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It is widely established that flow boiling, being a direct cooling technique also employing the latent heat of the fluid, has the potential to be more efficient than being useful in single-phase conventional cooling methods. This results in considerable potential for thermal management in many fields like microelectronics, space technology, thermal power plants, etc. The increasing demand for heat dissipation, consequent to component miniaturization, has pushed the development of new strategies for enhancing heat transfer efficiency, such as employment of functionalized surfaces. This review aims to describe in detail the current status of technology related to flow boiling heat transfer enhancement via micro/nanoscale surface functionalization. Key objectives are an increased nucleation site density and enhanced bubble dynamics. The vast majority of findings show favorable heat transfer performance, evidenced by an earlier onset of boiling (ONB), an improved flow boiling heat transfer coefficient (HTC), and an ameliorated critical heat flux (CHF). Increased pressure drop is a serious concern in certain application cases. Nanoscale textures mainly enhance capillary wicking to nucleation sites, thus being more effective in combination with microscale textures that define fixed nucleation sites. Degradation effects need to be more thoroughly and systematically characterized for application cases. Extra effects related to the manufacturing process can be easily overlooked, but one should be aware of their possible existence when drawing conclusions. Finally, the implementation of enhanced surfaces in mainstream applications is hindered by the absence of general predictive design tools for different channel configurations/materials, fluids, and operating conditions. A more universal understanding of the basic mechanisms involving texture geometry is needed in this aspect. [ABSTRACT FROM AUTHOR]

Despite numerous studies on nanofluids in microchannel heat sinks (MCHSs), they are not yet commercialized due to long-term stability issues and high maintenance costs. Therefore, this study explores the impact of nanofluids and nanoparticle clustering on single-phase convective heat transfer inside microchannels under laminar conditions. Water and commercially available water-based nanosuspensions, including Al 2 O 3 -water (30–60 nm), TiO 2 -water (5–30 nm), and polystyrene-water (50 nm), are circulated through silicon MCHS having rectangular channels integrated into a closed flow loop. To assess the in situ and real-time nanoparticle clustering during heat transfer experiments, Light Extinction Spectroscopy (LES) is applied as a non-intrusive measurement technique on nanofluids without any fluid sampling. Our findings reveal the appearance of nanofluid discoloration with no measurable increase in heat transfer coefficient. This unexpected change is attributed to the interplay of abrasion, erosion, and corrosion phenomena, likely triggered by the clustering of nanoparticles within the silicon microchannels—a novel insight into the complex dynamics of nanofluid behavior (an increase in the De Brouckere mean diameter from 11 nm to 107.3 nm over a 2.5 h period for TiO 2 nanoparticles). The resulting material loss could not be mitigated by altering the nanoparticle material, which may impede heat transfer enhancement under tested conditions. [ABSTRACT FROM AUTHOR]

• Laser cutting with dynamic beam shaping is investigated. • Illustrative diagram for cutting with dynamic beam shaping is presented. • Melt flow evolution for fusion cutting with longitudinal linear oscillation is studied. • Cutting front geometry for flame cutting with linear oscillation is examined. A dramatic increase in the industrial demands for tailoring the laser beam profile in laser cutting has led to significant performance improvements thanks to the recently emerging dynamic beam shaping technology. Nonetheless, the underlying reasons for such process improvements are not fully understood yet, and mainly theoretical assumptions are available. This paper uses several high-speed videography strategies to evaluate and compare the melt flow dynamics and cutting front geometry during 4 kW fiber laser cutting of 15 mm plates with static and oscillating beams. The absorption behavior on the cutting front is analyzed through trim-cut observation, and a substantial difference of around 40% is observed between these two laser beam intensity distributions. Moreover, a distinctive molten material ejection mechanism is found during linear beam oscillation, being beneficial for fusion cutting and detrimental for flame cutting. Finally, based on the experimental results, a former explanatory model from the literature is enhanced and further ways to optimize the beam profile during laser cutting are proposed. [ABSTRACT FROM AUTHOR]

The development of measurement techniques capable to be used when the optical access is limited is critical for a large series of applications ranging from combustion to aeronautics. In this paper it will be shown that backward scattering glory can be used to measure droplet size in the micrometric range using a unique optical access. The goal of this paper is first to present the glory as a measurement technique and to develop a wave optic model capable to predict the diameter of a droplet from its glory pattern. Indeed the novelty of this work consists in the fact that, considering the edge rays as major contributors to the glory pattern generated by a spherical droplet, Young's fringe analogy can be directly used to determine its size. The model is numerically validated for droplet size ranging from 10 μm to 1 mm and it is applied to multiple droplets glory pattern. The simplicity of the wave optics model allows both a more straightforward interpretation of the physical phenomena and a quick implementation into data inversion algorithms. [ABSTRACT FROM AUTHOR]

This work attempts to solve two fundamental problems that can be encountered when solving kinetic equations using quadrature based moment methods (QBMM). These two problems are considered crucial problems since they are observed even in the simplest particulate systems. On the one hand, we have that particle trajectory crossing (PTC) events can lead to non-physical accumulation of particles in the solution when the system of transport equations is weakly hyperbolic (i.e. delta-shock formation). On the other hand, the fact that particle segregation events can create moment sets close to the frontier of moment space. These, in turn, can produce non-physical high-velocity fluxes that will significantly reduce the computational efficiency. Given that both problems are fundamental, we limit the framework to one-dimensional free transport of particles. For the first problem we create a novel hyperbolic quadrature moment closure. Although we lack formal analytic proof for a quadrature order (N) higher than three, the numerical examples presented clearly show a well-behaved solution, absent of delta-shocks up to an eight-order quadrature. Our solution to the second problem consists of two minor, though effective, modifications to the general numerical algorithm frequently used for solving kinetic equations using QBMM. The performance of the new algorithm is remarkably stable and efficient, shown by a numerical example. • 1-D hyperbolic quadrature method of moments closure for kinetic equations. • Quadrature order can be chosen arbitrary. • Effective algorithm for suppressing the birth and contamination of high velocity fluxes. [ABSTRACT FROM AUTHOR]

Droplet impacting and freezing on solid surfaces are ubiquitous in nature and crucial to many industrial applications, while the underlying mechanism of this phenomenon remains elusive. In this paper, the effect of surface wettability on dynamic behaviors and freezing mechanism of a water droplet impacting on a solid substrate at a fixed Weber number of 200 has been experimentally investigated at various surface temperatures from −35.2 to −10 °C. With the decrease in temperature of the superhydrophobic surface, the complete rebound behavior moves to partial rebound and ultimately to full adhesion, mainly due to the competition between the fluid dynamics and heat transfer processes. An intense prompt splashing is achieved by altering the surface wettability toward superhydrophobicity. Raising the water repellency enhances the lift of the lamella rim during the initial droplet spreading. The receding velocity on superhydrophobic surfaces is about ten times larger than that on hydrophobic surfaces, while it is barely affected by the surface and droplet temperatures. Three distinct freezing shapes are observed in the experiments, namely, spherical ice, irregular ice, and central cap ice, and those can be explained through the analysis of the differences between the time for the onset of freezing and receding time. Some cases of droplet freezing after full receding, also known as spherical ice, include two freezing stages. First, the liquid–gas interface freezes within a short period. Next, the remaining liquid freezes upward because the evaporation through the liquid–gas interface leads to the temperature of this interface being lower than the liquid–surface interface temperature. A phase diagram described by the final equilibrium contact factor and the surface temperature reveals the conditions to form different freezing processes or to remain a liquid state. [ABSTRACT FROM AUTHOR]

For more than 20 years, the use of nanofluids to enhance heat transfer in microchannel heat sinks (MCHSs) has been the subject of a large number of scientific articles. Despite the great potentialities reported in several works, the presence of controversial results and the lack of understanding of heat transfer enhancement mechanisms prevent further advancement in the use of nanofluids as coolants. This article reviews the scientific literature focused on several aspects of nanofluids that have a role in the heat transfer enhancement within the MCHSs: nanofluid stability, thermal conductivity, and particle clustering, as well as the particle–surface interactions, i.e., abrasion, erosion, and corrosion. We also include the most relevant works on the convective heat transfer and MCHSs operated with nanofluids in our review. [ABSTRACT FROM AUTHOR]

Spray cooling is a heat transfer technology that has already shown its advantages and limitations. There has been increasing interest from academia and industry in combining this technology with nanofluids as coolants, owing to their potential for heat transfer enhancement. Nevertheless, there is a lack of understanding of the physical mechanism leading to this enhancement with the presence of technical problems that prevent the use of nanofluids in spray cooling applications. In this study, we investigate the effect of water-based TiO 2 nanofluids on both spray characteristics and heat transfer using an industrial full-cone pneumatic nozzle. For this purpose, three mass concentrations (0.05 wt.%, 0.1 wt.%, and 0.2 wt.%) were prepared and tested. We monitored the droplet sizes and velocity profiles with a particle dynamics analysis system. Moreover, the temporal temperature decrease of a heated aluminum block from 190 to 65 °C was measured via an infrared camera to calculate the heat transfer rate and heat transfer coefficient. The presence of nanoparticles is shown not to substantially alter the spray characteristics. Moreover, heat transfer is augmented mainly in the boiling regime due to more nucleation sites formed by the deposited nanoparticles. However, in the non-boiling regime, the contribution of adsorbed nanoparticles to the heat transfer enhancement diminishes. Overall, the aluminum block is cooled down 6%, 12%, and 25% faster than the DI water by the nanofluids at 0.05 wt.%, 0.1 wt.%, and 0.2 wt.%, respectively, including boiling and non-boiling regimes. [ABSTRACT FROM AUTHOR]

An experimental technique is introduced for measuring the contact angle of small colloidal particles at planar fluid interfaces. The presented light scattering-based method relies on performing two spectral transmittance measurements: one on a particle monolayer standing at the fluid interface and the other on a dispersion of the same particles in a homogeneous medium. The observed shift between the two transmitted spectra is explained in terms of the phase shift parameter, which is then used to determine the particle position relative to the interface and hence the contact angle. The applicability of the technique is demonstrated through simulations and experiments. [ABSTRACT FROM AUTHOR]

Investigation of the hydrodynamics within microfluidic chips is crucial for cutting-edge integrated liquid cooling systems due to the coupling between the temperature and velocity fields. Therefore, in this experimental work, we examine the spatial periodicity of the laminar velocity fields and pressure drops inside offset strip fin (OSF) and square pin fin (SPF) arrays at Reynolds numbers between 50 and 292 under isothermal conditions. The velocity fields are characterized using the μ PIV technique, and an advanced image stitching algorithm is applied to obtain the streamwise velocity fields. These stitched velocity fields serve two key purposes: evaluation of the flow development length and validation of the flow periodicity due to the periodic nature of the fin arrays. The velocity measurements are compared to the DNS results, and the friction factors acquired from pressure drop measurements are accurately predicted by the correlations based on the periodic flow assumption owing to the rapid flow development. For the first time, to the authors' knowledge, the consistent monotonic decay of flow perturbations is experimentally evidenced to occur via a single exponential mode. Finally, based on our validation, we confirm the feasibility of using the unit-cell approach to significantly reduce the computational costs compared to simulations that resolve the entire geometry. • The spatial periodicity of the laminar velocity fields and pressure drops are examined inside microchannels with periodic fins. • The velocity fields are characterized using the μ PIV technique and an advanced image stitching algorithm is applied. • The flow development length is evaluated to assess the extent of the flow periodicity and validate its assumption. • The velocity measurements are compared to the DNS results, and the correlations accurately predict the friction factors. • For the first time, a consistent monotonic decay of flow perturbations is evidenced to occur via a single exponential mode. [ABSTRACT FROM AUTHOR]

A better understanding of the wind–wave direction effect on the wind field and on the momentum transfer at the air–sea interaction will aid in the development of numerical atmospheric and wave models, important for wind energy studies among other applications. So far, the effect of opposed wind–wave directions on the wind field is not fully understood, even though it is shown to be an occurring phenomenon at the air–sea interface. To investigate the effects of different wind–wave alignments on the wind field and on the momentum transfer, the Wind Gallery of the von Karman Institute (VKI) was modified in order to perform aligned and opposed wind–wave direction experiments. The measurements performed to define the wind field and momentum transfer consisted of a Particle Image Velocimetry (PIV) setup with one laser and one camera. Based on this setup, differences in horizontal and vertical wave perturbation velocity as well as in phase-averaged wave-perturbation stress or momentum were observed between aligned and opposed wind–wave directions in terms of location, structure and vertical depth. This study thus provides support that the wind field and phase-averaged wave-perturbation momentum are dependent on the wind–wave alignment, which in the future should be accounted for in numerical models. • Wave perturbation velocity is dependent on the wind–wave directions. • Wave perturbation velocity differs in terms of height, structure, and location for different wind–wave directions. • No tilting downwind of the wave perturbation velocity for opposed wind–wave directions. • The wave perturbation velocity reaches higher altitudes for opposed wind–wave directions. [ABSTRACT FROM AUTHOR]

Existing energy balance models, which estimate maximum droplet spreading, insufficiently capture the droplet spreading from low to high Weber and Reynolds numbers and contact angles. This is mainly due to the simplified definition of the viscous dissipation term and incomplete modeling of the maximum spreading time. In this particular research, droplet impact onto a smooth sapphire surface is studied for seven glycerol concentrations between 0% and 100%, and 294 data points are acquired using high-speed photography. Fluid properties, such as density, surface tension, and viscosity, are also measured. For the first time according to the authors' knowledge, we incorporate the fluid viscosity in the modeling of the maximum spreading time based on the recorded data. We also estimate the characteristic velocity of the viscous dissipation term in the energy balance equation. These viscosity-based characteristic scales help to formulate a more comprehensive maximum droplet spreading model. Thanks to this improvement, our model successfully fits the data available in the literature for various fluids and surfaces compared to the existing models. [ABSTRACT FROM AUTHOR]

Cryogenic cavitation affects the operation of liquid propulsion systems during the first phase of a launch. Its effects within orifices or turbopumps can range from mild instabilities to catastrophic damages to the structures, jeopardizing the launch itself. Therefore, to ensure the proper designing of propulsion systems, cavitation phenomena cannot be neglected. Although hydrodynamic cavitation has been studied for decades, the impact of the nature of the fluid has been sparsely investigated. Therefore, this review, beginning from the basic concepts of cavitation, analyzes the literature dedicated to hydrodynamic cryogenic cavitation through an orifice. Our review provides a clear vision of the state-of-the-art from experimental and modeling viewpoints, identifies the knowledge gaps in the literature, and proposes a way to further investigate cryogenic cavitation in aerospace science. [ABSTRACT FROM AUTHOR]

A tool-based hybrid laser-electrochemical micromachining process involves concurrent application of two process energies i.e. electrochemical and laser in the same machining zone by means of a hybrid tool which serves as an ECM tool as well as a multimode waveguide. It is a relatively novel process finding applications in defect-free machining of difficult-to-cut materials without affecting their microstructure. In order to understand the physical phenomena occurring during this process, in-situ observations are required. Therefore, in this work, a real time observation was carried out of a novel tool-based hybrid laser electrochemical micromachining process. A combination of high-speed imaging and Large Scale Particle Image Velocimetry (LSPIV) was used to visualize the tool-based hybrid laser-ECM process in real time. It also allowed to carry out experimental investigations on the by-products and bubble generation which have a direct effect on process performance in terms of accuracy and efficiency. The real-time on-machine observations are unique of its kind and they will facilitate the understanding of underlying mechanisms governing this hybrid laser-electrochemical micromachining process. This will ultimately help in improving the quality of parts manufactured. This research is also a step forward towards making these physics-based hybrid processes deterministic by employing high-speed imaging in a closed loop control. [ABSTRACT FROM AUTHOR]

This manuscript presents the design of a capacitive void fraction sensor for cryogenic LN2 two phase flows and its validation at room conditions. The capacitive void fraction sensor is first designed by means of Electric Field Analysis (EFA) simulations taking into account specific technical constraints coming from the test section in which it should be accommodated. Then it is manufactured and validated using a proper combination of fluids (Polydimethylsiloxane (PDMS) and air) having a dielectric constant ratio similar to the one encountered in LN2/GN2 two phase flows. The validation is performed through comparison with void fraction measured by means of optical visualizations and shows how the capacitive measurement technique robustness allows obtaining reasonable accurate values of void fraction also for the substitute fluid case. The sensor presented in this manuscript was used to evaluate the void fraction during LN2 chilldown of a rectangular cooling channel and the results are presented in the second part of this work. [ABSTRACT FROM AUTHOR]

The flow periodicity in an additively manufactured microchannel heat transfer device is analysed using the micro-particle image velocimetry technique. A metal offset strip fin (OSF) microchannel is additively manufactured using laser powder bed fusion to overcome the limitations of conventional production techniques in manufacturing small complex features. This study demonstrates the capability of measuring the velocity field in an OSF microchannel up to 1 mm deep with a resolution of 10 μ m and a velocity uncertainty smaller than 0.5% for laminar flows with a Reynolds number below 10. Higher measurement uncertainties are encountered in regions close to the channel walls due to particle deposition. A methodology aiming to evaluate and minimise the uncertainties is presented. The experimentally obtained velocity fields are compared to a numerical simulation of the flow in the offset strip fin microchannel. The results verify that flow periodicity can be observed and the extent of the flow development region can be studied. Nevertheless, in regions with a large velocity gradient, a considerable uncertainty on the velocity measurements exists due to manufacturing imperfections. • Measuring two-dimensional velocity fields up to 1 mm deep inside the microchannel. • Offset strip fin microchannels are additively manufactured by laser powder bed fusion. • Streamwise flow periodicity and the development of the flow field are observed. [ABSTRACT FROM AUTHOR]

The present work proposes an extension of the existing analytical development on the radial spread of a liquid jet over a horizontal surface to the case of a thin radial flow. When the gap, H , between the jet nozzle and the plate is reduced the discharging area may be smaller than the inlet area leading to an increase of the main flow velocity downstream of the thin cylindrical opening. This increase of velocity, defined here as 1 α , can be related to the relative gap of the nozzle H R with R the nozzle pipe radius. Numerical computations with a volume of fluid method were realised with for H R ranging from 0.2 to 3 and with flow rates Q of 3 and 6 l min − 1 . The results of these computations allowed us to express α with respect to H R . Taking in account the flow acceleration allowed us to extend the set of equation from the jet impacting flow to the thin cylindrical opening flow. The liquid layer thickness and the surface velocity differ with a maximum error of 4% between the flow predicted by the model and computations. Main discrepancies appear in the region close to the nozzle where the analytical model assumption of a constant velocity outside the boundary layer is not valid. However, further downstream the model and the computations are in good agreement. [ABSTRACT FROM AUTHOR]

• Particle size distribution and concentration of PS-DI colloids are measured via LES. • Sensitivity of LES to refractive index uncertainties & system noise is investigated. • Experimentally characterized noise profile is embedded into numerical simulations. • Wavelength spectrum limited to high SNR region improves the LES results. • In-situ calibration of complex refractive index enhances the quality of LES results. • Both mono and bi-modal size distributions are experimentally retrieved. The measurement of particle size distribution (PSD) in colloids and nanofluids presents many challenges, especially when it requires to be conducted in-situ and real-time. Our work aims to assess the capabilities of Light Extinction Spectroscopy (LES) technique to determine concentration and volumetric PSD (vPSD) of colloids and nanofluids. Numerical simulations are performed to verify robustness of LES data inversion algorithm and to identify the most relevant uncertainty sources. Experiments are carried out on well-dispersed water based colloids containing Polystyrene particles with median diameters at 120 nm and 300 nm. LES results are compared with supplier's data, 3D-DLS measurements, and SEM images. Via sensitivity analyses, LES sensitivity to particle complex refractive index spectrum and to system noise level are examined. To improve accuracy, we limit LES data inversion at shorter wavelengths where high SNR is acquired. The noise is experimentally characterized and embedded into numerical simulations to better mimic the real conditions. We show that while noise level affects mostly vPSDs' peak height, small inaccuracies in complex refractive index spectrum deteriorate the stability of the inversion, especially for smaller vPSDs. To overcome the latter, we carry out a preliminary in-situ calibration on particle complex refractive index spectrum. Accordingly, results are substantially improved with maximal discrepancies smaller than 3% for median and volume mean diameters, and lower than 13% for number concentrations. Besides, the retrieval of bi-modal distribution is promising. In conclusion, our methodology can be considered as a guideline to evaluate the applicability and accuracy of the LES technique in colloids. [ABSTRACT FROM AUTHOR]

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids' use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids', is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids' rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified. [ABSTRACT FROM AUTHOR]

Spray drying and electrospraying are well-established drying processes that already have proven their value in the pharmaceutical field. However, there is currently still a lack of knowledge on the fundamentals of the particle formation process, thereby hampering fast and cost-effective particle engineering. To get a better understanding of how functional particles are formed with respect to process and formulation parameters, it is indispensable to offer a comprehensive overview of critical aspects of the droplet drying and particle formation process. This review therefore closely relates single droplet drying to pharmaceutical applications. Although excellent reviews exist of the different aspects, there is, to the best of our knowledge, no single review that describes all steps that one should consider when trying to engineer a certain type of particle morphology. The findings presented in this article have strengthened the predictive value of single droplet drying for pharmaceutical drying applications like spray drying and electrospraying. Continuous follow-up of the particle formation process in single droplet drying experiments hence allows optimization of manufacturing processes and particle engineering approaches and acceleration of process development. [ABSTRACT FROM AUTHOR]

• The success of the 3PT depends on the sensors distance and the sampling frequency. • The 3PT is not applicable at the orifice exit because of the non-homogeneous flow. • The far-field pressure affects the evolution of the void fraction and speed of sound. Speed of sound and void fraction are two key parameters in the characterisation of two-phase flows. However, accurate measurements require either intrusive or complex techniques. This paper reports on the Three Pressure Transducers (3PT) technique, which derives the speed of sound by measuring pressure fluctuations and which, thanks to its robustness and simplicity, could be applicable in harsh conditions. Therefore, the aim of this paper is to study in detail the feasibility of this technique against its limits and constraints in a cavitating flow. First, a numerical assessment of the technique is proposed to determine both the optimal transducers configuration and the sampling frequency. Then, the implemented algorithm was applied to a two-phase air–water mixture with well-known properties. Finally, the 3PT algorithm was used to study the behaviour of a cavitating flow induced by an orifice. This last application highlighted the possibility to use this technique to characterize the bubble flow generated by an orifice without the use of any optical access and by using a very compact experimental arrangement. The results obtained are also qualitatively compared to the images of the flow simultaneously acquired by a high-speed camera. [ABSTRACT FROM AUTHOR]