Journal Of Applied Fluid Mechanics
Volume:7 Issue: 8, Aug 2024

The blade profile selection is paramount for the efficient operation of straight Darrieus wind turbines in terms of torque and power generation. In this work, we have used the Kline-Fogleman Airfoil (KFA) design for the wind turbine blades. The concept of KFA design aims to cause flow separation, vortex formation, and reattachment establishment before the trailing edge. Thus, geometric tests on have been performed on the baseline airfoil NACA0015 as one of the best profiles for operating a straight Darrius wind turbine. A two-dimensional Computational Fluid Dynamic (CFD) model using the two-equation Shear Stress Transport k-ω (SST k-ω) turbulent model was developed in ANSYS/FLUENT software to assess the aerodynamic efficiency of the modified airfoil. Two designs (KFA-2 and KFA-4) were tested initially in the static case. The effects of the opening step angle and its curvature diameter were studied for an angle of attack’s range of -20° to +20°. The rounded KFA-4 design with an opening step angle of 93.6° led to a significant improvement in the lift-to-drag ratio thus, aerodynamic efficiency. Finally, the straight KFA-4 design with the opening step angle of 93.6° revealed a the most advantageous effects on the operation of a straight Darrieus wind turbine for a Tip Speed Ratio less than 1.6 (TSR<1.6). It allowed a noticeable reduction of the dead zone and TSR corresponding to the nominal power, thus consequently improving the starting torque and delaying torque stall.

To reduce the fluid resistance on the surface of flow-through components and improve energy utilization efficiency, a biomimetic fitting structure model is constructed based on the ridge-like features of beluga skin. The SST k-ω model is employed to numerically simulate the drag reduction characteristics of three biomimetic structures (fitting structure, V-shaped structure, and arc structure) included in the design. The variations of the fitting structure’s viscous resistance and pressure drop resistance with different widths and depths are compared. The drag reduction mechanism of the fitting structure surface is studied based on the pressure stress, velocity field, and shear stress. The results demonstrate that the fitting structure exhibits the best drag reduction performance. The fitting structure with a width of 30 mm and a depth of 0.7 mm achieves an optimal drag reduction effect of 4.18%. The fitting structure exhibits a large low shear stress region, which increases the thickness of the bottom boundary layer, thereby reducing surface velocity and viscous resistance.

The study analyzes the unique behavior of two-phase flows when incorporating nanofluids containing aluminum trioxide (Al2O3) and copper (Cu) nanoparticles in a vertical channel. The main goal is to investigate the behavior of air-nanofluid mixtures in this setting, with potential implications for industrial and exploration applications. Research in this area could provide valuable insights into the dynamics of these flows and their impact on heat transfer, fluid dynamics, and material science. This study includes an analysis of upwelling dynamics, the effect of fluid characteristics on bubble growth, and the system's thermal efficiency. Using numerical and quantitative visualization techniques, we seek to understand the behavior of these particles at the interface between the liquid and gas phases by integrating Al2O3 and Cu nanoparticles into the VOF approach. Because of their superior thermal conductivity, copper nanoparticles have a higher volumetric density and provide more efficient heat transfer, leading to quick and efficient thermal dissipation. Smaller nanoparticles offer an increased surface area-to-volume ratio, which improves heat transfer capabilities and ensures uniform heat dissipation throughout the material. Consequently, copper nanoparticles emerge as the preferred choice for applications necessitating high thermal transfer and optimal performance. These results significantly impact the design of more efficient heat exchangers and optimize recovery techniques by elucidating the interactions between these nanoparticles and the surrounding fluids. Furthermore, the selection of smaller copper nanoparticles further amplifies thermal transfer, maximizing performance across diverse applications.

An uneven mixing of hydrogen-blended natural gas will lead to hydrogen embrittlement in distribution pipelines, thereby affecting the quality of terminal gas, and thus highlighting the importance of ensuring the uniformity of hydrogen and natural gas mixing. In this study, FLUENT software was used to simulate three different hydrogen filling modes, namely, T-tube, bending-tube, and static mixer, and the mechanisms underlying the mixing of hydrogen and natural gas under different filling modes were analyzed. In addition, we assessed the influences of gas velocity, hydrogen blending ratio, and mixer length and blade angle on the mixing effects of a static mixer. The results revealed that among the three mixing methods assessed, the static mixer has the best overall mixing effect. Increasing gas velocity was found to have no significant effect on the mixing of hydrogen and natural gas. With an increase of hydrogen blending ratio, the mixing uniformity of hydrogen and natural gas increased from 99.49% to 99.95%, whereas there was an increase from 84.12% to 99.05% when the length of static mixer was increased, and an increase from 59.53% to 99.78% in response to an increase in blade angle. Our findings in this study can provide a methodological reference for increasing the mixing uniformity of hydrogen and natural gas in hydrogen-blended natural gas pipeline networks, and thereby contribute to the safe and rapid development of the hydrogen energy industry.

Strict emission regulations together with reducing fossil fuels resources lead to more attention on new combustion strategies and alternative fuels such as biodiesel which is renewable, environmentally friendly and more cost-effective than other fuels. In this study, CONVERGE CFD software coupling with chemical kinetics mechanism is used to numerically investigation of natural gas (NG)/biodiesel dual fuel engine. The discussed biodiesel consists of 25% methyl decanoate (MD), 25% methyl-9-decanoate (MD9D) and 50% diesel. A comparative study of NG/diesel and NG/biodiesel fueled cases is performed to highlight the combustion characteristics of biodiesel. For all simulated cases, it is supposed that 5% of energy is supplied by high reactive fuel (i.e., Diesel or Biodiesel) and 95% is coming with low reactive fuel (i.e., Natural Gas). Results revealed that in full load condition, using biodiesel/NG led to 86% lower carbon monoxide (CO) and 91% unburned hydrocarbons (UHC). On the other hand, peak pressure and maximum in-cylinder temperature increased 5% and 83 K, respectively which led to 0.6% efficiency improvement. according to the results of different injection timing, when it was advanced from -4 to -20 crank angle degree after top dead center (CAD ATDC) for biodiesel/NG and diesel/NG, the indicated mean effective pressure (IMEP) and gross thermal efficiency (GTE) reached at their peaks 18.3 bar and 48.2% at -12 CAD ATDC, 18.05 bar and 47.7% at -8 CAD ATDC respectively.

This paper investigates the effects of the chordwise fence on the spanwise change in aerodynamic characteristics of an aircraft wing with a different taper ratio for varying angles of attack. The investigation was carried out for the tapered wing with different taper ratios of 0.41, 0.6, and 0.75. The wing is tested in a subsonic, low turbulence wind tunnel at the free stream Reynolds number of 2.3×105 for various angles of attack ranging from α = 0° to 45°. The baseline wing model is attached to a fence of different diameters of 1.5 and 2.5 mm at a plane equal to the root and tip chord. There are pressure ports spread across the span of the wing, and the corresponding surface pressure is measured using the MPS 4264 miniature pressure scanner. The surface pressure measured is analyzed further for the variation of the aerodynamic characteristics of the wing. The presence of a fence on the tapered wing forms an efficient flow control device that delays the flow separation, thereby delaying stall angles and preventing the steep transition of the favorable pressure gradient to the adverse pressure gradient at the stall. The presence of a fence on the wing surface has considerably increased the lift coefficient, and the stall is significantly delayed for a least taper ratio wing. The fence has suppressed the interaction of the leading-edge vortices with the tip vortices; thereby, the spanwise flow from the root chord to the tip chord is controlled.

As a common industrial valve solution in slurry pipeline systems, Y-type slurry valves must be designed with properly shaped valve seat sealing rings to alleviate particle erosion and keep the valve operating in optimal condition. In this study, we investigate four different valve seat shapes with varying inner angles to understand the effect of the seat structure on particle erosion in Y-type slurry valves. We conducted computation fluid dynamics discrete element method simulations to analyze the impact of different valve seat shapes on the characteristics of particle–liquid two-phase flows and erosion. We also performed a detailed analysis of velocity and pressure distributions for liquid phase and particle velocity distributions as well as surface erosion on key valve components. We observed the primary erosion areas to be the back surface of the valve body, the inner surface of the valve seat, and the front surface of the valve disc, and as a result, we investigated the erosion distribution in these areas with different valve seat structures. Our simulation results indicate that the variation in particle diameter and the inner angle of the valve seat sealing ring have significant impacts on erosion in the studied areas.

To understand the stratification of the smoke layer in a road tunnel, numerical simulations are employed to model tunnel fires with varying heat release rates. The different simulations cases are carried out with FDS (Fire dynamic simulations). These simulations are conducted to examine the influence of tunnel slope and longitudinal airflow on the smoke stratification along the downstream side of tunnel. The aim is to explore the relationship between longitudinal airflow and temperature ratio taking into account the tunnel slope. As a result, a quantitative analysis, based on Newman's theory, is conducted to assess the clarity of the smoke layer stratification, a Froude number (Fr = 0.63) is obtained. The slopes in tunnels can have a substantial impact on smoke flow during a fire, primarily driven by thermal buoyancy and the stack effect. With a slope less than 1.5°, the stratification improves. Similarly, clear stratification occurs when the longitudinal airflow is less than 1 m/s. However, a balance between inertia force and buoyancy force is crucial for maintaining clear stratification. Increasing both the longitudinal airflow and the tunnel slope serves to disturb the stratification of the smoke layer.

Computational particle fluid dynamics method is utilized to study the influence of polydisperse and monodisperse particle size distribution, fuel addition, and biomass mixing ratio on the gas-solid flow behavior in a pilot-scale circulating fluidized bed (CFB). Numerical results show that a polydisperse system with different particle sizes can enhance the fluidization quality and the uniformity of the particle volume fraction in comparison with a monodisperse system with uniform particle sizes. When fuel is present in the CFB, the disturbance at the circulation inlet is eliminated and the particle aggregation effect at the wall is reduced. Furthermore, the particle volume fraction, pressure, and particle velocity distributions change only slightly as the biomass increased from 0% to 20% or from 50% to 100% of the total fuel mass. However, as the biomass ratio increases from 20% to 50%, the pressure drop in the riser decreases and the back-mixing degree at the riser wall weakens.

Buzz is an unwanted and inevitable phenomenon occurring due to the subcritical operation of intake which needs a comprehensive understanding. The buzz pattern in axisymmetric intakes differs from 2D counterpart and requires further investigation. The current study emphasizes the ways of buzz formation and its sustenance at supersonic speeds. In the present study URANS simulations have been done for various throttling ratios to simulate the engine demand conditions. It has been found that the onset of intake buzz happens for anything above the throttling ratio of 0.54. An active flow control technique using plasma actuator was used here to mitigate the influence of buzz. The study also emphases on the impact of plasma power densities on the intake performance.

The present study investigated the transmission of exhaled particles generated by coughing inside a car cabin, considering eleven different window opening configurations. The results indicated that particle dispersion and removal were mainly affected by the airflow, which was largely determined by the window opening configurations. Notably, efficient ventilation and a large number of open windows did not necessarily result in lower infection risk. Given the complex structure and formation of intricate airflow patterns within the cabin, airborne particles could spread throughout the cabin and deposit on the interior walls. As particles tended to escape or deposit rapidly within the first 10 s, precautionary measures were necessary during this time frame following a passenger's coughing activity. Furthermore, closing the window adjacent to the driver effectively reduced the proportion of exhaled particles passing through the driver's breathing zone due to the rear-in and front-out airflow pattern, thus mitigating the risk of infection.

To investigate the gas-liquid two-phase flow characteristics in an emergency rescue drainage pump, the MUSIG model was adopted to analyze the effect of the gas phase on the internal flow characteristics of the pump. The results show that the gas phase predominantly accumulated in the impeller region, with significant tendencies for large diameter bubbles to fragment into smaller diameter bubbles. The bubbles of the impeller blades converged towards the middle zone of the blade near the hub, forming an air pocket that obstructed the flow passage through the impeller. Such findings ultimately resulted in a loss of pump performance. Moreover, as the diameter of inlet bubble increased, there was a greater tendency for the gas phase to converge into a concentration distribution, leading to unfavorable flow conditions in the pump. This phenomenon ultimately led to a decline in pump performance and may have resulted in the loss of water conveyance functionality. Meanwhile, the Ω method was used to investigate the vortex flow within the drainage pump under different gas contents. As the inlet gas volume fraction increased, the vortex area expanded and the vortex tended to fragment into multiple smaller pieces, resulting in the formation of more complex structures.

Present computational simulation studied H2-CH4 combustion characteristics in a specific gas turbine combustor used for power generation. Across four thermal loads (1.1-4.4 bar) and varying hydrogen fraction (0-50% by volume), changes in flame temperature, reaction zone stability, and flow field are scrutinized. Results show coherent thermal patterns and stable flame fronts across all conditions, indicating hydrogen addition does not deteriorate combustion when blended with methane. Flame temperatures increase by approximately 40 K with increasing hydrogen fraction. Acceptable NOx emissions are observed, peaking at 6.20 ppm with 50 % H2 at 168 kW. The combustor enables reliable operation for blends up to 50% hydrogen. These results suggest potential for increasing legislated hydrogen blending limits for more sustainable gas turbine power generation. By expanding the viable envelope for hydrogen-methane mixtures, this work contributes to understanding combustion of decarbonized fuels in gas turbines. However, as results are limited to the investigated combustor geometry, generalized conclusions cannot be drawn at this stage. Nonetheless, this study represents an incremental advancement in knowledge that may inform future research on sustainable power generation and decarbonization efforts.

The circumferential non-uniform tip clearance (CNTC) due to casing out-of-roundness adversely affects the turbine aerodynamic performance due to machining and assembly errors, thermal deformation, and improper active clearance control (ACC), etc. Moreover, the asymmetric computational domain caused by casing out-of-roundness presents difficulties for conventional numerical techniques that consider rotational periodicity. Since previous traditional methods using split computational domains have the disadvantages of high interpolation error and high time cost, an efficient fast-moving mesh (FMM) method based on an algebraic approach is proposed in this paper. This method is first validated by using a single-stage turbine with elliptical casing. The results show that the FMM has the advantages of high accuracy, high efficiency, and easy operation, which helps to solve the CNTC problem quickly in scientific research or engineering applications. Then, the effects of CNTC induced by the elliptical casing on the flow field and aerodynamic performance are investigated by using an in-house code that integrates the FMM method. Finally, the effect of stator row interference on the aerodynamic performance in the turbine stage with an elliptical casing is demonstrated. The results show that different types of elliptical casings have a significant effect on the aerodynamic performance. However, the variation law is not consistent (decreasing by 0.538% or increasing by 0.212%). Importantly, the novel finding of this paper is that this discrepancy is jointly determined by the interaction of multiple secondary flows (passage vortex, scraping vortex, etc.) at different spans, not just related to the variation of the tip leakage vortex (TLV) with tip size.  Furthermore, this study is the first to indicate that the stator row interference can mitigate the extent of performance degradation due to elliptical casings by suppressing the development of secondary flows. These results may provide theoretical support for blade tip gap design and can also serve as a reasonable reference for the effective application of ACC in engineering. Finally, low-order harmonic components with high amplitudes are also innovatively found in the rotor row with a CNTC. These components may cause low-engine-order (LEO) resonances that endanger the safe operation of engines.

The performance and life of centrifugal pumps can be severely impacted by cavitation. In this study, a vortex generator was placed on the blades of the first-stage impeller of a BB4 multistage centrifugal pump to investigate its effect on the local flow field. The response surface method was employed to design various solutions based on the vortex generator's arrangement position, height, and rotation angle and the optimum parameters were φ = 20°, θ = 10°, and h = 1.2114 mm. The results showed that a pressure rise occurred at the corresponding position of the vortex generator, and the pressure variation ranges from 1.99%-8.91%, which is closely related to the height parameter of the vortex generator. In the flow field near the vortex generator, the velocity increased and then decreased along the vertical direction of the blade wall, reaching a maximum height of 1.5 mm. Additionally, the low-velocity zone formed at the end of the vortex generator gradually became larger with the downward movement of the arrangement position, and its velocity varies in the range of about 1.5 m/s-15.7 m/s, and the pressure varies in the range of about 14,000 Pa-125,000 Pa. This research is crucial for understanding the cavitation control principle of vortex generators in centrifugal pumps.