1,721,207 research outputs found
The effect of electrolyte concentration on counter-current gas–liquid bubble column fluid dynamics: Gas holdup, flow regime transition and bubble size distributions
No full textElectrolytes are well-known to suppress the coalescence, stabilize the homogeneous flow regime and increase the gas holdup; however, there is a lack of studies concerning the influence of electrolyte concentration on counter-current bubble column fluid dynamics. In this paper, we contribute to the existing discussion by experimentally investigating the effect of electrolyte concentration (sodium chloride, NaCl) on gas holdup, flow regime transition and bubble size distributions in a large-diameter and large-scale counter-current bubble column (5.3 m in height and 0.24 m inner diameter). We considered gas superficial velocities in the range of 0.004–0.20 m/s and liquid superficial velocities up to −0.06 m/s. Air was used as the dispersed phase and various aqueous solutions of NaCl—up to the critical concentration—were used as the liquid phase. The gas holdup measurements were used to investigate the bubble column fluid dynamics and to analyze the flow regime transition. The image analysis was applied, near the sparger and in the developed region, to study the bubble size distributions in batch and in counter-current modes. The presence of NaCl—up to a critical concentration—stabilizes the homogeneous flow regime, increases the gas holdup and shifts the bubble size distribution toward lower bubble diameters. The counter-current mode destabilizes the homogeneous flow regime when using tap water and stabilizes the homogeneous flow regime when using aqueous solution of NaCl. The results suggest that the changes in the bubble size distributions stabilize/destabilize the homogeneous flow regime and, thus, increase/decrease the gas holdup. © 2016 Institution of Chemical Engineer
Role of k-w closure coefficients for free jet turbulence modelling
No full textThe present work is focused on the prediction of the fluid dynamics behaviour for natural gas burners characterized by low NOX emissions by using a Computational Fluid Dynamics (CFD) analysis. One of the major critics addressed to CFD simulations for such complex physics is linked to the accuracy of turbulence models not able to properly predict the behaviour of the jet entering the chamber. For this reason some well known turbulence models are examined in this work in order to validate the CFD approach for both high and low Reynolds jet. The authors want to show that a case by case understanding of the flow regime and the physics may help in extending the range of validity of RANS turbulence models based on the eddy viscosity concept and, in particular, of the k-two-equations turbulence models given by Wilcox in 1988 and 1998. The experimental data used as reference, come from two set of tests related to different isothermal flow behaviour: high Reynolds number (Re= 68000) and low Reynolds number (Re= 5400). Based on the evidence that at low Reynolds number the hypothesis of homogeneous and isotropic turbulence eddies for the dissipative scale is no longer valid, a modification of k-turbulence model’s closure coefficients has been proposed for the low Reynolds case, accordingly to the hypothesis of free-shear mixing (hereafter referred to as “Briggs’s approach”). This leads to a better agreement with the experimental data
Holdup and local flow properties in a counter-current bubble column
No full textGas-liquid bubble column reactors are met in several industrial plants in the chemical and oil industry fields. The understanding of the global hydrodynamics, the local flow phenomena and the bubble properties are of fundamental importance in the comprehension of the flow dynamics as well as the mass transfer phenomena. This paper describes the experimental results obtained in a counter-current circular column of 240 mm in terms of the fluid dynamic behavior. The counter-current flow studied concerns an upward flow of air and a downward flow of liquid at ambient temperature and pressure. Air is introduced by means of a spider sparger distributor. Different liquids have been used as working fluids: water, water with NaCl (at different concentrations) and mixtures of water and ethanol. The following range of operating conditions was analyzed: superficial air velocities up to 20 cm/s and superficial liquid velocities up to - 11 cm/s, corresponding to a global air volume fraction (the holdup) up to 26%. The experimental investigations concerns (i) flow visualization, (ii) local data from a double fiber optical probe and (iii) holdup measurements. The images obtained from an optical camera were used to observe the general flow patterns and obtain information concerning the bubble shape. The data obtained from the double fibre optical probe were used to study the local flow characteristics at different radial position for different operating conditions. In particular, the local void fractions, the bubble velocities, the bubble mean diameters and the bubble diameter distributions and are presented and discussed. The bed expansion technique was used to obtain the gas holdup measurements for every operating condition. The gas holdup measurements are discussed, compared with existing correlations and used for investigating the flow regime transitions. Finally, the gas holdup and the local void fraction measurements data are compared and used for understanding the local hydrodynamics
Analisi numerica dello scambio termico nel flusso monofase in condotti internamente microalettati
No full textAnalisi numerica con modellazione RANS e LES del campo termofluidodinamico in un canale corrugato
No full textInfluence of electrolyte concentration on holdup, flow regime transition and local flow properties in a large scale bubble column
Full text linkWe experimentally investigate the influence of the electrolyte concentration on holdup, flow regime transition and local flow properties in a large scale bubble column, with air and water as working fluids. The column is 0.24 m inner diameter, 5.3 m height and the air is introduced by a spider sparger up to a superficial gas velocity of 0.2 m/s. The influence of five NaCl concentrations are investigated by using gas holdup and optical probe measurements. The gas holdup measurements are used for analysing the flow regime transition between the homogeneous and the transition regime and the optical probe is used for studying the local flow characteristics at different radial positions. The presence of NaCl - up to a critical concentration - increases the gas holdup. The increase in the gas holdup is due to the inhibition of the coalescence between the bubbles and, thus, the extension of the homogeneous regime. The results are in agreement with the previous literature on smaller bubble columns
L'analisi computazionale come strumento di thermal management nella progettazione di componenti elettronici
No full text- …
