Internal Flow Changes Research Articles

Three-dimensional nonlinear vortex-induced vibrations of risers under internal gas-liquid and external shear flows

A novel three-dimensional (3D) dynamic model is proposed to explore the nonlinear vortex-induced vibration (VIV) characteristics of top-tension risers under the combined action of internal gas-liquid two-phase flow and external shear flow. The van der Pol equation is used to evaluate the interaction between the riser and the external shear flow. The nonlinear governing equations are established through Hamilton's principle, and they are discretized by the Galerkin method and solved via the Runge-Kutta method. The VIV responses calculated by the proposed nonlinear dynamic model are compared with the previously experimental and computational fluid dynamics (CFD) results, demonstrating the effectiveness and accuracy of the present theoretical method. The influences of the internal flow effect, the tension and the shear parameter of the external flow on the VIV responses of the riser are analyzed. The results show that when the internal flow effect is not considered, the transition of the external flow from uniform flow to shear flow leads to an increase in excited modes and a decrease in resonance responses of the riser in the cross-flow (CF) direction. Furthermore, with the increase of the external shear flow velocity, resonances at different positions of the riser are not synchronized, which is different from that of the riser in uniform flow. In the case of the riser conveying gas-liquid two-phase flow and being subjected to external uniform or shear flow, the increase in the velocity of the liquid phase leads to an increase in the maximum response amplitudes of the riser in the in-line (IL) and axial directions. When the internal flow changes from single-phase to gas-liquid two-phase, the maximum VIV responses of the riser in the IL and axial directions tend to decrease regardless of whether the riser is in uniform flow or shear flow.

Effect of the Flow Passage Area Ratio on a Double Slit-Type Pintle Injector

Due to the structural characteristics of the pintle injector, a recirculation zone is generated under the pintle, so the pintle tip is directly exposed to a high-temperature environment. Therefore, the cooling technique and protective coating on the pintle tip are essential for improving the reusability and durability of the pintle injector. Although various cooling methods are known, this study applied a forced flow passage cooling method by adding internal structures called inserts to the pintle injector. The characteristics of the insert were analyzed experimentally and numerically based on change of its area ratio. When the internal flow changes, the spray angle, droplet size, and cooling performance also change in accordance with an increase in the insert’s central area. When the insert structure is installed in the pintle post, upward flow may occur, which can worsen the spray angle and droplet distribution characteristics. Nevertheless, owing to the insert structure, the surface temperature of the pintle post is reduced dramatically as intended. Thus, an optimal design technique for cooling by changing the area ratio of the first- and second-row slits of the pintle post can be suggested to overcome the limitations of the insert structure. The results of this study will be helpful in understanding the spray characteristics of the insert structure and optimize the design of cooling channels for reusable pintle injectors in the future.

Influence of stall fences on the performance of Wells turbine

The Wells turbine is one of the candidates for use in Oscillating Water Column (OWC) type wave energy conversion devices. The narrow operating range of a Wells turbine limits its application to a small range of ocean environments. At a high angle of attack, the turbine performance drops suddenly due to a phenomenon known as stall. The present study introduces stall fences in the Wells turbine blade to postpone stall and widen the operating range. The stall fences are defined by the length, height, and thickness in percentage of blade chord length. For the present study, dimensions of the stall fences are determined by using a surrogate-based optimization technique. The modified and reference turbine are numerically studied by solving the Reynolds-Averaged-Navier-Stokes equations in the commercial CFD software ANSYS CFX 16.1. The comparison of fenced turbine with reference turbine shows 16.6% improvement in operating range at the cost of peak torque developed by the turbine. The peak-to-average power ratio in the stall-free range is reduced by 16.7% when stall fences are used. The change in internal flow due to the presence of stall fences is analyzed in detail in the present work.

Experimental investigation on heat transfer and pressure drop of internal flow in corrugated tubes

The operating conditions of the pre-heater zone of an automotive vehicle waste heat recover heat exchanger occurs in, or close to, the transitional flow regime. For such an application the use of corrugated tubes in a cross flow tube heat exchanger is of particular interest. However, not much work has been done in the transitional flow regime in corrugated tubes where the internal flow changes from laminar to turbulent with heat transfer at high heat fluxes. Therefore, more data is needed. Hence, the main purpose of this work is the characterization of the heat transfer and pressure drop of internal flow in corrugated tubes. To this end an experimental setup was developed and validated for the study of a smooth tube and two corrugated tubes that have an internal diameter of 5.75 mm and a heating length of 0.38 m. The experiments were conducted in laminar, transitional and turbulent regime, with Reynolds numbers in the range from 429 to 6212, under both adiabatic and diabatic flow conditions. The heat flux imposed on the tube wall ranged from 5.5 kW/m2 to 21.1 kW/m2. The flow was hydrodynamically fully developed at test section inlet. The results revealed that the friction factor of the corrugated tubes presented a smoother transition than in the case of the smooth tube, being the diabatic and adiabatic friction factor for the corrugated tubes quite similar from laminar to turbulent regime. The corrugated tubes are more effective under transitional flow regime. The highest heat transfer augmentation occurs at Re ≈ 2000, specifically, the Nusselt number augmentation (Nuc/Nus) increases up to 4.7 for the corrugated tube with helical pitch p = 6 mm and up to 3.8 for the corrugated tube with p = 12 mm.

Leap in love with resection of emotion and direction of serial novel of Dong-A Ilbo - Focusing on What did you get -

In this study, I consider Yeom Sang-sub’s What did you get in point of a serial novel.What did you get is Dong-A Ilbo’s serial novel, so it has continuity and differentiation of Dong-A Ilbo’s other serial novel. Continuity is that Dong - A Ilbo s discourse is revealed in the novel. In the growth novels and detective novels, Dong-A Ilbo revealed their worldview. Similarly, a romance novel makes a sacred love affair between people as its content. However, the novel contents reveal the difference. The character in the novel does not get the ability to love. In Donga Ilbo s love discourse, unrestrained emotions are eliminated as bad things. And love is considered sacred, so lovers should be a excellent person who can do divine love. After all, lovers should be able to contribute to society through their love. Emotions not leading to home and marriage were considered bad. The characters of What did you get are the same. They are faithful to their feelings, but the external gaze is critical. So they internalize the external gaze and critically see their emotions. The problem, then, they will not be able to make love. The external gaze is only focused on criticizing emotions, and it never raises the ability to make love. Consequently there is a crack between Dong-A Ilbo s discourse and romance novel’ content. Dong-A Ilbo tried to show the figure of personality person who can love. However, what was actually revealed was the inability of charater and the impossible of love. So it became clear that the romance novel is no longer suitable for the Dong-A Ilbo. Changes in the trends of the novel consisting of Dong-A Ilbo paper space innovation were influenced by these internal flow changes. Discourse no longer affects novels and is exposed to the it’s limit.

A parametric study of electrothermal flow inside an AC EWOD droplet

A fully coupled electrothermal flow model is developed to simulate the electrothermal flow inside a droplet under AC electro-wetting on dielectric (EWOD) with a needle–planar-electrode configuration. The variations of electrothermal flow velocity and droplet temperature with frequency are compared with a classical electrothermal model and a previous experimental study. The investigated frequency range is extended to include dielectrophoretic (DEP) force dominant regime, and differences of characteristics of velocity and temperature fields in the Coulomb and DEP force dominant regimes are discussed and compared. Effects of solution concentration and needle electrode position and size are also examined. The results show that peak frequency to achieve the maximum electrothermal flow, underestimated by the classical model is correctly predicted by the present fully coupled model. After peak frequency, the velocity decreases till the valley frequency where the minimum electrothermal flow can be achieved, and then increases again, and finally keeps almost unchanged. After the valley frequency, the rotation of internal flow changes from counterclockwise to clockwise. When frequency is smaller, the smaller the solution concentration, the larger the electrothermal flow, while when frequency is larger, the result is opposite. The peak frequency and its corresponding electrothermal flow velocity increase with the increase of solution concentration. The needle electrode size has no influence on peak frequency, but the maximum electrothermal flow velocity at the peak frequency increases with decreasing electrode size. The smaller is the distance of needle electrode from the substrate, the larger are the peak frequency and the corresponding maximum electrothermal flow velocity. The numerical model developed and the results obtained in this study are useful for the design of droplet based electrothermal flow mixer in microfluidics.

μPIV-Analysis of Taylor flow in micro channels

Micro particle imaging velocimetry (μPIV) has been used to characterize two-phase flows in micro channels. Phase internal flow in large segments as well as in small segments has been investigated for the contribution of interface friction to the formation of the internal flow field. According to experimental conditions (flow rate, volume ratio of the phases, channel path geometry, viscosity), the decisive contribution of either liquid/wall or liquid/liquid interface friction to the phase internal flow changes locally. This results in partially alternating internal flow directions between micro droplets and separation medium. For internal flow field evaluation, algorithms of PIV image analysis have been extended for algorithms of droplet recognition, mapping and transformation into a single coordinate system before displacement analysis. Simulations by computational fluid dynamics (CFD) are in good agreement with the measured flow fields.