Search Results (11)

In centrifugal pump open cavitation tests, cavitation regulation valves are indispensable. During valve regulation, the irregular shape of the flow cross-section easily induces cavitation, significantly affecting the test results. This study investigates and designs a novel cavitation regulation valve. The valve core is composed of several identical valve flaps. By restricting the movement direction and distance of the valve flaps, the shape of the flow cross-section remains circular under different valve openings, ensuring optimal fluid flow conditions. This study examines the influence of the number of valve flaps on the flow state. The results indicate that, as the number of valve flaps increases, the flow cross-section approaches a circular shape, reducing the number of bubbles and improving the valve flow state. When the number of valve flaps increases to 20, the flow state shows no significant difference from a circular flow cross-section. Additionally, this study investigates the impact of the valve inlet and outlet shapes on the flow state. The findings reveal that the rounded corner structure experiences severe cavitation inside and at the rear end of the valve. The chamfered corner structure generates bubbles earlier than the initial valve structure but exhibits a stronger ability to resist pressure fluctuations. Both the chamfered inlet and chamfered outlet structures help suppress cavitation, with the chamfered outlet structure exhibiting lower-pressure fluctuations and stronger cavitation resistance. Therefore, the novel cavitation regulation valve with a circular flow cross-section can effectively enhance fluid flow conditions and suppress valve cavitation, demonstrating significant engineering application value. Full article

Bubbles in pipes are widely present in marine engineering, transmission, and fluid systems with complex environments. This paper divides tubes into short, longer, and long tubes due to different lengths. In short tubes, the formation, development, and stability of spark bubbles are deeply analyzed through numerical simulation and experimental measurement, and the morphology and period of vortex rings generated in the surrounding fluid are studied. The results show that bubbles in tubes are significantly elongated compared with those in free fields. Changing the parameters of tubes can affect the size and oscillation speed of vortex rings. Secondary cavitation is found in asymmetric positions in longer tubes. The conditions, positions, and periods of multiple secondary cavitations are summarized in a series of experiments on long tubes. It is found that bubbles in tubes are related to the ${\gamma }_t$ and ${\gamma }_L$ tube parameters. More secondary cavitation is easily generated in thinner and longer tubes. In addition, the pumping effect brought about by the movement of bubbles in tubes is studied. By designing reasonable tube parameters, the life cycle of bubbles can be changed, and the pumping efficiency can be improved. This study provides important theoretical support for the reliability of the movement of bubbles and surrounding fluid in tubes and lays a foundation for the optimization and promotion of this technology in practical applications. Full article

Pump-jet propellers are currently the mainstream propulsion method for underwater vehicles, and cavitation is an important factor limiting the high speed and miniaturization of pump-jet propellers. In order to explore the cavitation performance of high-speed pump-jet propellers, based on the modified SST turbulence model and the Zwart cavitation model, a three-dimensional numerical simulation of unsteady internal cavitation flow was carried out by comparing the impeller with specific speed n_s = 1920 using FLUENT 2020R2 software. At the same time, the occurrence and development process of cavitation under 0.95 Q, 1.0 Q, and 1.05 Q conditions were analyzed (Q is the mass flow), the changes in gas volume fraction in the impeller channel were captured, the distribution characteristics of cavitation under different NPSH values were explored, and the change law of cavitation with time was determined. The results show that, when NPSH dropped to 95 m, the impeller cavitation first occurred under the 1.05 Q operating condition, and the impeller cavitation volume fraction was 0.0379525. When NPSH dropped to 85 m, the impeller cavitation occurred under the 1.0 Q operating condition, and the impeller cavitation volume fraction was 0.0185164. When NPSH dropped to 80 m, the impeller cavitation occurred under the condition of 0.95 Q, and the volume fraction of the impeller cavitation was 0.013541. The high-speed pump-jet propeller had better anti-cavitation ability with a small flow rate. The cavitation distribution law under the three operating conditions was similar; cavitation was first generated on the impeller inlet edge and near the shroud, and the vacuoles with large volumes were mostly concentrated on the impeller inlet side. As the NPSH gradually decreased, the entire flow channel was gradually occupied by vacuoles. As the flow decreased, the corresponding NPSH also decreased. When NPSH dropped to 50 m, the volume fraction of the impeller under all three operating conditions reached around 0.4. As the cavitation only occurred on the suction surface, the volume fraction of the cavitation on the suction surface exceeded 0.8, at which time the impeller had already undergone severe cavitation. Within a complete cycle, bubbles first appeared at the inlet edge of the impeller (measured near the shroud) and gradually spread toward the middle and rear of the impeller, ultimately covering the suction surface of the impeller. Under the design condition, the experimental results of the model pump were consistent with the numerical simulation results, and the error was only 2.68%, thus verifying the reliability of the numerical simulation. The research results provide a reference for the in-depth study of the cavitation performance of high-speed pump-jet propellers and provide a good theoretical basis and practical significance in the engineering field for the high-speed and miniaturization process of high-speed pump-jet propellers. Full article

A microactuation process was developed with the help of four coils that generate a pulsatory rotating magnetic field. A small actuator stator, which contains a 46 mm acrylonitrile butadiene styrene (ABS) opened box and four coils with E-type ferrite cores, was constructed. Simulations were made for different Duty Cycles, 0.2, 0.5, 0.72 and 0.9, and distances above the E cores, between 0.01 and 6 mm. These simulations determined the magnetic bubble inflating distance, the saturation regions and the average forces that are responsible for nanofluid flow inside the ABS box. An electrical driving scheme was designed, and a drive was constructed to activate four inductive loads that generate a pulsatory rotating magnetic field. The electronic drive can change the actuation frequency (rotation speed) between 1 Hz and 25 Hz and can adjust the Duty Cycle between 5% and 95% (driving force). From simulations and experiments, it was observed that the Duty Cycle must be limited to 0.7 to avoid the magnetic nanofluid saturation at 45 mT. It was found that three applications use a pulsatory rotating magnetic field: a small motor, a small flat pump and a manipulating sheet matrix for displays or chemical droplets mixing. Full article

The energy transition, originating in the limitation of fossil resources and greenhouse gas (GHG) emission reduction, is the basis of many studies on renewable energies in different industrial applications. The diffusion absorption refrigeration machines are very promising insofar as they allow the use of renewable resources (solar, geothermal, waste gas, etc.). This technology is often considered an alternative to vapor compression systems in cooling and refrigeration applications. This paper aims to overview the thermal mechanisms related to modeling system energy sources and highlight the primary methodologies and techniques used. We study and analyze the technology’s current challenges and future directions and, finally, identify the gaps in the existing models to pave the way for future research. The paper also gives a classification of absorption refrigeration systems (ARS) to position and limit the scope of the study. The paper will help researchers who approach the various aspects to have a global synthetic analysis of the mechanisms characterizing the modeling of energy sources of absorption refrigeration machines. Full article

Over the years, Computational Fluid Dynamics (CFD) has been an integral part of most pump design processes. Unfortunately, as calculation schemes and flow investigations become more complicated, the cost of conducting numerical simulations also becomes more expensive in terms of computational time. To remedy this, cutting-edge technology, together with novel calculation techniques, are continuously introduced with the end target of producing more accurate results and faster computing time. In this paper, CFD simulations are run on a numerical model of a double-volute double-suction pump prepared using ANSYS Fluent Mosaic meshing technology. Poly-Hexcore, the first application of Mosaic technology, fills the bulk region with octree hexes, keeps a high-quality layered poly-prism mesh in the boundary layer, and conformally connects these two meshes with general polyhedral elements. This technology promises to provide a lower number of cells along with a significant increase in computing speed. In this paper, steady state results of the model with Mosaic Poly-Hexcore mesh with ~37% fewer cells produced comparable results with a similarly sized model prepared with multi-block structured hexagonal mesh. The predicted pump head, efficiency and shaft power under the design conditions were within 1% for both models, while calculation time was reduced by ~25%. Additional simulations using the Poly-Hexcore mesh showed that the model was able to closely predict the pump’s NPSH3 for $0.8Q_D$, $1.0Q_D$, and $1.2Q_D$ compared with the manufacturer’s data. Under cavitating flow conditions, the formation of vapor bubbles was observed on the suction side, starting at the leading edge of the blade and slowly forming as thin sheets towards the trailing edge as the suction pressure is reduced. Lastly, pressure fluctuations were observed from pressure coefficient data collected at several monitoring points in the volute and the impeller. It was seen that due to the interaction between the stationary casing and the rotating impeller, pulsations were equivalent to the blade passing frequency and its harmonics. Full article

Gas injection is a frequently used method for artificial lift and flow regime rectification in offshore production and transportation flowlines. The flow behaviour in such flowlines is complex and a better understanding of flow characteristics, such as flow patterns, void fraction/hold up distributions and pressure gradient is always required for efficient and optimal design of downstream handling facilities. Injection method and location have been shown to strongly affect downstream fluid behaviour that can have important implications for pumping and downstream facility design, especially if the development length between pipeline and downstream facility is less than L/D = 50 as reported by many investigators. In this article, we provide the results of an experimental investigation into the effects of the gas injection position on the characteristics of the downstream upwards vertical gas flow using a vertical riser with an internal diameter of 52 mm and a length of 10.5 m. A horizontal 40-m-long section connected at the bottom provides options for riser base or horizontal flow line injection of gas. The flowline gas injection is performed 40 m upstream of the riser base. A 16 by 16 capacitance wire mesh sensor and a gamma densitometer were used to measure the gas-liquid phase cross-sectional distribution at the riser top. A detailed analysis of the flow characteristics is carried out based on the measurements. These include flow regimes, cross-sectional liquid holdup distributions and peaking patterns as well as analysis of the time series data. Our findings show that flow behaviours differences due to different gas injection locations were persisting after a development length of 180D in the riser. More specifically, core-peaking liquid holdup occurred at the lower gas injection rates through the flowline, while wall-peaking holdup profiles were established at the same flow conditions for riser base injection. Wall peaking was associated with dispersed bubbly flows and hence non-pulsating as against core-peaking was associated with Taylor bubbles and slug flows. Furthermore, it was found that the riser base injection generally produced lower holdups. It was noted that the circumferential injector used at the riser base promoted high void fraction and hence low liquid holdups. Due to the bubbly flow structure, the slip velocity is smaller than for larger cap bubbles and hence the void fraction is higher. The measurements and observations presented in the paper provides valuable knowledge on riser base/flowline gas introduction that can directly feed into the design of downstream facilities such as storage tanks, slug catchers and separators. Full article

The pre-swirl inflow generated by guide vanes could improve the hydrodynamic performances of centrifugal pumps as long as the inflow matches the patterns of internal flow of the impeller. In this work, we present a numerical investigation on the internal flow in a centrifugal impeller subjected to inflow artificially constructed with simple pre-swirling; unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are performed at the designed flow rate with five values of rotating velocity of the inflow, i.e., U_rot/U_ref = −0.5, −0.3, 0.0, 0.3 and 0.5, where U_rot and U_ref denote the rotating and normal velocity component at the entrance of the inflow tube, respectively. The primary objective of this work is to reveal the three-dimensional characteristics of internal flow of the impeller as influenced by the superimposed pre-swirl inflow, and to identify the propagation of inflow within the impeller. The numerical data are presented and analyzed in terms of the streamline fields, the distributions of various velocity components along the circumferential and axial directions, the pressure distribution and limiting streamlines on the surfaces of a blade. Numerical results reveal that separation occurs around the leading edge of the blades and occasionally at the trailing edge, and the internal flow is more uniform in the central region of the channels. A noticeable fluctuation of both radial and circumferential velocities is observed at the outlet of the impeller as it is subjected to counter-rotating inflow, and the greatest fluctuation is close to the hub instead of the middle channel and shroud as for the co-rotating inflow. The boundary layer flow of suction surface is more sensitive to the inflow; occasional small-scale separation bubble occurs on the suction surface around the leading edge for some blades, and reattachment of separated flow is reduced for the counter-rotating inflow. Full article

To understand the formation mechanism and evolution process of the perpendicular cavitation vortex (PCV) of an axial flow pump for off-design conditions, turbulent cavitating flows were numerically investigated using the rotation curvature-corrected shear stress transport (SST-CC) turbulence model and the Zwart–Gerber–Belamri cavitation model. In this work, the origin and evolution of a PCV were analyzed through a high-speed photography experiment and numerical simulation. The results showed that the PCV came from a secondary tip leakage vortex (S-TLV) and was aggregated by the action of the re-entrant jet, combined with the cavitation bubbles driven by the radial flow to form the cavitation vortex (CV). With the joint action of leakage jet lifting and TLV entrainment, the PCV was reoriented and gradually became perpendicular to the chord direction. Then, the PCV and TLV collided, mixed, and entrained, which formed a strong pressure pulsation. The PCV was gradually divided into upper and lower parts. One part was combined with the residual part of the TLV and flowed to the next blade, and the other part flowed out of the impeller area along the axial direction. At the same time, the generation, evolution, and dissipation of the PCV formed high pulsation amplitudes and frequencies in the middle and rear above the blade suction. Full article

When a pump-jet propeller rotates at high speeds, a tip vortex is usually generated in the tip clearance region. This vortex interacts with the main channel fluid flow leading to the main energy loss of the rotor system. Moreover, operating at a high rotational speed can cause cavitation near the blades which may jeopardize the propulsion efficiency and induce noise. In order to effectively improve the propulsion efficiency of the pump-jet propeller, it is mandatory to research more about the energy loss mechanism in the tip clearance area. Due to the complex turbulence characteristics of the blade tip vortex, the widely used Reynolds averaged Navier–Stokes (RANS) method may not be able to accurately predict the multi-scale turbulent flow in the tip clearance. In this paper, an unsteady numerical simulation was conducted on the three-dimensional full flow field of a pump-jet propeller based on the DES (detached-eddy-simulation) turbulence model and the Z-G-B (Zwart–Gerber–Belamri) cavitation model. The simulation yielded the vortex shape and dynamic characteristics of the vortex core and the surrounding flow field in the tip clearance area. After cavitation occurred, the influence of cavitation bubbles on tip vortices was also studied. The results revealed two kinds of vortices in the tip clearance area, namely tip leakage vortex (TLV) and tip separation vortex (TSV). Slight cavitation at J = 1.02 led to low-frequency and high-frequency pulsation in the TLV vortex core. This occurrence of cavitation promotes the expansion and contraction of the tip vortex. Further, when the advance ratio changes into J = 0.73, a third type of vortex located between TLV and TSV appeared at the trailing edge which runs through the entire rotational cycle. This study has presented the dynamic characteristics of tip vortex including the relationship between cavitation bubbles and TLV inside the pump-jet propeller, which may provide a reference for the optimal design of future pump-jet propellers. Full article

This paper provides an illustrated description of a proposed LiBr-H₂O vapour absorption refrigerator which uses a thermally activated solution pumping mechanism that combines controlled variations in generator vapour pressure with changes it produces in static-head pressure difference to circulate the absorbent solution between the generator and absorber vessels. The proposed system is different and potentially more efficient than a bubble pump system previously proposed and avoids the need for an electrically powered circulation pump found in most conventional LiBr absorption refrigerators. The paper goes on to provide a sample set of calculations that show that the coefficient of performance values of the proposed cycle are similar to those found for conventional cycles. The theoretical results compare favourably with some preliminary experimental results, which are also presented for the first time in this paper. The paper ends by proposing an outline design for an innovative steam valve, which is a key component needed to control the solution pumping mechanism. Full article