Penetration characteristics of a liquid droplet impacting on a narrow gap:Experimental and numerical analysis

Experimentalists are limited in the amount of information they can derive from drop impact experiments on porous surfaces because of the short timescales involved and the normally opaque nature of porous materials. Numerical simulations can supplement experiments and provide researchers with previously unattainable information such as velocity and pressure profiles, and quantification of fluid volume flow rates into the pores. Ethanol drops, 2.0 mm in diameter, are impacted on a narrow gap at Weber numbers that match the impact of water drops, also 2.0 mm in diameter, on the same gap size in a previous study. The experiments show the ethanol drops cleaving at all Weber numbers tested, while the water drops completely enter the gap at low Weber numbers and only cleave at higher Weber numbers. A volume of fluid numerical model of the experiments is constructed in OpenFOAM and used to probe the interior of the drops during impact. For the water drop, a high-pressure region fills the drop during impact which continuously drives liquid into the gap. For the ethanol drops, the high-pressure region is smaller and quickly attenuates, which results in a near-zero vertical velocity at the entrance of the gap. Compared to water, the lower surface tension of ethanol causes these drops to spread further upon impact, recoil less, and overall have less liquid over the gap, which promotes cleaving. Against a superficial thought, when the penetration of liquids into porous materials is to be maximized, a higher surface tension liquid is therefore desirable

Numerical investigation of liquid film instabilities and evaporation in confined oscillating slug-plug flows.

An enhanced volume of fluid (VOF)-based numerical simulation framework that accounts for conjugate heat transfer between solid and two-phase flow regions and phase-change due to boiling/condensation, is utilised in order to investigate the effect of flow oscillation amplitude and frequency on the liquid film evaporation and instability formation in slug-plug flows within heated channels, in saturated flow boiling conditions. Various series of parametric numerical simulations are performed, for different values of flow oscillation amplitude and frequency for a variety of working fluids. For one of the working fluids two different channel diameters are also tested. The oscillations in each case are induced by applying an oscillating pressure boundary condition at the inlet of the channel, keeping the pressure constant at the outlet, after an initial period of constant pressure drop between the inlet and the outlet. Capillary ridges that are initiated at the liquid film, in the vicinity of the leading edge of the considered vapour slugs, are identified as a result of the imposed oscillations, which are translated in the form of capillary waves towards the rear end of the bubbles. It is shown that the formation frequency as well as the geometric characteristics of the generated ridges, are directly related to the corresponding frequency and amplitude of the induced flow oscillations. Furthermore, it is shown that in the initial stages of the bubble fate after the application of the oscillations liquid film evaporation is enhanced with the increase of the oscillation amplitude while it degrades as the frequency of the oscillation becomes higher. However, for large oscillation amplitudes and channel diameters, liquid jets penetrate into the elongated bubbles leading in a lot of cases to bubble break-up

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

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

An Enhanced VOF Method Coupled with Heat Transfer and Phase Change to Characterise Bubble Detachment in Saturated Pool Boiling

The present numerical investigation identifies quantitative effects of fundamental controlling parameters on the detachment characteristics of isolated bubbles in cases of pool boiling in the nucleate boiling regime. For this purpose, an improved Volume of Fluid (VOF) approach, developed previously in the general framework of OpenFOAM Computational Fluid Dynamics (CFD) Toolbox, is further coupled with heat transfer and phase change. The predictions of the model are quantitatively verified against an existing analytical solution and experimental data in the literature. Following the model validation, four different series of parametric numerical experiments are performed, exploring the effect of the initial thermal boundary layer (ITBL) thickness for the case of saturated pool boiling of R113 as well as the effects of the surface wettability, wall superheat and gravity level for the cases of R113, R22 and R134a refrigerants. It is confirmed that the ITBL is a very important parameter in the bubble growth and detachment process. Furthermore, for all of the examined working fluids the bubble detachment characteristics seem to be significantly affected by the triple-line contact angle (i.e., the wettability of the heated plate) for equilibrium contact angles higher than 45°. As expected, the simulations revealed that the heated wall superheat is very influential on the bubble growth and detachment process. Finally, besides the novelty of the numerical approach, a last finding is the fact that the effect of the gravity level variation in the bubble detachment time and the volume diminishes with the increase of the ambient pressure

Penetration characteristics of a liquid droplet impacting on a narrow gap: Experimental and numerical analysis

Experimentalists are limited in the amount of information they can derive from drop impact experiments on porous surfaces because of the short timescales involved and the normally opaque nature of porous materials. Numerical simulations can supplement experiments and provide researchers with previously unattainable information such as velocity and pressure profiles, and quantification of fluid volume flow rates into the pores. Ethanol drops, 2.0 mm in diameter, are impacted on a narrow gap at Weber numbers that match the impact of water drops, also 2.0 mm in diameter, on the same gap size in a previous study. The experiments show the ethanol drops cleaving at all Weber numbers tested, while the water drops completely enter the gap at low Weber numbers and only cleave at higher Weber numbers. A volume of fluid numerical model of the experiments is constructed in OpenFOAM and used to probe the interior of the drops during impact. For the water drop, a high-pressure region fills the drop during impact which continuously drives liquid into the gap. For the ethanol drops, the high-pressure region is smaller and quickly attenuates, which results in a near-zero vertical velocity at the entrance of the gap. Compared to water, the lower surface tension of ethanol causes these drops to spread further upon impact, recoil less, and overall have less liquid over the gap, which promotes cleaving. Against a superficial thought, when the penetration of liquids into porous materials is to be maximized, a higher surface tension liquid is therefore desirable

The Effect of Hydraulic Diameter on Flow Boiling within Single Rectangular Microchannels and Comparison of Heat Sink Configuration of a Single and Multiple Microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms

Numerical Investigation of Droplet Impact on Smooth Surfaces with Different Wettability Characteristics: Implementation of a dynamic contact angle treatment in OpenFOAM

The “Direct Numerical Simulations” (DNS) of droplet impact processes is of great interest and importance for a variety of industrial applications, where laboratory experiments might be difficult, costly and time-consuming. Furthermore, in most cases after validated against experimental data, they can be utilised to further explain the experimental measurements or to extend the experimental runs by performing “virtual” numerical experiments. In such “DNS” calculations of the dynamic topology of the interface between the liquid and gas phase, the selected dynamic contact angle treatment is a key parameter for the accurate prediction of the droplet dynamics. In the present paper, droplet impact phenomena on smooth, dry surfaces are simulated using three different contact angle treatments. For this purpose, an enhanced VOF-based model, that accounts for spurious currents reduction, which has been previously implemented in OpenFOAM CFD Toolbox, is utilised and further enhanced. Apart from the already implemented constant and dynamic contact angle treatments in OpenFOAM, the dynamic contact angle model of Kistler, that considers the maximum advancing and minimum receding contact angles, is implemented in the code. The enhanced VOF model predictions are initially compared with literature available experimental data of droplets impacting on smooth surfaces with various wettability characteristics. The constant contact angle treatment of OpenFOAM as well as the Kistler’s implementation show good qualitative and quantitative agreement with experimental results up to the point of maximum spreading, when the spreading is inertia dominated. However, only Kistler’s model succeeds to accurately predict both the advancing and the recoiling phase of the droplet impact, for a variety of surface wettability characteristics. The dynamic contact angle treatment fails to predict almost all stages of the droplet impact. The optimum version of the model is then applied for 2 additional series of parametric numerical simulations that identify and quantify the effects of surface tension and viscosity, in the droplet impact dynamics

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

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

Fluid flow and heat transfer in microchannel devices for cooling applications: experimental and numerical approaches

Microchannel heat sinks are pointed to have a great potential in cooling systems. This paper presents a systematic study to develop a microchannel heat sink to be used in PV panels cooling. A systematic experimental approach is used to optimize the heat sink geometry. Then the potential advantage of using flow boiling conditions is explored in both numerical and experimental approaches. The results show that a heat exchanger with thin walls and wide channels can dissipate a greater amount of heat. Comparing the results obtained for one and two-phase flow conditions, one must conclude that although in the boiling tests the heat transfer coefficient was higher, the cooling method with single-phase flow using water dissipated a greater amount of heat, which was mainly due to flow instabilities. In this context, the numerical work clearly evidences that boiling can be an advantage in microchannel heat sinks, as long as the flow is controlled. The work also shows that the considered numerical simulation tool is sensitive enough to quantify the heat transfer enhancement due to boiling within the examined microchannel paths