Showing papers by "Marco Marengo published in 2017"

Curvature effect on droplet impacting onto hydrophobic and superhydrophobic spheres

Abstract: Droplet impact on hydrophobic and superhydrophobic solid surfaces finds numerous applications, while the wide range of the parameters affecting its outcome necessitate a thorough study to reveal th...

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

Abstract: 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.

A benchmark study for the crown-type splashing dynamics of one- and two-component droplet wall–film interactions

Abstract: The present paper investigates experimentally the impact dynamics of crown-type splashing for miscible two- and one-component droplet wall–film interactions over a range of Weber numbers and dimensionless film thicknesses. The splashing outcome is parametrised in terms of a set of quantifiable parameters, such as crown height, top and base diameter, wall inclination, number of fingers, and secondary droplet properties. The results show that the outcome of a splashing event is not affected by the choice of similar or dissimilar fluids, provided the dimensionless film thickness is larger than 0.1. Below this threshold, distinctive features of two-component interactions appear, such as hole formation and crown bottom breakdown. The observation of different crown shapes (e.g. V-shaped, cylindrical, and truncated-cone) confirms that vorticity production induces changes in the crown wall inclination, thus affecting the evolution of the crown height and top diameter. The evolution of the crown base diameter, instead, is mainly dependent on the relative importance of liquid inertia and viscous losses in the wall-film. The maximum number of liquid fingers decreases with increasing wall, film thickness, due to the enhanced attenuation of the effect of surface properties on the fingering process. The formation of secondary droplets is also affected by changes in the crown wall inclination. In particular, for truncated-cone shapes the occurrence of crown rim contraction induces a large scatter in the secondary droplet properties. Consequently, empirical models for the maximum number and mean diameter of the secondary droplets are derived for V-shaped crowns, as observed for the hexadecane-Hyspin interactions.

Sensible Heat Transfer during Droplet Cooling: Experimental and Numerical Analysis

Abstract: This study presents the numerical reproduction of the entire surface temperature field resulting from a water droplet spreading on a heated surface, which is compared with experimental data. High-speed infrared thermography of the back side of the surface and high-speed images of the side view of the impinging droplet were used to infer on the solid surface temperature field and on droplet dynamics. Numerical reproduction of the phenomena was performed using OpenFOAM CFD toolbox. An enhanced volume of fluid (VOF) model was further modified for this purpose. The proposed modifications include the coupling of temperature fields between the fluid and the solid regions, to account for transient heat conduction within the solid. The results evidence an extremely good agreement between the temporal evolution of the measured and simulated spreading factors of the considered droplet impacts. The numerical and experimental dimensionless surface temperature profiles within the solid surface and along the droplet radius, were also in good agreement. Most of the differences were within the experimental measurements uncertainty. The numerical results allowed relating the solid surface temperature profiles with the fluid flow. During spreading, liquid recirculation within the rim, leads to the appearance of different regions of heat transfer that can be correlated with the vorticity field within the droplet.

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

Abstract: 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.

U-PHOS Project: Development of a Large Diameter Pulsating Heat Pipe Experiment on board REXUS 22

Abstract: U-PHOS Project aims at analysing and characterising the behaviour of a large diameter Pulsating Heat Pipe (PHP) on board REXUS 22 sounding rocket. A PHP is a passive thermal control device where the heat is efficiently transported by means of the self-sustained oscillatory fluid motion driven by the phase change phenomena. Since, in milli-gravity conditions, buoyancy forces become less intense, the PHP diameter may be increased still maintaining the slug/plug typical flow pattern. Consequently, the PHP heat power capability may be increased too. U-PHOS aims at proving that a large diameter PHP effectively works in milli-g conditions by characterizing its thermal response during a sounding rocket flight. The actual PHP tube is made of aluminum (3 mm inner diameter, filled with FC-72), heated at the evaporator by a compact electrical resistance, cooled at the condenser by a Phase Change Material (PCM) embedded in a metallic foam. The tube wall temperatures are recorded by means of Fibre Bragg Grating (FBG) sensors; the local fluid pressure is acquired by means of a pressure transducer. The present work intends to report the actual status of the project, focusing in particular on the experiment improvements with respect to the previous campaign.

Fluid-flow pressure measurements and thermo-fluid characterization of a single loop two-phase passive heat transfer device

Abstract: A Novel Single Loop Pulsating Heat Pipe (SLPHP), with an inner diameter of 2 mm, filled up with two working fluids (Ethanol and FC-72, Filling Ratio of 60%), is tested in Bottom Heated mode varying the heating power and the orientation. The static confinement diameter for Ethanol and FC-72, respectively 3.4 mm and 1.7mm, is above and slightly under the inner diameter of the tube. This is important for a better understanding of the working principle of the device very close to the limit between the Loop Thermosyphon and Pulsating Heat Pipe working modes. With respect to previous SLPHP experiments found in the literature, such device is designed with two transparent inserts mounted between the evaporator and the condenser allowing direct fluid flow visualization. Two highly accurate pressure transducers permit local pressure measurements just at the edges of one of the transparent inserts. Additionally, three heating elements are controlled independently, so as to vary the heating distribution at the evaporator. It is found that peculiar heating distributions promote the slug/plug flow motion in a preferential direction, increasing the device overall performance. Pressure measurements point out that the pressure drop between the evaporator and the condenser are related to the flow pattern. Furthermore, at high heat inputs, the flow regimes recorded for the two fluids are very similar, stressing that, when the dynamic effects start to play a major role in the system, the device classification between Loop Thermosyphon and Pulsating Heat Pipe is not that sharp anymore.

Experimental Study of a Sodium Two-Phase Thermosyphon

Abstract: The main objective of this work is to study experimentally the thermal characteristics and performance of twophase thermosyphons that operates with sodium as the working fluid, aiming their application to high temperature (700 to 1000 °C) solar receptor technologies. Two thermosyphons were fabricated and tested under eighteen different operation conditions, to verify the influence of the parameters: inclination angle, evaporator position, condenser geometry, and length of the adiabatic section. It was shown that the sodium thermosyphon can be used in hybrid applications, ie, for the integration of concentrated solar power systems to conventional fossil fuel power plants. For both heat sources and for different configurations studied, this device was able to transfer more than 1000 W.

Upgraded Pulsating Heat Pipe Only For Space (U-Phos): Results of the 22nd Rexus Sounding Rocket Campaign

Abstract: A large tube may still behave, to a certain extent, as a capillary in a micro-gravity environment. This very basic concept is here applied to a two-phase passive heat transfer devices in order to obtain a new family of hybrid wickless heat pipes. Indeed, a Loop Thermosyphon, which usually consists of a large tube, closed end to end in a loop, evacuated and partially filled with a working fluid and intrinsically gravity assisted, may become a capillary tube in space condition and turn its thermo-fluidic behavior into a so called Pulsating Heat Pipe (PHP), or better, a Space Pulsating Heat Pipe (SPHP). Since the objective of the present work is to experimentally demonstrate the feasibility of such a hybrid device, a SPHP has been designed, built, instrumented and tested both on ground and microgravity conditions on the 22 ESA REXUS Sounding Rocket Campaign. Ground tests demonstrate that the device effectively work as a gravity assisted loop thermosyphon, whether the sounding rocket data clearly reveal a change in the thermal hydraulic behavior very similar to the PHP. Since a microgravity period of approximately 120s is not sufficient to reach a pseudo steady state regime, further investigation on a longer term weightless condition is mandatory.

Simulation of droplet spreading on micro-CT reconstructed 3D real porous media using the volume-of-fluid method

Abstract: Droplet impact on porous media has a broad range of applications such as material processing, drug delivery and ink injection etc. The simulation studies of such processes are rather limited. To represent the spreading and absorption process of the droplet on porous materials, robust numerical schemes capable of accurately representing wettability as well as capillary effects need to be established. The current work, presents one of the first studies of droplet impact on a real porous media geometry model extracted from a micro-CT scan. The process involves processing of CT image and subsequent threshold based on the structures segmentation. The porous geometry is extracted in the form of a STL (STereoLithography) model, which, with the aid of dedicated software like ANSA and SnappyHexMesh, is converted to an unstructured mesh for successful discretization of the flow domain. The solution algorithm is developed within the open source CFD toolbox OpenFOAM. The numerical framework to track the droplet interface during the impact and the absorption phases is based on previous work. The volume-of-fluid (VOF) method is used to capture the location of the interface, combined with additional sharpening and smoothing algorithms to minimise spurious velocities developed at the capillary dominated part of the phenomenon (droplet recession and penetration). A systematic variation of the main factors that affect this process are considered, i.e. wettability, porous size, impact velocity. To investigate the influence of porous structures on droplet spreading, the average porosity of the media is varied between 18.5% and 23.3% . From these numerical experiments, we can conclude that the droplet imbibition mainly depends on the porous wettability and secondly that the recoiling phase can be observed in the hydrophobic case but not in the hydrophilic case.

Single loop pulsating heat pipe with non-uniform heating patterns: fluid infrared visualization and pressure measurements

Abstract: A novel Single Loop Pulsating Heat Pipe (SLPHP) filled at 60% filling ratio with pure ethanol, with an inner diameter of 2mm is tested in Bottom Heated mode varying the heating power. The system is designed with two sapphire tubes mounted between the evaporator and the condenser allowing simultaneous fluid flow high-speed visualizations and IR analysis. Furthermore, two highly accurate pressure transducers carry out local pressure measurements just at the ends of one of the sapphire inserts. Additionally, three heating elements are controlled independently, in such a way to heat up the device varying the distribution of the heating location at the evaporator. It is found that peculiar heating distributions promote the slug/plug flow motion in a preferential direction, increasing the overall performance of the device. Pressure measurements point out that the flow patterns are strictly related to the pressure drop between the evaporator and the condenser. Furthermore, the IR visualization highlights interesting phenomena related to the liquid film dynamics during the device operations, which represent a very useful information for future numerical modeling of Pulsating Heat Pipes.

A novel lumped parameter model for Loop Heat Pipes – validation and parametric analysis

Abstract: A one dimensional lumped parameter model has been developed within the general framework of the open source software Octave, in order to describe the physical behaviour of a Loop Heat Pipe. By means of the electro-thermal-hydraulic analogy, this model gives the values of temperature and pressure for every part of the device, in response to varying boundary conditions. Furthermore, a novel approach in describing the phase change at the condenser has been adopted, differentiating the vapour quality variation over time. The code is initially validated against both simulation and experimental data found in literature. Since the present work aims to produce a design tool for the automotive industry, a parametric analysis on the geometrical characteristics of a Loop Heat Pipe is then performed, identifying and quantifying the most influential design parameters.

Etude d’un caloduc oscillant plat testé avec un fluide remouillant sous champ de gravité variable

Abstract: Résumé – Dans cette article, l’étude expérimentale d’un caloduc oscillant (PHP) plat avec un canal à section carrée testé sous hyper et microgravité est analysée. Les fluides testés sont l’eau pure et un mélange eau-butanol offrant les propriétés au fluide d’être « remouillant » (tension de surface augmentant avec la température). Les résultats montrent que, si les deux fluides ont tendance à s’assécher lors des phases de microgravité, le mélange est sujet à des mouvements de fluide rapides et intenses le long du PHP bien plus fréquents que pour l’eau pure, permettant des transferts de masse et de chaleur plus efficaces. La mouillabilité est un critère qui semble essentiel à ce comportement.

Characterization of a Space Pulsating Heat Pipe on board REXUS 22 Sounding Rocket

Abstract: Micro-gravity environments might extent the concept of capillary behavior structure to large diameter tubes. A Loop Termosyphon, in its traditional use, is a two-phase thermal device which usually consists of a large tube, closed end to end in a loop, evacuated and partially filled with a working fluid and intrinsically gravity assisted. With the concept stated here, it may become a capillary tube in space condition and turn its thermo-fluidic behavior into a Pulsating Heat Pipe (PHP), or better, a Space Pulsating Heat Pipe (SPHP). The present work presents the results of the experimental campaign of a SPHP effectuated both on ground and in microgravity conditions. A SPHP has been designed and implemented on board of a sounding rocket launched in the 22nd ESA REXUS Sounding Rocket Campaign. This type of sounding rocket launch guarantees approximately 120s of microgravity environment. The ground tests prior to the launch demonstrated that the device effectively work as a gravity assisted loop thermosyphon, while the experimental data from the microgravity period during the sounding rocket launch reveal a change in the thermal hydraulic behavior similar to the PHP. The micro-g environment provided by the rocket was not sufficient to reach a pseudo steady state regime, therefore, further investigation on a longer term weightless condition is mandatory.

Thermo-hydraulic characterization of semi-transparent Flat-Plate Pulsating Heat Pipes in variable gravity regimes

Abstract: Pulsating Heat Pipes are thermally driven two-phase passive devices mainly based on phase change phenomena (film evaporation, flow boiling, film condensation), and influenced by capillary and gravity forces. They consist of a meandering capillary tube closed in a loop, evacuated and partially filled with a working fluid at saturation conditions. Once the heat load is applied, the fluid motion starts and an oscillating pattern of alternating vapour bubbles and liquid plugs forms inside the tube. As it is widely accepted, complex flow patterns, ranging from slug flow to annular flow, occur in the adjacent tubes of pulsating heat pipes (PHP), initiated by local pressure instabilities ((Khandekar et al. 2003) and (Liu et al. 2007). Such flow patterns have obviously effects on the total heat flux transferred from the heated to the cooled ends of the PHP. Many parameters have also a direct influence on their operation (Charoensawan et al. 2003): number of turns, PHP dimensions, filling ratio and physical properties of the working fluid, applied heat power, etc. One of the most important parameter is the channel internal diameter permitting liquid/vapor phase division into liquid slugs and vapor bubbles separated by menisci due to capillary forces. And, more particularly in the context of this study, the inclination with respect to gravity or, generally, the change in the value of acceleration (for example, for tests on board of an aircraft during a parabolic flight campaign and/or under microgravity conditions for space application, (Gu et al. 2005), (Ayel et al. 2015) and (Mangini et al. 2015). This paper presents some results obtained during the ESA 64th Parabolic Flight Campaign during which six similar Flat Plate Pulsating Heat Pipes (FPPHP) have been tested. One example of FPPHP can be seen on Fig. 1.The FPPHP were milled from copper plates (length: 204 mm, with varying widths and thicknesses according to the channel dimension, see below) with a single square shaped groove, forming a series of 11 Uturns in the evaporator (see Fig. 1). Every PHP channels were square shaped, with dimensions D varying from 1.5 mm to 3 mm. Three condenser lengths (5, 10 and 15 cm) were also tested for the 2.5 mm channel FPPHP. A containment channel link to a revervoir previously emptied has been set in order to avoid introduction of non-condensable gazes in the device (Fig. 1). Thus, considering six devices tested during 3 days of flight, each day 2 separate FPPHP were tested with doubled instrumentation. Two visible cameras (Canon® EOS 100D and 550D, 50 Hz) recorded movies allowing visualizations of fluid flow motions in the overall channels of both PHP. Ten T-type thermocouples of 0.5 mm (±0.5 K) monitor the temperature of each section in the PHPs: three for each FPPHP evaporator (TEV1-TEV6); two in the water cooling loop and two thermocouples instrument the air temperatures. Two pressure sensors (GE PTX5076-TA-A3-CA-HO-PS, 5 bars absolute, ±200 Pa) allow recording of local fluid pressure at the top of the condenser zones. A g-sensor (DE-ACCM3D, ±0.1g) is used to measure the gravity level variations during each parabolic flight.

Introduction of a new Dynamic simulation screening tool to support early stage building design

Abstract: The paper introduces a newly designed dynamic building energy simulation screening tool to help integrate the use of advanced simulation techniques to early stage building design and feasibility studies. The tool will help the design process to move toward an integrated design approach, including energy analyses and expertise from the first stages of design when time constraints and information requirements are still a hindrance for the use of other existing simulation tools. The paper focus on the integration of the user input and output interfaces and automatic model generation algorithms while referring to previously existing papers in term of model definition, case studies and validation. The tool is able to simulate building energy performances starting from a limited number of inputs received through a specifically designed user interface supported by databases and suggested values. Based on those inputs, a simplified building model is generated and simulated in EnergyPlus and results are post processed and visualized on the user interface. The tool is fully web based and can be used through any web enabled device as only the input and output interfaces are managed by the user device, with all other components being allocated to the server. The tool is able to run building performance simulations based on a simplified building model description with a limited number of inputs and in a short span of time, ranging from minutes to less than an hour. Nonetheless, the simulations are still returning results with an acceptable margin of accuracy compared to a detailed simulation, considered to generate useful information during early stage design and still higher compared to the use of traditional stationary models. The proposed tool will help the design process evolve toward an integrated approach and adapt to the foreseeable changes in regulations and market demand of lowand zero-carbon buildings.

Break-up Mechanisms and Conditions for Vapour Slugs Within Mini-Channels

Abstract: In the present investigation an enhanced Volume Of Fluid (VOF) based numerical simulation framework is applied for the conduction of parametric numerical simulations, aiming to investigate observed break-up phenomena of vapour slugs, within circular mini-channel branches of a hybrid thermosyphon / pulsating heat pipe device, during microgravity experiments. The simulation results identify three prevailing break-up regimes. The effect of fundamental controlling parameters in the resulting break-up characteristics is also examined. An entrainment of a liquid droplet at the trailing edge of the vapour slug, that is responsible for its subsequent “full” break-up, is identified from the simulations. Moreover, it is quite interesting that the value of the applied heat flux, does not seem to influence the break-up regime and its main characteristics.

Experimental and numerical study on sensible heat transfer at droplet/wall interactions

Abstract: The present study addresses a detailed experimental and numerical investigation on the impact of water dropletson smooth heated surfaces. High-speed infrared thermography is combined with high-speed imaging to couple the heat transfer and fluid dynamic processes occurring at droplet impact. Droplet spreading (e.g. spreading ratio) and detailed surface temperature fields are then evaluated in time and compared with the numerically predicted results. The numerical reproduction of the phenomena was conducted using an enhanced version of a VOF- based solver of OpenFOAM previously developed, which was further modified to account for conjugate heat transfer between the solid and fluid domains, focusing only on the sensible heat removed during droplet spreading. An excellent agreement is observed between the temporal evolution of the experimentally measured and the numerically predicted spreading factors (differences between the experimental and numerical values were always lower than 3.4%). The numerical and experimental dimensionless surface temperature profiles along the droplet radius were also in good agreement, depicting a maximum difference of 0.19. Deeper analysis coupling fluid dynamics and heat transfer processes was also performed, evidencing a strong correlation between maximum and minimum temperature values and heat transfer coefficients with the vorticity fields in the lamella, which lead to particular mixing processes in the boundary layer region. The correlation between the resulted temperature fields and the droplet dynamics was obtained by assuming a relation between the vorticity and the local heat transfer coefficient, in the first fluid cell i.e. near the liquid-solid interface. The two measured fields revealed that local maxima and minima in the vorticity corresponded to spatially shifted local minima and maxima in the heat transfer coefficient, at all stages of the droplet spreading. This was particularly clear in the rim region,which therefore should be considered in future droplet spreading models. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.5024

Drop Impact onto a Metallic Porous Layer: Effect of Liquid Viscosity and Air Entrapment

Abstract: The drop impact onto porous surfaces has important applications in many fields, such as painting, paper coating, drug delivery and cosmetic sprays. In most of these applications, the optimisation of the deposition process is carried out empirically, without a proper understanding of the physics and a theoretical modelling of the spreading and the imbibition phenomena. The purpose of this study is to analyse droplet impacts on metallic meshes to define a general modelling strategy of the impact regimen on particular 2D regular porous surfaces. The application of this structure is relevant in process like filtration but also in the medical field, considering for example reconstructive surgery. By analysing the impact of droplets of water, acetone and a mixture of glycerol and water, having a diameter and an impact velocity in a range of 1.5-3mm and 2-4m/s, respectively, on meshes with a pore size ranging between 25 and 400 μm, a regime map was built considering 6 different impact outcomes. The outcomes were characterised by a deposition of the droplet on the substrate, or a partial imbibition, or a total imbibition. By increasing the impact velocity, a splash region was defined, which is still characterised by a final deposition, a partial imbibition and a total imbibition. It is found that the most influencing parameters are closely linked to the liquid properties and the impact velocity, more specifically liquid surface tension plays a major role in defining the impact outcome. In the case of Acetone, the lower surface tension brings to an almost instantaneous total imbibition whereas the experiments conducted using water and glycerol solution, showed a major distribution of the deposition regimes with respect to the other outcomes, due to the effect of a higher viscosity. It was found that the geometrical characteristics of the mesh such as pore size and wire diameter, play an important role as well in defining the total imbibition outcome. Finally, the defined transition maps, shows that for a certain combination of physical properties and initial condition, the outcome of the droplet impact is predictable.