Sukanta K. Dash

Mixing in a tundish and a choice of turbulence model for its prediction

The Navier‐Stokes equation and the species continuity equation have been solved numerically in a boundary fitted coordinate system comprising the geometry of a single strand bare tundish. The solution of the species continuity equation predicts the time evolution of the concentration of a tracer at the outlet of the tundish. The numerical prediction of the tracer concentration has been made with nine different turbulence models and has been compared with the experimental observation for the tundish. It has been found that the prediction from the standard k‐e model, the k‐e Chen‐Kim (ck) and the standard k‐e with Yap correction (k‐e Yap), matches well with that of the experiment compared to the other turbulence models as far as gross quantities like the mean residence time and the ratio of mixed to dead volume are concerned. It has been found that the initial transient development of the tracer concentration is best predicted by the low Reynolds number Lam‐Bremhorst model (LB model) and then by the k‐e RNG model, while these two models under predict the mean residence time as well as the ratio of mixed to dead volume. The Chen‐Kim low Reynolds number (CK low Re) model (with and without Yap correction) as well as the constant effective viscosity model over predict the mixing parameters, i.e. the mean residence time and the ratio of mixed to dead volume. Taking the solution of the k‐e model as a starting guess for the large eddy simulation (LES), a solution for the LES could be arrived after adopting a local refinement of the cells twice so that the near wall y+ could be set lower than 1. Such a refined grid gave a time‐independent solution for the LES which was used to solve the species continuity equation. The LES solution slightly over predicted the mean residence time but could predict fairly well the mixed volume. However, the LES could not predict both the peaks in the tracer concentration like the k‐e, RNG and the Lam‐Bremhorst model. An analysis of the tracer concentration on the bottom plane of the tundish could help to understand the presence of plug and mixed flow in it.

Performance evaluation of a green process for microalgal CO2 sequestration in closed photobioreactor using flue gas generated in-situ

In the present study, carbon-dioxide capture from in situ generated flue gas was carried out using Chlorella sp. in bubble column photobioreactors to develop a cost effective process for concomitant carbon sequestration and biomass production. Firstly, a comparative analysis of CO2 sequestration with varying concentrations of CO2 in air-CO2 and air-flue gas mixtures was performed. Chlorella sp. was found to be tolerant to 5% CO2 concentration. Subsequently, inhibitory effect of pure flue gas was minimized using various strategies like use of high initial cell density and photobioreactors in series. The final biofixation efficiency was improved by 54% using the adopted strategies. Further, sequestered microalgal biomass was analyzed for various biochemical constituents for their use in food, feed or biofuel applications.

A numerical and experimental investigation on the coefficients of discharge and the spray cone angle of a solid cone swirl nozzle

Abstract Numerical and experimental investigations have been made on the coefficient of discharge C d and the spray cone angle ψ of a swirl spray solid cone pressure nozzle. The theoretical predictions are made from a numerical computation of flow in the nozzle using the standard k – e model of turbulence. The values of C d and ψ have been evaluated from the radial distributions of velocity components of liquid flow at the nozzle exit. The experiments have been carried out to measure the values of C d and ψ of a solid cone spray nozzle at different operating conditions to validate the numerical predictions. It has been established, from a fair agreement between the theoretical and experimental results, that the adaptation of the standard k – e model for turbulence in nozzle flow serves well the purpose of predictions of C d and ψ within the range of operating parameters studied in the present work. It has been observed that the coefficient of discharge and the spray cone angle remain almost constant with the Reynolds number Re of the flow at inlet to the nozzle. The coefficient of discharge C d is almost uninfluenced by inlet swirl number S in its lower range, but decreases with S in its higher range. The spray cone angle ψ , on the other hand, always increases with an increase in S. For a given Re , an increase in flow ratio q r (the ratio of flow rate through inlet central port to the total flow through the nozzle) increases the value of C d and decreases the value of ψ . However, the influence of q r on C d is prominent at lower values of D 2 / D 1 (the ratio of the diameters of inlet axial port to the swirl chamber of the nozzle). An increase in the value of C d takes place with a decrease in D 2 / D 1 mainly in the range of higher q r and for values of D 2 / D 1 less than 0.17. The spray cone angle ψ , on the other hand, is almost uninfluenced with D 2 / D 1 , except in the situation when ψ increases with an increase in D 2 / D 1 , from 0.38 to 0.75 mainly in the lower range of flow ratio q r .

Effect of outlet positions and various turbulence models on mixing in a single and multi strand tundish

The Navier‐Stokes equation and the species continuity equation have been solved numerically in a boundary fitted coordinate system comprising the geometry of a large scale industrial size tundish. The solution of the species continuity equation predicts the time evolution of the concentration of a tracer at the outlet of a single strand bare tundish. The numerical prediction of the tracer concentration has been made with three different turbulence models; (a standard k‐e, a k‐e RNG and a Low Re number Lam‐Bremhorst model) which favorably compares with that of the experimental observation for a single strand bare tundish. It has been found that the overall comparison of k‐e model with that of the experiment is better than the other two turbulence models as far as gross quantities like mean residence time and ratio of mixed to dead volume are concerned. However, it has been found that the initial transient development of the tracer concentration is best predicted by the Lam‐Bremhorst model and then by the R...

Formation of Air Core in Nozzles With Tangential Entry

The process of formation of air core and its development with time, inside one cylindrical and one conical nozzle having two tangential entries, has been analyzed experimentally and numerically. Experiments have been carried out using Plexiglas nozzles and water in ambient air; the air core has then been photographed for different nozzles and flow parameters. Numerical simulations have been performed using a finite volume method that employs unstructured grids with cell-wise local refinement and an interface-capturing scheme to predict the shape of the air core. The shape of the air core inside the cylindrical nozzle is found to be helicoidal at steady state for higher inlet velocity, whereas the shape of the free surface remains nearly cylindrical for low inlet velocity. In the conical nozzle, the air core is nearly axisymmetric in experiments. So only two-dimensional simulations are performed; the air core widens at the end of conical section as it approaches nozzle exit

Employment of different turbulence models to the design of optimum steel flows in a tundish

The Navier‐Stokes equation and the species continuity equation have been solved numerically in a boundary fitted coordinate system comprising the geometry of a large scale industrial size tundish. The solution of the species continuity equation predicts the time evolution of the concentration of a tracer at the outlets of a six strand billet caster tundish. The numerical prediction of the tracer concentration has been made with six different turbulence models (the standard k‐e, the k‐e RNG, the Low Re number Lam‐Bremhorst model, the Chen‐Kim high Re number model (CK), the Chen‐Kim low Re number model (CKL) and the simplest constant effective viscosity model (CEV)) which favorably compares with that of the experimental observation for a single strand bare tundish. It has been found that the overall comparison of the k‐e model, the RNG, the Lam‐Bremhorst and the CK model is much better than the CKL model and the CEV model as far as gross quantities like the mean residence time and the ratio of mixed to dead...

Energy and Exergy Balance in the Process of Pulverized Coal Combustion in a Tubular Combustor

A theoretical model of exergy balance, based on availability transfer and flow availability, in the process of pulverized coal combustion in a tubular air-coal combustor has been developed to evaluate the total thermodynamic irreversibility and second law efficiency of the process at various operating conditions. The velocity, temperature, and concentration fields required for the evaluation of flow availability have been computed numerically from a two-phase separated flow model on a Eulerian-Lagrangian frame in the process of combustion of pulverized coal particles in air. The total thermodynamic irreversibility in the process has been determined from the difference in the flow availability at the inlet and outlet of the combustor. A comparative picture of the variations of combustion efficiency and second law efficiency at different operating conditions, such as inlet pressure and temperature of air, total air flow rate and inlet air swirl, initial mean particle diameter, and length of the combustor, has been provided to shed light on the trade-off between the effectiveness of combustion and the lost work in the process of pulverized coal combustion in a tubular combustor.

Process integration for microalgal lutein and biodiesel production with concomitant flue gas CO2 sequestration: a biorefinery model for healthcare, energy and environment

In this study, a green microalgal feedstock based biorefinery was developed by process optimization and integration with a view to sequestering flue gas CO2 and synthesizing lutein and lipid for environmental, healthcare and biofuel applications, respectively. Out of the four microalgal cultures tested in a 2 L airlift photobioreactor, Chlorella minutissima showed comparatively higher productivities of both lutein (2.37 ± 0.08 mg L−1 d−1) and lipid (84.3 ± 4.1 mg L−1 d−1). Upon optimization of the critical process parameters using artificial neural network modeling and the particle swarm optimization (ANN-PSO) technique, the productivities of lutein and lipid were enhanced to 4.32 ± 0.11 mg L−1 d−1 and 142.2 ± 5.6 mg L−1 d−1 respectively, using pure CO2 sequestered at a rate of 1.2 ± 0.03 g L−1 d−1. One of the most interesting findings was that the lutein and lipid productivities were not significantly affected by the use of toxic flue-gas, when diluted to 3.5% CO2 with air, under the same process conditions, suggesting the possible commercial usefulness of flue-gas carbon. Another major achievement is that a single step ethanol–hexane based extraction procedure, followed by parallel saponification and trans-esterification, resulted in the simultaneous recovery of 94.3% lutein and 92.4% fatty acid methyl ester. Therefore, the potential industrial significance of this study lies in the development of an integrated biorefinery that may prove to be a sustainable technology platform towards addressing some contemporary challenges in healthcare, energy and environment through concomitant production of microalgal lutein as a nutraceutical and biodiesel as an alternative fuel, coupled with flue gas CO2 sequestration.

Numerical predictions on the influences of the air blast velocity, initial bed porosity and bed height on the shape and size of raceway zone in a blast furnace

A numerical model has been developed to predict the shape and size of the raceway zone (a void space) created by the force of the blast air injected through the tuyeres in the packed coke bed of a blast furnace. The model is based on the solution of conservation equations of both gas and solid phases as interpenetrating continua on a Eulerian–Eulerian frame. A modified k–e model has been adopted for gas phase turbulence. The solid phase constitutive equation is characterized by the solid pressure, bulk viscosity and shear viscosity, which are evaluated from the kinetic theory of random motions of granular materials in a fluid flow. The influences of the air blast velocity, initial porosity of the coke bed and the bed height on the shape and size of the raceway zone have been predicted.

Initiation of air core in a simplex nozzle and the effects of operating and geometrical parameters on its shape and size

Abstract Experimental investigations have been carried out to recognize the phenomenon of initiation of air core in a simplex type swirl spray pressure nozzle and to determine the influences of nozzle geometry and nozzle flow on the size of the fully developed air core. It has been recognized that below a certain Reynolds number at inlet to the nozzle, liquid flows full through the nozzle without the formation of an air core, while, above a certain Reynolds number at inlet to the nozzle, the formation of a fully developed central air core of cylindrical shape takes place in the nozzle. A transition flow with the development of a central air core takes place within the nozzle in between the two limiting inlet Reynolds numbers. These two limiting Reynolds numbers Re L1 and Re L2 are found to be an inverse function of the ratio of orifice to swirl chamber diameter ( D o / D s ) and a direct function of the ratio of tangential entry port to swirl chamber diameter ( D p / D s ). It has been observed that the fully developed central air core is cylindrical in shape with a sudden bulging in diameter at the entrance to the orifice. The air core diameter in both converging and orifice sections of the nozzle increases with an increase in the inlet Reynolds number ( Re ) at its lower range, while it becomes independent with Re at its higher range. The ratio of air core to orifice diameter (da 2 / D o ) increases with an increase in D o / D s or swirl chamber cone angle ( α ), and a decrease in D p / D s or L o / D s (the ratio of the orifice length to swirl chamber diameter). The ratio of air core diameter in the orifice to air core diameter in the swirl chamber of the nozzle (da 2 /da 1 ) is an inverse function of D o / D s but a direct function of D p / D s . Empirical relations of the ratios of air core to orifice diameter, da 1 / D o and da 2 / D o , with the pertinent dimensionless controlling parameters have been established.