1,720,983 research outputs found
Performance Evaluation Criteria for Triangular Microchannels with Smoothed Corners
Full text linkThe manufacturing capabilities available today for microchannels make it possible to produce in a comparatively easy, quick and cheap way several cross-sections, possibly allowing modifications of the macro-geometry. Another way is smoothing the corners of polygona cross-sections: this eliminates low-gradient areas and increases transport phenomena, i.e. frictional losses and heat transfer. Several ways of assessing the relative performance of the new shape in comparison to the original have been suggested over the years, among which are Performance Evaluation Criteria (PEC), as proposed by Bergles and Webb. PEC are based on a first-law analysis and aim at extremising the thermal power, heat exchange area, inlet temperature difference or pumping power under varying constraints. In this work equilateral triangular microchannels with uniform wall temperature are considered, through which a liquid flows in fully-developed, steady laminar regime. The cross-sectional area has its corners progressively rounded, and the velocity and temperature profiles are determined, in order to compute the Poiseuille, Nusselt and Stanton numbers, which are then employed in computing the objective functions for some PEC
Numerical Modeling of Anti-Icing Operations Over Super-Hydrophobic Surfaces
No full textHydrophobic and super-hydrophobic coatings are currently considered promising tools to enhance the performances of thermal devices for in-flight icing protection. Icing protection is mostly obtained via heating of the critical surfaces: the simulation is quite complex, including the need for CFD solution of the flow around the wing, including the tracking of the cloud supercooled droplet, the simulation of the evolution of the interaction between droplets and airfoil (including coalescence into rivulets and continuous film, evaporation due to the de-icing heat flux, freezing on impact or later...). However, current commercial numerical prediction tools for in-flight icing simulation are based on the Messinger model [1], although extended to 3-D arbitrary surfaces [2], coupled with Lagrangian or Eulerian [3] droplet flow field analysis. Messinger's model assumes that the runback water layer is a continuous film driven by the shear stress and provides reliable and accurate results in several applications. Unfortunately, it intrinsically neglects any wettability effect and thus cannot assess hydrophobic coating performances. The main difficulty in the simulation of such coatings is that it involves an inherently multi-scale problem: wettability operates at a small scale, at most of the order of the single impinging droplet, but the local impinging mass flow, shear stresses, and heat transfer convective coefficients require state-of-the-art CFD computation around a whole wing or a whole aircraft. Since it is not practical to manage the large-scale computations with a grid fine enough to resolve the single droplet evolution (e.g., via VOF approach), a kind of intermediate bridge is required to derive average integral corrections from the small scale and transfer them to a coarser grid for the standard CFD solution of the large scale thermal and flow field. Here, a hierarchical approach is followed: first, a high-fidelity, small-scale model defines statistical distributions of relevant properties (rebounding droplet fractions, average runback water velocities, wet area fractions, coalesced droplet diameter distribution, including heat transfer and phase changes) as a function of the local fluid dynamic conditions; as a second step, statistical correlations are extracted by this small-scale computation campaign, providing integral corrections to the larger-scale CFD simulation. The high-fidelity simulation tool, an individual-based droplet phenomenological model, was described and validated in [4], and here is improved with regard to the phase change modeling (for both the ice beads freezing and the droplet evaporation due to de-icing system heat flux). The present work is then focused on the statistical models derived by the high- fidelity results, with special attention to the distribution of the small scale droplets (i.e., below the radius where coalescences become the main growth effect), following the approach described in [5] under different conditions. Such small droplets are of special interest for the antiicing devices, which ideally should operate mostly under such regime, allowing for quick evaporation and minimization of runback water rivulets. The statistical model is coupled with a CFD simulation of the heat and fluid flow around an airfoil under icing condition and a lagrangian droplet tracing code, allowing for thoughtful final validation versus literature experimental data provided in [6]. Finally, a parametrical analysis is conducted commenting on the usefulness of hydrophobic coatings of different properties for anti-icing operations under various environmental conditions. It demonstrates that it may offer a valuable aid in obtaining dry clean surfaces with no or little runback water with a relatively low energy consumption
Optimal geometrical duct configurations for microchannel heat sinks under constraints
No full textThis work aims at studying the influence of smoothing the corners of microchannels of rectangular cross-section on their thermal performance. A laminar, steady incompressible flow is assumed with thermal boundary conditions of type H1 (uniform axial heat flux and uniform temperature on the heated perimeter of each cross-section), and several aspect ratios are considered, from unity (square channel) to 0.03 (thin slot). The analysis is carried out in terms of performance evaluation criteria, namely the case FG1a (maximize heat duty) and entropy generation minimization. Four geometrical constraints are applied to the geometry (fixed reference length, fixed heated perimeter, fixed cross-sectional area and fixed hydraulic diameter) and the results are commented both separately and through a combination of the outcomes of first- and second-law analysis. It turns out that smoothing of the corners is beneficial in terms of entropy generation when the cross-sectional area or the heated perimeter are constrained, whereas it worsens -although only slightly - the heat duty. The best results are obtained for high aspect ratios and high reference irreversibility ratios
Thermal Transport in the Entry Region of a Microchannel in the Presence of Electro-osmotic Flow
Full text linkThe motion of a polar fluid in microducts induced by an external electric field,
known as electro-osmotic flow, enables fluid circulation without the need for mechanical
devices. This feature makes it particularly appealing for the thermal management of electronic
components, as microchannels with almost any cross-sectional shape can be easily integrated
on the chips. Therefore, it is essential to evaluate how the channel’s geometry influences heat
transfer performance. This study investigates the thermal entry region and fully developed
electro-osmotic flow in a microchannel with a rectangular cross-section and smoothed corners,
with one adiabatic wall and uniform temperature elsewhere. The paper proposes correlations for
the Poiseuille and Nusselt numbers, taking into account the aspect ratio and non-dimensional
smoothing radius under fully developed thermal and hydrodynamic conditions, which can be
valuable for practical design purposes. The study also emphasizes how Joule heating may lead
to the reversal of heat flow and how the thermal entry length is linearly dependent on the
logarithm of the non-dimensional Joule heating in thermally developing flow. Additionally, it
demonstrates that smoothing the corners may increase the local Nusselt number over sharp
corners, but it may also shorten the thermal entry length
Numerical simulation of dropwise condensation over hydrophobic surfaces using vapor-diffusion model
No full textDropwise condensation of humid air over hydrophilic and hydrophobic surfaces is numerically investigated using a phenomenological, Lagrangian model. Mass flux through droplets free surface is predicted via a vapor-diffusion model. Validation with literature experimental data is successfully conducted at different air humidities and air velocities. The accuracy of the implemented condensation model is compared with a standard analogy between convective heat and mass transfer, showing that the latter is not able to predict heat transfer performances in the investigated air velocity range
Numerical analysis of surface coatings performances for in-flight icing device performance enhancement
No full textRecent developments in manufacturing techniques offer a wide range of high-performance coatings designed to improve heat and mass transfer processes. Superhydrophobic surfaces may enhance the performances of anti and de-icing devices for in-flight ice applications. Numerical modeling of the effect of such devices requires the analysis of the behavior of an ensemble of individual droplets, rather than assuming a continuous film as in standard icing codes. Here an individual-based method (IBM) is adopted. Droplets are assumed as the smaller element of the simulations, and phenomenological sub-models describe their behavior through impact on the surface, possible rebound, growth via coalescence or phase change (either freezing or evaporation), transition from still droplet to moving ones, the transition from running droplets to rivulets. The model is validated versus literature experimental data, showing the capability of the approach. It appears that hydrophobicity is not yet enough to build a passive anti-icing system but can significantly enhance the performances of typical thermal devices
Numerical simulation of film instability over a corrugated sheet
Full text linkThe evolution of a liquid layer flowing down a corrugated sheet driven by gravity, which is the characteristic configuration of structured packing, is numerically analysed via the solution of the governing lubrication equation, which reduces the 3D physical problem to a 2D mathematical problem. Disjoining pressure is used to model contact line dynamics and surface wettability, while full implementation of capillary pressure allows to investigate contact angles up to 60°. The effect of corrugation is introduced via the definition, in the governing lubrication equation, of non-uniform gravity acceleration. Furthermore, the additional capillary force, arising from variations in the solid surface curvature, is also implemented. Different corrugation geometries and flow conditions, the latter being defined by the reference film Capillary number, are investigated, looking for configurations that allow enhancement of the liquid-gas interface area at low liquid flow rate. Such an analysis provides a novel approach in the design of structured packing, widely used in carbon capture via chemical absorption
Numerical modelization of contact angle hysteresis of falling droplet under enhanced lubrication approximation
Full text linkMoving contact lines are involved in several engineering applications: in in-flight icing phenomenon, the eventual transition from droplet to rivulet or continuous film regime is crucial for the prediction of ice accretion over the aircraft surface; absorption process through structured packing is also characterized by a thin film flowing over the corrugated sheets. Disjoining pressure together with the assumption of a thin precursor film is largely used in numerical simulations of thin films and moving droplets in order to model the dynamics of moving contact lines and the surface wettability properties, in terms of imposed static contact angle. The disjoining pressure model was largely validated in case of falling films with the well known Voinov-Hoffman-Tanner law. On the other side, the capability of the disjoining pressure to model the contact angle hysteresis, which is a crucial parameter for predicting moving droplets behavior, has not been discussed yet. Here, numerical simulations of both falling films and moving droplets under lubrication approximation are conducted and the disjoining pressure model is used to predict the contact line dynamics. After verification of the full curvature implementation for a 1D falling film, the effective contact angle hysteresis is estimated for a moving droplet under different flow conditions and the transition from droplet to rivulet regime detected
Bifurcation analysis of liquid films over low wettability surfaces
Full text linkThin liquid layer evolution over a solid substrate and film instability phenomena are involved in a number of engineering applications: in chemical absorption through structured packing, the corrugated sheets are covered by the liquid solvent, offering an enhanced interface surface between the solvent and the gas solute; in coating process, the liquid pattern influences the resulting coating quality; in condensation over finned dehumidifier, heat transfer performances are influenced by the evolving liquid layer, which may arrange as a droplets population or an ensemble of rivulets. Here, the evolution of a liquid layer flowing down an inclined plate bounded by lateral walls, which is the simplest configuration describing hydrodynamics inside structured packing, is numerically investigated. An in-house code, previously developed and largely validated in case of film instability and rivulet buildup, is used in order to solve governing lubrication equations. The full implementation of capillary pressure allows to simulate contact angles up to 60 . Film break is observed due to instability induced by lateral walls, if the imposed liquid flow rate exceeds a critical value, leading to the formation of a rivulet pattern. Fixing the size of the investigated physical domain, the number of observed rivulets, which strongly influences the resulting wetted area, is traced as a function of the flow characteristics (identified by the Bond number), the substrate wettability and the liquid properties; the corresponding bifurcation diagram is presented
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