3,153 research outputs found
Large Eddy Simulation of Liquid Metal Turbulent Mixed Convection in a Vertical Concentric Annulus
No full textIn the present study, turbulent forced and mixed convection heat transfer to a liquid metal flowing upwards in a concentric annulus is numerically investigated by means of large eddy simulation (LES). The inner-to-outer radius ratio is 0.5. The Reynolds number based on bulk velocity and hydraulic diameter is 8900, while the Prandtl number is set to a value of 0.026. A uniform and equal heat flux is applied on both walls. LES has been chosen to provide sufficiently accurate results for validating Reynolds-averaged turbulence models. Moreover, with the thermal sublayer thickness of liquid metals being much larger than the viscous hydrodynamic one, liquid metals present a separation between the turbulent thermal and hydrodynamic scales. Thus, with the same grid resolution, it is possible to perform a LES for the flow field and a “thermal” direct numerical simulation (DNS) for the temperature field. Comparison of the forced convection results with available DNS simulations shows satisfying agreement. Results for mixed convection are analyzed and the differences with respect to forced convection at the same Reynolds number are thoroughly discussed. Moreover, where possible, a comparison with air is made
Modeling of the fluid dynamics and SO2 absorption in a gas–liquid reactor
No full textThis paper illustrates a computational fluid dynamic (CFD) model of a counter-current Open Spray Tower desulphurisation reactor and its application in the simulation of a full-scale industrial equipment. The raw flue gas flows upward while a suspension of water and limestone is sprayed downward from different heights. Thereby sulfur dioxide is washed out of the gas.
The two-phase gas–liquid flow inside the equipment has been simulated with an Euler–Lagrange approach using a commercial CFD code, while a model for the SO2 absorption has been developed and implemented in the software through dedicated modules. Physical absorption is modeled using dual-film theory and appropriate empirical and semi-empirical correlations. The aqueous phase chemistry accounts for the instantaneous equilibrium reactions of eight dissolved species into a slurry droplet. The model is used to simulate an industrial plant at different operating conditions. The numerical results are in good agreement with the measured values of pressure drop and sulphur removal efficiency
Improvement of SO2 removal with application of wall rings and advanced CFD modeling. The case of FGD unit Megalopolis
No full textMegalopolis SES is a lignite-fired steam-electric power plant (850 MW) located in southern Greece. The plant consists of four units. Unit III is retrofitted with a WFGD unit in order to meet the emission values of LCP 2001/80/EC. The WFGD flue gas treatment system removes SO2 by scrubbing the flue gas with a limestone / gypsum process. Flue gas is treated in the absorber by passing the flue gas stream through proper spray levels. The gas flows upward through the absorber countercurrent to the spray liquor flowing downward through the absorber. Slurry is pumped through banks of spray nozzles to atomize it to fine droplets and uniformly contact the gas. The droplets absorb SO2 from the gas, facilitating the reaction of the SO2 with reagent in the slurry. The quantity of liquid sprayed relative to flue gas is related to the SO2 collection efficiency needed and is referred to as liquid-to-gas (L/G) ratio. Higher L/G ratios improve SO2 removal by exposing the gas to more absorbing liquor. However, higher L/G ratios also consume more power. A CFD modelling of the absorber has been used to define gas and liquid distribution inside of the absorber with the purpose to combine the optimised SO2 removal with the lowest possible power consumption. The paper shows the results of the CFD study on Megalopolis III WFGD containing the fluid dynamic optimisation of the nozzles arrangement including the wall ring (patented by Alstom) location
Direct numerical simulation and RANS comparison of turbulent convective heat transfer in a staggered ribbed channel with high blockage
No full textA turbulent convective flow of an incompressible fluid inside a staggered ribbed channel with high blockage at ReH≈4200 is simulated with direct numerical simulation (DNS) and Reynolds-averaged NavierStokes (RANS) techniques. The DNS results provide the reference solution for comparison of the RANS turbulence models. The kε realizable, kω SST, and v2f model are accurately analyzed for their strengths and weaknesses in predicting the flow and temperature field for this geometry. These three models have been extensively used in literature to simulate this configuration and boundary conditions but with discordant conclusions upon their performance. The v2f model performs much better than the kε realizable while the kω SST model results to be inadequate
CFD modeling of the Dry-Sorbent-Injection process for flue gas desulfurization using hydrated lime
No full textMultiphase Euler–Lagrange CFD simulation applied to Wet Flue Gas Desulphurisation technology
No full textThe gas and liquid hydrodynamics inside an Open Spray Tower has been simulated using a commercial CFD code, while a model that treats the absorption process of SO2 has been developed and implemented in the software through dedicated modules. Besides SO2 absorption also evaporation of slurry droplets and droplet-wall interaction are considered, the latter modeled with an empirical correlation and implemented in a sub-module.
The continuous gas phase has been modeled in an Eulerian framework, while the dispersed liquid phase with a Lagrangian approach by tracking a large number of particles through the computational domain. Physical absorption of SO2 has been modeled using dual-film theory and appropriate empirical and semi-empirical correlations. The model for aqueous phase chemistry considers instantaneous equilibrium reactions of eight dissolved species into a slurry droplet, namely: SO2aq, CO2aq, H+, OH-, HSO3-, SO3--, HCO3-, CO3--. The empirical droplet-wall interaction model handles impact, deposition and splashing events occurring when a liquid particle hits an internal element of the scrubber. A pilot plant OST has been simulated and the numerical results show good agreement with the experimental values of pressure drop, temperature and sulphur dioxide removal efficiency
Numerical simulation of the turbulent convective buoyant flow of sodium over a backward- facing step
No full textA forced convective and a buoyancy-aided turbulent liquid sodium flow over a
backward-facing step with a constant heat flux applied on the indented wall is simulated.
Linear eddy viscosity models are used for the Reynolds stresses. Turbulent heat fluxes are
modelled with a single gradient diffusion hypotheses with two different approaches to evaluate
the turbulent Prandtl number. Moreover, the influence of turbulence on heat transfer to sodium
is also assessed through simulations with zero turbulent thermal diffusivity. The results are
compared with DNS data from literature. The velocity and turbulent kinetic energy profiles
predicted by all models are in good agreement with the DNS data. The local Nusselt number
trend is qualitatively well captured, however, its magnitude is underestimated by all models
for the mixed convection case. For forced convection, the heat transfer is overestimated by all
heat flux models. The simulation with neglected turbulent heat transfer shows the best overall
agreement for the forced convection case. For the mixed convection best agreement is obtained
using a correlation to locally evaluate the turbulent thermal diffusivity
Investigation of a turbulent convective buoyant flow of sodium over a backward- facing step
No full textThe influence of buoyancy-aided mixed convection on the heat transfer downstream of a sudden expansion of a plane channel is investigated by means of the steady-state Reynolds averaged Navier Stokes (RANS) simulation. A linear eddy viscosity model is used to compute the Reynolds stresses. The turbulent heat fluxes are modelled with a single gradient diffusion hypotheses using a local correlation to evaluate the turbulent Prandtl number. The velocity, turbulent kinetic energy and Reynolds shear stress profiles predicted are in good agreement with available DNS data. The local Stanton number trend is qualitatively well captured, however, its magnitude is underestimated for the mixed convection cases. Compared to the case of forced convection, the flow field and heat transfer change significantly. An influence of buoyancy is reported at very low Richardson numbers (Ri). A steady increase in heat transfer with rising influence of buoyancy is observed
CASTOR study on technological requirements for flue gas clean-up prior to CO2-capture
No full textThe objective of this study has been to investigate the feasibility and costs of reducing the concentrations of constituents such as SO2 and NO2 in coal derived flue gases down to or approaching the levels required by the amine scrubber suppliers. This has been carried out for three separate boilers 600MW, 1000MW and 380MW firing respectively low sulphur bituminous coal, a pre-dried German lignite and a conventional high moisture lignite from Greece
THERMO-CHEMICAL EULER-LAGRANGE CFD ANALYSIS APPLIED TO WET FLUE GAS DESULPHURISATION TECHNOLOGY
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