Showing papers by "Jyeshtharaj B. Joshi published in 2013"

Large Eddy simulation for dispersed bubbly flows : a review

Abstract: Large eddy simulations (LES) of dispersed gas-liquid flows for the prediction of flow patterns and its applications have been reviewed. The published literature in the last ten years has been analysed on a coherent basis, and the present status has been brought out for the LES Euler-Euler and Euler-Lagrange approaches. Finally, recommendations for the use of LES in dispersed gas liquid flows have been made.

Entrainment phenomenon in gas–liquid two-phase flow: A review

Abstract: The gas–liquid separation equipments are aimed to be designed for maximum efficiency of phase separation. In order to maximize their capacity the flow rates are required to be optimized for the capital cost of equipment. This leads to the situation where the gas phase leaves the separation interface with high velocities and carry liquid phase along with it in the form of droplets reducing the equipment efficiency. This is known as entrainment or carryover. Depending on the nature of the separation interface i.e., turbulence intensity, bubble dynamics, the size and velocity distribution of liquid fragments, droplets at the separation interface varies. This is the main source of empiricism involved in the analysis of such equipments. The mechanics of motion of the dispersed liquid phase in bulk of gas is relatively well studied. In the present paper the various experimental, analytical and numerical investigations carried out to address the issues of entrainment/carryover are carefully analyzed. Further, a critical review has been presented for bringing out a coherent theme and a current status of the subject under reference.

Hydrodynamic study of flow in the rotor region of annular centrifugal contactors using cfd simulation

Abstract: Annular centrifugal extractors are widely used in industry for the extraction of radioactive fuel in nuclear fuel reprocessing, emulsion polymerization, crystallization, and other processes. It is a single compact unit, consisting of a mixer and a settler, that operates under the action of centrifugal force. Although the equipment is widely used, the design procedures are still empirical due to the complexity of fluid mechanics in the mixer and settler zones. For the development of rational and reliable design procedures, it is important to understand the flow patterns in these zones. This article presents a hydrodynamic study of flow within the rotor using computational fluid dynamics (CFD) with standard k-ϵ, SST k-ω, and RSM turbulence models. The rotor diameter was varied over a wide range of 125 to 375 mm and the rotor speed over 20 to 175 rad/s. A comparison is presented between the CFD predictions and experimental measurements reported in the published literature. The hydraulic performance of a roto...

Heat transfer and flow pattern in co‐current downward steam condensation in vertical pipes‐I: CFD simulation and experimental measurements

Abstract: The condensation of pure steam flowing inside a vertical tube has been extensively studied during the last nine decades. Considerable amount of experimental and analytical efforts can be found due to the significance of this subject in practice. In the present work (Part I), experimental investigations have been performed over a range of pressure (0.1 < P < 0.35 MPa) and internal tube diameter (Di = 10, 20 and 43 mm). A two-dimensional computational fluid dynamic (CFD) simulations have been carried out commercial software Fluent 6.2 [Fluent 6.2, “User's Manual to FLUENT 6.2,” Fluent Inc., Lebanon, USA, 2005]. CFD results were used to predict the temperature profiles, pressure drop and the heat transfer coefficient, which was in close agreement with the experimental values. The film characteristics predicted by the CFD simulations have been compared qualitatively with the photographic images. Further, the CFD model developed in Part I extended for the analysis of all the experimental data reported in the published literature. © 2012 Canadian Society for Chemical Engineering

Core safety of Indian nuclear power plants (NPPs) under extreme conditions

Abstract: Nuclear power is currently the fourth largest source of electricity production in India after thermal, hydro and renewable sources of electricity. Currently, India has 20 nuclear reactors in operation and seven other reactors are under construction. Most of these reactors are indigenously designed and built Heavy Water Reactors. In addition, a 300 MWe Advanced Heavy Water Reactor has already been designed and in the process of deployment in near future for demonstration of power production from Thorium apart from enhanced safety features by passive means. India has ambitious plans to enhance the share of electricity production from nuclear. The recent Fukushima accident has raised concerns of safety of Nuclear Power Plants worldwide. The Fukushima accident was caused by extreme events, i.e., large earthquake followed by gigantic Tsunami which are not expected to hit India’s coast considering the geography of India and historical records. Nevertheless, systematic investigations have been conducted by nuclear scientists in India to evaluate the safety of the current Nuclear Power Plants in case of occurrence of such extreme events in any nuclear site. This paper gives a brief outline of the safety features of Indian Heavy Water Reactors for prevention and mitigation of such extreme events. The probabilistic safety analysis revealed that the risk from Indian Heavy Water Reactors are negligibly small.

Heat transfer and flow pattern in co-current downward steam condensation in vertical pipes-II: Comparison with published work

Abstract: The condensation of pure steam flowing downward inside a vertical tube has been extensively studied. Considerable amount of experimental and analytical efforts can be found due to the significance of this subject in practice. In this work, a critical review of the most important experimental, analytical and computational fluid dynamic (CFD) investigations have been presented. CFD simulations for the geometries of Goodykoontz and Dorsch [Goodykoontz and Dorsch, NASA TN D-3326, 1966; Goodykoontz and Dorsch, NASA TN D-3953, 1967], Kim and No [Kim and No, Int. J. Heat Mass Transf. 2000;43:4031–4042] and the present work have been performed and compared with the experimental data reported in these investigations. CFD predictions of the pressure drop and the heat transfer coefficient (HTC) were in close agreement with the experimental values. A preliminary regime map has been constructed for downward flow steam condensation inside pipes. Finally, all the published semi-empirical correlations for the HTC have been critically analysed and compared with the CFD predictions. An attempt has been made to make specific recommendations. © 2012 Canadian Society for Chemical Engineering

Evaluation of local energy dissipation rate using time resolved PIV

Abstract: Numerous industrial approaches input large quantities of power to achieve a desired outcome. The question is whether that power is most efficiently transformed into process to achieve the specific purpose of heat, mass transfer, mixing etc. To optimize heat/mass transfer, small scale eddies should have maximum energy and they should be close to the interface. In contrast, optimum mixing performance is achieved by large eddies distributed uniformly over entire volume of contactor. In summary, the 'Energy budget'; i.e. how the supplied energy is distributed between eddies of different sizes is very important. Such a distribution is represented by the turbulent energy spectrum. In the current work, we utilize high speed particle image velocimetry (PIV) to record the variation of liquid velocity over a plane in time-resolved fashion. The local energy spectrum is obtained by taking FFT of velocity time series at each point in planar PIV data. A model energy spectrum by Pope ('Turbulent Flows' Cambridge University Press, 2000) was fitted to experimental spectrum giving the energy flux (equivalent to the turbulent energy dissipation rate) at each point. The salient feature of this method is the ability to calculate local energy dissipation rate without using the estimates of velocity gradients. The results obtained in current work are less susceptible to the noise in PIV velocity data. The local energy spectrum and energy dissipation rate measurements will enable us to put a close check on the energy budget in the process equipment. It will allow tailoring the turbulence to the specific application, significantly enhancing the operating efficiency.

In-flight collision behaviour of droplets on a spherical particle falling under gravity

Abstract: Droplet-particle interaction is critical in design of many industrial processes. In the current work, a systematic experimental study of the impact of water droplets on a moving glass particle falling under gravity at NTP was investigated by a high speed imaging technique. The droplet and particle impact velocities were varied in the experiment independently to observe the effect on the collision outcome. In the entire operating range, the droplets were observed to undergo inelastic collision resulting into complete deposition onto the particle surface forming a thin film. Significant deflection in the particle trajectory during collision was observed at higher droplet Weber number however this deflection was observed to decrease when particle velocity was increased. Two dimensional CFD simulations were also carried out using the Volume of Fluid method in conjunction with the dynamic meshing technique resulting in qualitative agreement with the experimental observations.

Crystal growth and morphological study of adu and its calcined product

Abstract: Ammonium diuranate (ADU) and its calcined product uranium oxides (UO 3 ) are important intermediates for nuclear fuel production. The morphology and Particle Size Distribution (PSD) of ADU powder produced at various stages of precipitation have been observed to study the particle growth of ADU. It has been observed that the ADU particles are basically agglomerates of submicron platelets. The microstructure of ADU is retained in UO 3 even after calcination. Effect of calcination temperature has been studied on tap density, surface area, porosity, O/ U ratio and morphology of uranium oxides. It has been observed that O/U ratio, bulk density and tap density of the uranium oxide are reduced with an increase in temperature. Surface area and porosity increase with temperature, pass through maxima and then reduce. These observations have been explained using high resolution SEM.

Influence of grid-generated turbulence on detachment of a bubble anchored to a vertical cylindrical surface: Application to mineral flotation Systems

Abstract: Flotation recovery of coarse particles (typically more than 100 micrometers in diameter) is adversely influenced by the liquid motion resulting from energy input associated with mixing of the gas and solid phases. In particular, the collected particles can become detached from the bubble as the particle-bubble aggregate passes through regions of different turbulence levels within the flotation cell. The dynamics of the bubble-particle-turbulence interaction is almost impossible to visualize within a real flotation system as the phases are in constant motion which changes with time and position. In this study, a stationary vertical (flat-ended) needle of size 1.24mm has been used to generate a bubble which is anchored to the top of the vertically-oriented cylindrical surface. An oscillating grid device was then applied to generate a known turbulent field around the anchored bubble. The strength of the turbulence needed to detach bubble of different size was measured. Particle image velocimetry (PIV) combined with laser induced fluorescence (LIF) technique using fluorescent tracer particles was applied to quantify the instantaneous velocity field around the disturbed bubble. The detachment of the bubble was then modeled following the same approach as that used for particle-bubble flotation detachment. The detachment process reported in this study has allowed us to obtain detailed quantitative analysis of the instantaneous flow field and can be directly related to the dynamics occurring within flotation cells.

Collision behaviour of a small spherical particle on a large stationary droplet

Abstract: In the present work, collision behaviour of a solid particle on an unconfined gas-liquid interface e.g. droplet was studied at low Weber number range. A glass ballotini particle (1.17 mm) was impacted on a stationary water droplet (3.15 mm) at the Weber number range of 0.2 to 3.6 and the collision process was captured by high speed imaging technique. It was observed, at lower to intermediate impact velocity range, that the particle was partially submerged into the droplet and slide along the convex interface whilst at higher impact velocity, the complete penetration was observed. Based on the forces acting on the particle at the interface, a simple model is proposed providing a satisfactory agreement with the experimental observations. Of all the forces involved, surface tension force was found to dominate the collision process in all the cases investigated. A 3D CFD model has also been developed incorporating the dynamic meshing technique with multiphase Volume of Fluid method which qualitatively agrees observed interface penetration behaviour.