In the paper, we identify characteristic phases of the auto-oscillation cycle of unsteady cloud cavitation on a twodimensional symmetric hydrofoil by analyzing random (uncorrelated) instantaneous two-component velocity fields from a representative PIV database. By applying a certain criterion to the amplitude of the fluctuating velocity, we were able to successfully isolate individual realizations from the entire ensemble into characteristic phases of the quasi-periodic process of attached cavity length pulsations accompanied by detachments of cloud cavities. This ultimately allowed us to phase average the selected velocity fields, to perform phase-by-phase tracking of large-scale vapor and vortex structures developing in the unsteady cavitating flow and, finally, to calculate spatial distributions of vapor concentration and turbulence characteristics in each of these phases. All this provided important information on the phase-averaged turbulence structure, typical of unsteady cloud cavitation, and opened up new opportunities for further in-depth analysis of the interactions of vapor structures and flow turbulence. In particular, it was established that the breakup of the sheet cavity interface followed by the shedding of a large-scale cloud occurs on average at a distance of 36-39% of the chord length from the hydrofoil leading edge. The reverse flow was reliably detected downstream of the cavitation sheet closure and between the suction surface and detached cloud cavity, which directly indicates the presence of a large-scale vortex induced by the circulation of liquid due to the coordinated action of the main flow and re-entrant jet at the moment of the attached cavity breakup.

This study investigates a low-concentration, parabolic-trough photovoltaic–thermal collector for industrial heat and electricity cogeneration applications. A fully coupled 3-D optical, electrical, and thermal model of the collector was developed in COMSOL, based upon which a full-scale prototype was designed, constructed and tested in Monterrey, Mexico. A comparison between modelling predictions and experimental results at steady-state conditions revealed an RMSE of about 4% for the thermal efficiency and 1% for the electrical efficiency. The experimental results revealed maximum (zero reduced temperature) thermal and electrical efficiencies of 60% and 8%, corresponding to peak thermal and electrical power of 1.6 kWth and 210 Wel, respectively, from the 3.6 m2 collector Additionally, the collector was characterized following international standards: dynamic testing guidelines for thermal (ISO 9806) and electrical characterisation (IEC 62670-3) for a more rigorous analysis. Results obtained demonstrates that the developed simulation model is suitable for predicting the performance and may be used as basis for optimizing the collector.

The hydrodynamic characteristics of liquid-liquid flows in a micro-mixing plate with a ‘heart/spade’ geometry and a hydraulic diameter of ∼ 0.36 mm at the contraction point are studied experimentally in the Reynolds number range of 250 - 1,000 . Localised optical observations of the two-phase flow within the micro-mixing device units are performed using a high-speed camera in combination with an LED-induced fluorescence imaging technique. A qualitative interpretation of the instantaneous images enabled the development of a regime map with three stable flow patterns, each with a metastable transitional state. The formation of secondary flows and recirculation zones resulted in the breakup of interfaces and fragmentation of droplets. The droplet size distribution of an MTBE-water dispersion is studied across a broad range of the dispersed phase (water) ratios, ϕ _d = 0.091 - 0.714 . The resulting Sauter mean diameter of water droplets is used to evaluate the improvement in the specific surface area and was correlated as a function of the energy dissipation rate and the Reynolds and Weber numbers.

We study experimentally the agitation of viscoplastic CarbopolTM 980 (C980) fluids in a 2.5-L vertical unbaffled cylindrical vessel using central dual-impeller stirring systems, examining the mixing characteristics of different combinations of standard six-blade Rushton turbines (RTs) and downward-pumping 45 degrees-pitched four-blade turbines (PBTs) of identical diameter D = 41 mm. We consider the effects of rotational speed (as expressed by the modified power-law Reynolds number, Rem), impeller separation (G/D = 0.73 and 1.22) and a variety of arrangements of the rotors on the mixing characteristics of these systems. Phenomena of cavern segregation and internal flow compartmentalisation are revealed and explained in terms of expected flow patterns. Dual-RT stirrers are found to produce highly symmetrical flow patterns, influenced by the impeller separation, along with strong time-dependent compartmentalisation between the internal flows of both caverns. This flow compartmentalisation can be, however, avoided in dual-PBT systems due to the downward-pumping nature of this system, which also allows for the achievement of a state of full tank homogeneity through cavern engulfment. Comparing the total equivalent volumes of the caverns, Vctot, for the examined dual-impeller systems, the mixing effectiveness, as characterised by the growth of Vctot with Rem, is maximised for the RT-PBT and PBT-RT arrangements with a separation of G/D = 1.22, for which Vctot is 70-100 % larger at Rem = 18.5 compared to the dual-RT and dual-PBT arrangements. Although the RT-PBT arrangement performs well, it does not achieve a homogeneous mixing state of the viscoplastic fluid throughout the entire vessel within a reasonable time. Thus, the most efficient mixing system is that featuring a PBT rotor overlying a RT. The findings demonstrate the significant influence of the selection and geometrical arrangement of dual-impeller systems on the mixing of viscoplastic fluids.

Abstract Recent advancements in laser and imaging systems, as well as in computational processing capabilities, have made quantitative optical imaging, which is often combined with laser illumination, highly adaptable, robust and reliable. Laser-based diagnostic techniques, such as planar laser-induced fluorescence (PLIF), offer the possibility of simultaneous spatiotemporally resolved measurements of temperature fields in the liquid phase at boiling conditions. In this paper, we examine the applicability of two-colour PLIF (2cPLIF), where the ratio between individual fluorescent emissions from uniformly dispersed dyes is used to take temperature measurements in the liquid phase in the presence of moving vapour-liquid interfaces typical of boiling flow. The implementation of 2cPLIF necessitates uniformity in the concentration of different dyes across the flow field. However, in the case of a multiphase flow such as boiling, thermophoresis can lead to inhomogeneous dye distributions. To overcome this challenge, a single-dye multispectral planar laser-induced fluorescence (SDMS-PLIF) method has been developed, which employs fluorescent emissions in different spectral bands of the same dye (Nile Red). The spectral characteristics of Nile Red were measured using a spectrometer to identify its temperature-sensitive bands over a wide range of dye concentrations, from 0.3 to 30 mg/L. Following this, we demonstrate the measurement capabilities of SDMS-PLIF thermography as applied to a boiling flow in a miniaturised vertical square channel, gaining insight into the thermohydrodynamic interactions between vapour bubbles and a heated wall.

The accumulation of hydrates presents an important flow-assurance challenge. Herein, we performed an experimental study to quantify the thickness of a hydrate layer growing laterally along the interface of a water drop submerged in the bulk phase of cyclopentane at sub-zero temperatures. Spatially-resolved velocity and temperature measurements in both liquid phases were taken independently using particle image velocimetry, one- or two-color planar laser-induced fluorescence. We found that the appearance of hydrates leads to a displacement of the temperature maximum away from the phase boundary, which is typically observed very close to the interface due to the recirculating motion of water inside the drop. Making an assumption that, when hydrates are absent, this recirculating motion is retained in time and the maximum position is, thus, unchanged, we conclude that this shift should indicate the hydrate layer should mostly protrude into the drop, and its thickness was evaluated to be around 0.38 mm.

Passive methods of flow and cavitation control appear to offer some of the best prospects in the field of hydraulic engineering and marine applications. In this article, we aimed at an experimental examination of the effect of wall roughness/wettability on the occurrence of cavitation and turbulence structure in the cross flow around and in the wake of a circular cylinder in two characteristic regimes. For this, we used three test bodies with different surface morphologies: smooth (reference), micro-scale irregularities (rough) and regular large-scale (of the order of a millimeter) texture (finned). Using high-speed imaging to observe vapor cavities, we revealed that cavitation is noticeably suppressed by both types of roughness. Applying the method of vapor phase detection (Pervunin et al., 2021), this finding was then quantitatively confirmed through an in-depth analysis of an ensemble of instantaneous velocity fields measured by PIV, indicating that modification of wall morphology is an effective method of cavitation control. The procedure of statistical vector filtration (Heinz et al., 2004) allowed us to remove outliers from the velocity fields and, thus, calculate various turbulence characteristics, including higher-order moments (i.e., the coefficients of skewness and excess). Wall irregularities were found to significantly affect the turbulence structure of the wake flow, but the higher-order moments downstream of the modified-surface cylinders turned out to be unexpectedly insensitive to a change in the flow regime, as opposed to the smooth one. Regardless of the type of surface morphology, the influence of roughness on the mechanism of formation of large-scale vortices and their characteristics was weakened. However, it caused overall disorganization of liquid motion in the cylinder wake, thus making local flow conditions highly unsteady. In addition, this process became more chaotic with an increase in the scale of irregularities.

The boiling of dielectric fluids in miniaturised channels has emerged as one of the most promising solutions to high- density electronics cooling. Although significant breakthroughs have been made, a fundamental understanding of the hydrodynamic and thermal performance in relevant flows is still lacking, which calls for further development of state-of-the-art measurement techniques and their deployment for the provision of high-quality, detailed experimental information. To this end, we conducted an experimental investigation of flow boiling in a miniaturised vertical square channel with a hydraulic diameter of 5 mm, using HFE-7100 as the working fluid. A whole-field thermographic imaging approach referred to as single-dye multispectral planar laser-induced fluorescence (SDMS-PLIF) was developed and applied to measure spatiotemporally-resolved temperature fields. For its implementation, Nile Red, a thermosensitive lipophilic dye, was dissolved in HFE-7100 as a fluorophore. The measurements were taken at selected conditions to achieve a nucleate boiling regime in a laminar flow. Time-lapse temperature maps provided direct evidence for bubble-induced mixing observed as thermal plumes, where a portion of hot fluid is advected above the thermal boundary layer at the heated wall and penetrates the colder bulk phase in the flow core.

Active flow control in microfluidic devices to broaden the range of desired conditions is a significant practical concern. In this study, we experimentally investigate liquid-liquid parallel flow under the dispersed phase flowrate pulsations in a T-junction microchannel. Sinusoidal flow rate disturbances with variations in amplitude and period were applied to dispersed phases of different viscosities. The study involves flow visualization and velocity field measurements using the micro-PIV technique for both free and excited flow conditions. The analysis of flow pattern maps, velocity fields, velocity gradients, and phase-averaged velocity profiles in a T-junction region and far downstream provided insights into the evolution of disturbances, which is different in low- and high-viscosity liquids. In the less viscous liquid, transverse waves amplify longitudinal ones due to relaxation of the liquid-liquid interface, while in the high-viscosity dispersed phase, longitudinal waves are damped significantly, with the transverse wave amplitude remaining almost constant. As a result, we revealed two distinct mechanisms of disturbance wave propagation and the loss of parallel flow stability, which are both dependent on the Ohnesorge number of the dispersed phase. Destabilization of the parallel flow led to the formation of plugs with a narrow length distribution. Single-mode, double-mode, triple-mode, and multi-mode plug formation regimes were identified. The results of the study can potentially be used to extend the range of segmented flow regimes and generate plugs of a desired length.

Liquid cyclopentaneis frequently used in hydrate formation studiesas an analogue of natural gas because cyclopentane hydrates are stableabove the ice melting point at ambient pressure. In this study, hydrategrowth was established on a sessile water drop of 11 mm in diameterand 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane.The hydrate formation mechanism and growth processes were observedoptically over an extended range of subcooling temperatures from 5.1to 15.2 degrees C, with the cyclopentane bulk temperature maintainedin different experimental runs between 2.6 and -7.5 degrees C.Qualitative and quantitative comparisons were performed to confirmthe absence of ice freezing during hydrate formation, and thus, thelack of contamination of the latter from the former in the experiments.Different transformations in the hydrate film morphology were registeredfrom macroscopic observation over the considered range of subcoolingtemperatures, with the hydrate crystals composing the film takingthe form of polyhedral, dendritic, or spherulitic structures. It wasalso found that the hydrate growth rate varied depending on the subcoolingtemperature, with the variation of the growth rate as a function ofthis temperature changing from a power to an (approximately) linearlaw with an increase in the degree of subcooling. We postulate thathydrate film growth can be governed by different mechanisms, whoseroles change over the range of explored subcooling temperatures. The mechanism of cyclopentane hydrateformation and evolutionat sub-zero bulk temperatures depends on the thermodynamic drivingforce (subcooling temperature) and leads to a different relationshipbetween the hydrate growth rate and the subcooling temperature, withthe hydrate crystalline film morphology altering from polyhedral todendritic and eventually to spherulitic as the subcooling temperatureincreases.

Thermo-hydraulic characteristics of supercritical CO2 (SCO2) flows in horizontal tubes with half-wall heat-flux conditions are investigated numerically, which is a common practice such as applications in solar parabolic trough collectors, while the heat transfer performance and the underlying mechanisms have not been fully understood. In heated flows, buoyancy acts to inhibit heat transfer when the top half of the tube wall is heated, however, when the bottom half of the tube wall is heated, this inhibition is alleviated, and the synergy between the temperature gradient and velocity fields improves thanks to the secondary flow in the near-wall region at the bottom wall. As a result, the heat transfer coefficient is ∼95% higher (on average) than in the case when the top half of the tube wall is heated. When the bottom half of the tube wall is cooled, buoyancy is expected to enhance heat transfer, while the synergy between the temperature gradient and velocity fields is supressed by the secondary flow in the near-wall region at the bottom of the tube. Conversely, when the top half of the tube wall is cooled, the buoyancy effect inhibits heat transfer, while the synergy between the temperature gradient and velocity fields is improved by the secondary flow in the near-wall region at the top of the tube, which eventually leads to an increase of ∼21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall is cooled. Finally, the heat transfer discrepancy due to different heat flux conditions revealed in this study are employed in a heat exchanger model, indicating that the thermal performance of this device can be increased by ∼6% through an appropriate arrangement of the hot and cold flows without additional costs.

This paper presents the results of a statistical analysis of turbulent structure in a free bubbly jet at a Reynolds number of 12 500 based on PIV measurements of the carrier-phase velocity. Distributions of higher-order statistical moments for velocity fluctuations (coefficients of skewness and excess) and the turbulence energy spectra for single-phase and gas-saturated jets were obtained after statistical filtering of instantaneous velocity fields. The influence of the dispersed phase (bubbles with an average diameter of 0.8 mm) with a volumetric gas fraction of 0, 1, 2, and 3% on the specified characteristics of the continuous phase was analyzed.

The accumulation of hydrates in oil and gas pipelines can lead to flow blockage during either normal operation or shutdowns (i.e., in static conditions) and, therefore, presents an important flow assurance challenge. We investigate experimentally the role of heat transfer on the lateral growth of hydrates over a sessile water drop submerged in cyclopentane at sub-zero temperatures. Spatiotemporally-resolved temperature measurements in cyclopentane were performed using a one-colour planar laser-induced fluorescence (PLIF) method. It is found that the temperature and heat transfer coefficient in the bulk cyclopentane phase above the water drop interface are both higher during hydrate formation and growth compared to when hydrates are absent, and this can be attributed to the hydrate growth phenomenon, though the mechanics are yet not fully understood.

We perform large-eddy simulations to study a cavitating flow over a two-dimensional hydrofoil section—a scaled-down profile (1:13.26) of guide vanes of a Francis turbine—using the Schnerr–Sauer cavitation model with an adaptive mesh refinement in intensive phase transition flow areas. In the test case, the guide vane is tilted at an angle of attack of 9° to the direction of the flow, in which the Reynolds number, based on the hydrofoil chord length, equals 1.32×106, thus providing a strong adverse pressure gradient along the surface. The calculated time-averaged turbulence characteristics are compared with those measured by particle image velocimetry to verify that the flow is correctly reproduced in numerical simulations using the procedure of conditional averaging proposed and tested in our previous investigation. A re-entrant jet is identified as the primary source of vapor cloud shedding, and a spectral analysis of the cavitating flow over the profile midsection is conducted. Two characteristic frequencies corresponding to the cases, when an attached cavity detaches completely (as a whole) and two partially from the hydrofoil, are found in the flow. The study reveals that the natural frequency of partial cavity shedding is three times higher than that of full detachments. The examined regime exhibits an oscillatory system with two oscillation zones related to cavitation surge instability and unsteady cloud cavitation resulting from the re-entrant jet. Conditional averaging correlates cavitation structures with pressure distributions, forces, and torque on the guide vane. This modeling approach captures the fine details of quasi-periodic cavitation dynamics, providing insights into unsteady sheet/cloud cavitation and offering a method for developing control strategies.

The article presents results of a statistical analysis of the original PIV data for the cavitating flow around the 2D symmetric hydrofoil mimicking a guide vane of a Francis turbine which were previously reported in Timo-shevskiy et al. (2020). We employ the procedure of statistical vector filtration (Heinz et al., 2004) to eliminate vector outliers from the measured velocity fields, which allows us to retrieve genuine spatial distributions of higher-order statistical moments of velocity fluctuations, and the method of vapor phase detection (Pervunin et al., 2021) to extract time-averaged fields of vapor in the cavitating flow. The spatial distributions of the probability of vapor phase occurrence allowed us to unambiguously distinguish the extension of the flow area occupied by the dispersed phase (cavitation) together with the time-averaged concentration of the vapor phase in the three characteristic regimes of the cavitating flow as well as different features of cavitation evolution, including the location and length of an attached cavity and the place of detachments of cloud cavities in the case of unsteady cloud cavitation. The development of cavitation is also reflected in the fields of higher-order sta-tistical moments of turbulent fluctuations by the example of coefficients of asymmetry and excess. With a transition to unsteady cloud cavitation, the distribution of velocity fluctuations transforms, taking a nonequi-librium shape, with an increase in magnitudes of the asymmetry and excess coefficients over a significant flow area. In a region of the attached cavity, around the place of formation of vapor clouds and downstream, where the cloud cavities are carried away by the flow, the distribution of velocity fluctuations becomes bimodal. Through visual inspection of the raw PIV images, realizations related to two characteristic phases of the cavity oscillation cycle were extracted, providing quasi-phase-averaged data. As a result, the bimodal shape of the distributions in certain areas of the unsteady cavitating flow was demonstrated to be most probably linked with periodic detachments and downstream advection of the vapor clouds as the predominant fundamental process determining the flow structure and dynamics.

The thermo-hydraulic characteristics of heated supercritical CO2 (SCO2) flows are investigated numerically in a vertical pipe from first-and second-law perspectives, and the influence of the flow direction, mass flux and heat flux (both distribution and average value) are evaluated. Two mass flux (254 kg/(m(2)center dot s) and 400 kg/(m(2)center dot s)) and three average heat flux (30 kW/m(2), 50 kW/m(2) and 70 kW/m(2)) conditions are simulated at an inlet temperature of 288 K and a pressure of 8.0 MPa (corresponding pseudo-critical temperature of 308 K) in a 4-mm diameter pipe. The simulation results reveal that the heat transfer is enhanced and the irreversibility is reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorates and the irreversibility is increased in upward flows. Both higher heat fluxes and lower mass fluxes also further hinder heat transfer in the upward flows, and multiple peaks are observed in the axial wall temperature profile. Moreover, it is found that the heat-flux distribution has a significant effect on the heat transfer performance of upward flows; the heat transfer further deteriorates and the irreversibility is further increased when a linearly decreasing heat-flux distribution is applied to the wall, while the heat transfer deterioration is alleviated when a linearly increasing heat-flux distribution is used. An analysis of the heat transfer mechanism indicates that the turbulence production in the core region of the supercritical flow is suppressed, and the accumulation of gas-like fluid in the near-wall region is promoted by the buoyancy effect in upward flows, leading to severe heat transfer deterioration and a sharp increase in the wall temperature, which is similar to the critical heat-flux phenomenon in subcritical boiling. The present study provides insights into the heat transfer characteristics of SCO2 flows, as well as practical guidance on the design and optimisation of relevant components and equipment.(c) 2022 Elsevier Ltd. All rights reserved.