The present study puts forward a novel thermal storage operation scheme (Strategy 2) for SA-LAES systems integrated with electric heating during valley periods, thereby achieving long-term stable operation of the energy storage system via the alternating utilization of two full molten salt tanks and one empty molten salt tank. Theoretical and case studies are conducted based on thermodynamic and economical technical indicators, and detailed comparisons are made with the traditional strategy (Strategy 1) of only changing the solar multiple and capacity. The results show the exergy destruction of Strategy 1 mainly occurs in air turbines under off-design operating conditions. The exergy destruction of Strategy 2 is mainly in the electric heater part. The single-day analysis found that the new strategy has a more significant advantage in terms of exergy efficiency than Strategy 1 when the direct normal irradiance (DNI) is below 650 W/m2. In the case study, the levelized cost of energy (LCOE) values for Nanning, Beijing, and Hami regions in Strategy 2 are 0.118 $/kWh, 0.077 $/kWh, and 0.062 $/kWh, respectively. The LCOE values of Strategy 2 are decreased by 10.97 %, 27.49 %, and 3.40 % compared to Strategy 1, respectively. The payback periods for investments in the Beijing and Hami regions have decreased by 67.63 % and 16.41 %, respectively. The research presented in this article provides an economical, efficient, and stable operating strategy for thermal storage in the SA-LAES system.

Power generation gas turbine performance enhancement in hot ambient temperature conditions through axial compressor design optimization

The output power and efficiency of gas turbines are profoundly influenced by ambient temperatures, with higher temperatures resulting in a notable decline in power output. This issue becomes particularly critical during periods of heightened power demand in hot climates. There are two common ways to solve this problem: incorporating auxiliary systems, such as inlet air cooling, or undertaking a comprehensive redesign of the gas turbine core engine. However, these methods sometimes are costly and unfeasible, particularly in regions characterized by arid conditions or constrained budgets for engine modifications. An alternative strategy has been developed to optimize axial compressor performance in order to improve gas turbine power in high-temperature environments, through an innovative approach. This approach centers on the concept of “re-staggering” the stator blades as a retrofit upgrade for existing gas turbines, enabling an increase in compressor inlet mass flow rate while maintaining peak efficiency at 45 °C. An in-house optimization tool was developed, incorporating a genetic algorithm and artificial neural network, to ascertain the optimal configuration of the re-stagger angles. This optimization process was initially coupled with a mean-line solver and subsequently refined using a through-flow model to enhance the preliminary design. The refined compressor design was then subjected to a comprehensive 3D Computational Fluid Dynamics analysis. Additionally, “end-bend” and “elliptical leading edge” techniques were employed to sustain compressor efficiency and ensure stability across various design and off-design operating conditions. Following a thorough structural analysis, the newly designed compressors were manufactured and tested at two distinct sites in Iran (Yazd and Bampur). The measured data demonstrated the commendable performance and stability of the redesigned compressors, aligning closely with numerical predictions. Notably, the results showcased a substantial increase in gas turbine power output ranging from 7.0% to 8.0% at hot ambient conditions, all while preserving efficiency and stability. Encouragingly, the upgraded compressor also exhibited superior performance even at lower temperatures, such as 15 °C. Consequently, this uprating concept presents a practical and cost-effective solution for power plant operators seeking a significant boost in power output, especially during hot weather conditions or in arid regions prone to drought.

An Overview of Active Control Techniques for Vortex Rope Mitigation in Hydraulic Turbines

This review addresses the current state of research into active control and suppression of vortex rope in hydroturbines under off-design operating conditions. Only active control methods that can be “switched on” when required under off-design operating conditions are considered in this work. The review focuses on air addition into the flow, as well as various auxiliary fluid jets. It includes all the best practices for vortex rope suppression in numerical and experimental studies. It can be inferred from the review that a modern flow control system should be comprehensive, designed for a specific hydroturbine geometry, and obtain feedback from the flow. Injecting ~2% of air from the impeller fairing cone appears optimal for suppressing pressure pulsations without significant efficiency loss. The cost of air injection is rarely estimated, but the use of an automatic venting system can minimize overheads and potentially improve efficiencies at low gas contents. Fluid jets ranging from 3% to 12% of the main flow rate can efficiently suppress pressure pulsations, but their high energy requirements limit their use. Azimuthal perturbation of the flow appears promising as it does not require significant energy loss, but practical implementation remains challenging as one needs to accurately know the system dynamics and be capable of real-time manipulation of the flow.

Off-design condition optimization of organic Rankine cycle based on genetic algorithm

Organic Rankine cycle (ORC) has been considered as one of the most promising technologies in industrial waste heat utilization and power generation. During the actual operation of ORC system, due to the fluctuation of cooling and heat sources, the system operates under off-design conditions in most cases. In this paper, thermodynamic model, heat transfer process description and power equipment model are established to evaluate the operating parameters of ORC for the off-design conditions. Evaporation temperature and condensation temperature are taken as independent parameters for the operation of ORC system. Genetic algorithm is adopted to optimize the independent parameters under the maximum net output power. The results show that the effect of optimizing independent parameters is to make the working fluid at the outlet of the preheater as close as possible to a saturated liquid state, and the working fluid at the inlet of the screw expander should be in a saturated gas state. With the optimal power output increasing by 19.1% for every 5 °C increase in hot water inlet temperature, 9.2% for every 20 kg/s increase in hot water mass flow rate, and 3.9% for every 1 °C decrease in cooling water temperature. The optimization method of off-design operating conditions has good system performance and good engineering application prospects.

Comparison of optimal power production and operation of unmoored floating offshore wind turbines and energy ships

Abstract. As the need to transition from global reliance on fossil fuels grows, solutions for producing green alternative fuels are necessary. These fuels will be especially important for hard-to-decarbonize sectors such as shipping. Mobile offshore wind energy systems (MOWESs) have been proposed as one such solution. These systems aim to harness the far-offshore wind resource, which is abundant and yet untapped because of installation and grid-connection limitations. Two classes of MOWES have been proposed in the literature: unmoored floating offshore wind turbines (UFOWTs) and energy ships (ESs). Both systems operate as autonomous power-to-X (PtX) plants, powered entirely by wind energy, and so can be used to produce synthetic green fuels such as hydrogen or ammonia, or for other energy-intensive applications such as direct air carbon capture. The two technologies differ in form; UFOWTs are based on a conventional FOWT but include propellers in place of mooring lines for course keeping, while ESs operate like a sailing ship and generate power via hydro-turbines mounted on the underside of the hull. Though much research and development is necessary for these systems to be feasible, the promise of harnessing strong winds far offshore, as well as the potential to avoid siting regulatory challenges, is enticing. This paper develops models of each MOWES concept to compare their power production on a consistent basis. The performance of the technologies is examined at steady-state operating points across relative wind speeds and angles. An optimization scheme is used to determine the values of the control variables which define the operating point for each set of environmental conditions. Results for each model show good agreement with published results for both UFOWTs and ESs. Model results suggest that UFOWTs can generate more power than ESs under ideal environmental conditions but are very sensitive to off-design operating conditions. In above-rated wind speeds, the UFOWT is able to produce as much power as a conventional, moored FOWT, whereas the ES cannot, since some power is always consumed to spin the Flettner rotors. The models developed here and their results may both be useful in future works that focus on the routing of UFOWTs or holistically designing a mobile UFOWT. Although differences in the performance of the systems have been identified, more work is necessary to discern which is a more viable producer of green electrofuels (e-fuels).

Investigations on ORC radial inflow turbine three-dimensional geometry design and off-design performance prediction

Radial inflow turbine is a critical component in the ORC system, which not only influences the system's thermodynamic performance directly but also dominates the system investment cost. Besides, the ORC system is usually operated under off-design operating conditions. Therefore, the reasonable design of the radial inflow turbine and the effective prediction of turbine off-design performance are very important for the ORC system. So based on the authors' previous studies, approaches to designing the turbine's three-dimensional geometry and to predicting the turbine's off-design performance have been discussed in this paper. At first, the parametric modeling for the turbine rotor has been established and the influence of geometric control parameters on turbine performance has been investigated, then the reasonable three-dimensional geometry design of the turbine has been determined. After that, based on the suitable turbine geometry, the off-design performance of the turbine has been investigated. Through the polynomial fitting method, new turbine performance prediction models have been constructed. And the effectiveness of these models has been validated under different organic fluid conditions. This paper presents new perspectives on ORC turbine three-dimensional geometry design and off-design performance prediction, which aims to promote its standardization.

Investigation of a Novel Turbine Housing to Produce a Nonuniform Spanwise Flow Field at the Inlet to a Mixed Flow Turbine and Provide Variable Geometry Capabilities

Abstract Automotive engine downsizing has placed an increased focus on the ability of the turbocharger to provide adequate boost levels across the full engine operating range. To achieve the desired levels of turbocharger performance, the turbine must be capable of operating effectively at the intended design point and also at off-design conditions. Mixed flow turbines (MFTs) provide a potential method to improve performance at off-design conditions and during transient engine operation. A unique feature of an MFT is the spanwise variation of incidence angle at the rotor leading edge. This results in additional flow separation from the blade suction surface near the hub under a wide range of operating conditions. The flow separation generates additional loss and has a detrimental impact on turbine performance. A novel design of a turbine volute similar to a conventional twin-entry turbine volute was examined. The novel turbine volutes were designed to produce a spanwise variation in flow conditions at the rotor inlet. The primary objective was to reduce the incidence angle and increase the mass flowrate at the hub side of the passage relative to the shroud side, as it has previously been identified that this can be beneficial for MFT performance. A number of different volute geometries were examined by numerical analysis to determine the impact of key parameters on turbine performance. The results indicated that generating a suitable spanwise flow distribution could produce a moderate improvement in turbine efficiency at off-design operating conditions. The novel volute design also provided a means of achieving a degree of variable geometry operation to further improve off-design performance. Turbine performance was examined under the variable geometry operation and an improvement in turbine power output at low-speed, off-design conditions was achieved. This was analogous to operating with a conventional pivoting vane variable geometry system and had the potential to benefit performance during transient engine operation.

Improve Ship Propeller Efficiency via Optimum Design of Propeller Boss Cap Fins

This paper aims to cost-effectively improve the energy efficiency of large vessels in shipping by the optimum design of propeller boss cap fins (PBCFs). First, a model propeller of the modern four-blade propeller in a Ro-Ro ship, with no boss cap fin in its original design, is experimentally and numerically investigated. The computational fluid dynamics (CFD) model reproduced all the experiments very well. Then, the CFD model is used to conduct a comprehensive optimum design of PBCFs for the down-scaled propeller. Besides the commonly used rectangular PBCFs, nine airfoils are investigated, due to their favorable lift-to-drag ratio and great potential of being effective PBCFs. The best performing profile, among the 10 shapes, is chosen as the PBCF for further optimization. Finally, the optimum design of the PBCFs for the propeller/rudder system is achieved. It was found to yield remarkable efficiency gains for the modern propeller/rudder system under both design and off-design operation conditions, mainly due to the suppressed hub vortex and partly due to the extra thrust. The yield strength analysis confirmed that the optimum design is feasible in practice and can be used in industrial vessels. The generalized criteria for the optimum design of PBCFs also benefit other propeller/rudder systems for cost-effective energy saving.