Lateral Secondary Flow Research Articles

Application of microreactor constructed with micro-mixing channel featuring 3D lateral secondary flow structure for ultra-small particle size preparation of magnetic chitosan nanodrug carriers

Magnetic chitosan nanoparticles serve as reliable carriers for drug delivery, but conventional macroscopic synthesis methods are associated with a number of disadvantages. Efficient micromixing is a crucial aspect of the controlled synthesis of nanoparticles. However, enhancing the mixing efficiency of micromixers at low Reynolds numbers (Re) remains a significant challenge. Fortunately, the addition of lateral structures to the microchannel sidewalls has been demonstrated to be an effective method for enhancing mixing performance at low Re. In this work, the secondary flow effect of 3D lateral structure was numerically studied, and the reason why the multi-dimensional secondary flow effect can effectively promote mixing was analyzed. A high-efficiency microreactor (3D(S)-SWM) was constructed by combining the spherical (S) 3D lateral structure with the square wave (SW) channel. In comparison to other microreactors, 3D(S)-SWM exhibits a shorter full mixing cycle and a smaller pressure drop. Magnetic chitosan nanoparticles were prepared in one step by using the 3D(S)-SWM. The introduction of the 3D lateral structure enables the rapid preparation of magnetic chitosan nanoparticles at low Re. The production of a product with a particle size as low as 18–26 nm, while maintaining superparamagnetic. The nanoparticles prepared can effectively load drugs and achieve sustained drug release. This has significant implications for the targeted transport of drug carriers and their metabolism in vivo. It is anticipated that 3D(S)-SWM will expand the scope of drug delivery applications and facilitate the treatment of diseases.

Influence of mixed dimples on aerodynamic performance of high-load compressor cascade

In this study, the effect of mixed dimples on the aerodynamic performance of a high-load compressor cascade, named National Advisory Committee for Aeronautics 65-K48, at different attack angles was numerically and experimentally investigated. Mixed dimples with four columns and eight rows were arranged at 10% to 32% of the chord length of the half-blade height. The performance parameters of the cascade outlet section and vortex structure inside the cascade were exploited. The results reveal that the boundary layer is disturbed, and the boundary layer transition occurs ahead of time, induced by the mixed dimples. The anti-separation ability of the fluid was enhanced, and the separation bubbles on the suction surface were eliminated. Meanwhile, the lateral secondary flow was inhibited, and the intensities of the passage and concentrated shedding vortices were consequently reduced. Compared with the prototype cascade, the total pressure loss of the dimpled cascade at all attack angles was reduced. The total pressure loss can be reduced by nearly 20% at –9°.

A 3D homogeneous microreactor with high mixing intensity at wide Re range for MOF preparation and POCT application

Most microreactors can only work under high Re or extremely low Re conditions to obtain enough mixing efficiency, the inefficient reaction intensity undoubtedly limits its application scenarios. In this study, a novel 3D homogeneous microreactor (3D-HM) was reported that demonstrate a strong mixing intensity over a wide range of Re conditions. The 3D geometries of shunt and lateral secondary flow structure were optimized in details, and then were added to the Zigzag mixing channel to promote mixing index (MI) (over 94 % from Re = 0–10, and more than 98 % for Re > 10). Compared with the traditional T-shaped, Split and Recombine micromixer, a shorter mixing distance (Lmix) and mixing time (τmix) were needed to realize fully mixing. The 3D-HM was used for the preparation of Zeolitic imidazolate framework (ZIF-8), the particle size, yield and drug loading capacity of ZIF-8 prepared using the 3D-HM under a milder Re conditions were better than those prepared by traditional and existing microfluidic methods under a harsh Re conditions. The promotion to diffusion mixing with 3D-HM channel was then verified using the enzymatic catalytic reaction driven by syringe pump at low Re and capillary pump, respectively. Enzyme and substrates in the laminar flow in the 3D-HM are more likely to bind each other than that in the T-shaped diffusion mixing. In addition, the laminar diffusion mixing process was scaled up by microfluidic channel design, so that the results of enzymic catalytic reaction was visualized for Point-of-care Testing (POCT) application. The MI, Lmix and τmix directly determine the homogeneous microreaction intensity. The proposal of the 3D-HM may provide more possibilities for the application of micromixing-based reactors.

The effects of swirling purge leakage on aerothermal performance of flat and non-axisymmetric endwalls

The relative rotation of turbine rotor determines the velocity triangle of the purge flow, which significantly influences the aerothermal performance of turbine endwalls. In this paper, a turbine cascade model is constructed to simulate the relative motion effect between the rotor endwall and the purge leakage. The effects of the swirl ratio of the purge leakage on the aerothermal performance of two endwall schemes (flat endwall and non-axisymmetric endwall) are investigated and compared. The results show that due to the relative motion, the purge leakage's flow direction and migration trajectory shift towards the blade suction side. The cooling jets tend to push the boundary layer towards the suction side, thus enhancing the lateral secondary flows. At a low MFR, with the decrease of swirl ratio, both endwalls' film cooling effectiveness is gradually reduced, with the aerodynamic performance and heat transfer coefficient changing very little. At a high MFR, with the decrease of swirl ratio, the aerodynamic loss and heat transfer coefficient of both endwalls are gradually increased, while the endwall film cooling effectiveness is gradually decreased. Globally, compared to the flat endwall, the non-axisymmetric endwall can dramatically enhance the film cooling effectiveness while effectively reducing the aerodynamic loss and heat transfer levels for all swirl ratios and MFRs investigated.

The Steady Wake of a Wall-Mounted Rectangular Prism with a Large-Depth-Ratio at Low Reynolds Numbers

The wakes of wall-mounted small (square) and large (long) depth-ratio rectangular prisms are numerically studied at Reynolds numbers of 50–250. The large depth-ratio significantly alters the dominance of lateral secondary flow (upwash and downwash) in the wake due to the reattachment of leading-edge separated flow on the surfaces of the prism. This changes the wake topology by varying the entrained flow in the wake region and changing the distribution of vorticity. Thus, the magnitude of vorticity significantly decreases by increasing the prism depth-ratio. Furthermore, the length of the recirculation region and the orientation of near wake flow structures are altered for the larger depth-ratio prism compared to the square prism. Drag and lift coefficients are also affected due to the change of pressure distributions on the rear face of the prism and surface friction force. This behavior is consistently observed for the entire range of Reynolds numbers considered here. The wake size is scaled with Re1/2, whereas drag coefficient scaled with Re−0.3.

Study on the Horizontal Distribution Law of Flood Water and Sediment Factors under the Effect of Vegetation on a Curved Beach

The sediment-laden floodplain flood is affected by beach vegetation and the shape of curved compound channels. The laws of water and sediment exchange and deposition distribution in beach troughs are very complex and play a significant role in the formation and development of secondary suspended rivers, the adjustment of beach horizontal gradients, and even the evolution of flood control situations. This study used a combination of experimental simulations and theoretical research to carry out a generalized model test of floodplain flood evolution, analyzing the transverse distribution characteristics of sediment-laden flow and sediment factors in a curved compound channel under the conditions of beach vegetation, proposing a theoretical model of transverse distribution of velocity and sediment concentration that is based on the momentum equation considering the inertial force of the lateral secondary flow and river curvature. The results showed the following: (1) The model test results for floodplain flood in the compound channel with curved vegetation showed that the main stream was not only concentrated in the main channel but also appeared near the foot of the left and right bank levees and formed flood discharge along the embankment, as the beach siltation was mainly concentrated in the beach lip; (2) The arrangement of full vegetation on the beach had a uniform effect on the velocity distribution of the beach, which can reduce the phenomenon of excessive velocity at the foot of the beach and increase the velocity effect in the main channel; and (3) Through five numerical examples, the lateral velocity distribution model of a curved compound channel with beach vegetation was tested and, in general, the analysis model was consistent with the experimental results. The research results will provide a theoretical basis for river management and have great significance for enriching the basic theory of water and sediment movement and promoting the integration of hydraulics, river dynamics, and ecology.

Numerical analysis on non-uniform flow and heat transfer of supercritical cryogenic methane in a heated horizontal circular tube

This paper numerically studied non-uniform flow and heat transfer of supercritical cryogenic methane in a heated horizontal circular tube. Numerical results indicated that the peak value of heat transfer coefficient appeared near the vicinity of pseudo-critical point. The Reynold number monotonically increased with the augment of bulk fluid temperature, however, the Prandtl number exhibited both peak and trough values. Compared with mass fluxes, operating pressures had less influence on the heat transfer performances. Moreover, the flow stratification phenomenon and lateral secondary flows were developed as a result of the combined interaction of buoyancy and gravity. The “flow acceleration” caused by the variation of bulk fluid density had important effect on the decrease of heat transfer capacity. Further analysis demonstrated that the Blasius and Han models were suggested to predict the pressure drop and heat transfer of supercritical cryogenic methane.

Numerical analysis on nonuniform heat transfer of supercritical pressure water in horizontal circular tube

A numerical simulation is conducted in this paper aiming to further reveal the nonuniform heat transfer mechanism of supercritical pressure water in horizontal circular tube. The governing equations are solved by finite volume method, and the renormalization group k-ε turbulence model (RNG k-ε model) with enhanced wall treatment is utilized to predict the coupled wall-to-fluid heat transfer. A good agreement between the numerical results and experimental data indicates high accuracy and reliability of the numerical method. On this basis, the nonuniform heat transfer characteristics of supercritical water and corresponding mechanism are analyzed, and the lateral secondary flow and the criteria for onset of buoyancy effect on heat transfer are also discussed. The numerical results exhibit that: (1) the RNG k-ε turbulence model with enhanced wall treatment is recommended; (2) the strong circumferential nonuniformity in heat transfer exists in horizontal tube due to the flow asymmetry, and the variation of heat flux and pressure can bring on a remarkable change in heat transfer; (3) the lateral secondary flow results from the buoyancy and thermal acceleration, and the interactive influence between secondary flow and thermophysical properties of supercritical water exerts significant effects on heat transfer; and (4) the criterion of Jackson–Hall Gr/Re2.7 accurately evaluates the onset of buoyancy effect and predicts heat transfer deterioration for horizontal flow of supercritical water.

An analytical model for lateral depth-averaged velocity distributions along a meander in curved compound channels

This paper presents an analytical method for modeling the lateral depth-averaged velocity distribution along a half-meander in a curved compound channel. An equation is derived from the momentum equation and the flow continuity equation which contains a velocity term with both streamwise velocity variation and lateral secondary flow variation. A velocity variation parameter is proposed in the main channel and on the floodplain for a series of test sections. To study the validity of these equations experiments were conducted in a large scale meandering compound channel at Sichuan University, China. Based on the experimental data, the generation mechanism of secondary flow in the main channel along half a meander is analyzed. It is shown that the secondary current is enhanced by the centrifugal force and the floodplain flow. Due to the discontinuity of the flow depth and the effect of meandering in the main channel flow, a region divided method is adopted. A new boundary condition is proposed by introducing the angle between the main channel flow and the floodplain flow, and it is shown that this gives better modeling results in cross-over sections. The modeling results indicate that the proposed method, which uses the new boundary condition and includes both the streamwise velocity variation and the lateral secondary flow variation, can model the lateral depth-averaged velocity distributions more accurately. Finally, variations in the velocity term between the main channel and floodplain are discussed and analyzed.

A study on the characteristics of upward air–water two-phase flow in a large diameter pipe

An adiabatic upward co-current air–water two-phase flow in a vertical large diameter pipe (inner diameter, D: 0.2 m, ratio of pipe length to diameter, L/ D: 60.5) was experimentally investigated under various inlet conditions. Flow regimes were visually observed, carefully analyzed and classified into five, i.e. undisturbed bubbly, agitated bubbly, churn bubbly, churn slug and churn froth. Void fraction, bubble frequency, Sauter mean diameter, interfacial area concentration (IAC) and interfacial direction were measured with four-sensor optical probes. Both the measured void fraction and the measured IAC demonstrated radial core-peak distributions in most of the flow regimes and radial wall peak in the undisturbed bubbly flow only. The bubble frequency also showed a wall-peak radial distribution only when the bubbles were small in diameter and the flow was in the undisturbed bubbly flow. The Sauter mean diameter of bubbles did not change much in the radial direction in undisturbed bubbly, agitated bubbly and churn bubbly flows and showed a core-peak radial distribution in the churn slug flow due to the existence of certain amount of large and deformed bubbles in this flow regime. The measurements of interfacial direction showed that the main and the secondary bubbly flow could be displayed by the main flow peak and the secondary flow peak, respectively, in the probability density function (PDF) of the interfacial directional angle between the interfacial direction and the z-axis, η zi . The local average η zi at the bubble front or rear hemisphere ( η zi F and η zi R ) reflected the local bubble movement and was in direct connection with the flow regimes. Based on the analysis, the authors classified the flow regimes in the vertical large diameter pipe quantitatively by the cross-sectional area-averaged η zi at bubbly front hemisphere ( η zi F ¯ ). Bubbles in the undisturbed bubbly flow moved in a vertical way with some swerving motions and those in other flow regimes moved along the lateral secondary flow with an averaging net upward velocity.