Search

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]

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]

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]