On the modelling of population balance in isothermal vertical bubbly flows - average bubble number density approach

To model the spatial evolution of the geometrical structure of the gas bubbles in isothermal vertical bubbly flow conditions, the population balance approach has been employed and merged with the three-dimensional two-fluid model. The population balance is realized by incorporating an average bubble number density transport equation into a commercial computational fluid dynamics (CFD)codeANSYS CFX 10. The coalescence and breakage effects of the gas bubbles are formulated according to the bubble coalescence by random collision driven by turbulence and wake entrainment while for bubble breakage by the impact of turbulent eddies. Three models representing these coalescence and breakage mechanisms proposed by Wu et al. [1], Hibiki and Ishii [2] and Yao and Morel [3] are assessed. Local radial distributions of the five primitive variables in bubbly flows: void fraction, Sauter mean diameter, interfacial area concentration, and gas and liquid velocities, are compared with two experimental data of Liu and Bankoff [4,5] and Hibiki et al. [6]. Close agreements between the predictions and measurements demonstrated the capability of the average bubble number density transport equation in modelling bubbly flow conditions

Modelling horizontal gas-liquid flow using averaged bubble number density approach

In this study, the internal phase distributions of gas-liquid bubbly flow in a horizontal pipe have been predicted using the population balance model based on Average Bubble Number Density approach. Four flow conditions with average gas volume fraction ranging from 4.4% to 20% have been investigated. Predicted local radial distributions of void fraction, interfacial area concentration and gas velocity have been validated against the experimental data. In general, satisfactory agreements between predicted results and measured values have been achieved. For high superficial gas velocity, it has been ascertained that peak local void fraction of 0.7 with interfacial area concentration of 800 m-1 can be encountered near the top wall of the pipe. Some discrepancies have nonetheless been found between the numerical and experimental results at certain locations of the pipe. The insufficient resolution of the turbulent model in fully accommodating the strong turbulence in the current pipe orientation and the inclusion of additional interfacial force such as the prevalent bouncing force among bubbles remain some of the outstanding challenging issues need to be addressed in order to improve the prediction of horizontal gas-liquid bubbly flow

A study of drag force in isothermal bubbly flow

Driven by the extensive demands of simulating highly concentrated gas bubbly flows in many engineering fields, numerical studies have been performed to investigate the neighbouring effect of a swarm of bubbles on the interfacial drag forces. In this study, a novel drag coefficient correlation (Simonnet et al., 2007) in terms of local void fraction coupled with the population balance model based on average bubble number density (ABND) has been implemented and compared with Ishii-Zuber densely distributed fluid particles drag model. The predicted local radial distributions of three primitive variables: gas void fraction, Sauter mean bubble diameter, and gas velocity, are validated against the experimental data of Hibiki et al. (2001). In general, satisfactory agreements between predicted and measured results are achieved by both drag force models. With additional consideration for closely packed bubbles, the latest coefficient model by Simonnet et al. (2007) shows considerably better performance in capturing the reduction of drag forces incurred by neighbouring bubbles

Numerical investigation on the performance of coalescence and break-up kernels in subcooled boiling flows in vertical channels

In order to accurately predict the thermal hydraulic of two-phase gas-liquid flows with heat and mass transfer, special numerical considerations are required to capture the underlying physics: characteristics of the heat transfer and bubble dynamics taking place near the heated wall and the evolution of the bubble size distribution caused by the coalescence, break-up, and condensation processes in the bulk subcooled liquid. The evolution of the bubble size distribution is largely driven by the bubble coalescence and break-up mechanisms. In this paper, a numerical assessment on the performance of six different bubble coalescence and break-up kernels is carried out to investigate the bubble size distribution and its impact on local hydrodynamics. The resultant bubble size distributions are compared to achieve a better insight of the prediction mechanisms. Also, the void fraction, bubble Sauter mean diameter, and interfacial area concentration profiles are compared against the experimental data to ensure the validity of the models applied

Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2

The use of MgB2 in superconducting applications still awaits for the development of a MgB2-based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured Hc2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.Comment: 12 pages, 3 figure

Surface induced selective delamination of amphiphilic ABA block copolymer thin films

This is the result of an ongoing collaboration with Dr. N. Sommerdijk’s Biomaterials group at the University of Eindhoven (the Netherlands) and illustrates the close collaboration that exists in pursuing the design and application of novel polymeric materials between the two groups. This details work on a physical phenomenon (selective delamination) and key materials (amphiphilic block copolymers) that have subsequently been applied in the design of novel biomaterials. These results have appeared in a larger body of work including Advanced Materials, Angewandtie Chemie International Edition and the Journal of Materials Chemistry

Mindfulness-Based Interventions in Recurrent Ovarian Cancer: A Mixed-Methods Feasibility Study.

A recurrence of cancer is a traumatic and stressful experience, and a number of approaches have been proposed to manage or treat the associated psychological distress. Meditative techniques such as mindfulness may be able to improve an individual's ability to cope with stressful life events such as cancer diagnosis or treatment. This single-arm mixed-methods study primarily aimed to determine the feasibility of using a mindfulness-based intervention in managing psychosocial distress in recurrent ovarian cancer. Twenty-eight participants took part in a mindfulness-based program, involving six group sessions, each lasting 1.5 hours and delivered at weekly intervals. The study found that the mindfulness-based intervention was acceptable to women with recurrent ovarian cancer and feasible to deliver within a standard cancer care pathway in a UK hospital setting. The results suggested a positive impact on symptoms of depression and anxiety, but further study is needed to explore the effectiveness of the intervention

Co-combustion characteristics and kinetics of microalgae chlorella vulgaris and coal through TGA

Published online: 26 Dec 2018The combustion characteristics and kinetics of microalgae (Chlorella vulgaris) and sub-bituminous coal blends (CCBs) are studied by a thermogravimetric analyzer (TGA), and those of pure Chlorella vulgaris (C. vulgaris) and coal were also taken respectively as control groups. The microalgae to coal blending ratio (MCR) is 3/7, 5/5 and 7/3. The results showed that three stages were observed during the combustion of CCBs. And the main combustion of CCBs was occurred at the second stage ranged from 254.6 ~ 389.4°C to 698.7 ~ 741.0°C. Both of the ignition temperature (Ti) and the final temperature detected when stabilization of samples mass occurred (Tf) were decreased as the C. vulgaris content increased in the CCB. The maximum combustion rate (Rmax) of C. vulgaris was maximum. The average reaction rate (Rv) was firstly decreased, and then increased as the content of C. vulgaris in CCBs increasing. With the increasing content of C. vulgaris, both of the ignition index (Di) and the comprehensive combustion characteristic index for the blends (SM) were increased. Some deviations from their expected characteristics indicate interaction. As the heating rate (β) increases, Ti, the peak temperature (Tp), the reaction rate at the peaks (Rp), Rv and Tf were all increased significantly, while the residual mass (Mr) was first increased, and then decreased. For CCBs, the activation energy (E) was the first decreased, and then increased, and the minimum E was obtained as MCR = 5/5. Among all the samples, E of pure coal was the minimum one. Finally, kinetic triplets were determined by the Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and master-plots method, they are respectively E = 62.90, 108.99, 85.28, 92.27, 104.98 kJ/mol, the reaction order (n) = 1.4, 4.1, 2.7, 3.2, 4 and the frequency factor (A) = 6.38 × 105, 1.05 × 106, 2.29 × 104, 8.73 × 104, 2.93 × 106 min−1 for the coal, blends with MCR of 3/7, 5/5, 7/3 and C. vulgaris combustion at β = 20°C/min.Chunxiang Chen, Qing Nian Chan, Paul R. Medwell and Guan Heng Yeo

Adapting to Change: The State of Singapore Private Enterprise in China

Academy of International Business. Southeast Asia Regional Conference 2013, December 5-7, Bali, Indonesia</h3