Search Results (111)

The transient operation of pump turbines generates significant flow-induced instabilities, prompting a comprehensive numerical investigation using the SST $k-\omega$ turbulence model to examine these instability effects throughout the complete operating range in turbine mode. This study specifically analyzes the evolutionary mechanisms of unsteady flow dynamics under ten characteristic off-design conditions while simultaneously characterizing the pressure fluctuation behavior within the vaneless space (VS). The results demonstrate that under both low-speed conditions and near-zero-discharge conditions, the VS and its adjacent flow domains exhibit pronounced flow instabilities with highly turbulent flow structures, while the pressure fluctuation amplitudes remain relatively small due to insufficient rotational speed or flow rate. Across the entire turbine operating range, the blade passing frequency (BPF) dominates the VS pressure fluctuation spectrum. Significant variations are observed in both low-frequency components (LFCs) and high-frequency, low-amplitude components (HF-LACs) with changing operating conditions. The HF-LACs exhibit relatively stable amplitudes but demonstrate significant variation in the frequency spectrum distribution across different operating conditions, with notably broader frequency dispersion under runaway conditions and adjacent operating points. The LFCs demonstrate significantly higher spectral density and amplitude magnitudes under high-speed, low-discharge operating conditions while exhibiting markedly reduced occurrence and diminished amplitudes in the low-speed, high-flow regime. This systematic investigation provides fundamental insights into the flow physics governing pump-turbine performance under off-design conditions while offering practical implications for optimizing transient operational control methodologies in hydroelectric energy storage systems. Full article

To investigate the impact of sand-laden flow on energy loss in Francis turbines, this study integrates entropy generation theory with numerical simulations conducted using ANSYS CFX. The mixture multiphase flow model and the SST k-ω turbulence model are employed to simulate the solid–liquid two-phase flow throughout the entire flow passage of the turbine at the Gengda Hydropower Station (Minjiang River Basin section, 103°17′ E and 31°06′ N). The energy loss characteristics under different off-design conditions are analyzed on the basis of the average sediment concentration during the flood season (2.9 kg/m³) and a median particle diameter of 0.058 mm. The results indicate that indirect entropy generation and wall entropy generation are the primary contributors to total energy loss, while direct entropy generation accounts for less than 1%. As the guide vane opening increases, the proportion of wall entropy generation initially rises and then decreases, while the total indirect entropy generation exhibits a non-monotonic trend dominated by the flow pattern in the draft tube. Entropy generation on the runner walls increases steadily with larger openings, whereas entropy generation on the draft tube walls first decreases and then increases. The variation in entropy generation on the guide vanes remains relatively small. These findings provide technical support for the optimal design and operation of turbines in sediment-rich rivers. Full article

The electric submersible pump (ESP) is a critical component in subsurface resource extraction systems, yet the presence of gas in the working medium significantly affects its performance. To investigate the impact of impeller perforation on gas–liquid mixing and internal flow characteristics, unsteady numerical simulations were conducted based on the Euler–Euler multiphase flow model. The transient evolution of the gas phase distribution, flow behavior, and liquid phase turbulent entropy generation rate was analyzed under an inlet gas volume fraction of 5%. Results show that under part-load flow conditions, impeller perforation reduces the amplitude of dominant frequency fluctuations and enhances periodicity, thereby mitigating low-frequency disturbances. Under design flow conditions, it leads to stronger dominant frequencies and intensified low-frequency fluctuations. Gas phase distribution varies little under low and design flow rates, while at high flow rates, gas accumulations shift from the midsection to the outlet with rotor rotation. As the flow rate increases, liquid velocity rises, and flow streamlines become more uniform within the channels. Regions of high entropy generation coincide with high gas concentration zones: they are primarily located near the impeller inlet and suction side under low flow, concentrated at the inlet and mid-passage under design flow, and significantly reduced and shifted toward the impeller outlet under high flow conditions. The above results indicate that the perforation design of ESP impellers should be optimized according to operating conditions to improve gas dispersion paths and flow channel geometry. Under off-design conditions, perforations can enhance operational stability and transport performance, while under design conditions, the location and size of the perforations must be precisely controlled to balance efficiency and vibration suppression. Full article

This paper reviews the evolution of flow control methods for bladed linear cascades, focusing on passive techniques like riblets, grooves, vortex generators (VGs), and blade slots, which have proven effective in reducing drag, suppressing flow separation, and enhancing stability. The review outlines key historical developments that have improved flow efficiency and reduced losses in cascades. Bio-inspired designs, including riblets and grooves, help stabilize the boundary layer, reduce loss coefficients, and improve flow turning, which is vital for controlling drag and secondary flow effects. Vortex generators, fences, and slotted wingtips enhance stall margins and suppress corner separation, improving performance under off-design conditions. These methods are optimized based on aerodynamic parameters such as Reynolds number and boundary layer characteristics, offering substantial efficiency gains in high-performance compressors. Advancements in computational tools, like high-fidelity simulations and optimization techniques, have provided deeper insights into complex flow phenomena, including turbulence and vortex dynamics. Despite these advancements, challenges remain in fully optimizing these methods for diverse operating conditions and ensuring their practical application. This review highlights promising strategies for improving flow control efficiency and robustness, contributing to the design of next-generation turbomachinery. Full article

To meet the load requirements of the power grid, the hydroelectric power plants need to extend the operational load range of the turbine units, which are often operated under off-design operating conditions. This new challenge significantly changes the flow characteristics of the hydro turbine units. Strong vibrations and high stresses caused by pressure pulsations at various loads directly lead to severe damage to the runner blades, threatening the safe operation of the hydropower unit. In this study, the detailed flow dynamics analysis under three loading conditions of a large-scale Francis turbine, i.e., 33.3%, 66.6%, and 100% of the Francis turbine’s rated power, is investigated with computational fluid dynamics (CFD) calculations. The pressure files at different operating conditions are adopted to carry out the corresponding flow-induced strength analysis of the Francis runner prototype. The pressure distributions and flow velocity distributions at these three typical operating conditions are studied, and the maximum stress of the runner gradually increases with the power output of the turbine, but it is only around one-third of the yield stress of the runner material. It reveals that the runner is safe to operate in the extended operation range from a 33.3% to 100% of the rated power load. The analysis approach in this work can be applied to other hydraulic machinery including Francis turbines, pumps and pump–turbines. Full article

Gas turbines are widely used in power generation due to their efficiency, flexibility, and low environmental impact. Modeling, especially in thermodynamics, is crucial for the designer and operator of a gas turbine. An advanced and rigorous thermodynamic model is essential to accurately predict the performance of a gas turbine under on-design operating conditions, off-design or failure. Such models not only improve understanding of internal processes but also optimize performance and reliability in a wide variety of operational scenarios. This article presents the development of a thermodynamic model simulating the off-design performance of a gas turbine. The mathematical relationships established in this model allow for quick calculations while requiring a limited amount of data. Only nominal data are required, and some additional data are needed to calibrate the model on the turbine under study. A key feature of this model is the development of an innovative relationship that allows direct calculation of the mass flow of air entering the turbine and, thus, the performances of the turbine according to atmospheric conditions (such as pressure, temperature, and relative humidity) and the position of the compressor inlet guide vanes (IGV). The results of the simulations, obtained using code implemented in MATLAB (R2014a), demonstrate the efficiency of the model compared to experimental data. Indeed, the model relationships exhibit high determination coefficients (R² > 0.95) and low root mean square errors (RMSE). Specifically, the simulation results for the air mass flow rate demonstrate a very high determination coefficient (R² = 0.9796) and a low root mean square error (RMSE = 0.0213). Full article

The guide vane mixed-flow pump is a crucial component in medium-to-low-head pumping stations. The guide vanes are mostly fixed in traditional designs. The efficiency of these pumps under off-design operating conditions tends to be low, leading to higher energy consumption. This study explores the design of an adjustable guide vane for the conventional guide vane of a mixed-flow pump at a certain pumping station. Through numerical simulations and two sets of three-factor, five-level orthogonal experiments, we investigate the impact of flow rate, guide vane angle, and impeller angle on efficiency. Through numerical simulation, we identify the optimal relationships between an impeller angle of ±2° and 0° and guide vane angles of ±6°, ±3°, and 0°, focusing on the entropy production rate (EPR) as a key performance metric. The results demonstrate that adjustable guide vanes significantly improve the performance of mixed-flow pumps under off-design conditions. Efficiency increases by up to 17.71% at high flow rates, and by up to 5.48% at low flow rates. Energy consumption is notably reduced. As the flow rate and impeller blade angle vary, the adjustable guide vane rotates to match with the impeller, enhancing flow adaptation, expanding the high-efficiency operating range, and reducing overall energy consumption. Full article

Decarbonization of industrial processes by using high-temperature heat pumps is one of the most important pillars towards sustainable energy goals. Most heat pumps are based on the standard Carnot cycle which includes an expansion valve leading to irreversible dissipation and energetic losses. Especially for high-temperature applications, these losses increase significantly, and a replacement of the conventional throttle valve with an ejector, which is an alternative expansion device, for partial recovery of some of the pressure lost during the expansion, is investigated in this paper. However, designing such a device is complicated as the flow inside is subject to multiphase and supersonic conditions. Therefore, this paper aims to streamline an approach for designing ejectors for high-temperature heat pumps using numerical simulations. To showcase the application of the design procedure, an ejector, which is used to upgrade a standard cycle high-temperature heat pump with the synthetic refrigerant R1233zdE, is developed. To design the ejector heat pump, an interaction between a fast 1D design tool, a 1D heat pump cycle simulation, and a 2D CFD simulation is proposed. An ejector is designed for a sink temperature of 130 °C, which can potentially increase the COP of the heat pump by around 20%. Preliminary measurements at off-design conditions at 100 °C sink temperature are used to validate the design procedure. The pressure distribution inside the ejector is well captured, with relative errors around 4%. However, the motive nozzle mass flow was underpredicted by around 30%. To summarize, the presented approach can be used for designing ejectors of high-temperature heat pumps, although the numerical modeling has to be further developed by validation with experiments to improve the prediction of the motive mass flow. Full article

The present study introduces a conceptual design of a small axial-flow fan. Both individual and combined effects of blade stagger angle and winglet on the performance of the fan design are investigated in design and off-design operating conditions using a computational flow methodology. A stepwise solution, in which a proper stagger angle adjustment of a specifically generated blade profile is followed by appending a winglet at the tip of the blade with consideration of different geometrical parameters, is proposed to improve the performance characteristics of the fan. The initial model comparison analysis demonstrates that a three-dimensional, Reynolds-averaged Navier–Stokes (RANS) equation-based renormalization group (RNG) k–ε turbulence modeling approach coupled with the multiple reference frame (MRF) technique which adapts multi-block topology generation meshing method successfully resolves the rotating flow around the fan. The results suggest that the use of a proper stagger angle with the winglet considerably increases the fan performance and the fan attains the best total efficiency with an additional stagger angle of +10° and a winglet, which has a curvature radius of 6.77 mm and a twist angle of −7° for the investigated dimensioning range. The present study also underlines the effectiveness of passive flow control mechanisms of the stagger angle and winglets for energy-efficient axial-flow fans. Full article

The worldwide concern regarding the harmful effects of old polluting and toxic propellants has led to increased interest in new, green propellants and higher efficiency thrusters. This fact requires that a new generation of turbopumps, fit for these propellants, is developed. This paper focuses on the design of a radial inflow turbine, which was developed to power a single-shaft turbopump system for a 30 kN upper stage expander cycle thruster engine. The objective was to create a high-efficiency, compact, cheap-to-manufacture, 3D-printable turbine suitable to simultaneously power the methane and Oxygen pumps that feed the thruster. The total power consumed by the pumps for which this turbine was designed is 152 kW. The solution proposed in this paper includes measures such as elimination of the bladed diffuser, which was carried out to reduce the weight and the overall dimensions of the turbine. Comparing it with an axial turbine with the same power output, it has lower overall dimensions because it does not require a direction change at the inlet to the turbine bladed components, it does not require a stator to work, and its casing has a conical shape and is not cylindrical like the axial construction one. The proposed design has been analysed by CFD, which revealed that it can power the pumps. Analysis performed in off-design conditions indicated that the turbine has the best efficiency if the rotation speed and mass flow are varied at the same time. A breadboard model of the turbopump for which the turbine in this paper has been designed has been built using plastic and tested at pressures up to 6 bars using compressed air. The results indicate that above 1.5 bars of inlet pressure the turbine can overcome the internal resistances of the components and the rotor starts to spin. No indication of imbalance of the rotor was observed at maximum test pressure. Two configurations of the seals between the turbine and the adjacent pump have been tested, indicating that labyrinth seals must be doubled by floating ring seals. Full article

The performance of heat exchangers in the pressurized water test loop is a critical factor in ensuring the achievement of irradiation parameters for fuel assemblies and the safety of experimental operations. The effect of the heat exchange area margin on the heat exchangers in the pressurized water test loop for the fuel assembly during the steady-state irradiation is analyzed. Additionally, optimization methods for determining the margin of heat exchange area and corresponding design strategies are further investigated. It shows that the effect of the heat exchange area margin on the heat exchange power is less affected by the inlet temperature of the primary water and is primarily influenced by the flow rate of the primary water. A decrease in the flow rate of the primary water reduces the compensatory effect of the cooling section on power and enhances the weakening effect of the regeneration section on power. Meanwhile, the correspondence between the margin of the regeneration section and the cooling section, established based on design conditions, can be applicable when there are changes in the inlet temperature of the primary water, but it is not suitable when there are changes in the flow rate of the primary water. When the flow rate of the primary water decreases, the cooling section margin required to compensate for the decrease in power caused by the regeneration section margin will increase significantly. In addition, short-circuiting the heat exchange tubes in the regeneration section can effectively enhance the heat transfer capability. Furthermore, setting the heat exchange area margins of the regeneration and cooling sections to zero can serve as a termination condition for iterative calculations in the verification of regenerative heat exchangers under off-design conditions. Full article

Blowers are widely used in tasks such as ventilation, exhaust, drying, cooling, heat dissipation, or conveying medium, and they usually consume a lot of energy. There is an inevitable gap between the rotating impeller and static volute casing due to manufacturing tolerance and thermal deformation. The circulating flow in the gap has an important effect on the performance of the blower. In this study, computational fluid dynamics (CFD) was used to investigate the performance of the blower under different flow conditions and gaps, and the accuracy of the numerical simulation was verified by performance experiments. The results show that the flow separation under low flow conditions in the impeller channel can be suppressed by the circulating flow. However, the efficiency of the blower is decreased because a part of the power is used to maintain the circulating flow. Under design conditions, efficiency is reduced by 5.3~8.2%, depending on the gap sizes. Due to the increased flow rate in the impeller channel caused by the gap circulating flow, the net flow rate of the impeller under design conditions is about 12% higher than the inlet flow rate of the blower. Therefore, it leads to an increase of about 12% in impeller efficiency calculated by the net flow rate compared with the inlet flow rate. Finally, the flow field distribution on the impeller channel under different gap conditions was compared, and the effects of the gap on the blower performance were analyzed from the perspective of flow field structure. Full article

Recently, supercritical CO₂ (S-CO₂) has been extensively applied for the recovery of waste heat from flue gas. Although various cycle configurations have been proposed, existing studies predominantly focus on the steady analysis and optimization of different S-CO₂ structures under design conditions, and there is a noticeable deficiency in off-design research, especially for the innovative S-CO₂ cycles. Thus, in this work aimed at the proposed novel S-CO₂ power cycle, off-design characteristics and corresponding control strategies are investigated for the waste heat recovery. Based on the design parameters of the S-CO₂ cycle, structural dimensions of printed circuit heat exchangers (PCHEs) and shell-and-tube heat exchangers are determined, and design values of turbines and compressors are specified. On this basis, off-design models for these key components are formulated. By manipulating variables such as cooling water inlet temperature, cooling water mass flow rate, flue gas inlet temperature and flue gas mass flow rate, cycle performances of the system are analyzed under off-design conditions. The simulation results show that when the inlet temperature and the mass flow rate of cooling water vary separately, the thermal efficiency both can reach the maximum value of 28.43% at the design point. For the changes in heat source parameters, the optimum point is slightly deviated from the design condition. Amidst the fluctuations in flue gas inlet temperature, the thermal efficiency optimizes to a peak of 28.56% at 530 °C. In the case of variation in the flue gas mass flow rate, the highest thermal efficiency 28.75% can be obtained. Furthermore, to maintain the efficient and stable operation of the S-CO₂ power cycle, the corresponding control strategy of the cooling water mass flow rate is proposed for the cooling water inlet temperature variation. Generally, when the inlet temperature of cooling water increases from 23 °C to 27 °C, the cooling water mass flow should increase from 82.3% to 132.7% of the design value to keep the system running as much as possible at design conditions. Full article

The ORC-based micro-cogeneration systems exploiting a solar source to generate electricity and domestic hot water (DHW) simultaneously are a promising solution to reduce CO₂ emissions in the residential sector. In recent years, a huge amount of attention was focused on the development of a technological solution allowing improved performance of solar ORC-based systems frequently working under off-design conditions due to the intermittence of the solar source availability and to the variability in domestic hot water demand. The optimization efforts are focused on the improvement of component technology and plant architecture. The expander is retained as the key component of such micro-cogeneration units. Generally, volumetric machines are adopted thanks to their better capability to deal with severe off-design conditions. Among the volumetric expanders, scroll machines are one of the best candidates thanks to their reliability and to their flexibility in managing two-phase working fluid. Their good efficiency adds further interest to place them among the best candidate machines to be considered. Nevertheless, similarly to other volumetric expanders, an additional research effort is needed toward efficiency improvement. The fixed built-in volume ratio, in fact, could produce an unsteady under- or over-expansion during vane filling and emptying, mainly when the operating conditions depart from the designed ones. To overcome this phenomenon, a dual intake port (DIP) technology was also introduced for the scroll expander. Such technology allows widening the angular extension of the intake phase, thus adapting the ratio between the intake and exhaust volume (so called built-in volume ratio) to the operating condition. Moreover, DIP technology allows increasing the permeability of the machine, ensuring a resulting higher mass flow rate for a given pressure difference at the expander side. On the other hand, for a given mass flow rate, the expander intake pressure diminishes with a positive benefit on scroll efficiency. DIP benefits were already proven experimentally and theoretically in previous works by the authors for Sliding Rotary Vane Expanders (SVRE). In the present paper, the impact of the DIP technology was assessed in a solar-assisted ORC-based micro-cogeneration system operating with scroll expanders and being characterized by reduced power (hundreds of W). It was found that the DIP Scroll allows elaboration of a 32% higher mass flow rate for a given pressure difference between intake and expander sides for the application at hand. This leads to an average power increase of 10% and to an improvement of up to 5% of the expander mechanical efficiency. Such results are particularly interesting for micro-cogeneration ORC-based units that are solar-assisted. Indeed, the high variability of hot source and DHW demand makes the operation of the DIP expander at a wide range of operating conditions. The experimental activity conducted confirms the suitability of the DIP expander to exploit as much as possible the thermal power available from a hot source even when at variable temperatures during operation. Full article

The efficiency of supersonic combustion is largely dependent on inlet and injection parameters. Additional energy input is required in some off-design conditions, and nanosecond discharge actuation can be a solution. In the present study, a phenomenological model of a nanosecond-pulsed surface dielectric barrier discharge (NS-SDBD) actuator was developed to analyze the combustion enhancement effect for a supersonic combustor with transverse H₂ injection. A seven-reaction H₂–air combustion model was adopted for the numerical simulation. Dynamic mode decomposition (DMD) was employed to acquire temperature perturbation in spatial and temporal domains. The results show that the actuator provides additional temperature-increment and species transportation through compression waves. The combustion enhancement effect is mainly attributed to the flow perturbation in the shear layer, which promotes the turbulent diffusion of fuel. Given the same power input, the combustion efficiency at the shockwave reflection point is increased by 17.5%, and the flame height is increased by 15.4% at its maximum. Full article