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This paper investigates the effect of changing the bending radius of pipes on the maldistribution of velocity and turbulence of thermal fluid when flowing through a u-shaped tube bundle used in compact heat exchangers, among other applications. The study included three bending radii corresponding to successive rows of the actual tube bundle of a compact heat exchanger. Both liquid flow velocities recommended for compact heat exchangers and velocities elevated from the recommended ones were adopted. The results of the study were obtained by Computational Fluid Dynamics (CFD) and the performed experiment using the Particle Image Velocimetry (PIV) method. The limits of maldistribution were indicated by parameterizing this phenomenon with related geometric and flow values (turbulent flow intensity factor, flow velocity, pipe diameter, and bending radius). An increase in flow velocity above the recommended values did not result in a significant increase in turbulent flow intensity factor for u-tubes with large d/rg values. The shortest distance at which the return to steady-state flow conditions in a straight section of pipe downstream of an elbow took place was determined. This distance was 17d for geometry rg = 0.009 m, with velocity vp = 1.44 m/s. The localization of the areas of highest and lowest fluid velocity in the elbow element of the u-tube for extreme values of rg was opposite. This fact has an exploitable significance (non-uniform erosive effect of thermal fluid on pipes in different rows). [ABSTRACT FROM AUTHOR]

• A CFD model of powder spraying in modified Wurster apparatus was developed. • The model was validated using a digital PIV method. • The Euler–Lagrange approach using the discrete phase model (DPM) was applied. • The model enabled the prediction of air-powder mixture movement in the device. • The powder velocity was mainly determined by the spouting airflow rate. The aim of these studies was to examine the suitability of the Euler–Lagrange approach for modelling the powder spraying process performed with a pneumatic nozzle used for dry coating in spout-fluid bed apparatus. Simulation results were validated using the digital PIV (Particle Image Velocimetry) method and measurement equipment constructed by the authors. The validation process led to the determination of the optimal Ansys Fluent solver settings, which ensure a satisfactory accuracy of the simulation results (the average relative error of all simulation points at the level of 10.5%). The experimental results are most accurately reflected by simulation results obtained with the use of the standard k − ε model that has the discrete random walk (DRW) model activated. The detailed analysis of the operation of the spout-fluid bed apparatus allowed for the examination of the influence of hydrodynamic parameters (the mass flow rate of the gas spraying the powder and plasticizer, as well as spouting gas) on the velocity and concentration of the dispersed phase in the device. The scientific investigation covered the analysis of the velocity profiles and the volume fraction of the coating powder in different zones of the column. [ABSTRACT FROM AUTHOR]

Energy efficiency is a key issue of sustainable development. During the design of industrial devices, it strives to achieve the highest possible energy efficiency. In the industrial systems, two-phase flow is a difficult task, especially the prediction, and maintenance of the two-phase flow regime. That is why this research proposes the evaluation and choice of an algorithm that will give a hint of the device design for which the hydrodynamic conditions of the two-phase mixture flow may be evaluated. The tests were carried out in a rectangular vertical narrow channel, as this type of device is in common use. The work aimed to show which algorithm is better for such evaluation. Parameters such as pressure drop, heat, mass, and momentum transfer are influenced by the phase velocity field. Still, various models are used for the determination of the velocity field. Therefore, there is a problem of choosing a model that will give the results closest to the real conditions. Flow visualization gives the non-invasive determination of the actual velocity field. An analysis of the velocity field was performed, which showed that for different two-phase flow regimes there are differences for given algorithms. The following algorithms were used to determine the velocity vector field: adaptive correlation method and adaptive particle image velocity method were used which are the parts of the general digital particle image velocimetry. The determination of the velocity fields in the quantitative and qualitative assessment of a given two-phase flow regime was obtained. The result of the research is the evaluation of algorithms for characterization two-phase gas-liquid flow. [ABSTRACT FROM AUTHOR]

The paper presents the results of research on liquid flow maldistribution in the shell side of a shell-and-tube heat exchanger (STHE). This phenomenon constitutes the reason for the formation of the velocity reduction area and adversely affects heat transfer and pressure drop. In order to provide details of the liquid distribution in STHE, two visualization methods were utilized. First, computational fluid dynamics (CFD) code coupled with the k-ε model and the laser-based particle image velocimetry (PIV) technique was applied. The tests were carried out for a bundle comprising 37 tubes in an in-line layout with a pitch dz/t = 1.5, placed in a shell with Din = 0.1 m. The STHE liquid feed rates corresponded to Reynolds numbers Rein equal to 16,662, 24,993, and 33,324. The analysis demonstrated that the flow maldistribution in the investigated geometry originates the result of three main streams in the cross-section of the shell side: central stream, oblique stream, and bypass stream. For central and oblique streams, the largest velocity reduction areas were formed in the wake of the tubes. On the basis of the flow visualization, it was also shown that the in-line layout of the tube bundle helps to boost the wake region between successive tubes in a row. Additionally, unfavorable vortex phenomena between the last row of tubes and the lower part of the exchanger shell were identified in the investigations. The conducted studies confirmed the feasibility of both methods in the identification and assessment of fluid flow irregularities in STHE. The maximum error of the CFD method in comparison to the experimental methods did not exceed 7% in terms of the pressure drops and 11% in the range of the maximum velocities. [ABSTRACT FROM AUTHOR]

Flow structure maps are the most useful tool in the process of identifying structures formed in multiphase flows. They can be defined as a graphical way to present the transition boundaries of flow structures, depending on the characteristic parameters of the phase transformation or flow, such as the vapor quality, the void fraction, the mass flow density, or the velocities of individual phases. Maps are usually two-dimensional drawings, described with a minimum of two selected parameters or quantities describing the phenomenon. The oldest maps of flow structures concerned adiabatic flows, mainly two-phase water-air systems or three-phase water-oil-air systems in conventional channels. Maps of nonadiabatic flow structures are simple and allow the selection of an appropriate model to determine the heat transfer coefficient as well as the flow resistance of the refrigerant. This has a major impact on the design of flow devices, where two- and even threephase flows. This paper includes reviews of proposed maps of multiphase flow structures by various authors. [ABSTRACT FROM AUTHOR]

This paper presents experimental studies on the optimization of two-phase fluid flow in an airlift pump. Airlift pumps, also known as mammoth pumps, are devices applied for vertical transport of liquids with the use of gas. Their operating principle involves the existence of a density gradient. This paper reports the results of experimental studies into the hydrodynamic effects of the airlift pump. The studies involved optical imaging of two-phase gas-liquid flow in a riser pipe. The visualization was performed with high-speed visualization techniques. The studies used a transparent model of airlift pump with a rectangular cross-section of the riser. The assessment of the airlift pump operation is based on the image grey-level analysis to provide the identification of two-phase flow regimes. The scope of the study also involved the determination of void fraction and pressure drops. The tests were carried out in a channel with dimensions 35 × 20 × 2045 mm with the gas flux range 0.2–15.0 m3/h. For the assessment of the two-phase flow pattern Probability Density Function (PDF) was applied. On the basis of the obtained results, a new method for selecting the optimum operating regime of airlift pump was derived. This method provides the finding of stability and efficiency of liquid transport. It can also be applied to determine the correlation between the total lifting efficiency and the required gas flux for proper operation of the airlift pump. [ABSTRACT FROM AUTHOR]

Highlights • The CFD model is used for the description of liquid atomization. • Impact of the model parameters on the calculated droplets velocity is investigated. • The secondary phenomena have the greatest impact on the droplets velocity. • The results are compared with our own experimental data. • The calibrated model is used to describe the plasticizer atomization. Abstract The aim of this study was to develop a CFD model for initial qualitative analysis of the operation of the pneumatic nozzle used for dry coating. The calibration consisted in comparing the droplets velocity yielded by the model with our own experimental data. The impact of the sheet and ligament constants as well as of the application of secondary coalescence and breakup models on dispersed phase velocity were taken into consideration. It turned out that the secondary phenomena had the largest impact on the dispersed phase velocity. Only after they had been taken into consideration was the agreement between the computation results and empirical data satisfactory. Afterwards, the best set-up of the model was used for describing the plasticizer atomization in a draft tube of a spout-fluid bed apparatus. It was concluded that this process should be carried out with the atomizing gas at a pressure of 2 ∙ 105 Pa. [ABSTRACT FROM AUTHOR]

[Display omitted] • A large (30) number of geometrical configurations of the rod were analyzed. • The application of the rod inside the cyclone significantly improved its performance. • The use of the rod in each of the variants increased the value of overall separation efficiency. • The use of the rod allowed to reduce the Eu in each of the proposed geometries. • The use of the rod stabilized the flow in the area of the inner vortex. The study explored areas related to the effect of installing an additional element in the axis of the cyclone separator (in the form of a rod with a circular cross-section) on its performance. 30 geometrical variants (the variable parameters included rod diameter and length) were used for this purpose. The research was carried out using three methods – CFD (based on LES and DPM models), experimental research and stereo-PIV. The use of three research methods made it possible to conduct the process of validating the results. The placement of a rod with a circular cross-section on the axis of the cyclone separator significantly improved its performance (overall separation efficiency, Stk 50 and Eu). The most beneficial variant led to an increase in the overall separation efficiency by 8.2% and a reduction in Stk 50 by 25.5%. In the case of a pressure drop (expressed as Eu), all variants generated a lower value of Eu than the base variant - the maximum reduction observed was 23.9% (as compared to the base model). Additionally, to comprehensively study the effect of using an additional element on the flow field, the mean and fluctuating velocity and pressure fields were analyzed for individual variants and compared with the standard geometry. Furthermore, it was observed that the use of a rod stabilized the flow in the region confined to the inner vortex, increased the symmetry of the flow, and improved the performance of cyclone separators. [ABSTRACT FROM AUTHOR]

This article reports the results of a study into the effect of operating parameters on the occurrence and course of gas–liquid two-phase phenomena during the fogging process carried out with the use of a conical pressure-swirl nozzle. Four alternatives of the stub regulation angles and four values of pressure of air supply to the nozzle were tested as part of the current research. The range of the investigated variables was common for the operation of fumigators used to prevent the spread of SARS-CoV-2 virus. The liquid flow rate (weighting method), the field of velocity, and turbulent flow intensity factor, as well as velocity profiles over the section of 1 m from the nozzle were determined using the particle image velocimetry (PIV) technique. The obtained results were correlated with the measurements of the diameters of spray droplets using the laser light scattering (LLS) technique. On the basis of this research, a dependence between the nozzle parameters and the spray cone pattern was identified in terms of dynamics and droplet diameter distribution. As a result of the research, a wide range of parameters were identified in which the fogging process was carried out in a stable and repeatable manner. There were exceptions to this rule only in the cases when there was a deficiency of the liquid necessary to generate a two-phase mixture. [ABSTRACT FROM AUTHOR]

• 15 variants of different geometrical configurations of the vortex finder were analyzed (LES and experimental). • An optimally configured geometry of the vortex finder increased overall separation efficiency by 16%. • The diameter of the vortex finder had significant effect on Eu (max. values increased by about 660%). • The length of the vortex finder mildly affected Eu. The aim of this study was to analyse the effect of varying the vortex finder diameter and its insertion length on the flow field and overall performance in a square cyclone. A total of 15 different vortex finder configurations with 5 different diameters, each having 3 different insertion lengths, were analyzed. An in-depth analysis of flow field inside the separator was conducted by using stereoscopic particle image velocimetry (Stereo-PIV) and large eddy simulation (LES). The former makes use of two cameras that focused simultaneously over the region of interest to record the details of the flow field, whereas the latter approach directly resolves a large part of the flow and provides the three-dimensional time-dependent details of the turbulent flows. The results extracted from PIV and LES – viz. mean tangential, mean axial, and mean radial velocities – were compared, and good agreement was observed between the two. In addition, the separation efficiency and Euler number (Eu) were evaluated experimentally and by using LES. Conclusive results indicate that the geometric shapes of the vortex finder have a significant effect on the performance of square cyclone separators. Decreasing the diameter of the vortex finder resulted in a considerable increase in Eu compared to the base variant (maximal increase in Eu amounted to about 660%). However, increasing the diameter of the vortex finder decreased Eu by about 55%. An optimally configured geometry of the vortex finder increased overall separation efficiency by 16%. [ABSTRACT FROM AUTHOR]

Counteracting the spread of the SARS-CoV-2 virus is a current and important problem. A crucial issue in this area is the disinfection of various surfaces, as well as the air itself. For this purpose, devices such as foggers, which have different designs, are used. The appropriate size of droplets and their distribution determines the effectiveness of disinfectants. The paper presents droplet size distributions and characteristic mean droplet diameters, which are obtained with the use of a conical pressure-swirl atomizer. For the purpose of the analysis, the laser diffraction method was used. The influence of gas pressure and the distance from the atomizer's orifice on the spray angle and the distance between the nozzles on the spray curtain was also demonstrated. • The laser diffraction method was used. • The droplet size distributions and droplet mean diameters were analyzed. • The effect of gas pressure and the distance from the orifice were presented. • The spray angle was determined. • The distribution of the nozzles on the curtain's support frame was analyzed. [ABSTRACT FROM AUTHOR]