Showing papers by "Marco Marengo published in 2020"

Droplet Impact on Suspended Metallic Meshes: Effects of Wettability, Reynolds and Weber Numbers

Abstract: Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh.

How Wettability Controls Nanoprinting.

Abstract: Using large scale molecular dynamics simulations, we study in detail the impact of nanometer droplets of low viscosity on flat substrates versus the wettability of the solid plate. The comparison between the molecular dynamics simulations and different macroscopic models reveals that most of these models do not correspond to the simulation results at the nanoscale, in particular for the maximal contact diameter during the nanodroplet impact (${D}_{\mathrm{max}}$). We have developed a new model for ${D}_{\mathrm{max}}$ that is in agreement with the simulation data and also takes into account the effects of the liquid-solid wettability. We also propose a new scaling for the time required to reach the maximal contact diameter ${t}_{\mathrm{max}}$ with respect to the impact velocity, which is also in agreement with the observations. With the new model for ${D}_{\mathrm{max}}$ plus the scaling found for ${t}_{\mathrm{max}}$, we present a master curve collapsing the evolution of the nanometer drop contact diameter during impact for different wettabilities and different impact velocities. We believe our results may help in designing better nanoprinters since they provide an estimation of the maximum impact velocities required to obtain a smooth and homogenous coverage of the surfaces without dry spots.

Numerical investigation of quasi-sessile droplet absorption into wound dressing capillaries

Abstract: The key concept in wound dressing design and development is the fact that keeping a wound moist accelerates healing. Therefore, the selection of the appropriate wound dressing type is vital. The absorption of wound exudate by wound dressings can be considered as a microfluidic phenomenon that can be investigated either by performing high resolution laboratory experiments or by utilizing high resolution Computational Fluid Dynamics numerical simulations. As an initial step, in the present paper, the effects of the pore size (wound dressing porosity), the liquid (wound exudate) viscosity, and the initial droplet diameter are numerically investigated using a simplified analog of the phenomenon that consists of a quasi-sessile droplet being absorbed by a single cylindrical pore. For this purpose, an enhanced Volume Of Fluid model, developed in the general context of OpenFOAM, is validated and applied. It is found that distinct droplet absorption rates exist with specific relationships derived using best-fit lines that can predict the absorption rates for particular values of pore size and liquid viscosity. For the examined Eo and Oh number ranges (0.0015 < Eo < 0.15 and 0.0035 < Oh < 0.095), these distinct droplet absorption rates are directly linked with four different droplet evolution regimes that are grouped in a well-defined flow map. Finally, it is shown that the resulting liquid absorption rates are not significantly affected by the initial droplet diameter and that an appropriate wound dressing porosity can be selected by an estimation of the wound exudate physical properties.

Experimental study of thermal performance in a closed loop pulsating heat pipe with alternating superhydrophobic channels

Abstract: A pulsating heat pipe (PHP) with alternating hydrophilic/superhydrophobic channels was tested at vertical position, bottom heat mode and different heat power inputs. The device consists in a copper tube (internal/external diameters of 3.18/4.76 mm), bent into a planar serpentine of ten channels and five U-turns. The tube is partially functionalized with a superhydrophobic coating, in such a way to create an alternation of hydrophilic and superhydrophobic surfaces on the straight tubes along the loop, in the condenser zone. The aim is to investigate how the alternated wettability affects the start-up, the fluid motion along the tubes and the overall thermal performance of the device, which is compared to another PHP, having the same geometry and under the same working conditions, but completely hydrophilic. Distilled water, at 50% filling ratio is the working fluid. Power inputs varying from 20 to up to 350 W, in a stepwise increasing and decreasing fashion, are applied to the PHP. The condenser temperature is kept constant, at 20 °C. The device is monitored by sixteen thermocouples and one pressure transducer, mounted in contact to the fluid in the condenser region. Data analysis shows that the alternating wettability of tube sections strongly affects the flow motion, the start-up and the overall performance. In general, the alternating PHP presents a worse overall thermal performance: the thermal resistance is always higher and the start-up is achieved at higher heating power levels. Temperature at superhydrophobic surfaces exhibit a flat trend, suggesting that the flow was blocked in the functionalized area surface, while, for the hydrophilic inserts, more pronounced temperature fluctuations are observed. It is believed that the superhydrophobic coating hinders the liquid film formation, decreasing locally the flow motion. On the other hand, the enhancement of the inner wettability improves the flow motion, as the liquid film that covers the inner surface acts as lubricant.

Heat Transport Capacity of an Axial-Rotating Single-Loop Oscillating Heat Pipe for Abrasive-Milling Tools

Abstract: In order to enhance heat transfer in the abrasive-milling processes to reduce thermal damage, the concept of employing oscillating heat pipes (OHPs) in an abrasive-milling tool is proposed. A single-loop OHP (SLOHP) is positioned on the plane parallel to the rotational axis of the tool. In this case, centrifugal accelerations do not segregate the fluid between the evaporator and condenser. The experimental investigation is conducted to study the effects of centrifugal acceleration (0–738 m/s2), heat flux (9100–31,850 W/m2) and working fluids (methanol, acetone and water) on the thermal performance. Results show that the centrifugal acceleration has a positive influence on the thermal performance of the axial-rotating SLOHP when filled with acetone or methanol. As for water, with the increase of centrifugal acceleration, the heat transfer performance first increases and then decreases. The thermal performance enhances for higher heat flux rises for all the fluids. The flow inside the axial-rotating SLOHP is analyzed by a slow-motion visualization supported by the theoretical analysis. Based on the theoretical analysis, the rotation will increase the resistance for the vapor to penetrate through the liquid slugs to form an annular flow, which is verified by the visualization.

A Novel Loop Heat Pipe Based Cooling System for Battery Packs in Electric Vehicles

Abstract: A novel cooling method for Electric Vehicles battery modules by means of Loop Heat Pipe and graphite sheets is proposed. The Loop Heat Pipe is a passive two-phase system and as such it reduces the parasitic power consumed by the EV thermal management. A validated lumped parameter mathematical model has been created describing the thermo-fluid-dynamic problem and used to simulate the performance of the cooling system during highway driving and ultra-fast charging conditions. The numerical predictions show a clear potential to contain the cells’ temperature below 40°C even during ultra-fast charging, with a 3.3K peak temperature reduction in comparison to a conventional liquid cooling method. Moreover, this system adds only 8% of the battery pack mass and it shows potential parasitic power reductions of one order of magnitude.

A fast response performance simulation screening tool in support of early stage building design

Abstract: This paper aims to introduce the development of a simplified dynamic building energy simulation screening tool aimed to support early stage building design and feasibility studies while considering the lack of resources typical of those stages of the design process. The structure and main characteristics of the tool are discussed, providing insight on how they can benefit the integrated design process of more sustainable buildings. Results generated by the tool are shown to be comparable with the results of the simplified models, proving to be useful in the integration of energy aspects during the initial stages of building design.

Wettability Effect On Flow Boiling Characteristics Within Micro- passages

Abstract: A numerical investigation on the effect of wettability characteristics on a single bubble growth during saturated flow boiling conditions within a microchannel, is conducted in the present paper. The numerical simulations are conducted with the open-source toolbox OpenFOAM, utilising a user-enhanced Volume OF Fluid (VOF) solver. The proposed solver enhancements involve a treatment for spurious velocities dampening (a well-known defect of VOF methods), an improved dynamic contact angle treatment to accurately account for wettability effects as well as the implementation of a phase-change model in the fluid domain, accounting for conjugate heattransfer with a solid domain. The predictions of the simulations show that the local Nusselt number (Nu) is more depended on wettability characteristics for low heat fluxes, and less dependent on higher heat fluxes. In more detail, it seems that the local, instantaneous heat transfer coefficient is higher for super-hydrophilic cases in comparison to hydrophilic. However, as the applied heat flux increases, hydrophilic and super-hydrophilic cases show a similar heat transfer enhancement with respect to the single-phase heat transfer in the considered micro-channel. Finally, superhydrophobic cases, show lower heat transfer performance with respect to the single-phase case. This is due to the fact that a vapour blanket is rapidly formed immediately after the nucleation, acting as an insulator of the heated solid surface.

Effect of Channel Aspect Ratio on Flow Boiling Characteristics within Rectangular Micro-passages

Abstract: A numerical investigation on the effect of channel aspect ratio on a single bubble growth during saturated flow boiling conditions within square microchannels, is conducted in the present paper. The open-source toolbox OpenFOAM is applied for the simulations, utilising a user-enhanced Volume OF Fluid (VOF) solver. The solver enhancements include a treatment for spurious velocities dampening, the implementation of an improved dynamic contact angle sub-model for accurate prediction of wettability effects as well as the implementation of a phase-change model in the fluid domain, accounting for conjugate heat-transfer with a solid domain. It is shown that the variation of the aspect ratio of a microchannel has a significant effect in the local heat transfer coefficient, due to an enhancement of the single-phase heat transfer in combination with the alteration of the underpinned bubble growth dynamics, which result in different contributions of contact line versus liquid film evaporation.