Increasing Reynolds Number Research Articles

Influence of different geometrical dimple configurations on flow behaviour and thermal performance within a 3D circular pipe

Numerical analysis has been performed to evaluate the heat transfer characteristics and per-formance of a circular pipe with geometrical dimple patterns. Using computational fluid dy-namics (CFD) codes, we examine the effects of geometrical configurations on the flow and thermal behavior of circular pipes with concavity (dimple) diameters. Fluid mixing and flow perturbation are facilitated by perforations across the pipe core and wall regions, thereby im-proving thermal efficiency. In addition, a concavity with a diameter of 4 mm enhances heat transfer. Based on the results of the study, the disrupted pipe wall and pipe core region pro-duce swirls and transverse vortices in the flow that provide superior heat transfer compared to conventional (smooth) pipes. In an increasing Reynolds number (Re), mixing, secondary, and separation flows become larger. Performance evaluation factor (PEF) values increased at low Reynolds numbers when dimple diameter was 1mm. As a result of these improved pipes, heat exchanger efficiency may improve in industrial applications, a key factor for energy con-servation.

Hydrothermal analysis of archimedean spiral single and dual channel heat sink for CPU cooling

Engineering modeling and simulation represent a predominantly design tool in the modern manufacturing industry in which the actual system is reproduced using a mathematical and scientific model. This requires CPUs with higher computational capacities. However, increas-ing the computational capacities of CPU and GPU imposes challenges in the cooling process due to space limitations. CPU liquid cooling system has attracted more interest as an efficient heat dissipation tool. This work presents computational modeling of the conjugate heat and flow for the CPU liquid heat sink cooling. An Archimedean spiral channel is grooved into the cold plate of the heat sink. Single and dual channel passes are used in this work. The out-er diameter of the cold plate is 105 mm and the channel depth is 5 mm for both single and dual-channel configurations. The conjugate heat sink model was constructed to have four different domains: CPU (alumina), glue layer (ethoxy), cold plate (copper), and liquid cool-ant (water). To incorporate the effect of turbulence, the flow rate varied to cover a range of Reynolds number from 3000 up to 15000 at a constant inlet temperature of 25 °C. The used turbulence model was the Shear Stress Transport (k-ω) to better capture the viscous, high-fre-quency flow fluctuation in the near-wall region. The bottom surface of the CPU is subjected to 450 W of heat energy. The results showed that the channel configuration and Reynolds number have a decisive impact on controlling the CPU temperature. The CPU temperature decrease as Reynolds number increases, however, the pressure drop increases at an exponen-tial rate. These findings are supported by Darcy–Weisbach equation for internal flow in which the pressure drop depends on the square of the average fluid velocity and it was noticed that the pressure drop in the dual channel was three times higher than that in the single channel. The hydrothermal performance of the Archimedean spiral channel rapidly decreased with Reynolds number and the single-channel had a slightly better performance compared with the dual-channel.

Turbulent Heat Transfer of Hybrid Nanofluid with Nano-Encapsulated Phase Change Material in a Square Channel

Investigation of turbulent mixed convection of different working fluids, namely, pure water, CuO/water nanofluid, n-octadecane/water nanofluid (or slurry), and a hybrid water-based suspension of CuO and n-octadecane nanoparticles in a uniformly heated vertical square channel is carried out. The shear stress transport/kappa-omega model is applied to simulate the turbulent flow. Eulerian–Lagrangian two phase model and homogeneous model are employed to simulate the nanofluid flow. It is shown that Eulerian–Lagrangian two-phase model predicts higher Nusselt number and friction factor than single-phase model and the results of two-phase model are closer to experimental data. It is found that a new hybrid nanofluid with dispersed nano-encapsulated phase change material particles provides better thermal and hydrodynamic performances compared to other nanofluids. Slurry containing 5% n-octadecane provides an improvement of nearly 37.2%, while hybrid nanofluid (1% CuO + 5% n-octadecane) offers added convective heat transfer enhancement of almost 60%. With increasing volume fraction, friction factor becomes slightly larger than that of pure water. For 5% n-octadecane/water and 1% CuO + 5% n-octadecane/water nanofluids, friction factor increases by 2.5% and 1.7%, respectively. Due to dominance of thermal entropy generation, total entropy generation reduces by increasing Reynolds number and volume fraction.

Numerical Simulation of Graphene-Nanoplatelet Nanofluid Convection in Millimeter-Sized Automotive Radiator

The depletion of fossil fuels and environmental considerations in transportation sector motivate the researchers to enhance the efficiency and performance of the automotive systems. However, the poor thermal performance of conventional coolant poses a limitation in the development of energy-efficient vehicle due to the cooling constraint. In the present study, a comprehensive numerical study is conducted to scrutinize the convective performance of graphene nanoplatelets (GnP) nanofluid in millimeter-sized automotive radiator, aiming to enhance the understandings on the underlying physical significance of the suspension of graphene-based nanoparticle in water for the performance enhancement of automotive radiator. The temperature-dependent thermophysical properties of GnP-water nanofluid is predicted via existing correlations, while a modified viscosity correlation is developed. ANSYS Fluent is employed in the present numerical simulation to investigate the effects of various pertinent parameters such as Reynolds number, nanoparticles aspect ratio, tube aspect ratio and tube hydraulic diameter on the heat transfer performance of the radiator. Double precision and second-order upwind scheme with inclusion of viscous heating, and convergent criteria of 10˗6 are adopted for the present simulation. It is observed that the convective performance of the radiator is significantly enhanced by increasing Reynolds number and nanoparticle volume fraction while decreasing the aspect ratios of nanoparticle and radiator tube, with an enhancement rate as much as 1816%. Therefore, it is evident that the suspension of GnP intensifies the heat transfer performance of millimeter-sized automotive radiator, which could possibly lead to a more efficient radiator that is smaller and lighter.

An Experimental Study of Heating Heavy Fuel Oil by Hot Air using Helical Fins in a Double-Pipe Heat Exchanger

In present work, the effect of helical fins with height 20 mm and pitch 100 mm on the performance of the double pipe heat exchange investigated experimentally. The heat exchanger had been chosen as a heat recovery device in bricks factory to recover heat from exhaust gases to preheat the heavy fuel oil. Air had been chosen as a hot gas instead of furnace exhaust gases because of the similar thermal properties and it flows in the inner pipe with different velocity (20, 27, and 34) m/s and inlet temperature (200, 225) °C while, heavy fuel oil enters shell side with counter flow at mass flow rates (0.06, 0.08, and 0.1) kg/s and constant inlet temperature of 40 °C. Both of pipes and helical fins are made from stainless steel. The experimental results show that increasing Reynolds number from 139139 to 333934 led to increase heat transfer rate, oil outlet temperature, overall heat transfer coefficient and effectiveness of heat exchanger with (8%, 4%, 5% and 5%) respectively while, increasing oil mass flow rate from (0.06 to 0.1) kg/s cause decreasing oil outlet temperature and effectiveness of with (10% ,14%) respectively. The results shown the double pipe heat exchanger with helical fins produce heat transfer rate and oil outlet temperature higher than smooth double pipe heat with average 15% and 8.6%, as well as increasing Nusselt number, Overall heat transfer coefficient and effectiveness of heat exchanger with average 26%,15% and 13% respectively. The results clear that, for 225°C exhaust air and velocity 27 m/s, the application of double pipe with helical fins by preheat heavy fuel oil could reduce the viscosity and reduce electric power required and save 1.69 MW annually for one factory.

An experimental and numerical investigation of thermofluidic properties on the novel curved microchannel heat sink with slotted section

The current work proposes the development of a novel curved microchannel heat sink by modifying the existing flat microchannel heat sink for enhancing the heat dissipation in the electronic device without significant pressure drop penalty. The inspiration for the curved channel is taken from a NACA airfoil. The microchannel comprises two sections: a curved section and a slotted straight section. Experimental and numerical investigations are performed on the proposed geometry to study the thermofluidic properties for Reynolds number (Re) ranging from 200 to 500 using DI water as working fluid. Fluid flow and heat transfer are analysed by carrying out a full domain numerical simulation in ANSYS Fluent 19.0. The thermofluidic properties of the curved microchannel heat sink (CMCHS) and straight microchannel heat sink (SMCHS) are compared numerically by keeping the geometrical parameters (hydraulic diameter, foot area and fin area) and inlet parameters (inlet velocity and heat flux) identical. The average heat transfer coefficient (havg) of CMCHS is enhanced over SMCHS due to the formation of vorticities in the slotted region owing to the fluid intermixing and boundary layer disruption. The presence of curved section leads to reducing pressure drop penalty pertaining to gravitational effects. Together, the curved section and slotted section enhance the performance factor of CMCHS over SMCHS. The vortices formed increase as the Reynolds number increases, enabling an increase in the heat transfer coefficient. Therefore, the performance factor also increases as the Reynolds number is increased. An enhancement of ≈ 67.6 % in havg is achieved for Re = 500 in CMCHS compared to SMCHS with DI water as the working fluid.

Theoretical and numerical analyses of turbulent plane Couette flow controlled using uniform blowing and suction

We theoretically and numerically investigate the control effect of the uniform blowing and suction (UB/US) in a turbulent plane Couette flow under a constant velocity difference condition in terms of the net energy saving. First, the effective range of UB/US amplitude is derived theoretically. According to the theoretical analysis, the effective range of UB/US amplitude decreases as the Reynolds number increases. The upper bound of the ratio between UB/US amplitude and the wall velocity, Vwc/Uw, is about 0.45% even at very low Reynolds numbers, i.e., Rew∼1000, while it is less than 0.1% at practically high Reynolds numbers, i.e., Rew>107. Next, using a direct numerical simulation (DNS), we investigate the control effect of UB/US in a turbulent plane Couette flow at Rew=3200. In the cases with smaller UB/US amplitude, i.e., Vw/Uw≤0.1%, little control effect can be observed. In the cases with larger UB/US amplitude, i.e., Vw/Uw≥0.3%, although not only a significant decrease of the contribution from the Reynolds shear stress (RSS) but also relaminarization of flow field can be observed, the total drag is increased due to the increase of the contribution from the UB/US beyond the decrease of the contribution from the RSS. Based on the present study, we can draw a conclusion that overall drag reduction by UB/US is hardly possible for turbulent Couette flows.

Electrohydrodynamic peristaltic microfluidic flow with thermal and molecular analysis featuring sloped configurations

This study examines the simultaneous impacts of thermal and molecular transfer of non-Newtonian fluid propelled by the collective impact of electroosmosis and peristaltic pumping via inclined channel. Internal frictional heating and heat generation are also taken into consideration. The electrohydrodynamic phenomenon is described by the nonlinear Poisson–Boltzmann equation. The problem is modulated mathematically through a system of coupled non-linear differential equations in wave frame of reference. Using the regular perturbation method around a small wave number δ, we obtain sequential solutions for stream function ψ, electric potential ϕ, velocity u, pressure gradient dpdx, temperature θ and concentration Ω distributions. The influences of newly arising parameters such as the electroosmotic parameter me, relaxation time parameter λ1, retardation time parameter λ2, wave number δ, Reynold’s number Re, heat generation parameter β, Prandtl number Pr, Schmidt number Sc, Froude number Fr and Soret number Sr on physical characteristics are analyzed graphically. There is an inverse correlation between the parameters of relaxation time and retardation time and their effects on axial velocity, concentration, and temperature distributions. In particular, the temperature of the fluid increases as the EDL parameter, relaxation time parameter, viscous dissipation parameter, and Reynolds number increase, but decreases as the retardation time parameter increases. Moreover, the axial pressure gradient increases as Reynolds number and inclination angle increase, while it decreases as Froude number increases. The current model has applicability in the chemical industry, reservoir engineering, micro-fabrication, and biomedical applications, where electroosmotic effects on heat and mass movement are crucial.

Effects of Blockage Ratio on the Spatiotemporal Dynamics of Turbulent Flow Separation Around a Square Cylinder at Moderate Reynolds Numbers

Abstract The effects of blockage ratio (BR) on turbulent flows around square cylinders at moderate Reynolds numbers are investigated using a time-resolved particle image velocimetry (TR-PIV). The blockage ratios range from 2.5% to 15%, and the Reynolds numbers based on the freestream velocity and cylinder thickness are 3000, 7500, and 15,000. The flow dynamics are examined in terms of the mean flow, Reynolds stresses, frequency spectra, reverse flow area, and proper orthogonal decomposition (POD). The results show that the wake characteristics are nearly independent of the Reynolds number and blockage ratio. Spectral analyses of the velocity fluctuations demonstrate that the von Kármán (VK) shedding frequency is independent of the Reynolds number and blockage ratio, however, the Kelvin–Helmholtz (KH) frequencies increase with increasing Reynolds number and blockage ratio. The probability density function of the reverse flow area shows unimodal and bimodal distributions for the lower (BR ≤ 5%) and higher (BR ≥ 10%) blockage ratios, respectively, and the mean reverse flow area and its standard deviation decrease with increasing blockage ratio. The results also show that the contributions from the first POD mode pair to the total energy increase with blockage ratio but independent of the Reynolds number. The POD mode coefficients show significant cycle-to-cycle variation at lower blockage ratios, suggesting that the energetic structures are comparatively less organized at lower blockage ratios. The spectra of the velocity fluctuations, reverse flow area, and POD mode coefficients all show dominant peaks at the fundamental shedding frequency.

On the stability of a pair of vortex rings

The growth of perturbations subject to the Crow instability along two vortex rings of equal and opposite circulation undergoing a head-on collision is examined. Unlike the planar case for semi-infinite line vortices, the zero-order geometry of the flow (i.e. the ring radius, core thickness and separation distance) and by extension the growth rates of perturbations vary in time. The governing equations are therefore temporally integrated to characterize the perturbation spectrum. The analysis, which considers the effects of ring curvature and the distribution of vorticity within the vortex cores, explains several key flow features observed in experiments. First, the zero-order motion of the rings is accurately reproduced. Next, the predicted emergent wavenumber, which sets the number of secondary vortex structures emerging after the cores come into contact, agrees with experiments, including the observed increase in the number of secondary structures with increasing Reynolds number. Finally, the analysis predicts an abrupt transition at a critical Reynolds number to a regime dominated by a higher-frequency, faster-growing instability mode that may be consistent with the experimentally observed rapid generation of a turbulent puff following the collision of rings at high Reynolds numbers.

An experimental investigation on the aerodynamic characteristics and vortex dynamics of a flying wing

AbstractIn this paper, we present a detailed experimental investigation mainly on the vortical flow fields and the associated vortex breakdown phenomena over a non-slender flying wing (sweep angle, ${\rm{\Lambda }}$ = 53°). In the process, the aerodynamic coefficients were also determined using a six-component force balance. Surface oil flow visualisation, surface pressure measurements and particle image velocimetry (PIV) measurements, in various crossflow planes and in a longitudinal plane passing through the leading-edge vortex core, were carried out at various Reynolds numbers to understand the flow field over the non-slender flying wing. Aerodynamic characteristics of the flying wing show local peaks and valleys in the pitching moment coefficient. The surface flow visualisation reveals that the nonlinearity of the pitching moment curve is due to the complex nature of vortical flow structures. The flow visualisation also demonstrates the presence of a wave-like surface pattern, and its size is found to reduce with increasing Reynolds numbers. The present PIV measurements confirm that this wave-like surface pattern is associated with vortex breakdown phenomena. These measurements also reveal that the vortex breakdown has not reached the apex of the wing, even at post-stall angle-of-attack. For pre-stall ( $\alpha $ = 20°) flow regimes, it is observed that the location of the vortex breakdown moves downstream as the Reynolds number increases, but this influence is minimised at near-stall ( $\alpha $ = 25°) and post-stall ( $\alpha $ = 30°) flow regimes. Reconstructed velocity field using the first 10 dominant proper orthogonal decomposition (POD) modes reveals that the nature of the vortex breakdown over the flying wing is a spiral-type vortex breakdown.

Effect of fluid elasticity and shear thinning on heat transfer from a heated sphere at moderate Reynolds numbers

The drag and heat transfer characteristics of an isothermal sphere in a FENE-P-type viscoelastic fluid have been studied in the steady axisymmetric flow regime. The governing equations, namely, mass, momentum, and energy equations, together with FENE-P type viscoelastic constitutive equations, have been solved using the finite volume method based opensource CFD code OpenFOAM over the following ranges of conditions: Reynolds number, 1 ≤ Re ≤ 100 ; Prandtl number, 1 ≤ Pr ≤ 50 ; Weissenberg number, 0 ​ ≤ Wi ≤ 10 and polymer extensibility parameter, 10 ≤ L 2 ≤ 500 for a fixed value of the polymer viscosity ratio of β = 0.5 . At low Reynolds numbers, fluid viscoelasticity causes an upward and downward shifting in the axial velocity profiles with reference to that in a Newtonian fluid along the upstream and downstream axis of the sphere, respectively. However, such a shift progressively diminishes with the increasing Reynolds number. Furthermore, a “velocity overshoot” and/or a “negative wake” is also observed in a viscoelastic polymer solution at low Reynolds numbers, and this is accentuated with the increasing Weissenberg number. The separation of boundary layers is seen to be suppressed with the increasing Weissenberg number. The drag coefficient decreases with the Reynolds number irrespective of the fluid type and the corresponding dependence on the Weissenberg number is found to be non-monotonic depending upon the values of Re and L 2. The average Nusselt number always increases with the increasing values of Re, Pr, and Wi and with the decreasing value of L 2. A limited number of simulations have also been carried out to show the effect of the temperature-dependent viscosity of the fluid on the flow dynamics and heat transfer characteristics. Finally, simple correlations for the drag coefficient and average Nusselt number are presented which not only capture the functional dependence but also can be used for the interpolation of the present results for the intermediate values of the governing parameters in a new application.

PARAMETRIC STUDY AND DEVELOPMENT OF A CORRELATION FOR NUSSELT NUMBER IN NANOFLUID JET IMPINGEMENT

Numerical simulations for nanofluid ( water with Al<sub>2</sub>O<sub>3</sub> nanoparticles) jet impinging perpendicularly on a flat circular heated p late have been performed. A heated p late is subjected to constant heat flux boundary condition. A two-phase modified mixture mo del was used for the prediction of heat transfer coefficient, and comparisons are made with standard mixture model. Present results for average Nusselt number are validated with experimental data available in the literature. Though a standard mixture model predicted heat transfer with accepted accuracy, it was found that accuracy of modified mixture model is better (around 5% improvement) compared to standard mixture model. Thereafter, parametric study was performed considering nozzle exit Reynolds number (Re), spacing ratio (<i>H/D</i>), nanoparticle volume fraction (φ), and nanoparticle diameter (dp) on heat transfer prediction. The results reveal that particle diameter in the range 10-100 nm has no effect on the Nusselt number, Furthermore, heat transfer increased with increasing Reynolds number and volume fraction. However, spacing ratio shows first increasing and then, decreasing trend (similar to a log-normal distribution curve) in the prediction of heat transfer. Finally, a new correlation was developed for Nusselt number using nonlinear regression analysis. In the correlation, a two-phase multiplier was used, which is the ratio between two-phase Nusselt number (Nu<sub>nf</sub> ) and single-phase Nusselt number (Nu<sub>sp</sub>). The simplified correlation is found to predict data with maximum error of 8.9%, average error of 2.74% and <i>R</i><sup>2</sup> = 0.955.

Исследование сжимаемых ламинарных потоков в трехмерном прямоугольном канале

The problem of the flow of compressible laminar gas flows through a three-dimensional channel with a sudden expansion in the shape of a square parallelepiped is formulated. The Navier-Stokes equations are solved. When solving equations, spatial derivatives are approximated with a high order of accuracy. The pseudo-implicit right-hand side algorithm is used to integrate equations over time. The computational process is parallelized using MPI technology and is carried out on the Uran supercomputer. The range of Reynolds numbers from 100 to 600 is considered. The changes in the coordinates of attachment points in various planes depending on the Reynolds number are shown. With increasing Reynolds number, the magnitudes of small separation zones are first increasing and then decreasing, while large zones only increase. The field of the vortex vector is analyzed for different Reynolds numbers. The center of rotation shifts, and disturbances penetrate further into the channel. The profiles of the longitudinal velocity at various points of the channel are presented. A tendency towards a symmetrical state is revealed for all Reynolds numbers considered in the work.

EXPERIMENTAL STUDY OF HEAT TRANSFER EFFECTS DUE TO IMPINGING SLOT JETS CONFINED BY INCLINED PLATES

In this study, the effects of heat transfer in turbulent flow fields of impinging slot jet confined by inclined plates were investigated experimentally. Temperature distributions on the impingement plates were obtained with thermal camera. Temperature measurements were carried out on the impingement plate in four different impinging slot jet flow setups confined by plates with inclination angles of θ = 0°, 15°, 30°, and 45°, with an interplate spacing of 0.5 ≤ <i>H/W </i>≤ 6, and a Reynolds number range of 10,000 ≤ Re ≤ 30,000. The effects of confinement plate inclination angle, intervals between the plates, and Reynolds number on the Nusselt number distributions of the impingement plates were examined. It was observed that Nusselt number values increased with increasing Reynolds number and decreased with increasing intervals between the plates. There are secondary top points in the Nusselt number distributions on the impingement plate for low inclined confinement plates (θ = 0° and θ = 15°) and small interplate spacings (<i>H/W</i> = 0.5 and <i>H/W</i> = 1), while there is an increase in the Nusselt numbers at the ends of the impingement plate for high inclined confinement plates (θ = 30° and θ = 45°) and small interplate spacings (<i>H/W</i> = 0.5 and <i>H/W</i> = 1).

USING A NONUNIFORM MAGNETIC FIELD TO ENHANCE HEAT TRANSFER BEFORE A SUDDEN COMPRESSION IN A 2D MILLI-CHANNEL

The advancement of electronic devices has made heat dissipation challenging, but heat convection shows promise as a solution. However, obstacles like resistors in the way of a straight channel can slow the flow and weaken heat transfer, particularly where the horizontal and vertical walls meet at sudden compression. This study examines numerically using nonuniform magnetic fields to enhance thermal energy transfer in the mentioned critical regions in a sudden compression inside a two dimensional milli-channel. The study includes placing single or multiple dipoles either beneath the lower heated wall (where the compression occurs) or over the upper wall. The effects of number, the longitudinal and vertical locations of dipoles, and the inlet flow Reynolds number are examined. The ferrofluid used in this study is EMG-805. The findings of this study demonstrate that heat transfer improves when single or multiple dipoles are positioned downstream of the step wall on the heated wall. The location of dipoles is critical. For example, the best minimum local Nusselt number (70.7% increase with respect to the base case) is achieved when the single dipole is at <i>a</i> = 49 mm (from the inlet). Increasing the number of dipoles in the thermally weak region improves heat transfer. For instance, by having three dipoles of equal strength in that area, the minimum local Nusselt number is enhanced by 90.1%, resulting in a Nusselt number before the step that surpasses the Nusselt number immediately after the step. Furthermore, as the Reynolds number increases, the effects of the magnetic field disappear.

Effect of Nozzle Shape on Fluid Flow and Heat Transfer characteristics of an Impinging Jet System – A Numerical Study

Impinging jet is one of the most efficient techniques to achieve a high heat transfer coefficient and is used in many engineering applications. The present study focuses on the effect of nozzle shape on fluid behavior and heat transfer characteristics. For the current investigation, circular, square, rectangular, and elliptical nozzles with identical hydraulic diameters are used with Reynolds number Re ranging from 15,000–35,000. The circular nozzle results are validated with the published numerical and experimental data. In the current study, it is found that as the Reynolds number increases, the value of the averaged Nusselt number increases in all circumstances. When examining the different nozzle shapes, the value of the averaged Nusselt number is higher when an elliptical nozzle is used. The contours of the surface Nusselt number and velocity streamlines are also presented. The contour shows that the heat flux is highest in the stagnation zone and gradually decreases to the sides because they are outside the impingent coverage. Moreover, the area between the jets has a low heat flux. The heat transfer in the impinging zone is initially raised as the jet-induced crossflow increases and achieves a peak value, and then reduced stream-wise because of the crossflow effect.

Effects of cavitation and hydraulic flip on liquid film formed by jet impinging on the wall

The technology of the liquid film formed by jet impinging on the wall is widely applied in the aerospace, steel quenching, and cleaning. In this paper, the spreading and evolution of the liquid film are experimentally studied. The effects of the cavitation and hydraulic flip on the film are examined, and it is identified that they are a serious problem of the nozzle design. Results demonstrate that the jets formed by using a nozzle with 120° contraction angle and 3.5 mm outlet length sequentially produce the cavitation and hydraulic flip as the Reynolds number increases. Small contraction angle or long outlet length promotes the stability of the discharge coefficient and jet states and inhibits the occurrence of the cavitation and hydraulic flip. For the flip jet, the jet cross section is axially switched. Several patterns of the liquid film, such as the gravity flow, gravity flow with dry patch formation, rivulet flow with outward streams, and outward flow with triple rivulets, etc., are observed as the jet regime and inclination angle change. Particularly, for the film formed by the cavitation jet, the rivulets and dry patches emerge in the tail of the film; meanwhile, a lot of splashing droplets are generated. For the film generated by the flip jet, the bifurcation of the film shapes occurs. An impressive flow feature is that the two sprays are formed when the flip jet impinges on the wall, which is caused by the collision of the fluids in the secondary impingement zones.

Inertial migration of polymer micelles in a square microchannel.

Using a hybrid simulation approach that combines a lattice-Boltzmann method for fluid flow and a molecular dynamics model for polymers, we investigate the inertial migration of star-like and crew-cut polymer micelles in a square microchannel. It is found that they exhibit two types of equilibrium positions, which shift further away from the center of the microchannel when the Reynolds number (Re) increases, as can be observed for soft particles. What differs from the behaviors of soft particles is that here, the blockage ratio is no longer the decisive factor. When the sizes are the same, the star-like micelles are always relatively closer to the microchannel wall as they gradually transition from spherical to disc-like with the increase of Re. In comparison, the crew-cut micelles are only transformed into an ellipsoid. Conversely, when the hydrophobic core sizes are the same, the equilibrium position of the star-like micelles becomes closer to that of the crew-cut micelles. Our results demonstrate that for polymer micelles with a core-shell structure, the equilibrium position is no longer solely determined by their overall dimensions but depends on the core and shell's specific dimensions, especially the hydrophobic core size. This finding opens up a new approach for achieving the separation of micelles in inertial migration.

Computational analysis on fluid flow characteristics in a modified annular dump combustor of a ship

The improvement of systems of aeronautical and marine propulsion able to meet the forthcoming drastic certification requirements on the reduction of the level of pollutants emission heavily relies on the development of new concepts of combustion chamber technologies. In view of this, a novel numerical investigation has been performed on the basis of characteristics of the flow of a fluid passing over a simple annular-shaped dump combustor and a modified dump combustor with certain central restrictions like rectangular and spherical shapes. The restrictions are located at a fixed distance of 0.14 m from the inlet. Flow characteristics and patterns are presented sequentially in the form of streamline contours, velocity contour, pressure contour, axial velocity profiles, and centerline velocity profiles. The above-mentioned flow characteristics are studied at several locations in the flow field for various Reynolds numbers (Re), ranging from 103 to 106 for all considered models. The flow regulating mass and momentum conservation partial differential governing equations along the x-axis and y-axis are discretised with the help of the control volume-based finite volume approach. The SIMPLE algorithm and second-order upwind scheme are used to solve iteratively the discretised equations for the case of a fixed aspect ratio of 3 and for a central abstraction of 120 %. The analysis corroborates and concludes that more numbers of bubbles are formed in a geometrically modified dump combustor than in a plain dump combustor. Additionally, it has been found that the number of recirculating bubbles increases along with the value of the Reynolds number. Consequently, the core recirculation zone is improved, which ensures the desirable mixing of fluids in the ship combustor and helps significantly in the reduction of NOx emission and combustion residuals. Furthermore, as the Reynolds number (Re) increases, the variation in axial velocity also raises for all considered Reynolds numbers, indicating an improved chance of better mixing. At a Re of 106, the axial velocity profile reaches its peak for both the modified combustors, although at different locations. Specifically, the magnitude of the axial velocity profile is 1.05 at the position of 0.14 m for the rectangular restriction and 0.10 m for the spherical restriction. Consequently, a higher Reynolds number may be recommended for optimal mixing.