Fluid-phase velocity fluctuations in gas-solid flows

Fluid-phase velocity fluctuations in fixed beds and freely evolving suspensions are quantified using particle-resolved direct numerical simulation (PR-DNS). The flow regime corresponds to homogeneous gas-solid systems typically encountered in fluidized beds, with high solid to fluid density ratios and particle diameters being greater than dissipative length scales. The contribution of turbulent and pseudo-turbulent fluctuations to the level of fluid-phase velocity fluctuations is quantified in flow past fixed particle assemblies. The simulations are then extended to freely evolving suspensions with elastic and inelastic collisions. For the parameter ranges considered here (volume fraction 0.1 and 0.2, particle to fluid density ratio 100 and 1000, coefficient of restitution in the range 0.7-1.0), the level of gas-phase velocity fluctuations in freely evolving suspensions differs by only 10% from the value for a fixed bed at the same solid volume fraction and mean slip Reynolds number. Quantification of the Reynolds stress indicates that the second moments of the gas-phase velocity fluctuations are anisotropic, corresponding to unidirectional axisymmetric velocity fluctuations. The anisotropy increases with Reynolds number to a maximum in the range between 10 to 40, and then decreases. In addition, the anisotropy decreases with increasing solid volume fraction for all cases considered in this study. The Reynolds stress is decomposed into isotropic and deviatoric parts, and their dependence on solid volume fraction and Reynolds number is quantified and explained

Analysis of gas-solid flow using particle-resolved direct numerical simulation: flow physics and modeling

Gas-solid flows are encountered in many industrial processes such as pneumatic conveying, fluid catalytic cracking, CO2 capture and fast pyrolysis process. In spite of several experimental and numerical studies performed to understand the physics governing observed phenomena in gas-solid flows, and to propose accurate closure models for computational fluid dynamics (CFD) simulations using the averaged conservation equations, there are several challenges in gas-solid flows that yet need to be addressed. In many of the industrial processes, the solid-to-fluid density ratio is of the order of 100 to 1000, and the particle diameter ranges from 50 to 500 micron. The interaction of heavy and large particles with the carrier phase leads to the formation of a boundary layer around each particle that in turn gives rise to interphase momentum transfer at the fluid-solid interface. The rate of work done by the carrier flow to sustain the interphase transfer of momentum leads to generation of velocity fluctuations in both the gas phase and the solid phase. Gas-phase velocity fluctuations enhance gas-particle heat transfer and the mixing of chemical species. Additionally, fluctuating motion of solid particles together with microscale hydrodynamic instabilities give rise to formation of mesoscopic particle clusters in gas-solid flows. The particle clusters then modify the hydrodynamic field and then the interconnected phenomena mentioned above dynamically modify the response of the system. Furthermore, if there exists a particle size distribution in the dispersed phase, the differences in the gas-particle and particle-particle drag forces lead to the segregation phenomenon. In this study, particle-resolved direct numerical simulation (PR-DNS) is used to address some aspects of the challenges noted above, and to propose closure models for device-scale CFD calculations. First, the level of gas-phase velocity fluctuations is quantified, and its dependence on flow parameters is explained. An algebraic Reynolds stress model is proposed by decomposing the Reynolds stress into isotropic and deviatoric parts. Also the influence of solid particles with isotropic turbulent flow has been addressed using PR-DNS. In addition, in this study the slip velocity between two particle size classes in a bidisperse mixture is quantified, which is the key signature of segregation of particle size classes. The predictive capability of two-fluid closure models in predicting the slip velocity between particle size classes is also assessed. PR-DNS is used to propose a bidisperse gas-particle drag model that improves the prediction of the mean slip velocity between the two particle size classes. In addition, the mechanism of transfer of kinetic energy from the mean flow to fluid-phase and particle velocity fluctuations in a homogeneous bidisperse suspension is explained. This mechanism of transfer of energy is important because particle velocity fluctuations affect the particle-particle drag, which jointly with the gas-particle drag on each particle class determines the mean slip velocity between the two particle classes. In this study we have also used PR-DNS to quantify the mean drag force on particle clusters that are statistically consistent with those observed in experiments. A clustered particle drag model has been proposed based on our PR-DNS results. To address the effect of filtering the hydrodynamic field on flow statistics, which is used in LES of gas-solid flows, we have shown that the source and sink of kinetic energy in particle velocity fluctuations obtained from the PR-DNS are different from those predicted by the LES approach. These differences lead to a different level of kinetic energy in the solid phase obtained from the two approaches, and thus the flow characteristics that depend on solid-phase kinetic energy, such as formation and evolution of particle clusters, may not be comparable between the PR-DNS and LES approaches. In this study we have also used PR-DNS to quantify the growth rate of mixing length in a particle-laden mixing layer, and the corresponding mechanism is identified by using a scaling analysis

Mechanism of kinetic energy transfer in homogeneous bidisperse gas-solid flow and its implications for segregation

Most gas–solid flows encountered in nature and industrial applications are polydisperse, and the segregation or mixing of particle classes in polydisperse gas–solid flows is a phenomenon of great practical importance. A statistically homogeneous gas–solid flow with a bidisperse distribution (in size or density) of particles is a canonical representation of polydisperse flows. A key feature that distinguishes the bidisperse flow from its monodisperse counterpart is the exchange of momentum and kinetic energy between the particle classes due to collisions, which are important for applications outside the very dilute regime. The average exchange of linear momentum between particle classes due to collisions occurs through the particle–particle drag term. The conservation equations for average momentum corresponding to each particle class can be used to deduce the average slip velocity between the particle size and density classes, which is the signature of particle segregation. In this canonical problem, the steady value of particle mean slip velocity results from a balance between three terms, each in turn involving the body force or the mean fluid pressure gradient, the gas–particle drag, and the particle–particle drag. The particle–particle drag depends on the particle velocity fluctuations in each class [Louge, M. Y. et al., “The role of particle collisions in pneumatic transport,” J. Fluid Mech. 231, 345–359 (1991)], thereby coupling the mean and second–moment equations. For monodisperse gas-solid flows the transfer of kinetic energy from the mean to second-moment equations was explained by Subramaniam and co-workers who proposed the conservation of interphase turbulent kinetic energy transfer principle [Xu, Y. and Subramaniam, S., “Consistent modeling of interphase turbulent kinetic energy transfer in particle-laden turbulent flows,” Phys. Fluids 19(8), 085101 (2007)], and this was subsequently verified by particle–resolved direct numerical simulation [Mehrabadi, M. et al., “Pseudo-turbulent gas-phase velocity fluctuations in homogeneous gas-solid flow: Fixed particle assemblies and freely evolving suspensions,” J. Fluid Mech. 770, 210–246 (2015)]. The principle shows that the power supplied to the mean flow to maintain the mean slip velocity between solid and gas phases partitions into transfer terms that supply power to maintain fluid and particle velocity fluctuations. One question this paper seeks to answer is what the role of particle–particle drag is in this transfer process in bidisperse flows, given that the particle–particle drag does not appear in the mixture mean momentum conservation equation for the solid phase. The conservation equations for mean momentum and kinetic energy in each particle class are coupled through interphase and interclass exchange terms. This coupling between the mean momentum and kinetic energy equations due to interphase and interclass interactions is explained by extending the conservation of interphase turbulent kinetic energy transfer principle originally proposed for monodisperse gas-solid flows to bidisperse suspensions. This explains the role of particle–particle drag in the partitioning of kinetic energy in velocity fluctuations between particle classes and provides insight into segregation and mixing of particle classes in industrial devices

Evaluation the cytotoxic effect of cytotoxin-producing Klebsiella oxytoca isolates on the HEp-2 cell line by MTT assay

Background: The cytotoxic effects on epithelial cells of the human are not observed in other strains of Klebsiella spp and are only observed in K. oxytoca strains. MTT assay was used to evaluate cytotoxic activity. In this study, colorimetric method was used to evaluate the cytotoxic effect of cytotoxin-producing isolates on Hep-2 cell line and determines the percentage of surviving cells. Materials and methods: In this study, we collected a total of 75 K. oxytoca strains isolate and we detected the production of toxins and their cytotoxic effects on HEp-2 cells. Colorimetric method such as MTT assay was used to evaluate the cytotoxic effect of cytotoxin-producing isolates on Hep-2 cell line and determines the percentage of surviving cells. Results: Nine isolates had cytotoxic effects on HEp-2 cells. The results of MTT assay showed that the isolated strains were different from the control stain in terms of toxinogenicity and cytotoxic effects on HEp-2 cells at the studied dilutions (1:3, 1:6, 1:12, 1:24, 1:48, and 1:96). Conclusions: In the current study, Percentage of Hep-2 surviving cells exposed to 1:3, 1:6, 1:12, 1:24, 1:48, and 1:96 supernatant dilutions of cytotoxin-producing Klebsiella oxytoca isolates was different

Identification of Klebsiella pneumoniae Carbapenemase-producing Klebsiella oxytoca in Clinical Isolates in Tehran Hospitals, Iran by Chromogenic Medium and Molecular Methods

Objectives: Production of carbapenemase, especially Klebsiella pneumoniae carbapenemases (KPC), is one of the antibiotic resistance mechanisms of Enterobacteriaceae such as Klebsiella oxytoca. This study aimed to investigate and identify KPC-producing K. oxytoca isolates using molecular and phenotypic methods. Methods: A total of 75 isolates of K. oxytoca were isolated from various clinical samples, and were verified as K. oxytoca after performing standard microbiological tests and using a polymerase chain reaction (PCR) method. An antibiotic susceptibility test was performed using a disc diffusion method according to the Clinical and Laboratory Standards Institute guidelines. CHROMagar KPC chromogenic culture media was used to examine and confirm the production of the carbapenemase enzyme in K. oxytoca isolates; in addition, PCR was used to evaluate the presence of blaKPC gene in K. oxytoca strains. Results: Of a total of 75 K. oxytoca isolates, one multidrug resistant strain was isolated from the urine of a hospitalized woman. This strain was examined to assess its ability to produce carbapenemase enzyme; it produced a colony with a blue metallic color on the CHROMagar KPC chromogenic culture media. In addition, the blaKPC gene was confirmed by PCR. After sequencing, it was confirmed and deposited in GenBank. Conclusion: To date, many cases of KPC-producing Enterobacteriaceae, in particular K. pneumoniae, have been reported in different countries; there are also some reports on the identification of KPC-producing K. oxytoca. Therefore

Antipseudomonal activity of Artemisia quettensis Podlech essential oil and its synergy with imipenem

Context: The problems associated with hospital infections caused by Pseudomonas aeruginosa, and the emergence of new and the re-emergence of old infectious diseases have become increasingly evident. Therefore, medicinal plants take precedence over the development of new antibacterial agents. The combination effects of antibiotics and plant compounds might be an appropriate solution for microbial resistance and useful method for assessment of synergistic interactions for inhibition of bacterial growth. This study is an experimental design for the discovery and finding of natural and harmless compounds for the treatment of infectious diseases. Aim: To determine the antibacterial potency of Artemisia quettensis essential oil, and in combination with imipenem, to inhibit the growth of Pseudomonas aeruginosa. Methods: The essential oil was obtained through hydrodistillation from aerial parts of the plant and analysis using GC and GC-MS. To demonstrate the in vitro antibacterial activity of the essential oil against Pseudomonas aeruginosa (ATCC 27853) disc diffusion assay was used, either alone or in combination with a standard antibiotic. Results: The most dominant components were homoadamantane (9.38%), Camphor (7.91%) and Eugenol (10.46%). The oil and antibiotic showed high antibacterial activity against Pseudomonas aeruginosa with minimal inhibitory concentration (MIC) 0.5 µL/mL and 16 µg/mL and minimal bactericidal concentration (MBC) 4 µL/mL and 32 µL/mL, respectively. The synergistic effect of the oil and antibiotic showed MIC 0.2 µL/mL and 4 µg/mL and MBC 2 µL/mL and 8 µL/mL, respectively. This study showed that Artemisia quettensis oil has significant antibacterial activity against Pseudomonas aeruginosa infections. Conclusions: The essential oil exhibited synergism with imipenem displaying the ability to enhance the activity of this compound and it may be useful in the fight against emerging microbial drug resistance

Identification of cytotoxin-producing Klebsiella oxytoca strains isolated from clinical samples with cell culture assays

BACKGROUND: Klebsiella oxytoca is an opportunistic pathogen which damages intestinal epithelium through producing cytotoxin tilivalline. This toxin plays a role in the pathogenesis of bacteria and is the main virulence factor which leads to antibiotic-associated hemorrhagic colitis progress. MATERIALS AND METHODS: In this study, we collected a total of 75 K. oxytoca strains isolated from the stool, urine, blood, wounds, and sputum and evaluated them in terms of the production of toxins; we detected their cytotoxic effects on HEp-2 cells. RESULTS: Of all the isolates, five K. oxytoca strains isolated from the stool cultures, two strains isolated from the blood cultures, one strains isolated from the wound cultures, and one strains isolated from the urine cultures had cytotoxic effects on HEp-2 cells. The strains isolated from sputum cultures had no cytotoxic effects on HEp-2 cells. CONCLUSIONS: In the current study, the majority of strains isolated from the stool of the patients included cytopathic effects on HEp-2 cells. Copyright © 2017 Elsevier Ltd. All rights reserved