Micromixing efficiency in static mixer

Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

This study investigated the heat transfer and fluid flow characteristics of laminar and turbulent regimes in microchannels using a combined experimental and numerical approach. The objective of the study was to evaluate the influence of Reynolds number on thermal and hydraulic performance and to assess the applicability of conventional heat transfer correlations at microscale dimensions. An experimental setup consisting of a heated microchannel test section, controlled flow delivery system, and precision temperature and pressure measurement instruments was employed to obtain data for a wide range of Reynolds numbers covering both laminar and turbulent flow regimes. Complementary computational fluid dynamics simulations were conducted using appropriate governing equations and boundary conditions to predict velocity, temperature, and pressure fields within the microchannel. The results showed that the Nusselt number increased with Reynolds number in both flow regimes, with turbulent flow exhibiting significantly higher heat transfer rates than laminar flow. However, the turbulent regime was also associated with substantially higher friction factors and pressure drops, indicating increased pumping power requirements. The experimental and numerical results demonstrated close agreement, validating the reliability of the numerical model. The study further revealed that microscale effects such as entrance length influence and enhanced convection caused deviations from classical theoretical predictions developed for macroscale channels. Overall, the findings established that optimal microchannel thermal performance was achieved at moderate turbulent Reynolds numbers, where heat transfer enhancement outweighed hydraulic penalties. The study provided valuable insights for the design and optimisation of microchannel heat exchangers used in electronics cooling, energy systems, and other high heat flux applications.

Low Reynolds number k-ϵ model is employed to demonstrate the Bejan's numerical heat and mass flow visualization in turbulent boundary layer regime. The governing turbulence kinetic energy and its dissipation rate equations are included along with conservation equations. The turbulent flow equations are highly nonlinear and coupled in nature, hence, an unconditionally stable Crank–Nicolson finite difference approach is deployed. The computer-generated heatlines, masslines, streamlines, and isotherms for different control parameters are analyzed in the turbulent flow regime. Average momentum, thermal, and concentration diffusion rates are increased upon the increment in in laminar regime and are decreasing with rising in turbulent regime. The stream, heat and masslines deviate more in turbulent regime when compared to laminar regime. Further, the kinetic energy and dissipation rate contours are deviated more in turbulent heat and mass flow regime for different values of . Mainly, in this research paper, the authors made an attempt to demonstrate the turbulent heat and mass flow visualization about a vertical plate using boundary layer approximations through LRN k-ϵ turbulence model.

Free flow above permeable domains is a common problem in both natural and industrial environments. The classical problem of turbulent flows above forests and urban areas, known as the canopy flow problem (Finnigan 2000), was addressed by hundreds of publications. A library search for the keywords ‘canopy and flow’ in the ISI database returned 1312 results in April 2007 and 1845 results in April 2009, about 40% increase in the last 2years alone. In fact, this search underestimates the actual number of publications since research of flow above permeable domains covers a much wider range of scenarios. Vegetated flows in rivers and wetlands, flow above gravel streambeds, and flow above coral reefs are a few environmental examples; industrial examples include flows in coating processes, chemical reactors, and electronic cooling; and examples of a much smaller length scale include flows above bio-films and inside bio-tissues. Studies of flows over permeable domains include both laminar and turbulent flow regimes. Where most of the porous media studies focus on laminar and creeping flows, terrestrial, ocean, and atmospheric flows are all turbulent. Combinations such as turbulent channel flows above laminar flow inside dense gravel beds are also common. Most of the past and present research activities are often classified by the following four disciplines: porous media flows (e.g., Whitaker 1999), atmospheric canopy flows (e.g., Finnigan 2000), vegetated flow (e.g., Nepf 1999), and fluvial flows. Studies of fluvial flows were recently addressed in a special issue on double averaging applications (Nikora and Rowinski 2008). The following special issue with its 13 publications covers both laminar and turbulent flow regimes while emphasizing on fundamental questions of formulating the transport conditions at the interface between the fluid and the porous domains. The most famous publication on laminar flows above porous interfaces is the study by Beavers and Joseph (1967) (cited 771 times, ISI, April 2009). By investigating the flow above porous domains, they developed a linear relationship between the velocity gradient above the interface and the interface slip velocity. However, numerous studies showed that the Beavers and Joseph condition is not general enough and a coupled solution of both the free flow and

Local heat transfer to impinging liquid jets in the initially laminar, transitional, and turbulent regimes

A visualization of thermal diffusion in a horizontal pipe flow is conducted to elucidate the axial diffusion characteristics of two fluids of differing temperatures, particularly those flows between the laminar and turbulent flow regimes. Tracer particles are used to detect the thermal diffusion interface as well as to measure streamwise velocity profiles. We found that the Hagen–Poiseuille parabolic velocity profile established a long axial diffusion length that drives thermal convection in the pipe cross-sectional plane. This situation results in a 3D interface at Reynolds number, Re = 600, and a vertically stratified interface at Re = 1900. In turbulent flows at Re = 5700, an anti-diffusion spot appears in the front section before undergoing rapid mixing under turbulence. From the visualized particle images, the apparent axial coefficients of diffusion were measured and compared with the friction properties of the pipe. Furthermore, particle image velocimetry (PIV) explored the two-way coupling mechanism of between the temperature and the velocity within the thermal diffusion process.

Efficient production of hydrogen (H2) derivatives, such as e-methanol, is essential to achieving carbon-neutrality targets. In the present computational fluid dynamics study, we investigate synthetic methanol (CH3OH) production via direct carbon dioxide (CO2) hydrogenation in a milli-duct with a rectangular cross section. The duct involves catalytic wall reactions with CO2 and H2 as reactants. Direct numerical simulations are carried out at bulk Reynolds numbers of Reb= 100, 500, 1100, and 2200 spanning laminar and turbulent flow regimes. We investigate a high catalyst loading, corresponding to gas hourly space velocity values of 0.086, 0.44, 0.96, and 1.93 m3/(kgcat h) at these Reynolds numbers, respectively, to study the impact of turbulence on methanol production in the limit of fast, transport-limited chemistry. The numerical results consist of the following features: (1) In the turbulent case (Reb=2200), the reaction rate reaches local minima/maxima in the turbulent ejection/sweep structures observed in the proximity of the catalytic walls, respectively. (2) Turbulent mixing may significantly enhance methanol production under transport-limited chemistry, increasing the methanol mass flow rate at the outlet up to 56.4% compared to laminar flow at a matched Reynolds number. (3) The single-pass conversion efficiency of CO2 and the methanol yield across the duct decrease monotonically as the Reynolds number increases (lower residence time). In contrast, the methanol mass flow rate at the outlet increases as Reb increases (higher reactant inflow).

Thermographic flow visualization is a contactless, non-invasive technique to visualize the boundary layer flow on wind turbine rotor blades, to assess the aerodynamic condition and consequently the efficiency of the entire wind turbine. In applications on wind turbines in operation, the distinguishability between the laminar and turbulent flow regime cannot be easily increased artificially and solely depends on the energy input from the sun. State-of-the-art image processing methods are able to increase the contrast slightly but are not able to reduce systematic gradients in the image or need excessive a priori knowledge. In order to cope with a low-contrast measurement condition and to increase the distinguishability between the flow regimes, an enhanced image processing by means of the feature extraction method, principal component analysis, is introduced. The image processing is applied to an image series of thermographic flow visualizations of a steady flow situation in a wind tunnel experiment on a cylinder and DU96W180 airfoil measurement object without artificially increasing the thermal contrast between the flow regimes. The resulting feature images, based on the temporal temperature fluctuations in the images, are evaluated with regard to the global distinguishability between the laminar and turbulent flow regime as well as the achievable measurement error of an automatic localization of the local flow transition between the flow regimes. By applying the principal component analysis, systematic temperature gradients within the flow regimes as well as image artefacts such as reflections are reduced, leading to an increased contrast-to-noise ratio by a factor of 7.5. Additionally, the gradient between the laminar and turbulent flow regime is increased, leading to a minimal measurement error of the laminar-turbulent transition localization. The systematic error was reduced by 4% and the random error by 5.3% of the chord length. As a result, the principal component analysis is proven to be a valuable complementary tool to the classical image processing method in flow visualizations. After noise-reducing methods such as the temporal averaging and subsequent assessment of the spatial expansion of the boundary layer flow surface, the PCA is able to increase the laminar-turbulent flow regime distinguishability and reduce the systematic and random error of the flow transition localization in applications where no artificial increase in the contrast is possible. The enhancement of contrast increases the independence from the amount of solar energy input required for a flow evaluation, and the reduced errors of the flow transition localization enables a more precise assessment of the aerodynamic condition of the rotor blade.

We investigate both laminar and turbulent flow regimes in a rough-walled rock fracture via numerical CFD simulations. While previous studies were limited to either fully viscous Darcy or inertial Forchheimer laminar flow regimes, we chose to cover the widest possible Reynolds number range of 0.1–10^6. We introduce CFD simulation of a turbulent flow for rough-walled fractures, implementing RANS approach to turbulence modeling. We focus on 2D fracture geometries and implement changes in both shear displacement and wall roughness, systematically examining their effect on fracture permeability and friction factor in a manner similar to the fundamental studies of the flow in rough-walled pipes. For a curvilinear fracture, laminar flow becomes non-stationary between Re , {sim } , 10^2–10^3, earlier for larger shear displacement and wall roughness. Laminar–turbulent transition starting at Re_{textrm{cr}} , {sim } , 2300 may lead to a sharp drop in permeability depending on the fracture geometry; this gap vanishes for larger shear displacement and wall roughness. Depending on the fracture geometry, bottlenecks closing with shear become a major negative factor for the overall permeability.

Entropy generation due to natural convection has been calculated for a wide range of Rayleigh number (Ra) in both laminar (104 ≤ Ra ≤ 108) and turbulent (1010 ≤ Ra ≤ 1012) flow regimes, for diameter ratio of 2 ≤ D/d ≤ 5, for an isothermal vertical cylinder fitted with annular fins. In the laminar regime, the entropy generation was predominantly caused by heat transfer (conduction and convection) and the viscous contribution was negligible with respect to heat transfer. But in the turbulent regime, entropy generation due to fluid friction is significant enough although heat transfer entropy generation is still dominant. The results demonstrate that the degree of irreversibility is higher in case of finned configuration when compared with unfinned one. With the deployment of a merit function combining the first and second laws of thermodynamics, we have tried to delineate the thermodynamic performance of finned cylinder with natural convection. So, we have defined the ratio (I/Q)finned/(I/Q)unfinned. The ratio (I/Q)finned/(I/Q)unfinned gets its minimum value at optimum fin spacing where maximum heat transfer occurs in turbulent flow, whereas in laminar flow the ratio (I/Q)finned/(I/Q)unfinned decreases continuously with the increase in number of fins.

Two-scale model for quantifying the effects of laminar and turbulent mixing on algal growth in loop photobioreactors

Numerical simulation of planar hydraulic fracture propagation with consideration to transition from turbulence to laminar flow regime

An analysis is made for thermophoretic transport of small particles through a free-convection boundary layer adjacent to a cold, vertical deposition surface. The gas-particle, boundary layer equations are solved numerically for both laminar and turbulent flow. The numerical results indicate that, for a fixed set of boundary conditions and physical properties, the particle concentration at the wall in the laminar flow is very close to that in turbulent flow. A simple expression is suggested relating the particle transport rate to the heat transfer coefficient for the laminar and turbulent flow regimes.

Experimental study and predictions of pressure losses of fluids modeled as Herschel–Bulkley in concentric and eccentric annuli in laminar, transitional and turbulent flows

An improved prediction of frictional pressure drop using an accurate shear rate equation for high-yield stress drilling fluids at the surface and downhole conditions